JP2018185278A - Optical frequency domain refractometry device and optical frequency domain refractometry method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical frequency domain refractometry device and an optical frequency domain refractometry method with which it is possible to improve the accuracy of measuring strain or temperature distribution over the wide range of an optical fiber being measured.SOLUTION: Weight filters 61, 62 consist of a weight characteristic A that changes linearly, in which a position between minimal positions z, zon an optical fiber being measured where the error of a signal whose linearity of wavelength sweep has been corrected is minimal is a variable, and a weight characteristic B in which a position outside of the minimal positions z, zon the optical fiber being measured is a variable. The differential value of amplitude of the weight characteristic B relative to a position on the optical fiber being measured is continuous with the differential value of amplitude of the weight characteristic A at the minimal positions z, zand is zero at a position on the optical fiber being measured that corresponds to the zero frequency of a measured interference signal, and is zero at a position on the optical fiber being measured that corresponds to a half of a sampling frequency in linearization means 51, 52.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical frequency domain reflection measurement apparatus and an optical frequency domain reflection measurement method for measuring distortion or temperature distribution of an optical fiber to be measured using a wavelength swept light source, and in particular, the optical frequency sweep characteristic of a wavelength swept light source is a straight line. The present invention relates to an optical frequency domain reflectometry apparatus and an optical frequency domain reflectometry method for correcting an error that occurs in a case where the frequency is not.

<Basic configuration>
Conventionally, strain and temperature of an optical fiber have been measured using an optical frequency domain reflectometry (OFDR). A basic configuration of a conventional optical frequency domain reflection measurement method is shown in FIG. The swept light source 1 outputs light that has been wavelength swept so that the optical frequency changes linearly with time. The measurement interferometer 4 branches the input light into two, inputs one light to the optical fiber to be measured included in the measurement interferometer 4, and reflects the reflected light from the optical fiber to be measured and the other light (reference light) Light) and output.

  For example, as shown in FIG. 31 (b), the input light is branched into two by the optical coupler 41, and one light is input to the first terminal 42 a of the optical circulator 42. The light input to the first terminal 42 a of the optical circulator 42 is output from the second terminal 42 b and input to the measured optical fiber 43. The reflected light from the measured optical fiber 43 is input to the second terminal 42b of the optical circulator 42 and output from the third terminal 42c. The light output from the third terminal 42 c of the optical circulator 42 and the other light (reference light) branched by the optical coupler 41 are combined by the optical coupler 45 and output.

  The light output from the measurement interferometer 4 is input to the light receiver 11. The light receiver 11 converts the input light into an electrical signal proportional to the intensity. Thereby, the beat due to the interference between the reflected light from the optical fiber 43 to be measured and the reference light is output as an electric signal. The electrical signal output from the light receiver 11 is converted into a digital signal by the A / D converter 12, and Fourier transform is performed by the Fourier transform unit 60.

As shown in FIG. 32A, assuming that the measurement optical fiber 43 has three reflection points, point A, point B, and point C, the distance from the near-end O point of the measurement optical fiber 43 to each point is determined. each _{L a, L B,} and _{L C.} The optical path length of the light output from the optical coupler 41 and reflected at the near end O point of the measured optical fiber 43 and reaching the optical coupler 45, and the optical path length of the reference light output from the optical coupler 41 and reaching the optical coupler 45 Are equal to each other, the light reflected at the point A of the optical fiber 43 to be measured is combined with the reference light by the optical coupler 45 with a time delay of t _A = 2nL _A / c compared to the reference light.

Here, n is the refractive index of the optical fiber 43 to be measured, and c is the speed of light. Similarly, the light reflected at the points B and C is delayed by t _B = 2nL _B / c and t _C = 2nL _C / c. For example, at the position of the input terminal of the optical coupler 45, the optical frequency ν _R of the reference light, the optical frequency ν _A of the reflected light from the point _A , the optical frequency ν _B of the reflected light from the point _B , and the reflected light from the C point The time change of the optical frequency ν _C is as shown in FIG. When the optical frequency change amount per unit time of the output light of the sweep light source 1 is S, the beat frequency f _A due to the interference between the reflected light from the point A and the reference light is expressed by the following formula (1).

Similarly, the beat frequency due to the interference between the reflected light from the points B and C and the reference light is expressed by the following equations (2) and (3).

Therefore, when the digital signal converted by the A / D converter 12 is Fourier-transformed, beats of frequencies f _A , f _B , and f _C proportional to the distances L _A , L _B , and L _C as shown in FIG. A signal is observed. It is assumed that the reflectance at each point is sufficiently small, and multiple reflections are ignored. As described above, the reflected light distribution in the longitudinal direction of the measured optical fiber 43 can be measured by the optical frequency domain reflection measurement method.

<Configuration including linearization processing>
The optical frequency domain reflection measurement method requires a swept light source in which the light frequency changes linearly with respect to time, but the actual light source has a deviation from the straight line in the sweep characteristics. In particular, in the case of an external cavity laser that mechanically sweeps the wavelength, it is difficult to change the optical frequency completely linearly.

  The wavelength sweep by the sweep light source includes, for example, a sweep in which the light wavelength changes linearly with time and a sweep in which the light wavelength changes sinusoidally. In the case of a sine wave sweep, it is possible to obtain a sweep that is close to a straight line by using only a region of the sine wave that is relatively close to a straight line, but there is a problem that the usable wavelength range becomes narrow. . For this reason, a method has been conventionally proposed in which an auxiliary interferometer different from the measurement interferometer including the optical fiber to be measured is prepared and the nonlinearity of the wavelength sweep is corrected.

  The configuration of the optical frequency domain reflection measurement method including the linearization processing is shown in FIG. The light branching unit 2 branches the light from the sweep light source 1 into two and inputs the light to the auxiliary interferometer 3 and the measurement interferometer 4 respectively.

  The auxiliary interferometer 3 branches the input light into two, and multiplexes them by giving different delay times. For example, as shown in FIG. 33B, the input light to the auxiliary interferometer 3 is branched into two by an optical coupler 31a, one via a delay fiber 32 of a predetermined length, and the other without a delay fiber. Are respectively input to the optical coupler 34 and multiplexed.

  The linearization means 5 performs linearization processing for correcting the nonlinearity of the wavelength sweep of the sweep light source 1 on the output signal of the measurement interferometer 4 using the output signal of the auxiliary interferometer 3. For example, as shown in FIG. 33 (c), when the output of the auxiliary interferometer 3 is converted into an electrical signal by the light receiver 11a, a sine wave beat signal having a frequency proportional to the sweep speed of the sweep light source 1 is obtained. .

  The sampling timing calculation means 13 outputs a timing at which the phase of the sine wave beat signal is at a constant interval. For example, when the zero cross point of the sine wave is detected by the comparator, the rising edge of the comparator output has an interval of 2π in the phase of the sine wave.

  The sampling means 15 samples the output of the measurement interferometer 4 converted into an electrical signal by the light receiver 11b at a timing when a predetermined delay time δt is added by the delay adding means 14 to the timing from the sampling timing calculating means 13. Convert to digital signal.

  FIG. 33 (c) shows a configuration in which the sampling means 15 performs A / D conversion according to the output of the sampling timing calculation means 13, but after the output of the measurement interferometer 4 is A / D converted at a constant sampling frequency. The same effect can be obtained by a configuration in which resampling is performed in accordance with the timing of the sampling timing calculation means 13 by digital processing. Further, the sampling timing calculation means 13 may detect the timing at which the phase of the sine wave becomes a constant interval by digital processing after the output signal of the auxiliary interferometer 3 is A / D converted. When the sampling timing is calculated by digital processing, the phase interval to be detected can be easily set to an arbitrary value other than 2π.

  Qualitatively, since the beat frequency of the output of the auxiliary interferometer 3 increases when the sweep speed of the sweep light source 1 is high, the linearization means 5 samples the output signal of the measurement interferometer 4 at a high frequency. On the other hand, when the sweep speed of the sweep light source 1 is slow, the beat frequency of the output of the auxiliary interferometer 3 becomes low, and the linearization means 5 samples the output signal of the measurement interferometer 4 at a low frequency. Thereby, a measurement signal corresponding to the case where the sweep speed is constant can be obtained from the measurement interferometer 4.

  Quantitatively, as shown in Non-Patent Document 1, when the delay time by the delay adding means 14 is set to δt = τ / 2, the first-order error term is canceled, and the error due to the nonlinearity of the sweep speed is eliminated. Reduced. Here, τ is the delay time difference between the two optical paths of the auxiliary interferometer 3. Hereinafter, only the first-order error term due to nonlinear sweep is handled. The measurement signal from the measurement interferometer 4 in which the nonlinearity of the wavelength sweep is corrected by the linearization means 5 is Fourier-transformed by the Fourier transform unit 60, and the measurement result of the optical frequency domain reflection measurement method is obtained.

<Application of optical frequency domain reflectometry>
In the measured optical fiber 43, light is continuously reflected in the longitudinal direction by Rayleigh scattering or a fiber Bragg diffraction grating (FBG) provided in the measured optical fiber 43. When distortion is applied in the longitudinal direction of the optical fiber 43 to be measured, the phase of light reflected by Rayleigh scattering or FBG changes. Therefore, by observing the phase of the beat signal in the frequency domain obtained by the optical frequency domain reflection measurement method, it is possible to measure the longitudinal distribution of minute distortions in the optical fiber 43 to be measured.

  When the temperature of the optical fiber 43 to be measured changes, the phase of the reflected light due to Rayleigh scattering or FBG changes as in the case where distortion is applied in the longitudinal direction of the optical fiber 43 to be measured. Therefore, by observing the phase of the beat signal in the frequency domain obtained by the optical frequency domain reflection measurement method, the temperature distribution in the longitudinal direction can be measured instead of the distortion of the optical fiber 43 to be measured.

  Patent Document 1 discloses a method of measuring the position or shape of an optical fiber to be measured by an optical frequency domain reflection measurement method using a multi-core fiber having a plurality of cores. Also in Patent Document 1, the non-linearity of the laser sweep is corrected with respect to the signal from the interrogator network using the signal from the interferometer in the laser monitoring network, and minute distortion is accurately measured. Therefore, nonlinear correction of the wavelength sweep is necessary.

Special table 2013-505441 gazette

Eric D. Moore and Robert R. McLeod, "Correction of sampling errors due to laser tuning rate fluctuations in swept-wavelength interferometry," Optics Express, vol. 16, no. 17, pp. 13139-13149, 2008.

  An error that occurs when the optical frequency sweep characteristic is not a straight line can be corrected by the method of Non-Patent Document 1. However, what can be corrected by the method of Non-Patent Document 1 is limited to a beat signal from a measurement interferometer having a frequency corresponding to a set specific delay time δt. That is, when measuring the strain distribution of the optical fiber to be measured having a predetermined length, the error can be corrected only at a specific position of the optical fiber to be measured corresponding to the specific delay time δt. However, there is a problem that the effect of correcting the error is low. In particular, when the optical fiber to be measured is long, there is a problem that the error becomes large at a position far from a specific position.

