Optical fiber sensing system and method based on time domain vernier effect

Abstract

The invention relates to the technical field of optical fiber sensing, and discloses an optical fiber sensing system and method based on a time domain vernier effect. The system comprises a first laser, a second laser, a first coupler, a second coupler, a third coupler, a fourth coupler, a sensing optical fiber, a common optical fiber, a reference optical fiber, an optical fiber delay line, a first detector, a second detector and a third detector. Two Mach-Zehnder interferometers are formed by a sensing optical fiber, a common optical fiber and a reference optical fiber to obtain two beams of interference signals, and meanwhile the other beam of light is reversely input into the Mach-Zehnder interferometer where the reference optical fiber is located to form the other beam of interference signals. Reverse interference signals and the first two beams of interference signals are added to form a reference envelope and a sensing envelope respectively, and sensing signals are demodulated through relative offset of the reference envelope and the sensing envelope. According to the invention, accurate measurement of external environment changes can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber sensing technology, and in particular to an optical fiber sensing system and method based on time domain vernier effect. BACKGROUND

[0002] In the fields of industrial infrastructure health monitoring, aerospace, power systems, and petrochemical industry, accurate and real-time monitoring of physical parameters such as temperature and strain (stress) is crucial. Traditional electrical sensors (such as resistance strain gauges and thermocouples) gradually fail to meet the requirements of modern industry for high reliability, high precision, and long-term stability due to their inherent shortcomings such as susceptibility to electromagnetic interference, corrosion in harsh environments, difficulty in implementing long-distance distributed measurement, and high cost in large-scale deployment. In this context, optical fiber sensing technology emerged and showed great potential for development. This technology uses optical fiber as the medium for sensing and transmission, and uses light waves as the carrier. By detecting the changes in the intensity, phase, wavelength, and polarization state of light in the optical fiber caused by external environmental changes (such as temperature and strain), the information of the measured parameters can be demodulated. Compared with traditional electrical sensors, optical fiber sensors have the advantages of inherent electromagnetic interference resistance, corrosion resistance, small size, light weight, and ease of implementation of wavelength division multiplexing and distributed sensing. In the field of optical fiber sensing, micro-nano optical fibers or optical fiber gratings are generally used to measure strain and temperature, but both methods have low sensitivity and rely on expensive equipment such as wide-spectrum light sources and spectrometers. Moreover, if the sensitivity needs to be improved, the dependence on wide-spectrum light sources and spectrometers will be even greater.

[0003] Therefore, in order to improve the sensing accuracy and reduce the requirements of the system on the spectrometer, the existing optical fiber sensing system needs to be improved. SUMMARY

[0004] In order to solve the problem of dependence on light sources and spectrometers in the existing optical fiber sensing technology and improve the sensing accuracy, the present application proposes an optical fiber sensing system and method based on time domain vernier effect.

[0005] In order to solve the above technical problems, the technical solution adopted by the present application is as follows: an optical fiber sensing system based on time domain vernier effect, comprising a first laser, a second laser, a first coupler, a second coupler, a third coupler, a fourth coupler, a sensing optical fiber, a common optical fiber, a reference optical fiber, an optical fiber delay line, a first detector, a second detector, and a third detector. The output of the first laser is connected to the first input of the first coupler, the first output of the first coupler is connected to one end of the sensing fiber, and the other end of the sensing fiber is connected to the first input of the second coupler; the second output of the first coupler is connected to one end of the common fiber, the other end of the common fiber is connected to the first input of the third coupler, the first output of the third coupler is connected to the second input of the second coupler, and the second output of the third coupler is connected to the first input of the fourth coupler. The third output terminal of the first coupler is connected to one end of the reference optical fiber, and the other end of the reference optical fiber is connected to the second input terminal of the fourth coupler. The output of the second coupler is connected to the first detector, the first output of the fourth coupler is connected to the second detector, the second output is connected to the second laser, and the second input of the first coupler is connected to the third detector. The fiber delay line is mounted on the common fiber.

[0006] The wavelength difference between the first laser and the second laser is 1~30nm.

[0007] The wavelength of the first or second laser is 1550nm.

[0008] The first coupler is a 2×3 fiber optic coupler, the second coupler is a 1×2 fiber optic coupler, the third coupler is a 1×2 fiber optic coupler, and the fourth coupler is a 2×2 fiber optic coupler.

