JP5557513B2 - Chromatic dispersion measuring apparatus and chromatic dispersion measuring method using the same - Google Patents

Description

  The present invention relates to a technical field such as a chromatic dispersion measuring device for measuring chromatic dispersion of an optical pulse, and more particularly to an optical pulse propagating through an optical transmission line of an optical fiber network in a high-speed optical communication system having a transmission rate of several tens of Gbit / s. The present invention relates to a chromatic dispersion measuring apparatus for measuring chromatic dispersion and a chromatic dispersion measuring method using the same.

  In recent years, data communication has been shifting to one via an optical fiber, and along with this, the data transmission speed has been dramatically increased. In the near future, in such a high-speed optical communication system via an optical fiber, it is considered to use ultrashort optical pulses and communicate at a transmission rate of several tens of Gbit / s or higher, which is faster than the current transmission rate. Has been.

By the way, when performing data communication in a high-speed optical communication system, there is a problem that crosstalk and transmission errors always occur.
However, as the data transmission speed becomes higher, the width of each light pulse and the interval between the light pulses before and after each other become narrower, and when performing the above-described reliable data communication with the above-described crosstalk and transmission error, This is a very important issue as explained below.

  The speed at which light travels through the material is determined by the refractive index of the material, and the higher the refractive index, the slower the light speed. In materials such as glass, semiconductors, and optical crystals, the refractive index changes depending on the frequency of light (wavelength in air), so the speed of light depends on the wavelength. It is known that due to the wavelength dependence of the refractive index, the waveform of the light pulse is distorted while the light pulse travels through the material, and the time width of the light pulse is increased. Furthermore, in an optical waveguide represented by an optical fiber, the effective refractive index of the optical waveguide is determined according to the shape and size of each of the core and the cladding, and the speed of light depends on the wavelength. Therefore, the structure of the optical waveguide is also a factor that increases the time width of the optical pulse. Thus, the characteristic that the speed of light differs according to the wavelength of light is hereinafter referred to as wavelength dispersion or simply dispersion.

As described above, while traveling through the optical fiber, the waveform of the optical pulse is distorted or the time width of the optical pulse is widened due to the above-described wavelength dispersion. Since this interval is wider than chromatic dispersion, it is not a big problem.
However, if the data transmission speed is higher than several tens of Gbit / s, the chromatic dispersion becomes wider than the interval between the front and rear optical pulses, and the front and rear optical pulses interfere with each other, causing crosstalk and transmission errors. End up. For this reason, simply trying to increase the transmission speed with the current technology cannot realize data communication with higher speed and higher reliability.

In order to remove (or control) chromatic dispersion in the high-speed optical communication system described above, first, chromatic dispersion of various optical components used in the high-speed communication system is measured, and the characteristics of chromatic dispersion of each member are grasped. There is a need.
For example, there is a wavelength dispersion measuring apparatus using a spectrum shearing interferometer using a frequency shifter that measures the spectrum phase of various components in order to obtain chromatic dispersion from the change of the spectrum phase (see, for example, Patent Document 1).
In this spectrum shearing interferometer, in order to uniquely measure the spectrum phase, the cos component and the sin component of the optical pulse are converted into a horizontal polarization component and a vertical polarization component, respectively, and polarization separation is performed, so that two orthogonal components are obtained. Therefore, an interferometer is configured using a spatial optical system.

In a spectrum shearing interferometer, an optical pulse is propagated by linearly polarized light in an optical fiber constituting a part of the interferometer.
In this spectrum shearing interferometer, in order to generate the orthogonal two components of the cos component and the sin component, it is necessary to convert linearly polarized light into circularly polarized light.
This circularly polarized light is formed by superimposing two orthogonally polarized lights, ie, horizontally polarized light and vertically polarized light orthogonal to each other in the vertical and horizontal directions. There is a 90 ° phase difference between horizontally polarized light and vertically polarized light.
Therefore, by using a polarization beam splitter to spatially separate circularly polarized light into horizontal polarized light and vertical polarized light, two orthogonal components of a cos component and a sin component can be obtained.

As described above, for measurement of chromatic dispersion, it is necessary to obtain two orthogonal components of a cos component and a sin component in a plurality of wavelength bands.
In contrast, in an optical fiber, only light of a specific wavelength corresponding to the optical length of the optical fiber is propagated without changing from circularly polarized light to elliptically polarized light, and light of other wavelengths is transmitted from circularly polarized light to elliptically polarized light. The orthogonal two components cannot be maintained in a stable state, and the orthogonal two components cannot be obtained with high accuracy.

  For this reason, by using a spatial optical system in the optical path related to the separation of the orthogonal two components so that the circularly polarized light does not change to elliptically polarized light, the circularly polarized light can be stably propagated to the light of all applicable wavelengths, and cos An orthogonal two component of a component and a sin component is generated with high accuracy.

JP 2007-85981 A

However, although the chromatic dispersion measuring apparatus of Patent Document 1 can accurately obtain two orthogonal components of an optical pulse, since it uses a spatial optical system, input / output of light between the optical fiber and the spatial optical system. As a result, optical loss occurs. Due to this light loss, there is a problem that the intensity of light is lowered and the measurement sensitivity is reduced.
Further, since the chromatic dispersion measuring apparatus of Patent Document 1 uses a spatial optical system, there is a problem that the configuration of the apparatus is complicated and the size cannot be reduced due to the necessity of arranging components necessary for the spatial optical system.

  Therefore, the present invention has been made in view of the above problems, and provides a chromatic dispersion measuring device and the like that can reliably and highly stably measure the chromatic dispersion of an optical pulse, which can reduce the size of the device. With the goal.

  In order to solve the above problem, the invention according to claim 1 is an incident path (incident optical fiber 1) configured of an optical fiber having polarization maintaining characteristics that propagates an optical signal to be measured incident from a measurement target. And the measured optical signal is incident from the first incident end connected to the incident path, and the measured optical signal incident from the first incident end is converted into the first measured optical signal and the second measured optical signal. The first measured optical signal is output from the first output end, and the second measured optical signal having the same polarization direction as that of the first measured optical signal is output to the second output end. And an optical branching unit (optical branching unit 2) for generating a frequency difference between the first optical signal to be measured and the second optical signal to be measured when the optical signal is emitted and connected to the first outgoing end The first optical signal to be measured is propagated, and is composed of an optical fiber having polarization maintaining characteristics. A first branch path (first optical fiber 3) and a second branch path (connected to the second output end, which propagates the second optical signal to be measured, and is made of an optical fiber having polarization maintaining characteristics ( The phase of the optical measurement optical signal that is provided in one of the second optical fiber 4) and one of the first branch path and the second branch path and propagates through the provided branch path is 0 degrees and 90 degrees. An optical phase shifter (optical phase shift unit 9) that alternately changes by value, the first optical signal to be measured that is incident from a second incident end connected to the first branch path, and the second branch path The optical phase obtained by combining the second measured optical signal incident from the third incident end connected to the first optical signal, and interference between the first measured optical signal and the second measured optical signal The measurement light signal of the branch path provided by the shifter and the measurement light of the other branch path Interference element of the first optical component (cos component) when the phase difference between the signals is 0 degree, the measurement optical signal of the branch path where the optical phase shifter is provided, and the measurement optical signal of the other branch path When the phase difference between them is 90 degrees, the second optical component (sin component) interference element is the first combined optical signal to be measured from the third output end, and the second combined measured signal from the fourth output end. As an optical signal, a probe coupling path (probe) composed of an optical multiplexing unit (optical coupling unit 5) that emits and an optical fiber that is connected to the third emission end and propagates the first optical signal to be measured. A path coupling optical fiber 6), and a reference coupling path (reference path coupling optical fiber 7) that is connected to the fourth emission end and is configured to propagate the second combined optical signal to be measured. , A fourth incident end connected to the probe coupling path The first combined optical signal to be measured is incident, the frequency range through which the first combined optical signal to pass is swept, and the spectral component of the frequency range is extracted from the first combined optical signal to be measured An optical frequency sweeping unit (optical frequency sweeping unit 10) that emits the result of frequency decomposition as a component measured optical signal from the fifth emitting end, and a fifth incident end connected to the reference coupling path The second combined optical signal to be measured is incident, the frequency range through which the second combined optical signal to pass is swept, and the second combined optical signal to be measured is fixed within the frequency range from the second combined optical signal to be measured A spectral component of a specific frequency is extracted, and the extracted result is output as a reference light signal from the sixth emitting end (optical frequency extracting unit 13) and connected to the fifth emitting end, and the component measured Optical fiber that propagates optical signals And a reference emission light path (reference emission optical fiber 14) constituted by an optical fiber that is connected to the sixth emission end and propagates the reference light signal. And detecting the component light to be measured from the sixth incident end connected to the probe output light path, converting the component light signal to be converted into an electric signal, and using the conversion result as a probe signal. The reference light signal is incident from a part (probe light detection unit 12) and a seventh incident end connected to the reference emission light path, the reference light signal is converted into an electric signal, and the conversion result is referred to as a reference signal. The reference light detection unit (reference light detection unit 15) that synchronizes with the phase change of the optical phase shifter, the probe signal of the first light component and the second light component, the first light component and the first light component Reference signal of two light components And a control unit (control unit 16) that obtains chromatic dispersion by differentially measuring the spectral phase obtained from the probe signal based on the spectral phase obtained from the reference signal as a reference. Features.

  In order to solve the above problem, the invention according to claim 2 is provided in any one of the first branch path and the second branch path, and the optical path length difference between the first branch path and the second branch path. It further has an optical delay unit (optical delay unit 8) for adjusting the light intensity.

  In order to solve the above problem, the invention according to claim 3 is that the optical delay unit is provided in one of the first branch path and the second branch path, and the optical phase shifter is provided in the other. Features.

  In order to solve the above-mentioned problem, the invention according to claim 4 is characterized in that the optical delay unit and the optical phase shifter are integrated into one of the first branch path and the second branch path (optical delay / optical phase shift). Part 47).

  In order to solve the above-mentioned problem, the invention according to claim 5 is characterized in that the control unit receives the probe signal of the first optical component and the reference signal, and the probe of the second optical component. And a second receiver for receiving the signal and the reference signal.

  In order to solve the above-mentioned problem, the invention according to claim 6 is characterized in that the control unit measures the first optical component and the first signal for each of the probe signal and the reference signal as a measurement unit for each measurement frequency sweep in the measurement range. Two light components are acquired as a data pair in time series.

  In order to solve the above problem, the invention according to claim 7 is the polarization component that is measured first in the first light component and the second light component constituting the data pair for each of the probe signal and the reference signal. The time for performing the phase change is set shorter than the time for performing the phase change to the polarization component to be measured later.

  In order to solve the above-mentioned problem, the invention according to claim 8 is that the control unit performs either one of the first light component or the second light component in one sweep in the measurement frequency sweep in the measurement range. The process of acquiring only one of the probe signal and the reference signal is performed alternately and repeatedly for the first light component and the second light component.

  In order to solve the above-described problem, the invention according to claim 9 is configured such that the probe coupling path is configured by an optical fiber having polarization maintaining characteristics, and the first combined measured light is provided at a stage subsequent to the probe coupling path. A first polarization controller for controlling a polarization direction of the signal, and the first polarization controller and the optical frequency sweep unit are connected by an optical fiber having polarization maintaining characteristics; and the reference coupling path Is composed of an optical fiber having polarization maintaining characteristics, and a second polarization controller for controlling the polarization direction of the second combined optical signal to be measured is inserted after the reference coupling path, and the second polarization The wave controller and the optical frequency extraction unit are connected by an optical fiber having polarization maintaining characteristics.

In order to solve the above problem, the invention according to claim 10 is a chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 9,
A branching unit is provided in a portion for evaluating the chromatic dispersion of the optical transmission line to be measured, and the polarization control unit controls the polarization of the optical signal to be measured obtained from the branching unit to a linearly polarized wave. A spectrum obtained from the reference signal of the first light component and the second light component by aligning the polarization axis propagating in the measuring device and making the measured optical signal incident on the chromatic dispersion measuring device via the incident path. Using the change in phase as a reference, the change in spectral phase of the optical signal under measurement is obtained by differentially measuring the change in spectral phase obtained from the probe signal of the first light component and the second light component, The chromatic dispersion in the measurement object is evaluated.

