Optical fiber sensor and optical fiber sensing method

Abstract

To avoid an influence of fading without sacrificing spatial resolution excessively by determining an area of a search range for each distance from an incidence end on an optical fiber.SOLUTION: Light information acquisition means acquires a distribution of strength I(x) and a phase P(x) of signal light relative to a distance (x) from an incidence end of an optical fiber of signal light from a beat signal for each light pulse. Each position search range acquisition means acquires a search range LS(x) for a gauge length xGL for each position of an optical fiber from the distribution of the strength I(x) of signal light acquired by the light information acquisition means. Precision deterioration avoiding means determines two points for calculating a phase difference ΔP(x) at the distance (x) from the incidence end of the optical fiber of the signal light on the basis of the gauge length xGL and the search range LS(x). Vibration information demodulation means acquires the phase difference ΔP(x) between the two determined points and demodulates vibration information from the phase difference ΔP(x).SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] This invention relates to an optical fiber sensor and an optical fiber sensing method.

Background Art

[0002] With the development of optical fiber communication, technologies using the optical fiber itself as a sensing medium have been actively studied. In particular, optical fiber sensing using scattered light enables long-distance distributed sensing, which is different from electrical sensors that measure at a point.

[0003] Phase-sensitive Optical Time Domain Reflectrometry (φ-OTDR), known as a distributed optical fiber sensor using Rayleigh scattered light, has attracted attention in a wide range of fields due to its wide measurement range and high measurement sensitivity (see, for example, Non-Patent Document 1 or 2).

[0004] However, in φ-OTDR, since the Rayleigh scattered light intensity behaves randomly depending on the position on the optical fiber, the phase measurement accuracy also behaves randomly depending on the position (see, for example, Non-Patent Document 3). In particular, fading occurs at positions where the scattered light intensity is weak, resulting in distortion of the measurement waveform (see, for example, Non-Patent Document 4).

[0005] As methods for solving this problem, a method of frequency multiplexing probe light (see, for example, Non-Patent Document 5) and a method of modulating the optical phase of probe light (see, for example, Non-Patent Document 6) are known. In recent years, methods for suppressing the influence of fading only by signal processing, such as a method by analyzing the frequency domain of fading (see, for example, Non-Patent Document 7), a method using Nearest Neighbor Analysis (see, for example, Non-Patent Document 8), and a method by learning using teacher data (see, for example, Non-Patent Document 9), have also been proposed.

[0006] Furthermore, there is also an adaptive gauge length method proposed by the inventors of this application, in which the optical phase at a certain position is substituted with the optical phase at the position with the highest intensity in its vicinity (see, for example, Patent Document 1 or 2, or Non-Patent Document 10). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-159915 [Patent Document 2] Japanese Patent Publication No. 2021-103107 [Non-patent literature]

