Strain change measuring device and strain change measuring method

Abstract

This invention provides a strain change measurement device and a strain change measurement method, aiming to determine the distribution of vibration applied to each core wire after branching at a high sampling rate. The strain change measurement device of this invention, at a reference time, incident a first chirped pulse light with a frequency that varies linearly with time onto the fiber under test branched by a coupler, obtaining a signal of first scattered light relative to the first chirped pulse light. At each monitoring time, a second chirped pulse light that reverses the chirp of the first chirped pulse light on the time axis is incident on the fiber under test, obtaining a signal of second scattered light relative to the second chirped pulse light. The correlation between the waveform that reverses and shifts the signal of the first scattered light on the time axis and the waveform of the second scattered light signal is determined to be the maximum offset, and this offset is used to calculate the change in strain in the fiber under test from the reference time to each monitoring time.

Description

Technical Field

[0001] This invention relates to an apparatus and method for measuring strain changes in branched optical fibers. Background Technology

[0002] Methods for measuring fiber strain caused by vibrations along the long side of the fiber include OTDR (optical time domain reflectometry) and OFDR (optical frequency domain reflectometry) methods (see, for example, Non-Patent Document 1). In the OTDR method, a pulsed test light is incident on the fiber under test from the incident end, and the phase of the Rayleigh scattered light backscattered from various locations is measured. The strain applied to the fiber can be quantitatively measured in a distributed manner based on the phase change (see, for example, Non-Patent Document 2). In the OTDR method, after an incident pulse is incident on the fiber under test at a certain moment and the scattered light returns to the incident end, the next incident pulse can be incident on the fiber under test immediately, and measurements can be performed at other times. Therefore, strain measurement can be performed at a high sampling rate.

[0003] In the OFDR method, by incidenting continuous light with frequency scanning from the incident end onto the fiber under test and obtaining the complex amplitude of the backscattered Rayleigh light, the power spectrum of the Rayleigh scattered light at various locations is obtained based on the short-time Fourier transform. The strain applied to the fiber can be quantitatively measured according to the distribution of the spectral shift (see, for example, non-patent documents 3 and 4).

[0004] Furthermore, a method for measuring quasi-static strain or low-frequency vibration using frequency-scanning OTDR has been proposed (see, for example, non-patent documents 5 and 6). In this method, pulsed light is incident on the fiber under test while scanning the optical frequency, and the intensity of the scattered light after Rayleigh scattering is measured. To achieve the required sensitivity for target vibration detection, the Rayleigh scattered light power spectrum is measured by scanning a sufficient range of optical frequencies with adequate precision. Similar to the OFDR method, information such as strain is quantitatively measured based on the spectral shift.

[0005] However, the OTDR, OFDR, and frequency-scanning OTDR methods are independent of the specific implementation method, assuming that the fiber under test is not branched by an optical coupler. If the fiber under test is branched by an optical coupler, and the OTDR, OFDR, or frequency-scanning OTDR methods are still used, the scattered light from the fibers below the branch overlaps with each other, making it impossible to measure the strain of the fibers below the branch.

[0006] On the other hand, Patent Document 1 proposes a method that makes it possible to measure vibration distribution. Although Patent Document 1 mainly focuses on measuring the change in light loss at the lower part of the branch, it is also possible to measure strain changes caused by vibration, etc. by directly utilizing the method proposed in Patent Document 1.

[0007] In the method proposed in Patent Document 1, a structure is used in which a reflective element is present at the front end of each branch optical fiber to reflect light of the wavelength of the test light. The method utilizes the following approach: a signal of backscattered light propagating towards the reflective element, generated after the test light is reflected by the reflective element and propagates towards the incident end, is further reflected by the reflective element and propagates towards the incident end, thus being detected (hereinafter referred to as the ghost signal), to be added to the signal of ordinary scattered light (hereinafter referred to as the ordinary signal), which can be detected even without the reflective element. This backscattered light propagates towards the reflective element after the test light is reflected by the reflective element and propagates towards the incident end, and is thus detected as the total signal (hereinafter referred to as the total signal). If the number of branches of the branch optical fiber is set to an integer N, and each branch optical fiber is labeled with numbers from #1 to #N, and the distance from the incident end of branch optical fiber #n to the reflective element is set to L#n, then the ghost signal scattered at a distance z from the incident end of branch optical fiber #n can be visually considered as the ordinary signal scattered at a distance 2L#nz from the incident end. Therefore, if the scattered light power spectrum at each location of the total signal is calculated using the same method as when calculating the scattered light power spectrum at each location with only a normal signal, then, assuming no vibration or other disturbance is applied to the optical fiber during the propagation of the test light and the state of the scatterers, such as the spacing, remains unchanged, the calculated scattered light power spectrum at a distance z from the incident end is the same as the calculated scattered light power spectrum at a distance 2L#nz from the apparent distance of the incident end. Especially when the distances L#n from the incident ends of each branch optical fiber to the reflecting element are different, since the apparent distance 2L#nz is also different, if the offset is calculated in a way that maximizes the correlation value between the waveform of the scattered light power spectrum at a distance 2L#nz from the apparent distance of the incident end measured at the monitoring time and the scattered light power spectrum at a distance z measured at the reference time, then the optical loss variation can be measured based on the correlation value under this offset. Furthermore, since this offset is proportional to the changes in temperature and strain state, the changes in temperature and strain state relative to the reference time at the monitoring time can also be quantitatively measured. Therefore, it is also possible to measure vibrations as dynamic strain changes.