[0009] The fiber optic sensing system based on the time-domain vernier effect further includes a data processing module. This data processing module, along with a first detector, a second detector, and a third detector, is used to receive a first interference signal, a second interference signal, and a third interference signal output by the first detector, the second detector, and the third detector, respectively. It is also used to superimpose the first interference signal and the third interference signal to form a sensing envelope, and to superimpose the second interference signal and the third interference signal to form a reference envelope. Furthermore, it is used to determine the change in optical path difference between the sensing fiber and the reference fiber based on the relative offset between the sensing envelope and the reference envelope, and finally demodulate the sensing signal.

[0010] Furthermore, this invention also provides an optical fiber sensing method based on the time-domain vernier effect, implemented according to the aforementioned optical fiber sensing system based on the time-domain vernier effect, comprising the following steps: Step 1: Scan the optical path difference using an optical fiber delay line to obtain the first interference signal, the second interference signal, and the third interference signal detected by the first detector, the second detector, and the third detector, respectively; Step 2: Superimpose the first interference signal and the third interference signal to form a sensing envelope, and superimpose the second interference signal and the third interference signal to form a reference envelope; Step 3: calculating the offset of the sensing envelope relative to the reference envelope, calculating the optical path difference change of the sensing fiber relative to the reference fiber according to the offset, and calculating the sensing signal according to the optical path difference change of the sensing fiber relative to the reference fiber.

[0011] In the step 1, the first interference signal, the second interference signal and the third interference signal are further converted from time domain to space domain.

[0012] In the step 1, the method for converting from time domain to space domain is: determining the peak points of the interference signals, calculating the time difference T corresponding to two adjacent peak points; calculating the space domain coordinates corresponding to each point on the horizontal axis, and the calculation formula is: L=t λ / T; t represents the time coordinate corresponding to the horizontal axis of the interference signal, L represents the space domain coordinate corresponding to the time coordinate t, and λ represents the wavelength corresponding to the interference signal; In the step 3, the optical path difference change of the sensing fiber relative to the reference fiber is calculated according to the peak distance of the sensing envelope relative to the reference envelope.

[0013] The optical path difference change of the sensing fiber relative to the reference fiber is: =a ΔL; wherein, ΔL represents the peak distance of the sensing envelope relative to the reference envelope, and a is a calibration constant.

[0014] The sensing signal is a temperature signal or a strain signal.

[0015] Compared with the prior art, the present application has the following beneficial effects: The present application provides an optical fiber sensing system and method based on time domain vernier effect, two Mach-Zehnder interferometers are formed by using a sensing fiber, a common fiber and a reference fiber to obtain two interference signals, another interference signal is formed by using another light inputting into the Mach-Zehnder interferometer in which the reference fiber is located in the opposite direction, time domain interference signals are detected by a detector, a reference envelope and a sensing envelope are formed by adding the reverse interference signal and the first two interference signals respectively, the relative offset of the reference envelope and the sensing envelope is used to realize accurate demodulation of the external environment change, the obtained signal has a vernier-like amplification effect, and the sensing system of the present application does not need to rely on complex structure and precise instruments, and the sensitivity can be adjusted by adjusting the wavelength of the incident light, so that the present application can provide a new idea for constructing a high-sensitivity optical fiber sensor with simple structure, low price and wide application scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 A structural schematic diagram of a fiber sensing system based on time-domain vernier effect provided for embodiment one of the present application; Figure 2 A schematic diagram of the interference signals and envelope signals obtained by theoretical simulation in embodiment one of the present application; (a) and (d) in the figure correspond to the interference signals and envelope signals when the optical path difference is 0 μm, (b) and (e) correspond to the interference signals and envelope signals when the optical path difference is 0.5 μm, and (c) and (f) correspond to the interference signals and envelope signals when the optical path difference is 1 μm; Figure 3 A schematic diagram of the time-domain signals and spatial-domain signals of the interference signals obtained by experiment in embodiment one of the present application; (a), (b), (c) correspond to the time-domain signals of the first interference signal, the second interference signal and the third interference signal, respectively, and (d), (e), (f) correspond to the spatial-domain signals of the first interference signal, the second interference signal and the third interference signal, respectively; Figure 4 A schematic diagram of the reference envelope and the sensing envelope obtained by experiment in embodiment one of the present application; (a) is the reference envelope, and (b) is the sensing envelope.