[0008] [Non-Patent Document 1] Yuelan Lu, Tao Zhu, Liang Chen, and Xiaoyi Bao, "Distributed Vibration Sensor Based on Coherent Detection of Phase-OTDR," J. Lightwave Technol. 28, 3243-3249 (2010). [Non-Patent Document 2] Z. Pan, K. Liang, Q. Ye, H. Cai, R. Qu, and Z. Fang, "Phase-sensitive OTDR system based on digital coherent detection," in Optical Sensors and Biophotonics, J. Popp, D. Matthews, J. Tian, ​​and C. Yang, eds., Vol. 8311 of Proceedings of SPIE (Optica Publishing Group, 2011), paper 83110S. [Non-Patent Document 3] A. K. Wojcik, “Signal statistics of phase dependent optical time domain reflectometry,” Ph.D. dissertation, Dept. Electr. Eng. Comput. Sci., Texas A&M University, College Station, TX, USA 2006. Available: <http: / / hdl.handle.net / 1969.1 / 4873>. [Non-Patent Document 4] Healey, P, “Fading in heterodyne OTDR”, Electronics Letters 20(1), 30-32(1984). [Non-Patent Document 5] Y. Lu, X. Zhang, C. Liang, M. Chen, J. Wang, and Z. Meng, “Fading noise reduction in distributed vibration measurements utilizing multi-wavelength based on φ-OTDR”, in 26th International Conference on Optical Fiber Sensors, OSA Technical Digest(Optical Society of America, 2018), paper TuE21. [Non-Patent Document 6] X. Wang et al., "Interference-Fading-Free φ-OTDR Based on Differential Phase Shift Pulsing Technology," in IEEE Photonics Technology Letters, vol. 31, no. 1, pp. 39-42, 1 Jan.1, 2019. [Non-Patent Document 7] Yue Wu, Zinan Wang, Ji Xiong, Jialin Jiang, Shengtao Lin, and Yongxiang Chen, "Interference Fading Elimination With Single Rectangular Pulse in ヨ -OTDR," J. Lightwave Technol. 37, 3381-3387 (2019) [Non-Patent Document 8] Guojie Tu, Mengmeng Zhao, Zheng Tang, Kai Qian, and Benli Yu, "Fading Noise Suppression in φ-OTDR Based on Nearest Neighbor Analysis," J. Lightwave Technol. 38, 6691-6698 (2020). [Non-Patent Document 9] Fei Jiang, Zhenhai Zhang, Zixiao Lu, Honglang Li, Yahui Tian, Yixin Zhang, and Xuping Zhang, "High-fidelity acoustic signal enhancement for phase-OTDR using supervised learning," Opt. Express 29, 33467-33480 (2021) [Non-Patent Document 10] N. Yamashiro, Y. Kanda, H. Murai, and H. Sasaki, "Adaptive Gauge Length Method to Avoid Fading Effect for Phase-sensitive OTDR," in Optical Fiber Sensors Conference 2020 Special Edition, G. Cranch, A. Wang, M. Digonnet, and P. Dragic, eds., OSA Technical Digest (Optical Society of America, 2020), paper T2A.2. [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Here, the adaptive gauge length method described above substitutes the optical phase at a certain position with the optical phase at the position with the highest intensity in its vicinity. Therefore, it results in a degradation of spatial resolution depending on the width of the search range. In conventional adaptive gauge length methods, a single search range was determined over the entire optical fiber.

[0010] However, the average scattered light intensity differs between the near-entry end of the optical fiber (one end) and the near-termination end (the other end). Therefore, the required search range to avoid fading also differs. Despite this, conventional adaptive gauge length methods set the search range to avoid fading near the termination.

[0011] As a result, while the conventional adaptive gauge length method can avoid fading, it sets an excessive search range near the incident edge, which may degrade the spatial resolution.

[0012] This invention has been made in view of the above-mentioned problems, and the object of this invention is to provide an optical fiber sensor and an optical fiber sensing method that avoid the effects of fading without excessively impairing spatial resolution by determining the width of the search range for each distance from the incident end on the optical fiber. [Means for solving the problem]

[0013] To achieve the above-mentioned objectives, the optical fiber sensor of this invention comprises a light source unit that generates optical pulses as probe light, a light receiving unit that generates a beat signal by coherently detecting the signal light including backscattered light generated at the object to be measured by the probe light, and a calculation unit to which the beat signal is input. The calculation unit comprises optical information acquisition means, position search range acquisition means, precision It includes a deterioration avoidance means and a vibration information demodulation means.

[0014] The optical information acquisition means acquires, for each optical pulse, the distributions of the intensity I(x) and the phase P(x) of the signal light with respect to the distance x from the incident end of the optical fiber of the signal light from the beat signal. Each position search range acquisition means determines, for each position of the optical fiber, the gauge length x GL with respect to the search range L S (x). The accuracy deterioration avoidance means determines two points for calculating the phase difference ΔP(x) at the distance x from the incident end of the optical fiber of the signal light based on the gauge length x GL and the search range L S (x). The vibration information demodulation means acquires the phase difference ΔP(x) between the two determined points and demodulates the vibration information from the phase difference ΔP(x).

[0015] According to the preferred embodiment of the optical fiber sensor described above, each position search range acquisition means adopts, for each position of the optical fiber, the minimum search range in which the expected value of the number of sections where the phase error given by the signal component and the noise component of the backscattered light becomes greater than or equal to a predetermined maximum phase error is less than 1.

[0016] Also, each position search range acquisition means may be configured to acquire a search range that is shorter on the input end side of the optical fiber and longer on the terminal end side.

