A device and method for improving the dynamic strain range of phase-sensitive optical time-domain reflectometer

Abstract

The present invention relates to the field of optical fiber sensing technology, and specifically to a device and method for improving the dynamic strain range of a phase-sensitive optical time domain reflectometer. The device includes a semiconductor narrowband laser, a tunable optical fiber laser, a wavelength division multiplexer, an acousto-optic modulator, an erbium-doped optical fiber amplifier, an optical fiber circulator, a sensing optical fiber, a wavelength division multiplexer, a photoelectric balanced detector, a high-speed digitizer, a signal source, an optical fiber coupler, and an optical fiber isolator, wherein the semiconductor narrowband laser and the tunable optical fiber laser generate two narrow linewidth lasers of different wavelengths, and the two lasers of different wavelengths are combined into a dual-wavelength detection light source through a wavelength division multiplexer, and a dual-wavelength detection pulse is generated after pulse modulation and power amplification. In the data demodulation process, the minimum optical path difference square between two different wavelengths is used as the convergence standard, a two-dimensional boundary condition is set, and the actual phase is restored using a dual-wavelength linear programming phase unwrapping algorithm. The present invention improves the dynamic range of the phase-sensitive optical time domain reflectometer, reduces the superposition phase noise introduced by the dual-wavelength detection pulse, and also ensures the response bandwidth of the system.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to a device and method for improving the dynamic strain range of a phase-sensitive optical time domain reflectometer. Background Art

[0002] Phase-sensitive optical time-domain reflectometer is a distributed vibration / acoustic sensor, which is widely used in rail transit monitoring, oil and gas pipeline monitoring, geological activity monitoring, perimeter intrusion and other fields. It has the advantages of high sensitivity, accurate positioning, and quantitative restoration. Its principle is to modulate a single-wavelength narrow-linewidth laser into a pulse light and then inject it into the sensing optical fiber to poll the sensing optical fiber. When an external disturbance acts on the sensing optical fiber, the dynamic strain at the disturbance position causes the phase of the backward Rayleigh scattered light in the optical fiber to change, and the phase change is linearly related to the intensity of the external disturbance. By demodulating the phase, the phase-sensitive optical time-domain reflectometer system can quantitatively restore the characteristics of the disturbance while locating the disturbance event.

[0003] Dynamic strain range is an important performance parameter of phase-sensitive optical time-domain reflectometer, which is defined as the ratio of the maximum dynamic strain to the minimum dynamic strain that can be measured. Phase-sensitive optical time-domain reflectometer has high accuracy in measuring the range from pico-strain to nano-strain. The minimum measurable dynamic strain depends only on the noise of the system itself, but the maximum measurable dynamic strain is constrained by the π phase principle, making it difficult to measure large dynamic strain.

[0004] In digital phase demodulation, the phase change in a single sampling cycle is extracted by subtracting the phase of two adjacent points at the vibration position of the backward Rayleigh scattering curve. After multiple acquisitions, the continuous phase change process in the time domain is extracted. In the process of continuous sampling, the phase change between adjacent sampling points cannot exceed (-π, π], otherwise it will cause phase unwrapping errors. When monitoring large dynamic strains, the phase change will exceed (-π, π] due to excessive strain, and the phase demodulation will fail. Although the sampling rate can be increased to make the phase change between adjacent sampling points always within the range of ±π, the sampling rate has a theoretical upper limit due to the pulse polling time. In order to correctly restore the phase, the bandwidth of the signal to be measured can only be reduced.

