Optical fiber sensor and optical fiber sensing method

Abstract

This improves the degradation of the cross-correlation coefficient caused by the error between the set frequency and the actual frequency of the light source, and makes it possible to improve the degradation of the measurement accuracy of strain and temperature. [Solution] The light source unit has a frequency-stabilized wavelength-tunable laser that generates continuous light, and a first optical coupler splits the continuous light generated by the light source unit into two, sending one to a second optical coupler and the other to an optical frequency measurement unit. The optical frequency measurement unit measures the frequency of the continuous light generated by the light source unit, and the second optical coupler further splits the continuous light split by the first optical coupler into two, sending one as probe light to the optical pulse generation unit and the other as local light to a detection unit. The detection unit measures the frequency change Δf by means of coherent time domain reflectometry and a means for heterodyne detection of the Rayleigh scattered light and the local light resulting from Rayleigh scattering of pulsed light in the fiber under test. max and means for acquiring the

Description

[Technical Field]

[0001] The present invention relates to an optical fiber sensor and an optical fiber sensing method using coherent time domain reflectometry (COTDR). [Background technology]

[0002] With the development of optical fiber communications, there has been active research into distributed optical fiber sensing, which uses the optical fiber itself as the sensing medium. In particular, optical fiber sensing, which uses scattered light, differs from electrical sensors that measure at individual points in that it is capable of measuring in a distributed manner over long distances, making it possible to measure the physical quantities of the entire object being measured.

[0003] A typical distributed optical fiber sensing technique capable of measuring long-distance distribution is Optical Time Domain Reflectometry (OTDR), which sends an optical pulse into one end of the optical fiber and measures the backscattered light in the optical fiber over time. Backscattering in optical fibers includes Rayleigh scattering, Brillouin scattering, and Raman scattering. Of these, the one that measures spontaneous Brillouin scattering is called BOTDR (Brillouin OTDR).

[0004] Brillouin scattering is observed at frequencies shifted by approximately GHz toward the Stokes and anti-Stokes directions relative to the center frequency of an optical pulse incident on an optical fiber, and this spectrum is called the Brillouin gain spectrum (BGS). The frequency shift and spectral linewidth of the BGS are called the Brillouin frequency shift (BFS) and Brillouin linewidth, respectively. The BFS and Brillouin linewidth vary depending on the material of the optical fiber and the wavelength of the incident light. For example, in the case of a silica-based single-mode optical fiber, the magnitude of the BFS and Brillouin linewidth at a wavelength of 1.55 μm have been reported to be approximately 11 GHz and approximately 30 MHz, respectively. Furthermore, according to Non-Patent Document 1, the magnitude of the BFS due to strain and temperature changes in a single-mode fiber are 0.049 MHz / με and 1.0 MHz / °C, respectively, at a wavelength of 1.55 μm.

[0005] As described above, BFS is dependent on strain and temperature. For this reason, BOTDRs can be used for purposes such as diagnosing deterioration of large structures such as bridges and tunnels, temperature monitoring in plants, and monitoring areas at risk of landslides, and have become widely used (see, for example, Patent Document 1).

[0006] Coherent OTDR (COTDR) technology, which uses Rayleigh backscattering light, is a means of measuring strain and temperature with greater precision. With COTDR, the frequency of Rayleigh backscattering light changes at rates of -151 MHz / με and -1291 MHz / °C, respectively (Rayleigh backscattering spectra shift: RBS shift). As a result, COTDRs simply have three orders of magnitude greater sensitivity than BOTDRs. However, because COTDRs obtain the RBS shift by calculating the cross-correlation between arbitrary reference data and measured data, they measure the relative frequency change rather than the absolute frequency of Brillouin scattered light as with BOTDRs. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2023-000350 Summary of the Invention [Problem to be solved by the invention]

[0008] To calculate the RBS shift using COTDR, it is necessary to obtain the Rayleigh scattering intensity for each frequency. This requires a mechanism that can tune the optical frequency over a wide bandwidth. There are two methods for controlling the optical frequency: external modulation and direct modulation.

[0009] In the external modulation method, an optical frequency modulator, such as an SSB (Single Sideband) modulator, is used after the output of the light source. This external modulation method can accurately oscillate at the intended frequency. However, due to limitations imposed by the modulation bandwidth, it is difficult to expand the variable range of the optical frequency to correspond to the measurement range.