[0008] Existing technical documents

[0009] Patent Document 1: Japanese Patent Publication No. 2016-142618, Shingo Ohno, Tatsuya Okamoto, Kunihiro Toge, Tetsuya Manabe, "Testing Method and Apparatus for Long-Distance Optical Fibers with Branches"

[0010] Non-patent Document 1: Ali. Masoudi, T.P. Newson, “Contributed Review: Distributed optical fibre dynamic strain sensing,” Review of Scientific Instruments 87, 011501 (2016)

[0011] 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 (Optical Society of America, 2011), paper 83110S.

[0012] Non-patent Document 3: Da-Peng Zhou, Zengguang Qin, Wenhai Li, Liang Chen, and Xiaoyi Bao, “Distributed vibration sensing with time-resolved optical frequency-domain reflectometry,” Opt. Express 20, 13138 - 13145 (2012)

[0013] Non-patent Document 4: Da-Peng Zhou, Liang Chen, and Xiaoyi Bao, “Distributed dynamic strain measurement using optical frequency-domain reflectometry,” Appl. Opt. 55, 6735 - 6739 (2016)

[0014] Non-patent document 5: Yahei Koyamada, Mutsumi Imahama, Kenya Kubota, and Kazuo Hogari, "Fiber-Optic Distributed Strain and Temperature Sensing With VeryHigh Measurand Resolution Over Long Range Using C oherent OTDR," J.LightwaveTechnol.27,1142-1146(2009)

[0015] Non-patent literature 6: Sascha Liehr, Sven Munzenberger, and Katerina Kreber, “Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing,” Opt. Express 26, 10573-10588 (2018).

[0016] Non-patent document 7: Lihi Shiloh and Avishay Eyal, "Distributed acoustic and vibration sensing via optical fractional Fourier transform reflectometry," Opt.Express 23, 4296-4306 (2015)

[0017] In the method of Patent Document 1, OFDR or frequency-scanning OTDR is used as a basic technique for determining the Rayleigh scattering power spectrum. However, in strain measurement using OFDR, as described in Non-Patent Document 7, it is difficult to perform the desired frequency scan at a high speed.

[0018] Furthermore, in order to ensure excellent spatial resolution in OFDR, a sufficiently long time is required to measure the beat signal between the scattered light and the reference light, compared to the time required from the moment the test light, which has undergone frequency scanning, is incident until the scattered light, generated at the farthest point from the incident end (including the location on the surface), reaches the incident end. Based on this, the next frequency scanning light is incident and measured in the same amount of time. Since the above operation needs to be repeated to sample the vibration, the sampling rate during strain measurement is limited, and it is generally difficult to achieve a sampling rate on the order of kilohertz.

[0019] Furthermore, in frequency scanning OTDR, as described in Non-Patent Document 6, in order to measure the spectrum, the fineness of the optical frequency scan needs to be set to a very small value, and the second pulse needs to be incident and measured sequentially after the first pulse of the scanned pulse is incident from the incident end and the scattered light returns. Therefore, compared with the strain measurement technology of conventional OTDR, the sampling rate needs to be reduced, which is a significant disadvantage, especially in the measurement of long-distance optical fibers. Summary of the Invention

[0020] The purpose of this invention is to measure the distribution of strain changes in each core wire after branching using a high sampling rate.

[0021] In the strain change measuring device of the present invention

[0022] At a reference time, a first chirped pulse of light with a frequency that varies linearly with time is incident on the fiber under test branched by a coupler, and the signal of the first scattered light relative to the first chirped pulse light is obtained.

[0023] At each monitoring moment, a second chirped pulse, whose chirp is reversed in time with the first chirped pulse, is incident on the fiber being measured, and a signal of the second scattered light relative to the second chirped pulse is obtained.

[0024] The maximum offset is determined by finding the waveform that causes the signal of the first scattered light to be reversed and shifted on the time axis, and the waveform of the signal of the second scattered light. This offset is then used to calculate the change in strain in the measured optical fiber from the reference time to each monitoring time.

[0025] In the strain change measurement method of the present invention

[0026] The strain measurement device incident a first chirped pulse of light, with a frequency that varies linearly with time, onto the optical fiber being measured, which is branched by a coupler, and obtains the signal of the first scattered light relative to the first chirped pulse.

[0027] The strain measurement device incident a second chirped pulse, whose chirp is reversed on the time axis of the first chirped pulse, onto the optical fiber being measured, and obtains the signal of the second scattered light relative to the second chirped pulse.

[0028] The strain measurement device determines the maximum offset between the waveform that causes the signal of the first scattered light to be reversed and shifted on the time axis and the waveform of the signal of the second scattered light, and uses this offset to calculate the change in strain in the fiber being measured.

[0029] According to the present invention, it is possible to perform the distribution measurement of strain changes of each core wire after branching at a high sampling rate. Attached Figure Description

[0030] Figure 1 This illustrates an example of the system configuration of the present invention.

[0031] Figure 2 This represents a general outline of the strain change measurement method of the present invention.

[0032] Figure 3 This refers to the chirping situation when referring to something.

[0033] Figure 4 This indicates the chirping situation during monitoring.

[0034] Figure 5 This represents an example of an OTDR waveform when the incident pulse S0 is scattered by the scatterer S4 of the target interval S2 before reaching the reflecting end S3 of the fiber being measured S1.

[0035] Figure 6 This is an example of an OTDR waveform when an incident pulse S7, whose chirp is reversed relative to pulse 1, reaches the reflecting end S3 of the fiber being measured S1 and is reflected and then scattered by the scatterer S4 of the target interval S2.

