JP5641178B2 - Optical reflectometry measuring method and optical reflectometry measuring apparatus - Google Patents

Description

  The present invention relates to an optical reflectometry measurement method for calculating a birefringence distribution of an object to be measured such as an optical circuit, and further calculating a longitudinal distribution of polarization mode dispersion from the obtained birefringence distribution, and this method. The present invention relates to an optical reflectometry measuring apparatus for implementation.

  As one of the methods for measuring the polarization state distribution of the backscattered light, the polarization measurement time domain reflectometry measurement method (P-OTDR: Polarization-Optical Time Domain Reflectometry) and the optical frequency domain reflectometry measurement method (phase noise compensation OFDR) The method (PNC-OFDR: Phase Noise Compensation-Optical Frequency Domain Reflectometry) is known. (For example, refer nonpatent literature 1 and nonpatent literature 2.).

  In the P-OTDR method, the polarization state of the light backscattered light with respect to the propagation direction is measured by measuring the Stokes parameter of the backscattered light that is incident on the object to be measured and reflected from the object to be measured. Can do. However, in the existing P-OTDR technology, since the spatial resolution is 1 meter or more, this spatial resolution is backward for a fiber having a polarization mode dispersion (PMD) of 1 ps / √km or more. It is insufficient to measure the distribution of scattered light. On the other hand, according to the phase noise compensation OFDR method, the polarization state can be measured with an excellent spatial resolution, which is also effective for a fiber having high polarization mode dispersion.

If the polarization state of the light backscattered light with respect to the propagation direction is known, the distribution of polarization mode dispersion can be measured. That is, the differential group-velocity delay (DGD) of an arbitrary section to be measured can be measured (see, for example, Non-Patent Document 3). This method measures the Mueller matrix R (z) of the transmission path in which the measurement light is transmitted from the exit to the point z to be measured and returned to the optical receiver by reflection or scattering at the point z, and the optical reflectometry measurement method. It can obtain | require from the polarization state of the backscattered light measured by (1). By using the Mueller matrix R (z), a birefringence vector [β _R (z)] (hereinafter, [] represents a vector in the description) can be calculated. Assuming that the birefringence of the object to be measured is linear, the birefringence vector [β _R (z)] is used to calculate the group velocity delay difference of an arbitrary section of the object to be measured. Distribution can be calculated.

  In practice, however, the Mueller matrix R (z) cannot be calculated simply by measuring the polarization state of the returned light after the incident light is incident on the object to be measured and measuring only one polarization state. . Another incident light having a polarization state different from the polarization state of the incident light is again incident on the object to be measured, and the polarization state of the returned light needs to be measured.

  However, in the calculation of R (z), it is assumed that the polarization characteristics of the measurement target do not change between the first measurement and the next measurement. That is, if the polarization characteristics change due to the influence of the external environment, an unpredictable error occurs in the calculation of R (z), and the reliability of the final result is questioned.

  Therefore, if two polarized lights having different polarization states are simultaneously incident on the object to be measured and the respective polarization states can be measured at the same time, the above assumption is unnecessary and a reliable final result can be obtained. A technique for measuring two polarization states has become necessary.

“Polarization optical time domain reflectometry”, Alan J. Rogers, IEE Electronics Letters, 16 (13), pp. 489-490. "Measurement of polarization distribution of backscattered light of high PMD fiber by phase noise compensation OFDR method", Fan Xinyu, Yusuke Furushikiya, Fumihiko Ito, IEICE Technical Report, 108 (309), OCS2008-96, pp. 67-70, November 2008. `` Distributed Polarization-Mode-Dispersion Measurement in Fiber Links by Polarization-Sensitive Reflectometric Techniques '', Andrea Galtarossa, et.al., 20 (23), IEEE Photonics Technology Letters, pp. 1944-1946.

  As described above, the Mueller matrix R (z) is calculated when measuring the distribution of the polarization mode dispersion in the optical polarization state distribution measurement technique such as the existing P-OTDR method and the phase noise compensation OFDR method. Although it is necessary, if the polarization characteristics of the measurement target change due to the external environment, an unpredictable error will occur in the calculation of R (z), and the reliability of the final result will be questioned. A technique for measuring two polarization states has become necessary.