  The present invention has been made to solve such a conventional problem, and is an optical frequency domain reflection measuring apparatus capable of improving the measurement accuracy of strain or temperature distribution over a wide range of an optical fiber to be measured. And it aims at providing the optical frequency domain reflection measuring method.

In order to solve the above-described problems, an optical frequency domain reflection measurement apparatus according to the present invention includes a sweep light source that outputs wavelength-swept light as output light, and a part of the output light from the sweep light source as a delay fiber. An auxiliary interferometer that inputs and outputs as an auxiliary interference signal by interfering with the light output from the delay fiber and another part of the output light from the sweep light source, and the output light from the sweep light source Is input to the optical fiber to be measured, and the reflected light reflected by the optical fiber to be measured interferes with another part of the output light from the swept light source to output as a measurement interference signal An interferometer and first and second linearization means for outputting, as output signals, signals obtained by correcting nonlinearity of wavelength sweep of the swept light source with respect to the measurement interference signal using the auxiliary interference signal, The auxiliary The first and second linearization means each having a delay time for providing two different relative time differences between the interference signal and the measurement interference signal; and two from the first and second linearization means The first and second weight filters for adding the first and second weight characteristics to the output signal, and the two output signals to which the first and second weight characteristics are added are added to the Fourier signal. And an addition / Fourier transform means for outputting a converted frequency domain signal, wherein z ₀ is a position on the measured optical fiber corresponding to a zero frequency of the output signal. Corresponding to the delay times of the first and second linearization means, and the position on the optical fiber to be measured where the error of the two output signals due to the nonlinearity of the wavelength sweep of the swept light source is minimized. Each And ₁ and z _2, the position on the optical fiber to be measured corresponding to half the frequency of the sampling frequency of the output signal and z _n, said first and second weighting characteristics, the optical fiber to be measured A weight characteristic A that is defined in z ₁ ≦ z ≦ z ₂ and changes linearly with respect to z, z ₀ ≦ z <z ₁ , and z ₂ <z ≦ z, with the upper position z as a variable. a weight characteristic B defined in _n , and the differential value of the amplitude of the weight characteristic B with respect to z is continuous with the differential value of the amplitude of the weight characteristic A with respect to z in z ₁ and z ₂ . , And zero at z _{0 and} zero at z _n .

  With this configuration, the optical frequency domain reflectometry apparatus according to the present invention can correct the nonlinearity of the wavelength sweep and improve the measurement accuracy of strain or temperature distribution over a wide range of the optical fiber to be measured.

  Also, with this configuration, the optical frequency domain reflectometry apparatus according to the present invention can realize a weight filter with a small amplitude error from a desired weight characteristic with an FIR digital filter with a small number of taps, and with a small amount of calculation. The nonlinear influence of the wavelength sweep of the sweep light source can be reduced.

  In the optical frequency domain reflectometry apparatus according to the present invention, the first and second weight filters may be FIR filters having a filter delay time that is an integral multiple of the reciprocal of the sampling frequency.

  In this case, the continuity of the phase of the frequency characteristics of the first and second weighting filters is maintained at the turning point of the frequency characteristics at half the sampling frequency of the output signal of the first and second linearization means. Therefore, the optical frequency domain reflection measurement apparatus according to the present invention can accurately realize a weighting filter having a desired weighting characteristic with an FIR filter.

Further, in the optical frequency domain reflectometry apparatus in accordance with the present invention, the amplitude of the weight characteristic B may be zero in the z _n.

  With this configuration, the optical frequency domain reflectometry apparatus according to the present invention is capable of measuring the amplitude value and the amplitude at the turning point of the frequency characteristic at half the sampling frequency regardless of the delay times of the first and second weight filters. Therefore, a weighting filter having a desired weighting characteristic can be realized with high accuracy by the FIR filter.

Further, the optical frequency domain reflection measuring apparatus according to the present invention includes a sweep light source that outputs wavelength-swept light as output light, a part of the output light from the sweep light source is input to a delay fiber, and the delay fiber An auxiliary interferometer that interferes with the light output from the sweep light source and another part of the output light from the swept light source and outputs it as an auxiliary interference signal; and measures a part of the output light from the swept light source A measurement interferometer that inputs to an optical fiber and causes reflected light reflected by the optical fiber to be measured to interfere with another part of the output light from the swept light source and output as a measurement interference signal; and the auxiliary First and second linearization means for outputting, as output signals, signals obtained by correcting nonlinearity of wavelength sweeping of the swept light source with respect to the measurement interference signal using an interference signal, the auxiliary interference signal and Measurement For the first and second linearization means each having a delay time for providing two different relative time differences between the signals, and for the two output signals from the first and second linearization means The first and second weight filters that respectively add the first and second weight characteristics, and the two output signals to which the first and second weight characteristics are added are added to the Fourier transform of the frequency domain. And an addition / Fourier transform means for outputting a signal, wherein the position on the measured optical fiber corresponding to the zero frequency of the output signal is z _0, and the first and first The positions on the optical fiber to be measured corresponding to the delay times of the two linearization means and where the error between the two output signals due to the nonlinearity of the wavelength sweep of the swept light source is minimized are z ₁ and z ₂ , respectively. age, The position on the optical fiber to be measured corresponding to 1/2 of the sampling frequency of the serial output signal and z _n, said first and second weighting characteristic variables the position z on the optical fiber to be measured Defined as z ₁ ≦ z ≦ z ₂ , weight characteristics A changing linearly with respect to z, and weights defined as z ₀ ≦ z <z ₁ and z ₂ <z ≦ z _n The differential value of the weighting characteristic B with respect to z is continuous with the differential value of the weighting characteristic A with respect to z at z ₁ and z ₂ and z _0. And the amplitude of the weight characteristic B is zero at z _n .

  With this configuration, the optical frequency domain reflectometry apparatus according to the present invention can correct the nonlinearity of the wavelength sweep and improve the measurement accuracy of strain or temperature distribution over a wide range of the optical fiber to be measured.

  Also, with this configuration, the optical frequency domain reflectometry apparatus according to the present invention can realize a weight filter with a small amplitude error from a desired weight characteristic with an FIR digital filter with a small number of taps, and with a small amount of calculation. The nonlinear influence of the wavelength sweep of the sweep light source can be reduced.

  In the optical frequency domain reflectometry apparatus according to the present invention, the first and second weight filters may be FIR filters having a filter delay time that is an integral multiple of an inverse of the sampling frequency +1/2. Good.

  In this case, the continuity of the phase of the frequency characteristics of the first and second weighting filters is maintained at the turning point of the frequency characteristics at half the sampling frequency of the output signal of the first and second linearization means. Therefore, the optical frequency domain reflection measurement apparatus according to the present invention can accurately realize a weighting filter having a desired weighting characteristic with an FIR filter.

In the optical frequency domain reflectometry apparatus according to the present invention, the amplitude of the weight characteristic B of the first weight filter is 1 at the z ₀ and the weight characteristic of the second weight filter. The amplitude of B may be zero.

With this configuration, the optical frequency domain reflectometry apparatus according to the present invention reduces the nonlinearity of the wavelength sweep near z ₁ and also increases the dynamic noise necessary for weight increase filter processing and computation near the zero frequency. Increase in range can be suppressed.

  Moreover, in the optical frequency domain reflection measuring apparatus according to the present invention, the amplitude of the weight characteristic B may be represented by a trigonometric function having z as a variable.

In the optical frequency domain reflectometry apparatus according to the present invention, in the weight characteristic B, at least one of z ₀ ≦ z <z ₁ or z ₂ <z ≦ z _n is divided into two regions. The amplitude of the weight characteristic B may be expressed by a trigonometric function having z as a variable in each of the two regions, and the period of the trigonometric function in the two regions may be set equal.

  In the optical frequency domain reflectometry apparatus according to the present invention, the amplitude of the weight characteristic B may be represented by a quadratic function with z as a variable.

In the optical frequency domain reflectometry apparatus according to the present invention, in the weighting characteristic B, at least one of z ₀ ≦ z <z ₁ or z ₂ <z ≦ z _n is in two or three regions. The amplitude of the weight characteristic B is expressed by a quadratic function having the variable z in the two or three regions, and 2 of the quadratic function of the two or three regions. The second order differential values may be set equal.

Further, the optical frequency domain reflection measurement method according to the present invention includes a step of outputting, as output light, light that has been wavelength-swept from a swept light source, and a part of the output light from the swept light source is input to a delay fiber, A step of causing the light output from the delay fiber to interfere with another part of the output light from the swept light source and outputting as an auxiliary interference signal; and measuring a part of the output light from the swept light source A step of causing the reflected light that is input to the optical fiber and reflected by the optical fiber to be measured to interfere with another part of the output light from the swept light source to be output as a measurement interference signal; and the auxiliary interference signal First and second linearization steps for outputting, as output signals, signals obtained by correcting the nonlinearity of the wavelength sweep of the swept light source with respect to the measurement interference signal using the first and second steps, respectively, The linearization step includes the first and second linearization steps each having a delay time for providing two different relative time differences between the auxiliary interference signal and the measurement interference signal, and the first and second linearization steps. Two weight characteristics are added by adding a first weight characteristic and a second weight characteristic to the two output signals from the linearization step, and adding a first weight characteristic and a second weight characteristic, respectively. Adding the output signal and outputting a Fourier-transformed frequency domain signal, comprising the steps of: measuring a position on the optical fiber under measurement corresponding to the zero frequency of the output signal; and z _0, corresponding to each of said delay time of said first and second linearization means, error of two of the output signal due to the nonlinearity of the wavelength sweep of the swept light source is minimum the The position on the measuring optical fiber and z ₁ and z _2, respectively, the position on the optical fiber to be measured corresponding to half the frequency of the sampling frequency of the output signal and z _n, said first and second The weight characteristic is defined as z ₁ ≦ z ≦ z ₂ with the position z on the optical fiber to be measured as a variable, and the weight characteristic A changes linearly with respect to z, and z ₀ ≦ z <z _1. And a weight characteristic B defined in z ₂ <z ≦ z _n , and the differential value of the amplitude of the weight characteristic B with respect to z is the amplitude of the weight characteristic A in z ₁ and z ₂ . It is continuous with the derivative value with respect to z, is zero at z ₀ , and is zero at z _n .

  With this configuration, the optical frequency domain reflection measurement method according to the present invention can correct the nonlinearity of the wavelength sweep and improve the measurement accuracy of strain or temperature distribution over a wide range of the optical fiber to be measured.

  In addition, with this configuration, the optical frequency domain reflection measurement method according to the present invention can realize a weight filter with a small amplitude error from a desired weight characteristic with an FIR digital filter with a small number of taps, and with a small amount of calculation. The nonlinear influence of the wavelength sweep of the sweep light source can be reduced.

  The present invention provides an optical frequency domain reflection measurement apparatus and an optical frequency domain reflection measurement method capable of improving the measurement accuracy of strain or temperature distribution over a wide range of an optical fiber to be measured.

