A phase-sensitive optical time-domain reflectometry fiber optic sensing system and phase demodulation method

Abstract

The present invention discloses a phase-sensitive optical time-domain reflectometry fiber optic sensing system and phase demodulation method, the method comprising: obtaining two beat frequency signals; performing Hilbert transform and time translation transform on the two signals respectively; performing product and difference trigonometric function transform on the signals obtained above; performing addition and subtraction processing on the signals obtained after the transformation to obtain two signals; and performing inverse tangent transform on the two mutually orthogonal signals obtained to extract the phase. The method is used to demodulate and recover the phase information of acoustic and vibration signals in a distributed fiber optic sensing system, and can effectively reduce the impact of polarization fading noise caused by the mismatch between the polarization state of the signal light in the optical fiber and the polarization state of the reference light, improve the accuracy of the system vibration positioning and phase demodulation, and thus effectively improve the system's leakage and false alarm problems.

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 (PTDR) fiber-optic sensing is a powerful technology that enables fast-response and highly sensitive fully distributed vibration sensing. PTDR can detect and quantify very small strain changes by simply measuring the optical phase difference of backscattered light from adjacent locations in the fiber. It has applications in a wide range of fields, including fiber optic cable construction and maintenance, security, and traffic monitoring.

[0003] However, polarization fading noise in phase-sensitive optical time-domain reflectometry fiber-optic sensing systems is a key issue hindering their signal-to-noise ratio (SNR). Without a polarization diversity module, if the polarization state of the Rayleigh scattered signal and the reference light do not match, the signal strength at certain locations will approach zero, drowning the signal in the noise. Polarization fading degrades the system's demodulation results, reducing the signal-to-noise ratio (SNR), distorting the demodulation results, and leading to missed detections and false alarms, impacting system performance. Summary of the Invention

[0004] The purpose of the present invention is to provide a phase-sensitive optical time-domain reflectometry 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's missed alarm rate and false alarm rate, and improve the accuracy of the system's phase demodulation.

[0005] 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.

[0006] Light emitted by a narrow-linewidth laser is split into beam a and beam b via a first fiber coupler. Beam a is modulated by an acousto-optic modulator and a fiber amplifier before entering an optical circulator. The optical circulator then outputs the Rayleigh scattered light carrying vibration information returned from the sensing fiber. The light is split into a first P beam and a first S beam via a first polarization beam splitter. Beam b is split into a second P beam and a second S beam, each with perpendicular vibration directions, via a second polarization beam splitter.

[0007] A first beat signal generated by coupling the first P light and the second P light through the second optical fiber coupler is transmitted to a digital demodulation unit by the first photodetector, and a second beat signal generated by coupling the first S light and the second S light 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 signal and the second beat signal.

[0008] According to the above solution, 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:

[0009] The first beat frequency signal is obtained and the second beat frequency signal Perform Hilbert transform to get the signal and signal ;

[0010] Simulate and generate 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 ;

[0011] Signal Perform Hilbert transform to get the signal ;

[0012] For the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0013] Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0014] Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0015] For the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0016] Signal ,Signal Add the signal ;

[0017] Signal ,Signal Add the signal ;

[0018] Signal ,Signal Perform differential cross-multiplication to extract the phase.

[0019] According to the above solution, it is characterized in that the first beat frequency signal and the second beat frequency signal are respectively:

[0020]

[0021] Where, , 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.

[0022] According to the above scheme, it is characterized in that the signal and signal Respectively expressed as:

[0023] .

[0024] According to the above scheme, it is characterized in that the signal and signal Respectively expressed as:

[0025]

[0026] Where, and The amplitude and position-related phase corresponding to the next adjacent heterodyne signal respectively; is the difference angle between the polarization states of the signal light direction and the intrinsic light direction at that moment.

[0027] According to the above scheme, the signal Expressed as:

[0028] .