  The present invention has been made in view of the above circumstances, and can measure two polarization states at the same time, thereby obtaining high reliability with respect to the birefringence distribution by Mueller matrix calculation. An object of the present invention is to provide an optical reflectometry measurement method capable of measuring a polarization mode dispersion distribution with high reliability from a refractive index distribution, and an optical reflectometry measurement apparatus using this method.

In order to achieve the above object, the optical reflectometry measurement method according to the present invention has the following configuration.
(1) The output light of the light source is branched into two, one of the branched light is incident on the measurement target as measurement light, the reflected light is captured, and the reflected light and the reference light from the other branched light are combined. An optical reflectometry measurement method for measuring an interference signal by optically detecting the interference wave, wherein light having two different polarization states is generated from the measurement light and simultaneously incident on an object to be measured, A configuration in which an interference signal in two polarization states is simultaneously measured, a transfer function of the measurement target is calculated from the two interference signals measured at the same time, and a birefringence distribution of the measurement target is measured. .

(2) In the optical reflectometry measurement method of (1), the longitudinal distribution of polarization mode dispersion is calculated using the measurement result of the birefringence distribution.
(3) In the optical reflectometry measurement method according to (1), a configuration in which the polarization state of the measurement result is distinguished in the frequency domain by shifting the optical frequencies of the two incident lights having different polarization states. To do.

Moreover, the optical reflectometry measuring apparatus according to the present invention has the following configuration.
(4) The output light of the light source is branched into two, one of the branched lights is incident on the measurement target as measurement light, the reflected light is captured, and the reflected light and the reference light from the other branched light are combined. An optical reflectometry measuring apparatus that measures an interference signal by optically detecting the interference wave, and generates two lights having different polarization states from the measurement light and simultaneously enters the measurement object An incident means, an interference signal measuring means for simultaneously measuring an interference signal in the two polarization states, a birefringence of the object to be measured by calculating a transfer function of the object to be measured from the two interference signals measured simultaneously. It is set as the structure of the aspect which comprises the birefringence distribution analysis means which measures a rate distribution.

(5) In the optical reflectometry measuring device according to (4), the configuration further includes a polarization mode dispersion distribution analyzing means for calculating the longitudinal distribution of the polarization mode dispersion using the measurement result of the birefringence distribution. And
(6) In the optical reflectometry measuring apparatus of (4), the measurement light incident means shifts the optical frequencies of the two incident lights having different polarization states, and the birefringence distribution analysis means The polarization state is distinguished in the frequency domain.

  That is, in the optical reflectometry measurement method and apparatus according to the present invention, two lights having different polarization states are simultaneously incident on a measurement target, and the polarization state of backscattered light with respect to each incident light is simultaneously measured, From the result, the birefringence distribution to be measured and the polarization mode dispersion distribution are calculated.

  As described above, according to the present invention, two polarization states can be measured simultaneously, thereby obtaining high reliability for the birefringence distribution by Mueller matrix calculation. It is possible to provide an optical reflectometry measuring method capable of measuring a highly reliable polarization mode dispersion distribution and an optical reflectometry measuring apparatus using the method.

The block diagram which shows schematic structure of one Embodiment of the optical reflectometry measuring apparatus which concerns on this invention. The block diagram which shows the structure of the optical reflectometry measuring apparatus at the time of being based on the PNC-OFDR method as a specific example of embodiment shown in FIG. The image figure which shows the analysis content in the frequency analyzer of the measuring apparatus shown in FIG. The block diagram which shows schematic structure of the optical reflectometry measuring apparatus which measures distribution of a polarization state by the existing P-OTDR and PNC-OFDR measuring method. The block diagram which shows the specific structure of the optical reflectometry measuring apparatus which measures distribution of a polarization state by the existing PNC-OFDR measuring method.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of an embodiment of an optical reflectometry measuring apparatus to which a PMD (polarization mode dispersion) distribution measuring method according to the present invention is applied. In FIG. 1, the light S <b> 1 generated by the light source 11 is sent to the dual polarization generation device 12. The two-polarization generator 12 generates light having two polarization components from incident light S 1, and the output light S 2 is guided by the circulator 13 to the measured optical circuit 14.