[0005] Some researchers have applied the dual-wavelength detection method to phase-sensitive optical time-domain reflectometry, and used the traditional phase unwrapping method to extract phase information, thereby expanding the dynamic strain range. However, this method will introduce and superimpose the phase noise of two single wavelengths, causing the detection signal-to-noise ratio to drop sharply. In addition, some researchers have proposed a phase unwrapping algorithm that combines phase information and polarization information, but the improvement in the dynamic strain range is limited; some scholars have also proposed a DUI (Differential-Unwrapping-Integral) unwrapping algorithm, but the algorithm is complex and requires a lot of calculations. Summary of the invention

[0006] The present invention provides a device and a method for improving the dynamic strain range of a phase-sensitive optical time-domain reflectometer under the premise of ensuring the system response bandwidth.

[0007] The present invention adopts the following technical scheme: a device for improving the dynamic strain range of a phase-sensitive optical time-domain reflectometer, comprising a semiconductor narrowband laser and a tunable fiber laser, wherein the semiconductor narrowband laser is connected to a fiber 1*2 coupler 1, and the tunable fiber laser is connected to a fiber 1*2 coupler 2; the fiber 1*2 coupler 1 outputs a reference light and enters a port of a fiber 3dB coupler 1, and the fiber 1*2 coupler 2 outputs a reference light and enters a port of a fiber 3dB coupler 2; the fiber 1*2 coupler 1 and the fiber 1*2 coupler 2 respectively output different signal lights and enter a wavelength division multiplexer to combine two signal lights of different wavelengths into a dual-wavelength detection light source; the output end of the wavelength division multiplexer is connected to the optical fiber 1*2 coupler 1 according to the wavelength division multiplexer. The first optical fiber is connected to an acousto-optic modulator, an erbium-doped fiber amplifier and a port of a circulator, the b port of the circulator is connected to a sensing fiber, the tail end of the sensing fiber is connected to a fiber isolator, and the c port of the circulator is connected to a wavelength division multiplexer; the output end of the wavelength division multiplexer outputs backward Rayleigh scattered light with a wavelength of λ1 to the b port of a fiber 3dB coupler 1, and the backward Rayleigh scattered light with a wavelength of λ2 is input to the b port of a fiber 3dB coupler 2; the fiber 3dB coupler 1 is connected to a first photoelectric balanced detector, the fiber 3dB coupler 2 is connected to a second photoelectric balanced detector, the first photoelectric balanced detector and the second photoelectric balanced detector are connected to a high-speed digitizer, and the high-speed digitizer is connected to a host computer.

[0008] The wavelengths output by the semiconductor narrowband laser and the tunable fiber laser are λ1 and λ2 respectively, λ1 is fixed at 1550nm, the line width is less than 3kHz, and the wavelength tuning range of λ2 is 1480-1640nm, and the line width is less than 60kHz; the signal light with the wavelength λ1 output by the semiconductor narrowband laser enters the 1550nm channel input end of the wavelength division multiplexer; the signal light with the wavelength λ2 output by the tunable fiber laser enters the non-1550nm channel input end of the wavelength division multiplexer.

[0009] 90% of the output branches of the optical fiber 1*2 coupler 1 are used as signal light with a wavelength of λ1, and 10% of the branches are used as reference light; 90% of the output branches of the optical fiber 1*2 coupler 2 are used as signal light with a wavelength of λ2, and 10% of the branches are used as reference light.

[0010] The central wavelength of the wavelength division multiplexer is 1530nm-1570nm, the wavelength interval is 5nm, and the number of multiplexed channels is 9.

[0011] The acousto-optic modulator is connected to the acousto-optic modulator driver, and the acousto-optic modulator driver is connected to the signal source; the frequency shift of the acousto-optic modulator is 200MHz, the modulation pulse width generated by the signal source is 50-150ns, and the pulse repetition frequency is less than n / 2cL ,in, n is the refractive index of single-mode fiber, c is the speed of light in vacuum, L is the sensing optical fiber length.

[0012] The demultiplexer has the same technical parameters as the wavelength division multiplexer.

[0013] The bandwidth of the first photoelectric balanced detector and the second photoelectric balanced detector are both 350 MHz, the gain is 1000 V / W, and the noise equivalent power is 16 pw / √Hz.