It is a block diagram which shows an example of a structure of the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention. It is a block diagram which shows an example of a structure of the auxiliary interferometer with which the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention is provided. It is a block diagram which shows an example of a structure of the measurement interferometer with which the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention is provided. It is a block diagram which shows an example of a structure in the case of light-receiving by the polarization diversity system with which the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention is equipped. It is a block diagram which shows an example of a structure of the optical frequency domain reflection measuring apparatus which concerns on 1st Embodiment. It is a graph which shows an example of the setting of the weight by a weight filter. It is a graph which shows an example of the measurement range of a to-be-measured optical fiber. It is a block diagram which shows an example of a structure of the FIR filter as a weighting filter. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter which limited the amplitude to 0-1. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter which limited the amplitude to 0-1. It is a graph which shows the frequency characteristic (integer sample delay) of the weight filter of all the band straight lines. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weight filter of all the bands straight line. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of 1st Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of 1st Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the trigonometric function of 1st Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the quadratic function of 1st Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of 2nd Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of 2nd Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the trigonometric function of 2nd Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the quadratic function of 2nd Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of 3rd Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of 3rd Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the trigonometric function of 3rd Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the quadratic function of 3rd Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of 4th Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of 4th Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the trigonometric function of 4th Embodiment. It is a graph which shows the frequency characteristic (integer +1/2 sample delay) of the weighting filter using the quadratic function of 4th Embodiment. It is a graph which shows the variation of the frequency characteristic of the weighting filter of 4th Embodiment. It is a flowchart which shows the process of the optical frequency domain reflection measuring method by the optical frequency domain reflection measuring apparatus which concerns on 1st-4th embodiment. It is a block diagram which shows an example of the basic composition of the conventional optical frequency domain reflection measuring method. It is explanatory drawing which shows an example of the basic operation | movement of the conventional optical frequency domain reflection measuring method at the time of assuming three reflection points. It is a block diagram which shows an example of a structure of the conventional optical frequency domain reflection measuring method including a linearization process.

  Hereinafter, embodiments of an optical frequency domain reflection measurement apparatus and an optical frequency domain reflection measurement method according to the present invention will be described with reference to the drawings.

<Configuration>
FIG. 1 shows a basic configuration of an optical frequency domain reflection measuring apparatus according to the present invention. The optical frequency domain reflection measurement apparatus includes a swept light source 1, an optical branching unit 2, an auxiliary interferometer 3, a measurement interferometer 4, a first linearization unit 51, a second linearization unit 52, and a first linearization unit 52. 1 weight filter 61, second weight filter 62, and addition / Fourier transform means 7.

  The sweep light source 1 sweeps the wavelength of the output light within a prescribed sweep wavelength range and sweep speed. The wavelength may be swept one time, may be repeatedly swept in a predetermined cycle, or may be a sweep according to an external trigger signal (not shown). The sweep direction may be a sweep from a long wavelength to a short wavelength, a sweep from a short wavelength to a long wavelength, or a sweep in both directions may be used. For example, in an external cavity laser using a diffraction grating, the oscillation wavelength can be swept by changing the resonance wavelength by changing the angle of the diffraction grating or the mirror.

  In the optical frequency domain reflection measurement method, a sweep in which the optical frequency changes completely linearly with respect to time is ideal, but in reality, there is a deviation from the straight line in the sweep characteristic of the sweep light source. The wavelength sweep by the sweep light source includes, for example, a sweep in which the light wavelength changes linearly with time and a sweep in which the light wavelength changes sinusoidally. In the case of a sinusoidal sweep, by using only a region of a sine wave that is relatively close to a straight line, it can be regarded as a sweep that is close to a straight line.

  The light branching unit 2 branches the light from the sweep light source 1 into two and inputs the light to the auxiliary interferometer 3 and the measurement interferometer 4 respectively. Here, the light branching unit 2 splits the light from the sweep light source 1 into two, and then the auxiliary interferometer 3 and the measurement interferometer 4 each further split into two. However, the present invention is limited to this. Instead, a configuration in which the order of branching is reversed or four branches at a time may be employed.

  The auxiliary interferometer 3 branches the input light into two, and multiplexes them by giving different delay times. For example, as shown in FIG. 2, the input light to the auxiliary interferometer 3 is branched into two by an optical coupler 31b, one of which is reflected by a Faraday mirror 35 via a delay fiber 32 having a predetermined length. Propagate the path in the reverse direction. The other is transmitted through the optical fiber 33 without the delay fiber, reflected by the Faraday mirror 36, and propagates in the reverse direction on the same path. These two lights reflected by the Faraday mirrors 35 and 36 are combined by the optical coupler 31b and output as an auxiliary interference signal from a port different from the input side.

  That is, the auxiliary interferometer 3 inputs a part of the output light from the sweep light source 1 to the delay fiber 32, and outputs the light output from the delay fiber 32 and another part of the output light from the sweep light source 1. The auxiliary interference signal is output by causing interference. When an auxiliary interference signal is input to a light receiver that outputs an electric signal proportional to the light intensity, the auxiliary interference signal is square-detected to obtain a beat signal due to interference. By using the Faraday mirrors 35 and 36, it is possible to match the polarizations of the two lights at the time of multiplexing without using a polarization maintaining fiber or a polarization controller.

  The measurement interferometer 4 branches the input light into two, and inputs one light into the optical fiber to be measured included in the measurement interferometer 4. Then, the reflected light from the optical fiber to be measured and the other light (reference light) are combined and output. For example, as shown in FIG. 3, the input light to the measurement interferometer 4 is branched into two by the optical coupler 41, and one light is input to the first terminal 42 a of the optical circulator 42. The light input to the first terminal 42 a of the optical circulator 42 is output from the second terminal 42 b and input to the measured optical fiber 43. The reflected light from the measured optical fiber 43 is input to the second terminal 42b of the optical circulator 42 and output from the third terminal 42c. The light output from the third terminal 42c of the optical circulator 42 and the other light (reference light) branched by the optical coupler 41 are combined by the optical coupler 45 and output as a measurement interference signal.

  That is, the measurement interferometer 4 inputs a part of the output light from the swept light source 1 to the measured optical fiber 43, and separates the reflected light reflected by the measured optical fiber 43 and the output light from the swept light source 1. A measurement interference signal is output by interfering with a part of the signal. When a measurement interference signal is input to a light receiver that outputs an electrical signal proportional to the light intensity, the measurement interference signal is square-detected to obtain a beat signal due to interference.

  When the measured optical fiber 43 is a normal single mode fiber, the polarization of the light changes during propagation, so the polarization of the reflected light varies depending on the reflection position on the measured optical fiber 43. In this case, as shown in FIG. 4, a polarization diversity method is used in which the output light from the measurement interferometer 4 is separated into two polarized waves orthogonal to each other by the polarization beam splitter 47 and each is received.

  At this time, it is necessary to prevent the reference light from being orthogonal to either of the two polarization directions of the polarizing beam splitter 47, and it is desirable that the intensity of the reference light is separated approximately 1: 1 by the polarizing beam splitter 47. For this reason, at least the path of the reference light is configured by a polarization maintaining fiber, or the polarization controller is inserted into the path of the reference light to adjust the polarization of the reference light.

  When the optical frequency of the sweep light source 1 changes nonlinearly with time, the beat frequency due to interference between the reflected light from the predetermined position of the optical fiber 43 to be measured of the measurement interferometer 4 and the reference light changes with time. . The first linearization means 51 uses the auxiliary interference signal from the auxiliary interferometer 3 to determine the beat frequency due to the interference between the reflected light from the predetermined position of the optical fiber 43 to be measured of the measurement interferometer 4 and the reference light. The measurement interference signal from the measurement interferometer 4 is sampled so as to be constant.

  Specifically, the first linearization means 51 samples the beat signal of the measurement interference signal from the measurement interferometer 4 at a frequency proportional to the beat frequency of the auxiliary interference signal from the auxiliary interferometer 3. That is, the beat signal of the measurement interference signal from the measurement interferometer 4 is sampled at a time when the phase of the sine wave of the auxiliary interference signal from the auxiliary interferometer 3 is a constant interval.

  The second linearization means 52 has the same configuration as the first linearization means 51, and the measurement signal from the measurement interferometer 4 at a time when the phase of the sine wave of the auxiliary interference signal from the auxiliary interferometer 3 becomes a constant interval. The beat signal of the measurement interference signal is sampled. Thereby, the first linearization means 51 and the second linearization means 52 respectively output beat signals obtained by correcting the nonlinearity of the wavelength sweep of the sweep light source 1 with respect to the measurement interference signal using the auxiliary interference signal. It is possible.

  Here, the first linearization means 51 and the second linearization means 52 provide two different relative time differences between the auxiliary interference signal from the auxiliary interferometer 3 and the measurement interference signal from the measurement interferometer 4. Have a delay time of each. Specifically, the first linearization unit 51 and the second linearization unit 52 are configured to delay at least one of the auxiliary interference signal from the auxiliary interferometer 3 and the measurement interference signal from the measurement interferometer 4. The delay time is different between the first linearization means 51 and the second linearization means 52.

  The first weight filter 61 is a time domain filter having a predetermined frequency characteristic, and outputs a result obtained by adding the first weight characteristic to the output signal from the first linearization means 51. The second weight filter 62 is a time domain filter having a frequency characteristic different from that of the first weight filter 61, and has a second weight characteristic for the output signal from the second linearization means 52. Output the added result. Here, the amplitudes of the frequency characteristics of the first weight filter 61 and the second weight filter 62 correspond to weights corresponding to positions on the optical fiber 43 to be measured.

  The addition / Fourier transform means 7 adds the output signal to which the weight characteristic from the first weight filter 61 is added and the output signal to which the weight characteristic from the second weight filter 62 is added, and is subjected to Fourier transform. Outputs frequency domain signals.

  The optical frequency domain reflection measuring apparatus according to the present invention has a function of correcting a delay time difference between the output signal from the first linearization means 51 and the output signal from the second linearization means 52. There is also. In this case, in the time domain where the nonlinearity of the sweep of the optical frequency of the swept light source 1 is large, the error term after the linearization by the first linearization means 51 and the second linearization means 52 generated by this nonlinearity. It is desirable to correct the above-described delay time difference so that the error terms after linearization by the above are out of phase and cancel each other. In the following, linearization by the first linearization means 51 is also simply referred to as “first linearization”, and linearization by the second linearization means 52 is also simply referred to as “second linearization”.

  This delay time adjustment is performed by interpolating a delay of integer samples or between samples in at least one of the output signal from the first linearization means 51 and the output signal from the second linearization means 52. This can be realized by adding a delay less than the obtained sampling interval. Alternatively, delay time adjustment may be included in at least one of the first weight filter 61 and the second weight filter 62. Specifically, a delay time difference including a delay less than the sampling interval can be realized by adding a phase gradient to the frequency characteristics of the time domain filters constituting the weight filters 61 and 62.

(First embodiment)
Hereinafter, the configuration of the optical frequency domain reflection measurement apparatus according to the first embodiment of the present invention will be described. FIG. 5 shows the configuration of this embodiment in detail. The configuration and operation of the sweep light source 1, the optical branching unit 2, the auxiliary interferometer 3, and the measurement interferometer 4 are the same as the basic configuration of FIG.

