Optical fiber sensor, optical fiber sensing method, and program

Abstract

[Problem] To improve measurement performance in a wavelength-swept COTDR by compensating for the influence of wavelength dispersion of an optical fiber. [Solution] The present invention is provided with: a wavelength dispersion compensation means for compensating for the wavelength dispersion of the optical fiber with regard to the position xj, obtained by varying the frequency fi of a frequency-variable light source, of the optical fiber in the longitudinal direction in a reference distance coordinate system with fixed intervals, and to the intensity distribution S(fi, xj) of scattered light, which serves as a function of a frequency fi; and a cross-correlation function calculation means for acquiring light intensity distributions S1(fi, x) and S2(fi+Δf, x) of the scattered light at a first time point and a second time point from the intensity distribution S(fi, xj) of the scattered light obtained by the wavelength dispersion compensation means, calculating a cross-correlation function R(Δf, x) between the intensity distribution S1(fi, x) at the first time point and the intensity distribution S2(fi+Δf, x) at the second time point, and acquiring Δf that maximizes the cross-correlation function R(Δf, x).

Description

Optical fiber sensor, optical fiber sensing method, and program

[0001] This invention relates to an optical fiber sensor and optical fiber sensing method suitable for use in, for example, a wavelength-swept COTDR, and a program that can be used with them.

[0002] With the development of optical fiber communication, technologies that use optical fibers themselves as sensing media are being actively researched. In particular, optical fiber sensing, which utilizes scattered light, enables long-range distributed sensing, unlike electrical sensors that measure at a single point.

[0003] Strain (temperature) sensors using optical fibers as a sensing medium are broadly classified into BOTDR (Brillouin Optical Time Domain Reflectometry) which uses Brillouin scattered light, and wavelength-swept COTDR (Coherent Optical Time Domain Reflectometry) (see, for example, Patent Document 1) or frequency-domain optical reflectometry (OFDR: Optical Frequency Domain Reflectometry) which uses Rayleigh scattered light.

[0004] Among these, wavelength-swept COTDRs and OFDRs have high distortion (temperature) sensitivity due to the frequency shift of Rayleigh scattered light, allowing for more accurate measurements compared to general BOTDR methods.

[0005] International Publication No. 2015 / 059969

[0006] Here, since wavelength-swept COTDR is a technology that relies on wavelength sweeping of the light source, it is affected by the wavelength dispersion of the optical fiber, resulting in a deterioration of measurement accuracy. While wavelength dispersion compensation using auxiliary interferometers has been considered for OFDRs, it has not been considered for wavelength-swept COTDRs.

[0007] This is because the influence of wavelength dispersion on measurement is an influence in a limited area, such as an event where a large distortion occurs only in an extremely short section in long-distance measurement, so the influence has not been manifested so far. However, as the development of wavelength-scanning type COTDR progresses, it has become necessary to consider the influence of wavelength dispersion.

[0008] This invention has been made in view of such a situation. This invention provides an optical fiber sensor, an optical fiber sensing method, and a program that can be used for them, which improve measurement performance by compensating for the influence of wavelength dispersion of an optical fiber in a wavelength-scanning type COTDR.

[0009] In order to achieve the above object, the optical fiber sensor of this invention includes a frequency-variable light source, which branches continuous light generated by the frequency-variable light source into two, generates pulsed light from one of the two branches, and uses the other as reference light; a pulsed light generation unit; an optical fiber into which the pulsed light is input; scattered light obtained by scattering of the pulsed light in the optical fiber; and a scattered light information acquisition unit into which the reference light is input.

[0010] The scattered light information acquisition unit includes a light receiving unit and an arithmetic unit. The light receiving unit performs coherent detection on the reference light and the scattered light, and sends the obtained electrical signal to the arithmetic unit.

[0011] The arithmetic unit changes the frequency f ｉ , ｊ , １ , ｉ , ２ , ｉ of the frequency-variable light source, and for the intensity distribution S(f ｊ , x ｉ ), which is a function of the position x in the longitudinal direction of the optical fiber in a reference distance coordinate system at a constant interval obtained by the change, wavelength dispersion compensation means for compensating for the wavelength dispersion of the optical fiber; and from the intensity distribution S(f ｉ , x ｊ ) of the scattered light obtained by the wavelength dispersion compensation means, the intensity distributions S ｉ , x ｊ ) of the scattered light at the first time and the second time are acquired, and the intensity distribution S at the first time １ (f ｉ , x), S ２ [[ID=32]] (f ｉ + Δf, x) are obtained, and the intensity distribution S at the first time１ (f ｉ (x), and the intensity distribution S at the second time step. ２ (f ｉ The system includes a means for calculating the cross-correlation function R(Δf, x) of +Δf, x, and obtaining the Δf that maximizes the cross-correlation function R(Δf, x).