[0014] A modulation method for improving the dynamic range of a phase-sensitive optical time-domain reflectometer comprises the following steps:

[0015] Step 1: The output of the semiconductor narrowband laser is divided into two paths with a ratio of 90:10 through the optical fiber 1*2 coupler 1. The 90% branch enters the 1550nm channel input end of the wavelength division multiplexer as the signal light with a wavelength of λ1, and the 10% branch enters the a port of the optical fiber 3dB coupler 1 as the reference light; the output of the tunable fiber laser is divided into two paths with a ratio of 90:10 through the optical fiber 1*2 coupler 2. The 90% branch enters the non-1550nm channel input end of the wavelength division multiplexer as the signal light with a wavelength of λ2, and the 10% branch enters the a port of the optical fiber 3dB coupler 2 as the reference light.

[0016] Step 2: The wavelength division multiplexer combines the two signal lights of different wavelengths into a dual-wavelength detection light source, and then outputs it to the acousto-optic modulator to modulate it into a dual-wavelength pulse light.

[0017] Step 3: After the peak power of the dual-wavelength pulse light is amplified by the erbium-doped fiber amplifier, it becomes a dual-wavelength detection pulse; the dual-wavelength detection pulse enters the a port of the circulator, and then is injected into the sensing optical fiber through the b port of the circulator; the tail end of the sensing optical fiber is connected to the optical fiber isolator; the backward Rayleigh scattered light generated during the forward transmission of the dual-wavelength detection pulse returns to the b port of the circulator, and then is output to the wavelength division multiplexer through the c port of the circulator.

[0018] Step 4: The wavelength division multiplexer separates the backscattered Rayleigh lights of two different wavelengths. The backscattered Rayleigh light with wavelength λ1 is input into port b of the first fiber 3dB coupler, and the backscattered Rayleigh light with wavelength λ2 is input into port b of the second fiber 3dB coupler, and beats with the two 10% branch reference lights of different wavelengths.

[0019] Step 5: The two outputs of the optical fiber 3dB coupler 1 enter the first photoelectric balanced detector for photoelectric conversion, and the two outputs of the optical fiber 3dB coupler 2 enter the second photoelectric balanced detector for photoelectric conversion; the two analog electrical signals after photoelectric conversion are synchronously collected by a high-speed digitizer.

[0020] A demodulation method for a device for improving the dynamic range of a phase-sensitive optical time-domain reflectometer comprises the following steps:

[0021] Step 1: First, perform digital bandpass filtering on the two channels of collected data. The center frequency of the bandpass filter is 200 MHz and the passband bandwidth is 80 MHz.

[0022] Step 2: Perform digital orthogonal demodulation on the bandpass filtered data, cross-multiply the two-channel data with a pair of orthogonal signals of the same frequency generated by the host computer to generate sum frequency components and zero frequency components;

[0023] Step 3: Use a low-pass filter to remove the sum frequency component after digital orthogonal demodulation and retain the zero-frequency component containing phase and amplitude information. The cutoff frequency of the low-pass filter is 50MHz.

[0024] Step 4: Locate the external disturbance event by amplitude difference; extract the wrapping phase at the vibration position by inverse tangent function and difference operation;

[0025] Step 5: The actual phase change is restored through the dual-wavelength linear programming phase unwrapping method, and the external disturbance signal can be quantitatively restored.

[0026] The convergence criterion in the dual-wavelength linear programming phase unwrapping method in step 5 is:

[0027]

[0028] Its boundary conditions are: , .

[0029] Compared with the prior art, the present invention has the following significant effects:

[0030] 1. The present invention adopts two narrow linewidth lasers as dual-wavelength detection light sources, utilizes the optical path difference generated by two lasers of different wavelengths, takes the minimum optical path difference square as the convergence criterion, and uses a linear programming phase unwrapping algorithm to restore the true phase, thereby improving the dynamic strain range of the phase-sensitive optical time domain reflectometer.