[0010] On the other hand, direct modulation controls the oscillation frequency using the temperature and current of the laser. Direct modulation can achieve a wide frequency range of several nm or more by using a DFB laser array or DBR laser. However, the accuracy and repeatability of the oscillation frequency with direct modulation is inferior to that of external modulation due to the precision of the drive current and the temperature dependency of the laser.

[0011] With COTDR, errors in the oscillation frequency directly affect errors in the RBS shift amount, which in turn directly affects the measurement precision (accuracy) of strain and temperature measurements. For this reason, the accuracy and repeatability of the oscillation frequency are extremely important factors that determine measurement performance. Therefore, when using direct modulation, it is necessary to provide a separate means to stabilize or correct the oscillation frequency.

[0012] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical fiber sensor and an optical fiber sensing method that can improve the degradation of the cross-correlation coefficient caused by an error between the set frequency and the actual frequency of a light source, thereby improving the degradation of the measurement accuracy of strain and temperature. [Means for solving the problem]

[0013] In order to achieve the above object, the optical fiber sensor of the present invention comprises a light source section, a first optical coupler, a second optical coupler, an optical pulse generating section, a detecting section, and an optical frequency measuring section.

[0014] The light source unit has a frequency-stabilized wavelength-tunable laser that generates continuous light, the first optical coupler splits the continuous light generated by the light source unit into two, sending one to the second optical coupler and the other to the optical frequency measurement unit, the optical frequency measurement unit measures the frequency of the continuous light generated by the light source unit, the second optical coupler further splits the continuous light split by the first optical coupler into two, sending one as probe light to the optical pulse generation unit and the other as local light to the detection unit, and the pulse generation unit generates pulse light from the probe light and sends it to the optical fiber under test.

[0015] The detection unit detects the amount of frequency change Δf by means of heterodyne detection of Rayleigh scattered light and local light that are generated by Rayleigh scattering of the pulsed light in the optical fiber under test, and by means of coherent time domain reflectometry. max and means for acquiring the

[0016] According to a preferred embodiment of the optical fiber sensor of the present invention, the frequency change amount Δf maxmeans for changing the frequency of the frequency-stabilized wavelength-tunable laser by a predetermined frequency step width and acquiring intensity distributions S1(f, x) and S2(f+Δf, x) at a first time and a second time as a function of the position x in the longitudinal direction of the optical fiber under test and the frequency f; and means for calculating a cross-correlation function R(Δf, x) of the intensity distribution S1(f, x) at the first time and the intensity distribution S2(f+Δf, x) at the second time using the frequency measured by the optical frequency measurement unit, and determining Δf at which the cross-correlation function R is maximum as the frequency change amount Δf max and a means for acquiring the information as the information.

[0017] According to a preferred embodiment of the optical fiber sensor of the present invention, the optical frequency measurement unit is a delay interferometer, and measures the frequency of the continuous light based on the change in frequency of the light obtained by the delay interferometer and the oscillation frequency of the frequency-stabilized wavelength-tunable laser at the start of measurement.

[0018] According to a further preferred embodiment of the optical fiber sensor of the present invention, the detection unit further calculates the frequency change Δf max and a means for acquiring a temperature change or a strain change of the optical fiber under test between a first time and a second time.

[0019] In order to achieve the above-mentioned object, an optical fiber sensing method of the present invention includes the steps of: generating continuous light at a first time and a second time; branching the continuous light into a first continuous light and a second continuous light; branching the first continuous light into a probe light and a local light; generating pulsed light from the probe light and sending the pulsed light to an optical fiber under test; heterodyne detecting Rayleigh scattered light obtained by Rayleigh scattering of the pulsed light in the optical fiber under test and the local light; and detecting a frequency change Δf by coherent time domain reflectometry. max and a process for obtaining the

[0020] According to a preferred embodiment of the optical fiber sensing method of the present invention, the frequency change amount Δf maxThe step of acquiring the intensity distributions S1(f, x) and S2(f+Δf, x) at a first time and a second time as a function of the position x in the longitudinal direction of the optical fiber under test and the frequency f includes the steps of changing the frequency of the frequency-stabilized wavelength-tunable laser by a predetermined frequency step width, and acquiring the intensity distributions S1(f, x) and S2(f+Δf, x) at a first time and a second time as a function of the position x in the longitudinal direction of the optical fiber under test and the frequency f, and measuring the frequency of the continuous light from the second continuous light. Furthermore, the frequency of the measured continuous light is used to calculate a cross-correlation function R(Δf, x) between the intensity distribution S1(f, x) at the first time and the intensity distribution S2(f+Δf, x) at the second time, and the Δf at which the cross-correlation function R is maximized is defined as the frequency change amount Δf max Obtain as.