[0012] According to a preferred embodiment of the optical fiber sensor of this invention, the wavelength dispersion compensation means controls the frequency f of the frequency tunable light source. ｉ The time t obtained by changing ｊ , and the frequency f ｉ The intensity distribution of scattered light p(f) as a function of ｉ ,t ｊ A first means for obtaining ) and the time t ｊ The optical group velocity v in the optical fiber ｇ Using the position x in the aforementioned reference distance coordinate system ｊ A second means for converting to the position x in the reference distance coordinate system ｊ The position x in the distance coordinate system obtained by compensating for wavelength dispersion for each frequency. ｊ Using ', the intensity distribution of scattered light S(f ｉ , x ｊ A third means to obtain ( ) and the position x in the distance coordinate system ｊ Interpolating ' to obtain the position x in the aforementioned reference distance coordinate system ｊ Intensity distribution of scattered light S(f ｉ , x ｊ The system also includes a fourth means for obtaining ).

[0013] According to a further preferred embodiment of the optical fiber sensor of this invention, the third means uses x, which is a parameter D representing wavelength dispersion. ｊ ' = (1 + 2D × (f ｉ -f １ )) x ｊ According to the formula, the position x in the reference distance coordinate system ｊ position x in the aforementioned distance coordinate system ｊ Convert to '.

[0014] According to a further preferred embodiment of the optical fiber sensor of this invention, the fourth means performs cubic spline interpolation to determine the position x in the distance coordinate system. ｊ Interpolate the '.

[0015] According to a further preferred embodiment of the optical fiber sensor of this invention, the calculation unit further includes strain (temperature) information acquisition means for acquiring temperature changes or strain changes of the optical fiber between a first time and a second time from Δf acquired by the cross-correlation function calculation means.

[0016] Furthermore, the optical fiber sensing method of this invention performs the following steps at a first and second time point: splitting a continuous light into two, generating pulsed light from one of the two branches and using the other as a reference light; injecting the pulsed light into an optical fiber; and obtaining an electrical signal by coherently detecting the scattered light obtained from the pulsed light scattered in the optical fiber and the reference light, while changing the frequency of the continuous light. In addition, the longitudinal position x of the optical fiber in a reference distance coordinate system at regular intervals ｊ , and the frequency f ｉ The intensity distribution of scattered light S(f) as a function of ｉ , x ｊ Regarding the above, the process of compensating for the wavelength dispersion of the optical fiber and the intensity distribution S(f) of the scattered light obtained by the wavelength dispersion compensation means ｉ , x ｊ ) From this, the intensity distribution S of scattered light at the first time and the second time. １ (f ｉ ,x), S ２ (f ｉ The intensity distribution S at the first time step is obtained by obtaining +Δf,x). １ (f ｉ (x), and the intensity distribution S at the second time step. ２ (f ｉ The method includes a process for calculating the cross-correlation function R(Δf, x) of +Δf, x, and obtaining the Δf that maximizes the cross-correlation function R(Δf, x).

[0017] According to a preferred embodiment of the optical fiber sensing method of this invention, the process of compensating for wavelength dispersion involves the frequency f of the frequency-tunable light source. ｉ The time t obtained by changingｊ , and the frequency f ｉ The intensity distribution of scattered light p(f) as a function of ｉ ,t ｊ The first step is to obtain the time t ｊ The optical group velocity v in the optical fiber ｇ Using the position x in the aforementioned reference distance coordinate system ｊ A second step is to convert to the position x in the reference distance coordinate system. ｊ The position x in the distance coordinate system obtained by compensating for wavelength dispersion for each frequency. ｊ Using ', the intensity distribution of scattered light S(f ｉ , x ｊ The third step is to obtain the position x in the distance coordinate system. ｊ Interpolating ' to obtain the position x in the aforementioned reference distance coordinate system ｊ Intensity distribution of scattered light S(f ｉ , x ｊ The fourth step is to obtain ).

[0018] According to a further preferred embodiment of the optical fiber sensing method of this invention, in the third step, x is used with a parameter D representing wavelength dispersion. ｊ ' = (1 + 2D × (f ｉ -f １ )) x ｊ According to the formula, the position x in the reference distance coordinate system ｊ position x in the aforementioned distance coordinate system ｊ Convert to '.

[0019] According to a further preferred embodiment of the optical fiber sensing method of this invention, in the fourth step, the position x in the distance coordinate system is interpolated by cubic spline interpolation. ｊ Interpolate the '.

[0020] A further preferred embodiment of the optical fiber sensing method of this invention further includes a step of obtaining the temperature change or strain change of the optical fiber between a first time and a second time from Δf obtained by the cross-correlation function calculation means.