[0036] Figure 7 This represents an example of a system configuration using a coherent detector structure. Detailed Implementation

[0037] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments shown below. These embodiments are merely illustrative, and the present invention can be implemented in various ways with modifications and alterations based on the knowledge of those skilled in the art. Furthermore, in this specification and the accompanying drawings, the same reference numerals denote identical constituent elements.

[0038] (Invention Summary)

[0039] The following technique is proposed: In the case of an optical fiber branch to be measured using an optical coupler, a structure is provided at the front end of each branch fiber that reflects light of the wavelength of the test light, thereby measuring the strain change generated in each branch fiber after branching at a high sampling rate.

[0040] The proposed method is characterized in that, although the incident pulse light is based on the OTDR method, two light pulses with mutually reversed chirps are used separately as the incident pulse light.

[0041] Specifically, two types of light pulses are used as incident light: a chirped light pulse with a frequency that increases linearly with time and a chirped light pulse with the same frequency range but a reversed chirped slope. At a certain time A, one type of light pulse is incident and the OTDR waveform is measured; at a certain time B, the other type of light pulse is incident and the OTDR waveform is obtained. Furthermore, the waveform obtained at time A, which is inverted on the time axis, is further offset on the time axis in a manner that maximizes its correlation with the OTDR waveform obtained at time B within a predetermined range of amplitudes with spatial resolution. The change in strain applied to the predetermined range from time A to time B is calculated based on the offset.

[0042] If the specific steps described above are used, the relationship between the measurement distance and the sampling rate in the OTDR method is directly established, thus enabling the measurement of strain changes at high sampling rates.

[0043] (Invention Effects)

[0044] Whether already set up or newly set up, as long as there is a structure at the front end of the optical coupler that contains elements (reflective filters, mirrors) to reflect the test light, it is possible to measure the distribution of vibration applied to each core wire after the coupler branch at a high sampling rate. For example, if we consider the case of constructing a sensor network by arranging optical fibers on a plane, in the case of constructing a network without branches, if a fault such as a break occurs midway, sensing cannot be performed at all locations after that point. However, if a network using a branch structure with couplers is used, it can have redundancy, etc. Therefore, a method for measuring strain changes in such a branch structure sensor network is provided.

[0045] Furthermore, by connecting the incident ends of multiple sensor networks covering different regions to multiple output sides of a single-input multiple-output optical coupler, and connecting the output end of a photometer to a single input side of the optical coupler, a unit that can simultaneously perform distributed vibration measurements in different regions using a single photometer can also be provided.

[0046] As another example, if we consider the use of fiber optic communication networks for sensing, then in access network methods that connect base stations to service user residences, the main characteristics of Passive Optical Network (PON) systems using branch structures with optical couplers as passive optical devices are that the couplers themselves are inexpensive, do not require power supply, are small, and the optical fibers in the front part of the coupler branch can be shared. In PON systems, reflection filters that cut off wavelengths other than communication light are generally placed in front of each service user ONU. Therefore, the present invention can also be used as a method for measuring the distributed vibration of each optical fiber after the optical coupler branch in these PON systems at a high sampling rate.

[0047] (Implementation Example 1)

[0048] Figure 1 This describes the system configuration used in this embodiment. The strain measurement device in this embodiment includes: a laser 1, a modulator 2, a circulator 3, a photodiode 7, an AD board 8, and a computer 9. The strain measurement device is connected to the optical fiber 4 being measured.

[0049] Continuous light is emitted from laser 1, shaped into chirped pulses by modulator 2, and incident on the fiber 4 to be measured via circulator 3. The fiber 4 is branched into N branch fibers with core wire numbers from #1 to #N via optical coupler 5. Reflective elements 6 are positioned near the distal end of each core wire to reflect the chirped pulses. The distances from the incident end to the reflective elements 6 of each branch fiber are designed to satisfy a condition that they are mutually larger than the spatial resolution determined by the pulse width of the pulsed light, or, in the case of measuring existing branch fibers, strain changes are measured on the branch fibers that satisfy this condition. Backscattered light generated as the chirped pulses propagate in the fiber 4 is detected by photodiode 7 via circulator 3. The signal of the scattered light, after photoelectric conversion by photodiode 7, is digitized by AD board 8 and transmitted to computer 9. The signal transmitted to computer 9 is temporarily stored in storage unit 10. The calculation unit 11 uses the signals obtained from the steps at different times and stored in the storage unit 10 to calculate the time change of the strain applied to each core of the optical fiber 4 being measured.

[0050] like Figure 2 As shown, the proposal method specifically comprises steps S111 to S113.

[0051] In step S111, at a reference time, a chirped pulse of light with an instantaneous frequency that changes linearly with time is incident on the optical fiber being measured, and the scattered light signal I is obtained. ref And it is stored in storage unit 10.

[0052] In step S112, at the monitoring time, the chirped pulse light (after chirp reversal compared to step S111) is incident on the fiber being measured, and the scattered light signal I is obtained. p And save it in storage unit 10. Step S112 is repeated at multiple monitoring moments when strain changes are to be tracked.

[0053] In step S113, in the calculation unit 11, the I to be obtained at the reference time is... ref The waveform after being reversed and shifted on the time axis, and the I obtained at each monitoring time. p The correlation between the waveforms is maximized by calculating the correlation with the inverted I. ref The offset is calculated, and the strain change from the reference time to each monitoring time is calculated using the calculated offset. Figure 2 These steps are summarized.

[0054] The following is a detailed explanation of the principles.

[0055] In step S111, at the reference time T = T ref At the incident end, a chirped pulse light given by the time waveform of Equation (1) is generated and incident on the fiber 4 to be measured.