[0029] According to the above scheme, it is characterized in that the signal Expressed as:

[0030]

[0031] Where, is the phase change between two different backscattered Rayleigh signals caused by external vibration, is the time interval between two adjacent detection pulses;

[0032] Signal Expressed as:

[0033]

[0034] Signal Expressed as:

[0035] .

[0036] According to the above scheme, it is characterized in that the signal Expressed as:

[0037]

[0038] Signal Expressed as:

[0039]

[0040] Signal Expressed as:

[0041] .

[0042] According to the above scheme, it is characterized in that, according to the signal ,Signal The extracted phase is expressed as:

[0043] .

[0044] The present invention also provides a phase demodulation method for obtaining a first beat frequency signal. and the second beat frequency signal Performing demodulation, the method comprises:

[0045] The first beat frequency signal is obtained and the second beat frequency signal Perform Hilbert transform to get the signal and signal ;

[0046] Simulate and generate 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 ;

[0047] Signal Perform Hilbert transform to get the signal ;

[0048] For the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0049] Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0050] Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0051] For the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0052] Signal ,Signal Add the signal ;

[0053] Signal ,Signal Add the signal ;

[0054] Signal ,Signal Perform differential cross-multiplication to extract the phase.

[0055] Beneficial effects

[0056] 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 and difference, and differential cross-multiplication on two beat frequency signals. This can 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's missed alarm rate and false alarm rate, and improve the accuracy of the system's phase demodulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a schematic structural diagram 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.

[0058] Figure 2 This is a digital demodulation flow chart of an embodiment of the present invention.

[0059] 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

[0060] To make the purpose, technical solutions, and advantages of the embodiments of the present disclosure more clear, the technical solutions of the embodiments of the present disclosure will be clearly and completely described below in conjunction with the accompanying 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.

[0061] See also Figure 1This embodiment discloses a phase digital demodulation system for a phase-sensitive optical time-domain reflectometry fiber-optic 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. The connection method 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 fiber amplifier 4, the output end of the 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 fiber 7, the reflecting 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 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 input end of the second 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-maintaining fiber coupler 10 is connected to the second input end of the second polarization-maintaining fiber coupler 10, the second output end of the second polarization-maintaining fiber coupler 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 then connected to the digital demodulation unit 15. The digital demodulation unit 15 processes and analyzes the two beat frequency signals, including: Hilbert transform, trigonometric function product, addition and subtraction transform, differential cross multiplication, and phase extraction.

[0062] The working principle of the system is as follows: a narrow linewidth laser 1 emits a light source with an operating wavelength of 1550nm, which enters the 90:10 first coupler 2 and is divided into two paths. The path with higher power is used as the signal light, which is modulated into pulse light by an acousto-optic modulator 3 to generate a 200MHz frequency shift and modulate the optical signal into pulse light. The modulated pulse light is amplified by a fiber amplifier 4. 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 is passed 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 passes through the first polarization beam splitter 8 and is split into P light and S light. The other local oscillation light passes through the second polarization beam splitter 9 and is split into P light and S light with mutually perpendicular vibration directions. The first beat signal generated by the P light and P light being coupled through the second polarization-maintaining fiber coupler 10 enters the first photodetector 11. The second beat signal generated by the S light and S light being coupled through the third polarization-maintaining fiber coupler 12 enters the second photodetector 13. The first and second photodetectors 11 and 13 convert the optical signals into electrical signals, transmit the electrical signals to the data acquisition device 14, and then enter the digital demodulation unit 15.

[0063] Furthermore, the optical fiber amplifier 4 may be an erbium-doped optical fiber amplifier.