  Inside the optical circuit to be measured 14, when the polarization-adjusted light S2 is incident, backscattered signal light is generated. This signal light becomes reflected light S4 and is guided to the optical receiver 15 by the circulator 13. The optical receiver 15 receives the signal light S5 from the circulator 13 and detects its signal component. The detection signal obtained here is sampled by the sampling device 16 and sent to the first analysis device 17. . The first analysis device 17 analyzes the polarization state of the two-polarized backscattered light generated in the measured optical circuit 14 from the sampled signal component, and the analysis result is sent to the second analysis device 18. . The second analyzer 18 calculates the birefringence distribution by performing Mueller matrix calculation from the analysis result of the polarization state of the two-polarized backscattered light, and calculates and measures the PMD distribution from the birefringence distribution. .

Before describing the measurement method of the optical reflectometry measuring apparatus having the above-described configuration, an existing polarization state distribution measurement technique will be briefly described.
FIG. 4 is a block diagram showing a configuration of an optical reflectometry measuring apparatus for measuring a polarization state distribution based on, for example, an existing P-OTDR or PNC-OFDR measurement method. In FIG. 4, the same parts as those in FIG.

  In FIG. 4, the light S1 generated by the light source 11 is subjected to polarization adjustment by the polarization controller 12 '. Here, the polarization-adjusted light S2 ′ is guided by the circulator 13 to the optical circuit 14 to be measured. In this measured optical circuit 14, when the polarization-adjusted light S2 'is incident, backscattered signal light is generated. This signal light becomes reflected light S 4 ′ and is guided to the optical receiver 15 by the circulator 13. The optical receiver 15 receives the signal light S5 'from the circulator 13 and detects its signal component. The detection signal obtained here is sampled by the sampling device 16 and sent to the analyzing device 17'. . This analysis device 17 'analyzes the longitudinal distribution of the backscattered light polarization state generated in the measured optical circuit 14 from the sampled signal component by the following calculation process.

The polarization state of light can be expressed using Stokes parameters (x, y, z). Since it is a distribution measurement in the longitudinal direction of the optical circuit 14 to be measured, the polarization state is expressed as a function of distance. If the input polarization state is I ₁ (0), the measured polarization state U ₁ (z) can be expressed by the following equation (1).
U ₁ (z) = R (z) ・ I ₁ (0)… (1)
As is clear from the above equation, the Mueller matrix R (z) cannot be specified even if I ₁ (0) and U ₁ (z) are known. However, if a polarization state I ₂ (0) different from I ₁ (0) is input to the object to be measured, the polarization state U ₂ (z) can be measured. The measured polarization state U ₂ (z) can be expressed by the following equation (2).
U ₂ (z) = R (z) ・ I ₂ (0)… (2)
Since the Mueller matrix R (z) can be specified by the simultaneous equations of the equations (1) and (2), the birefringence vector [β _R according to the following equation (3) is used by using the calculated value of R (z). (z)] can be calculated.
∂R (z) / ∂z = [β _R (z)] × R (z) (3)
Using [β _R (z)] obtained by equation (3), assuming that the birefringence of the object to be measured is linear, the group velocity delay difference from any section z ₀ to z of the object to be measured Can be calculated by the following equation (4).

以上の偏波状態分布測定方法において、入力した偏波状態I₁(0) とI₂(0) が同時ではなく前後であり、U₁(z) とU₂(z) を測定する間に外部環境の影響で被測定対象の偏波特性が変わると、ミュラーマトリックスR(z)の計算結果に誤差が生じてしまう。

In the polarization state distribution measurement method described above, the input polarization states I ₁ (0) and I ₂ (0) are not simultaneous but before and after, and U ₁ (z) and U ₂ (z) are measured. If the polarization characteristics of the measurement object change due to the influence of the external environment, an error occurs in the calculation result of the Mueller matrix R (z).

The present invention is a measurement method in order to eliminate the above influence. In the optical reflectometry measuring apparatus for PMD distribution measurement having the configuration shown in FIG. 1, the first polarized light I ₁ having two different polarization states is used in the two-polarization generation apparatus 12 instead of the conventional polarization controller 22. (0) and second polarized light I ₂ (0) are generated and combined and output. Here, a technique for distinguishing both polarized lights I ₁ (0) and I ₂ (0) is necessary so that measurement signals to be synthesized later can be distinguished. For example, when distinguishing by frequency, in the two-polarization generator 12, a frequency shifter (for example, an acoustooptic modulator) is provided at one of the output terminals of I ₁ (0) and I ₂ (0), and both frequencies This can be realized by making a difference in.