[0056] [Mathematical formula 1]

[0057]

[0058] In equation (1), the time reference t is used, which sets the timing of the start of the incident pulse at the incident end to t=0, and has t=TT. ref The relational relationship. rect(x) is a function that takes the value 1 when 0 < x < 1, and 0 when 0 > x and 1 < x. Let the imaginary unit be j. τ p It is the optical pulse width, designed to accompany t as it travels from 0 through τ. p The instantaneous frequency changes linearly from ν0 to (ν0-δν) p ).

[0059] Figure 3 This represents the chirping situation. Equation (1) represents the time variation of the complex amplitude of the electric field of the incident pulse at the incident end. Therefore, if the distance from the incident end is set as z, the time variation of the complex amplitude of the electric field at any position of the incident pulse propagating in the positive z direction becomes equation (S0). However, it should be noted that in equation (S0), the overlap with the test light traveling in the negative z direction after being reflected by the reflecting element or the overlap with the scattered light is not considered. Let the speed of light in the optical fiber be c.

[0060] [Mathematical expression S0]

[0061]

[0062] The following calculation calculates the complex amplitude at the incident end when the scattered light generated by the propagation of the test light in the branched fiber #n (n = 1, 2, ..., N) travels in the negative z direction and propagates to the position at the incident end. The distance z from the incident end is defined as D < z < (D + τ). p The complex amplitude of the light scattered by the scatterer in the interval C / 2) is taken as the object. If the position of the scatterer i is set as a distance D+d from the incident end... i Let the scattering cross-section be a i Then the time waveform of the scattered light that is backscattered and returns to the incident end is given by equation (2).

[0063] [Mathematical formula 2]

[0064]

[0065] On the rightmost side of equation (2), the absolute value of the term that depends on each branch fiber #n is set as s. #n (D, t), let the deflection angle be θ #n (D, t). In equation (2), only the case where the distance z from the incident end is D < z < (D + τ) is considered. p The light scattered by the scatterer in the interval C / 2) is actually light scattered from scatterers in other intervals. Therefore, the sum of the scattered light from all scatterers becomes a waveform that is continuous in the horizontal time direction in the same way as the usual OTDR waveform without chirped pulses.

[0066] Furthermore, the following continues to focus on the data from D < z < (D + τ). p The principle of the proposed method is explained by the scattering of the scatterer in the interval C / 2), but the essence of the conclusion remains unchanged as it is when the discussion is extended to all intervals. Furthermore, in equation (2), * is used as the symbol for the convolution integral. In the following mathematical notation, * also only represents the convolution integral. The convolution integrals of f(t) and g(t) as functions of time t are defined by equation (S1).

[0067] [Mathematical expression S1]

[0068]

[0069] Furthermore, the expression for convolution integrals is also used as described in (S2). The expression on the left side of equation (S2) is used in equation (2).

[0070] [Mathematical expression S2]

[0071] f(t)*g(t)=(f(t)*g(t))(t)=(f*g)(t) Formula (S2)

[0072] The photodiode receives the scattered light described in equation (2) and outputs a signal I. ref In fact, it becomes the sum of the scattered light from each branch of the optical fiber, given by equation (3) using the impulse response h(t) of the photodiode and the t-independent proportionality coefficient A.

[0073] [Mathematical Expression 3]

[0074]

[0075] In equation (3), the time required for the scattered light from the incident end to further propagate to the photodiode 7 via the circulator remains unchanged regardless of whether this is considered or not, and is therefore considered zero. This required time is also ignored in the following discussion. Furthermore, the time required for the photoelectric converted electrical signal to be digitized by the AD board 8 and stored in the storage unit 10 of the computer 9 remains unchanged regardless of whether this is considered or not, and is therefore considered zero. This required time is ignored in the following discussion. Furthermore, the signal degradation that occurs during the process of digitizing the photoelectric converted electrical signal by the AD board 8 and storing it in the storage unit 10 of the computer 9 can be ignored by selecting an AD board 8 with sufficient time resolution, voltage resolution, and bandwidth. Therefore, equation (3) can be directly used as the waveform stored in the storage unit 10 of the computer 9 in step S111.

[0076] In step S112, at monitoring time T = T k A chirped pulse light, given by the time waveform of equation (4), is generated at the incident end and incident on the fiber 4 to be measured. However, the monitoring time needs to be such that the scattered light at the monitoring time does not overlap with the scattered light at the reference time or other monitoring times when it returns to the incident end.

[0077] [Mathematical Expression 4]

[0078]

[0079] In equation (4), the design is as follows: the timing of the start of the incident pulse at the incident end is set as t = 0 as the time reference, and the time from t to τ... p The instantaneous frequency changes from (ν0-δν) p The linear change is ν0. The convolution integral with the delta function is performed to shift the time axis.

[0080] Figure 4This represents the chirping situation. It exhibits the property of chirping being time-reversed relative to the chirped pulse incident at the reference time. Equation (4) represents the time variation of the complex amplitude of the electric field at the incident end of the incident light, but the time reference t in Equation (4) is re-evaluated to satisfy t = TT. k It should be noted that the t = TT used in equations (1) to (3) must be satisfied. ref The time reference t is different. By resetting the time reference t each time an incident pulse is received, the OTDR waveforms of the reference time or each monitoring time, with the timing of the incident pulse being received at the incident end as the origin, can be processed separately for subsequent explanation of the principle.