[0031] 2. The dual-wavelength linear programming phase unwrapping algorithm of the present invention avoids the superposition of the phase noise of each single wavelength in the dual-wavelength detection method, so that the dual-wavelength detection method can also reach the noise level of traditional single-wavelength detection.

[0032] 3. The present invention improves the dynamic strain range of the phase-sensitive optical time-domain reflectometer while ensuring the response bandwidth of the system.

[0033] 4. The system structure of the present invention is simple and has strong anti-noise capability, so that the phase-sensitive optical time-domain reflectometer can better meet the monitoring needs of a large dynamic strain range. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of the system structure of the present invention;

[0035] Figure 2 This is the data demodulation process of the present invention. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0037] Attached Figure 1The system structure diagram of the device and method for improving the dynamic strain range of a phase-sensitive optical time-domain reflectometer of the present invention is shown, which includes a semiconductor narrow-band laser 1, a tunable fiber laser 2, a fiber 1*2 coupler 1 3, a fiber 1*2 coupler 2 4, a wavelength division multiplexer 5, an acousto-optic modulator 6, an erbium-doped fiber amplifier 7, a fiber circulator 8, a wavelength division multiplexer 9, a fiber 3dB coupler 1 10, a fiber 3dB coupler 2 11, a first photoelectric balanced detector 12, a second photoelectric balanced detector 13, a high-speed digitizer 14, a sensing fiber 15, a fiber isolator 16, a signal source 17, an acousto-optic modulator driver 18, and a host computer 19; wherein, the semiconductor narrow-band laser 1 The output of is divided into two paths of 90:10 through the optical fiber 1*2 coupler 3, and the 90% branch enters the 1550nm channel input end of the wavelength division multiplexer 5 as the signal light with a wavelength of λ1, and the 10% branch enters the a port of the optical fiber 3dB coupler 10 as the reference light; the output of the tunable fiber laser 2 is divided into two paths of 90:10 through the optical fiber 1*2 coupler 4, and the 90% branch enters the non-1550nm channel input end of the wavelength division multiplexer 5 as the signal light with a wavelength of λ2, and the 10% branch enters the a port of the optical fiber 3dB coupler 11 as the reference light; the wavelength division multiplexer 5 combines the two signal lights of different wavelengths into a dual-wavelength detection light source, and outputs it to the acousto-optic modulator 6, which is modulated as Dual-wavelength pulse light; a signal source 17 generates a modulated pulse in channel 1, which is input into an acousto-optic modulator driver 18, and the acousto-optic modulator driver 18 is connected to the acousto-optic modulator 6; the dual-wavelength pulse light is amplified by the erbium-doped fiber amplifier 7 to become a dual-wavelength detection pulse; the dual-wavelength detection pulse enters the a port of the circulator 8, and then is injected into the sensing optical fiber 15 through the b port of the circulator 8; the tail end of the sensing optical fiber 15 is connected to the optical fiber isolator 16; the backward Rayleigh scattered light generated during the forward transmission of the dual-wavelength detection pulse returns to the b port of the circulator 8, and then is output to the wavelength division multiplexer 9 through the c port of the circulator 8; the wavelength division multiplexer 9 separates the two different wavelengths of backward Rayleigh scattered light, the wavelength is λ1 The backward Rayleigh scattered light with a wavelength of λ2 is input into the b port of the optical fiber 3dB coupler 10, and the backward Rayleigh scattered light with a wavelength of λ2 is input into the b port of the optical fiber 3dB coupler 11, and beats with the two 10% branch reference lights of different wavelengths; the two outputs of the optical fiber 3dB coupler 10 enter the first photoelectric balanced detector 12 for photoelectric conversion, and the two outputs of the optical fiber 3dB coupler 11 enter the second photoelectric balanced detector 13 for photoelectric conversion; the two analog electrical signals after photoelectric conversion are synchronously collected by the high-speed digitizer 14 and transmitted to the host computer 19 for data demodulation; the signal source 17 channel 2 generates a trigger pulse synchronized with channel 1 to trigger the high-speed digitizer 14.