The first and second polarized lights I ₁ (0) and I ₂ (0) having different polarization states are simultaneously incident on the measured optical circuit 14 via the circulator 13. In the measured optical circuit 14, signal light back-scattered for each of the first and second polarized lights I ₁ (0) and I ₂ (0) is generated. The signal light is output as reflected light S 4, guided to the optical receiver 15 by the circulator 13, and detected by the optical receiver 15.

A signal generated in the detection output is sampled by the sampling device 16, and the first analyzing device 17 analyzes the polarization state of the two-polarized backscattered light. Specifically, the polarization states U ₁ (z) and U ₂ (z) are calculated separately for the first and second polarized lights I ₁ (0) and I ₂ (0). The second analysis device 18 uses the equations (3) and (4), and z ₀ to z based on the polarization states U ₁ (z) and U ₂ (z) calculated by the first analysis device 17. DGD (group velocity delay difference) is calculated to obtain a birefringence distribution, and the PMD distribution in the longitudinal direction of the optical circuit 14 to be measured is obtained as a measurement result from the calculation result.

  By doing so, it is possible to simultaneously measure the polarization states of the two backscattered light, thereby obtaining high reliability for the birefringence distribution by the Mueller matrix calculation, and further from this birefringence distribution. A highly reliable polarization mode dispersion distribution can be measured.

(Example)
FIG. 2 is a block diagram showing a specific configuration of an optical reflectometry measuring apparatus based on a PNC-OFDR measuring apparatus as an embodiment according to the present invention.
In FIG. 2, the output light S11 from the light source 21 swept by the optical frequency is branched into two systems by an optical directional coupler 22A, one of which is used as reference light S12 and the other is used as measurement light S13. The reference light S12 is adjusted by the polarization controller 23A so that the branching ratio between the s-polarization direction and the p-polarization direction is 1: 1, and is combined with the later-described light S17 by the optical directional coupler 22B as the light S18. .

  On the other hand, the measurement light S13 branched by the optical directional coupler 22A is further branched into two systems by the optical directional coupler 22C. One branched light S20 is subjected to polarization adjustment by the polarization controller 23B, and the other branched light S21 is shifted to a frequency different from that of the other branched light S20 by the frequency shift device 30, and then is polarized by the polarization controller 22C. Receives polarization adjustment. The lights S22 and S23 whose polarizations have been adjusted by the polarization controllers 22B and 22C are combined by the optical directional coupler 22D, whereby the measurement light S14 having two polarization components is obtained. The measurement light S14 is guided by the circulator 24 to the measured optical circuit 25.

  In the measured optical circuit 25, when the light S13 is incident, signal light S16 back-scattered inside is generated. The signal light S16 is extracted by the circulator 24, and is combined with the light S18 from the polarization controller 23A by the optical directional coupler 22B as light S17. This combined light is branched in the s-polarization direction and the p-polarization direction by the polarization beam splitter 26 and detected by the optical receivers 27A and 27B, respectively. The sampling beater 28 samples the s-polarized interference beat signal and the p-polarized interference beat signal generated at the detection outputs of the optical receivers 27A and 27B. The sampling data is analyzed by the frequency analyzer 29 to measure the backscattered light amplitude distribution from each position in the measured optical circuit 25 and the distribution of the difference between the s direction optical phase and the p direction optical phase. The polarization state of the backscattered light is completely measured.

When the frequency analyzer 29 measures the backscattered light amplitude distribution from each position in the optical circuit 25 to be measured and the distribution of the difference between the s-direction optical phase and the p-direction optical phase, the measurement result is the PMD distribution measurement / analysis apparatus. 31 for analysis of PMD distribution measurement.
Before describing the measurement method of the optical reflectometry measuring apparatus having the above-described configuration, an existing PNC-OFDR measurement technique will be briefly described.