[0081] Calculate the complex amplitude of the test light scattered from the branch fiber #n (n = 1, 2, ..., N) as given by equation (4). Based on the test light being reflected by the reflective element 6 of the branch fiber #n (n = 1, 2, ..., N), the distance z from the incident end is D < z < (D + τ). p The scattering body in the interval C / 2) is backscattered, and the backscattered light is reflected by the reflecting element 6. The complex amplitude of the light returning to the incident end is taken as the object. Let L be the distance from the incident end to the reflecting element 6. #n .

[0082] First, for simplicity, before considering the situation where the distribution of the scatterer changes due to vibration applied to the fiber 4 under test relative to the reference time, we consider the situation where the fiber 4 under test is stationary without vibration applied relative to the reference time. In the case of no vibration applied, the time waveform of the scattered light that is backscattered and returns to the incident end is given by equation (5).

[0083] [Mathematical Expression 5]

[0084]

[0085] Similar to equation (2), it should be noted in equation (5) that only the distance z from the incident end is considered to be D < z < (D + τ). p The light scattered by the scatterer in the interval C / 2). Based on the calculation of the complex conjugate of equation (5), if the time is reversed, the relationship of equation (6) is obtained.

[0086] [Mathematical Expression 6]

[0087]

[0088] As can be seen from equation (6), the waveform on the left side of equation (6) is shifted in time by -τ relative to the waveform given by equation (2). p -4L #nThe waveform of / C. That is, the OTDR waveform of equation (5) and the OTDR waveform of equation (2) have a relationship in which their shapes are reversed in time. This aspect can be used Figure 5 and Figure 6 Understand qualitatively.

[0089] like Figure 5 As shown, when a chirped pulse S0 is incident on the fiber S1 being measured, if we consider the scattered light scattered from scatterers S41, S42, S43, and S44 located in interval S2 before being reflected by the reflecting element S3, the scattered light becomes S51, S52, S53, and S54 when the timing of the scattering from the first S41 is taken as a reference. Therefore, their overlapping waveform is S6.

[0090] On the other hand, such as Figure 6 As shown, when the chirped S7 relative to S0 is incident on the fiber S1 to be measured, the scattered light that is backscattered from the scatterers located in section S2 based on the temporary reflection by the reflective element S3 and returns to the incident end based on the reflection by the reflective element S3 is the same as the scattered light that is not reflected by the reflective element S3, but is backscattered from the scatterers S91, S92, S93, and S94 of fiber section S8, which are located at positions that are mirror images of the scatterers S41, S42, S43, and S44 of fiber section 2 relative to the reflective element S3 and are located further away from the incident end than the reflective element S3, and returns to the incident end. The reflectivity of the reflective element is set to 1. Therefore, the waveform of the scattered light generated by the scatterers S41 to S44 in the target interval S2 after the incident pulse S7 reaches the reflecting end S3 and is reflected is the same as the waveform of the scattered light S11 that overlaps with the scattered light S91, S92, S93, and S94 scattered from the scatterers S91, S92, S93, and S94 in the interval S8, namely 101, 102, 103, and 104.

[0091] It can be known Figure 5 The scattered light S6 shown is Figure 6 The scattered light S11 shown becomes a shape that is reversed relative to time. In fact, within the chirped pulse, the following properties still hold: there are a large number of scatterers, the distribution of which extends before and after interval S2, and the scattering coefficient is different for each scatterer, but in the case of an incident reversed chirp, a reversed shape is obtained.

[0092] However, L #n The value of is different in each branch of the fiber, therefore the test light is reflected by the reflective elements of each branch of the fiber, and the distance z from the incident end is D < z < (D + τ). pThe scatterer in the interval C / 2) is backscattered, and the backscattered light is reflected by the reflecting element. The timing of its return to the incident end varies for each branch fiber. At the timing of the aforementioned scattered light returning to the incident end from a certain branch fiber #X, the signal I output by the photodiode is... p，#X It becomes formula (7).

[0093] [Mathematical Expression 7]

[0094]

[0095] Here, it should be noted that the subscript #X is used to specify which core wire is being matched for timing. Hereafter, unless it is specifically stated which core wire the timing is being matched with, the received signal at the monitoring time will simply be recorded as I. p If D is defined as in equation (8) #n , #x Then equation (7) can be transformed using equation (6) as in equation (9).

[0096] [Mathematical Expression 8]

[0097] D #n，#X =2(L #n -L #X )+D formula (8)

[0098] [Mathematical Expression 9]

[0099]

[0100] Next, as a general case, consider the situation where the distribution of the scattering body changes due to the application of vibration to the fiber 4 being measured relative to the reference time. Regarding the fiber core #n, at the reference time, the scattering body located near the incident end at a distance D moves to a distance D+ΔD based on the sum of vibrations up to that point. #n Nearby, and with a length greater than τ p Uniform vibration at a scale of C / 2, i.e., dynamic strain ε #n (D) is applied at a distance D+ΔD #n Nearby, and the length from the incident end to the reflecting element 6 varies from L according to the sum of the vibrations applied to the whole. #n Change to L #n +ΔL #n .

[0101] At this point, based on the test light being reflected by the reflective element 6 of the branch fiber #n, the distance z from the incident end is calculated to be (D+ΔD). #n )<z<(D+ΔD #n +τ pThe time change E of the complex amplitude of the electric field of the backscattered light in the interval C / 2) is the light reflected back by the reflecting element 6 and returned to the incident end. #n [D, t, ΔD] #n ΔL #n , ε #n (D)]. In ΔD #n =ΔL #n =ε #n When (D) = 0, E #n [D, t, ΔD] #’ ΔL #n , ε #n (D)] is E given by equation (5) #n Generalization of (D, t).