In order to solve the above-mentioned problem, the invention according to claim 11 is a chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 9,
An optical signal subjected to polarization control is incident on the incident end of the measurement target for evaluating chromatic dispersion, and the polarization of the measured optical signal emitted from the output end of the measurement target is controlled to be linearly polarized, Aligned with the polarization axis propagating in the chromatic dispersion measuring device, the measured optical signal is incident on the chromatic dispersion measuring device through the incident path, and the reference signal of the first light component and the second light component is used. Using the obtained spectral phase change as a reference, the spectral phase change obtained from the probe signal of the first optical component and the second optical component is differentially measured to obtain the spectral phase change of the optical signal to be measured. And chromatic dispersion in the measurement object is evaluated.

According to this invention, since the interferometer of the chromatic dispersion measuring device is configured by the first branch path and the second branch path of the optical fiber having the polarization maintaining characteristic without using the spatial optical system, the conventional example As described above, light loss due to input / output of light between the optical fiber and the spatial optical system does not occur, and the light intensity of the optical signal to be measured is not reduced.
Further, according to the present invention, the first branch path and the second branch constituting the interferometer are propagated through the apparatus while maintaining the polarization of the optical signal to be measured by the optical fiber having the polarization maintaining characteristic. By switching the phase difference of the second measured optical signal propagated to the second branch path to 0 degree and 90 degrees in time series for the first measured optical signal propagated to the first branched path in the path, The first optical component (cos component) from the first and second optical signals to be measured in the same polarization state which are stable, and the second optical component (sin component) having a phase different by 90 ° from the first optical component. Each interference element can be extracted, and the chromatic dispersion of the optical pulse can be measured with higher accuracy and higher sensitivity than in the past.
In addition, according to the present invention, since the spatial optical system is not used, the configuration of the apparatus is simplified, and there is no need to arrange parts required in the spatial optical system, and the apparatus can be downsized. It becomes possible.
Further, according to the present invention, the change in the spectrum phase at a specific frequency is measured in parallel with the measurement of the change in the spectrum phase accompanying the frequency sweep, and the change in the spectrum phase at the specific frequency is used as a reference. Because it is configured to measure the difference in spectral phase change at each frequency, phase fluctuations of the optical fiber spectrum shearing interferometer due to fluctuations in the optical path length of the optical fiber can be further reduced, and highly stable measurement can be performed. .

It is a figure for demonstrating the measurement of variation | change_quantity (DELTA) phi ((nu)) of a spectral phase from an optical pulse. It is a block diagram which shows the structural example of the chromatic dispersion measuring apparatus by 1st Embodiment. In the first embodiment, the frequency sweep operation of the optical frequency sweep unit 10, the phase shift operation of the optical phase shift unit 8 corresponding to this, and the sampling of the electrical signal from the probe light detection unit 12 in the control unit 16 It is a wave form diagram which shows the timing of the operation | movement in. In the second embodiment, the frequency sweep operation of the optical frequency sweep unit 10, the phase shift operation of the optical phase shift unit 9 corresponding to this, and the sampling of the electrical signal from the probe light detection unit 12 in the control unit 16 It is a wave form diagram which shows the timing of the operation | movement in. It is a block diagram which shows the structural example of the chromatic dispersion measuring apparatus by 4th Embodiment. In the frequency sweep operation of the optical frequency sweep unit 10 in the fifth embodiment, the phase shift operation of the optical phase shift unit 9 corresponding to this, and the sampling of the electric signal from the probe light detection unit 12 in the control unit 16 It is a wave form diagram which shows the timing of operation | movement. It is a block diagram which shows the structural example of the optical frequency sweep part 10 or the optical frequency extraction part 13 by 6th Embodiment. It is a figure explaining the measuring method which measures the dispersion parameter of the optical pulse which propagates an optical fiber transmission line using the chromatic dispersion measuring apparatus of this invention. It is a figure explaining the measuring method which measures the dispersion parameter of the optical pulse which propagates an optical component using the wavelength dispersion measuring apparatus of this invention.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings. In this embodiment, for example, a main optical fiber transmission line between Tokyo and Osaka, a metro optical fiber network in an urban area, etc., and an optical pulse propagating through an optical fiber transmission line such as an optical network using wavelength division multiplexing transmission. It is an embodiment when the present invention is applied to a chromatic dispersion measuring apparatus for evaluating chromatic dispersion characteristics and a chromatic dispersion measuring method using the chromatic dispersion measuring apparatus.

[Chromatic dispersion]
First, chromatic dispersion measured by the chromatic dispersion measuring apparatus in the present embodiment will be described.

  In this embodiment, as described above, the spectral phase of an optical pulse propagating through an optical fiber transmission line is measured, and the characteristics of chromatic dispersion generated in the optical fiber transmission line are evaluated. In particular, this embodiment will be described with reference to an embodiment suitable for evaluating the chromatic dispersion characteristics of an optical fiber transmission line used in a high-speed optical communication system of about 10 GBit / s to 40 GBit / s.

  In order to evaluate the chromatic dispersion of the optical fiber transmission line, the frequency-wave number relationship, that is, the dispersion relationship is important. From this relationship, the speed at which light propagates through the optical fiber transmission line can be obtained. This speed refers to the speed at which the center of gravity of the light pulse moves, and is called “group speed”. The wavelength dependence of the group velocity represents chromatic dispersion.

  This group velocity is given as the slope (derivative coefficient) of the frequency-wavenumber characteristic curve. In vacuum and air, the frequency-wavenumber characteristic is a straight line, and the group velocity is constant regardless of the frequency. In a substance such as a metal, the frequency-wave number characteristic is not a straight line, and the group velocity changes according to the frequency. Since the optical fiber transmission line through which the optical pulse propagates is mainly made of glass, chromatic dispersion occurs according to the characteristics of the glass, and chromatic dispersion occurs according to the shape and dimensions of the core and cladding. The group velocity will change according to the frequency (which may be referred to as wavelength).

  Here, since the optical pulse includes not only a single wavelength but also various wavelength components, if the group velocity depends on the wavelength, the width of the optical pulse increases as it propagates in the optical fiber transmission line, and the optical pulse The waveform of the pulse is distorted, and the signals are overlapped by the front and rear optical pulses to cause crosstalk, resulting in an error.

Since chromatic dispersion increases in proportion to the length of a medium such as an optical fiber that propagates, the distortion of optical pulses increases as the length of the path composed of optical fibers and optical components increases. Becomes a serious problem.
Therefore, compensating for chromatic dispersion is an important issue when constructing and operating an optical network. In order to compensate for the chromatic dispersion, it is necessary to evaluate the degree of the chromatic dispersion.

A dispersion parameter D, which is a chromatic dispersion characteristic of the optical fiber transmission line, is expressed by the following equation (1). The unit of the dispersion parameter is, for example, ps / nm / km. In this equation (1), Δτ _g is a group delay time difference, L is a distance through which light propagates, and Δλ is a wavelength difference.

In the present embodiment, the dispersion parameter D is, for example, the length of an optical fiber transmission line or an optical component. Since the distance L through which light propagates is known, the dispersion parameter D can be calculated if the group delay time difference with respect to the wavelength difference is obtained.
The wavelength difference is expressed as the following equation (2) by the frequency difference Δν. In equation (2), ν is the frequency and c is the speed of light.

  By substituting equation (2) into equation (1), equation (3) shown below is obtained.

By the way, the frequency of light is very high, and it is impossible to measure the vibration of the electric field of the light by electrical measurement, for example, the frequency of light having a wavelength of 1500 nm corresponds to 200 THz (terahertz). . Therefore, an interferometer is used as means for measuring the phase of light.
In this interferometer, incident light is split in two directions by a beam splitter, and each light passes through an independent path and is then combined again. The phase difference due to the divided light propagating through each path can be measured as the intensity of the interference light after the combination.

Therefore, in the present embodiment, a part of the optical pulse itself propagating through the optical fiber transmission line is extracted, and a part of the extracted optical pulse is Δν ₀ by a frequency shifter (AOFS: acousto-optic frequency shifter). Using the optical pulse shifted in frequency, the interference fringe obtained by interfering with the original optical pulse is polar-coordinated into intensity and phase, so that the frequency derivative of the phase of the original optical pulse can be obtained. Delay time can be measured.

Hereinafter, it demonstrates more concretely using figures.
FIG. 1A is a diagram showing the waveform of an optical pulse propagating through an optical fiber transmission line, and is a time waveform of an optical pulse with the horizontal axis representing time t and the vertical axis representing signal intensity I. In the example shown in the figure, it is assumed that the optical pulse repeats ON and OFF every 25 ps (40 Gbit / s). Assuming that the spectral phase after propagation through the optical fiber transmission line or optical component is φ (ν), and the change in spectral phase with respect to the frequency difference of Δν is Δφ (ν), the group delay time difference Δτ _g is expressed by the following equation ( 4). Here, the spectral phase is generally called optical pulse chirp, and describes how the phase changes as a function of frequency. Here, the phase generated by the optical fiber transmission line is described. It shows a change.

  By substituting Equation (4) into Equation (3), a relational expression represented by Equation (5) shown below is derived.

  The dispersion parameter D in the optical fiber transmission line or the optical component is obtained by calculating the change in the spectral phase with respect to the frequency difference Δν with respect to the optical pulse propagated in the optical fiber transmission line or the optical component by the equation (5). Thus, chromatic dispersion can be evaluated. Since higher-order chromatic dispersion such as dispersion slope is expressed as the frequency dependence of the dispersion parameter, chromatic dispersion of all orders can be evaluated by equation (5).

FIG. 1B shows the dependence characteristic of the phase ν of the optical pulse on the frequency ν. The optical pulse of the optical fiber transmission line to be measured has a second-order dispersion, and the phase changes as shown by a parabola having an upward convex shape as shown in FIG. When the frequency is slightly shifted by Δν _{0, the} spectral phase also changes by Δφ due to this shift, as shown by the dotted line in the figure. The value of the change Δφ in the spectral phase is equivalent to a value obtained by differentiating the original optical pulse (solid line in the figure) that has not been frequency shifted. Therefore, the group delay time difference Δτ _g can be obtained by dividing the spectral phase change Δφ by the frequency shift amount Δν ₀ as shown in the equation (4).

[Spectral shearing interferometer]
In a spectrum shearing interferometer, an input optical pulse is split into two by maintaining the same polarization direction by two branch paths provided in the interferometer, and propagates through one of the branch paths. Is given a frequency shift.
FIG. 1C shows an intensity spectrum waveform of an optical pulse. This figure shows the dependence of the frequency ν on the electric field R. As shown by the solid line, the original optical pulse that has not been frequency shifted has a spectrum in which the electric field R is maximum (peak) at the center frequency ν _0. I understand. On the other hand, when the frequency is slightly shifted by Δν ₀ (illustrated by a dotted line), the peak of the intensity spectrum is shifted, but the waveform does not change. That is, since the value of the electric field R does not change even if the frequency is slightly shifted, the power spectrum of the original optical pulse indicated by the absolute value is approximated by the power spectrum of the optical pulse whose frequency is slightly shifted by Δν _0. be able to.

For this reason, the spectrum of the interference fringe caused by this frequency shift is acquired, and the change Δφ (ν) of the spectral phase with respect to the frequency difference Δν shown in FIG. 1B, that is, a value equivalent to the value obtained by differentiating the optical pulse. Can be measured as As a result, the dispersion parameter can be calculated by substituting the obtained spectral phase change Δφ (ν) into the equation (5) to evaluate the chromatic dispersion. For more information on this spectral shearing interferometer, see OPTICS LETTERS Vol.19, No.4, pp.287-289, February 15, 1994, "Analysis of ultrashort pulse-shape measurement using linear interferferometers", Victor Wong. and Ian Walmsley).
Hereinafter, in addition to the configuration shown in the above-mentioned reference, the basic principle of chromatic dispersion evaluation by the spectrum shearing interferometer according to the present embodiment including unambiguous phase measurement by detecting orthogonal two components will be described. .

An optical pulse that has propagated through an optical fiber transmission line or an optical component to be evaluated for chromatic dispersion is used as an optical signal to be measured. Separate into branch paths. The optical signal under measurement propagating to one of these two branch paths is phase-shifted by π / 2, and the first optical signal to be measured propagating through two branch paths, for example, the first branch path and the second optical path propagating through the second branch path. The interferometer is configured to multiplex the two optical signals to be measured.
The time waveform of the electric field of the first optical signal to be measured propagating through the first branch path is expressed by the following equation (6). In this equation (6), t is time and ν ₀ is the center frequency of the signal under measurement.