[0021] Further, the program of this invention has a frequency-variable light source, branches the continuous light generated by the frequency-variable light source into two, generates pulsed light from one of the two branches, and includes a pulsed light generation unit using the other as reference light, an optical fiber into which the pulsed light is input, scattered light obtained by scattering of the pulsed light in the optical fiber, and a scattered light information acquisition unit for the optical fiber sensor into which the reference light is input, and the scattered light information acquisition unit of the optical fiber sensor is related to the frequency f ｉ of the frequency-variable light source, the position x ｊ in the longitudinal direction of the optical fiber in the reference distance coordinate system obtained by changing the frequency f ｉ and the intensity distribution S(f ｉ , x ｊ ) of the scattered light as a function of the frequency f ｉ , x ｊ . From the intensity distribution S １ (f ｉ , x), S ２ (f ｉ + Δf, x) of the scattered light at the first time and the second time obtained by the wavelength dispersion compensation means for compensating the wavelength dispersion of the optical fiber and the intensity distribution S １ (f ｉ , x) at the first time and the intensity distribution S ２ (f ｉ + Δf, x) at the second time, the cross-correlation function R(Δf, x) is calculated, and the cross-correlation function calculation means functions to obtain Δf at which the cross-correlation function R(Δf, x) becomes maximum.

[0022] According to a preferred embodiment of the program of this invention, the wavelength dispersion compensation means is related to the intensity distribution p(f ｉ , t ｊ ) of the scattered light as a function of the time t ｉ and the frequency f ｉ obtained by changing the frequency f ｊ of the frequency-variable light source, a first means for obtaining the intensity distribution p(f ｊ , t ｇ ), a second means for converting the time t ｊ to the position x ｊ in the reference distance coordinate system using the group velocity v ｇ of light in the optical fiber, and the position x ｊ in the reference distance coordinate systemThe position x in the distance coordinate system obtained by compensating for wavelength dispersion for each frequency. ｊ Using ', the intensity distribution of scattered light S(f ｉ , x ｊ A third means to obtain ( ) and the position x in the distance coordinate system ｊ Interpolating ' to obtain the position x in the aforementioned reference distance coordinate system ｊ Intensity distribution of scattered light S(f ｉ , x ｊ It will function as a fourth means to obtain ).

[0023] According to a further preferred embodiment of the program of this invention, the third means uses x, which is a parameter D representing wavelength dispersion. ｊ ' = (1 + 2D × (f ｉ -f １ )) x ｊ According to the formula, the position x in the reference distance coordinate system ｊ position x in the aforementioned distance coordinate system ｊ Convert to '.

[0024] According to a further preferred embodiment of the program of this invention, the fourth means interpolates the position x in the distance coordinate system by cubic spline interpolation. ｊ Interpolate the '.

[0025] According to a further preferred embodiment of the program of this invention, the scattered light information acquisition unit is further configured to function as a strain (temperature) information acquisition unit that acquires temperature changes or strain changes of the optical fiber between a first time and a second time from Δf acquired by the cross-correlation function calculation unit.

[0026] According to the optical fiber sensor and optical fiber sensing method of this invention, and the program that can be used therewith, measurement performance is improved by compensating for the effects of chromatic dispersion of the optical fiber.

[0027] This is a schematic diagram of an optical fiber sensor used in a typical wavelength-swept COTDR. This is a schematic diagram to explain the principle of a typical wavelength-swept COTDR. This is a diagram to explain the effect of wavelength dispersion. This is a schematic diagram of an optical fiber sensor used in the wavelength-swept COTDR of this invention. This is a diagram to explain the results of applying processing in the wavelength dispersion compensation means. This is a diagram showing the measurement results of the strain distribution.

[0028] The embodiments of this invention will be described below with reference to the figures, but the shapes, sizes, and arrangements of each component are only shown in a general manner to the extent that the invention can be understood. Furthermore, preferred configuration examples of this invention will be described below, but these are merely examples. Therefore, this invention is not limited to the following embodiments, and many changes or modifications can be made to achieve the effects of this invention without departing from the scope of the configuration of this invention.

[0029] (Typical Wavelength-Swept COTDR) A typical wavelength-swept COTDR will be explained with reference to Figure 1. Figure 1 is a schematic diagram of an optical fiber sensor used in a typical wavelength-swept COTDR.

[0030] A typical wavelength-swept COTDR is used, for example, to measure strain and temperature. The optical fiber sensor is composed of a pulse light generation unit 10, a circulator 20, an optical fiber 30, and a scattered light information acquisition unit 40.

[0031] The pulsed light generation unit 10 is configured to include, for example, a frequency-variable light source 12, a brancher 13, an intensity modulator 14, a function generator 16, and an optical amplifier 18.