[0102] E #n [D, t, ΔD] #n ΔL #n , ε #n (D) becomes relative to E due to the following two effects. #n The waveform of (D, t) offset in the direction of parameter t.

[0103] Impact 1:

[0104] Due to the accompanying change ΔD #n and change ΔL #n The variation in the propagation time of the test light and the scattered light, and the effect caused by the change in the timing of the scattered light reaching the incident end.

[0105] Impact 2:

[0106] Since the scatterers in the aforementioned interval are spaced apart from each other by (1+ε) #n (D) times, as the effect of the scattered light waveform change obtained from the interference pattern of the scattered light from each scatterer.

[0107] The following is an approximate evaluation of the magnitude of each influence.

[0108] • The magnitude of influence 1:

[0109] Since we consider the scattered light at a visual distance of 2L#nD from the incident end, the timing of the scattered light reaching the incident end will change by 2(2ΔL) due to the round-trip portion of the light. #n -ΔD) / c.

[0110] • The magnitude of influence 2:

[0111] Consider the scattered light power spectrum within the aforementioned interval. If the scattered light power spectrum at the reference time before the applied vibration is set as σ(D, ω), then according to the same consideration method as OFDR, if the interval between the scatterers at the monitoring time is (1+ε) #n (D) times, then the power spectrum of the scattered light σ(D, ω, ε) #n (D) becomes equation (10).

[0112] [Mathematical Expression 10]

[0113] σ(D,ω,∈ #n (D))=σ(D,(1+∈ #n (D))ω) Equation (10)

[0114] In this invention, a chirped pulse is used. Since there is a linear relationship between each frequency ω and time t, the power spectrum of the scattered light can correspond to the time waveform as shown in equation (11).

[0115] [Mathematical Expression 11]

[0116]

[0117] If equations (10) and (11) are used, the time waveform at the monitoring moment becomes equation (12).

[0118] [Mathematical Expression 12]

[0119]

[0120] Consider the rightmost transformation of equation (12) where the product between infinitesimal terms can be neglected. The time waveforms of equations (11) and (12) correspond to the square of the amplitude of the electric field of the scattered light, thus implying that due to ε #n The effect of (D) causes the time waveform of the squared amplitude to shift negatively with respect to the parameter t by ν0τ. p ε #n (D) / δν p If we take this as a reference, we can also know E. #n [D, t, ΔD] #n ΔL #n , ε #n (D)] Due to ε #n The effect of (D) relative to ε #n (D, t), except for the phase component which is independent of the angular frequency ω, is shifted negatively by ν0τ with respect to the parameter t. p ε #n (D) / δν p .

[0121] If the magnitudes of influence 1 and influence 2 are taken into account, then a phase component independent of angular frequency ω is used. Equation (13) is obtained.

[0122] [Mathematical Expression 13]

[0123]

[0124] Compare the magnitudes of the terms in the rightmost delta function of equation (13). If we consider measurements under actual conditions, the approximation of equation (14) can be used at most locations on the core wire #n. In addition, if equation (14) does not hold, implementation example 2, etc., described later, can also be used.

[0125] [Mathematical Expression 14]

[0126]

[0127] Using equation (14), equation (13) can be transformed into equation (15).

[0128] [Mathematical Expression 15]

[0129]

[0130] From equation (15), it can be seen that the ΔD can be approximately ignored. #n ΔL #n The effect of the change. If equation (15) is used, the test light is reflected by the reflective element of a branch fiber #X, and the distance z from the incident end is D+ΔD. #X <z<D+ΔD #X +τ p The scatterer in the C / 2 interval is backscattered. At the precise moment when the backscattered light is reflected back to the incident end by the reflecting element, the photodiode outputs a signal I. p，#X It becomes formula (16).

[0131] [Mathematical Expression 16]

[0132]

[0133] In equation (16), the parts that depend on n and m but have no effect in the following discussion are replaced by the new use of the real number θ. #n，#m This is used to represent the mathematical expression, which is then simplified.

[0134] In step S113, the intensity I of the scattered light measured at the reference time is calculated. ref (D, t) and the intensity of scattered light I measured at the monitoring time p，#XThe correlation of (D, t) is used to calculate the distance z from the incident end of the branch fiber #X at the reference time, which is D < z < D + τ. p Dynamic dependent variable ε at the monitoring time of interval C / 2 #X (D). At this point, the correlation function CORR is defined by equation (17) for correlation calculation. R is a real number representing the offset.

[0135] [Mathematical Expression 17]

[0136]

[0137] Regarding L #X The calculation can focus on the strong signal peak generated when the test light reflected by the reflective elements of each core wire returns to the incident end. The scattered light intensity I at the reference time is calculated as in equation (18). ref (D, t) and the intensity of scattered light I at the monitoring time p，#X The correlation value C of (D, t) #X (D, R). FT stands for Fourier transform, and IFT stands for inverse Fourier transform.

[0138] [Mathematical Expression 18]

[0139]

[0140] The term containing the product of the complex amplitudes of scattered light from different cores is so small that it can be ignored when integrated. In addition, the term containing the product of the complex amplitudes of scattered light from the same cores but completely different intervals is so small that it can be ignored when integrated. Therefore, equation (18) can be transformed into equation (19).

[0141] [Mathematical Expression 19]

[0142]

[0143] Therefore, if the relevant value C is made #X (D, R) becomes the maximum offset. The value of R is set as R. max Then R max Satisfying equation (S3).