[0017] Figure 5 A schematic diagram of the relative offset amount of the reference envelope and the sensing envelope under different axial strain conditions in embodiment one of the present application and a fitting curve; (a)-(e) correspond to the strain of 0 με, 0.67 με, 1.33 με, 2.00 με and 2.67 με, respectively; (f) is the result fitting of the strain and the offset amount obtained by 3 repeated experiments.

[0018] In the figure: 1 is a first laser, 2 is a first coupler, 3 is a sensing optical fiber, 4 is a second coupler, 5 is a common optical fiber, 6 is a third coupler, 7 is a fourth coupler, 8 is a reference optical fiber, 9 is a first detector, 10 is a second detector, 11 is a second laser, 12 is a third detector, and 13 is an optical fiber delay line. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme of the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0020] Embodiment one As Figure 1As shown, the embodiment one of the present application provides a fiber sensing system based on time domain vernier effect, comprising a first laser 1, a second laser 11, a first coupler 2, a second coupler 4, a third coupler 6, a fourth coupler 7, a sensing fiber 3, a common fiber 5, a reference fiber 8, a fiber delay line 13, a first detector 9, a second detector 10 and a third detector 12.

[0021] The output end of the first laser 1 is connected with the first input end of the first coupler 2, the first output end of the first coupler 2 is connected with one end of the sensing fiber 3, the other end of the sensing fiber 3 is connected with the first input end of the second coupler 4; the second output end of the first coupler 2 is connected with one end of the common fiber 5, the other end of the common fiber 5 is connected with the first input end of the third coupler 6, the first output end of the third coupler 6 is connected with the second input end of the second coupler 4, the second output end of the third coupler 6 is connected with the first input end of the fourth coupler 7; the third output end of the first coupler 2 is connected with one end of the reference fiber 8, the other end of the reference fiber 8 is connected with the second input end of the fourth coupler 7; the output end of the second coupler 4 is connected with the first detector 9, the first output end of the fourth coupler 7 is connected with the second detector 10, the second output end is connected with the second laser 11, the second input end of the first coupler 2 is connected with the third detector 12; the fiber delay line 13 is arranged on the common fiber 5.

[0022] Specifically, in the embodiment, the wavelength difference between the first laser 1 and the second laser 11 is 1-30nm.

[0023] Further, in the embodiment, the wavelength of the first laser 1 is 1547nm, and the wavelength of the second laser 11 is 1550nm, in addition, the first laser 1 and the second laser 11 can also be transposed, that is, the wavelength of the first laser 1 is 1550nm, and the wavelength of the second laser 11 is 1547nm.

[0024] Further, in the embodiment, the first coupler 2 is a 2x3 fiber coupler, the second coupler 4 is a 1x2 fiber coupler, the third coupler 6 is a 1x2 fiber coupler, and the fourth coupler 7 is a 2x2 fiber coupler.

[0025] In the embodiment, the sensing fiber 3, the common fiber 5 and the reference fiber 8 form a sensing arm, a common arm and a reference arm, wherein the sensing fiber 3 and the common fiber 5 form a first Mach-Zehnder interferometer for sensing, and the common fiber 5 and the reference fiber 8 form a second Mach-Zehnder interferometer, and the two Mach-Zehnder interferometers are arranged in parallel. The 1547nm laser output by the first laser 1 enters the sensing fiber 3, the common fiber 5 and the reference fiber 8 after passing through the first coupler 2, and the fiber delay line 13 arranged on the common fiber 5 can scan the optical path difference of the two Mach-Zehnder interferometers, so that the first detector 9 and the second detector 10 arranged at the output ends of the second coupler 4 and the fourth coupler 7 can obtain the interference signals of the two Mach-Zehnder interferometers. In addition, the 1547nm laser output by the second laser 11 enters the second Mach-Zehnder interferometer in reverse through the fourth coupler 7, and the interference signal is detected by the third detector 12 after being output by the first coupler 2.