[0032] The variable frequency light source 12 generates continuous light at a frequency set by the scattered light information acquisition unit 40. The continuous light generated by the variable frequency light source 12 is sent to the branching switch 13.

[0033] The brancher 13 splits the continuous light sent from the frequency-variable light source 12 into two at a predetermined branching ratio. One of the two branches obtained by the brancher 13 is sent to the intensity modulator 14, and the other is sent to the scattered light information acquisition unit 40 as reference light.

[0034] As the intensity modulator 14, for example, an acoustic-optical modulator (AOM) is used. The intensity modulator 14 generates optical pulses from the continuous light sent from the brancher 13, with pulse widths and pulse periods determined by electrical signals input from a function generator 16 or the like. The pulse width can be determined according to the desired spatial resolution. The pulse period can be set at intervals longer than the time it takes for the optical pulse to travel back and forth through the optical fiber 30. When an AOM is used as the intensity modulator 14, the frequency of the optical pulse changes from the frequency of the continuous light input to the intensity modulator 14 due to the optical Doppler effect. The optical pulses generated by the intensity modulator 14 are sent to the optical amplifier 18.

[0035] The optical amplifier 18 amplifies the optical pulses sent from the intensity modulator 14 and sends them to the circulator 20. While it is common to insert an optical filter after the optical amplifier 18 to remove the spontaneous emission (ASE) noise generated by the optical amplifier 18, a detailed explanation and illustration of this are omitted here.

[0036] In this manner, the pulsed light generation unit 10 generates light pulses, which are sent to the circulator 20. The pulsed light generation unit 10 also generates reference light, which is sent to the scattered light information acquisition unit 40.

[0037] The circulator 20 has first to third ports. Light input to the first port is output from the second port, light input to the second port is output from the third port, and light input to the third port is output from the first port.

[0038] In this example, the optical pulses sent from the pulsed light generation unit 10 are input to the first port of the circulator 20. The optical pulses input to the first port of the circulator 20 are output from the second port. The optical pulses output from the second port of the circulator 20 are sent to the optical fiber 30.

[0039] The optical fiber 30 is the object of measurement for strain and temperature. The optical pulse (input light) input to the optical fiber 30 is Rayleigh scattered as it propagates from the input end, which is the end of the optical fiber 30 on the circulator 20 side, towards the other end, which is the termination. The backscattered light (hereinafter also simply referred to as scattered light) propagates in the opposite direction to the input light, from the scattering point towards the input end of the optical fiber, is output from the optical fiber 30, and is sent to the circulator 20.

[0040] The scattered light sent to the circulator 20 is input to the second port of the circulator 20. The scattered light input to the second port of the circulator 20 is output from the third port. The scattered light output from the third port of the circulator 20 is sent to the scattered light information acquisition unit 40.

[0041] The scattered light information acquisition unit 40 is configured to include a light receiving unit 50 and a calculation unit 60.

[0042] The light-receiving unit 50 generates an electrical signal by coherently detecting the scattered light generated in the optical fiber 30 using a reference light. The light-receiving unit 50 is composed of a coherent receiver 52, a photoelectric converter 54, and an analog-to-digital (AD) converter 56. The coherent receiver 52 performs coherent detection of the scattered light using the reference light. For example, a 90° optical hybrid coupler can be used as the coherent receiver 52.

[0043] When the modulation in the intensity modulator 14 involves a frequency shift, the detection in the coherent receiver 52 becomes heterodyne detection, and when the modulation in the intensity modulator 14 does not involve a frequency shift, it becomes homodyne detection. Alternatively, the scattered light received by the light receiving unit 50 may be amplified before being input to the coherent receiver 52.

[0044] The combined light obtained by coherent detection in the coherent receiver 52 is output from the coherent receiver 52 and sent to the photoelectric converter 54.

[0045] The photoelectric converter 54 is, for example, a PD (Photo Diode) and converts the combined light generated by the coherent receiver 52 into an electrical signal. The electrical signal generated by the photoelectric converter 54 is sent to the AD converter 56.

[0046] The AD converter 56 converts the electrical signal generated by the photoelectric converter 54 from an analog signal to a digital signal. The digital signal obtained by the AD conversion in the AD converter 56 is sent to the arithmetic unit 60.

[0047] The arithmetic unit 60 includes, for example, RAM (Random Access Memory) 62, ROM (Read Only Memory) 64, storage means 66, and CPU (Central Processing Unit) 70, and can be configured in any suitable configuration for performing digital signal processing, such as a commercially available personal computer (PC).