[0017] According to a further preferred embodiment of the optical fiber sensor of this invention, the accuracy deterioration avoidance means, for each position x of the optical fiber, on the incident end side, when s is changed from x - x GL to x - x GL - L S (x), the incident end side position where the minimum value of the scattered light intensity in a plurality of optical pulses becomes the maximum, and for each position x of the optical fiber, on the terminal end side, when s is changed from x + x GL to x + x GL + L SWhen the value is changed up to (x), the terminal position where the minimum scattered light intensity of multiple light pulses is maximized is determined as the two points used to calculate the phase difference ΔP(x).

[0018] Furthermore, the optical fiber sensing method of this invention comprises the following steps. First, an optical pulse is generated as probe light. Next, a beat signal is generated by coherent detection of the signal light, including the backscattered light generated in the optical fiber by the probe light. Next, for each optical pulse, the distribution of the signal light intensity I(x) and phase P(x) with respect to the distance x from the incident end of the optical fiber is obtained from the beat signal. Next, from the distribution of the signal light intensity I(x), the gauge length x is determined for each position in the optical fiber. GL Search range L for S Get (x). Next, gauge length x GL and search range L S Based on (x), two points are determined at a distance x from the incident end of the optical fiber of the signal light to calculate the phase difference ΔP(x). Next, the phase difference ΔP(x) between the two determined points is obtained, and the vibration information is demodulated from the phase difference ΔP(x).

[0019] According to the preferred embodiment of the optical fiber sensing method described above, the search range L S In the process of obtaining (x), the smallest search range is obtained for each optical fiber position such that the expected value of the number of intervals in which the phase error given by the signal and noise components of the backscattered light is greater than or equal to a predetermined maximum phase error is less than 1.

[0020] Also, the search range L S In the process of obtaining (x), a configuration may be used in which the search range obtained is shorter at the input end of the optical fiber and longer at the termination end.

[0021] Furthermore, according to a further preferred embodiment of the optical fiber sensing method of this invention, in determining the two points for calculating the phase difference ΔP(x), for each position x of the optical fiber, s is set xx on the incident end side. GL From, xx GL -L SWhen (x) is varied, the incident end position where the minimum scattered light intensity of multiple light pulses is maximized is determined, and for each position x in the optical fiber, s is set to x+x on the terminal side. GL From x+x GL +L S When it is changed up to (x), The terminal position where the minimum scattered light intensity of multiple light pulses is maximized is determined as the two points used to calculate the phase difference ΔP(x). [Effects of the Invention]

[0022] According to the optical fiber sensor and optical fiber sensing method of this invention, by determining the width of the search range for each distance from the incident end on the optical fiber, the effects of fading can be avoided without excessively impairing spatial resolution. [Brief explanation of the drawing]

[0023] [Figure 1] This is a schematic diagram illustrating the basic principles of fading suppression. [Figure 2] This is a schematic diagram illustrating the optical fiber sensor of this invention. [Figure 3] Figure (1) illustrates the operation of the optical fiber sensor of this invention. [Figure 4] Figure (2) illustrates the operation of the optical fiber sensor of this invention. [Figure 5] Figure (3) illustrates the operation of the optical fiber sensor of this invention. [Modes for carrying out the invention]

[0024] The embodiments of this invention will be described below with reference to the figures, but these are merely schematic representations to the extent that the invention can be understood. Furthermore, preferred configurations of this invention will be described below, but these are merely examples. Therefore, this invention is not limited to the following embodiments, and many modifications or alterations can be made to achieve the effects of this invention without departing from the scope of the configuration of this invention.

[0025] (Basic principles of fading suppression) Refer to Figure 1 to explain the basic principle of fading suppression. Figure 1 is a schematic diagram illustrating the basic principle of fading suppression.

[0026] The optical fiber sensor comprises a light source unit 10, an optical circulator 20, an optical fiber 30, and a measurement unit 40. The measurement unit 40 further comprises a light receiving unit 50 and a calculation unit 60. This vibration-sensing optical fiber sensor is used, for example, in an OTDR (Optical Timer Detection) system.

