JP2018048917A - Optical fiber test device and optical fiber test method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber test device and an optical fiber test method that make it possible to determine a position of a bending or of occurrence of strain in a test target optical fiber by making access to only one end of the optical fiber.SOLUTION: An optical fiber test device according to the present invention comprises: light incidence means that makes pulsed light incident on one end of a few-mode optical fiber being a test target optical fiber in arbitrary propagation modes; light separation means that separates Rayleigh backscattered light of the pulsed light in a certain position of the test target optical fiber, which returns back to the one end of the test target optical fiber, for each of the propagation modes; and calculation means that detects a phase difference between the Rayleigh backscattered light beams which the light separation means has separated for each of the propagation modes, and obtains a distribution of the phase difference in a lengthwise direction of the test target optical fiber.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a test apparatus and a test method for measuring a transfer function of a fumode optical fiber in a distributed manner.

  A fu-mode optical fiber is an optical fiber that can propagate light using a plurality of waveguide modes. A sensing technique has been proposed in which a fusing mode optical fiber is used as a sensor medium to detect side pressure, distortion, and vibration by detecting a phase change between two different propagation modes that pass through the sensor medium.

In Non-Patent Document 1, two fringe modes, a fundamental mode (LP ₀₁ mode) and a first higher-order mode (LP ₁₁ mode) are input to a fu-mode optical fiber, and interference fringes between these output modes are analyzed. By doing so, a phase difference between modes that changes due to perturbation applied to the optical fiber such as lateral pressure and strain is detected.

In Non-Patent Document 2, pulse light and continuous light are incident from both directions using both ends of a fu-mode optical fiber, and an interaction (Brillouin scattering) that occurs at the point where the pulse light and continuous light overlap in the optical fiber. Observing strain and temperature distribution information in the optical fiber under test by observation. At this time, the mode input to the optical fiber is selectively excited (for example, the incident light from one is set to the LP ₀₁ mode and the other is set to the LP ₁₁ mode), and the Brillouin frequency shift which shows different behavior depending on the mode state of interaction. By observing, it becomes possible to separate the parameters under test with high accuracy.

  On the other hand, in Non-Patent Document 3, OTDR (Optical Time) that measures optical characteristics in the longitudinal direction by irradiating pulsed light from one end of a fu-mode optical fiber and observing backward Rayleigh scattered light generated in the optical fiber. Domain Reflectometry) has been proposed.

M.M. R. Layton and J.M. A. Bucaro, “Optical fiber acoustic sensor moderating mode-mode interference”, App. Opt. , Vol. 18, no. 5, pp. 666-670, 1979. An Li, Yifei Wang, Jiang Fang, Ming-Jun Li, Byon Yoon Kim, and William Shir, and "Few-mode fiber multi-distance and parasitometer sensor wrench." Lett. 40, 1488-1491 (2015). Masataka Nakazawa, Masato Yoshida, and Toshihiko Hirooka, “Measurement of moderation of feed-moderation--Old-fibre-Tim. Express 22, 31299-31309 (2014).

The optical fiber bending sensor described in Non-Patent Document 1 observes interference fringes between modes output from one end of the basic (LP ₀₁ ) mode and the secondary (LP ₁₁ ) mode as test light. By doing so, the bending state can be detected. However, with such a method, if there are multiple bends in the optical fiber under test, the phase change between the modes observed at the output end is the cumulative value of the phase difference produced by each bend. There has been a problem that it is impossible to obtain distributed information along the longitudinal direction of the optical fiber.

  In addition, since the apparatus configuration described in Non-Patent Document 2 needs to input pulsed light and continuous light using both ends of the fu-mode optical fiber, it requires access to both ends of the optical fiber, There was a problem with efficiency.

  Further, the apparatus configuration described in Non-Patent Document 3 performs measurement using only one end of the fu-mode optical fiber. However, after separating the back Rayleigh scattered light generated in the fu-mode optical fiber into each mode, the photodetector In this case, only the intensity information can be acquired, and the phase difference between distributed modes in the longitudinal direction of the optical fiber cannot be detected.

  In view of the above problems, an object of the present invention is to provide an optical fiber test apparatus and an optical fiber test method capable of specifying the position of bending or strain of an optical fiber by accessing only one end of the optical fiber under test. .