[0038] In the above technical solution, the wavelengths of the semiconductor narrowband laser 1 and the tunable fiber laser 2 are λ1 and λ2 respectively, λ1 is fixed at 1550nm, and the line width is less than 3KHz, and the wavelength tuning range of λ2 is 1480nm-1640nm, and the line width is less than 60KHz.

[0039] In the above technical solution, two signal lights of different wavelengths are combined through the wavelength division multiplexer 5 as a dual-wavelength detection light source. The central wavelength of the wavelength division multiplexer 5 is 1530nm-1570nm, the wavelength interval is 5nm, and the number of multiplexed channels is 9. In actual applications, only two of the channels are occupied at the same time, one channel is 1550nm, and the other channel is one of the remaining channels except 1550nm. By adjusting the wavelength interval of the two channels, the degree of expansion of the dynamic range can be adjusted.

[0040] In the above technical solution, the dual-wavelength detection light source is modulated into a dual-wavelength pulse light by an acousto-optic modulator 6, the frequency shift of the acousto-optic modulator 6 is 200MHz, the acousto-optic modulator 6 is provided with a signal source 17 and an acousto-optic modulator driver 18, the signal source 17 modulates the pulse width to be 50-150ns, and the pulse repetition frequency is less than n / 2cL , where n is the refractive index of the single-mode fiber, c is the speed of light in vacuum, and L is the length of the sensing fiber.

[0041] In the above technical solution, the peak power of the dual-wavelength pulse light is amplified by the erbium-doped fiber amplifier 7 as a dual-wavelength detection pulse, and the pulse peak power is greater than 28dBm. The operating wavelength of the erbium-doped fiber amplifier 7 is 1528nm-1570nm. Within this range, the erbium-doped fiber amplifier 7 has a relatively flat gain.

[0042] In the above technical solution, the system detection pulse is injected into the sensing optical fiber through the port of the circulator 8b, and the dual-wavelength backward Rayleigh scattered light returns to the port of the circulator 8c.

[0043] In the above technical solution, the de-wavelength division multiplexer 9 separates the dual-wavelength backward Rayleigh scattered light returned from the detection optical fiber, and the de-wavelength division multiplexer 9 and the wavelength division multiplexer 5 have the same technical parameters.

[0044] In the above technical scheme, the two separated backward lights of different wavelengths and the 10% reference lights of two different wavelengths are coherently beat in the optical fiber 3dB coupler 10 and the optical fiber 3dB coupler 2 11. After the beat, coherent detection is achieved through the first photoelectric balanced detector 12 and the second photoelectric balanced detector 13. The bandwidth of the two photoelectric balanced detectors is 350MHz, the gain is 1000V / W, the noise equivalent power is 16pw / √Hz, and the wavelength response range is 1100nm-1700nm.

[0045] In the above technical solution, the dual-channel high-speed digitizer 14 collects two electrical signals and transmits them to the host computer 19 for demodulation, with a sampling rate of 1Gsa / s and a resolution of 12 bits. When demodulating data, firstly, the two collected data are digitally bandpass filtered; then the bandpass filtered data are digitally orthogonally demodulated, and the two data are cross-multiplied with a pair of orthogonal signals of the same frequency generated by the host computer; then a low-pass filter is used to remove the sum frequency component after digital orthogonal demodulation, and the zero-frequency component containing phase and amplitude information is retained; finally, the external disturbance event is located by amplitude difference, and the wrapped phase at the vibration position is extracted by inverse tangent function and differential operation, and the wrapped phase is brought into the dual-wavelength linear programming phase unwrapping algorithm to restore the actual phase change, so that the external disturbance signal can be quantitatively restored.