[0027] The light source unit 10 is configured, for example, with a laser light source 11, an optical fiber coupler 12, an intensity modulator 13, an optical amplifier 14, an optical filter 15, and a frequency shifter 16.

[0028] The laser light source 11 generates laser light as continuous light in the communication wavelength band. It is preferable to use a so-called narrow-linewidth laser with stable frequency and a linewidth of 10 kHz or less as the laser light source 11. The wavelength of the laser light can be arbitrary, but it is preferable to use 1550 nm for low loss in a standard single-mode optical fiber. The laser light generated by the laser light source 11 is sent to the optical fiber coupler 12.

[0029] The optical fiber coupler 12 splits the laser beam into a probe beam and a local beam. One of the split probe beams is sent to the intensity modulator 13. The other split local beam is sent to the frequency shifter 16.

[0030] The intensity modulator 13 converts the probe light into an optical pulse using an electrical pulse input from outside the intensity modulator 13, thereby generating an optical pulse. This optical pulse is sent to the optical amplifier 14. The optical pulse generated by the intensity modulator 13 has, for example, a pulse width of 100 ns and a repetition frequency of 5 kHz. The optical pulse takes 5 ns to propagate 1 m through the optical fiber 30, which is the object to be measured. When observing the backscattered light generated in the optical fiber 30 as the signal light, a delay of 10 ns per meter occurs because the time required for forward propagation and backward propagation is needed. For example, when the pulse width is 100 ns and the repetition frequency is 5 kHz... The spatial resolution is 10m, and the maximum measurement distance is 20km.

[0031] For example, an acoustic-optical modulator (AOM) can be used as the intensity modulator 13.

[0032] The probe light generated by the intensity modulator 13 is amplified by the optical amplifier 14. This is because the stronger the intensity of the probe light, the stronger the intensity of the backscattered light in the optical fiber 30. The probe light amplified by the optical amplifier 14 is sent to the optical filter 15.

[0033] The optical filter 15 filters out the spontaneously emitted light (ASE: Amplified) generated by the optical amplifier 14. The noise from spontaneous emissions is removed. The probe light, from which the noise has been removed by the optical filter 15, is sent to the optical fiber 30 via the optical circulator 20.

[0034] The probe light sent to the optical fiber 30 propagates through the optical fiber 30. Backscattered light is generated as the probe light propagates. This signal light, including the backscattered light, is sent to the light receiving unit 50 via the optical circulator 20. Although not shown or explained here, an optical amplifier and an optical bandpass filter (BPF) are often provided before the light receiving unit 50 to amplify the backscattered light.

[0035] On the other hand, the local light sent to the frequency shifter 16 has its optical frequency shifted at the frequency shifter 16. This frequency shifter 16 adjusts the optical frequency of the local light according to the detection method. For example, when using heterodyne detection, it is used to create a frequency difference between the local light and the scattered light. When using homodyne detection, it is used to match the frequencies of the local light and the scattered light. Depending on the frequency difference between the local light and the scattered light, a configuration without the frequency shifter 16 may be used.

[0036] For example, when an AOM is used in the intensity modulator 13, the frequency of the optical pulse generated by the intensity modulator 13 changes from the frequency of the laser light input to the intensity modulator 13 due to the optical Doppler effect. For this reason, a frequency shifter 16 may be provided when using a homodyne detection method. On the other hand, a frequency shifter 16 may not be provided when using a heterodyne detection method.

[0037] The light-receiving unit 50 generates an electrical signal by coherently detecting the backscattered light generated in the optical fiber 30 using probe light.

[0038] The light-receiving unit 50 is comprised of a coherent receiver 52, a balanced photodiode (PD) 54, and an analog-to-digital (A / D) converter 56.

[0039] The coherent receiver 52 performs coherent detection of backscattered light using a reference light. For example, a 90° optical hybrid coupler can be used as the coherent receiver 52. The output from the coherent receiver 52 is sent to the balanced PD 54.

[0040] The balanced PD54 performs balanced detection of the output from the coherent receiver 52. This generates I-phase (cosine wave) and Q-phase (sine wave) beat signals that contain information about the intensity and phase of the backscattered light. These I-phase and Q-phase beat signals are sent to the A / D converter 56.