FIG. 5 is a block diagram showing a configuration of an optical reflectometry measuring apparatus for measuring the polarization state distribution based on the PNC-OFDR measurement method. In FIG. 5, the same parts as those in FIG.
5 is different from the apparatus of FIG. 2 in that the measurement light S13 is directly sent to the polarization controller 23B without branching, and the measurement light S14 ′ whose polarization is adjusted here is sent by the circulator 24 to the optical circuit 15 to be measured. , Sampling by the sampling device 28, and analyzing the sampling data by the frequency analyzing device 29 ′, so that the backscattered light amplitude distribution and the s direction from each position in the optical circuit 25 to be measured The distribution of the difference between the optical phase and the p-direction optical phase is measured to measure the polarization state of the backscattered light.

  That is, in the measuring apparatus shown in FIG. 5, the backscattered signal light S16 generated inside the optical circuit 15 to be measured is extracted by the circulator 24, and is combined with the light S18 by the optical directional coupler 22B as the light S17. The polarization beam splitter 26 branches the signal into the s-polarization direction and the p-polarization direction, and is detected by the optical receiver 27A and the optical receiver 27B, respectively. The interference beat signal in the direction is sampled by the sampling device 28. Then, by analyzing the sampling data by the frequency analyzer 29 ', the backscattered light amplitude distribution from each position in the optical circuit to be measured 25 and the distribution of the difference between the s-direction optical phase and the p-direction optical phase are measured. In addition, the polarization state of the backscattered light is completely measured.

However, since there is still only one degree of freedom in the calculation of the Mueller matrix R (z) required for PMD distribution measurement, the polarization state of the incident light S14 is changed, and the backscattered light in the new polarization state. It is necessary to measure the polarization state.
In the measurement apparatus shown in FIG. 5, the new polarization state of the incident light S14 can be realized by adjusting the polarization state of the measurement light S13 with the polarization controller 23B. However, the measurement is performed twice for the PNC-OFDR measurement. The polarization state may change between the first measurement and the next measurement due to the influence of the external environment, resulting in an unpredictable error in the calculation of R (z) and the reliability of the final result. Sex will be asked.

Therefore, it is considered that if the incident light having two different polarization states is simultaneously incident on the object to be measured and the respective polarization states can be measured simultaneously, a reliable final result can be obtained.
The measuring apparatus shown in FIG. 2 pays attention to the above points, and enables the two PM polarization states in the PNC-OFDR measurement to be simultaneously measured so that the optical PMD distribution can be accurately measured.

  That is, the first characteristic point is that the measurement light S13 is branched into light S20 and S21 by the optical directional coupler 22C, and one of the branched light S20 is subjected to polarization adjustment by the polarization controller 23B, and the light S22. It becomes. In order to distinguish the other branched light S21 from the branched light S20, the frequency F is shifted by a frequency shift device 30 (for example, an acoustooptic modulator). The reason for making a distinction here is to enable the frequency analyzer 29 to determine whether the data is generated data by the light S20 or the generated data by the light S21 at a later data processing stage. The light whose frequency (ΔF) is shifted adjusts the polarization through the polarization controller 23C and becomes the light S23. It should be noted that the polarization states of the light S22 and the light S23 need not be the same. The light S22 and the light S23 whose polarization states are adjusted are combined by the optical directional coupler 22D to become the light S14.

  The second feature is that the sampling device 28 collects data, the frequency analysis device 29 performs fast Fourier transform (FFT), the horizontal axis is frequency, and the vertical axis is amplitude term or phase term. Data processing is performed for the relationship when the FIG. 3 shows an image of the analyzed data.

  The frequency of the Fourier transformed data (amplitude term and phase term) is in two zones. That is, in the signal generated by the light S20, the Fourier-transformed data becomes the left zone, and has a frequency from −f to f (f is the maximum beat frequency depending on the object to be measured). Here, the Fourier-transformed data is called D1. On the other hand, the data generated by the light S21 is the right zone and has a frequency from ΔF−f to ΔF + f. Here, it is called data D2.

  The frequency of the data D2 is shifted to the right by ΔF from the frequency of the data D1. This frequency ΔF is shifted by the frequency shift device 30. What should be noted about the design of the frequency ΔF is that the two zones do not overlap. That is, ΔF−f> f must be satisfied.