In this equation (6), the time waveform of the first optical signal to be measured representing the orthogonal two components is shown, the upper row is the cos component which is one of the orthogonal two components, and the lower row is the orthogonal two components. The sin component which is the other component of is shown. Each of | E ₁ ^cos (t) | and | E ₁ ^sin (t) | represents the absolute value of the envelope of the electric field. Here, the cos component, which is the first optical component in the first optical signal to be measured, and the sin component, which is the second optical component having a phase different by π / 2 with respect to the cos component, have the same polarization direction.
Ψ is a phase in time domain notation and includes a term related to chromatic dispersion. In the equation (6), the phase component depending on the signal modulation format is omitted.
In the spectrum interferometer, the time waveform of the electric field of Expression (6) is dispersed, that is, Fourier transformed, and converted to a spectrum with the center frequency ν ₀ as the origin, and the following Expression (7) is obtained.

Next, a frequency shift of Δν ₀ is given to the second measured optical signal propagating through the second branch path with respect to the center frequency ν ₀ . This frequency shift Δν ₀ is very small, there is the relationship of ν ₀ »Δν _0. The time waveform of the electric field of the second optical signal to be measured propagating through the second branch path is expressed by the following equation (8). Corresponding to the first measured optical signal, the phase difference of the cos component in the second measured optical signal is 0 degree and the phase difference of the sin component is 90 degrees, and the sin component has a phase difference of π / 2. Is added. Here, in order to obtain the power spectrum of the interference component (interference fringe) by the sin component, a phase difference of π / 2 is given to the second optical signal to be measured, and the sin component is used. Further, the cos component, which is the first light component in the second optical signal to be measured, and the sin component, which is the second light component having a phase different by π / 2 with respect to the cos component, have the same polarization direction.

When the above equation (8) is Fourier transformed into a spectrum with the center frequency ν ₀ as the origin, the following equation (9) is obtained.

As described above, since the frequency shift Δν ₀ is very small, the following approximate expression of Expression (10) is applied to each of the cos component and the sin component in Expression (9).

The first optical signal to be measured propagating on the first branch path and the second optical signal to be measured propagating on the second branch path are combined again by the coupling unit, and the combined optical signal to be measured is optically detected after the multiplexing. When detected by the unit, a power spectrum due to interference between the first measurement optical signal and the second measured optical signal is obtained. The power spectra of the interference components of the cos component and the sin component after recombination are expressed as | E ^cos (ν) | and | E ^sin (ν) |, respectively, and the polarization directions in the electric field spectra of the equations (7) and (9) Are obtained by superimposing the cos components of the first optical signal to be measured and the second optical signal to be measured, and the sin components to obtain the square of the absolute value, thereby obtaining the interference component of the cos component and the interference component of the sin component. Each power spectrum is calculated | required like the following formula | equation (11).

In the above equation (11), assuming that the frequency shift Δν ₀ is equal to the frequency difference Δν in the equations (4) and (5), the term representing the phase difference as in the following equation (12) is the frequency difference Δν. It is assumed that it is equal to the change Δφ (ν) of the spectral phase with respect to.

  Next, the equation (11) is modified to obtain the terms of the cos component and the sin component corresponding to the change amount Δφ (ν) of the spectrum phase as shown in the following equation (13). Here, when performing the recombination, the polarization directions of the first measured optical signal and the second measured optical signal are the same, and the cos component and the sin in the first measured optical signal and the second measured optical signal The polarization direction of the component is also the same.

The interference fringe component in the interference between the cos component and the sin component in the first measured optical signal and the second measured optical signal is included in | E ^cos (ν) | and | E ^sin (ν) | in Equation (13). ing.
Further, in this embodiment, when recombining the first measured optical signal and the second measured optical signal to obtain the above equation (13), the first measured optical signal and the second measured light One of the signals is alternately shifted in phase between 0 degrees and 90 degrees with respect to the other. This phase shift is performed by alternately applying a phase shift voltage, which is a voltage for controlling the amount of phase shift, to the optical phase shift unit that is an optical phase shifter. Even after this phase shift, the polarization directions of the first measured optical signal and the second measured optical signal are the same.
As a result, the cos component (upper stage) and the sin component (lower stage) in the signal under measurement of Expression (8) are obtained alternately. In this embodiment, the optical phase shift unit is provided in the second branch path, and as described above, the phase shift voltage is applied to the phase of the first optical signal to be measured that propagates through the first branch path. The phase difference of the second optical signal to be measured propagating through the second branch path is alternately changed by two values of 0 degree (cos component acquisition) and 90 degrees (sin component acquisition). As a result, in the multiplexing of the first measured optical signal and the second measured optical signal, the cos component interference component and the sin component interference component are alternately obtained. Then, by substituting the power spectrum of the interference component of the pair of cos component and sin component obtained alternately into the following equation (14), the change amount Δφ (ν) of the spectrum phase with respect to the frequency difference Δν is It is uniquely determined in the range of 0 to 2π by tan ⁻¹ of the valence function. Here, the pair of the cos component and the sin component is a unit for obtaining the change Δφ (ν) of the spectrum phase for each frequency.

  In Expression (14), in order to obtain the rightmost expression, the cos component in each of the first measured optical signal that propagates through the first branch path and the second measured optical signal that propagates through the second branch path, and The following equation (15) was applied assuming that the power of the sin component was equal.

The spectral phase change Δφ (ν) is periodically folded in the range of 0 to 2π, and is unfolded by unwarp processing to release the phase folding.
As described above, the spectral phase change Δφ (ν) with respect to the frequency difference Δν is measured using the spectrum shearing interferometer, and the dispersion parameter D is calculated by substituting it into the equation (5) to transmit the optical fiber. Evaluation of chromatic dispersion characteristics in the road.

On the other hand, in the measurement of the dispersion parameter D using a spectrum interferometer, the dispersion parameter D can be calculated by obtaining a group delay time by differentiating the measured spectrum phase φ (ν). However, when this frequency differentiation is performed, the measurement noise in the spectrum phase is also differentiated at the same time, and sharp spike noise obtained by differentiating the measurement noise is superimposed on the group delay time, thereby reducing the accuracy in calculating the dispersion parameter. It will be.
As described above, the spectrum interferometer has a drawback that the dispersion parameter cannot be accurately detected. For this reason, in the present invention, the spectral dispersion is measured with high accuracy by measuring the change Δφ (ν) of the spectral phase with respect to the frequency difference Δν using a spectrum shearing interferometer.

[Reduction in fluctuation of phase difference between the first branch path and the second branch path]
In general, the optical path length of an optical fiber varies due to a change in ambient temperature. As the ambient temperature rises, it expands, while when the ambient temperature falls, it shrinks, which changes the physical length of the optical fiber. Further, the effective refractive index of the optical fiber also changes with the ambient temperature, and the optical path length difference changes.
Due to the change in the optical path length of the optical fiber due to the change in the ambient temperature described above, the optical path length difference fluctuates due to the difference in the optical path length between the first branch path and the second branch path constituting the spectrum shearing interferometer. Will do. As a result, the phase difference between the first branch path and the second branch path fluctuates in time, and the interference fringe between the first measured optical signal and the second measured optical signal cannot be stably measured, Measurement of the change in the spectral phase cannot be performed with high accuracy.

In order to stabilize the interference fringe measurement with the spectrum shearing interferometer against the ambient temperature change described above, fluctuations in the phase difference between the first branch path and the second branch path, that is, fluctuations in the optical path length difference are measured. It is necessary to reduce the influence on the measurement according to the measurement accuracy.
If the fluctuation of the optical path length difference between the first branch path and the second branch path is Δl, and the fluctuation occurring in the phase difference between the first branch path and the second branch path at the frequency ν is ΔΨ (ν), The fluctuation ΔΨ (ν) is expressed by the following equation (16).

  Here, the optical path lengths of the first branch path and the second branch path need to be adjusted in advance to be equal to each other. When the optical path lengths of the first branch path and the second branch path are different, the fluctuation of the optical path length becomes large, the measurement accuracy of the interference fringe becomes very bad, and the change amount Δφ (ν) of the spectrum phase cannot be obtained. . As described above, the variation in the optical path length difference includes the variation in the physical length of the optical fiber and the variation in the effective refractive index.

In the present embodiment, the change in spectral phase Δφ (ν) at each frequency is differentially measured with reference to the change in spectral phase at a specific frequency ν _s in the measurement frequency range. The fluctuation ΔΨ (ν) −ΔΨ (ν _s ) generated in the phase difference that appears in the difference measurement can be expressed by the following equation (17).

The center frequency ν ₀ of the measurement frequency range is used as the specific frequency ν _s described above, and this measurement frequency range is set as one channel of an ITU (International Telecommunication Union) grid at 100 GHz intervals. For example, assuming that the measured optical signal is in the C band or the L band, the minimum value of the center frequency ν ₀ is 184 THz. At this time, since ν−ν ₀ is within 50 Hz in absolute value, the following relational expression (18) is established.

By measuring the difference Δφ (ν) of the spectrum phase at each frequency in the measurement frequency range with reference to the change Δφ (ν ₀ ) of the spectrum phase at the center frequency ν, the above equation (18) As shown in the relational expression, the fluctuation of the phase difference can be reduced to 0.027% or less as compared with the case of directly measuring the change in the spectrum phase at each frequency. As a result, the spectral phase change is measured passively by measuring the difference in the spectral phase difference without performing active stabilization especially corresponding to the temperature change. Can be stabilized.

[Configuration and function of wavelength dispersion measuring apparatus]
<First Embodiment>
Next, the configuration and function of the chromatic dispersion measuring apparatus according to the present embodiment will be described with reference to FIG. FIG. 2 is a block diagram illustrating a configuration example of the chromatic dispersion measuring apparatus according to the present embodiment.
The chromatic dispersion measuring apparatus includes an incident optical fiber 1 as an incident path, an optical branching unit 2, a first optical fiber 3 as a first optical branching path, a second optical fiber 4 as a second optical branching path, and an optical coupling unit 5. , A probe path coupling optical fiber 6 as a probe path coupling path, a reference path coupling optical fiber 7 as a reference path coupling path, an optical delay unit 8, an optical phase shift unit 9 as an optical phase shifter, light Frequency sweep unit 10, probe emission optical fiber 11, probe light detection unit 12, optical frequency extraction unit 13, reference light emission optical fiber 14, reference light detection unit 15, control unit 16, phase control line 17, sweep frequency control line 18 , A reference frequency control line 19, a probe detection control line 20, and a reference detection signal line 21 are provided.

  The incident optical fiber 1 has one end that receives an optical pulse from an optical fiber transmission line or an optical component that is an evaluation target for evaluating chromatic dispersion, and the other end that is connected to the incident end (first incident end) of the optical branching unit 2. ing. Here, an optical pulse propagated through an optical fiber transmission line or an optical component to be evaluated for evaluating chromatic dispersion is incident on a chromatic dispersion measuring device via an incident optical fiber 1, and the incident optical pulse is used as an optical signal to be measured. To do.

The optical branching unit 2 branches the optical signal to be measured input from the incident end in two directions, with respect to the first optical fiber 3 having one end connected to one outgoing end (first outgoing end). The light beam is guided as a first measured light signal, and the other light beam is guided as a second measured light signal to the second optical fiber 4 having one end connected to the other emitting end (second emitting end). Here, the first optical signal to be measured has a time waveform shown in Expression (6) and has a frequency spectrum shown in Expression (7). The second optical signal to be measured has a time waveform shown in Expression (8) and has a frequency spectrum shown in Expression (9).
Further, the optical branching unit 2 includes a first measured optical signal that is emitted from one emission end to the first optical fiber 3 and a second measured optical signal that is emitted from the other emission end to the second optical fiber 4. In the meantime, a carrier frequency difference is generated.

In the present embodiment, as a result of the occurrence of the carrier frequency difference, for example, it is emitted to the second optical fiber 4 so as to have a frequency different from the frequency of the first optical signal to be measured emitted to the first optical fiber 3. A frequency shift Δν ₀ is given as a carrier frequency difference to the second optical signal to be measured. On the other hand, the first measured optical signal emitted to the first optical fiber 3 has no frequency change.

The optical branching unit 2 uses, for example, an acousto-optic frequency shifter. A zero-order light output port of the acousto-optic frequency shifter is connected to one end of the first optical fiber 3, and a primary light output port is connected to one end of the second optical fiber 4. Acousto-optic frequency shifter, when the high frequency of .DELTA..nu ₀ is supplied, outputs a first measured optical signal from the zero-order light output ports are not frequency-shifted, whereas, frequency by frequency .DELTA..nu ₀ from the primary light output port The shifted second optical signal to be measured is output. Since the optical branching unit 2 recombines in an optical coupling unit 5 described later to acquire an interference component, the optical branching unit 2 emits the first and second measured optical signals with the same polarization direction.