  Furthermore, the optical frequency domain reflection measurement apparatus of this embodiment includes the light receivers 11a and 11b, the A / D converters 12a and 12b, the instantaneous phase calculation unit 17, the timing calculation unit 18, and the first delay addition unit. 21, second delay adding unit 22, first resampling unit 23, second resampling unit 24, first weight filter 61, second weight filter 62, and adder 27, The Fourier transform unit 60 and the control unit 70 are provided.

  Here, the light receivers 11a and 11b, the A / D converters 12a and 12b, the instantaneous phase calculation unit 17, the timing calculation unit 18, the first delay addition unit 21, and the first resampling unit 23 are the first The linearization means 51 is comprised. On the other hand, the light receivers 11a and 11b, the A / D converters 12a and 12b, the instantaneous phase calculation unit 17, the timing calculation unit 18, the second delay addition unit 22, and the second resampling unit 24 are the second linear. Forming means 52 is configured. The light receivers 11 a and 11 b, the A / D converters 12 a and 12 b, the instantaneous phase calculation unit 17, and the timing calculation unit 18 are shared by the first linearization unit 51 and the second linearization unit 52. The first weight filter 61, the second weight filter 62, the adder 27, and the Fourier transform unit 60 constitute the adder / Fourier transform unit 7.

  The output light from the auxiliary interferometer 3 is converted into an electrical signal by the light receiver 11a. The light receiver 11 a outputs a current or voltage proportional to the light intensity, and outputs a beat signal due to interference between the two lights combined by the auxiliary interferometer 3. Since the auxiliary interferometer 3 combines two lights having different delay times, a sine wave signal having a frequency proportional to the optical frequency sweep rate of the swept light source 1 is obtained. The electric signal output from the light receiver 11a is converted into a digital signal at a constant sampling frequency by being input to the A / D converter 12a.

  The instantaneous phase calculation unit 17 calculates the instantaneous phase of the sine wave beat signal from the output of the A / D converter 12a. The instantaneous phase calculation unit 17 includes, for example, a complex coefficient FIR filter that passes at least a positive frequency region corresponding to a sine wave beat signal and blocks a negative frequency region corresponding to a sine wave beat signal, and a complex coefficient And a processing unit using an arctangent function for calculating the phase of the complex number output from the FIR filter.

  The timing calculation unit 18 outputs a timing at which the instantaneous phase calculated by the instantaneous phase calculation unit 17 becomes a constant interval as a sampling timing. Here, it is necessary to detect the timing at which the instantaneous phase becomes a constant interval in consideration of the fact that the instantaneous phase is folded back to a value from −π to π, for example. Alternatively, the restoration phase may be detected at a fixed interval after the instantaneous phase is restored.

  The first delay adding unit 21 adds a first delay time to the timing output from the timing calculating unit 18 and outputs the result as the first sampling timing. Similarly, the second delay addition unit 22 adds the second delay time to the timing output from the timing calculation unit 18 and outputs the result as the second sampling timing.

  On the other hand, the output light from the measurement interferometer 4 is converted into an electric signal by the light receiver 11b. The light receiver 11b outputs a current or voltage proportional to the light intensity, and outputs a beat signal due to interference between reflected light from the optical fiber 43 to be measured and reference light.

  The electrical signal output from the light receiver 11b is A / D converted at a constant sampling frequency to be converted into a digital signal, and input to the first resampling unit 23 and the second resampling unit 24.

  The first resampling unit 23 outputs an input signal at a time corresponding to the first sampling timing as a first digital signal. The second resampling unit 24 outputs an input signal at a time corresponding to the second sampling timing as a second digital signal.

  Since the time corresponding to each sampling timing does not necessarily coincide with the sampling time of the A / D converter 12b, each resampling unit 23, 24 interpolates and outputs the A / D converted digital signal. Specifically, each of the resampling units 23 and 24 uses a finite impulse response (FIR) digital filter for a finite number of A / D converted digital signals near the time corresponding to each sampling timing. Each interpolated digital signal at the time corresponding to the sampling timing is calculated.

  The first digital signal is input to the first weight filter 61, and the second digital signal is input to the second weight filter 62. The adder 27 adds the outputs of the weight filters 61 and 62. The Fourier transform unit 60 performs a Fourier transform on the output from the adder 27 and outputs the result.

  In the case of the polarization diversity configuration shown in FIG. 4, for the two outputs of the polarization beam splitter 47, a light receiver, an A / D converter, a first resampling unit, a second resampling unit, a first The weight filter, the second weight filter, the adding unit, and the Fourier transform unit are arranged.

  The control unit 70 is constituted by, for example, a microcomputer or a personal computer including a CPU, ROM, RAM, HDD, and the like, and controls operations of the above-described units constituting the optical frequency domain reflection measurement apparatus. Furthermore, the control unit 70 determines the first delay time, the second delay time, the filter coefficient of the first weight filter 61, and the filter coefficient of the second weight filter 62 according to the length of the optical fiber 43 to be measured. It is also possible to include an operation for setting. Thereby, the optical frequency domain reflection measuring apparatus which can measure the optical fibers 43 to be measured having various lengths can be configured. Instantaneous phase calculation unit 17, timing calculation unit 18, first delay addition unit 21, second delay addition unit 22, first resampling unit 23, second resampling unit 24, first weight filter 61, The second weight filter 62, the addition unit 27, and the Fourier transform unit 60 can be configured by digital circuits such as FPGA and ASIC, and can also be configured by software by executing a predetermined program. A configuration in which hardware processing by a digital circuit and software processing by a predetermined program are appropriately combined is also possible.

<Delay time setting>
The optical path lengths of the two optical fibers 32 and 33 of the auxiliary interferometer 3 (reciprocal optical path length in the case of the reflection type) are respectively L _a and L _b , and the optical path length of the reference light of the measurement interferometer 4 (in the case of the reflection type). L _r is the round-trip optical path length), and z = z ₀ is the position on the measured optical fiber 43 where the optical path length in the optical fiber on the measured optical fiber 43 side of the measurement interferometer 4 is equal to the optical path length L _r of the reference light. = 0. The other delay time on the auxiliary interferometer 3 side is assumed to be equal to the delay time on the measurement interferometer 4 side.

The optical path length of light reflected at the position z on the measurement of the measurement interferometer 4 optical fiber 43 becomes 2z + L _r. Therefore, the delay time t _ab of the beat signal of the auxiliary interferometer 3, the delay time t _1r of the beat signal of the light reflected at the position z ₁ on the optical fiber 43 to be measured and the reference light in the measurement interferometer 4, the measurement interferometer 4, the delay time t _2r of the beat signal between the light reflected at the position z ₂ on the measured optical fiber 43 and the reference light is expressed by the following equations (4) to (6).

Here, n is the refractive index of the optical fiber, and c is the speed of light. The first delay time δt ₁ and the second delay time added to the auxiliary interferometer 3 side so that the frequency error of the beat signal of the measurement interference signal due to nonlinear sweep becomes zero at the positions z ₁ and z ₂ on the optical fiber 43 to be measured. The delay time δt ₂ of ₂ is expressed by the following equations (7) to (10). When a delay time is added to the measurement interferometer 4 side, δt ₁ and δt ₂ have opposite signs.

<Setting the weight>
The first linearized error term ψ ₁ and the second linearized error term ψ ₂ generated by the non-linearity of the sweep of the sweep light source 1 are expressed by the following equations (11) and (12). The

Here, z is a variable indicating the position (distance) on the optical fiber 43 to be measured. The delay time of the first linearization by the first delay adding unit 21 is such that the error in the frequency of the beat signal of the measurement interference signal due to the non-linear sweep of the sweep light source 1 at the position z ₁ on the measured optical fiber 43 is minimal ( Or zero). The delay time of the second linearization by the second delay adding unit 22 is such that the error in the frequency of the beat signal of the measurement interference signal due to the non-linear sweep of the sweep light source 1 at the position z ₂ on the optical fiber 43 to be measured. It shall be set to be minimal (or zero). The positions z ₁ and z ₂ set in this way are hereinafter also referred to as minimum positions z ₁ and z ₂ . That is, the minimum positions z ₁ and z ₂ correspond to the delay times of the first and second linearization means 51 and 52, respectively, and two output signals from the first and second linearization means 51 and 52 are obtained. Are positions on the measured optical fiber 43 at which errors due to the nonlinearity of the wavelength sweep of the sweep light source 1 are minimized.

Here, in order to cancel out the error terms ψ ₁ and ψ ₂ , r ₁ (z) is obtained as the _first linearized signal by the first weight filter 61, and the second linearity is obtained by the second weight filter 62. Consider a configuration in which a weight characteristic of r ₂ (z) is added to a signal after conversion and added. When the weights r ₁ (z) and r ₂ (z) are set so that the following formulas (13) and (14) are established, the signal added with the weight maintains the amplitude of the beat signal of the measurement interference signal. However, the error terms ψ ₁ and ψ ₂ can be canceled with each other.

The weights r ₁ (z) and r ₂ (z) obtained from the above equations (13) and (14) are expressed by the following equations (15) and (16).

The weights r ₁ (z) and r ₂ (z) in Equation (15) and Equation (16) are linear functions with z as a variable, as shown in FIG. Since the signs of r ₁ (z) and r ₂ (z) are different in the region of z <z ₁ and z> z ₂ , the addition processing for the outputs of the weight filters 61 and 62 in the adder 27 is substantially subtracted. There arises a problem that noise increases due to processing (S / N ratio deteriorates). Furthermore, since r ₁ (z) or r ₂ (z) is larger than 1, a problem arises in that the dynamic range required for calculation increases. If the interval between z ₁ and z ₂ is increased, the maximum value of the weight is reduced and the region of z <z ₁ and z> z ₂ is also reduced, so this problem is alleviated, but in the vicinity of the middle of z ₁ and z ₂ . Higher order nonlinear errors may remain.

Also, weights other than those in the equations (15) and (16) can be set in the region of z <z ₁ and z> z ₂ . For example, as shown in FIG. 6B, if r ₁ (z) and r ₂ (z) are set to constant values of weight 1 or weight 0 in the region of z <z ₁ and z> z ₂ , Although the effect of reducing non-linear errors due to weighted addition by the filters 61 and 62 and the adding unit 27 cannot be obtained, an increase in noise due to the subtraction process and an increase in dynamic range necessary for calculation do not occur.

Therefore, appropriate z ₁ and z ₂ can be determined in consideration of these effects. Although z ₁ and z ₂ may be set outside the measurement range of the optical fiber 43 to be measured as shown in FIG. 7A, high-order nonlinear errors may remain. Therefore, it is desirable to set z ₁ and z ₂ at both ends of the measurement range of the optical fiber 43 to be measured as shown in FIG. Further, as shown in FIG. 7C, z ₁ and z ₂ may be set inside both ends of the measurement range of the measured optical fiber 43 to reduce the maximum value of higher-order nonlinear errors and the like. .