[0041] The A / D converter 56 converts the I-phase and Q-phase beat signals into digital signals. The beat signals of phase I and phase Q, converted into barrel signals, are input to the calculation unit 60.

[0042] For example, a commercially available personal computer (PC) can be used as the arithmetic unit 60. Here, as an example, the arithmetic unit 60 is a CPU (Central Processing Unit). The CPU will be described as comprising a CPU 70, a RAM (Random Access Memory) 62, a ROM (Read Only Memory) 64, and a storage means 66. The CPU 70 implements each of the functional means described later by executing the program stored in the ROM 64. The processing results of each functional means are temporarily stored in the RAM 62.

[0043] The arithmetic unit 60 includes functional means such as optical information acquisition means 72, search range acquisition means 74, accuracy degradation avoidance means 76, and vibration information demodulation means 78. The operation of each of these functional means depends on the configuration of the light receiving unit 50. Here, as a coherent receiver 52, an example of heterodyne detection using, for example, an optical 90° hybrid coupler will be described.

[0044] The optical information acquisition means 72 calculates the signal light intensity (also called scattered light intensity) I(x, k) and the signal light phase (also called scattered light phase) P(x, k) from the cosine and sine waves of the beat signal obtained by the A / D converter 56. Here, x is the distance from the incident end of the optical fiber 30, and k is the number of the optical pulse repeatedly incident as probe light.

[0045] In calculating the scattered light intensity I(x,k) and scattered light phase P(x,k), the optical information acquisition means 72 first generates the optical complex field of the backscattered light from the cosine wave and the sine wave.

[0046] Since the complex optical field of backscattered light is also a beat signal, it needs to be down-converted. Therefore, the complex amplitude of the backscattered light is down-converted. One method of this down-conversion is to multiply the complex optical field of the backscattered light by a counter-rotating complex sine wave with a beat frequency and then apply a low-pass filter (LPF).

[0047] Subsequently, the scattered light intensity I(x,k) is obtained by calculating the square of the absolute value of the complex amplitude obtained by downconversion. Furthermore, the scattered light phase P(x,k) is obtained by calculating the phase of the complex amplitude.

[0048] The search range acquisition means 74 determines the search range L in the adaptive gauge length method so that fading does not occur. S Determine the search range L. s If the search range L is too wide, the effects of fading can be avoided, but the spatial resolution will deteriorate. S If the search range L is too narrow, the degradation of spatial resolution can be minimized, but the effects of fading cannot be avoided. Therefore, an appropriate search range L is necessary. S It is necessary to set the appropriate search range L. S The setting method can be simply by checking the measurement results, or it can be the quantitative setting method disclosed in Patent Document 2.

[0049] Generally, in a φ-OTDR that measures phase, a certain gauge length x GL For this, the vibration waveform at position x on the optical fiber is P(x+x GL / 2,k)-P(xx GL It is calculated using the formula / 2,k). At this time, I(x+x GL / 2,k) and I(xx GL If the (2,k) value is weak, it will be affected by fading.

[0050] Therefore, for each position x, s is set to xx on the incident end side. GL From, xx GL -L SWhen the value is changed to the specified range, the incident end position y1 at which the minimum scattered light intensity I(s,k) of multiple light pulses is maximized is derived. Furthermore, for each position x, s is set to x+x towards the terminal side. GL From x+x GL +L S When the value is changed to the minimum, the terminal position y2 at which the scattered light intensity I(s,k) of multiple light pulses is maximized is derived. Then, the vibration information demodulation means performs For a given pulse k and optical fiber position x, P(y2,k)-P(y1,k) is calculated, and this calculation result is unwrapped in the direction of the optical pulse k to demodulate the vibration information applied to the optical fiber. In this way, the adaptive gauge length method avoids the effects of fading by substituting the phase at a certain position with the phase at the position with the highest intensity in the vicinity.

[0051] (Embodiment) An embodiment of the optical fiber sensor of this invention will be described with reference to Figures 2 to 5. Figure 2 is a schematic diagram illustrating the optical fiber sensor of this invention. Figures 3 to 5 are diagrams illustrating the operation of the optical fiber sensor of this invention.