  In order to achieve the above object, an optical fiber testing apparatus according to the present invention detects a complex amplitude for each propagation mode of backward Rayleigh scattered light by making an optical pulse incident only on one end of an optical fiber to be tested. The amount of phase change between propagation modes in the longitudinal direction of the fiber is detected in a distributed manner.

Specifically, the optical fiber testing apparatus according to the present invention is:
A light incident means for injecting pulsed light in an arbitrary propagation mode to one end of a fu mode optical fiber which is an optical fiber to be tested;
Light separating means for separating the backward Rayleigh scattered light of the pulsed light at an arbitrary position of the optical fiber under test returned to the one end of the optical fiber under test for each propagation mode;
A calculating means for detecting a phase difference of the backward Rayleigh scattered light separated by the light separating means for each propagation mode, and obtaining a distribution of the phase difference with respect to a longitudinal direction of the optical fiber under test;
Is provided.

Further, the optical fiber testing method according to the present invention includes:
A light incident procedure in which pulsed light is incident on one end of a fu mode optical fiber, which is an optical fiber under test, in an arbitrary propagation mode;
A light separation procedure for separating the backward Rayleigh scattered light of the pulsed light at an arbitrary position of the optical fiber under test returned to the one end of the optical fiber under test for each propagation mode;
A calculation procedure for detecting a phase difference of light separated for each propagation mode in the separation procedure and obtaining a distribution of the phase difference with respect to a longitudinal direction of the optical fiber to be tested.
I do.

  The phase difference between the propagation modes changes in a portion where the fu-mode optical fiber is bent or distorted. In the present invention, since the phase difference between propagation modes is acquired in a distributed manner in the longitudinal direction of the optical fiber, the position of bending or distortion can be specified from the position where the phase difference changes. Therefore, the present invention can provide an optical fiber test apparatus and an optical fiber test method capable of specifying the position of bending or strain of the optical fiber by accessing only one end of the optical fiber under test.

  The optical fiber testing apparatus and method according to the present invention is characterized in that the calculation means calculates a distribution of a transfer function with respect to a longitudinal direction of the optical fiber under test from the distribution of the phase difference. The characteristics of the optical fiber under test can be simulated using the acquired transfer function.

  The computing means of the optical fiber test apparatus according to the present invention determines that the propagation mode of the pulsed light and the backward Rayleigh scattered light are the backward Rayleigh scattered light from the arbitrary position in consideration of the group velocity of the propagation mode. It is characterized by doing. In the measured optical fiber, the backward Rayleigh scattered light has a different group velocity for each mode. For this reason, in order to accurately acquire the phase difference between modes at an arbitrary point, it is necessary to convert a signal sampled at the same timing into distance information, and correction in consideration of the group velocity is required.

  The optical fiber test apparatus according to the present invention further includes a polarization acquisition unit that measures a polarization state for each propagation mode of the backward Rayleigh scattered light, and the calculation unit is configured for each polarization measured by the polarization acquisition unit. A phase difference is obtained. The phase of the propagation mode may change in the polarization state. For this reason, it is desirable to acquire the phase difference for each polarization in order to accurately acquire the phase difference between the modes at an arbitrary point.

  Note that the calculation means of the optical fiber testing apparatus according to the present invention can also be realized by a computer and a program. The program is a program for causing a computer to function as the calculation means. Further, the program can be recorded on a recording medium or provided through a network.

  The present invention can provide an optical fiber test apparatus and an optical fiber test method capable of specifying the position of bending or strain of an optical fiber by accessing only one end of the optical fiber to be tested.

It is a figure explaining the optical fiber test device concerning the present invention. It is a figure explaining operation | movement of a mode selection coupler. It is a figure explaining the optical fiber test method which concerns on this invention. It is a figure explaining the optical fiber test method which concerns on this invention. It is a figure explaining the optical fiber test device concerning the present invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
FIG. 1 is a diagram illustrating an optical fiber test apparatus 301 according to the present embodiment. The optical fiber test apparatus 301
A light incident means 11 for injecting pulsed light in an arbitrary propagation mode to one end of a fu mode optical fiber which is an optical fiber to be tested 1-7;
A light separating means 12 for separating the backward Rayleigh scattered light of the pulsed light at an arbitrary position of the optical fiber under test returned to the one end of the optical fiber under test 1-7 for each propagation mode;
The light separation means 12 includes a calculation means 13 for detecting the phase difference of the backward Rayleigh scattered light separated for each propagation mode and acquiring the distribution of the phase difference with respect to the longitudinal direction of the optical fiber 1-7 to be tested.