<Actual characteristics of weight filter and improvement method>
Hereinafter, a configuration in which the weight filters 61 and 62 are realized by time domain filters will be considered. As the time domain filter, a finite impulse response (FIR) digital filter (hereinafter also simply referred to as “FIR filter”) shown in FIG. 8 is widely used. Here, _{x i} is the input data to the FIR filter in the drawing, reference numeral 63 is a delay circuit for delaying one sample of the input _{data, a 1, a 2, a} 3, a 4, ···, a n is A filter coefficient, reference numeral 64 indicates a multiplier that multiplies each filter coefficient by input data, and reference numeral 65 indicates an adder that adds outputs from the multipliers 64. The rightmost adder 65 in the figure outputs a value y _i obtained by adding the outputs from all the multipliers 64.

  Since the FIR filter has a finite time response, generally an error occurs in the frequency characteristics. If there is an error in the frequency characteristics of the weight filters 61 and 62, the error term after the first linearization and the error term after the second linearization cannot be completely canceled as described above, and weighted addition is performed. An error remains in the later signal. If the number of taps of the FIR filter is increased, more accurate frequency characteristics can be realized. However, since there is a problem that the amount of calculation and the hardware scale increase, it is required to obtain desired characteristics with as few taps as possible. .

  The coefficient of the FIR filter can be calculated by inverse Fourier transforming the frequency characteristic of the filter. Here, by adding a phase gradient of φ (f) = − 2πτf (f is a frequency) to the frequency characteristics of the zero phase, a constant filter delay time τ that does not depend on the frequency f on the time axis is attached to the FIR filter. Can do.

Here, since the actual sampling frequency for the beat signal of the measurement interference signal of each resampling unit 23, 24 varies depending on the beat frequency of the auxiliary interference signal, it is not usually constant. The sampling frequency is regarded as a constant value f _s and the first and second digital signals are filtered. Note that by providing a buffer memory (not shown) between each resampling unit 23, 24 and each weight filter 61, 62, the actual sampling frequency of the input data to each weight filter 61, 62 can be made constant. . Filter delay time tau of the FIR filter for integer multiple of the sampling period 1 / _{f s} of the input data (k times) becomes τ = k / _{f s.} That, φ (f) = - 2πkf / f s becomes, φ _(f s / 2) and phi integral multiple of _(-f s / 2) difference of 2 [pi (k times), i.e. the same phase.

The discrete Fourier transform inevitably causes a return from the frequency f _s / 2 to the frequency −f _s / 2. However, if φ (f _s / 2) and φ (−f _s / 2) are in phase, The continuity of the phase is maintained at this turning point. This relationship is not satisfied when the filter delay time τ is not an integer multiple of the sampling period, the phase is different in phase and frequency -f s _{/ 2} at the frequency f _{s /} 2, the frequency -f from the frequency f _{s /} 2 _A phase discontinuity occurs at the turning point to _s / 2. In particular, when the filter delay time τ is an integral multiple of the sampling period + ½ times (k + ½), the difference between φ (f _s / 2) and φ (−f _s / 2) is an integral multiple of 2π + 1 / 2 times (k + 1/2 times), that is, an opposite phase. Note that phase continuity is maintained regardless of the filter delay time τ at the connection point in the positive and negative frequency regions corresponding to the zero frequency of the two output signals from the first and second linearization means 51 and 52. It is.

As for the amplitude characteristic corresponding to FIG. 6B, the frequency characteristic of the FIR filter when the filter delay time τ is an integral multiple of the sampling period is as shown in FIG. Hereinafter, the horizontal axis represents the frequency normalized by the sampling frequency f _s , which is a value proportional to the position z on the measured optical fiber 43. Here, z ₁ = 0.10 and z ₂ = 0.25, and the number of taps of the FIR filter is 32. The solid lines in the figure indicate the actual amplitude characteristics Ar ₁ and Ar ₂ of the FIR filter, and the broken lines in the figure indicate the ideal weight characteristics r ₁ and r ₂ shown in FIG. 6B.

The weight filters 61 and 62 preferably have a weight characteristic in which the amplitude changes linearly (linear function) at a position between the _two minimum positions z ₁ and z _2. However, in the frequency characteristic of an actual FIR filter, A perfect linear amplitude characteristic cannot be realized, and some undulation occurs. z ₁ to z amplitude error [Delta] r ₁ of the amplitude characteristic between the _two, (the difference between the ideal weight characteristics of the amplitude characteristics of the FIR filter _Ar 1, Ar ₂ and the straight line _r 1, _{r 2)} [Delta] r ₂ FIG. 9 (b )become that way. Ideal weight characteristics _r 1, _{r 2} as shown in FIG. 6 _(b), since the differential value of the amplitude at z 1, _{z 2} are discontinuous, particularly _z 1, _{z 2} near the amplitude error Δr ₁ and Δr ₂ are increased.

In the weighting characteristics r ₁ and r ₂ in FIG. 6B, the amplitude is at the turning point of the zero frequency and the normalized frequency 0.5 (a frequency that is ½ of the sampling frequency f _s of the input data of the FIR filter). Since it is constant, the continuity of the differential value of the amplitude is maintained. Further, since the filter delay time τ is an integral multiple of the sampling period, the continuity of the phase at the turning point of the normalized frequency 0.5 is maintained. From these facts, the amplitude errors Δr ₁ and Δr ₂ near the zero frequency and the normalized frequency 0.5 are small.

With respect to the amplitude characteristic corresponding to FIG. 6B, the frequency characteristic of the FIR filter when the filter delay time τ is an integral multiple of the sampling period + ½ times is as shown in FIG. In this filter delay time τ, phase continuity is not maintained at the turning point of the normalized frequency 0.5, so that Ar ₁ , Ar ₂ and ideal weight characteristics r ₁ , r ₂ near the normalized frequency 0.5. the difference between the increases, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 10 (b) with it, [Delta] r ₂ is also increased.

FIG. 11 (a) shows the frequency characteristics of the FIR filter when the filter delay time τ is an integral multiple of the sampling period with respect to the amplitude characteristics in which the entire band corresponding to FIG. 6 (a) is a straight line. In the weighting characteristics r ₁ and r ₂ of FIG. 6A, the differential values of the amplitudes at z ₁ and z ₂ are continuous, but the differential values of the amplitudes at the turning points of the zero frequency and the standardized frequency 0.5. Since it is non-zero, the continuity of the differential value of the amplitude cannot be maintained. For this reason, undulations are seen in the amplitude characteristics near zero frequency and normalized frequency 0.5 in Ar ₁ and Ar ₂ . Along with this, the amplitude error [Delta] r ₁ by undulation even between _z 1 to z ₂ as shown in FIG. 11 (b), Δr ₂ is seen.

FIG. 12A shows the frequency characteristics of the FIR filter when the filter delay time τ is an integral multiple of the sampling period + 1/2 times with respect to the amplitude characteristics in which the entire band corresponding to FIG. 6A is a straight line. In this filter delay time τ, phase continuity is not maintained at the turning point of the normalized frequency 0.5, so that Ar ₁ , Ar ₂ and ideal weight characteristics r ₁ , r ₂ near the normalized frequency 0.5. the difference between the increases, 12 amplitude error [Delta] r ₁ between _z 1 to z ₂ as (b) with it, [Delta] r ₂ is also increased.

Therefore, in the amplitude characteristic constituted by a straight line as shown in FIGS. 6A and 6B, there is a discontinuity of the differential value of the amplitude somewhere, so the filter delay time τ is set to an integral multiple of the sampling period. Even in this case, there is a problem that the amplitude errors Δr ₁ and Δr ₂ of the amplitude characteristic of the FIR filter become large.

In contrast, weight filter 61 and 62 of the present embodiment, amplitude error [Delta] r ₁ of the filter characteristic between _z 1 to z ₂ in FIR filter tap number is limited, in order to reduce [Delta] r _{2, z 1} the amplitude characteristic of ~z except between _two sections is obtained by changing from a straight line (linear function). In the frequency characteristics of the weight filters 61 and 62, when the amplitude value and the differential value of the amplitude with respect to the position z on the measured optical fiber 43 become discontinuous, the amplitude errors Δr ₁ and Δr ₂ become large. For this reason, in this embodiment, z <z ₁ and z> z so that the amplitude value of the frequency characteristic and the differential value of the amplitude of the weight filters 61 and 62 are continuous in the entire band including z ₁ and z _{2. 2} weight characteristics are set.

That is, the weight characteristics of the first and second weight filters 61 and 62 are such that z ₁ ≦ z ≦ z ₂ between the _two minimum positions z ₁ and z ₂ with the position z on the measured optical fiber 43 as a variable. , Defined in terms of weight characteristic A that changes linearly with respect to position z, and z ₀ ≦ z <z ₁ and z ₂ <z ≦ z _n outside the _two minimal positions z ₁ and z _2. Weighted characteristic B.

The amplitude value of the weighting characteristic B and the differential value of the amplitude with respect to the position z on the measured optical fiber 43 are continuous with the amplitude value and the differential value of the amplitude of the weighting characteristic A at the _two minimum positions z ₁ and z ₂ .

Further, in order to avoid differential discontinuity of the amplitude with the aliasing component of the frequency characteristic, the differential value of the amplitude of the weight characteristic B is a position z on the measured optical fiber 43 corresponding to the zero frequency and the normalized frequency 0.5. it is zero at _{0, z n.} The conditions at z = 0, z ₁ , z ₂ and z _n are expressed by the following formulas (17) to (20).

Here, z _n is a z value corresponding to a normalized frequency of 0.5. The conditions of the equations (17) to (20) are realized using a nonlinear function because 0 ≦ z <z ₁ and z ₂ <z ≦ z _n cannot be satisfied by the linear amplitude characteristic. For example, using the trigonometric function with the position z on the measured optical fiber 43 as a variable, as in the following formulas (21) and (22), 0 ≦ z <z ₁ and z ₂ <z ≦ z _n The weights r ₁ (z) and r ₂ (z) as the weight characteristic B can be realized. _{Incidentally, r} 1 (z) and _r 2 (z) is the _{z 1} ≦ _z ≦ _{z 2} is realized by a linear function.

Further, as shown in the following formulas (23) and (24), 0 ≦ z <z ₁ and z ₂ <z ≦ z using a quadratic function having the position z on the measured optical fiber 43 as a variable. It is also possible to realize the weights r ₁ (z) and r ₂ (z) at _n .

When the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using trigonometric functions are shown in FIG. 13A and the weight filter of the present embodiment using a quadratic function. The frequency characteristics of 61 and 62 are as shown in FIG. 14A, and the undulation is reduced as compared with the amplitude characteristics of FIGS. In the scales of FIG. 13A and FIG. 14A, Ar ₁ , Ar ₂ and r ₁ , r ₂ are almost overlapped and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 13 (b) and FIG. 14 (b), Δr ₂ is also greatly reduced.

This is because, as described above, the phase continuity is maintained at the turning point of the normalized frequency 0.5 (1/2 of the sampling frequency f _s ), and the linear amplitude is between z ₁ and z _2. By using a trigonometric function or a quadratic function in an interval other than between z ₁ and z ₂ while maintaining the characteristics, the discontinuity of the differential value of the amplitude including the turning point at the zero frequency and the normalized frequency 0.5 can be reduced. This is because it was excluded.