[0021] According to a preferred embodiment of the optical fiber sensing method of the present invention, the measurement of the frequency of the continuous light is performed based on the change in frequency of the light obtained by the delay interferometer and the oscillation frequency at the start of measurement of the frequency-stabilized wavelength-tunable laser that generates the continuous light.

[0022] According to a further preferred embodiment of the optical fiber sensing method of the present invention, the frequency change amount Δf at which the cross-correlation function R is maximized is further max The temperature change or strain change of the optical fiber under test between the first time and the second time is acquired from the above. [Effects of the Invention]

[0023] The optical fiber sensor and optical fiber sensing method of the present invention can improve the degradation of the cross-correlation coefficient caused by the error between the set frequency and the actual frequency of the light source, and can also improve the degradation of the measurement accuracy of strain and temperature. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram illustrating the basic configuration of a COTDR using an external modulation method. [Figure 2] 1A and 1B are schematic diagrams for explaining a waveform acquired by a COTDR and a measurement method. [Figure 3]FIG. 1 is a schematic diagram illustrating the basic configuration of a COTDR using an internal modulation method. [Figure 4] FIG. 1 shows the effect on the measurement results when the oscillation frequency differs from the desired value. [Figure 5] 1 is a schematic diagram for explaining an optical fiber sensor according to the present invention; [Figure 6] FIG. 1 is a schematic diagram showing a Mach-Zehnder interferometer as an example of a delay interferometer. [Figure 7] FIG. 2 shows the effect on the measurement results when the oscillation frequency differs from the desired value. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and positional relationship of each component are merely shown in a schematic manner to enable understanding of the present invention. Furthermore, while preferred configuration examples of the present invention will be described below, they are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes and modifications can be made that achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

[0026] (Basic configuration of COTDR using external modulation method) The basic configuration of a COTDR using an external modulation method will be described with reference to Fig. 1. Fig. 1 is a schematic diagram for explaining the basic configuration of a COTDR using an external modulation method.

[0027] The light source unit 101 is configured to include a frequency-stabilized laser 110 as a light source, an optical frequency shifter 120, and a modulation signal generator .

[0028] The frequency-stabilized laser 110 outputs light at a stable and single optical frequency. The frequency-stabilized laser 110 can stably output a single frequency by locking the oscillation frequency to the absorption line of hydrogen cyanide or acetylene, which have molecular absorption lines around 1550 nm, for example.

[0029] The light output from the frequency-stabilized laser 110 is sent to the optical frequency shifter 120, which imparts an arbitrary frequency shift to the light sent to the optical frequency shifter 120. An SSB modulator, for example, can be used as the optical frequency shifter 120. The optical frequency shifter 120 modulates the light with an electrical signal sent from the modulation signal generator 130, enabling accurate frequency shifting.

[0030] In the light source unit 101, the absolute frequency is determined by the frequency stabilized laser 110, and accurate frequency shifting by the optical frequency shifter 120 enables light to be output with high accuracy and reproducibility.

[0031] The light output from the light source unit 101 is sent to the optical coupler 200. The light sent to the optical coupler 200 is split into two by the optical coupler 200, one of which is used as probe light and the other as local light. The probe light is sent to the optical pulse generating unit 300. The local light is sent to the detecting unit 600.

[0032] The optical pulse generating section 300 comprises a first optical pulse generator 311, a second optical pulse generator 312, a polarization scrambler 330, and an optical amplifier 340.

[0033] The first optical pulse generator 311 and the second optical pulse generator 312 generate short-pulse optical pulses with a high extinction ratio from the probe light sent to the optical pulse generating unit 300 in response to the electrical pulses generated by the first electrical pulse generator 321 and the second electrical pulse generator 322, respectively. The generated short-pulse optical pulses with a high extinction ratio pass through the polarization scrambler 330 and the optical amplifier 340, and are then sent from the optical pulse generating unit 300 to the optical fiber 500 under test via the circulator 400.

[0034] The Rayleigh scattered light returning from the optical fiber under test 500 to the input side passes through the circulator 400 and is sent to the detection unit 600 .

[0035] The detection unit 600 includes an optical coupler 610, a balanced photodiode (PD) 620, an analog-to-digital converter (ADC) 630, and a signal processing unit 640.