[0144] [Mathematical expression S3]

[0145]

[0146] Therefore, the dynamic strain ε #X (D) can be calculated by equation (20).

[0147] [Mathematical Expression 20]

[0148]

[0149] By repeatedly performing the above calculations at each monitoring time, the change in strain ε at each monitoring time relative to the reference time can be calculated. #X (D) can measure the time variation of vibration as a strain variable and distribute it in each core wire.

[0150] To summarize the steps, in step S111, at the reference time, a chirped pulse light with complex amplitude according to equation (1) is used to transmit the signal I of the scattered light according to equation (3). ref The signal I of the scattered light according to formula (16) is stored in storage unit 10. In step S112, at the monitoring time, the chirped pulse light with complex amplitude according to formula (4) is used to transmit the signal I of the scattered light according to formula (16). p Stored in storage unit 10. In step S113, the I stored in storage unit 10 is used. ref and I p Find the offset that maximizes the correlation value defined by equations (17) and (18), and calculate the change in strain at the monitoring time relative to the reference time based on the found offset. Figure 2 These steps are summarized in an abstract way.

[0151] In addition, in the above steps, Equations (17) and (18) are used to calculate the correlation between the signal of the ordinary scattered light that can be detected even without the reflective element 6 at the reference time (hereinafter referred to as the ordinary signal) and the signal of the backscattered light that propagates in the direction of the reflective element 6 after the test light is reflected by the reflective element 6 at each monitoring time, which is further reflected by the reflective element 6 and propagates in the direction of the incident end, and is thus detected (hereinafter referred to as the ghost signal). However, the correlation between the ghost signal at the reference time and the ordinary signal at each monitoring time can also be calculated.

[0152] Figure 1 The device structure is based on a direct detection structure, but as Figure 7 As shown, even coherent detection structures can be implemented. Although the device structure becomes more complex when using a coherent detection structure, it has the advantages of improving the signal-to-noise ratio of scattered light detection and enabling measurement over longer distances.

[0153] Continuous light is emitted from laser 501 and branched into two paths by coupler 502. One path is used as a probe light, and the other as a reference light. Chirped pulse light is generated from the probe light by modulator 503. The generated chirped pulse light is incident on the fiber under test 505 via circulator 504. The fiber under test 505 is branched by optical coupler 506, resulting in N branch fibers from branch fiber #1 to branch fiber #N. At the distal end of each branch fiber, there is a reflective element 507 that reflects the generated chirped pulse light. The Rayleigh backscattered light from the fiber under test 505 and the reference light are incident on optical 90-degree mixer 508. Two of the four outputs from the optical 90-degree mixer corresponding to the in-phase components are incident on balanced detector 509, and two outputs corresponding to the quadrature components are incident on balanced detector 510. The output of the in-phase component obtained from photoelectric conversion is incident on the AD conversion board 511, and the output of the quadrature component is incident on the AD conversion board 512, converting them into digital signals. The digitized signals are stored in the storage unit 514 of the computer 513. By calculating the sum of the squares of the in-phase component and the quadrature component stored in the storage unit 514, the intensity of the scattered light described in equations (3) and (16) can be calculated. The calculation unit 515 uses the data stored in the storage unit 514 to perform calculations.

[0154] In addition, Figure 1 and Figure 7 In any structure, by significantly increasing the bandwidth of the photodiode, FT[h(t)] in equation (18) can be approximated as 1.

[0155] (Implementation Example 2)

[0156] In Implementation Example 1, vibration is tracked by measuring the change in strain at each monitoring time relative to a reference time. However, if the strain change from the reference time to the monitoring time becomes large, the value obtained by calculating the absolute value of equation (15) cannot be used as |E|. #n [D, t, ΔD] #n ΔL #n , ε #n (D)]|relative to|E #n (D, t)| offset in the direction of parameter t with respect to ε #n (D) An approximation of the waveform of a proportional quantity. To address this problem, in this embodiment, a method is described that uses the scattered light waveform obtained at the previous monitoring time as a reference for each monitoring time.

[0157] First, the monitoring time is expressed as t. k (k∈N). For all k, satisfy t (k-1) <t k <t (k+1) The numbering is marked in a specific way.

[0158] In Embodiment Example 1, the pulse that reverses the chirp with other monitoring times is used only at the reference time. However, in this embodiment, equation (1) is used when the number k is odd, and equation (4) is used when the number k is even. That is, the chirp-reversing pulses are alternately incident. The monitoring time t will be... k Let the obtained scattered light be I. k，#X (D, t).

[0159] Calculate from time t using the following steps. (k-1) to t k Change in dependent variable ε #X (D, k).

[0160] ·The case where k is even

[0161] Similar to Example 1 of Implementation, time t is calculated using equation (21). k Relevant value C k，#X .

[0162] [Mathematical Expression 21]

[0163]

[0164] If we calculate the value of R that gives the maximum value for the relevant values ​​in equation (21), and set it as R... max Then R max It satisfies equation (22).

[0165] [Mathematical Expression 22]

[0166]

[0167] Therefore, the change in the dependent variable ε #X (D, k) can be calculated using equation (23).

[0168] [Mathematical Expression 23]

[0169]

[0170] ·The case where k is odd

[0171] Similar to the case where k is even, the correlation value is calculated using equation (21), and the value of R that gives the maximum value of the correlation value is calculated and set as R. max R max Satisfy equation (24).

[0172] [Mathematical Expression 24]

[0173]

[0174] Therefore, the change in the dependent variable ε#X (D, k) can be calculated using equation (25).