The optical coupling unit 5 has one incident end (second incident end) connected to the other end of the first optical fiber 3 and the other input end (third incident end) connected to the other end of the second optical fiber 4. Has been. The optical coupling unit 5 has one outgoing end (third outgoing end) connected to the probe path coupling optical fiber 6 and the other outgoing end (fourth outgoing end) connected to the reference path coupling optical fiber 7. Has been.
The optical coupling unit 5 combines the first measured optical signal input from one incident end and the second measured optical signal input from the other incident end, and combines the combined measured signals. An optical signal is emitted from one output end to the probe path coupling optical fiber 6 and from the other output end to the reference path coupling optical fiber 7.

In addition, an optical delay unit 8 is inserted in the path of the first optical fiber 3. The optical delay unit 8 is provided in an optical fiber having a shorter optical path length than the other for the purpose of making the optical path length difference between the first optical fiber 3 and the second optical fiber 4 the same. A delay for adjustment to cancel is given to the optical signal under measurement.
Thus, by providing the optical delay unit 8 and eliminating the optical path length difference, fluctuations in the optical path length that occur between the first optical fiber 3 and the second optical fiber 4 can be reduced. The measurement accuracy of the change Δφ (ν) in the spectral phase in 14) can be improved.
If the optical path length difference between the first optical fiber 3 and the second optical fiber 4 does not affect the measurement accuracy, there is no need to provide the optical delay unit 8.

  Further, an optical phase shift unit 9 is inserted in the path of the second optical fiber 4. The optical phase shift unit 9 alternately shifts the phase of the second optical signal to be measured propagating through the second optical fiber 4 to 0 degrees and 90 degrees in a fixed first period. That is, the optical phase shift unit 9 sets the phase difference between the first measured optical signal propagating through the first optical fiber 3 and the second measured optical signal propagated through the second optical fiber 4 to a constant first phase. It is alternately switched between 0 degrees and 90 degrees in one cycle. Here, the optical phase shift unit 9 shifts the phase of the second measured optical signal with respect to the first measured optical signal, but the polarization direction of the second measured optical signal is changed to the first measured optical signal after the shift. It is emitted as the same optical signal.

As a result, when the phase difference between the first measured optical signal propagating through the first optical fiber 3 and the second measured optical signal propagating through the second optical fiber 4 is 0 degree, the cos component detection mode, 90 The case of the degree can be set to the sin component detection mode. A cos component and a sin component in the first optical signal to be measured are expressed by Expression (6). Further, the second measured optical signal having a phase difference of 0 degree is defined as a cos component, and the signal having a phase difference of 90 degrees is defined as a sin component, which are represented by an upper stage and a lower stage of Equation (8), respectively.
Here, as described above, the polarization directions of the first measured optical signal and the second measured optical signal emitted from the optical branching unit 2 are the same, and the optical phase with respect to the first measured optical signal. The polarization direction of the second optical signal to be measured whose phase difference is 0 degree or 90 degrees by the shift unit 9 is also the same. Therefore, by switching the phase difference generated in the optical phase shift unit 9 between 0 degree and 90 degrees, it is possible to select which of the cos component and the sin component in the two orthogonal components is detected. In the present embodiment, when the second measured optical signal is shifted by 0 degree with respect to the first measured optical signal, interference of the cos component of the first measured optical signal and the second measured optical signal occurs. When the second measured optical signal is shifted by 90 degrees with respect to the first measured optical signal, interference of the sine components of the first measured optical signal and the second measured optical signal occurs. That is, when the phase shift difference of the second measured optical signal is 0 degree, the optical coupling unit 5 uses the interference component in the cos component of the first measured optical signal and the second measured optical signal as the combined measured optical signal. When the phase shift difference is 90 degrees, the interference component in the sine component of the first measured optical signal and the second measured optical signal is output as a combined measured optical signal.

For example, a phase shifter using an electro-optic crystal (for example, LiNb0 ₃ ) can be used for the optical phase shift unit 9, and by changing applied phase shift voltages (V ₀ and V ₉₀ described later), The amount of phase shift can be switched between 0 degrees and 90 degrees.
In the present embodiment, the optical delay unit 8 is connected to the first optical fiber 3 and the optical phase shift unit 9 is connected to the second optical fiber 4. An optical delay unit 8 is inserted in one of the optical fiber 3 and the second optical fiber 4 having a short optical path length, and an optical phase shift unit 9 is connected to the other.
As described above, by inserting each of the optical delay unit 8 and the optical phase shift unit 9 into the optical paths of different optical fibers, the residual reflected light is transmitted between the optical delay unit 8 and the optical phase shift unit 9. Can be prevented from reciprocating. For this reason, it is possible to remove the spectrum ripple generated by the reciprocal resonance of the residual reflected light.

  The optical frequency sweep unit 10 has an incident end (fourth incident end) connected to the other end of the probe path coupling optical fiber 6 and an emission end (fifth emission end) connected to one end of the probe emission optical fiber 11. Yes. The optical frequency sweep unit 10 is, for example, a tunable bandpass filter, and a predetermined measurement frequency range is generated by a trigger signal indicating the start of a frequency sweep period (a frequency sweep period within a set measurement frequency range). A sweep is performed to change the frequency at. Here, the optical frequency sweeping unit 10 changes the center frequency of the band pass frequency width to be passed in time series in the range of the measurement frequency. The optical frequency sweep unit 10 performs processing for extracting an interference element having a frequency corresponding to the bandpass frequency width from the combined optical signal to be measured incident from the probe path coupling optical fiber 6, that is, the combined optical signal to be measured. Perform frequency decomposition (spectral decomposition) of the signal. The optical frequency sweep unit 10 emits a component-measured optical signal after frequency decomposition (a signal indicating a spectral intensity for each frequency, a probe light signal) to the probe emission optical fiber 11 from the emission end. The frequency used for frequency number resolution of the combined optical signal to be measured that is obtained by combining the first optical signal to be measured and the second optical signal to be measured is the center frequency in the bandpass frequency range. By the frequency decomposition, an interference element (interference component) of a cos component or a sin component for each frequency is detected.

The optical frequency extraction unit 13 has an incident end (fifth incident end) connected to the other end of the reference path coupling optical fiber 7, and an emission end (sixth emission end) connected to one end of the reference emission optical fiber 14. Yes. The optical frequency extraction unit 13 obtains an interference element of a specific frequency (in this embodiment, the center frequency ν ₀ of the measured optical signal) component from the combined measured optical signal incident from the reference path coupling optical fiber 7. The extraction process, that is, the extraction of the component of the specific frequency ν ₀ in the combined optical signal to be measured is performed.
The optical frequency extraction unit 13 sweeps the frequency to change the spectrum phase of each bandpass frequency width while the optical frequency sweeping unit 10 changes the center frequency of the bandpass frequency width in time series in the frequency sweep period. During the measurement, the frequency to be extracted is fixed at a specific frequency ν ₀ . Optical frequency extraction unit 13, from the exit end of the reference optical signal extracted by the specific frequency [nu ₀ (signal indicating a spectral intensity of a specific frequency [nu _0), emitted with respect to the reference outgoing optical fiber 14.
The optical frequency extraction unit 13 fixes the frequency to be extracted to a specific frequency ν ₀ as described above. However, for example, a tunable bandpass filter that can adjust the frequency to be extracted may be used. When changing the specific frequency, the user inputs data of the specific frequency, so that the control unit 16 sets the specific frequency to the optical frequency extraction unit 13 via the reference frequency control line 19. Output a control signal. Then, the optical frequency extraction unit 13 sets the frequency indicated by the control signal input from the control unit 16 through the reference frequency control line 19 as a specific frequency.
In this embodiment, the specific frequency is set as the center frequency of the signal under measurement, but the specific frequency may be another frequency of the signal under measurement. A specific frequency may be selected so that the phase fluctuation can be reduced most according to the measurement conditions.

The probe light detection unit 12 has an incident end (sixth incident end) connected to the other end of the probe outgoing optical fiber 11. The probe light detection unit 12 converts the component measured optical signal incident from the probe output optical fiber 11 into an electrical signal, and outputs the conversion result to the control unit 16 as a probe signal that is an interference signal.
Here, when the optical phase shift unit 9 sets the phase shift amount to 0 degree, the component measured optical signal is an interference element of the cos component of the corresponding frequency, and the optical phase shift unit 9 determines the phase shift amount. In the case of 90 degrees, it is an interference component of the sin component of the corresponding frequency.

The reference light detection unit 15 has an incident end (seventh incident end) connected to the other end of the reference outgoing optical fiber 14. The reference light detection unit 15 converts the reference light signal incident from the reference emission optical fiber 14 into an electrical signal, and outputs the conversion result to the control unit 16 as a reference signal that is an interference signal.
Here, when the optical phase shift unit 9 sets the phase shift amount to 0 degree, the reference optical signal is a cos component interference element having a specific frequency ν ₀ , and the optical phase shift unit 9 determines the phase shift amount. When the angle is 90 degrees, it is an interference element of a sine component having a specific frequency ν ₀ .

  In the present embodiment, each of the incident optical fiber 1, the first optical fiber 3, and the second optical fiber 4 uses a polarization maintaining optical fiber (PMF) having polarization maintaining characteristics. . The polarization axes of the incident optical fiber 1, the first optical fiber 3, and the second optical fiber 4 are all aligned in the same direction, and the polarization directions of the first measured optical signal and the second measured optical signal are changed. It is made the same and it is made to inject into the optical coupling part 5 which combines a 1st to-be-measured optical signal and a 2nd to-be-measured optical signal. Therefore, the polarization directions of the first optical signal to be measured and the second optical signal to be measured that are multiplexed by the optical coupling unit 5 are made the same. For this reason, the optical signal to be measured uses a polarization controller (not shown), and the polarization of the optical signal to be measured immediately after propagation through the optical fiber transmission line to be evaluated for chromatic dispersion is linearly polarized and its polarization axis is incident. After being aligned with the polarization axis (for example, the slow axis) of the optical fiber 1, the light enters the incident optical fiber 1. A polarization maintaining optical fiber may also be used for the probe path coupling optical fiber 6, the reference path coupling optical fiber 7, the probe emission optical fiber 11, and the reference emission optical fiber 14.

The control unit 16 synchronizes with the trigger signal indicating the start point of the frequency sweep cycle input from the optical frequency sweep unit 10 via the sweep frequency control line 18, and with respect to the optical phase shift unit 9, for each first cycle. The phase shift voltage is supplied to the optical phase shift unit 9 via the phase control line 17 in order to change the phase shift voltage to be applied alternately to perform the phase shift.
That is, when the measurement frequency is n points, the cos component and the sin component are paired with respect to one frequency, so that the voltage is alternately switched every first cycle obtained by dividing the frequency sweep cycle by 2n. Is performed in synchronization with the trigger signal.
In addition, the control unit 16 alternately receives cos component and sin component interference signals from the probe light detection unit 12 via the probe detection signal line 20 in synchronization with the first period. Then, the control unit 16 uses the cos component and sin component interference elements acquired in time series as a pair and uses them as power spectrum data for calculating the dispersion parameter at each frequency.
Further, the control unit 16 alternately receives the cos component and the sin component interference signal of the specific frequency ν ₀ from the reference light detection unit 15 through the reference detection signal line 21 in synchronization with the first period. . Then, the control unit 16 uses the acquired cos component and sin component interference elements as a pair and uses them as power spectrum data for calculating a dispersion parameter at a specific frequency ν ₀ .

Next, the control unit 16 obtains the power spectrum of the cos component and the power spectrum of the sin component by using the level of the probe signal that is the input interference signal as the power spectrum, and obtains the power spectrum of Expression (11). .
Further, the control unit 16 obtains a power spectrum of a pair of cos components and sin components for each frequency according to Equation (13) obtained by modifying Equation (11), and uses Equation (14) to calculate the phase change amount for each frequency. Δφ (ν) can be obtained. .
Similarly, the control unit 16 uses the level of the reference signal that is an input interference signal as the power spectrum, obtains the square of the power spectrum of the cos component and the power spectrum of the sin component, and obtains the power spectrum of Expression (11). .
Further, the control unit 16 obtains a power spectrum of a pair of cos component and sin component of the specific frequency ν ₀ by the equation (13) obtained by modifying the equation (11), and the specific frequency ν by the equation (14). ₀ of the spectral phase of the variation [Delta] [phi ([nu ₀₎ can be obtained.

