Phase-sensitive optical time domain reflection optical fiber sensing system and phase demodulation method

Abstract

The invention discloses a phase-sensitive optical time domain reflection optical fiber sensing system and a phase demodulation method. The method comprises the following steps: acquiring two paths of beat frequency signals; performing Hilbert transform and time translation transform on the two paths of signals respectively; performing trigonometric function transformation of integration and difference on the obtained signals; performing addition and subtraction processing on the signals obtained after transformation to obtain two paths of signals; and performing arc tangent transformation on the obtained two paths of mutually orthogonal signals to extract phases. The method is used for demodulating and recovering phase information of a sound signal and a vibration signal in a distributed optical fiber sensing system, can effectively reduce the influence of polarization fading noise caused by mismatching of the polarization state of signal light in an optical fiber and the polarization state of reference light, and improves the accuracy of vibration positioning and phase demodulation of the system. Therefore, the missed and false alarm problems of the system are effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber signal processing, and in particular to a phase-sensitive optical time-domain reflection optical fiber sensing system and a phase demodulation method. Background Art

[0002] Phase-sensitive optical time-domain reflectometry fiber optic sensing system is a very powerful technology that can achieve fast response and high sensitivity of fully distributed vibration sensing. Phase-sensitive optical time-domain reflectometry fiber optic sensing system can detect and quantify very small strain changes by simply measuring the optical phase difference of backscattered light from adjacent positions in the optical fiber. It has been applied in many fields such as optical cable construction and maintenance, safety protection, and traffic monitoring.

[0003] However, the polarization fading noise in the phase-sensitive optical time-domain reflectometry fiber optic sensing system is a key issue that affects the improvement of its signal-to-noise ratio. When the polarization diversity module is not used, if the polarization state of the Rayleigh scattering signal does not match the polarization state of the reference light, the signal strength at certain positions will be close to 0, causing the signal to be submerged in the noise. The polarization fading phenomenon will cause the demodulation result of the system to deteriorate, the signal-to-noise ratio (SNR) to decrease, and the demodulation result to be distorted, resulting in system leakage and false alarms, affecting system performance. Summary of the invention

[0004] The purpose of the present invention is to provide a phase-sensitive optical time domain reflection fiber optic sensing system and a phase demodulation method to eliminate the polarization fading phenomenon caused by the mismatch between the polarization state of the signal light and the polarization state of the reference light in the optical fiber, effectively reduce the system underreporting rate and false alarm rate, and improve the accuracy of the system phase demodulation.

[0005] In order to solve the above technical problems, the present invention provides a phase-sensitive optical time-domain reflectometry fiber-optic sensing system based on polarization diversity technology, comprising a narrow linewidth laser, a first fiber coupler, an acousto-optic modulator, a fiber amplifier, an optical circulator, a first polarization beam splitter, a second polarization beam splitter, a second fiber coupler, a third fiber coupler, a first photodetector, a second photodetector, a data acquisition device and the digital demodulation unit, which are sequentially arranged along the optical path direction; The light emitted by the narrow linewidth laser is divided into a beam a and a beam b by the first optical fiber coupler; the beam a is modulated by the acousto-optic modulator and the optical fiber amplifier and then enters the optical circulator, and then outputs to the sensing optical fiber through the optical circulator. The Rayleigh scattered light carrying vibration information returned by the sensing optical fiber is divided into a first P light and a first S light by the first polarization beam splitter; the beam b is divided into a second P light and a second S light with vibration directions perpendicular to each other by the second polarization beam splitter; A first beat frequency signal generated after the first P light and the second P light are coupled through the second optical fiber coupler is transmitted to the digital demodulation unit by the first photodetector, and a second beat frequency signal generated after the first S light and the second S light are coupled through the third optical fiber coupler is transmitted to the digital demodulation unit by the second photodetector; the digital demodulation unit performs phase extraction processing on the first beat frequency signal and the second beat frequency signal. According to the above scheme, it is characterized in that the digital demodulation unit performs phase extraction processing on the first beat frequency signal and the second beat frequency signal, including: The first beat frequency signal is obtained and the second beat frequency signal Perform Hilbert transform and get the signal and signal ; Simulate the generation of two beat frequency signals with the first one , the second beat frequency signal is the orthogonal signal of adjacent pulse periods , ,Signal , The first beat frequency signal , the second beat frequency signal Same frequency, different time ; Signal Perform Hilbert transform to get the signal ; For the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; For the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Add the signal ; Signal ,Signal Add the signal ; Signal ,Signal Perform differential cross-multiplication to extract the phase.