[0046] In the above technical solution, the data demodulation process is as shown in the attached Figure 2 As shown, the specific steps include:

[0047] Step 1: First, perform digital bandpass filtering on the two channels of collected data. The center frequency of the bandpass filter is 200 MHz and the passband bandwidth is 80 MHz.

[0048] Step 2: Perform digital orthogonal demodulation on the bandpass filtered data, cross-multiply the two-channel data with a pair of orthogonal signals of the same frequency generated by the host computer to generate sum frequency components and zero frequency components;

[0049] Step 3: Use a low-pass filter to remove the sum frequency component after digital orthogonal demodulation and retain the zero-frequency component containing phase and amplitude information. The cutoff frequency of the low-pass filter is 50MHz.

[0050] Step 4: Locate the external disturbance event through amplitude difference; extract the phase change at the vibration position through the inverse tangent function and differential operation to form a continuous phase change process in the time domain, whose value may not satisfy the π phase principle and may be wrapped;

[0051] Step 5: The actual phase change introduced by the dynamic strain is restored through the dual-wavelength linear programming phase unwrapping algorithm, and the external disturbance signal can be quantitatively restored.

[0052] In the above technical solution, the principle and specific process of the dual-wavelength linear programming phase unwrapping algorithm are as follows:

[0053] 1. First, the optical path lengths of lasers with wavelengths of λ1 and λ2 under the same strain are:

[0054] (1)

[0055] (2)

[0056] Among them, φλ1 (t) and φ λ2 (t) represents the wrapped phase value obtained by step 5 of the data demodulation process, τ(t) and ʋ(t) represent the phase wrapping coefficients, respectively. Since λ1 and λ2 are transmitted in the same optical fiber and measure the same strain at the same time, all optical paths should be equal. However, considering the existence of noise, R λ1 and R λ2 There will be a small difference between them, so at any time, there is always a pair of integers τ and ʋ such that R λ1 and R λ2 The difference between them is the smallest, and the calculation formula is shown in formula (3):

[0057] (3)

[0058] 2. After determining the convergence criteria of the linear programming, it is also necessary to determine the boundary conditions such as equations (4) and (5) to prevent the problem of non-convergence during the linear programming solution process, where λ ds =λ1*λ2 / (λ1+λ2), round() means rounding operation:

[0059] (4)

[0060] (5)