In FIG. 3, the left zone (data D1) is amplitude distribution A ₁ (z) and phase distribution P ₁ (z) data, and the right zone (data D2) is amplitude distribution A ₂ (z) and phase distribution P. ₂ (z) data. By discarding the data D2, the amplitude distribution A ₁ (z) and the phase distribution P ₁ (z) are extracted. Here, the backscattered light amplitude distribution from each position in the measured optical circuit 25 due to the incidence of the light S20 and the distribution of the difference between the s-direction light phase and the p-direction light phase are measured, and the backscattered light due to the incidence of the light S20. The polarization state U ₁ (z) of is completely measured.

By discarding the data D1 and shifting the frequency F on the horizontal axis, the amplitude distribution A ₂ (z) and the phase distribution P ₂ (z) are extracted. Here, the backscattered light amplitude distribution from each position in the measured optical circuit 25 by the incidence of the light S21 and the distribution of the difference between the s-direction light phase and the p-direction light phase are measured, and the backscattered light by the incidence of the light S21. The polarization state U ₂ (z) of is completely measured.

By using the input polarization states I ₁ (0) and I ₂ (0) and the polarization states U ₁ (z) and U ₂ (z), R (z ), Calculate the birefringence vector [β _R (z)] using equation (3) using the calculated R (z), and use the obtained [β _R (z)] Assuming that the birefringence of is linear, the DGD from an arbitrary section z ₀ to z of the object to be measured is calculated by equation (4).

By doing in this way, it becomes possible to measure the polarization state of backscattered light simultaneously, and to obtain a PMD distribution measurement result with high reliability.
Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some configurations may be deleted from all the components shown in the embodiment. Furthermore, you may combine the component covering different embodiment suitably.

   DESCRIPTION OF SYMBOLS 11 ... Light source, 12 ... Two polarized wave production | generation apparatus, 12 '... Polarization controller, 13 ... Circulator, 14 ... Optical circuit to be measured, 15 ... Optical receiver, 16 ... Sampling apparatus, 17 ... 1st analyzer (bipolar) Analyzing the backscattered light polarization state of the wave), 17 '... analyzing device (analyzing the longitudinal distribution of the backscattered light polarization state), 18 ... second analyzing device (calculating and measuring the PMD distribution), 21 ... light source, 22A, 22B, 22C, 22D ... Optical directional coupler, 23A, 23B ... Polarization controller, 24 ... Circulator, 25 ... Optical circuit to be measured, 26 ... Polarization beam splitter, 27A, 27B ... Optical receiver, 28 ... Sampling device, 29, 29 '... frequency analysis device, 30 ... frequency shift device, 31 ... PMD distribution measurement analysis device.

Claims (2)

The output light of the light source is branched into two, one of the branched lights is incident on the object to be measured as measurement light, the reflected light is taken in, the reflected light and the reference light from the other branched light are combined, and the interference wave An optical reflectometry measurement method for measuring an interference signal by detecting light,
Generate two lights of different optical frequencies having different polarization states from the measurement light and simultaneously enter the object to be measured,
Measuring two interference signals in different polarization states at the same time while distinguishing them in the frequency domain,
Calculating a transfer function of the object to be measured from the two interference signals measured simultaneously, and measuring a birefringence distribution of the object to be measured ;
Calculate the longitudinal distribution of polarization mode dispersion using the measurement result of the birefringence distribution,
The difference ΔF between the optical frequencies of the two lights incident on the measurement target is set such that ΔF> 2 × f, where f is the maximum beat frequency by the measurement target in the two interference signals. To measure optical reflectometry. The output light of the light source is branched into two, one of the branched lights is incident on the object to be measured as measurement light, the reflected light is taken in, the reflected light and the reference light from the other branched light are combined, and the interference wave An optical reflectometry measuring device that measures an interference signal by detecting light,
Measurement light incident means for generating two lights of different optical frequencies having different polarization states from the measurement light and simultaneously entering the light to be measured;
Interference signal measuring means for simultaneously measuring two interference signals in the different polarization states while distinguishing them in the frequency domain;
Birefringence distribution analysis means for calculating a birefringence distribution of the measurement object by calculating a transfer function of the measurement object from the two interference signals measured simultaneously ;
Polarization mode dispersion distribution analyzing means for calculating a longitudinal distribution of polarization mode dispersion using the measurement result of the birefringence distribution , and
The difference ΔF between the optical frequencies of the two lights incident on the measurement target is set such that ΔF> 2 × f, where f is the maximum beat frequency by the measurement target in the two interference signals. Optical reflectometry measuring device.

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