[0006] According to the above scheme, it is characterized in that the first beat frequency signal and the second beat frequency signal are respectively:

[0007] In the formula, , and Represent the light field amplitudes of signal light and intrinsic light respectively; Indicates the difference angle between the polarization states of the signal light direction and the intrinsic light direction; The frequency shift of the optical signal introduced by the AOM; For time; is the phase of the interference field.

[0008] According to the above scheme, it is characterized in that the signal and signal Respectively expressed as: .

[0009] According to the above scheme, it is characterized in that the signal and signal Respectively expressed as:

[0010] In the formula, and The amplitude and position-related phase of the next adjacent heterodyne signal correspond to each other; is the difference angle between the polarization states of the signal light direction and the intrinsic light direction at that moment.

[0011] According to the above scheme, the signal It is expressed as: .

[0012] According to the above scheme, it is characterized in that the signal It is expressed as:

[0013] In the formula, is the phase change between two different backscattered Rayleigh signals caused by external vibrations, is the time interval between two adjacent detection pulses; Signal It is expressed as:

[0014] Signal It is expressed as: .

[0015] According to the above scheme, it is characterized in that the signal It is expressed as:

[0016] Signal It is expressed as:

[0017] Signal It is expressed as: .

[0018] According to the above scheme, it is characterized in that according to the signal ,Signal The extracted phase is expressed as: .

[0019] The present invention also provides a phase demodulation method for acquiring a first beat frequency signal. and the second beat frequency signal Performing demodulation, the method comprises: The first beat frequency signal is obtained and the second beat frequency signal Perform Hilbert transform and get the signal and signal ; Simulate the generation of two beat frequency signals with the first one , the second beat frequency signal is the orthogonal signal of adjacent pulse periods , ,Signal , The first beat frequency signal , the second beat frequency signal Same frequency, different time ; Signal Perform Hilbert transform to get the signal ; For the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; For the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Add the signal ; Signal ,Signal Add the signal ; Signal ,Signal Perform differential cross-multiplication to extract the phase.

[0020] Beneficial Effects The present invention realizes digital demodulation of the phase of a phase-sensitive optical time domain reflectometry fiber optic sensing system based on polarization diversity technology by performing a series of algorithms such as Hilbert transform, product sum and difference, differential cross multiplication, etc. on two beat frequency signals. The polarization fading phenomenon caused by the mismatch between the polarization state of the signal light and the polarization state of the reference light in the optical fiber can be eliminated, the system underreporting rate and false alarm rate can be effectively reduced, and the accuracy of the system phase demodulation can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the structure of a phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to an embodiment of the present invention.

[0022] Figure 2 It is a digital demodulation flow chart of an embodiment of the present invention.

[0023] In the figure: 1-narrow linewidth laser, 2-first fiber coupler, 3-acousto-optic modulator, 4-fiber amplifier, 5-bandpass filter, 6-three-port circulator, 7-sensing fiber, 8-first polarization beam splitter, 9-second polarization beam splitter, 10-second polarization-maintaining fiber coupler, 11-first photodetector, 12-third polarization-maintaining fiber coupler, 13-second photodetector, 14-data acquisition device, 15-digital demodulation unit. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the embodiments of the present disclosure clearer, the technical solution of the embodiments of the present disclosure will be clearly and completely described below in conjunction with the drawings of the embodiments of the present disclosure. Obviously, the described embodiments are part of the embodiments of the present disclosure, not all of the embodiments. Based on the described embodiments of the present disclosure, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present disclosure.