[0061] 3. Based on the convergence criteria of formula (3) and the boundary conditions of formula (4) and formula (5), after solving τ(t) and ʋ(t) by the linear programming method, they are substituted into formula (1) or formula (2) to obtain the change of optical path under the action of external dynamic strain. Since the change of optical path is linearly related to the intensity of the external disturbance signal, the external disturbance can be quantitatively restored after calibration. It can be seen that the dual-wavelength linear programming phase unwrapping algorithm is only related to the phase values ​​obtained by the two single-wavelength measurements at the current moment, and has nothing to do with the phase values ​​at the previous moment. Therefore, the response bandwidth of the system is guaranteed, making the system response bandwidth close to the Nyquist frequency.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A device for improving the dynamic strain range of a phase-sensitive optical time-domain reflectometer, characterized in that: It comprises a semiconductor narrowband laser (1) and a tunable fiber laser (2), wherein the semiconductor narrowband laser (1) is connected to a first optical fiber 1*2 coupler (3), and the tunable fiber laser (2) is connected to a second optical fiber 1*2 coupler (4); The reference light outputted by the optical fiber 1*2 coupler 1 (3) enters the port a of the optical fiber 3dB coupler 1 (10), and the reference light outputted by the optical fiber 1*2 coupler 2 (4) enters the port a of the optical fiber 3dB coupler 2 (11); The optical fiber 1*2 coupler 1 (3) and the optical fiber 1*2 coupler 2 (4) respectively output different signal lights and enter the wavelength division multiplexer (5) to combine the two signal lights of different wavelengths into a dual-wavelength detection light source; The output end of the wavelength division multiplexer (5) is connected to the acousto-optic modulator (6), the erbium-doped fiber amplifier (7) and the a port of the circulator (8) in sequence, the b port of the circulator (8) is connected to the sensing fiber (15), the tail end of the sensing fiber (15) is connected to the optical fiber isolator (16), and the c port of the circulator (8) is connected to the wavelength division multiplexer (9); The output end of the wavelength division multiplexer (9) outputs the backward Rayleigh scattered light with a wavelength of λ1 to the b port of the optical fiber 3dB coupler 1 (10), and the backward Rayleigh scattered light with a wavelength of λ2 is input to the b port of the optical fiber 3dB coupler 2 (11); The optical fiber 3dB coupler 1 (10) is connected to the first photoelectric balanced detector (12), the optical fiber 3dB coupler 2 (11) is connected to the second photoelectric balanced detector (13), the first photoelectric balanced detector (12) and the second photoelectric balanced detector (13) are connected to the high-speed digitizer (14), and the high-speed digitizer (14) is connected to the host computer (19); The wavelengths output by the semiconductor narrowband laser (1) and the tunable fiber laser (2) are λ1 and λ2 respectively. The optical path difference generated by the two lasers with different wavelengths is utilized, and the minimum square of the optical path difference is used as the convergence criterion. A linear programming phase unwrapping method is used to restore the true phase, thereby improving the dynamic strain range of the phase-sensitive optical time domain reflectometer. The convergence criterion in the linear programming phase unwrapping method is: Its boundary conditions are: ， ， in, and represents the optical path length of lasers with wavelengths λ1 and λ2 under the same strain, φ λ1 (t) and φ λ2 (t) represents the wrapped phase value of different wavelengths, τ(t) and ʋ(t) represent the phase wrapping coefficient, λ ds =λ1*λ2 / (λ1+λ2).

2. The device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 1, characterized in that: The wavelength of λ1 is fixed at 1550nm, with a line width of less than 3kHz; the wavelength tuning range of λ2 is 1480-1640nm, with a line width of less than 60kHz; the signal light with a wavelength of λ1 output by the semiconductor narrowband laser (1) enters the 1550nm channel input end of the wavelength division multiplexer (5); and the signal light with a wavelength of λ2 output by the tunable fiber laser (2) enters the non-1550nm channel input end of the wavelength division multiplexer (5).

3. The device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 1, characterized in that: 90% of the branches output by the optical fiber 1*2 coupler 1 (3) are used as signal light with a wavelength of λ1, and 10% of the branches are used as reference light; 90% of the branches output by the optical fiber 1*2 coupler 2 (4) are used as signal light with a wavelength of λ2, and 10% of the branches are used as reference light.

4. The device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 1, characterized in that: The central wavelength of the wavelength division multiplexer (5) is 1530nm-1570nm, the wavelength interval is 5nm, and the number of multiplexed channels is 9.

5. The device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 1, characterized in that: The acousto-optic modulator (6) is connected to an acousto-optic modulator driver (18), and the acousto-optic modulator driver (18) is connected to a signal source (17); the frequency shift of the acousto-optic modulator (6) is 200 MHz, the modulation pulse width generated by the signal source (17) is 50-150 ns, and the pulse repetition frequency is less than n / 2cL ,in, n is the refractive index of single-mode fiber, c is the speed of light in vacuum, L is the sensing optical fiber length.

6. The device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 1, characterized in that: The wavelength division multiplexer (9) has the same technical parameters as the wavelength division multiplexer (5).

7. The device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 1, characterized in that: The bandwidth of the first photoelectric balanced detector (12) and the second photoelectric balanced detector (13) are both 350 MHz, the gain is 1000 V / W, and the noise equivalent power is 16 pw / √Hz.