  The light incident means 11 includes a light source 1-1, an optical branching means 1-2, an optical pulsing means 1-3, an electric pulse generating means 1-4, an optical circulator 1-5, and a mode selection coupler 1-6. The light separation means 12 includes an optical circulator 1-5 and a mode selection coupler 1-6. The arithmetic means 13 includes an optical 90-degree hybrid (1-8, 1-9), a balanced receiver (1-10 to 1-13), an A / D converter 1-14, and an arithmetic processing means 1-15. Including.

Light source 1-1 oscillates a continuous light having an optical frequency [nu _1. The light branching unit 1-2 branches the continuous light oscillated from the light source into probe light and first and second local light. The optical pulse forming means 1-3 pulses the probe light branched by the optical branching means. The electric pulse generating means 1-4 generates an electric pulse for driving the optical pulse converting means with a desired pulse width and a repetition frequency, and generates an electric pulse for outputting a trigger signal to the A / D converter. The optical circulator 1-5 extracts backward Rayleigh scattered light returning from the optical fiber under test to the incident end side. The mode selection coupler 1-6 inputs a probe pulse (pulse light) to the optical fiber under test in a desired propagation mode, and converts the backward Rayleigh scattered light returning from the optical fiber under test to the incident end side in each propagation mode. To separate. The optical fibers 1-7 to be tested are fumode optical fibers.

  The first light 90-degree hybrid 1-8 outputs interference light having a relative phase difference of π / 2 with respect to the first propagation mode of the separated backward Rayleigh scattered light and the first local light. . The second light 90-degree hybrid 1-9 outputs interference light having a relative phase difference π / 2 with respect to the second propagation mode of the separated backward Rayleigh scattered light and the second local light. . The balanced receiver 1-10 detects the in-phase component of the interference light output from the first optical 90-degree hybrid. The balanced receiver 1-11 detects the orthogonal component of the interference light output from the first optical 90-degree hybrid. The balanced receiver 1-12 detects the in-phase component of the interference light output from the second optical 90-degree hybrid. The balanced receiver 1-13 detects an orthogonal component of the interference light output from the second optical 90-degree hybrid. The A / D converter 1-14 converts the electrical signal output from the balanced receiver into a digital value.

  The computing means 1-15 computes the distribution of the transfer function with respect to the longitudinal direction of the optical fiber 1-7 to be tested from the phase difference distribution. Specifically, when calculating the transfer function for each section of the optical fiber under test, the computing means 1-15 uses the phase calculated by analyzing the backward Rayleigh scattered light from a certain point z and the point z + ΔL. By calculating the phase difference calculated by analyzing the rear Rayleigh scattered light, the phase difference in the optical fiber length ΔL in a specific section is distributedly measured.

The following explains theoretically that the transfer matrix in the optical fiber under test can be measured in a distributed manner. In the following, for the sake of simplicity of explanation, it is assumed that the fuse mode optical fiber can propagate only the LP ₀₁ mode and the LP ₁₁ mode. Strictly speaking, there are two LP ₁₁ modes orthogonal to each other, but for convenience, only one of them is referred to as LP ₁₁ for the sake of convenience.

The continuous light emitted from the first light source 1-1 having the optical frequency ν ₁ is branched into the probe light and the first and second local light by the light branching means 1-2, and the probe light is the electric pulse generating means. Based on the electrical signal generated in 1-3, the pulse is generated by the optical pulsing unit 1-4 to generate a probe pulse. At this time, the optical pulsing means 1-4 may shift the optical frequency of the probe light. For example, when an acousto-optic modulator that is pulse-driven is used as the optical pulsing means 1-4, the optical frequency is shifted by the frequency f of the acoustic wave applied to the acousto-optic modulator, and the optical frequency of the probe light is ν ₁ ±. f (the sign is determined by the incident probe light and the traveling direction of the acoustic wave). Here, the frequency of the pulsed light is ν ₂ (= ν ₁ ± f). After passing through the optical circulator 1-5, the probe pulse is input to the mode selection coupler 1-6. As shown in FIG. 2, since the mode selection coupler can selectively excite the output propagation mode according to the input port, the probe pulse having the desired propagation mode is incident on the optical fiber under test. can do.