[0026] Further, the fiber sensing system based on the time-domain vernier effect in the embodiment further comprises a data processing module, which is connected with the first detector 9, the second detector 10 and the third detector 12, and is used for receiving the first interference signal, the second interference signal and the third interference signal output by the first detector 9, the second detector 10 and the third detector 12 respectively, and is further used for superimposing the first interference signal and the third interference signal to form a sensing envelope, and superimposing the second interference signal and the third interference signal to form a reference envelope; and is used for determining the change amount of the optical path difference of the sensing fiber 3 relative to the reference fiber 8 according to the relative shift of the sensing envelope and the reference envelope, and finally demodulating to obtain a sensing signal.

[0027] During the scanning of the fiber delay line 13 from the minimum value to the maximum value, the optical path differences of the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer change with time. According to the interference condition of the Mach-Zehnder interferometer, the interference signal interferes constructively when the optical path difference is an integer multiple of the wavelength, and interferes destructively when the optical path difference is an odd multiple of half the wavelength. Therefore, the interference signals detected in the three detectors can be regarded as sinusoidal signals, and the maximum value of the interference signal corresponds to interference construction, and the minimum value corresponds to interference destruction. Therefore, the relationship between the time axis t of the interference signal and the optical path can be established, that is, the time difference between the two interference constructive points (wave peaks) corresponds to the optical path difference in the spatial domain, which is equal to the wavelength of the incident light. Therefore, the interference signal about time can be converted into the interference signal about optical path, that is, the time domain signal of the interference signal is converted into the spatial domain signal, which is conducive to eliminating the error problem caused by the non-uniform scanning speed of the fiber delay line 13.

[0028] Now it is assumed that the three interference signals measured by the three detectors are functions of the optical path difference, which can be expressed as: ; (1) wherein, , , respectively represent the first interference signal, the second interference signal and the third interference signal, which are measured by the first detector, the second detector and the third detector respectively, and respectively represent the wave numbers of the first interference signal and the second interference signal, , , represents the optical path difference between the first interference signal and the second interference signal. and respectively represent the wavelengths of the first laser 1 and the second laser 11.

[0029] If the change of the environment causes the change of the length or the refractive index of the sensing fiber 3, it will cause the change of the optical path difference between the sensing fiber 3 and the other two optical fibers. If the change is very small, the interference signal passing through the first Mach-Zehnder interferometer will be difficult to measure. In the embodiment, the interference signal output from the first Mach-Zehnder interferometer, i.e. the first interference signal and the reverse interference signal corresponding to the second Mach-Zehnder interferometer, i.e. the third interference signal are superimposed. Since the wavelengths of the two Mach-Zehnder interferometers are slightly different, the superposition of the first interference signal and the third interference signal will generate an envelope signal. The superposition signal can be represented as: ; (2) The generated envelope signal is denoted as a sensing envelope, which can be represented as: ; (3) When the optical path difference in the sensing fiber 3 changes, the optical path difference changes to , the phase change in the sensing fiber 3 is: , and the reference envelope remains unchanged. Therefore, the phase change of the sensing envelope relative to the reference envelope is Since the phase change of the envelope signal has a linear relationship with the spatial displacement, the ratio of the drift amount of the envelope signal to the drift amount of the first interference signal, i.e. the amplification factor, is: ; (4) It can be seen that the small change in the optical path difference of the sensing fiber 3 will cause a large displacement of the envelope signal, and therefore the magnified measurement of the optical path difference in the sensing fiber 3 can be achieved by measuring the offset of the sensing envelope relative to the reference envelope. At the same time, since the magnification is related to the wavelengths of the two lasers, the magnification can be changed by adjusting the wavelengths, but it needs to be noted that although the high magnification improves the sensitivity of the system as a whole, it also reduces the measurement range and the anti-interference ability of the system, and therefore the wavelengths of the two incident lights selected in the embodiment are the result of being limited by the laboratory environment and conditions and other factors. Since the present application is demodulated according to the relative displacement of the reference envelope and the measurement envelope, the initial optical path difference of the reference fiber 8 and the sensing fiber 3, and the non-uniformity of the scanning speed of the fiber delay line 13 will not affect the measurement results.

[0030] In order to more intuitively show this process, in the embodiment, the drift of the lower envelope is shown by theoretical calculation under different optical path difference changes The wavelengths of the first laser 1 and the second laser 11 are 1547nm and 1550mn respectively, that is, the first interference signal of 1547nm in the first Mach-Zehnder interferometer and the reverse third interference signal of 1550nm in the second Mach-Zehnder interferometer are superimposed to form the sensing envelope, and the second interference signal of 1547nm in the second Mach-Zehnder interferometer and the reverse third interference signal of 1550nm in the second Mach-Zehnder interferometer are superimposed to form the reference envelope.