8. A modulation method for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 1, characterized in that: The following steps are included: Step 1: The output of the semiconductor narrowband laser (1) is divided into two paths with a ratio of 90:10 through the optical fiber 1*2 coupler 1 (3), and the 90% branch is used as the signal light with a wavelength of λ1 to enter the 1550nm channel input end of the wavelength division multiplexer (5), and the 10% branch is used as the reference light to enter the a port of the optical fiber 3dB coupler 1 (10); The output of the tunable fiber laser (2) is divided into two paths with a ratio of 90:10 through the optical fiber 1*2 coupler 2 (4), and the 90% branch is used as the signal light with a wavelength of λ2 to enter the non-1550nm channel input end of the wavelength division multiplexer (5), and the 10% branch is used as the reference light to enter the a port of the optical fiber 3dB coupler 2 (11); Step 2: The wavelength division multiplexer (5) combines the two signal lights of different wavelengths into a dual-wavelength detection light source, and then outputs the combined light to the acousto-optic modulator (6) to modulate the light into a dual-wavelength pulse light; Step 3: After the peak power of the dual-wavelength pulse light is amplified by the erbium-doped fiber amplifier (7), it becomes a dual-wavelength detection pulse; the dual-wavelength detection pulse enters the a port of the circulator (8), and is then injected into the sensing optical fiber (15) through the b port of the circulator (8); the tail end of the sensing optical fiber (15) is connected to the optical fiber isolator (16); the backward Rayleigh scattered light generated during the forward transmission of the dual-wavelength detection pulse returns to the b port of the circulator (8), and is then output to the wavelength division multiplexer (9) through the c port of the circulator (8); Step 4: The wavelength division multiplexer (9) separates the backscattered Rayleigh lights of two different wavelengths. The backscattered Rayleigh light of wavelength λ1 is input to port b of the first optical fiber 3dB coupler (10), and the backscattered Rayleigh light of wavelength λ2 is input to port b of the second optical fiber 3dB coupler (11), and beats with the two 10% branch reference lights of different wavelengths. Step 5: The two outputs of the optical fiber 3dB coupler 1 (10) enter the first photoelectric balanced detector (12) for photoelectric conversion, and the two outputs of the optical fiber 3dB coupler 2 (11) enter the second photoelectric balanced detector (13) for photoelectric conversion; the two analog electrical signals after photoelectric conversion are synchronously collected by a high-speed digitizer (14).

9. A demodulation method for the device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer as claimed in claim 1, characterized in that: The following steps are included: Step 1: First, perform digital bandpass filtering on the two channels of collected data. The center frequency of the bandpass filter is 200 MHz and the passband bandwidth is 80 MHz. Step 2: Perform digital orthogonal demodulation on the bandpass filtered data, cross-multiply the two-channel data with a pair of orthogonal signals of the same frequency generated by the host computer to generate sum frequency components and zero frequency components; Step 3: Use a low-pass filter to remove the sum frequency component after digital orthogonal demodulation and retain the zero-frequency component containing phase and amplitude information. The cutoff frequency of the low-pass filter is 50MHz. Step 4: Locate the external disturbance event by amplitude difference; extract the wrapping phase at the vibration position by inverse tangent function and difference operation; Step 5: The actual phase change is restored through the dual-wavelength linear programming phase unwrapping method, and the external disturbance signal can be quantitatively restored.

10. The demodulation method of the device for improving the dynamic strain range of a phase sensitive optical time domain reflectometer according to claim 9, characterized in that: The convergence criterion in the dual-wavelength linear programming phase unwrapping method in step 5 is: Its boundary conditions are: ， ， in, and represents the optical path length of lasers with wavelengths λ1 and λ2 under the same strain, φ λ1 (t) and φ λ2 (t) represents the wrapped phase value of different wavelengths, τ(t) and ʋ(t) represent the phase wrapping coefficient, λ ds =λ1*λ2 / (λ1+λ2).

Images