[0064] 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. The digital demodulation unit combines the two phases to obtain the vibration change):

[0065] S1, respectively obtain the first beat frequency signal and the second beat frequency signal The signal (obtained through the data acquisition device 14) is subjected to Hilbert transform to obtain the signal and signal ;

[0066] The first beat frequency signal and the second beat frequency signal Respectively expressed as:

[0067]

[0068] Where, , 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;

[0069] Signal and signal Respectively expressed as:

[0070]

[0071] S2, simulate and generate 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 ;

[0072] Signal and signal Respectively expressed as:

[0073]

[0074] Where, and The amplitude and position-related phase corresponding to the next adjacent heterodyne signal respectively; is the difference angle between the polarization states of the signal light direction and the eigenlight direction at that moment;

[0075] S3, signal Perform Hilbert transform to get the signal ;

[0076] Signal Expressed as:

[0077]

[0078] S4, for the first beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0079] Signal Expressed as:

[0080]

[0081] S5. Signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0082] Signal Expressed as:

[0083]

[0084] S6, signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0085] Signal Expressed as:

[0086]

[0087] S7, for the second beat frequency signal ,Signal Perform trigonometric transformation of product and difference to obtain the signal ;

[0088] Signal Expressed as:

[0089]

[0090] In the above equation, can be regarded as a constant and ignored, while the signal 、 、 、 Contains high frequency terms and , so the above four formulas can be further processed through the following different steps

[0091] S8, signal ,Signal Add the signal ;

[0092] Signal Expressed as:

[0093]

[0094] S9, signal ,Signal Add the signal ;

[0095] Signal Expressed as:

[0096]

[0097] S10, signal ,Signal Perform differential cross-multiplication (inverse tangent transform) to extract the phase.

[0098] The extracted phase is expressed as:

[0099]

[0100] This embodiment provides a phase-sensitive optical time-domain reflectometry fiber sensing system and phase demodulation method based on polarization diversity technology. These methods are used for phase demodulation and analysis of coherent photoelectric signals in a phase-sensitive optical time-domain reflectometer system. This system can digitally demodulate the phase of the phase-sensitive optical time-domain reflectometer system, reducing the impact of polarization fading noise on the signal demodulation results. This allows for real-time analysis of distributed optical fiber vibration sensing signals, effectively acquiring information such as the external vibrations affecting the optical fiber. Using a polarization diversity module, the Rayleigh scattered signal is split into S light and P light, which interfere with the S and P light of the reference light, respectively. At certain locations on the optical fiber, when the intensity of the received S-path coherent light approaches zero, the intensity of the received P-path coherent light increases, and vice versa. The algorithm proposed in this invention can jointly process the P and S light signals demodulated at the same location, selecting the signal with the greater intensity, achieving resistance to polarization fading, effectively reducing the impact of polarization fading noise, and improving the signal-to-noise ratio and the accuracy of the demodulation results, thereby resolving the issues of missed and false alarms in DAS systems.

[0101] 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, or 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.

[0102] It will be easily understood by those skilled in the art that the above description is merely 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 scope of protection 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: The optical fiber coupler comprises a narrow linewidth laser, a first optical fiber coupler, an acousto-optic modulator, an optical fiber amplifier, an optical circulator, a first polarization beam splitter, a second polarization beam splitter, a second optical fiber coupler, a third optical fiber coupler, a first photodetector, a second photodetector, a data acquisition device, and a digital demodulation unit, which are sequentially arranged along the optical path. Light emitted by a narrow-linewidth laser is split into beam a and beam b via a first fiber coupler. Beam a is modulated by an acousto-optic modulator and a fiber amplifier before entering an optical circulator. The optical circulator then outputs the Rayleigh scattered light carrying vibration information returned from the sensing fiber. The light is split into a first P beam and a first S beam via a first polarization beam splitter. Beam b is split into a second P beam and a second S beam, each with perpendicular vibration directions, via a second polarization beam splitter. A first beat signal generated by coupling the first P light and the second P light through the second optical fiber coupler is transmitted to a digital demodulation unit by the first photodetector, and a second beat signal generated by coupling the first S light and the second S light 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 signal and the second beat signal; The digital demodulation unit performs phase extraction processing on the first beat frequency signal and the second beat frequency signal, including: Performing Hilbert transform on the obtained first beat frequency signal I1 and the second beat frequency signal I2 respectively to obtain a signal I3 and a signal I4; Simulate and generate two orthogonal signals I1′ and I2′ with adjacent pulse periods to the first beat signal I1 and the second beat signal I2. The signals I1′ and I2′ have the same frequencies as the first beat signal I1 and the second beat signal I2, respectively, and have a time difference of Δt. Performing Hilbert transform on signal I2′ to obtain signal I4′; Performing trigonometric function transformation of product and difference of the first beat frequency signal I1 and the signal I1′ to obtain a signal I5; Performing trigonometric function transformation of product and difference of signals I3 and I1′ to obtain signal I6; Performing product and difference trigonometric function transformation on signals I4 and I4′ to obtain signal I7; Performing trigonometric function transformation of product and difference of the second beat frequency signal I2 and the signal I4′ to obtain a signal I8; Adding the signal I5 and the signal I7 to obtain the signal I9; Subtract signal I6 and signal I8 to obtain signal I 10 ; For signal I9 and signal I 10 Perform differential cross-multiplication or inverse tangent transformation to extract the phase.