Then, the control unit 16 subtracts the spectral phase change Δφ (ν ₀ ) of the specific frequency acquired in the same first period from the spectral phase change Δφ (ν) of each frequency to obtain the change amount. The difference Δε (ν) is calculated.
The phase fluctuation in the change difference Δε (ν) is a change in spectral phase Δφ (ν ₀ ) of a specific frequency obtained in the same first period from the change Δφ (ν) in the spectrum phase of each frequency. Therefore, the phase fluctuation component in each first period is canceled and reduced as shown in Expression (17) and Expression (18).
Further, the change amount Δφ (ν) of the spectrum phase when the frequency ν is the frequency ν ₀ and the change amount Δφ (ν ₀ ) of the spectrum phase of the specific frequency ν ₀ measured in the same first period are as follows. It becomes the same value. Therefore, the change difference Δε (ν) obtained from the change Δφ (ν) of the spectrum phase when the frequency ν is the frequency ν ₀ is “0”.
The control unit 16 uses the change difference Δε (ν ₀ ) in the phase change Δφ (ν ₀ ) when the frequency ν is the frequency ν ₀ as a reference, and uses each frequency as a change in phase relative to this reference. The change difference Δδ (ν) at ν is output.
Then, the control unit 16 substitutes the change difference Δε (ν), which is a relative change in the phase, into the equation (5) as the change Δφ (ν), so that the frequency ν is the frequency ν ₀ . A dispersion parameter for each frequency is calculated based on the change difference Δε (ν ₀ ) in the change Δφ (ν ₀ ) of the spectrum phase at that time.

  As described above, in this embodiment, the start of the frequency sweep in the measurement frequency range is notified by the trigger signal supplied from the optical frequency sweep unit 10.

Further, the control unit 16 sets 2n times the first cycle as a frequency sweep cycle, generates a trigger signal indicating the start of this frequency sweep cycle, outputs a trigger signal to the optical frequency sweep unit 10, and in the range of measurement frequencies, A configuration for controlling frequency sweeping may be employed.
In addition, by using the trigger signals for the start point and end point of the frequency sweep cycle without making the frequency sweep cycle continuous, it is not necessary to set the start point and end point at the same time. During this period, a time for returning the frequency to the initial value of the sweep can be provided. For this reason, as in the case where the end point and the start point are the same, in order to change from the start point to the initial value, the first first period in the frequency sweep period may be shortened depending on the frequency change time. As a result, the measurement time can be controlled with higher accuracy.

Next, the operation of measuring the optical signal under measurement of the chromatic dispersion measuring apparatus shown in FIG. 2 in this embodiment will be described with reference to FIG. FIG. 3 shows the operation of the frequency sweep operation of the optical frequency sweep unit 10, the phase shift operation of the optical phase shift unit 9 corresponding to this, and the sampling of the interference signal from the probe light detection unit 12 in the control unit 16. It is a wave form diagram which shows a timing. Here, the control unit 16 performs the probe signal input from the probe light detection unit 12 and the reference signal input from the reference light detection unit 15 at the same timing. Therefore, in FIG. 3, the sampling timing chart is the same for both the probe signal and the reference signal.
That is, FIG. 3A shows the output timing of the trigger signal output from the optical frequency sweep unit 10 with the vertical axis representing voltage and the horizontal axis representing time. In FIG. 3A, the H level (V _H ) and L level (V _L ) of the trigger signal output from the optical frequency sweep unit 10 is controlled using TTL control (TTL (Transistor Transistor Logic) interface). ).

In FIG. 3B, the vertical axis represents frequency, the horizontal axis represents time, and shows the time change of the resonance frequency output in the sweep of the optical frequency sweep unit 10. In FIG. 3B, ν ₁ is a sweep start frequency (the lowest frequency in the measurement frequency range), and ν ₂ is a sweep stop frequency (the maximum frequency in the measurement frequency range). For this reason, the frequency ν ₁ to the frequency ν ₂ are in the measurement frequency range, that is, the frequency sweep range.
FIG. 3C is a diagram illustrating a waveform of the phase shift voltage that changes the phase difference applied to the optical phase shift unit 9 in the first period, with the vertical axis representing voltage and the horizontal axis representing time. The phase shift voltage V ₀ is a voltage when the phase difference is 0 degree (cos component), and the phase shift voltage V ₉₀ is a voltage when the phase difference is 90 degrees (sin component). The time for applying the phase shift voltage of the cos component and the sin component is the same period, that is, the first period Δt.
In FIG. 3D, the vertical axis represents voltage, the horizontal axis represents time, and the control unit 16 displays the probe signal from the probe light detection unit 12 and the reference signal from the reference light detection unit 15 in time series. It is a figure which shows the timing of the sampling period received as data.
3 (c) and 3 (d), in order to clearly describe the first period, the horizontal time scale of FIG. 3 (a) and FIG. Is shown.

The optical frequency sweep unit 10 generates a trigger signal and outputs the trigger signal to the control unit 16 in the frequency sweep period of time “T _{l + 1} −T _l ”, and performs frequency decomposition of the combined optical signal to be measured. Therefore, a sweep process for linearly increasing the frequency from the frequency ν1 to the frequency ν2 is started. Here, if the user measures the sweep change before the actual measurement and finds that the linearity of the swept frequency over time is not achieved, the frequency of the sweep is calibrated and the frequency sweep nonlinearity is corrected To do. In this embodiment, the frequency is swept from the low frequency side to the high frequency side. However, the frequency may be swept from the high frequency side to the low frequency side. The timing control is not limited to TTL control, and for example, a CMOS (Metal Oxide Semiconductor) interface may be used.
The optical frequency extraction unit 15 extracts the component of the fixed specific frequency ν ₀ in the frequency sweep cycle of “T _{1 + 1} −T ₁ ”, and uses the extracted component as a reference light signal to the reference light detection unit 15. Output.

When the trigger signal is supplied, the control unit 16 alternately outputs the phase shift voltage V ₀ and the phase shift voltage V ₉₀ to the optical phase shift unit 9 every first period Δt in synchronization with the trigger signal. Start processing. In the present embodiment, the voltage is supplied from the phase shift voltage V _0, but it may be supplied from the phase shift voltage V ₉₀ .
As a result, the optical phase shift unit 9 sets the phase of the second measured optical signal propagating through the second optical fiber 4 to 0 degree or 90 degrees by the supplied phase shift voltage V ₀ and phase shift voltage V _90. Change.
For each first period Δt, the light detection unit 11 alternately uses a component measured optical signal having a cos component interference element and a component measured optical signal having a sin component interference element at each frequency as an interference signal. Supply to the control unit 16.
The probe light detector 12 alternately uses a component measured optical signal having a cos component interference element and a component measured optical signal having a sin component interference element at each frequency as an interference signal for each first period Δt. , Supplied to the control unit 12.
The reference light detector 15 alternately interferes with the component measured optical signal having the cos component interference element and the component measured optical signal having the sin component interference element at the specific frequency ν ₀ for each first period Δt. The signal is supplied to the control unit 16 as a signal.

Then, the control unit 16 samples the component optical signal to be measured in the central portion of the first period Δt, for example, in synchronization with the first period Δt, thereby alternately including components having cos component interference elements at each frequency. A pair of interference signals of the optical signal to be measured and the component optical signal to be measured having a sin component interference element can be obtained. That is, by changing the phase of the second optical signal to be measured from 0 degrees to 90 degrees, a pair of cos component and sin component interference elements can be obtained. As a result, a pair of interference elements of n cos components and sin components of each frequency ν and specific frequency ν is obtained from the 2n first period Δt within the measurement frequency range.
As a result, as described above, the control unit 16 obtains the phase change Δ (ν ₀ ) of the specific frequency ν ₀ measured in the same first period as the phase change Δφ (ν). Then, the control unit 16 subtracts the spectral phase change Δφ (ν ₀ ) of the specific frequency ν ₀ measured in the same first period from the spectral phase change Δφ (ν), and the obtained difference The change parameter Δδ (ν ₀ ) is changed into the phase change Δφ (ν) and substituted into the equation (14) to calculate the dispersion parameter.

As described above, the control unit 16 corresponds to the discrete data, that is, the first period Δt, with respect to the change in the spectrum power and the spectrum phase at each frequency of the component measured optical signal by the sampling period shown in FIG. The probe signal obtained by sampling the component measured signal at the frequency ν output from the optical frequency sweep unit 10 at the timing and the reference signal obtained by sampling the measured signal at the frequency ν0 output from the optical frequency extraction unit 13 are acquired.
At this time, since the optical frequency sweeping unit 10 sweeps by gradually changing the amount of change in the frequency with respect to the first period Δt for switching the phase shift voltage, the frequency generated between a pair of measurements of the cos component and the sin component. The change is slight, and it can be considered that the frequency is constant between a pair of data.
For example, when 1000 points are measured alternately for each of the cos component and the sin component, the frequency change in the measurement frequency range accompanying switching of the phase shift voltage is only 1/2000 of the entire frequency sweep range. If the frequency sweep cycle T _{1 + 1} -T ₁ is 1 s (seconds), the first cycle Δt, which is the phase shift voltage switching time, is 0.5 ms.
The control unit 16 sets the sampling period of the component optical signal to be measured to 0.5 ms similarly to the first period, and synchronizes the data sampling timing with the switching of the phase shift voltage. Here, the sampling timing is delayed by, for example, Δt / 2 with respect to the change timing of the phase shift voltage so that the component measured optical signal can be reliably sampled.

Sampling of the 2000 component optical signals to be measured is alternately assigned to sampling of the cos component and the sin component by 1000 points. As a result, the sampling interval of two orthogonal components (sampling interval = first period × 2) is 1/1000 of the entire frequency sweep range (frequency measurement range). When the number of sampling points is fixed, the phase shift voltage output from the optical phase shift unit 9 and the sampling period of the component measured optical signal are proportional to the frequency sweep period. That is, the first period Δt is obtained by dividing the frequency sweep period by the number of frequencies corresponding to the resolution of the frequency to be measured.
Therefore, if the frequency sweep period is shortened, the sampling period is correspondingly shortened. When the spectrum shape is complicated, it is necessary to further increase the number of sampling points and improve the resolution. Even when this resolution is improved, the sampling period is shortened in the case of the same frequency sweep period.

The optical signal to be measured follows an ITU grid at 100 GHz intervals, and the frequency sweep range ν ₂ −ν ₁ is 100 GHz. For example, when the measured optical signal is assigned to the C band 31st channel of the ITU grid, the frequency ν ₁ and the frequency ν ₂ are 193.05 THz and 193.15 THz, respectively.
Each of the cos component and the sin component for each probe signal and reference signal is acquired at 1000 points, and is set to 100 MHz as a sampling interval of two orthogonal components, that is, a cycle twice the first cycle Δt.
In order to improve the measurement accuracy of the change Δφ (ν) in the spectral phase, it is necessary to obtain the phase at each sampling point with high accuracy. For this reason, it is preferable that the bandpass frequency width in the optical frequency sweep unit 10 be narrower than half of the sampling interval.
The narrower the bandpass frequency width, the higher the resolution of frequency resolution, and the change Δφ (ν) of the spectrum phase with respect to the frequency can be measured with high accuracy.
However, by narrowing the bandpass frequency width, the amount of light incident on the probe light detector 12 is reduced, and the influence of measurement noise is increased.

In this embodiment, the bandpass frequency width is set to 1/4 of the sampling frequency, that is, 25 MHz. At this time, the finesse of the bandpass optical filter used for the optical frequency sweep unit 10 is a value obtained by dividing the peak interval by the full width at half maximum of the transmission peak, that is, 4000.
When phase fluctuation is measured using a heterodyne spectrum interferometer, the time constant required for the phase to fluctuate about 180 degrees is about 5 s when the length of the optical fiber used for the path of the interferometer is about 1 m. Therefore, when the frequency sweep period is set to 1 sec, the influence of the phase fluctuation of the interferometer is expected to be small.
When it is necessary to further reduce the phase fluctuation in order to improve the measurement accuracy, the frequency sweep cycle may be further shortened. For example, by reducing the frequency sweep period to about 0.1 sec, the influence of phase fluctuation in frequency dispersion can be ignored.