[0052] The optical fiber sensor of this invention differs in that it is equipped with position search range acquisition means 75 instead of the search range acquisition means 74 shown in Figure 1, which was used to explain the basic principle of fading suppression. Other configurations are the same as those described with reference to Figure 1, so redundant explanations may be omitted.

[0053] Figure 3 shows an example of φ-OTDR when the adaptive gauge length method is not used. Figure 3(A) shows the scattered light intensity as a function of the optical fiber position. In Figure 3(A), the horizontal axis shows the position from the incident end of the optical fiber [unit: km], and the vertical axis shows the scattered light intensity in an arbitrary unit (au). Figure 3(B) shows the measurement results of φ-OTDR. In Figure 3(B), the horizontal axis shows the position from the incident end of the optical fiber [unit: km], and the vertical axis shows time [unit: ms]. In Figure 3(B), the grayscale indicates the phase.

[0054] Here, we show the results of applying a 100Hz sine wave near the end of an optical fiber with a length of 25km.

[0055] In φ-OTDRs where the adaptive gauge length method is not used, vertical lines due to noise can be seen at various positions, as shown in Figure 3(B).

[0056] Figure 4 shows the measurement results of φ-OTDR in an optical fiber sensor using the conventional adaptive gauge length method shown in Figure 1. In Figures 4(A) and (B), the horizontal axis represents the position from the incident end of the optical fiber [unit: km], and the vertical axis represents time [unit: ms]. In Figures 4(A) and (B), the intensity of the colors indicates the phase. Note that Figures 4(A) and (B) show an enlarged view of the range from 22 km to 25 km.

[0057] Here, the search range L in Figures 4(A) and (B) S These are all constant. Figure 4(A) shows the search range L S Figure 4(B) shows the case where the distance is 24m, and the search range L S This shows the case where the distance is 27m. In Figure 4(A), fading occurs around 22.5km and 24.3km. On the other hand, in Figure 4(B), it can be seen that the effects of these fadings have been eliminated.

[0058] Therefore, in φ-OTDR using the conventional adaptive gauge length method, the search range L S Approximately 27 meters is needed.

[0059] However, this required search range L S The 27m range is the search range necessary to avoid the effects of fading beyond 20km, and is considered excessive for the vicinity of the optical fiber's input end. Therefore, in the optical fiber sensor of this invention, each position search range acquisition means 75 acquires a search range L for each position. S To decide.

[0060] The signal component of scattered light is IS , noise component is I N In this case, the phase error φ is given by the following equation (1).

[0061]

number

[0062] Noise component is I N When this is the case, the maximum phase error φ m Signal component I that gives S0 This is given by equation (2) below.

[0063]

number

[0064] Therefore, noise component I N Given a phase error φ, the maximum phase error φ m To achieve the following, the following equation (3) must be satisfied.

[0065]

number

[0066] Furthermore, generally speaking, the intensity distribution P(I) of Rayleigh scattered light in an optical fiber follows an exponential distribution shown in equation (4) below.

[0067]

number

[0068] Therefore, the phase error in a given interval is given by the probability p of equation (5) below. m (=Pr(I S SO )) and it becomes φm or greater.

[0069]

number

[0070] In this case, the optical fiber length is L, and the distance over which the distribution can be considered independent is L. d , search range L S Therefore, from the theory of adaptive gauge length, the following equation (6) shows that the phase error is φ in the entire optical fiber as a result of using the adaptive gauge length method. m This can be estimated as the number of sections that exceed this limit.

[0071]

number

[0072] Therefore, the search range L S However, when the following equation (7) is satisfied, the expected value is that the phase error is φ m The number of sections where this occurs is less than 1, and the phase error is φ throughout the entire optical fiber. m This does not result in any problems, and the effects of fading can be suppressed.

[0073]

number

[0074] By rearranging equation (7) above, we obtain the following equation (8).

[0075]

number

[0076] If the smallest search range Ls that satisfies equation (8) above is adopted, the phase error is φ m This minimizes the degradation of spatial resolution without exceeding the limit.

[0077] Therefore, each position search range acquisition means 75 uses the above formula (5), Let L be the scattered light intensity around each position in the optical fiber, and define the search range L for each position. S The search range acquisition means 75 for each position is calculated. S The subsequent processing after the decision can be carried out in the same way as before, so redundant explanations will be omitted.