[0048] Here, the CPU 70 is described as realizing predetermined functional means described later by reading and executing a program stored in the ROM 64, but it is not limited to this. The processing of each functional means is temporarily stored in the RAM 62, and the processing results of the arithmetic unit 60 and the intensity distribution S(f,x) at each time point are stored in the storage means 66.

[0049] Note that the input, output, and communication means provided by the PC are not shown in the diagrams or explanations.

[0050] Furthermore, the pulse light generation unit 10, circulator 20, optical fiber 30, and scattered light information acquisition unit 40 of the optical fiber sensor can be configured in any suitable conventional known configuration, except for the functional means implemented by the scattered light information acquisition unit 40. Description of such a suitable conventional known configuration may be omitted.

[0051] This document describes a general optical fiber sensing method using a wavelength-swept COTDR, including the functions of each functional means.

[0052] Figure 2 is a schematic diagram illustrating the principle of a typical wavelength-swept COTDR. In a typical wavelength-swept COTDR, the intensity distribution of scattered light is measured at two time points: a first time point and a second time point.

[0053] Figure 2(A) shows the input light to the optical fiber 30 and the scattering centers at the first time point. Here, the scattered light from each scattering center retains the phase of the input light.

[0054] Figure 2(B) shows the input light to the optical fiber 30 and the scattering center at the second time point. If distortion or temperature changes occur in the optical fiber 30 between the first and second time points, the position of the scattering center changes, and the relative phase of the scattered light changes.

[0055] Figure 2(C) shows the input light to the optical fiber 30 and the scattering center at the second time point. The input light to the optical fiber 30 at the second time point has a different frequency f (wavelength) than the input light at the first time point. As described above, if distortion or temperature changes occur between the first and second time points, the position of the scattering center changes and the relative phase of the scattered light changes. However, by changing the frequency f (wavelength) of the input light, the phase relationship can be restored to that of the first time point.

[0056] The cross-correlation function calculation means 74, as a functional means, stores the intensity distribution S of scattered light at a first time, which is stored in the storage means 66. １ (f, x) and the intensity distribution S of scattered light at the second time step. ２ The (f, x) is read out. Then, the cross-correlation function calculation means 74 uses the following equation (1) to calculate the intensity distribution S at the first time. １ (f, x) and the intensity distribution S at the second time step. ２ Calculate the cross-correlation function R(Δf, x) of (f + Δf, x).

[0057] Here, position x, a variable in the intensity distribution of scattered light, represents the distance from the incident end of the optical fiber 30 being measured. Position x is the group velocity v of light in the optical fiber 30, calculated from the observation time t at the AD converter 56. ｇ Using this, x = v ｇ It is converted using the formula t / 2.

[0058]

[0059] Here, the Δf at which R(Δf, x) is maximized is the desired frequency change Δf that can return the phase relationship at the second time to the phase relationship at the first time. ｍａｘ That is the case. Furthermore, the frequency change Δf ｍａｘ The method for obtaining the intensity distribution S at the first time step is not limited to this example. １ (f, x) and the intensity distribution S at the second time step. ２ Other methods may be used if it is possible to calculate the amount of deviation of f that brings (f + Δf, x) closest to the given value.

[0060] The strain (temperature) information acquisition means 76 uses the Δf obtained by the cross-correlation function calculation means 74. ｍａｘ Using the following equation (2), strain (temperature) information is obtained, specifically the strain change Δε and the temperature change ΔT. ０ This is the frequency of continuous light at a first time, generated by a frequency-tunable light source.

[0061]

[0062] (Wavelength-swept COTDR of this invention) In a wavelength-swept COTDR, the frequency of the input light is f ｉ And at time t ｊ The Rayleigh backscatter observed is at the frequency f of the input light. ｉ The group velocity of light v in the optical fiber 30 relative to this group velocity v ｇ Therefore, position x = v ｇ t ｊ This is scattered light generated at 2 / 2. Therefore, even though the intensity of Rayleigh scattered light observed at the same time is affected by the wavelength dispersion of the optical fiber 30, the frequency of the input light f ｉ This causes the location of the scattered light to shift.

[0063] The effect of chromatic dispersion will be explained with reference to Figure 3. Figure 3 is a diagram illustrating the effect of chromatic dispersion, with time (Time [ns]) on the horizontal axis and Rayleigh scattering intensity (Rayleigh Scattering [a.u.]) on the vertical axis. Figure 3 shows the observed Fresnel reflection intensity at the end of the optical fiber 30 for three different incident frequencies: 194.1713 THz (Curve I), 194.6723 THz (Curve II), and 195.1713 THz (Curve III).