[0031] When the optical path differences of the sensing fiber 3 and the reference fiber 8 are 0μm, 0.5μm and 1μm respectively, the changes of the interference signals and the envelope signals are as shown in the following table. Figure 2 Since the period corresponding to the horizontal axis of the envelope signal is related to the wavelength of the laser, the time is converted into distance by coordinate conversion of the horizontal axis, and the conversion principle is to set the coordinates of the two peak values of the reference envelope as When the optical path difference is 0μm, the two envelopes coincide, when the optical path difference is 0.5μm, the drift of the sensing envelope relative to the measurement envelope is 258.33μm, and when the optical path difference is increased to 1μm, the relative drift of the two envelope signals is 516.67μm. This means that the sensitivity is increased by more than 500 times. In addition, in the embodiment, the signal demodulation is not by traditional frequency spectrum demodulation but by time domain signal detected by the detector, which reduces the dependence of the sensing system of the embodiment on the performance of the instrument. In addition, the sensitivity of the system can be freely adjusted by adjusting the wavelengths of the two incident lights, to meet more use scenarios.

[0032] In addition, in the embodiment, the measurement is also carried out by experiment, and the wavelengths of the first laser 1 and the second laser 11 are 1547nm and 1550mn respectively, and the maximum scanning range of the fiber delay line 13 is Because the scanning speed of fiber delay line 13 is uneven, the time-domain signal corresponding to the acquired interference signal is also unstable. Therefore, based on the relationship that the peak-to-peak value of the interference signal equals the laser wavelength, the time-domain signal of the interference signal is converted into a spatial signal, such as... Figure 3 As shown, this can avoid the problem of uneven period of interference signal caused by uneven scanning speed of fiber delay line 13, thereby eliminating the measurement error caused by fiber delay line 13.

[0033] Then, the first interference signal and the third interference signal are superimposed to form the sensing envelope, and the second interference signal and the third interference signal are superimposed to form the reference envelope. The resulting envelope is as follows: Figure 4 As shown.

[0034] To further verify the performance of the sensing system, a tensile measurement experiment was conducted. The sensing fiber 3 was clamped on an electrically driven displacement stage for tensile testing. To ensure that the sensing fiber 3 could be effectively stretched, after clamping it tightly on the stage and activating the stage, a pre-tension of 100 μm was applied. Subsequently, axial strains of 0 με, 0.67 με, 1.34 με, 2.00 με, and 2.67 με were applied in five trials. The results are as follows: Figure 5 As shown. From Figure 5 As can be seen from (a)-(e), the relative displacement between the sensing envelope and the reference envelope gradually increases. The results of statistical analysis and fitting of the relative displacement are as follows: Figure 5 As shown in (f). From the fitted curve, it can be seen that the tensile sensitivity of the system for the experimental range is 397.09 μm / με. Since the scanning resolution of the fiber delay line 13 is 10 fs, the resolution of the sensing system of the present invention can be calculated to be approximately 5.14 nε. The minimum displacement of the electric displacement stage used in the experiment is 0.1 μm.

[0035] In this embodiment, as can be seen from formula (4), the signal amplification factor can be significantly increased by adjusting the wavelength of the incident light. For example, if the incident light of 1547nm is replaced with incident light of 1548nm and 1549nm respectively, the amplification factor will become 774 times and 1548 times. This can further reduce the system resolution to 3.43nε and 2.57nε, while the system structure does not need to be changed much, which can effectively avoid structural complexity.

[0036] In addition, it should be noted that although the sensing system of the present embodiment can significantly improve the sensitivity of the system by adjusting the wavelength of the incident light, when the single displacement including the signal is large, the system will be difficult to effectively identify the relative displacement of the sensing envelope and the reference envelope, for example, in the present experiment, due to the minimum stretching amount of 0.1 μm, 5 experiments in the stretching range can effectively distinguish that the relative displacement of the envelope brought by two stretching experiments does not exceed one period. Assuming that the incident light of 1547 nm is replaced by incident light of 1549 nm, and the above experiment is repeated, it will result in that the relative displacement of 0.1 μm and the relative displacement of 0.5 μm have already been different by 5 periods, which will result in that it is difficult to accurately identify the relative displacement of the two envelopes. Therefore, before considering further improving the sensitivity, the minimum change amount of the environment should be evaluated.