[0078] The measurement results using the optical fiber sensor of this invention will be explained with reference to Figure 5. Figure 5 shows the results obtained with the optical fiber sensor of this invention. Figure 5(A) shows the measurement results of φ-OTDR. In Figure 5(A), the horizontal axis represents the position from the incident end of the optical fiber [unit: km], and the vertical axis represents time [unit: ms]. Also, in Figure 5(A), the intensity of the color indicates the phase.

[0079] Figure 5(B) shows the search range L at each position of the optical fiber. S This is shown in Figure 5(B). In Figure 5(B), the horizontal axis represents the position from the input end of the optical fiber [unit: km], and the vertical axis represents the search range L. S The unit is shown in meters. Also, in Figure 5(B), the search range L of the optical fiber sensor of this invention is shown. S The curve I shows the search range L in a conventional optical fiber sensor using an adaptive gauge length method. S This is shown by the line II.

[0080] Here, the maximum phase error φ m The value was set to π / 4, and the average scattered light intensity at each position was calculated using the average of the scattered light intensity over a 100m radius surrounding each position. Furthermore, the noise component I was also considered. N This can be defined as the scattered light intensity observed when no light pulse is present.

[0081] As shown in Figure 5(A), the optical fiber sensor of this invention does not show the vertical lines caused by noise seen in Figures 3(B) and 4(A), thus avoiding the effects of fading. Furthermore, as shown in Figure 5(B), the search range Ls is approximately 10m near the input end and around 10km from the input end, compared to the 27m of the conventional technology, for a total optical fiber length of 25km. This indicates that excessive degradation of spatial resolution is suppressed.

[0082] The accuracy degradation avoidance means 76 sets s to xx on the incident end side for each position x. GL From, xx GL -L S When the value is varied up to (x), derive the incident end position y1 at which the minimum scattered light intensity I(s,k) of multiple light pulses is maximized. Also, for each position x, set s to x+x towards the terminal side. GL From x+x GL +L S When the value is varied up to (x), the terminal position y2 at which the minimum scattered light intensity I(s,k) of multiple light pulses is maximized is derived.

[0083] Then, the vibration information demodulation means calculates P(y2,k)-P(y1,k) for the optical pulse k and optical fiber position x, unwraps this calculation result in the direction of the optical pulse k, and demodulates the vibration information applied to the optical fiber. In this way, the adaptive gauge length method avoids the effects of fading by substituting the phase at a certain position with the phase at the position with the highest intensity in the vicinity.

[0084] The vibration information demodulation means 78 acquires the phase difference at each position x. Furthermore, it unwraps the phase difference ΔP obtained in this calculation in the k direction. From the distribution of the phase difference ΔP, the vibration information demodulation means 78 acquires vibration information at reception time t by an arbitrarily preferred conventional known method. This reception time t corresponds to position x in the longitudinal direction of the optical fiber. Therefore, the vibration information acquisition means 78 can acquire vibration information at position x in the longitudinal direction of the optical fiber.

[0085] In the optical fiber sensor and optical sensing method of this invention, the search range L is shortened on the incident end side where the average scattered light intensity is relatively large, and lengthened on the terminal end side where it is relatively small. S This is determined according to the magnitude of the signal component of the scattered light intensity. As a result, the optical fiber sensor and optical sensing method of this invention can avoid the effects of fading without excessively impairing spatial resolution. [Explanation of Symbols]

[0086] 10 Light source section 11 Laser light source 12 Fiber Optic Couplers 13. Intensity modulator 14. Optical amplifier 15 Light Filters 16 Optical frequency shifter 18. Optical Amplifier 20 Light Circulator 30 optical fibers 40 Measurement section 50 Light receiving part 52 Coherent Receivers 54. Balanced Photodiode (PD) 56 Analog-to-Digital (A / D) Converter 60 Arithmetic section 62 RAM 64 ROM 66 Memory means 70 CPU 72 Optical information acquisition means 74. Means for obtaining the search range 75 Means for obtaining each location search range 76 Measures to avoid accuracy deterioration 78 Vibration information demodulation means