On the other hand, when the filter delay time τ is not an integral multiple of the sampling period, for example, when the filter delay time τ is an integral multiple of the sampling period +1/2 times, the frequency characteristics of the weight filters 61 and 62 using the trigonometric function are shown in FIG. a) The frequency characteristics of the weight filters 61 and 62 of the present embodiment using a quadratic function are as shown in FIG. 16A, and ideal weight characteristics r ₁ and r in the vicinity of the normalized frequency 0.5. The difference from ₂ is large. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 15 (b) and FIG. 16 (b), Δr ₂ also becomes large.

This is because, as described above, when the filter delay time τ is an integer times + ½ of the sampling period, the turning point of the frequency characteristic in the normalized frequency 0.5 (half the sampling frequency f _s) This is because the phase becomes discontinuous.

  Therefore, this embodiment is useful when the filter delay time τ is an integral multiple of the sampling period.

  As described above, the optical frequency domain reflection measurement apparatus according to the present embodiment has the first and second linear shapes each having a delay time for giving two different relative time differences between the auxiliary interference signal and the measurement interference signal. And first and second weight filters 61 and 62 for adding weight characteristics to the two output signals from the first and second linearization means 51 and 52, respectively. By correcting the nonlinearity of the wavelength sweep, the measurement accuracy of the strain or temperature distribution can be improved over a wide range of the optical fiber 43 to be measured.

Further, in the optical frequency domain reflection measurement apparatus according to the present embodiment, the differential value of the amplitude of the weighting characteristic B with respect to the position z on the optical fiber 43 to be measured is the amplitude of the weighting characteristic A at the _two minimum positions z ₁ and z ₂ . And zero at a zero frequency and a normalized frequency of 0.5 (1/2 of the sampling frequency f _s ). Thereby, the optical frequency domain reflection measuring apparatus according to the present embodiment realizes a weight filter having small amplitude errors Δr ₁ and Δr ₂ from desired weight characteristics r ₁ and r ₂ with an FIR digital filter having a small number of taps. Therefore, the nonlinear influence of the wavelength sweep of the sweep light source 1 can be reduced with a small amount of calculation.

Further, in the optical frequency domain reflectometry apparatus according to this embodiment, first and second weighting filters 61 and 62, a FIR filter with integer multiples of the filter delay time of the reciprocal of the sampling frequency f _s. Thus, in the turning point of the frequency characteristic in the normalized frequency 0.5 (half the sampling frequency f _s), the continuity of the phase of the frequency characteristics of the first and second weighting filters 61 and 62 have maintained Therefore, the optical frequency domain reflection measurement apparatus according to the present embodiment can accurately realize a weighting filter having desired weighting characteristics r ₁ and r ₂ using an FIR filter.

(Second Embodiment)
Next, an optical frequency domain reflection measurement apparatus according to the second embodiment of the present invention will be described with reference to the drawings. Note that the description of the same configuration and operation as in the first embodiment will be omitted as appropriate.

In the present embodiment, to zero the amplitude of the differential value of the frequency characteristics of the weighting filters 61 and 62 in the normalized frequency 0.5 (half the sampling frequency f _s) as in the first embodiment without to zero the amplitude of the frequency characteristic of the weighting filter 61 at normalized frequency 0.5 (half the sampling frequency _{f s).}

Also in the present embodiment, the amplitude value of the weighting characteristic B and the differential value of the amplitude with respect to the position z on the optical fiber 43 to be measured are the amplitude value and the differential value of the amplitude of the weighting characteristic A at the _two minimum positions z ₁ and z ₂ . And continuous. In addition, the amplitude values of the weight characteristics of the weight filters 61 and 62 are continuous in the entire band including z ₁ and z ₂ .

Further, the differential value of the amplitude of the weight characteristic B is zero at the position z ₀ on the measured optical fiber 43 corresponding to the zero frequency. Moreover, as already mentioned, the amplitude of the weight characteristic B is zero at the position z _n on the measured optical fiber 43 corresponding to the normalized frequency 0.5 (half the sampling frequency f _s). The conditions at z = 0, z ₁ , z ₂ and z _n are expressed by the following formulas (25) to (28).

The conditions of the equations (25) to (28) are realized using a nonlinear function because 0 ≦ z <z ₁ and z ₂ <z ≦ z _n cannot be satisfied by a linear amplitude characteristic. For example, as shown in the following formulas (29) and (30), z ₂ <z ≦ z _n is _expressed as two regions z ₂ <z using a trigonometric function with the position z on the measured optical fiber 43 as a variable. The weights r ₁ (z) and r ₂ (z) can be realized by using the weight characteristic B divided into ≦ z _c2 and z _c2 <z ≦ z _n . Here, z _c2 is any value from z ₂ to z _n , and z ₂ <z ≦ z _n on the measured optical fiber 43 is divided into two regions by z _c2 .

For example, if the periods of two trigonometric functions that are continuous before and after z _c2 in Equation (29) and Equation (30) are set equal, the value of z _c2 is as shown in Equation (31) below. In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using a trigonometric function are as shown in FIG. Is seen. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is larger as shown in FIG. 17 (b).

When the filter delay time τ is an integral multiple of the sampling period +1/2 times, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. Compared with the amplitude characteristic, the waviness of the amplitude characteristic is reduced. In the scale of FIG. 19 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 19 (b).

Further, as in the following equations (32) and (33), 0 ≦ z <z ₁ and z ₂ <z ≦ z using a quadratic function having the position z on the measured optical fiber 43 as a variable. It is also possible to realize the weights r ₁ (z) and r ₂ (z) at _n . Here, z _c2 and z _c3 are any values from z ₂ to z _n . For the weight r ₁ (z), z ₂ <z ≦ z _n on the measured optical fiber 43 is divided into two regions by z _c2 , and for the weight r ₂ (z), on the measured optical fiber 43 z ₂ <z ≦ z _n is divided into two regions by z _c3 .

For example, when the second-order differential values of two quadratic functions that are continuous before and after z _c2 and before and after z _c3 in equation (32) and equation (33) are set equal, the values of z _c2 and z _c3 are The following equations (34) and (35) are obtained. In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using a quadratic function are as shown in FIG. A swell is seen. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 18 (b), Δr ₂ also becomes large.

Further, when the filter delay time τ is an integral multiple of the sampling period + ½ times, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIG. Compared with the amplitude characteristic of 18, the swell is reduced. In the scale of FIG. 20A, Ar ₁ , Ar ₂ and r ₁ , r ₂ are almost overlapped and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 20 (b).

In a normalized frequency 0.5 in the present embodiment (half the sampling frequency _{f s),} and the zero amplitude of the frequency characteristic of the weighting filter 61. As described above, when the filter delay time τ is set to (integral multiple of sampling period +1/2) times, the anti-phase, that is, the sign is inverted before and after the turning point of the normalized frequency 0.5, and the normalized frequency 0.5 The differential value of the amplitude becomes continuous at. Thereby, discontinuity of the differential value of the amplitude including the turning point is eliminated, and the amplitude errors Δr ₁ and Δr ₂ are reduced.

  Therefore, this embodiment is useful when the filter delay time τ is an integral multiple of the sampling period +1/2.

  As described above, the optical frequency domain reflection measurement apparatus according to the present embodiment corrects the nonlinearity of the wavelength sweep and corrects the distortion or temperature over a wide range of the optical fiber 43 to be measured, as in the first embodiment. Distribution measurement accuracy can be improved.

Further, in the optical frequency domain reflection measurement apparatus according to the present embodiment, the differential value of the amplitude of the weighting characteristic B with respect to the position z on the optical fiber 43 to be measured is the amplitude of the weighting characteristic A at the _two minimum positions z ₁ and z ₂ . And is zero at the zero frequency. The amplitude weighting characteristic B is zero at a normalized frequency of 0.5 (half the sampling frequency _{f s).} Thereby, the optical frequency domain reflection measuring apparatus according to the present embodiment realizes a weight filter having small amplitude errors Δr ₁ and Δr ₂ from desired weight characteristics r ₁ and r ₂ with an FIR digital filter having a small number of taps. Therefore, the nonlinear influence of the wavelength sweep of the sweep light source 1 can be reduced with a small amount of calculation.

Further, in the optical frequency domain reflection measuring apparatus according to the present embodiment, the first and second weight filters 61 and 62 have FIR filters having a filter delay time that is an integral multiple of the reciprocal of the sampling frequency f _s +1/2. It is. Thus, in the turning point of the frequency characteristic in the normalized frequency 0.5 (half the sampling frequency f _s), the continuity of the phase of the frequency characteristics of the first and second weighting filters 61 and 62 have maintained Therefore, the optical frequency domain reflection measurement apparatus according to the present embodiment can accurately realize a weighting filter having desired weighting characteristics r ₁ and r ₂ using an FIR filter.

(Third embodiment)
Subsequently, an optical frequency domain reflection measurement apparatus according to a third embodiment of the present invention will be described with reference to the drawings. Note that description of the configuration and operation similar to those of the first and second embodiments will be omitted as appropriate.

In the present embodiment, in order to reduce the amplitude errors Δr ₁ and Δr ₂ even when the filter delay time τ is not an integral multiple of the sampling period or an integral multiple + ½ times, the first embodiment and the second embodiment It is a combination of the embodiments.

Also in the present embodiment, the amplitude value of the weighting characteristic B and the differential value of the amplitude with respect to the position z on the optical fiber 43 to be measured are the amplitude value and the differential value of the amplitude of the weighting characteristic A at the _two minimum positions z ₁ and z ₂ . And continuous.

Further, the differential value of the amplitude of the weight characteristic B is zero at the position z _n on the measured optical fiber 43 corresponding to the zero frequency and the normalized frequency 0.5 (1/2 of the sampling frequency f _s ). In addition, in all bands including z ₁ and z ₂ , the amplitude value of the weight characteristic and the differential value of the amplitude of the weight filters 61 and 62 are continuous.

Further, in the present embodiment, the amplitude of the weight characteristic B is zero at the position z _n on the measured optical fiber 43 corresponding to the normalized frequency 0.5 (half the sampling frequency f _s). The conditions at z = 0, z ₁ , z ₂ and z _n are expressed by the following formulas (36) to (39).

The conditions of Expressions (36) to (39) are realized by using a nonlinear function because 0 ≦ z <z ₁ and z ₂ <z ≦ z _n cannot be satisfied by a linear amplitude characteristic. For example, as shown in the following formulas (40) and (41), z ₂ <z ≦ z _{n is set} to two regions z ₂ <z using a trigonometric function with the position z on the measured optical fiber 43 as a variable. The weights r ₁ (z) and r ₂ (z) can be realized by using the weight characteristic B divided into ≦ z _c2 and z _c2 <z ≦ z _n . Here, z _c2 is any value from z ₂ to z _n , and z ₂ <z ≦ z _n on the measured optical fiber 43 is divided into two regions by z _c2 .