[0175] [Mathematical Expression 25]

[0176]

[0177] In addition, in the above steps, equations (17) and (18) are used to calculate the monitoring time t. (k-1) The signal of ordinary scattered light (hereinafter referred to as the ordinary signal) that can be detected even without a reflective element and the monitoring time t k The correlation between the backscattered light (hereinafter referred to as the ghost signal) and the backscattered light (which propagates towards the incident end) generated after the test light is reflected by the reflective element and propagates towards the incident end is further reflected by the reflective element. However, the monitoring time t can also be calculated. (k-1) Ghost signal and monitoring time t k The correlation between the usual signals.

[0178] Regarding the experimental structure, since only the modulation method of the probe light is changed, it can still be used. Figure 1 , Figure 7 The structure.

[0179] Furthermore, the present invention is not limited to the above-described Embodiment 1 and Embodiment 2. During implementation, the constituent elements and signal processing methods can be modified to embody the invention without departing from its spirit. Additionally, the device (computer) of the present invention can also be implemented using a computer and a program, the program can be recorded on a recording medium, or it can be provided via a network.

[0180] Industrial applicability

[0181] This invention can be applied to the information and communication industry.

[0182] Explanation of reference numerals in the attached figures

[0183] 1. 501: Laser

[0184] 2. 503: Modulator

[0185] 3. 504: Circulator

[0186] 4, 505: The optical fiber being measured

[0187] 5. 506: Optical Coupler

[0188] 6. 507: Reflective element

[0189] 7: Photodiode

[0190] 8: AD board

[0191] 9. 513: Computer

[0192] 10, 514: Storage Department

[0193] 11, 515: Computing Department

[0194] 502: Coupler

[0195] 508: Light 90-degree mixer

[0196] 509, 510: Balance detectors

[0197] 511, 512: AD conversion board

Claims

1. A method for measuring strain change, characterized in that, The strain measurement device incident a first chirped pulse of light, with a frequency that varies linearly with time, onto the optical fiber being measured, which is branched by a coupler, and obtains the signal of the first scattered light relative to the first chirped pulse. The strain measurement device incident a second chirped pulse, whose chirp is reversed on the time axis of the first chirped pulse, onto the optical fiber being measured, and obtains the signal of the second scattered light relative to the second chirped pulse. The strain measurement device determines the maximum offset between the waveform that causes the signal of the first scattered light to be reversed and shifted on the time axis and the waveform of the signal of the second scattered light, and uses this offset to calculate the change in strain in the fiber being measured.

2. The strain change measurement method according to claim 1, characterized in that, The optical fiber under test has a reflective element at the front end of each branch fiber branched by the coupler. The first scattered light is the light scattered by the optical fiber being measured before reaching the reflecting element, hereinafter referred to as the normal signal. The second scattered light includes the backscattered light generated by the reflection of the second chirped pulse light after it is reflected by the reflective element, which is then reflected back towards the incident end. This backscattered light is further reflected by the reflective element and propagates towards the incident end, thus being detected as a signal, hereinafter referred to as the ghost signal. The offset is calculated by limiting the correlation value between the time waveform of the ghost signal and the time waveform that causes the normal signal to be reversed and offset in time within a specified interval determined by the chirped pulse, and maximizing the calculated correlation value.

3. The strain change measurement method according to claim 1, characterized in that, The optical fiber under test has a reflective element at the front end of each branch fiber branched by the coupler. The first scattered light includes the backscattered light generated by the reflection of the second chirped pulse light after it is reflected by the reflective element, which is then reflected back towards the incident end by the reflected second chirped pulse light. This backscattered light is further reflected by the reflective element and propagates towards the incident end, thus being detected as a signal, hereinafter referred to as the ghost signal. The second scattered light is the light scattered by the optical fiber being measured before reaching the reflecting element, hereinafter referred to as the normal signal. The offset is calculated by limiting the correlation value between the time waveform of the ghost signal and the time waveform that causes the normal signal to be reversed and offset in time within a specified interval determined by the chirped pulse, and maximizing the calculated correlation value.

4. The strain change measurement method according to any one of claims 1 to 3, characterized in that, The signal of the first scattered light is obtained in advance. The signal of the second scattered light was acquired at each monitoring moment when the vibration was tracked. The strain change of the second scattered light signal at each monitoring time relative to the pre-acquired first scattered light signal is calculated, and the distributed vibration measurement of the optical fiber under test is performed by obtaining the time change of strain based on the calculated strain change.

5. The strain change measurement method according to any one of claims 1 to 3, characterized in that, At each monitoring moment during vibration tracking, the first chirped pulse light and the second chirped pulse light are alternately incident on the fiber being measured, and the signals of the first scattered light and the second scattered light are obtained. The strain change of the second scattered light signal at the monitoring time relative to the strain change of the first scattered light signal obtained at a different time from the monitoring time is calculated, and the distributed vibration measurement of the optical fiber under test is performed by obtaining the time change of strain based on the calculated strain change.

6. A strain change measuring device, characterized in that, At a reference time, a first chirped pulse of light with a frequency that varies linearly with time is incident on the fiber under test branched by a coupler, and the signal of the first scattered light relative to the first chirped pulse light is obtained. At each monitoring moment, a second chirped pulse, whose chirp is reversed in time with the first chirped pulse, is incident on the fiber being measured, and a signal of the second scattered light relative to the second chirped pulse is obtained. The maximum offset is determined by finding the waveform that causes the signal of the first scattered light to be reversed and shifted on the time axis, and the waveform of the signal of the second scattered light. This offset is then used to calculate the change in strain in the measured optical fiber from the reference time to each monitoring time.

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