[0064] As shown in Figure 3, the timing at which reflected light from the end of the optical fiber 30 is observed depends on the frequency f of the input light due to the effect of wavelength dispersion. ｉ This varies depending on the method. Consequently, the position x of the intensity distribution S(f,x) of Rayleigh scattered light calculated using conventional methods shifts from the original scattered light generation position for each optical frequency f. As a result, in regions where the strain (temperature) change interval is short and this shift cannot be ignored, strain (temperature) measurement becomes impossible.

[0065] Therefore, the optical fiber sensor used in the wavelength-swept COTDR of this invention is equipped with wavelength dispersion compensation means in the calculation unit.

[0066] The wavelength-swept COTDR of this invention will be described with reference to Figure 4. Figure 4 is a schematic diagram of the optical fiber sensor used in the wavelength-swept COTDR of this invention. The wavelength-swept COTDR of this invention differs from the optical fiber sensor of a general wavelength-swept COTDR described with reference to Figure 1 in that the calculation unit is equipped with a wavelength dispersion compensation means 78. Other configurations are the same as those of a general wavelength-swept COTDR optical fiber sensor, so redundant explanations may be omitted here.

[0067] The wavelength dispersion compensation means 78 comprises the first to fourth means.

[0068] In the first step, the first means uses the input light frequency f ｉ , time t ｊ The scattered light intensity observed is p(f ｉ ,t ｊ )

[0069] Next, in the second step, the second means sets the position x in the reference distance coordinate system to the typical optical group velocity v of the optical fiber 30. ｇ Using x ｊ = v ｇ t ｊ Let's assume it's divided by 2. Note that the method for determining the position x in the reference distance coordinate system is not limited to this example, and any suitable method can be used.

[0070] Next, in the third step, the third means converts the position x in the reference distance coordinate system to the position x' in the distance coordinate system. This conversion is, for example, x ｊ ' = (1 + 2D × (f ｉ -f １ )) x ｊ This is done using the following formula. Then, the intensity of the scattered light p(f) at time t is used. ｉ ,t ｊ ) is the scattered light intensity s(f) at position x'. ｉ , x ｊ Let's assume it's ').

[0071] Here, D is a parameter representing wavelength dispersion. D is given, for example, by the delay amount when light with a wavelength interval of 1 nm is transmitted over 1 km.

[0072] Next, in the fourth step, the fourth means is S(f ｉ , x ｊ Interpolating ') to S(f ｉ , x ｊ This interpolation can be performed, for example, with cubic spline interpolation, but is not limited to this, and any suitable interpolation method can be used.

[0073] The position x' in the distance coordinate system obtained in the third step is not the position in a reference distance coordinate system with constant intervals, as a result of the wavelength dispersion compensation process. Therefore, interpolation in this fourth step converts it to the position x in a reference distance coordinate system with constant intervals.

[0074] The processing after the first to fourth steps in the wavelength dispersion compensation means 78 is the same as that of a general wavelength-swept COTDR. That is, the cross-correlation function calculation means 74 calculates Δf ｍａｘAfter obtaining the above, the strain (temperature) information acquisition means 76 acquires the strain change amount Δε and the temperature change amount ΔT.

[0075] The effects of the wavelength-swept COTDR of this invention, which includes a wavelength dispersion compensation means 78, will be explained with reference to Figures 5 and 6.

[0076] Figure 5 is a diagram illustrating the results of applying the processing in the wavelength dispersion compensation means 78. The horizontal axis represents time (Time [ns]) and the vertical axis represents the intensity of Rayleigh scattered light (Rayleigh Scattering [a.u.]). Similar to Figure 3, Figure 5 shows the observed Fresnel reflection intensity at the end of the optical fiber 30 for three different incident frequencies: 194.1713 THz (Curve I), 194.6723 THz (Curve II), and 195.1713 THz (Curve III).

[0077] As shown in Figure 5, the processing by the wavelength dispersion compensation means 78 compensates for the Fresnel reflection from the end of the optical fiber 30, which was misaligned in Figure 3, so that it is in the same position.

[0078] Figure 6 shows the measurement results of the strain distribution, with the horizontal axis representing the distance from the input end of the optical fiber 30 (Fiber Length [m]) and the vertical axis representing the strain (Strain [με]). Figure 6 shows the results when the wavelength dispersion compensation means 78 is applied (I) and when it is not applied (II). Figure 6 shows the results when strain is applied to a section of the optical fiber 30 under measurement, with a frequency sweep width of 1 THz and a frequency sweep step of 500 MHz.

[0079] As shown in II in Figure 6, when the wavelength dispersion compensation means 78 is not applied, the strain change is not measured. In contrast, as shown in I in Figure 6, when the wavelength dispersion compensation means 78 is applied, the strain change is measured.

[0080] Thus, with the wavelength-swept COTDR of this invention, measurement performance is improved, such as being able to measure strain changes even in cases where large strains occur only in very short intervals, by performing wavelength dispersion compensation.