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 first beat frequency signal and the second beat frequency signal are: Where E = E S ·E L , E S and E L represents the optical field amplitude of the signal light and the intrinsic light respectively; θ represents the difference angle between the polarization states of the signal light and the intrinsic light; f is the optical signal frequency shift introduced by the AOM; t is time; is the phase of the interference field.

3. The phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 2, characterized in that: Signal I3 and signal I4 are respectively expressed as:

4. The phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 2, characterized in that: Signal I1′ and signal I2′ are respectively expressed as: Where E′ and They correspond to the amplitude and position-related phase of the next adjacent heterodyne signal respectively; θ′ is the difference angle between the polarization states of the signal light direction and the intrinsic light direction at that moment.

5. The phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 2, characterized in that: Signal I4′ is represented by:

6. The phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 5, characterized in that: Signal I5 is represented as: Where, is the phase change between two different backscattered Rayleigh signals caused by external vibration, and Δt is the time interval between two adjacent detection pulses; Signal I7 is represented as: Signal I9 is ​​represented by:

7. The phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 6, characterized in that: Signal I6 is represented as: Signal I8 is represented by: Signal I 10 Expressed as:

8. The phase-sensitive optical time-domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 7, characterized in that: According to signal I9, signal I 10 The extracted phase is expressed as:

9. A phase demodulation method, implemented based on the phase-sensitive optical time domain reflectometry fiber optic sensing system based on polarization diversity technology according to claim 1, characterized in that: The method for demodulating the acquired first beat frequency signal I1 and second beat frequency signal I2 includes: Performing Hilbert transform on the obtained first beat frequency signal I1 and the second beat frequency signal I2 respectively to obtain a signal I3 and a signal I4; Simulate and generate two orthogonal signals I1′ and I2′ with adjacent pulse periods to the first beat signal I1 and the second beat signal I2. The signals I1′ and I2′ have the same frequencies as the first beat signal I1 and the second beat signal I2, respectively, and have a time difference of Δt. Performing Hilbert transform on signal I2′ to obtain signal I4′; Performing trigonometric function transformation of product and difference of the first beat frequency signal I1 and the signal I1′ to obtain a signal I5; Performing trigonometric function transformation of product and difference of signals I3 and I1′ to obtain signal I6; Performing product and difference trigonometric function transformation on signals I4 and I4′ to obtain signal I7; Performing trigonometric function transformation of product and difference of the second beat frequency signal I2 and the signal I4′ to obtain a signal I8; Adding the signal I5 and the signal I7 to obtain the signal I9; Subtract signal I6 and signal I8 to obtain signal I 10 ; For signal I9 and signal I 10 Perform differential cross-multiplication to extract the phase.

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