[0037] Embodiment Two The embodiment two of the present application provides a kind of optical fiber sensing method based on time domain vernier effect, according to the realization of the optical fiber sensing system based on time domain vernier effect described in embodiment one, including the following steps: Step 1: by optical fiber delay line 13 scanning optical path difference, obtains the first probe 9, second probe 10 and third probe 12 respectively detected first interference signal, second interference signal and third interference signal.

[0038] Further, in the step 1, the step 1 further includes the step of converting the first interference signal, the second interference signal and the third interference signal from time domain to space domain.

[0039] Specifically, in the step 1, the method for converting the three interference signals from time domain to space domain in the step 1 is: (1) determine the peak point of interference signal, calculate the time difference T corresponding to adjacent two peak points; (2) calculate the space domain coordinates corresponding to each point on the horizontal axis, and the calculation formula is: L=t λ / T; (5) t indicates the time coordinate corresponding to the horizontal axis of interference signal, L indicates the space domain coordinate corresponding to time coordinate t, and λ indicates the wavelength of corresponding interference signal; for the first interference signal and the second interference signal, λ=λ1, for the third interference signal, λ=λ2, wherein λ1 and λ2 respectively indicate the wavelength of the first laser 1 and the second laser 11.

[0040] Step 2: superimpose the first interference signal and the third interference signal to form a sensing envelope, and superimpose the second interference signal and the third interference signal to form a reference envelope.

[0041] Step 3: calculate the offset of the sensing envelope relative to the reference envelope, calculate the optical path difference change of the sensing fiber 3 relative to the reference fiber 8 according to the offset, and calculate the sensing signal according to the optical path difference change of the sensing fiber 3 relative to the reference fiber 8.

[0042] Further, in step 3, the optical path difference change of the sensing fiber 3 relative to the reference fiber 8 is calculated according to the peak distance of the sensing envelope relative to the reference envelope.

[0043] Specifically, the optical path difference change of the sensing fiber 3 relative to the reference fiber 8 is: =a ΔL; (6) Wherein, represents the optical path difference change of the sensing fiber 3 relative to the reference fiber 8, a is a calibration constant, and ΔL represents the peak distance of the sensing envelope relative to the reference envelope.

[0044] Further, in the embodiment, the sensing signal is a temperature signal or a strain signal. Due to the strain, the measurement of temperature is based on the change of the physical length and the refractive index of the sensing fiber 3 caused by stretching or thermal expansion, thereby affecting the optical path difference between the sensing fiber 3 and the reference fiber 8. Therefore, under the condition that the reference fiber 8 remains stable, the measured optical path difference change between the sensing fiber 3 and the reference fiber 8 is the optical path change of the sensing fiber 3, which is converted into a temperature signal or a strain signal, thereby realizing high-precision measurement of the temperature signal or the strain signal.

[0045] In summary, the present application provides an optical fiber sensing system and method based on time domain vernier effect, which realizes accurate demodulation of external environment changes through the superposition envelope of the time domain interference signal detected by the detector, and does not depend on complex structure and precise instruments. Theoretical and experimental results prove the vernier amplification effect of the sensor of the present application on the sensing signal. Moreover, the present application can also realize the adjustability of sensitivity by adjusting the wavelength of the incident light, so the present application can provide a new idea for constructing a high-sensitivity optical fiber sensor with simple structure, low price and wide application scenarios.