[0081] 10 Pulse light generation unit 12 Frequency-tunable light source 13 Brancher 14 Intensity modulator 16 Function generator 18 Optical amplifier 20 Circulator 30 Optical fiber 40 Scattered light information acquisition unit 50 Light receiving unit 52 Coherent receiver 54 Photoelectric converter 56 AD converter 60 Calculation unit 62 RAM 64 ROM 66 Storage means 70 CPU 74 Cross-correlation function calculation means 76 Distortion (temperature) information acquisition means 78 Wavelength dispersion compensation means

Claims

1. A pulse light generation unit having a frequency-variable light source, branching the continuous light generated by the frequency-variable light source into two, generating pulse light from one of the two branches, and using the other as reference light; an optical fiber into which the pulse light is input; a scattered light information acquisition unit that receives scattered light obtained by scattering of the pulse light in the optical fiber and reference light; the scattered light information acquisition unit includes a light receiving unit and a calculation unit; the light receiving unit performs coherent detection on the reference light and the scattered light, and sends the obtained electrical signal to the calculation unit; the calculation unit ｉ Regarding the position x in the longitudinal direction of the optical fiber in a reference distance coordinate system at regular intervals obtained by changing the frequency f ｊ of the frequency-variable light source, and regarding the intensity distribution S(f ｉ , x ｉ ) as a function of the frequency f ｊ ), wavelength dispersion compensation means for compensating the wavelength dispersion of the optical fiber; from the intensity distribution S(f ｉ , x ｊ ) of the scattered light obtained by the wavelength dispersion compensation means, the intensity distributions S １ (f ｉ , x), S ２ (f ｉ + Δf, x) at the first time and the second time are acquired, and the cross-correlation function R(Δf, x) of the intensity distribution S １ (f ｉ , x) at the first time and the intensity distribution S ２ (f ｉ + Δf, x) at the second time is calculated, and a cross-correlation function calculation means for acquiring Δf at which the cross-correlation function R(Δf, x) becomes maximum. An optical fiber sensor comprising 2. The wavelength dispersion compensation means controls the frequency f of the frequency-tunable light source. ｉ The time t obtained by changing ｊ , and the frequency f ｉ The intensity distribution of scattered light p(f) as a function of ｉ ,t ｊ A first means for obtaining ) and the time t ｊ The optical group velocity v in the optical fiber ｇ Using the position x in the aforementioned reference distance coordinate system ｊ A second means for converting to the position x in the reference distance coordinate system ｊ The position x in the distance coordinate system obtained by compensating for wavelength dispersion for each frequency. ｊ Using ', the intensity distribution of scattered light S(f ｉ , x ｊ A third means to obtain ( ) and the position x in the distance coordinate system ｊ Interpolating ' to obtain the position x in the aforementioned reference distance coordinate system ｊ Intensity distribution of scattered light S(f ｉ , x ｊ The optical fiber sensor according to claim 1, further comprising a fourth means for obtaining ).

3. In the third method described above, x is obtained using the parameter D that represents wavelength dispersion. ｊ ' = (1 + 2D × (f ｉ -f １ )) x ｊ According to the formula, the position x in the reference distance coordinate system ｊ position x in the aforementioned distance coordinate system ｊ The optical fiber sensor according to claim 2, which converts to '.

4. In the fourth means described above, the position x in the distance coordinate system is determined by cubic spline interpolation. ｊ The optical fiber sensor according to claim 3, which interpolates '.

5. The optical fiber sensor according to claim 1, further comprising a strain (temperature) information acquisition means that acquires the temperature change or strain change of the optical fiber between a first time and a second time from Δf acquired by the cross-correlation function calculation means.

6. At the first and second time points, the process of splitting a continuous light into two, generating pulsed light from one of the two branches and using the other as a reference light, the process of injecting the pulsed light into an optical fiber, and the process of obtaining an electrical signal by coherently detecting the scattered light from the pulsed light scattered in the optical fiber and the reference light, is performed while changing the frequency of the continuous light, and further, the longitudinal position x of the optical fiber in a reference distance coordinate system at regular intervals ｊ , and the frequency f ｉ The intensity distribution of scattered light S(f) as a function of ｉ , x ｊ Regarding the above, the process of compensating for the wavelength dispersion of the optical fiber and the intensity distribution S(f) of the scattered light obtained by the wavelength dispersion compensation means ｉ , x ｊ ) From this, the intensity distribution S of scattered light at the first time and the second time. １ (f ｉ ,x), S ２ (f ｉ The intensity distribution S at the first time step is obtained by obtaining +Δf,x). １ (f ｉ (x), and the intensity distribution S at the second time step. ２ (f ｉ A fiber optic sensing method comprising the steps of calculating the cross-correlation function R(Δf, x) of +Δf, x, and obtaining the Δf that maximizes the cross-correlation function R(Δf, x).