When a probe pulse is incident on the optical fiber under test, backward Rayleigh scattered light is generated at each point in the longitudinal direction of the optical fiber. When tested optical fiber was diffuser-mode optical fiber, LP ₀₁ mode and LP ₁₁ mode of the backward Rayleigh scattering light may be any state of the probe pulse LP ₀₁ mode or LP ₁₁ mode to be tested optical fiber And both return to the incident end. Back Rayleigh scattered light returning to the incident end is input to the mode selection coupler at 1-6, separated into LP ₀₁ mode and LP ₁₁ mode, and output from each port. At this time, the LP ₁₁ mode is converted to the LP ₀₁ mode. However, in the following description, it is described as LP ₁₁ mode backward Rayleigh scattered light.

  As described above, the mode selection coupler 1-6 can not only selectively excite the input mode of the probe pulse, but also outputs the backward Rayleigh scattered light input from the optical fiber to be tested by separating each mode for each port. be able to. It has been demonstrated that a mode selection coupler having such a function can be manufactured by a planar optical waveguide circuit, a spatial optical system, or a melt-drawn coupling of an optical fiber.

  The backward Rayleigh scattered lights of LP01 and LP11 separated by the mode selection coupler are input to the optical 90-degree hybrids 1-8 and 1-9 together with the first and second local lights, respectively, and are combined, and the in-phase of the interference light The component and the quadrature component are output from balanced receivers 1-10, 1-11 and 1-12, 1-13, respectively.

  FIG. 3 schematically shows the relationship between the probe pulse propagating through the optical fiber 1-7 to be tested and the backward Rayleigh scattered light generated at an arbitrary point of the optical fiber 1-7 to be tested.

When the probe pulse which has passed through the mode coupler 1-6 is incident on the test optical fiber in LP ₀₁ mode, the complex amplitude of the probe pulse LP ₀₁ mode at the input (z = 0)
It expresses. The first and second rows of the matrix show the LP01 and LP11 mode complex amplitudes at the input, respectively. Here, A ₁ and ν ₂ are the amplitude and optical frequency of the probe pulse, respectively. φ (t) is a phase component that varies with time depending on light source coherency and the like.

If the loss and mode coupling in the optical fiber can be ignored, the transfer function of the section n of the optical fiber to be tested can be expressed as follows.
Here, β ₀₁ and β ₁₁ represent the propagation constants of the LP ₀₁ mode and the LP ₁₁ mode, respectively, and ΔL represents the optical fiber length in the section n.

The complex amplitude of the probe pulse that has reached the point z ₁ via the first interval T ₁ is
It becomes.

The scattering matrix R by the Rayleigh scattering process is
And Here, φ represents a phase change applied to each element by the backward Rayleigh scattering process, and r represents an amplitude mode coupling coefficient. The subscripts φ and r represent the modes of probe light and backward Rayleigh scattered light. For example, 01-11 indicates that LP01 mode probe light is scattered and coupled to LP11 mode backscattered light. The scattering matrix R is uniform over the longitudinal direction of the optical fiber to be tested.

Back Rayleigh scattered light generated at point z ₁ is
It becomes.

Such backward Rayleigh scattering light generated in the can come back to the probe pulse and the incident end through the section T ₁ in the same manner (z = 0).

Therefore, the complex amplitude E _B of the backward Rayleigh scattered light generated at the point z ₁ and returning to the incident end (z = 0) is
Due to the reciprocity of the optical fiber
It becomes.

The complex amplitude in each mode of the backscattered light at the point z = 0 is expressed as follows.

