Signal processing device for performing signal processing method in phase otdr

Abstract

The present disclosure provides a signal processing device that temporally tracks phase offsets at respective locations of an optical fiber from scattered light vectors at the respective locations of the optical fiber, said scattered light vectors being obtained by using a phase OTDR, and corrects the scattered light vectors at the respective locations by using the phase offsets corresponding to measurement times of the scattered light vectors.

Description

Signal processing device for performing signal processing method in phase OTDR

[0001] The present invention relates to a phase OTDR (Optical Time Domain Reflectometer) that measures the phase of scattered light from each point in an optical fiber under test.

[0002] In phase-shift OTDR, interference fading occurs, which causes sensitivity degradation at multiple points along the optical fiber. Phase averaging at multiple points along the fiber is an effective method for solving this problem [Non-Patent Document 1]. In Non-Patent Document 1, Moving Rotate-Vector-Average (MRVA) is proposed and demonstrated to perform phase averaging at multiple points with high sensitivity.

[0003] Non-Patent Document 1 is based on the assumption that the amount of phase change due to vibration or temperature change can be considered to be the same at each point within the spatial averaging window. However, the situation assumed by existing MRVA, in which the amount of phase change can be considered to be the same at each point within the spatial averaging window, is only valid when the amplitude of vibration or temperature change is small, and does not hold when the amplitude is large for the following reasons.

[0004] (Reason 1) Even if the phase changes at different points are responses to the same temperature change or vibration, they contain different nonlinearities due to interference from different scattering centers, so the amount of phase change is not the same and becomes more noticeable as the amplitude of the vibration or temperature change increases.

[0005] (Reason 2) Even if the same temperature change or vibration occurs within the spatial averaging window, the phase change at point z is an accumulation of phase changes before point z, so the amount of phase change is not the same, and this becomes noticeable when the amplitude of the vibration or temperature change becomes large.

[0006] Due to these factors, there will be times when the directions of the scattered light vectors at multiple points do not match even after phase rotation by MRVA, and at those times the length of the spatial average vector will not be maximized, resulting in a deterioration of accuracy. Therefore, when vibrations or temperature changes become large, there will be times at many points when high-precision measurements cannot be made with existing MRVA.

[0007] Heng Qian, Bin Luo, Haijun He, Yin Zhou, Xihua Zou, Wei Pan, and Lianshan Yan, “Fading-free Φ-OTDR evaluation based on the statistical analysis of phase hopping,” Appl. Opt. 61, 6729-6735 (2022).

[0008] The present disclosure aims to prevent the occurrence of time accuracy degradation even when the optical fiber is subjected to large vibrations or temperature changes.

[0009] A signal processing device according to the present disclosure executes a signal processing method according to the present disclosure, in which the signal processing device tracks the phase offset of each point on an optical fiber over time from the scattered light vector at each point on the optical fiber obtained using phase OTDR, and corrects the scattered light vector at each point using the phase offset corresponding to the measurement time of the scattered light vector.

[0010] The signal processing device may estimate the phase offset at any time based on scattered light vectors within a predetermined time window around that time, and repeatedly estimate the phase offset while sweeping the time window in the time axis direction.

[0011] The signal processing device may select one point within a predetermined spatial window as a reference point. In this embodiment, the signal processing device may estimate the phase offset of points other than the reference point within a spatial window of a predetermined window length when the phase offset of the reference point is set to zero.

[0012] The signal processing device may select a plurality of the reference points. In this embodiment, the signal processing device may calculate a spatial average vector for each of the reference points by averaging the corrected scattered light vectors at each of the points within the spatial window, calculate a time change in phase at each of the points using the spatial average vector for each of the reference points, and average the time change in phase calculated for each of the reference points.

[0013] The above disclosures can be combined as much as possible.

[0014] According to the present disclosure, even when the optical fiber is subjected to large vibrations or temperature changes, it is possible to prevent the occurrence of time that causes accuracy degradation.

[0015] 1 shows an example of the configuration of a measurement system according to the present disclosure; FIG. 2 is an explanatory diagram of an optical pulse; FIG. 3 shows a measurement method according to Non-Patent Document 1; FIG. 4 shows an example of an embodiment of the measurement method according to the present disclosure; FIG. 5 shows an example of an embodiment of the measurement method according to the present disclosure.