For example, if the periods of two trigonometric functions that are continuous before and after z _c2 are set equal in Expression (40) and Expression (41), the value of z _c2 is as shown in Expression (42) below. In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. Compared with 12 amplitude characteristics, the swell is reduced. On the scale of FIG. 21 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 21 (b).

Further, when the filter delay time τ is an integral multiple of the sampling period +1/2 times, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. ~ Waviness is reduced as compared with the amplitude characteristic of FIG. In the scale of FIG. 23 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 23 (b). Case filter delay time τ is not an integral multiple or integral multiple +1/2 times the sampling period is similarly reduces the waviness of the amplitude characteristic, the amplitude error [Delta] r 1 between z ₁ ~z _{2, Δr 2} is reduced .

Further, as shown in the following formulas (43) and (44), 0 ≦ z <z ₁ and z ₂ <z ≦ z using a quadratic function having the position z on the measured optical fiber 43 as a variable. It is also possible to realize the weights r ₁ (z) and r ₂ (z) at _n . Here, z _c2 and z _c3 are any values from z ₂ to z _n . For the weight r ₁ (z), z ₂ <z ≦ z _n on the measured optical fiber 43 is divided into three regions by z _c2 and (z _c2 + z _n ) / 2, and the weight r ₂ (z) , Z ₂ <z ≦ z _n on the measured optical fiber 43 is divided into three regions by z _c3 and (z _c3 + z _n ) / 2.

For example, when the second-order differential values of two quadratic functions that are continuous before and after z _c2 and before and after z _c3 in Equation (43) and Equation (44) are set equal, the values of z _c2 and z _c3 are The following equations (45) and (46) are obtained. In the present embodiment, when the filter delay time τ is an integer multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using a quadratic function are as shown in FIG. Compared with the amplitude characteristic of FIG. 12, the waviness is reduced. In the scale of FIG. 22A, Ar ₁ , Ar ₂ and r ₁ , r ₂ are almost overlapped and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 22 (b).

Further, when the filter delay time τ is an integral multiple of the sampling period +1/2 times, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIG. Compared with the amplitude characteristics shown in FIGS. 9 to 12, the waviness is reduced. On the scale of FIG. 24A, Ar ₁ , Ar ₂ and r ₁ , r ₂ are almost overlapped and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 24 (b).

In this embodiment the normalized frequency 0.5 (half the sampling frequency f _s), have zero amplitude value and the differential value of the amplitude of the frequency characteristic of the weighting filter 61. Therefore, regardless of the phase before and after the normalized frequency 0.5, that is, in an arbitrary case where the filter delay time τ is not limited to an integral multiple of the sampling period or an integral multiple +1/2 times, the normalized frequency 0. At the turning point of 5, the differential value of the amplitude is continuous. Thereby, discontinuity of the differential value of the amplitude including the turning point is eliminated, and the amplitude errors Δr ₁ and Δr ₂ are reduced.

  Therefore, this embodiment is useful even when the filter delay time τ is arbitrary, and is a highly versatile form.

  As described above, the optical frequency domain reflectometry apparatus according to the present embodiment corrects the nonlinearity of the wavelength sweep and extends over a wide range of the optical fiber 43 to be measured, as in the first and second embodiments. The measurement accuracy of strain or temperature distribution can be improved.

Further, in the optical frequency domain reflection measurement apparatus according to the present embodiment, the differential value of the amplitude of the weighting characteristic B with respect to the position z on the optical fiber 43 to be measured is the amplitude of the weighting characteristic A at the _two minimum positions z ₁ and z ₂ . And zero at a zero frequency and a normalized frequency of 0.5 (1/2 of the sampling frequency f _s ). Further, the amplitude of the weight characteristic B is zero at the normalized frequency of 0.5.

Thereby, the optical frequency domain reflection measuring apparatus according to the present embodiment realizes a weight filter having small amplitude errors Δr ₁ and Δr ₂ from desired weight characteristics r ₁ and r ₂ with an FIR digital filter having a small number of taps. Therefore, the nonlinear influence of the wavelength sweep of the sweep light source 1 can be reduced with a small amount of calculation. Furthermore, the amplitude value and the differential value of the amplitude are continuous at the turning point of the frequency characteristic at the normalized frequency 0.5 regardless of the phase of the frequency characteristic of the FIR filter before and after the normalized frequency 0.5. The optical frequency domain reflection measurement apparatus according to the present embodiment can accurately realize a weighting filter having desired weighting characteristics r ₁ and r ₂ using an FIR filter.

(Fourth embodiment)
Subsequently, an optical frequency domain reflection measurement apparatus according to a fourth embodiment of the present invention will be described with reference to the drawings. In addition, about the structure and operation | movement similar to 1st-3rd embodiment, description is abbreviate | omitted suitably.

In the present embodiment, in order to make the weights r ₁ (z) and r ₂ (z) close to those in FIG. 6B in the low frequency region close to z = z ₀ = 0, the first or second or In addition to the configuration of the third embodiment, the amplitude value at zero frequency is set to 1 and 0.

Also in the present embodiment, the amplitude value of the weighting characteristic B and the differential value of the amplitude with respect to the position z on the optical fiber 43 to be measured are the amplitude value and the differential value of the amplitude of the weighting characteristic A at the _two minimum positions z ₁ and z ₂ . And continuous. In addition, in all bands including z ₁ and z ₂ , the amplitude value of the weight characteristic and the differential value of the amplitude of the weight filters 61 and 62 are continuous.

The amplitude of the weight characteristic B of the first weight filter 61 is 1 and the weight characteristic B of the second weight filter 62 at the position z ₀ on the measured optical fiber 43 corresponding to the zero frequency of the measurement interference signal. Has an amplitude of zero.

For example, 0 ≦ z <(amplitude value at the zero frequency of 1 and 0) the first or second or third embodiment with different filter characteristics in z ₁ has a first or in z ₁ ≦ z ≦ z _n It has the same filter characteristics as the second or third embodiment. It is shown here for the same filter characteristic as the third embodiment in z ₁ ≦ z ≦ z _n, but may be the same filter characteristic as the first or second embodiment in z ₁ ≦ z ≦ z _n. The conditions at z = 0, z ₁ , z ₂ and z _n are expressed by the following formulas (47) to (50).

The conditions of the equations (47) to (50) are realized by using a non-linear function because 0 ≦ z <z ₁ and z ₂ <z ≦ z _n cannot be satisfied by the linear amplitude characteristic. For example, as shown in the following formulas (51) and (52), 0 ≦ z <z ₁ and z ₂ <z ≦ z _n are set using a trigonometric function with the position z on the measured optical fiber 43 as a variable. Weights r ₁ (z) and r using weight characteristics B divided into four regions 0 ≦ z <z _c1 and z _c1 ≦ z <z ₁ and z ₂ <z ≦ z _c2 and z _c2 <z ≦ z _{n 2} (z) can be realized. Here, z _c1 is any value from 0 to z ₁ and z _c2 is any value from z ₂ to z _n . That is, 0 ≦ z <z ₁ on the measured optical fiber 43 is divided into two regions by z _c1 , and z ₂ <z ≦ z _n is divided into two regions by z _c2 .

For example, in the expressions (51) and (52), if the periods of two trigonometric functions continuous before and after z _c1 and before and after z _c2 are set equal, the values of z _c1 and z _c2 are expressed by the following expression (53 ) And formula (54). In this embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of this embodiment using the trigonometric function are as shown in FIG. Compared with 12 amplitude characteristics, the swell is reduced. On the scale of FIG. 25 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 25 (b), Δr ₂ is reduced.

In addition, when the filter delay time τ is an integral multiple of the sampling period +1/2 times, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. ~ Waviness is reduced as compared with the amplitude characteristic of FIG. In the scale of FIG. 27A, Ar ₁ , Ar ₂ and r ₁ , r ₂ are almost overlapped and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is reduced as shown in FIG. 27 (b).

Further, as in the following formulas (55) and (56), 0 ≦ z <z ₁ and z ₂ <z ≦ z using a quadratic function having the position z on the measured optical fiber 43 as a variable. It is also possible to realize the weights r ₁ (z) and r ₂ (z) at _n . Here, z _c1 is any value from 0 to z ₁ , and z _c2 and z _c3 are any values from z ₂ to z _n .

Regarding the weight r ₁ (z), 0 ≦ z <z ₁ on the optical fiber 43 to be measured is divided into three regions by z _c1 / 2 and z _c1 , and z ₂ <z ≦ z _n is z _c2 And (z _c2 + z _n ) / 2.

For the weight r ₂ (z), 0 ≦ z <z ₁ on the measured optical fiber 43 is divided into three regions by z _c1 / 2 and z _c1 , and z ₂ <z ≦ z _n is It is divided into three regions by z _c3 and (z _c3 + z _n ) / 2.

For example, if the second-order differential values of two quadratic functions that are continuous before and after z _c1 , before and after z _c2 , and before and after z _c3 in equation (55) and equation (56) are set equal, z _c1 to The value of z _c3 is as shown in the following formulas (57) to (59). In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using a quadratic function are as shown in FIG. Compared with the amplitude characteristic of FIG. 12, the waviness is reduced. On the scale of FIG. 26 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 26 (b), Δr ₂ is reduced accordingly.

Further, when the filter delay time τ is an integral multiple of the sampling period +1/2 times, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIG. 9 to 12 undulation of the amplitude characteristic is reduced as compared with the amplitude characteristic of FIG. On the scale of FIG. 28 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 28 (b), Δr ₂ is reduced.

In this embodiment, since the differential value of the amplitude at z = z ₁ is continuous and the amplitude at z = 0 is 1 and 0, the weights r ₁ (z) and r ₂ (z) are in the vicinity of z = z _1. 6 has characteristics close to FIG. 6A (an effect of reducing non-linear errors by weighted addition), and has characteristics close to FIG. 6B near z = 0 (suppresses increase in noise and necessary dynamic range). Effect). Regarding the influence of the filter delay time τ, the weights r ₁ (z) and r ₂ (z) have the same filter characteristics as those of the first, second, or third embodiment in z ₁ ≦ z ≦ z _n . Therefore, the same characteristics as those of the first, second, or third embodiment corresponding thereto are obtained.

Further, by changing the values of z _c1 , z _c2 , and z _c3 as shown in FIG. 29, the weights r ₁ (z) and r ₂ (z) are changed to the characteristics shown in FIGS. 25 (a) and 26 (a). And a characteristic in which the lower limit value of the weight is zero. Here, FIG. 29A shows a case where a trigonometric function is used, and FIG. 29B shows a case where a quadratic function is used. Similarly, by changing the values of z _c2 and z _c3 in the second or third embodiment, the weights r ₁ (z) and r ₂ (z) are changed to those shown in FIGS. a), the characteristics shown in FIGS. 21 (a) and 22 (a), and the characteristics in which the lower limit value of the weight is set to zero may be used.

  As described above, the optical frequency domain reflection measurement apparatus according to the present embodiment corrects the nonlinearity of the wavelength sweep and extends over a wide range of the optical fiber 43 to be measured, as in the first to third embodiments. The measurement accuracy of strain or temperature distribution can be improved.