[0036] In the detection unit 600, the local light and Rayleigh scattered light are combined by an optical coupler 610 and heterodyne detected by a balanced PD 620. The signal is then converted into a digital signal by an ADC 630, and a signal processing unit 640 calculates the RBS shift and converts it into strain and temperature.

[0037] Specifically, the frequency of the continuous light generated by the light source unit 101 is changed by a predetermined frequency step width, and the intensity distributions S1(f, x) and S2(f+Δf, x) at the first time and the second time are acquired as a function of the position x in the longitudinal direction of the optical fiber under test and the frequency f. Then, the cross-correlation function R(Δf, x) between the intensity distribution S1(f, x) at the first time and the intensity distribution S2(f+Δf, x) at the second time is calculated, and the Δf at which the cross-correlation function R is maximized is defined as the frequency change amount Δf. max The frequency shift amount Δf that maximizes this correlation number is obtained as max corresponds to RBS shift.

[0038] The waveform acquired by the COTDR and the measurement method will be described in detail with reference to Fig. 2. Fig. 2 is a schematic diagram for explaining the waveform acquired by the COTDR and the measurement method. Fig. 2(A) shows an image of the waveform measured by the COTDR. The COTDR measures the intensity waveform of Rayleigh scattered light according to the oscillation frequency of the light generated by the light source unit 101. Therefore, as shown in Fig. 2(A), three-dimensional information is obtained: optical frequency, optical fiber length (time), and intensity.

[0039] Let us consider the case where a strain (temperature) change occurs at the distance enclosed by the dashed line in Figure 2(A). Figure 2(B) shows an image of the waveform acquired at an arbitrary distance, with the horizontal axis representing the optical frequency (oscillation frequency) and the vertical axis representing the intensity of Rayleigh scattered light. In Figure 2(B), curve I represents the waveform at the first time before the strain (temperature) change, and curve II represents the waveform at the second time after the change. As such, with COTDR, the intensity waveform distribution appears to shift in the frequency direction depending on the strain (temperature). Therefore, the amount of frequency shift can be calculated by calculating the cross-correlation on the frequency axis between the waveforms before and after the strain (temperature) change. Figure 2(C) shows the result of calculating the cross-correlation on the frequency axis for the two waveforms represented by curves I and II in Figure 2(B).

[0040] The frequency change Δf with the highest correlation coefficient max corresponds to the RBS shift. The relationship between the RBS shift and the strain-temperature is expressed by the following equations (1) and (2), where f0 is the oscillation frequency, Δν is the frequency shift, Δε is the strain change, and ΔT is the temperature change.

[0041]

number

[0042]

number

[0043] From the above equations (1) and (2), the frequency change Δf max The coefficients of the strain change amount Δε and the temperature change amount ΔT can be calculated as −151 MHz / με and −1291 MHz / ° C., respectively.

[0044] Here, we explain an example in which an SSB modulator is used as an optical frequency shifter with a variable oscillation frequency, but the maximum modulation bandwidth of a typical SSB modulator is approximately 40 GHz. Therefore, the strain and temperature measurement ranges are calculated to be 264 με and 31°C, respectively, using the above equations (1) and (2).

[0045] As described above, the external modulation COTDR has a problem in that the measurement range is limited to an extremely narrow range.

[0046] (Basic configuration of COTDR using internal modulation method) The basic configuration of a COTDR using an internal (direct) modulation method will be described with reference to Fig. 3. Fig. 3 is a schematic diagram for explaining the basic configuration of a COTDR using an internal modulation method.

[0047] The internal modulation COTDR differs from the external modulation COTDR in the configuration of the light source section. The configuration other than the light source section is the same as the external modulation COTDR, so a duplicated explanation will be omitted.

[0048] The light source unit 102 is configured to include a frequency-stabilized wavelength-tunable laser 112 as a light source, and a wavelength control signal generator 140. The well-known wavelength-tunable laser, ITLA (Integrable Tunable Laser Assembly), is tunable over a range of approximately 5 THz, including the C and L bands (191.5 to 196.25 THz), which, when converted to strain and temperature, are 33,000 με and 3800°C, respectively, demonstrating a sufficient measurement range.