Further, in the present embodiment, the optical frequency sweep unit 10 has been described as sweeping the frequency linearly, but the sweep is stopped and the quadrature two components are measured during the sampling period of the paired cos component and sin component. You may make it perform a sweep in steps so that it may become the same frequency during a period.
With this configuration, there is no frequency fluctuation between the pair of cos component and sin component, the measurement accuracy of the interference signal can be improved, and the change Δφ (ν) of the spectrum phase can be obtained with high accuracy.

According to the above-described configuration, an interferometer is configured using an optical fiber having polarization maintaining characteristics, and the orthogonal two components are maintained in a stable state. Therefore, there is no need to spatially arrange components required in the spatial optical system, and the structure can be simplified and the apparatus can be miniaturized.
Furthermore, since the interferometer is configured without using a spatial optical system, there is no loss of light in the input / output of light between the optical fiber and the spatial optical system, and a decrease in light intensity is suppressed. The chromatic dispersion can be measured while maintaining the measurement sensitivity.
Further, the phase change Δφ (ν ₀ ) of the specific frequency ν ₀ measured in the same first period is subtracted from the phase change Δφ (ν) of each frequency in the frequency sweep period, and the obtained difference Since the dispersion parameter is calculated based on the difference Δδ (ν ₀ ), the phase fluctuation is reduced compared to the case where the spectral phase change Δφ (ν) of each frequency is directly measured. A stable dispersion parameter can be obtained by reducing the influence.

<Second Embodiment>
Next, a chromatic dispersion measuring apparatus according to the second embodiment will be described. The second embodiment has the same configuration as the first embodiment, but in the configuration of FIG. 2, the cos component and the sin component are received in parallel by the two reception ports provided in parallel to the control unit 16. It has the composition to do.
FIG. 4 shows the frequency sweep operation of the optical frequency sweep unit 10, the phase shift operation of the optical phase shift unit 9 corresponding thereto, the probe signal output from the probe light detection unit 12 in the control unit 16, and the reference light. FIG. 6 is a waveform diagram showing timing of an operation for performing sampling with a reference signal output from a detection unit 15; Here, the control unit 16 inputs the probe signal input from the probe light detection unit 11 and the reference signal input from the reference light detection unit 15 at the same timing. For this reason, in FIG. 4, the sampling timing chart has the same probe signal and reference signal.

FIG. 4A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 10, where the vertical axis is voltage and the horizontal axis is time. In FIG. 4A, the H level and L level of the trigger signal output from the optical frequency sweep unit 10 are set so as to conform to the TTL control.
In FIG. 4B, the vertical axis represents frequency, the horizontal axis represents time, and shows the time change of the resonance frequency output in the sweep of the optical frequency sweep unit 10. In FIG. 4B, ν ₁ is a sweep start frequency (the lowest frequency in the measurement frequency range), and ν ₂ is a sweep stop frequency (the maximum frequency in the measurement frequency range).
FIG. 4C is a diagram illustrating a phase shift voltage waveform in which the vertical axis is voltage, the horizontal axis is time, and the phase difference applied to the optical phase shift unit 9 is changed in the first period. The phase shift voltage V ₀ is a voltage when the phase difference is 0 degree (cos component detection mode), and the phase shift voltage V ₉₀ is a voltage when the phase difference is 90 degrees (sin component detection mode). The time for applying the phase shift voltage of the cos component and the sin component is the same period, that is, the first period Δt.
In FIG. 4D, the vertical axis represents voltage, the horizontal axis represents time, and the control unit 16 shows the probe signal output from the probe light detection unit 12 and the reference signal output from the optical frequency extraction unit 13. FIG. 5 is a diagram illustrating the timing of a sampling period received as time series data in parallel from the reception port P1 and the reception port P2. In this embodiment, the reception port P1 receives the component measured optical signal of the cos component and the reference signal of the specific frequency ν ₀ of the cos component, and the reception port P2 receives the component measured optical signal of the sin component and sin A reference signal having a specific frequency ν ₀ of the component is received.
4 (c) and 4 (d), in order to clearly describe the first period, the horizontal time scale of FIG. 4 (a) and FIG. 4 (b) is expanded, and only a part of the time range is displayed. Is shown.

When the phase shift voltage V ₀ is output from the probe light detection unit 12, the control unit 16 outputs the component measured optical signal of the cos component and the reference signal of the specific frequency ν ₀ of the cos component through the reception port P 1. received, while outputting the phase-shifted voltage V _90, receives a component to be measured optical signal sin component by receiving port P2, and a reference signal of a specific frequency [nu ₀ of sin component.
In the second embodiment, as in the first embodiment, the frequency sweep period is acquired at 1000 points for each of the cos component and the sin component. The frequency sweep period is 1 s as in the first embodiment, and the first period Δt, which is the phase shift voltage switching time, is 0.5 msec. The sampling period of each of the reception port P1 and the reception port P2 is 1 msec, and the sampling timing is shifted by 0.5 msec between the reception port P1 and the reception port P2.

The control unit 16 performs A / D (analog / digital) conversion, and sets the voltage levels of the probe signal that is an interference signal from the probe light detection unit 12 and the reference signal that is the interference signal from the reference light detection unit 15. Obtain as digital data. The control unit 16 independently includes an A / D conversion circuit that performs A / D (analog / digital) conversion processing on the probe signal and an A / D conversion circuit that performs A / D conversion processing on the reference signal.
Therefore, the operation speeds of the A / D conversion circuits for the probe signal and the reference signal in the control unit 16 may be a limiting factor when it is desired to shorten the sampling period. However, in this embodiment, by adopting dual reception of the reception port P1 and the reception port P2, the sampling rate of each reception port is half that of reception by only one port. The operating speed limit can be increased by a factor of two. In this case, the control unit 16 performs an A / D conversion process on the probe signal input from the reception port P1, and an A / D conversion process on the reference signal input from the reception port P1. A conversion circuit, an A / D conversion circuit that performs A / D conversion processing on a probe signal input from the reception port P2, and an A / D conversion circuit that performs A / D conversion processing on a reference signal input from the reception port P2. It has four A / D conversion circuits. The reception port P1 and the reception port P2 each have two systems of probe signals and reference signals, that is, two reception ports.
In addition, since it is not necessary to alternately distribute the measurement timings of the cos component and the sin component in one reception port, the data processing program is simplified and the data processing speed can be improved.

<Third Embodiment>
Next, a chromatic dispersion measuring apparatus according to the third embodiment will be described. The third embodiment has the same configuration as that of the first embodiment or the second embodiment, except that the period during which the phase shift voltage is supplied (first period) is measured with the cosine component, the sin component, It is set to a different length of time when measuring.
That is, in the measurement of the component optical signal to be measured output from the optical frequency sweep unit 10, the reference signal is a pair for obtaining the change Δφ (ν) of the spectrum phase input from the probe light detection unit 12 to the control unit 16. It is not necessary to measure the cos component and the sin component at the same time width in the sampling period. Similarly, a pair of cos component and sin component for obtaining a spectral phase change Δφ (ν ₀ ) of the specific frequency ν ₀ input from the reference light detection unit 15 to the control unit 16 are set to the same time width in the sampling period. There is no need to measure.

The control unit 16 acquires the probe signals of the two orthogonal components from the probe light detection unit 12 in the order of the cos component and the sin component, and the reference light of the reference signals of the two orthogonal components in the order of the cos component and the sin component. When acquiring from the detection unit 15, the time width for applying the phase shift voltage supplied to the optical phase shift unit 9 is measured by measuring the cos component while keeping the pair of sampling periods of the cos component and the sin component in the first embodiment as they are. The time is shortened, and this shortened time is added to the measurement time of the sin component.
As a result, the measurement interval between the cosine component and the sine component that are a pair of each of the probe signal and the reference signal is shortened, the amount of change in frequency associated with the time series switching of the cosine component and the sine component is reduced, and the measurement frequency Accuracy can be improved.
Therefore, it is possible to improve the measurement accuracy of the spectral phase change Δφ (ν) and the change Δφ (ν ₀ ) measured in frequency units, and obtain the dispersion parameter with high accuracy.

In the waveform diagrams of FIG. 3 (d) in the first embodiment and FIG. 4 (d) in the second embodiment, a cos component and a sin component which are a pair of two orthogonal components of the probe signal and the reference signal are shown. The sampling period to be acquired is 2Δt. In the cos component measurement, the time during which the control unit 16 applies the phase shift voltage V ₀ to the optical phase shift unit 9 is Δt−δ, and the time during which the phase shift voltage V ₉₀ is applied is Δt + δ. As a result, the sampling period for acquiring the cos component and the sin component as a pair of orthogonal two components is 2Δt, which is the same as in the first and second embodiments. In addition, the time from when the phase shift voltage is applied until the control unit 16 performs sampling, that is, the time from the start of the measurement time to the time when the control unit 16 samples the probe signal and the reference signal that are interference signals is expressed as a cos component. and sin into components both the time [delta] _0. Here, Δt-δ, with respect to Delta] t + [delta] and [delta] _0, the relationship shown in the following equation (19).

Accordingly, the sum of the time for applying the phase shift voltage to the cos component and the sin component of the pair of orthogonal two components of the probe signal and the reference signal is 2Δt, and the sampling period of the orthogonal two components is the first embodiment. And it becomes the same as that of 2nd Embodiment.
As to how to set δ and δ ₀ , the sampling condition for how much the frequency resolution is set, the response time from the change of the phase shift voltage of the optical phase shift unit 9 to the actual change of the phase Also, it is determined by the operating frequency of the A / D conversion circuit that samples the voltage signals from the probe light detection unit 12 and the reference light detection unit 15.
Note that, under the above-described sampling conditions (frequency sweep period 1S, orthogonal two component data points 1000), shortening the switching time from the cos component to the sine component to 1/10 or less is possible with a commercially available phase shifter, A / D It is possible to cope with this by using a conversion circuit.

<Fourth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the fourth embodiment will be described. FIG. 5 is a block diagram illustrating a configuration example of the fourth embodiment. The same components as those in the first embodiment are denoted by the same reference numerals, and components different from those in the first embodiment will be described below.
When using the optical delay unit 8 that eliminates the optical path length difference between the first optical fiber 3 and the second optical fiber 4 and the optical phase shift unit 9 that shifts the phase of the optical signal to be measured, in the case of the first embodiment The optical delay unit 8 and the optical phase shift unit 9 are connected to different optical fibers.

As can be seen from the comparison with the configuration of the first embodiment shown in FIG. 2, it is not necessary to provide each of the optical delay unit 8 and the optical phase shift unit 9 in different optical fibers. Therefore, in the fourth embodiment, Light having a function of combining and integrating the optical delay unit 8 and the optical phase shift unit 9 to delay the light propagation of the optical delay unit 8 and a function of shifting the phase difference of the light of the optical phase shift unit 9 The delay / optical phase shift unit 47 is provided on either the first optical fiber 3 or the second optical fiber 4. When either one of the first optical fiber 3 and the second optical fiber 4 has an optical path length shorter than the other, an optical delay / optical phase shift unit 47 is provided on one side to correct the optical path length difference with respect to the other. Thereby, the apparatus can be further downsized as compared with the first embodiment by using the optical delay / optical phase shift unit 47 in which the optical delay unit 8 and the optical phase shift unit 9 are integrated.
Further, by combining the optical delay unit 8 and the optical phase shift unit 9 and integrating them, the optical delay unit 8 and the optical phase shift unit 9 are separated and provided in the same optical fiber, Residual reflected light generated in the optical fiber can be reduced. For this reason, the spectral ripple due to the resonance of the residual reflected light shown in the first embodiment can be suppressed to a lower level.

<Fifth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the fifth embodiment will be described. The chromatic dispersion measuring apparatus according to the fifth embodiment can be implemented in any configuration of the first embodiment shown in FIG. 2 or the fourth embodiment shown in FIG. It explains using.
The fifth embodiment differs from the first embodiment in that the first embodiment is a probe signal that is an interference signal from the probe light detection unit 12 and a reference signal that is an interference signal from the reference light detection unit 15. In the fifth embodiment, two frequency sweep periods are used in one frequency sweep period, while a pair of cos component and sin component is time-sequentially performed in the same frequency sweep period. The probe signal and reference signal of the cos component are sampled, and the probe signal of the sin component and the interference signal of the reference signal are sampled in the other frequency sweep period. Here, the control unit 16 performs the probe signal input from the probe light detection unit 11 and the reference signal input from the reference light detection unit 15 at the same timing. Therefore, in FIG. 6, the sampling timing chart is the same for both the probe signal and the reference signal.