7. The process for compensating for the wavelength dispersion involves the frequency f of the tunable light source. ｉ The time t obtained by changing ｊ , and the frequency f ｉ The intensity distribution of scattered light p(f) as a function of ｉ ,t ｊ The first step is to obtain the time t ｊ The optical group velocity v in the optical fiber ｇ Using the position x in the aforementioned reference distance coordinate system ｊ A second step is to convert to the position x in the reference distance coordinate system. ｊ The position x in the distance coordinate system obtained by compensating for wavelength dispersion for each frequency. ｊ Using ', the intensity distribution of scattered light S(f ｉ , x ｊ The third step is to obtain ( ) and the position x in the distance coordinate system. ｊ Interpolating ' to obtain the position x in the aforementioned reference distance coordinate system ｊ Intensity distribution of scattered light S(f ｉ , x ｊ The optical fiber sensing method according to claim 6, further comprising a fourth step of obtaining ).

8. In the third step described above, x is calculated using the parameter D that represents wavelength dispersion. ｊ ' = (1 + 2D × (f ｉ -f １ )) x ｊ According to the formula, the position x in the reference distance coordinate system ｊ position x in the aforementioned distance coordinate system ｊ The optical fiber sensing method according to claim 7, which converts to '.

9. In the fourth step described above, the position x in the distance coordinate system is determined by cubic spline interpolation. ｊ The optical fiber sensing method according to claim 8, wherein ' is interpolated.

10. The optical fiber sensing method according to claim 6, further comprising a step of obtaining the temperature change or strain change of the optical fiber between a first time and a second time from Δf obtained by the cross-correlation function calculation means.

11. A pulse light generation unit that has a frequency-variable light source, branches continuous light generated by the frequency-variable light source into two, generates pulse light from one of the two branches, and uses the other as reference light; an optical fiber into which the pulse light is input; and a scattered light information acquisition unit that acquires scattered light obtained by scattering of the pulse light in the optical fiber and reference light, for the scattered light information acquisition unit of an optical fiber sensor, the position x in the longitudinal direction of the optical fiber in a reference distance coordinate system at regular intervals, obtained by changing the frequency f ｉ of the frequency-variable light source, and the intensity distribution S(f ｊ , x ｉ ), as a function of f ｉ , wavelength dispersion compensation means for compensating the wavelength dispersion of the optical fiber, and from the intensity distribution S(f ｊ , x ｉ ) of the scattered light obtained by the wavelength dispersion compensation means, the intensity distributions S ｊ (f １ , x), S ｉ (f ２ + Δf, x) at the first time and the second time are acquired, and the cross-correlation function R(Δf, x) between the intensity distribution S ｉ (f １ , x) at the first time and the intensity distribution S ｉ (f ２ + Δf, x) at the second time is calculated, and a program for functioning as a cross-correlation function calculation means for acquiring Δf at which the cross-correlation function R(Δf, x) becomes maximum.​​ 12. The wavelength dispersion compensation means is configured to use the frequency f ｉ of the tunable light source, which is varied, to obtain the intensity distribution p(f ｊ , t ｉ ) of the scattered light as a function of the frequency f ｉ and the time t ｊ . The first means for obtaining the intensity distribution p(f ｊ , t ｇ ) of the scattered light, the second means for converting the time t ｊ to the position x ｊ in the reference distance coordinate system using the group velocity v ｉ of light in the optical fiber, the third means for obtaining the intensity distribution S(f ｊ , x ｉ ') of the scattered light using the position x ｊ ' in the distance coordinate system obtained by compensating for the wavelength dispersion for each frequency f ｊ ', and the fourth means for interpolating the position x ｊ in the reference distance coordinate system to obtain the intensity distribution S(f ｉ , x ｊ ) of the scattered light at the position x ｊ in the reference distance coordinate system. The program according to claim 11, which functions as the fourth means.

13. In the third method described above, x is obtained using the parameter D that represents wavelength dispersion. ｊ ' = (1 + 2D × (f ｉ -f １ )) x ｊ According to the formula, the position x in the reference distance coordinate system ｊ position x in the aforementioned distance coordinate system ｊ The program according to claim 12 for converting to ''.

14. In the fourth means described above, the position x in the distance coordinate system is determined by cubic spline interpolation. ｊ The program according to claim 13 for interpolating '.

15. The program according to claim 11, which causes the scattered light information acquisition unit to function as a strain (temperature) information acquisition unit that acquires temperature changes or strain changes of the optical fiber between a first time and a second time from Δf acquired by the cross-correlation function calculation unit.

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