[0049] However, because this tunable laser controls the frequency (wavelength) with temperature or current, the accuracy of the oscillation frequency is about 1 GHz. Also, when stabilizing using the absorption lines of hydrogen cyanide molecules or acetylene molecules, as with an externally modulated light source, it is not possible to stabilize all oscillation frequencies, and only one oscillation frequency at the start of measurement can be stabilized.

[0050] For this reason, when measuring by sweeping the frequency using a COTDR, although the oscillation frequency is stable at the start of measurement, the accuracy varies by about 1 GHz when the frequency is varied. Figure 4 shows the effect on the measurement results when the oscillation frequency differs from the desired value. Figure 4(A) shows the oscillation frequency accuracy of the light source unit 102, with the horizontal axis representing the optical frequency and the vertical axis representing the number of sweep steps of the oscillation frequency.

[0051] Ideally, the oscillation frequency of the light source unit 102 is set at regular intervals Δf as shown by dotted line I, and oscillates at the set frequency. However, the actually output oscillation frequency has a slight error of, for example, about 1 GHz from the set frequency as shown by dotted line II.

[0052] Ideally, the intensity distribution waveform of the Rayleigh scattered light shown by curve III in Figure 4(B) would be sampled as indicated by the squares, but in reality it is arranged at set frequency intervals as shown in Figure 4(C), which means that the original intensity distribution waveform of the Rayleigh scattered light is distorted on the frequency axis.

[0053] This distortion of the intensity distribution waveform may lead to degradation or distortion of the correlation value in cross-correlation and may affect the accuracy of temperature measurement. Therefore, wavelength accuracy and reproducibility are the most important factors in a COTDR, and it is clear that the smaller the error between the set frequency and output frequency, the more desirable it is.

[0054] (Embodiment) The optical fiber sensor of the present invention will be described with reference to Fig. 5. Fig. 5 is a schematic diagram for explaining the optical fiber sensor of the present invention.

[0055] The optical fiber sensor of the present invention comprises a light source section 102, a first optical coupler 201, a second optical coupler 202, an optical pulse generating section 300, a detecting section 603, and a delay interferometer 700 as an optical frequency measuring section.

[0056] In the case of the internal modulation COTDR described with reference to Figure 3, a frequency-stabilized tunable laser is used as the light source, and a wide range of oscillation frequencies can be achieved by directly modulating the light source. However, as mentioned above, there are issues with frequency accuracy and reproducibility, and errors in the oscillation frequency may affect the correlation coefficient value and the measurement accuracy of strain and temperature.

[0057] Since this problem is caused by the difference between the set value and output value of the oscillation frequency, it can be solved if the actual value of the oscillation frequency is known. Therefore, in the optical fiber sensor of this invention, the oscillation frequency of the light source is actually measured and this value is used to generate the intensity distribution waveform of the Rayleigh scattered light.

[0058] Therefore, the optical fiber sensor of the present invention is configured with an optical frequency measurement unit. Any suitable conventionally known optical frequency measurement unit can be used as the optical frequency measurement unit, but when using a light source with a stable oscillation frequency at the start of measurement, it can be realized with only a delay interferometer, rather than using a wavelength meter that uses a reference light source and a high-performance interferometer, as is commercially available.

[0059] Therefore, here, an example will be described in which the optical frequency measurement unit is configured with a delay interferometer 700. The delay interferometer 700 can convert a change in oscillation frequency into a phase shift, and therefore functions as a relative frequency meter.

[0060] The light source unit 102 has the same configuration as the internal modulation COTDR described with reference to Fig. 3. That is, a frequency-stabilized tunable laser similar to the light source used in the internal modulation COTDR can be used as the light source. Assuming that the oscillation frequency is stabilized by hydrogen cyanide molecules or acetylene molecules only at the start of measurement, it can be combined with a delay interferometer to function as an absolute frequency measuring instrument.

[0061] The continuous light generated by the light source unit 102 is sent to the first optical coupler 201. The first optical coupler 201 splits the continuous light generated by the light source unit 102 into two, and sends one of the first continuous light beams to the second optical coupler 202 and the other of the second continuous light beams to the delay interferometer 700.

[0062] The second optical coupler 202 further branches the first continuous light obtained by branching it in two by the first optical coupler 201 into two, and sends one of the beams as probe light to the optical pulse generating unit 300 and the other as local light to the detecting unit 603. The configuration of the optical pulse generating unit 300 is the same as that of the external modulation type or internal modulation type COTDR described with reference to Figures 1 and 3, so a duplicated description will be omitted.