Therefore, the control unit 16 performs phase shift with respect to the optical phase shift unit 9 during the frequency sweep period for measuring the cos component in synchronization with the trigger signal output from the optical frequency sweep unit 10 in the two frequency sweep periods. A voltage V ₀ is supplied, and a phase shift voltage V ₉₀ is supplied to the optical phase shift unit 9 during a frequency sweep period in which a sin component is measured. Here, the lengths of the frequency sweep periods for detecting the probe signal and the reference signal of the cos component and the sin component are the same. Therefore, the period during which the control unit 12 supplies the phase shift voltage V ₀ to the optical phase shift unit 8 and the period during which the phase shift voltage V ₉₀ is supplied are the same as the first period Δt.
In the present embodiment, as described above, the first period Δt to which the same phase shift voltage is applied is equal to the frequency sweep period. That is, T _{l + 1} −T _l = T _{l + 2} −T _{l + 1} = Δt.

FIG. 6 shows the frequency sweep operation of the optical frequency sweep unit 9, the phase shift operation of the optical phase shift unit 9 corresponding thereto, and the probe signal and reference light detection unit 15 from the probe light detection unit 12 in the control unit 16. FIG. 6 is a waveform diagram showing operation timing in sampling of a reference signal from
FIG. 6A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 10, where the vertical axis represents voltage and the horizontal axis represents time. In FIG. 6A, the H level and L level of the trigger signal output from the optical frequency sweep unit 10 are set so as to conform to the TTL control.

In FIG. 6B, the vertical axis represents frequency, the horizontal axis represents time, and shows the time change of the resonance frequency output in the sweep of the optical frequency sweep unit 10. In FIG. 6B, ν ₁ is a sweep start frequency (the lowest frequency in the measurement frequency range), and ν ₂ is a sweep stop frequency (the maximum frequency in the measurement frequency range).
FIG. 6C is a diagram illustrating a phase shift voltage waveform in which the vertical axis is voltage, the horizontal axis is time, and the phase difference applied to the optical phase shift unit 9 is changed in the first period. The phase shift voltage V ₀ is a voltage when the phase difference is 0 degree (cos component detection mode), and the phase shift voltage V ₉₀ is a voltage when the phase difference is 90 degrees (sin component detection mode). The time for applying the phase shift voltage of the cos component and the sin component is the same period, that is, the first period Δt. In the present embodiment, the frequency sweep period (T _{1 + 1} −T ₁ , T _{1 + 2} −T _{1 + 1} ) and the first period have the same length.

In FIG. 6D, the vertical axis represents voltage, the horizontal axis represents time, the control unit 16 generates an interference signal from the light detection unit 11, and the control unit 16 performs cos component and sin component for each frequency sweep period. It is a figure which shows the timing of the sampling period which receives any one of these interference signals. In this embodiment, in two frequency sweep periods, the cos component is sampled in the first frequency sweep period, the sin component is sampled in the subsequent frequency sweep period, and the interference signal of the same frequency is used as a pair of orthogonal two-component data. Here, the control unit 16 uses the same measurement frequency (corresponding to the frequency resolution) in each of the frequency sweep period for performing the frequency decomposition of the interference component of the cos component and the frequency sweep period for performing the frequency decomposition of the interference component of the sin component. Is set in advance as a measurement cycle (sampling cycle), and the cos component and the sin component are sampled in the set measurement cycle.
6 (c) and 6 (d), in order to clearly describe the first period, the horizontal time scale of FIGS. 6 (a) and 6 (b) is expanded, and only a part of the time range is displayed. Is shown.

In the present embodiment, since the probe signal and the reference signal of the cos component and the sin component are measured with the same sampling period in different frequency sweep periods, the phase difference in the optical phase shift unit 9 is measured as in the first embodiment. No change in frequency due to mutual switching occurs, and cos component and sin component interference signals can be obtained at the same frequency, and the measured spectral phase changes Δφ (ν) and Δφ (ν ₀ ) are measured. Accuracy can be improved.
However, the optical path length difference between the first optical fiber 3 and the second optical fiber 4 fluctuates every frequency sweep by causing the first optical fiber 3 and the second optical fiber 4 to slightly expand and contract with the vibration of the apparatus. Then, the phase difference between the first optical fiber 3 and the second optical fiber 4 deviates from 0 degrees or 90 degrees. As a result, the orthogonality of the cos component and the sin component is reduced, and the measurement accuracy of the spectral phase changes Δφ (ν) and Δφ (ν ₀ ) is lowered. For this reason, it is necessary to increase the stability against vibration of the first optical fiber 3 and the second optical fiber 4 so that the optical path length between the first optical fiber 3 and the second optical fiber 4 does not change with time.

<Sixth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the sixth embodiment will be described. The chromatic dispersion measuring apparatus according to the sixth embodiment can be implemented in any configuration of the first to fifth embodiments, and will be described below with reference to FIG.
In the sixth embodiment, the polarization direction of the optical signal to be measured incident on the optical frequency sweep unit 10 and the optical frequency extraction unit 13 in FIG. 2 is changed to the band pass in the optical frequency sweep unit 10 and the optical frequency extraction unit 13. The control is performed to match the polarization axis of the optical filter, and the spectral decomposition characteristic is improved. In the following description, only the configuration different from the first to fifth embodiments will be described.
The optical frequency sweep unit 10 and the optical frequency extraction unit 13 use a band pass optical filter having a narrow band pass frequency width and a high finesse in order to improve the resolution with respect to the frequency.
As an example of this bandpass optical filter, there is an optical element configured by providing a resonator having a high Q value in an optical fiber.

As described above, when the bandpass optical filter itself is formed of an optical fiber, it is advantageous to further reduce the size and weight of the configuration of the chromatic dispersion measuring apparatus according to the first to fifth embodiments already described.
On the other hand, in a bandpass optical filter, when a polarization-maintaining optical fiber (hereinafter referred to as an optical fiber) is used without using a polarization-maintaining optical fiber, when strain is applied to the optical fiber due to a fixed state or the like, Polarization dependence with a different refractive index occurs with respect to the polarization direction of incident light. As a result, in the bandpass optical filter, there arises a problem that the bandpass frequency width is expanded depending on the polarization direction, and there are a plurality of bandpass frequency bands depending on the polarization direction instead of a single bandpass frequency band.

In order to solve the problem of polarization dependency in the bandpass optical filter, a polarization controller is provided in front of the optical frequency sweep unit 10 and the optical frequency extraction unit 13 configured by the bandpass optical filter, The polarization direction may be adjusted by adjusting the polarization direction so as to coincide with a specific polarization axis of the optical fiber constituting the bandpass optical filter. Here, it is assumed that the polarization controller matches the polarization direction of the incoming combined optical signal to be measured with the slow axis of the optical fiber used for the bandpass optical filter.
FIG. 7 shows a configuration example for adjusting the polarization direction of incident light so as to coincide with a single polarization axis (for example, slow axis) of an optical fiber constituting the bandpass optical filter. The optical fiber 81 in FIG. 7 corresponds to the probe path coupling optical fiber 6 in FIG. 2. When 84 in FIG. 7 is the optical frequency sweep unit, it corresponds to the optical frequency sweep unit 10 in FIG. The optical fiber 85 is a polarization non-maintaining optical fiber and corresponds to the probe outgoing optical fiber 11 in FIG. Further, when 84 is an optical frequency extraction unit, it corresponds to the optical frequency extraction unit 13 in FIG. 2, and the optical fiber 85 in FIG. 7 is a polarization non-maintaining optical fiber, and the reference outgoing optical fiber 14 in FIG. It corresponds.

In the present embodiment, for the purpose of controlling polarization, the incident side optical fiber 81 and the connecting optical fiber 83 in FIG. 7 are polarization maintaining optical fibers. In the polarization controller 82, the exit end of the optical fiber 81 is connected to the entrance end, and the entrance end of the connection optical fiber 83 is connected to the exit end. In addition, the polarization controller 82 matches the polarization direction of the combined optical signal to be measured incident via the optical fiber 81 with the slow axis of the optical fiber constituting the bandpass optical filter in the optical frequency sweep unit 84. The light is emitted to the connection optical fiber 83.
As a result, the optical frequency sweep unit 84 (or the optical frequency extraction unit) passes through the connection optical fiber 83 and the combined measured light whose polarization characteristics coincide with the slow axis of the optical fiber constituting the internal bandpass optical filter. A signal enters from the optical coupler 5.
When 84 is an optical frequency sweeping unit, the optical frequency sweeping unit 84 frequency-decomposes the combined measured optical signal by sweeping the bandpass frequency width in the measurement frequency range, and the component measured optical signal. As a result, the light is emitted to the probe light detection unit 12 through the optical fiber 85.
When 84 is an optical frequency extraction unit, the optical frequency extraction unit extracts a component of a specific frequency of the combined optical signal to be measured by sweeping the bandpass frequency width in the range of the measurement frequency. The measurement light signal is emitted to the reference light detector 15 via the optical fiber 85.

  As described above, the polarization controller 82 adjusts the polarization direction of the combined optical signal to be measured to the polarization axis of the optical fiber constituting the bandpass optical filter in the optical frequency sweep unit 84 (or optical frequency extraction unit). Therefore, it is possible to prevent the degradation of the spectral resolution characteristic due to the polarization direction, and to improve the accuracy of the frequency resolution characteristic.

When the optical fiber constituting the bandpass optical filter in the optical frequency sweep unit 84 (or the optical frequency extraction unit) is a polarization maintaining optical fiber, the optical fiber 81 is used as a polarization maintaining optical fiber. The controller 82 can be omitted. That is, the polarization direction of the incident-side optical fiber 81 and the polarization direction of the optical fiber constituting the bandpass optical filter in the optical frequency sweep unit 84 (or optical frequency extraction unit) may be aligned and directly connected. .
As described above, when all the light transmission paths of the chromatic dispersion measuring apparatus are configured with optical fiber-based optical components, the accuracy of frequency resolution (spectral resolution characteristics) in the optical frequency sweep unit 84 (or optical frequency extraction unit). Can be prevented.

<Seventh Embodiment>
Next, a chromatic dispersion measuring apparatus according to the seventh embodiment will be described. FIG. 8 is a diagram for explaining a measurement method for measuring a dispersion parameter of an optical pulse propagating through an optical fiber transmission line by using the chromatic dispersion measurement apparatus according to any one of the first to sixth embodiments. . In this figure, a chromatic dispersion measuring device 66 is a chromatic dispersion measuring device according to any one of the first to sixth embodiments.
The monitoring optical branching unit 62 is arranged at a position for evaluating chromatic dispersion in the path of the optical fiber transmission path 61, extracts an optical pulse propagating to the optical fiber transmission path 61 as a signal under measurement, and monitors the optical fiber. The light is emitted to the polarization controller 64 through 63.

  At this time, the monitoring optical branching unit 62 extracts a part of the power of the optical pulse propagating in the optical fiber transmission line 61 so as not to attenuate to such an extent that the propagation in the optical fiber transmission line 61 is affected. In the present embodiment, the monitoring optical branching unit 62 branches a part of the power of the optical pulse propagated to the optical fiber transmission line 61, for example, 10%, to the monitoring optical fiber 63 as a measured optical signal. That is, due to the branching of the monitoring optical branching unit 62, the power branching ratio of the optical pulse propagating through the optical fiber transmission line 61 and the optical signal under measurement propagating through the monitoring optical fiber 63 becomes 9: 1. For the monitoring optical fiber 63, for example, a standard dispersion single mode optical fiber is used.

The polarization controller 64 sets the polarization state of the optical signal to be measured to linear polarization, and the polarization axis thereof is the polarization axis (for example, the slow axis) of the incident optical fiber 65 (the incident optical fiber 1 in FIG. 2 or 5). Then, the measured optical signal is emitted to the incident optical fiber 65.
As the incident optical fiber 65, a polarization maintaining optical fiber is used, and the polarization axis is a polarization axis with respect to the polarization maintaining optical fibers (first optical fiber 3 and second optical fiber 4) inside the wavelength dispersion measuring device 66. Align.