[0025] See also Figure 1 The present embodiment discloses a phase digital demodulation system of a phase-sensitive optical time-domain reflectometry fiber sensing system based on polarization diversity technology, comprising a narrow linewidth laser 1, a first fiber coupler 2, an acousto-optic modulator 3, a fiber amplifier 4, a bandpass filter 5, a three-port circulator 6, a sensing fiber 7, a first polarization beam splitter 8, a second polarization beam splitter 9, a second polarization-maintaining fiber coupler 10, a first photodetector 11, a third polarization-maintaining fiber coupler 12, a second photodetector 13, a data acquisition device 14, and a digital demodulation unit 15, which are sequentially arranged along the optical path direction. The connection mode of the system is as follows: the output end of the narrow linewidth 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 the input end of the acousto-optic modulator 3, the output end of the acousto-optic modulator 3 is connected to the input end of the optical fiber amplifier 4, the output end of the optical fiber amplifier 4 is connected to the input end of the bandpass filter 5, the output end of the bandpass filter 5 is connected to the input end of the three-port circulator 6, the output end of the three-port circulator 6 is connected to the sensing optical fiber 7, the reflection end of the three-port circulator 6 is connected to the input end of the first polarization beam splitter 8, the second output end of the first optical fiber coupler 2 is connected to the input end of the second polarization beam splitter 9, and the first output end of the first polarization beam splitter 8 is connected to the first output end of the second optical fiber coupler 10. The first polarization beam splitter 8 is connected to the first input end of the third fiber coupler 10, the first output end of the second polarization beam splitter 9 is connected to the second input end of the second polarization-maintaining fiber coupler 10, the second output end of the second polarization beam splitter 9 is connected to the second input end of the third polarization-maintaining fiber coupler 10, the two output ends of the second polarization-maintaining fiber coupler 10 are connected to the two input ends of the first photodetector 11, the two output ends of the third polarization-maintaining fiber coupler 12 are connected to the two input ends of the second photodetector 13, the output end of the first photodetector 11 and the output end of the second photodetector 13 are connected to the data acquisition device 14, and the data acquisition device 14 is connected to the digital demodulation unit 15. The processing and analysis of the two-way beat frequency signals by the digital demodulation unit 15 include: Hilbert transform, trigonometric function product, addition and subtraction transform, differential cross multiplication, and phase extraction.

[0026] The working principle of the system is as follows: a narrow linewidth laser 1 emits a light source with an operating wavelength of 1550nm, enters the 90:10 first coupler 2 and is divided into two paths. The path with high power is used as the signal light, and the acousto-optic modulator 3 generates a 200M frequency shift and modulates the optical signal into a pulse light. The modulated pulse light passes through the fiber amplifier 4 to amplify the pulse light energy. Since the fiber amplifier 4 generates ASE noise, a bandpass filter 5 with an operating wavelength of 1550nm and a bandwidth of 1nm is used to filter out the ASE noise generated by the fiber amplifier 4. The signal light after the noise is filtered out by the bandpass filter passes through a three-port circulator 6, the three ports of the three-port circulator 6 are respectively connected to the signal light passing through the bandpass filter 5, the sensing fiber 7, and the backward Rayleigh scattered signal containing vibration information transmitted back through the sensing fiber 7. The transmitted signal light is divided into P light and S light after passing through the first polarization beam splitter 8, and the other local oscillation light is divided into P light and S light with mutually perpendicular vibration directions after passing through the second polarization beam splitter 9. The first beat frequency signal generated by the coupling of P light and P light through the second polarization-maintaining fiber coupler 10 enters the first photodetector 11, and the second beat frequency signal generated by the coupling of S light and S light through the third polarization-maintaining fiber coupler 12 enters the second photodetector 13. The first photodetector 11 and the second photodetector 13 convert the optical signal into an electrical signal, and after transmitting the electrical signal to the data acquisition device 14, it enters the digital demodulation unit 15.