Next, calculation of the phase difference between the backward Rayleigh scattered light in the LP01 mode and the LP11 mode returning to the incident end will be described.
The Rayleigh scattered light is separated into respective modes by the mode selection coupler 1-6, and the backward Rayleigh scattered light in the LP01 mode and the first local light are converted into the first light 90-degree hybrid 1-8, and the backward Rayleigh scattered in the LP11 mode. The second local light is input to the second optical 90-degree hybrid 1-9, and the backward Rayleigh scattered light and the local light are combined by the optical 90-degree hybrids 1-8 and 1-9. The in-phase component and the quadrature component are output from the balanced receivers 1-10, 1-11 and 1-12, 1-13.

The complex amplitude of the LP01 mode of the backward Rayleigh scattered light separated by the mode selection coupler 1-6 and input to the optical 90-degree hybrid 1-8 is
It is represented by Here, A ₁ and ν ₂ are the amplitude and the optical frequency, respectively. ψ ₁ (t) represents the relative phase difference of the LP01 mode of Rayleigh scattered light with respect to local light, and varies randomly with time due to phase noise in the optical fiber.

On the other hand, the complex amplitude of the LP11 mode of the backward Rayleigh scattered light separated by the mode selection coupler 1-6 and input to the optical 90-degree hybrid 1-9 is
It is represented by Here, A ₂ and ν ₂ are the amplitude and the optical frequency, respectively. ψ ₂ (t) represents the relative phase difference of the LP11 mode of the backward Rayleigh scattered light with respect to the local light, and varies randomly with time due to phase noise in the optical fiber.

The complex amplitudes of the first and second local lights input to the first optical 90-degree hybrid 1-8 and the second optical 90-degree hybrid 1-9 are:
It becomes.

On the other hand, the complex amplitude of the second local light input to the second light 90-degree hybrid is
It becomes. Here, Δφ _L1−L2 represents a relative phase difference depending on a difference in optical path length through which the first local light and the second local light propagate.

When the LP01 mode backscattered light type and the first local light are input to the first light 90-degree hybrid 1-8, the in-phase interference light output from the balanced photoreceivers 1-10 and 1-11 Component I ₁ and orthogonal component Q ₁ are respectively
and
It becomes.

On the other hand, when the LP11 backscattered light type and the second local light are input to the second light 90-degree hybrid 1-9, the interference light output from the balanced light receivers 1-12 and 1-13 In-phase component I ₂ and quadrature component Q ₂ of
It becomes.

From Equation (13) and Equation (14)
Is required.

From Equation (15) and Equation (16)
Is required. Therefore,
It becomes. The phases ψ ₁ (t) and ψ ₂ (t) of the backward Rayleigh scattered light and the local light naturally (randomly) change from measurement to measurement due to phase noise in the optical fiber. Therefore, it can be reduced by averaging the waveforms. Therefore, it can be regarded as constants (denoted as C) that do not depend on the position z, except for the first term of the equation (19). Therefore
Thus, a value reflecting the phase change in the section can be obtained. The optical fiber length ΔL in the evaluation section is set to be greater than the distance resolution of the measuring instrument.

Phase difference Δφ generated in the section ^{n (n) (z),} from the phase difference determined by observing the backward Rayleigh scattering from the point z _n, by observing the backward Rayleigh scattering from the point z _n-1 It is obtained by taking the difference of the required phase difference.

On the other hand, the LP01 mode and the LP11 mode of the backscattered light in the optical fiber under test have different group velocities. Therefore, in order to compare the phase difference Δφ between the LP01 and LP11 modes at a certain point, the signals sampled at the same timing are used. When converting to distance information, correction in consideration of the group velocity is required. When the group velocities of the LP01 mode and the LP11 mode in the optical fiber under test are v ₁ and v ₂ respectively,
Here, z ₀₁ and z ₁₁ represent the positions in the optical fiber where the LP01 mode and the LP11 mode of the backscattered light observed at time t are generated, respectively. For the LP11 mode of backscattered light, the test light pulse propagates in the LP01 mode, and the backscattered light propagates in the LP11 mode and returns to the incident end side. Therefore, the total group velocity is the group velocity in each mode. The average value of

  For example, as shown in FIG. 4, a difference in propagation constant between the LP01 mode and the LP11 mode is different in a portion where bending or distortion occurs in the fu mode optical fiber as compared with the straight line portion. That is, phase modulation occurs (it can be said that the mode dispersion is different), so that the location can be specified by observing the phase change by the method of the present invention.