As described above, in the present embodiment, when measuring the chromatic dispersion in the optical fiber transmission line 61, the chromatic dispersion is measured by using the optical pulse that actually propagates through the optical fiber transmission line 61. A dedicated light source is prepared, a measurement light pulse is incident from the light source on the incident end of the optical fiber transmission line 61, a measurement light pulse output from the output end of the optical fiber transmission line 61 is taken out, and this measurement is performed. Therefore, it is not necessary to take out a light pulse for measurement and measure chromatic dispersion.
Not only the measurement of chromatic dispersion in the entire optical fiber transmission line 61 but also the chromatic dispersion of the distance to that position can be measured at any position of the optical fiber transmission line 61, and the measurement position of chromatic dispersion can be freely set. The degree can be improved.
In addition, since the spatial optical system is not used, the device itself can be reduced in size, using an optical pulse that carries the device and propagates in an arbitrary position in any optical fiber transmission path and transmits information. In order to monitor this optical pulse, the optical fiber transmission line 61 is measured by measuring the change Δφ (ν), Δφ (ν ₀ ) of the spectral phase with respect to the minute frequency shift without requiring a light source for measurement. Can be evaluated.

<Eighth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the eighth embodiment will be described. FIG. 9 is a diagram for explaining a measurement method for measuring a dispersion parameter of an optical pulse propagating through an optical component using the chromatic dispersion measurement apparatus according to any one of the first to sixth embodiments. In this figure, a chromatic dispersion measuring device 78 is a chromatic dispersion measuring device according to any one of the first to sixth embodiments.
In this embodiment, when measuring the chromatic dispersion of the optical component to be measured, a special light source for measurement is not prepared, but usually an optical pulse transmitted for information transmission is output to the optical fiber transmission line. An optical transmitter is used as the light source 71. As described above, the light source 71 is a light source used in an optical fiber transmission line, and in this embodiment, an optical transceiver that converts an interference signal into an optical pulse is used. The light source 71 emits an optical pulse as an optical signal to be measured to the incident optical fiber 72.

The incident optical fiber 72 is configured by an optical fiber similar to that used in an optical fiber transmission line in which an optical component as the measurement target 74 is arranged. An incident light control unit 73 is inserted in the incident optical fiber 72.
The incident light control unit 73 controls the power and polarization state of the measured optical signal propagating through the incident optical fiber 72, and transmits the controlled measured optical signal to the measured object 74 via the incident optical fiber 72. The light is emitted to the incident end.
By controlling the power of the optical signal under measurement, it is possible to measure and evaluate the power dependency of chromatic dispersion in the object under measurement 74, that is, the relationship between the degree of chromatic dispersion and the power. Further, by controlling the polarization state of the optical signal to be measured, it is possible to measure and evaluate the dependency of chromatic dispersion on the polarization state, that is, the relationship between the polarization state and the degree of chromatic dispersion.

One end of the output optical fiber 75 is connected to the measurement target 74 at the output end, and the measurement target optical signal incident from the input end is output from the output end to the output optical fiber 75.
The outgoing optical fiber 75 is configured by an optical fiber similar to that used in an optical fiber transmission line in which an optical component as the measurement target 74 is arranged.
In the polarization controller 76, the other end of the outgoing optical fiber 75 is connected to the incident end, and the measured optical signal from the measured target 74 enters. The polarization controller 76 has one end of an incident optical fiber 77 connected to the output end. The incident optical fiber 77 uses a polarization maintaining optical fiber, and the polarization axis is polarized with the polarization maintaining optical fibers (the first optical fiber 3 and the second optical fiber 4) inside the wavelength dispersion measuring device 78. Align with the axis.

The polarization controller 76 sets the polarization state of the optical signal to be measured to linear polarization, and the polarization axis thereof is the polarization axis (for example, the slow axis) of the incident optical fiber 77 (the incident optical fiber 1 in FIG. 2 or FIG. 5). Then, the measured optical signal is emitted to the incident optical fiber 77.
Further, the measurement object 74 may be a reflective optical component. In the case of a reflective optical component, the incident end and the exit end of the measurement target 74 are the same, and the incident optical fiber 72 to the measurement target 74 and the emission optical fiber 75 from the measurement target 74 are the same. It will be connected via a circulator.

As described above, the device can be miniaturized without using a spatial optical system, and the optical signal propagated through the optical component to be measured can be measured at any place with the device being carried. By measuring the change Δφ (ν), Δφ (ν ₀ ) of the spectral phase with respect to a minute frequency shift as an optical signal, the wavelength of the optical component to be measured using the optical pulse that actually propagates in the optical component Dispersion can be evaluated.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes design and the like within a scope not departing from the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Incident optical fiber, 2 ... Optical branching part, 3 ... 1st optical fiber, 4 ... 2nd optical fiber, 5 ... Optical coupling part, 6 ... Optical fiber for probe path | route coupling, 7 ... Optical fiber for reference path | route coupling | bonding, DESCRIPTION OF SYMBOLS 8 ... Optical delay part, 9 ... Optical phase shift part, 10, 84 ... Optical frequency sweep part, 11 ... Probe outgoing optical fiber, 12 ... Probe light detection part, 13 ... Optical frequency extraction part, 14 ... Reference outgoing optical fiber, DESCRIPTION OF SYMBOLS 15 ... Reference light detection part, 16 ... Control part, 17 ... Phase control line, 18 ... Sweep frequency control line, 19 ... Reference frequency control line, 20 ... Probe detection signal line, 21 ... Reference detection signal line, 47 ... Optical delay Optical phase shift unit 61: optical fiber transmission path 62: monitoring optical branching unit 63: monitoring optical fiber 64, 76, 82: polarization controller 65, 72, 77: incident optical channel Driver, 66, 78 ... wavelength dispersion measuring apparatus, 71 ... light source, 73 ... incident light control unit, 74 ... object to be measured, 75 ... light-emitting optical fiber, 81, 85 ... optical fiber, 83 ... connection optical fiber

Claims (11)

An incident path composed of an optical fiber having polarization maintaining characteristics, which propagates an optical signal to be measured incident from a measurement target;
The measured optical signal is incident from the first incident end connected to the incident path, and the measured optical signal incident from the first incident end is two of the first measured optical signal and the second measured optical signal. The first measured optical signal is emitted from the first output end, and the second measured optical signal having the same polarization direction as the first measured optical signal is output from the second output end. An optical branching unit that emits and generates a frequency difference between the first optical signal to be measured and the second optical signal to be measured when exiting; and
A first branch path connected to the first output end, propagating the first optical signal to be measured, and configured by an optical fiber having polarization maintaining characteristics;
A second branch path that is connected to the second emission end, propagates the second optical signal to be measured, and includes an optical fiber having polarization maintaining characteristics;
Light that is provided in one of the first branch path and the second branch path, and alternately changes the phase of the optical measurement optical signal propagating through the provided branch path by binary values of 0 degrees and 90 degrees. A phase shifter;
The first measured optical signal incident from the second incident end connected to the first branch path and the second measured optical signal incident from the third incident end connected to the second branch path And the optical phase shifter obtained by the interference between the first optical signal to be measured and the second optical signal to be measured, and the optical signal of the branch path on which the optical phase shifter is provided and the other optical path When the phase difference between the measurement optical signals is 0 degree, the interference element of the first optical component, and the optical phase shifter between the measurement optical signal of the branch path where the optical phase shifter is provided and the measurement optical signal of the other branch path When the phase difference is 90 degrees, the interference component of the second light component is emitted from the third emission end as the first combined measured optical signal and output from the fourth output end as the second combined measured optical signal. An optical combiner,
A probe coupling path configured by an optical fiber connected to the third emission end and propagating the first combined optical signal to be measured;
A reference coupling path configured by an optical fiber connected to the fourth emission end and propagating the second combined optical signal to be measured;
The first combined optical signal to be measured is incident from a fourth incident end connected to the probe coupling path, the frequency range through which the first combined optical signal to be measured is passed, and the first combined signal is swept. An optical frequency sweeping unit that performs frequency decomposition to extract a spectral component in the frequency range from the measured optical signal, and outputs the result of the frequency decomposition as a component measured optical signal from the fifth emission end;
The second combined optical signal is incident from a fifth incident end connected to the reference coupling path, the frequency range allowing the second combined optical signal to pass is swept, and the second combined signal is swept. An optical frequency extraction unit that extracts a spectral component of a fixed specific frequency within the frequency range from the optical signal to be measured, and outputs the extracted result as a reference optical signal from the sixth emission end;
A probe output light path configured by an optical fiber connected to the fifth output end and propagating the component measured optical signal;
A reference emission light path configured by an optical fiber connected to the sixth emission end and propagating the reference light signal;
A probe light detection unit that receives the component measured optical signal from a sixth incident end connected to the probe emission light path, converts the component measured optical signal into an electrical signal, and uses the conversion result as a probe signal; ,
A reference light detector that enters the reference light signal from a seventh incident end connected to the reference emission light path, converts the reference light signal into an electric signal, and uses the conversion result as a reference signal;
Synchronize with the phase change of the optical phase shifter, and acquire each of the probe signals of the first and second optical components and the reference signals of the first and second optical components in time series. And a control unit that obtains chromatic dispersion by differentially measuring the spectral phase obtained from the probe signal based on the spectral phase obtained from the reference signal.   The optical delay unit is provided in any one of the first branch path and the second branch path, and further includes an optical delay unit that adjusts an optical path length difference between the first branch path and the second branch path. Item 2. The chromatic dispersion measuring apparatus according to Item 1.   The chromatic dispersion measurement apparatus according to claim 2, wherein the optical delay unit is provided in one of the first branch path and the second branch path, and the optical phase shifter is provided in the other.   3. The chromatic dispersion measuring apparatus according to claim 2, wherein the optical delay unit and the optical phase shifter are integrally provided in one of the first branch path and the second branch path. 4.   The control unit includes a first receiving unit that receives the probe signal and the reference signal of the first optical component, and a second receiving unit that receives the probe signal and the reference signal of the second optical component. The chromatic dispersion measuring apparatus according to claim 1, wherein the chromatic dispersion measuring apparatus is provided.   The control unit acquires the first light component and the second light component as a data pair in time series for each of the probe signal and the reference signal as a measurement unit for each measurement frequency sweep in the measurement range. The chromatic dispersion measuring device according to any one of claims 1 to 5.   In the first light component and the second light component constituting the data pair for each of the probe signal and the reference signal, the polarization component that is measured later is the time during which the phase of the polarization component that is measured earlier is changed. The chromatic dispersion measuring apparatus according to claim 6, wherein the chromatic dispersion measuring apparatus is set to be short with respect to a time during which the phase is changed. In the sweep of the measurement frequency in the measurement range, the control unit obtains the probe signal and the reference signal for only one of the first light component and the second light component in one sweep. the wavelength dispersion measuring apparatus according to claim 6 or claim 7, characterized in that is repeated alternately to the first light component and the second light component.   The probe coupling path is composed of an optical fiber having polarization maintaining characteristics, and a first polarization controller for controlling the polarization direction of the first combined optical signal to be measured is inserted in the subsequent stage of the probe coupling path. The first polarization controller and the optical frequency sweep unit are connected by an optical fiber having polarization maintaining characteristics, and the reference coupling path is configured by an optical fiber having polarization maintaining characteristics, and the reference A second polarization controller for controlling the polarization direction of the second combined optical signal to be measured is inserted in the subsequent stage of the coupling path, and the polarization is maintained between the second polarization controller and the optical frequency extraction unit. The chromatic dispersion measuring device according to claim 1, wherein the chromatic dispersion measuring device is connected by an optical fiber having characteristics. A chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 9,
A branching unit is provided in a portion for evaluating the chromatic dispersion of the optical transmission line to be measured, and the polarization control unit controls the polarization of the optical signal to be measured obtained from the branching unit to a linearly polarized wave. A spectrum obtained from the reference signal of the first light component and the second light component by aligning the polarization axis propagating in the measuring device and making the measured optical signal incident on the chromatic dispersion measuring device via the incident path. Using the change in phase as a reference, the change in spectral phase of the optical signal under measurement is obtained by differentially measuring the change in spectral phase obtained from the probe signal of the first light component and the second light component, A chromatic dispersion measuring method characterized by evaluating chromatic dispersion in the measurement object. A chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 9,
An optical signal subjected to polarization control is incident on the incident end of the measurement target for evaluating chromatic dispersion, and the polarization of the measured optical signal emitted from the output end of the measurement target is controlled to be linearly polarized, Aligned with the polarization axis propagating in the chromatic dispersion measuring device, the measured optical signal is incident on the chromatic dispersion measuring device through the incident path, and the reference signal of the first light component and the second light component is used. Using the obtained spectral phase change as a reference, the spectral phase change obtained from the probe signal of the first optical component and the second optical component is differentially measured to obtain the spectral phase change of the optical signal to be measured. And measuring chromatic dispersion in the measurement object.

Images