[0016] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0017] 1 shows an example of the configuration of a measurement system according to the present disclosure. The measurement system of this embodiment includes a measurement device 18 and a signal processing device 17. The measurement device 18 is a phase OTDR that measures the phase of scattered light from each point in the optical fiber under test. The phase OTDR of this embodiment uses optical frequency multiplexing to measure the phase at multiple different optical frequencies.

[0018] The frequency from CW light source 1 is f ０ A continuous light beam with a single wavelength of f is emitted and split into a reference light beam and a probe light beam by a coupler 2. The probe light beam is shaped into an optical pulse 4 shown in FIG. 2 by an optical modulator 3. The optical pulse 4 has a frequency of f ｉ = f ０ +Δf ｉ (i is an integer) and pulses whose pulse width is set to a value P corresponding to the spatial resolution of measurement in the longitudinal direction of the optical fiber are arranged in a sequence of i=1, 2, ..., N (N is an integer representing frequency multiplexing). ｉ is selected so that the scattered light intensities at each time and point are sufficiently separated so that they can be considered uncorrelated between different i. The pulse width P corresponds to the spatial resolution.

[0019] There is no specific designation for the type of optical modulator 3, and there may be more than one type as long as it can generate the optical pulse 4. For example, an SSB modulator or a frequency-tunable AO modulator may be used, and intensity modulation may be further performed using an SOA (semiconductor optical amplifier) ​​or the like to increase the extinction ratio in pulsing.

[0020] The pulsed probe light 4 is incident on the optical fiber 6 under test via the circulator 5. Light scattered at each point in the longitudinal direction of the optical fiber 6 under test returns to the circulator 5 as backscattered light and is incident on one input of the optical 90-degree hybrid 7. The reference light branched by the coupler 2 is incident on the other input of the optical 90-degree hybrid 7.

[0021] The internal configuration of the optical 90-degree hybrid can be any configuration as long as it has the functionality of an optical 90-degree hybrid. Two of the four outputs of the optical 90-degree hybrid are detected by a balanced detector 13 to obtain an analog in-phase component, or electrical signal 15. The remaining two outputs of the optical 90-degree hybrid are detected by a balanced detector 14 to obtain an analog quadrature component, or electrical signal 16.

[0022] The electrical signals 15 and 16 are sent to a signal processing device 17. The signal processing device 17 includes AD conversion function elements 17a and 17b and a processor 17c. The AD conversion function elements 17a and 17b are devices capable of sampling the frequency band of a signal without aliasing. The AD conversion function element 17a samples the electrical signal 15, and the AD conversion function element 17b samples the electrical signal 16. As a result, the I component and Q component of each optical frequency at each point z are input to the processor 17c.

[0023] 3 shows the measurement method of Non-Patent Document 1. Non-Patent Document 1 includes the following steps S11 to S16 in order. In this embodiment, an example is shown in which a spatial window having a window length of 2W is predetermined and set in the processor 17c.

[0024] There are no restrictions on the setting value of the spatial window length 2W, but the larger 2W is set, the more the precision and accuracy of the ultimately obtained distributed vibration measurement data improves, but the spatial resolution deteriorates. For this reason, the window length 2W is set based on the precision and accuracy of the distributed vibration measurement data required for measurement and the spatial resolution.

[0025] (Step S11) The measuring device 18 measures the I and Q components of a plurality of different optical frequencies using a phase-shift OTDR. (Step S12) The processor 17c calculates, for each point z, a scattered light vector r(z) on the IQ plane for each optical frequency from the I and Q components of each optical frequency.

[0026] (Step S13) Processor 17c estimates the phase offset within the spatial window corresponding to point z. Specifically, the phase offsets of multiple points z from point z-W to point z+W are estimated. Processor 17c then rotates the phase of the scattered light vector r(z) of multiple points z from point z-W to point z+W by -Φ(z). This makes it possible to correct the phase offsets of multiple points z from point z-W to point z+W.

[0027] (Step S14) The processor 17c averages the phase-rotated scattered light vector r(z) within a spatial window from point z−W to point z+W. As a result, the spatial average vector r at point z is calculated. ａｖｅ (z) can be calculated.

[0028] The processing of steps S13 and S14 is continued while sweeping the point z of the spatial window in the spatial axis direction. ａｖｅ (z) can be calculated.