  In addition to the configuration of the first and second weight filters 61 and 62 in any of the first to third embodiments, the amplitude of the weight characteristic B of the first weight filter 61 is 1 at zero frequency. Yes, and the amplitude of the weight characteristic B of the second weight filter 62 is zero.

Thereby, the optical frequency domain reflection measuring apparatus according to the present embodiment realizes a weight filter having small amplitude errors Δr ₁ and Δr ₂ from desired weight characteristics r ₁ and r ₂ with an FIR digital filter having a small number of taps. Therefore, the nonlinear influence of the wavelength sweep of the sweep light source 1 can be reduced with a small amount of calculation. Furthermore, the optical frequency domain reflectometry apparatus of the present embodiment reduces the nonlinearity of wavelength sweep near the minimum position z ₁ , and increases the dynamic noise necessary for weight increase filter processing and computation near the zero frequency. Increase in range can be suppressed.

(Optical frequency domain reflection measurement method)
Hereinafter, an example of the processing of the optical frequency domain reflection measurement method using the optical frequency domain reflection measurement apparatus according to the first to fourth embodiments will be described with reference to the flowchart of FIG.

  First, the sweep light source 1 outputs the wavelength-swept light as output light (step S1).

  Next, the auxiliary interferometer 3 inputs a part of the output light from the sweep light source 1 to the delay fiber 32, and outputs the light output from the delay fiber 32 and another part of the output light from the sweep light source 1. Are output as an auxiliary interference signal (step S2).

  Next, the measurement interferometer 4 inputs a part of the output light from the swept light source 1 to the measured optical fiber 43, and reflects the reflected light reflected by the measured optical fiber 43 and the output light from the swept light source 1. Another part is made to interfere and it outputs as a measurement interference signal (step S3).

  Next, the first and second linearization means 51 and 52 output, as output signals, beat signals obtained by correcting the nonlinearity of the wavelength sweep of the sweep light source 1 with respect to the measurement interference signal using the auxiliary interference signal. (First linearization step S41, second linearization step S42).

  Next, the first and second weight filters 61 and 62 output the two output signals from the first and second linearization means 51 and 52 (that is, the beat signal of the measurement interference signal whose nonlinearity is corrected). First weight characteristic and second weight characteristic are added to (step S51 for adding the first weight characteristic, step S52 for adding the second weight characteristic).

  Next, the addition / Fourier transform means 7 performs Fourier transform on the signal obtained by adding the two output signals to which the weight characteristics are added by the first and second weight filters 61 and 62, and outputs the result as a signal in the frequency domain ( Step S6).

DESCRIPTION OF SYMBOLS 1 Sweep light source 2 Optical branch part 3 Auxiliary interferometer 4 Measurement interferometer 7 Addition / Fourier transform means 11a, 11b Light receiver 12a, 12b A / D converter 17 Instantaneous phase calculation part 18 Timing calculation part 21 1st delay addition part 22 second delay adding unit 23 first resampling unit 24 second resampling unit 27 adding unit 32 delay fiber 43 optical fiber to be measured 51 first linearizing unit 52 second linearizing unit 60 Fourier transform unit 61 first weight filter 62 second weight filter

Claims (11)

A swept light source (1) for outputting wavelength-swept light as output light;
A part of the output light from the sweep light source is input to the delay fiber (32), and the light output from the delay fiber interferes with another part of the output light from the sweep light source to assist. An auxiliary interferometer (3) for outputting as an interference signal;
A part of the output light from the swept light source is input to a measured optical fiber (43), reflected light reflected by the measured optical fiber, another part of the output light from the swept light source, and A measurement interferometer (4) that outputs a measurement interference signal by interfering with
First and second linearization means (51, 52) for outputting, as output signals, signals obtained by correcting the nonlinearity of the wavelength sweep of the swept light source with respect to the measurement interference signal using the auxiliary interference signal. The first and second linearization means (51, 52) each having a delay time for providing two different relative time differences between the auxiliary interference signal and the measurement interference signal;
First and second weight filters (61, 62) for respectively adding first and second weight characteristics to the two output signals from the first and second linearization means;
An optical frequency domain reflection measuring device comprising: addition / Fourier transform means (7) for adding the two output signals to which the first and second weight characteristics are added and outputting a Fourier transformed frequency domain signal Because
The position on the measured optical fiber corresponding to the zero frequency of the output signal is z ₀ ,
Corresponding to each of the delay times of the first and second linearization means, the positions on the optical fiber to be measured at which the error of the two output signals due to the nonlinearity of the wavelength sweep of the sweep light source is minimized, respectively. and z ₁ and _{z 2,}
Z _n is a position on the optical fiber to be measured corresponding to a half of the sampling frequency of the output signal,
The first and second weight characteristics are:
A weighting characteristic A defined as z ₁ ≦ z ≦ z ₂ with the position z on the measured optical fiber as a variable, and linearly changing with respect to z;
and z ₀ ≦ z <z ₁ , and weight characteristic B defined in z ₂ <z ≦ z _n , and
The differential value of the amplitude of the weight characteristic B with respect to z is continuous with the differential value of the amplitude of the weight characteristic A with respect to z at the z ₁ and z ₂ , and is zero at the z ₀ , and The optical frequency domain reflection measuring apparatus, wherein z _n is zero.   2. The optical frequency domain reflectometry apparatus according to claim 1, wherein the first and second weight filters are FIR filters having a filter delay time that is an integral multiple of the reciprocal of the sampling frequency. The amplitude of the weight characteristics B are optical frequency domain reflectometry according to claim 1, characterized in that the zero in the z _n. A swept light source (1) for outputting wavelength-swept light as output light;
A part of the output light from the sweep light source is input to the delay fiber (32), and the light output from the delay fiber interferes with another part of the output light from the sweep light source to assist. An auxiliary interferometer (3) for outputting as an interference signal;
A part of the output light from the swept light source is input to a measured optical fiber (43), reflected light reflected by the measured optical fiber, another part of the output light from the swept light source, and A measurement interferometer (4) that outputs a measurement interference signal by interfering with
First and second linearization means (51, 52) for outputting, as output signals, signals obtained by correcting the nonlinearity of the wavelength sweep of the swept light source with respect to the measurement interference signal using the auxiliary interference signal. The first and second linearization means (51, 52) each having a delay time for providing two different relative time differences between the auxiliary interference signal and the measurement interference signal;
First and second weight filters (61, 62) for respectively adding first and second weight characteristics to the two output signals from the first and second linearization means;
An optical frequency domain reflection measuring device comprising: addition / Fourier transform means (7) for adding the two output signals to which the first and second weight characteristics are added and outputting a Fourier transformed frequency domain signal Because
The position on the measured optical fiber corresponding to the zero frequency of the output signal is z ₀ ,
Corresponding to each of the delay times of the first and second linearization means, the positions on the optical fiber to be measured at which the error of the two output signals due to the nonlinearity of the wavelength sweep of the sweep light source is minimized, respectively. and z ₁ and _{z 2,}
Z _n is a position on the optical fiber to be measured corresponding to a half of the sampling frequency of the output signal,
The first and second weight characteristics are:
A weighting characteristic A defined as z ₁ ≦ z ≦ z ₂ with the position z on the measured optical fiber as a variable, and linearly changing with respect to z;
and z ₀ ≦ z <z ₁ , and weight characteristic B defined in z ₂ <z ≦ z _n , and
The differential value of the amplitude of the weight characteristic B with respect to z is continuous with the differential value of the amplitude of the weight characteristic A with respect to z at the z ₁ and z ₂ and zero at the z ₀ ,
The optical frequency domain reflection measurement apparatus characterized in that the amplitude of the weighting characteristic B is zero at z _n .   5. The optical frequency domain reflectometry apparatus according to claim 4, wherein the first and second weighting filters are FIR filters having a filter delay time that is an integral multiple of the reciprocal of the sampling frequency +1/2. . 2. The amplitude of the weight characteristic B of the first weight filter is 1 and the amplitude of the weight characteristic B of the second weight filter is zero at the z ₀ . The optical frequency domain reflection measuring apparatus according to claim 5.   The optical frequency domain reflection measurement apparatus according to any one of claims 1 to 6, wherein the amplitude of the weight characteristic B is expressed by a trigonometric function having z as a variable. In the weight characteristic B, at least one of z ₀ ≦ z <z ₁ or z ₂ <z ≦ z _n is divided into two regions,
4. The amplitude of the weight characteristic B is expressed by a trigonometric function having the variable z in the two regions, and the period of the trigonometric function in the two regions is set to be equal. The optical frequency domain reflection measuring apparatus according to claim 6.   The optical frequency domain reflectometry apparatus according to any one of claims 1 to 6, wherein the amplitude of the weight characteristic B is expressed by a quadratic function having the variable z. In the weight characteristic B, at least one of z ₀ ≦ z <z ₁ or z ₂ <z ≦ z _n is divided into two or three regions,
The amplitude of the weight characteristic B is expressed by a quadratic function having the variable z in the two or three regions, respectively, and the second-order differential values of the quadratic functions of the two or three regions are equal. The optical frequency domain reflectometry apparatus according to any one of claims 3 to 6, characterized in that it is set. A step (S1) of outputting the wavelength-swept light from the sweep light source (1) as output light;
A part of the output light from the sweep light source is input to the delay fiber (32), and the light output from the delay fiber interferes with another part of the output light from the sweep light source to assist. Outputting as an interference signal (S2);
A part of the output light from the swept light source is input to a measured optical fiber (43), reflected light reflected by the measured optical fiber, another part of the output light from the swept light source, and And outputting as a measurement interference signal (S3),
First and second linearization steps (S41, S42) for outputting, as output signals, signals obtained by correcting the nonlinearity of wavelength sweep of the swept light source with respect to the measurement interference signal using the auxiliary interference signal. The first and second linearization steps have a delay time for providing two different relative time differences between the auxiliary interference signal and the measurement interference signal, respectively. (S41, S42),
Adding first and second weight characteristics to the two output signals from the first and second linearization steps, respectively (S51, S52);
Adding the two output signals to which weight characteristics are added in the step of adding the first and second weight characteristics, and outputting a Fourier-transformed frequency domain signal (S6). A reflection measurement method,
The position on the measured optical fiber corresponding to the zero frequency of the output signal is z ₀ ,
Corresponding to each of the delay times of the first and second linearization means, the positions on the optical fiber to be measured at which the error of the two output signals due to the nonlinearity of the wavelength sweep of the sweep light source is minimized, respectively. and z ₁ and _{z 2,}
Z _n is a position on the optical fiber to be measured corresponding to a half of the sampling frequency of the output signal,
The first and second weight characteristics are:
A weighting characteristic A defined as z ₁ ≦ z ≦ z ₂ with the position z on the measured optical fiber as a variable, and linearly changing with respect to z;
and z ₀ ≦ z <z ₁ , and weight characteristic B defined in z ₂ <z ≦ z _n , and
The differential value of the amplitude of the weight characteristic B with respect to z is continuous with the differential value of the amplitude of the weight characteristic A with respect to z at the z ₁ and z ₂ , and is zero at the z ₀ , and The optical frequency domain reflection measurement method, wherein z _n is zero.

Images