[0027] Furthermore, the optical fiber amplifier 4 may be an erbium-doped optical fiber amplifier.

[0028] See also Figure 2 , the processing steps of the digital demodulation unit 15 are as follows (vibration occurs somewhere in the optical fiber, causing the phase of the backscattered signal to change, and the digital demodulation unit jointly processes the two phases to obtain the vibration change): S1, respectively obtain the first beat frequency signal and the second beat frequency signal The signal (obtained by the data acquisition device 14) is subjected to Hilbert transform to obtain the signal and signal ; The first beat frequency signal and the second beat frequency signal Respectively expressed as:

[0029] In the formula, , and Represent the light field amplitudes of signal light and intrinsic light respectively; Indicates the difference angle between the polarization states of the signal light direction and the intrinsic light direction; The frequency shift of the optical signal introduced by the AOM; For time; is the phase of the interference field; Signal and signal Respectively expressed as:

[0030] S2, simulate and generate two-way beat frequency signals with the first way , the second beat frequency signal is the orthogonal signal of adjacent pulse periods , ,Signal , The first beat frequency signal , the second beat frequency signal Same frequency, different time ; Signal and signal Respectively expressed as:

[0031] In the formula, and The amplitude and position-related phase of the next adjacent heterodyne signal correspond to each other; is the difference angle between the polarization states of the signal light direction and the intrinsic light direction at that moment; S3, signal Perform Hilbert transform to get the signal ; Signal It is expressed as:

[0032] S4, for the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal It is expressed as:

[0033] S5. Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal It is expressed as:

[0034] S6. Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal It is expressed as:

[0035] S7, for the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal It is expressed as:

[0036] In the above equation, can be regarded as a constant and ignored, while the signal , , , It contains high frequency terms and , so the above four formulas can be further processed by the following different steps S8, signal ,Signal Add the signal ; Signal It is expressed as:

[0037] S9, signal ,Signal Add the signal ; Signal It is expressed as:

[0038] S10, signal ,Signal Perform a differential cross-multiplication operation (inverse tangent transform) to extract the phase.

[0039] The extracted phase is expressed as:

[0040] This embodiment provides a phase-sensitive optical time domain reflection optical fiber sensing system and phase demodulation method based on polarization diversity technology, which is used for phase demodulation analysis of coherent photoelectric signals of phase-sensitive optical time domain reflectometer system, and can realize digital demodulation of phase of phase-sensitive optical time domain reflectometer system, reduce the influence of polarization fading noise on signal demodulation results, and then realize real-time analysis of distributed optical fiber vibration sensing signals, so as to effectively obtain information such as external vibration of optical fiber. After using the polarization diversity module, the Rayleigh scattering signal is divided into S light and P light, which interfere with the S light and P light of the reference light respectively. At certain positions of the optical fiber, when the intensity of the coherent light received by the S path is close to 0, the intensity of the coherent light received by the P path will increase, and vice versa. The algorithm proposed by the present invention can jointly process the P light and S light signals demodulated from the same position point, select the signal with larger intensity, realize anti-polarization fading, effectively reduce the influence of polarization fading noise, improve the signal-to-noise ratio and the accuracy of the demodulation result, thereby solving the problem of underreporting and false alarm of DAS system.

[0041] It should be pointed out that, according to the needs of implementation, the various steps / components described in this application can be split into more steps / components, and two or more steps / components or partial operations of steps / components can be combined into new steps / components to achieve the purpose of the present invention.