[0029] (Step S15) The processor 17c calculates the spatial average vector r ａｖｅ The phase change at point z is calculated from the time change in the angle (z).

[0030] (Step S16) The processor 17c calculates the spatial difference between the phase change at z-D and the phase change at z+D, assuming a gauge length of 2D. This provides the phase change in the section (centered at z) bounded by the two points from z-D to z+D, making it possible to calculate the waveform of the temperature change and vibration occurring in that section.

[0031] In Non-Patent Document 1, the same phase offset value Φ is rotated at each point within the spatial window in step S13. However, when the amplitude of vibration or temperature change is large, the amount of phase change cannot be considered to be the same at each point within the spatial window. Therefore, when the amplitude of vibration or temperature change is large, the spatial average vector r ａｖｅ There is a possibility that the accuracy of (z) may be deteriorated.

[0032] The amount of phase change at each point within the spatial window can be considered as a temporal change in the effective phase offset (hereinafter referred to as the effective phase offset). Therefore, in the present disclosure, the effective phase offset at each point is tracked over time. In this embodiment, an example is shown in which a time window with a window length of 2T and a spatial window with a window length of 2W are pre-set in the processor 17c.

[0033] Specifically, processor 17c has the following functions: A first function is to track the effective phase offset at each point on optical fiber 6 over time from the scattered light vector at each point on optical fiber 6 obtained using phase OTDR; and a second function is to correct the scattered light vector at each point using the effective phase offset corresponding to the measurement time of the scattered light vector.

[0034] Since the processor 17c has the first function, it can obtain the effective phase offset in each time window. Then, since the processor 17c has the second function, it can correct the scattered light vector using the effective phase offset for each time window. In this way, the present disclosure maximizes the length of the spatially averaged vector over the entire measurement time, improving the accuracy of the phase calculation.

[0035] 4 shows an example embodiment of the measurement method of the present disclosure. In the present disclosure, processor 17c executes steps S21 to S26 instead of steps S12 to S16. Processor 17c executes a first function in steps S21 to S23 and executes a second function in steps S24 and S25.

[0036] (Step S21) The processor 17c calculates the scattered light vector r(z, t) at each point z and each time t.

[0037] (Step S22) The processor 17c determines one point within a predetermined spatial window as a reference point z Ｒ For example, the processor 17c selects a point from among a plurality of points within the spatial window from point z-W to point z+W where the average vector length of the scattered light vector r(z, t) is long between time t-T and time t+T in the time window as the reference point z. Ｒ Selected as.

[0038] (Step S23) The processor 17c estimates the effective phase offset at any time t based on the scattered light vector r(z, t) within a predetermined time window from time t-T to time t+T around the time t. The estimated value obtained in this way is referred to as the "effective phase offset estimated value Φ(z, t)." For example, the processor 17c estimates the effective phase offset at the reference point z Ｒ When the phase offset of the reference point z in the spatial window from point z−W to point z+W is set to zero, Ｒ This calculates the phase offset of the reference point z Ｒ The effective phase offset estimate Φ(z, t) at points other than z can be estimated with high accuracy.

[0039] (Step S24) The processor 17c rotates the scattered light vector at time t at each point from point z-W to point z+W in the spatial window by -Φ(z, t). The rotated scattered light vectors are then averaged within each spatial window. This results in a spatial average vector r ａｖｅ (z, t) is calculated.

[0040] (Step S25) The processor 17c rotates the scattered light vector at time t+Δt at each point from point z−W to point z+W in the spatial window by −Φ(z, t). The rotated scattered light vectors are then averaged within each spatial window. This results in a spatial average vector r ａｖｅ (z, t+Δt) is calculated.

[0041] (Step S26) The processor 17c calculates the spatial average vector r ａｖｅ (z, t) and r ａｖｅFrom the angle change of (z, t+Δt), the phase change Δθ from time t to time t+Δt at point z is calculated. ａｖｅ Calculate (z, t).

[0042] (Step S27) The processor 17c performs the same process at each point, and calculates the spatial phase difference S(z, t) between the two points as follows: S(z, t)=Δθ, where 2D is the gauge length. ａｖｅ (z+D, t)-Δθ ａｖｅ Calculate (z−D, t). Phase unwrapping may be performed as appropriate.