[0046] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A fiber optic sensing system based on the time-domain vernier effect, characterized in that, It includes a first laser (1), a second laser (11), a first coupler (2), a second coupler (4), a third coupler (6), a fourth coupler (7), a sensing fiber (3), a common fiber (5), a reference fiber (8), a fiber delay line (13), a first detector (9), a second detector (10), and a third detector (12). The output end of the first laser (1) is connected to the first input end of the first coupler (2), the first output end of the first coupler (2) is connected to one end of the sensing fiber (3), and the other end of the sensing fiber (3) is connected to the first input end of the second coupler (4); the second output end of the first coupler (2) is connected to one end of the common fiber (5), the other end of the common fiber (5) is connected to the first input end of the third coupler (6), the first output end of the third coupler (6) is connected to the second input end of the second coupler (4), and the second output end of the third coupler (6) is connected to the first input end of the fourth coupler (7); The third output end of the first coupler (2) is connected to one end of the reference fiber (8), and the other end of the reference fiber (8) is connected to the second input end of the fourth coupler (7); The output of the second coupler (4) is connected to the first detector (9), the first output of the fourth coupler (7) is connected to the second detector (10), the second output is connected to the second laser (11), and the second input of the first coupler (2) is connected to the third detector (12). The fiber delay line (13) is disposed on the common fiber (5).

2. The fiber optic sensing system based on the time-domain vernier effect according to claim 1, characterized in that, The wavelength difference between the first laser (1) and the second laser (11) is 1~30nm.

3. The fiber optic sensing system based on the time-domain vernier effect according to claim 1, characterized in that, The wavelength of the first laser (1) or the second laser (11) is 1550nm.

4. The fiber optic sensing system based on the time-domain vernier effect according to claim 1, characterized in that, The first coupler (2) is a 2×3 fiber optic coupler, the second coupler (4) is a 1×2 fiber optic coupler, the third coupler (6) is a 1×2 fiber optic coupler, and the fourth coupler (7) is a 2×2 fiber optic coupler.

5. The fiber optic sensing system based on the time-domain vernier effect according to claim 1, characterized in that, It also includes a data processing module, which, together with the first detector (9), the second detector (10), and the third detector (12), is used to receive the first interference signal, the second interference signal, and the third interference signal output by the first detector (9), the second detector (10), and the third detector (12), respectively; and is also used to superimpose the first interference signal and the third interference signal to form a sensing envelope, and superimpose the second interference signal and the third interference signal to form a reference envelope; And to determine the change in optical path difference of the sensing fiber (3) relative to the reference fiber (8) based on the relative offset between the sensing envelope and the reference envelope, and finally demodulate to obtain the sensing signal.

6. A fiber optic sensing method based on the time-domain vernier effect, characterized in that, The implementation of a fiber optic sensing system based on the time-domain vernier effect according to any one of claims 1 to 5 includes the following steps: Step 1: Scan the optical path difference through the fiber delay line (13) to obtain the first interference signal, the second interference signal and the third interference signal detected by the first detector (9), the second detector (10) and the third detector (12) respectively; Step 2: Superimpose the first interference signal and the third interference signal to form a sensing envelope, and superimpose the second interference signal and the third interference signal to form a reference envelope; Step 3: Calculate the offset of the sensing envelope relative to the reference envelope, calculate the change in optical path difference between the sensing fiber (3) and the reference fiber (8) based on the offset, and calculate the sensing signal based on the change in optical path difference between the sensing fiber (3) and the reference fiber (8).

7. The fiber optic sensing method based on the time-domain vernier effect according to claim 6, characterized in that, Step 1 further includes the step of converting the first interference signal, the second interference signal, and the third interference signal from the time domain to the spatial domain.

8. The fiber optic sensing method based on the time-domain vernier effect according to claim 7, characterized in that, In step 1, the method for converting from the time domain to the spatial domain is as follows: Determine the peak point of the interference signal and calculate the time difference T between two adjacent peak points; The formula for calculating the spatial coordinates of each point on the horizontal axis is as follows: L=t λ / T; t represents the time coordinate corresponding to the horizontal axis of the interference signal, L represents the spatial coordinate corresponding to the time coordinate t, and λ represents the wavelength of the corresponding interference signal; In step 3, the change in optical path difference between the sensing fiber (3) and the reference fiber (8) is calculated based on the peak distance between the sensing envelope and the reference envelope.

9. The fiber optic sensing method based on the time-domain vernier effect according to claim 8, characterized in that, The change in optical path difference between the sensing fiber (3) and the reference fiber (8) is: =a ΔL; in, ΔL represents the change in optical path difference between the sensing fiber (3) and the reference fiber (8), where a is a calibration constant and ΔL represents the peak distance between the sensing envelope and the reference envelope.

10. The fiber optic sensing method based on the time-domain vernier effect according to claim 6, characterized in that, The sensing signal is a temperature signal or a strain signal.

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