Claims

1. A light source unit that generates light pulses as probe light, A light receiving unit that coherently detects the signal light, including the backscattered light generated in the optical fiber by the probe light, to generate a beat signal, The calculation unit to which the beat signal is input and Equipped with, The aforementioned arithmetic unit, For each of the aforementioned optical pulses, an optical information acquisition means obtains from the beat signal the distribution of the signal light intensity I(x) and phase P(x) with respect to the distance x from the incident end of the optical fiber of the signal light, From the distribution of the signal light intensity I(x), the gauge length x is determined for each position of the optical fiber. ＧＬ Search range L for Ｓ Each means for obtaining the position search range to obtain (x), Gauge length x ＧＬ and the search range L Ｓ A means for avoiding accuracy degradation that determines two points used to calculate the phase difference ΔP(x) at a distance x from the incident end of the optical fiber of the signal light, based on (x), The phase difference ΔP(x) between the two determined points is obtained, and vibration information demodulation means is used to demodulate vibration information from the phase difference ΔP(x). Equipped with A fiber optic sensor characterized by the following features.

2. Each of the aforementioned location search range acquisition means is: For each position of the optical fiber, the smallest search range is obtained such that the expected value of the number of intervals in which the phase error given by the signal and noise components of the backscattered light is greater than or equal to a predetermined maximum phase error is less than 1. The optical fiber sensor according to feature 1.

3. Each of the aforementioned location search range acquisition means is: The search range obtained is shorter at the input end of the optical fiber and longer at the termination end. The optical fiber sensor according to feature 1.

4. The accuracy degradation avoidance means, for each position x of the optical fiber, on the incident end side, when s is changed from x - x ＧＬ to x - x ＧＬ - L Ｓ (x), the minimum value of the scattered light intensity with a plurality of optical pulses is the maximum value of the incident end side position, and for each position x of the optical fiber, on the end side, when s is changed from x + x ＧＬ to x + x ＧＬ + L Ｓ (x), the minimum value of the scattered light intensity with a plurality of optical pulses is the maximum value of the end side position, and the two points used to calculate the phase difference ΔP(x) are determined The optical fiber sensor according to any one of claims 1 to 3.

5. The process of generating optical pulses as probe light, The process involves coherently detecting the signal light, which includes backscattered light generated in the optical fiber by the probe light, to generate a beat signal. For each of the aforementioned optical pulses, the process of obtaining the distribution of the signal light intensity I(x) and phase P(x) with respect to the distance x from the incident end of the optical fiber of the signal light from the beat signal, From the distribution of the signal light intensity I(x), the gauge length x is determined for each position of the optical fiber. ＧＬ Search range L for Ｓ The process of obtaining (x), The gauge length x ＧＬ and the search range L Ｓ The process of determining two points for calculating the phase difference ΔP(x) at a distance x from the incident end of the optical fiber of the signal light, based on (x), The phase difference ΔP(x) between the two determined points is obtained, and the process of demodulating vibration information from the phase difference ΔP(x) is performed. Equipped with A method for sensing optical fibers characterized by the following features.

6. The search range L Ｓ In the process of obtaining (x), For each position of the optical fiber, the smallest search range is obtained such that the expected value of the number of intervals in which the phase error given by the signal and noise components of the backscattered light is greater than or equal to a predetermined maximum phase error is less than 1. The optical fiber sensing method according to feature 5.

7. The search range L Ｓ In the process of obtaining (x), The search range obtained is shorter at the input end of the optical fiber and longer at the termination end. The optical fiber sensing method according to feature 5.

8. In determining the two points for calculating the phase difference ΔP(x), For each position x in the optical fiber, s is set to x-x on the incident end side. ＧＬ From x - x ＧＬ -L Ｓ When the value is changed up to (x), the input end position where the minimum scattered light intensity of multiple light pulses is maximized is determined, and for each position x in the optical fiber, s is set to x + x on the terminal side. ＧＬ From x + x ＧＬ +L Ｓ When the value is changed up to (x), the terminal position where the minimum scattered light intensity of multiple light pulses is maximized is determined as the two points used to calculate the phase difference ΔP(x). The optical fiber sensing method according to any one of claims 5 to 7.

Images