[0043] Here, in step S23, the processor 17c repeatedly estimates the effective phase offset Φ(z, t) while sweeping the time window time t in the time axis direction. The processes from step S23 to step S25 may be continued while sweeping the time window time t in the time axis direction. As a result, the spatial average vector r at each time t is ａｖｅ (z) can be calculated.

[0044] Here, as a specific method for tracking the effective phase offset in step S23, a time window may be set, the average value of the phase offset at each time within the time window is calculated at each point, and the calculated value is used as the effective phase offset estimate at the center time of the time window, and this process may be continued while sweeping the time window in the time axis direction. Note that the effective phase offset estimate is not limited to the center time of the time window, and may be any other time within the time window or a time near the time window.

[0045] When estimating the effective phase offset value in step S23, the reference point z Ｒ (S22), and the vector of the other point is selected as the reference point z Ｒ It is possible to correct the effective phase offset so that it is aligned in the direction of the vector z. In other words, it is possible to consider the effective phase offset at the reference point to be zero. However, Ｒ If the vector length is short, sensitivity degradation and waveform distortion may occur due to the rotation angle update, so it is possible to take measures to select only points with long vector lengths as reference points.

[0046] Furthermore, since the vector length changes over time, in step S23,Ｒ The reference point z can also be updated over time. Ｒ In this case, the measurement method of the present disclosure includes the procedure shown in FIG.

[0047] 5 shows an embodiment of the measurement method of the present disclosure. In the present disclosure, instead of steps S21 to S27, the processor 17c executes steps S31 to S34. (Step S31) In step S22, the processor 17c calculates a plurality of reference points z Ｒ (Step S32) The processor 17c selects the reference point z Ｒ As a result, the spatial phase difference S(z, t) between the two points is calculated based on the reference point z Ｒ (Step S33) The processor 17c calculates the distance between the plurality of reference points z Ｒ The processor 17c averages the phase differences S(z, t) at all points and all times to obtain new phase differences S(z, t). (Step S34) The processor 17c performs the same process for all points and all times to obtain new phase differences S(z, t).

[0048] In this way, multiple reference points z Ｒ Each selected point is a reference point z Ｒ In this case, the time change of the phase in the interval from z-D to z+D (centered at z) is calculated, and finally, the time change of the phase is averaged to calculate the final time change of the phase.

[0049] Other Embodiments The signal processing device 17 of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

[0050] As described above, by using the present disclosure, even in phase OTDR measurements with a simple configuration, it is possible to calculate phase changes with high accuracy over the entire measurement time, even when large vibrations or temperature changes are applied to the optical fiber 6. For example, phase OTDR is known as a technique capable of long-distance measurements in distributed vibration measurement (DAS), and can be used in situations such as estimating the surrounding environment in DAS measurements using optical fiber laid for communication, but because vibrations of various amplitudes, large and small, are applied to optical fiber laid for communication, the present disclosure can be used in situations such as when it is desired to measure these vibrations with high accuracy and accurately estimate the surrounding environment.

[0051] 1: CW light source 2: Coupler 3: Optical modulator 4: Optical pulse 5: Circulator 6: Optical fiber to be measured 7: 90-degree optical hybrid 13, 14: Balanced detector 15, 16: Electrical signal 17: Signal processing device 17a, 17b: AD conversion function element 17c: Processor 18: Measuring device

Claims

1. A signal processing device that tracks the phase offset of each point on an optical fiber over time from the scattered light vector at each point on the optical fiber obtained using a phase OTDR (Optical Time Domain Reflectometer), and corrects the scattered light vector at each point using the phase offset that corresponds to the measurement time of the scattered light vector.

2. The signal processing device according to claim 1, wherein the phase offset at any given time is estimated based on scattered light vectors within a predetermined time window around that time, and the estimation of the phase offset is repeated while sweeping the time window in the time axis direction.

3. A signal processing device as described in claim 2, wherein one point within a predetermined spatial window is selected as a reference point, and the phase offset of points other than the reference point within a spatial window of a predetermined window length is estimated when the phase offset of the reference point is set to zero.

4. A signal processing device as described in claim 3, which selects a plurality of reference points within the spatial window, calculates a spatial average vector for each reference point by averaging the corrected scattered light vectors at each point within the spatial window, calculates the time change in phase at each point using the spatial average vector for each reference point, and averages the time change in phase calculated for each reference point.

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