  In addition, the elliptical core type and eccentric core type fu-mode fibers have a larger amount of phase change at the bent portion than that of the straight portion. Therefore, using these fibers as optical fibers under test makes the phase change amount more sensitive. Can be detected. Further, in a general single mode fiber, the same measurement can be performed by using a wavelength shorter than the cutoff wavelength of the LP11 mode and longer than the cutoff wavelength of the LP21 mode.

(Embodiment 2)
FIG. 5 is a diagram for explaining the optical fiber testing apparatus 302 of the present embodiment. The optical fiber test apparatus 302 further includes polarization acquisition means 14 that measures the polarization state for each propagation mode of the backward Rayleigh scattered light with respect to the optical fiber test apparatus 301 of FIG. The acquisition means 14 acquires a phase difference for each polarization measured.

  In the first embodiment, the configuration in which the polarization state of each mode is not acquired has been described. However, the phase information acquired may change due to changes in the polarization state. In such a case, as shown in FIG. 5, the polarization beam splitter (1-16, 1-17, 1-18, 1-19) which is the polarization acquisition means 14, and the optical 90-degree hybrid (1-8) ', 1-9'). The optical fiber test apparatus 302 measures the phase difference between the LP01 mode and the LP11 mode for each polarization. With the configuration as shown in FIG. 5, measurement independent of the polarization state is also possible.

(effect)
According to the optical fiber transfer matrix distribution measuring apparatus of the present invention, the transfer matrix of the optical fiber can be measured in a distributed manner, and, for example, optical fiber bending occurs at a plurality of locations from the observed phase differences between the modes. Even in such a case, each can be specified individually.

1-1: Light source 1-2: Optical branching means 1-3: Pulse forming means 1-4: Electric pulse generator 1-5: Optical circulator 1-6: Mode selection coupler 1-7: Optical fiber to be tested (Fuse) Mode optical fiber)
1-8, 9, 8 ′, 9 ′: Optical 90-degree hybrid 1-10-13: Balanced receiver 1-14: A / D converter 1-15: Arithmetic processing means 1-16-19: Polarization Beam splitter 11: Light incident means 12: Light separation means 13: Calculation means 14: Polarization acquisition means 301, 302: Optical fiber test apparatus

Claims (6)

A light incident means for injecting pulsed light in an arbitrary propagation mode to one end of a fu mode optical fiber which is an optical fiber to be tested;
Light separating means for separating the backward Rayleigh scattered light of the pulsed light at an arbitrary position of the optical fiber under test returned to the one end of the optical fiber under test for each propagation mode;
A calculating means for detecting a phase difference of the backward Rayleigh scattered light separated by the light separating means for each propagation mode, and obtaining a distribution of the phase difference with respect to a longitudinal direction of the optical fiber under test;
An optical fiber testing apparatus comprising:   2. The optical fiber testing apparatus according to claim 1, wherein the computing means computes a transfer function distribution with respect to a longitudinal direction of the optical fiber under test from the phase difference distribution.   The calculation means determines that the propagation mode of the pulsed light and the backward Rayleigh scattered light are backward Rayleigh scattered light from the arbitrary position in consideration of a group velocity of the propagation mode. Or an optical fiber testing apparatus according to 2; Further comprising polarization acquisition means for measuring a polarization state for each propagation mode of the backward Rayleigh scattered light,
The optical fiber test apparatus according to claim 1, wherein the calculation unit acquires a phase difference for each polarization measured by the polarization acquisition unit. A light incident procedure in which pulsed light is incident on one end of a fu mode optical fiber, which is an optical fiber under test, in an arbitrary propagation mode;
A light separation procedure for separating the backward Rayleigh scattered light of the pulsed light at an arbitrary position of the optical fiber under test returned to the one end of the optical fiber under test for each propagation mode;
A calculation procedure for detecting a phase difference of light separated for each propagation mode in the separation procedure and obtaining a distribution of the phase difference with respect to a longitudinal direction of the optical fiber to be tested.
Optical fiber testing method.   6. The optical fiber test method according to claim 5, wherein a distribution of a transfer function with respect to a longitudinal direction of the optical fiber under test is calculated from the distribution of the phase difference in the calculation procedure.