[0063] The delay interferometer 700 measures the frequency of the second continuous light, that is, the frequency of the continuous light generated by the light source unit 102 .

[0064] An example of a delay interferometer will be described with reference to Fig. 6. Fig. 6 is a schematic diagram showing a Mach-Zehnder interferometer as an example of a delay interferometer. The Mach-Zehnder interferometer includes first and second half mirrors 711 and 712, first to third mirrors 721 to 723, and a variable delay device 730.

[0065] If the electric field of the light input to the Mach-Zehnder interferometer is E0(t), it is expressed by the following equation (3).

[0066]

number

[0067] A0 is the amplitude, f0 is the frequency of the light source, and φ0 is the initial phase. In COTDR, the oscillation frequency is polarized, so the frequency f0 of the light source is a function of time, f0(t).

[0068] The input light is split into two by the first half mirror 711, and one of the two branches is given a propagation delay by the variable delay device 730, and then combined. The electric fields E1(t) and E2(t) of the two lights to be combined are expressed by the following equations (4) and (5).

[0069]

number

[0070]

number

[0071] Here, the delay amount in the variable delay device 730 is τ. When these two lights are multiplexed by the second half mirror 712 and subjected to balanced detection by the balanced PD 623, the output signal of the balanced PD 623 is given by the following equation (6).

[0072]

number

[0073] The above equation (6) means that the output of the delay interferometer 700 does not depend on time t, but depends only on the time-varying optical frequency f0(t) of the light source.

[0074] That is, the change in the optical frequency of the continuous light generated by the light source unit 102 is converted into a phase change by passing through the delay interferometer 700, and can be measured as a change in output intensity by receiving the light. In addition, the delay time τ of the delay interferometer 700 serves to adjust the relationship between the amount of change in oscillation frequency and the amount of phase change. For example, if the frequency variable range of the light source is 1 THz and the amount of phase change at 1 THz is desired to be π, the amount of delay is 0.5 fs (0.15 mm).

[0075] The first to third mirrors 721 to 723 are used to set the optical path. For example, the first mirror 721 is disposed between the first half mirror 711 and the variable delay device 730, the second mirror 722 is disposed between the variable delay device 730 and the second half mirror 712, and the second mirror 723 is disposed between the second half mirror 712 and the balance PD 623.

[0076] The detection unit 603 includes an optical coupler 610, a balance PD 620, an ADC 630, and a signal processing unit 640, which are used to detect Rayleigh scattered light and local light, as well as a balance PD 623 and an ADC 633, which are used to process the output of the delay interferometer 700.

[0077] The output of the balance PD 623 is sent to the signal processor 640 via the ADC 633, and is used to calculate the RBS shift and convert it into strain and temperature. The operation of the signal processor 640 is the same as that of the COTDR of the external modulation type or the internal modulation type described above, and in addition, the signal processor 640 processes the measured frequency of the continuous light generated by the light source unit 102. That is, the frequency change amount Δf is calculated by the COTDR using the measured actual oscillation frequency of the laser light generated by the light source 112, not the set frequency for the light source 112. max Get.

[0078] Here, delay interferometer 700 can measure the frequency change of the input light by setting an appropriate delay time τ, and furthermore, if the accuracy of the oscillation frequency at the start of measurement is guaranteed, it can function as an absolute frequency meter. A specific image of measuring the intensity waveform of Rayleigh scattered light by COTDR using delay interferometer 700 will be described with reference to FIG.

[0079] 7(A) shows the difference between the set frequency of the light source and the actual oscillation frequency, as in FIG. 4(A). The optical fiber sensor of the present invention can measure the actual frequency indicated by the square.

[0080] Therefore, the sampled values ​​of the Rayleigh scattered light intensity waveform in Figure 7(B) at each frequency can be aligned with the measured frequency as shown in Figure 7(C) rather than the set frequency, which results in the generation of a correct intensity distribution of the Rayleigh scattered light.