[0042] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology, characterized in that: It includes a narrow line width laser, a first fiber coupler, an acousto-optic modulator, a fiber amplifier, an optical circulator, a first polarization beam splitter, a second polarization beam splitter, a second fiber coupler, a third fiber coupler, a first photodetector, a second photodetector, a data acquisition device and the digital demodulation unit, which are sequentially arranged along the optical path direction; The light emitted by the narrow linewidth laser is divided into a beam a and a beam b by the first optical fiber coupler; the beam a is modulated by the acousto-optic modulator and the optical fiber amplifier and then enters the optical circulator, and then outputs to the sensing optical fiber through the optical circulator. The Rayleigh scattered light carrying vibration information returned by the sensing optical fiber is divided into a first P light and a first S light by the first polarization beam splitter; the beam b is divided into a second P light and a second S light with vibration directions perpendicular to each other by the second polarization beam splitter; A first beat frequency signal generated after the first P light and the second P light are coupled through the second optical fiber coupler is transmitted to the digital demodulation unit by the first photodetector, and a second beat frequency signal generated after the first S light and the second S light are coupled through the third optical fiber coupler is transmitted to the digital demodulation unit by the second photodetector; the digital demodulation unit performs phase extraction processing on the first beat frequency signal and the second beat frequency signal.

2. The phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 1, characterized in that: The digital demodulation unit performs phase extraction processing on the first beat frequency signal and the second beat frequency signal, including: The first beat frequency signal is obtained and the second beat frequency signal Perform Hilbert transform and get the signal and signal ; Simulate the generation of two beat frequency signals with the first one , the second beat frequency signal is the orthogonal signal of adjacent pulse periods , ,Signal , The first beat frequency signal , the second beat frequency signal Same frequency, different time ; Signal Perform Hilbert transform to get the signal ; For the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; For the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Add the signal ; Signal ,Signal Add the signal ; Signal ,Signal Perform differential cross-multiplication to extract the phase.

3. The phase-sensitive optical time domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 2, characterized in that: The first beat frequency signal and the second beat frequency signal are:

4. In the formula, , and Represent the light field amplitudes of signal light and intrinsic light respectively; Indicates the difference angle between the polarization states of the signal light direction and the intrinsic light direction; The frequency shift of the optical signal introduced by the AOM; For time; is the phase of the interference field.

5. The phase-sensitive optical time domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 3, characterized in that: Signal and signal Respectively expressed as: 。 6. The phase-sensitive optical time domain reflectometry optical fiber sensing system based on polarization diversity technology according to claim 3, characterized in that: Signal and signal Respectively expressed as:

7. In the formula, and The amplitude and position-related phase of the next adjacent heterodyne signal correspond to each other; is the difference angle between the polarization states of the signal light direction and the intrinsic light direction at that moment.

8. The phase-sensitive optical time domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 3, characterized in that: Signal It is expressed as: 。 9. The phase-sensitive optical time domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 6, characterized in that: Signal It is expressed as:

10. In the formula, is the phase change between two different backscattered Rayleigh signals caused by external vibrations, is the time interval between two adjacent detection pulses; Signal It is expressed as:

11. Signal It is expressed as: 。 12. The phase-sensitive optical time-domain reflectometry optical fiber sensing system based on polarization diversity technology according to claim 7, characterized in that: Signal It is expressed as:

13. Signal It is expressed as:

14. Signal It is expressed as: 。 15. The phase-sensitive optical time-domain reflectometry optical fiber sensing system based on polarization diversity technology according to claim 8, characterized in that: According to the signal ,Signal The extracted phase is expressed as: 。 16. A phase demodulation method, characterized in that: Used to obtain the first beat frequency signal and the second beat frequency signal Performing demodulation, the method comprises: The first beat frequency signal is obtained and the second beat frequency signal Perform Hilbert transform and get the signal and signal ; Simulate the generation of two beat frequency signals with the first one , the second beat frequency signal is the orthogonal signal of adjacent pulse periods , ,Signal , The first beat frequency signal , the second beat frequency signal Same frequency, different time ; Signal Perform Hilbert transform to get the signal ; For the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; For the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ; Signal ,Signal Add the signal ; Signal ,Signal Add the signal ; Signal ,Signal Perform differential cross-multiplication to extract the phase.

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