[0081] Since the measured frequency values ​​are not evenly spaced (≠Δf), it is desirable to process them into evenly spaced frequency data by performing interpolation at δf intervals where δf<Δf in order to properly calculate the subsequent cross-correlation coefficients. [Explanation of symbols]

[0082] 101, 102 Light source section 110 Frequency-stabilized laser 112 Frequency-stabilized wavelength-tunable laser 120 Optical Frequency Shifter 130 Modulation Signal Generator 140 Wavelength Control Signal Generator 200, 201, 202, 610 Optical Couplers 300 Optical pulse generator 311, 312 Optical pulse generator 321, 322 Electric pulse generator 330 Polarization Scrambler 340 Optical Amplifier 400 Circulator 500 Optical fiber under test 600, 603 Detector 620, 623 Balance PD 630, 633 ADC 640 Signal Processing Unit 700 Delay Interferometer 711, 712 Half mirror 721, 722, 723 Mirror 730 Variable Delay

Claims

1. a light source unit, a first optical coupler, a second optical coupler, an optical pulse generating unit, a detecting unit, and an optical frequency measuring unit; the light source unit includes a frequency-stabilized wavelength-tunable laser that generates continuous light, the first optical coupler splits the continuous light generated by the light source unit into two, and sends one to the second optical coupler and the other to the optical frequency measurement unit; the optical frequency measurement unit measures the frequency of the continuous light generated by the light source unit, the second optical coupler further splits the continuous light split by the first optical coupler into two, and sends one of the two as probe light to the optical pulse generating unit and the other as local light to the detecting unit; the pulse generating unit generates pulsed light from the probe light and sends it to the optical fiber under test; The detection unit means for heterodyne detecting Rayleigh scattered light resulting from Rayleigh scattering of the pulsed light in the optical fiber under test and local light; Coherent time domain reflectometry measures the frequency shift Δf ｍａｘ and a means of obtaining An optical fiber sensor comprising:

2. The frequency change amount Δf ｍａｘ The means to obtain a means for varying the frequency of the frequency-stabilized wavelength-tunable laser by a predetermined frequency step width, and acquiring intensity distributions S1(f, x) and S2(f+Δf, x) at a first time and a second time as a function of a position x in the longitudinal direction of the optical fiber and the frequency f; Using the frequency measured by the optical frequency measurement unit, a cross-correlation function R(Δf, x) between the intensity distribution S1(f, x) at the first time and the intensity distribution S2(f+Δf, x) at the second time is calculated, and Δf at which the cross-correlation function R is maximized is defined as the frequency change amount Δf ｍａｘ and means of obtaining The optical fiber sensor according to claim 1 , comprising:

3. the optical frequency measurement unit is a delay interferometer, The frequency of the continuous light is measured based on the change in frequency of the light obtained by the delay interferometer and the oscillation frequency of the frequency-stabilized wavelength-tunable laser at the start of measurement.

3. The optical fiber sensor according to claim 1 or 2.

4. The detection unit further The frequency change amount Δf obtained by the cross-correlation function calculation means ｍａｘ means for acquiring a temperature change or a strain change of the optical fiber to be measured between a first time and a second time; The optical fiber sensor according to claim 1 , comprising:

5. At the first time and the second time, a process for producing continuous light; splitting the continuous light into a first continuous light and a second continuous light; splitting the first continuous light into a probe light and a local light; generating pulsed light from the probe light and sending the pulsed light to an optical fiber under test; a step of heterodyne detecting Rayleigh scattered light resulting from Rayleigh scattering of the pulsed light in the optical fiber under test and local light; Coherent time domain reflectometry measures the frequency shift Δf ｍａｘ The process of obtaining An optical fiber sensing method comprising:

6. The frequency change amount Δf ｍａｘ The process of obtaining a step of varying the frequency of the frequency-stabilized wavelength-tunable laser by a predetermined frequency step width, and acquiring intensity distributions S1(f, x) and S2(f+Δf, x) at a first time and a second time as a function of the position x in the longitudinal direction of the optical fiber and the frequency f; measuring a frequency of the continuous light from the second continuous light; Using the measured frequency of the continuous light, a cross-correlation function R(Δf, x) between the intensity distribution S1(f, x) at the first time and the intensity distribution S2(f+Δf, x) at the second time is calculated, and Δf at which the cross-correlation function R is maximized is defined as the frequency change amount Δf ｍａｘ The process of obtaining The optical fiber sensing method of claim 5 , comprising:

7. The measurement of the frequency of the continuous light includes: This is performed based on the change in the frequency of the light obtained by the delay interferometer and the oscillation frequency at the start of measurement of a frequency-stabilized wavelength-tunable laser that generates continuous light.

7. The optical fiber sensing method according to claim 5 or 6.

8. moreover, The frequency change Δf at which the cross-correlation function R is maximized ｍａｘ and acquiring a temperature change or a strain change of the optical fiber to be measured between a first time and a second time. The optical fiber sensing method according to claim 5 .

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