Interference type low-frequency optical fiber sensing system

Abstract

The invention discloses an interference type low-frequency optical fiber sensing system, and relates to the technical field of optical fiber sensing. Comprising the following steps: acquiring phase signals of a sensing interferometer and an auxiliary interferometer; phase signals of the sensing interferometer and the auxiliary interferometer are added; performing time shifting operation on phase signals of the sensing interferometer and the auxiliary interferometer; adding the signals after time shifting; and subtracting the two added phase signals to eliminate the low-frequency drift noise of the laser so as to obtain a compensated external low-frequency disturbance signal. According to the invention, through time translation interference, an equal-arm-length interferometer is constructed on an algorithm, the complexity of the system is not increased, the detection real-time performance of the system is not sacrificed, the types and application scenes of sensing optical fibers are not limited, and laser low-frequency drift noise suppression can be carried out on a phase signal of a specified channel; the detection performance of the interference type optical fiber sensing system on low-frequency signals is improved, and flexibility and diversity are achieved. And a new thought is provided for application of interference type optical fiber sensing in the fields of seismic detection, underwater sound detection and the like.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber sensing, and more specifically, relates to an interferometric low-frequency optical fiber sensing system. Background Art

[0002] Interferometric fiber optic sensors, due to their unique characteristics such as high sensitivity, wide frequency response bandwidth, immunity to electromagnetic interference, and long detection distance, are widely used in perimeter security, structural health monitoring, pipeline monitoring, geophysical exploration, and other fields. In these applications, the acquisition of high-frequency signals is particularly important. In recent years, with the development of fields such as seismic detection and underwater acoustic detection, interferometric fiber optic sensors have new development prospects, but their low-frequency detection capabilities are facing higher requirements.

[0003] Laser low-frequency drift noise is the most significant factor affecting the low-frequency detection performance of fiber optic sensing systems. Its impact on high-frequency signals is minimal and can usually be eliminated using a low-pass filter. However, for low-frequency signals, a longer detection time is required to obtain the entire cycle of the low-frequency signal. In this case, the laser low-frequency drift noise can severely degrade the signal-to-noise ratio, causing the signal to be submerged and preventing effective detection.

[0004] To address the impact of the aforementioned laser's low-frequency drift noise, patent CN116387954A discloses a frequency locking method based on a combination of optical feedback and PDH. This method achieves frequency lock by adjusting the optical path between the laser and the optical resonator using a real-time error signal obtained from the optical feedback phase adjustment. While this method can achieve long-term frequency stability, it requires additional temperature control and vibration isolation measures to prevent the resonator and laser from being affected, increasing system complexity and cost and hindering integration. Patent CN117251676A discloses a method for decomposing the disturbance signal into high-frequency and low-frequency components, denoising each separately, and then reconstructing the phase signal. This method requires multiple modal decompositions and the addition of a predetermined number of Gaussian white noises of varying amplitudes, compromising the system's real-time performance. Patent CN117150228A discloses a method for obtaining low-frequency self-referenced noise based on spatial dispersion. However, low-frequency drift noise varies across different spatial channels, so this method can only partially suppress low-frequency noise and cannot eliminate the differences across different spatial channels. The literature Compensating for influence of laser-frequency-drift in phase-sensitive OTDR with twice differential method, Optics Express, 27(3): 3664-3671, 2019 proposed a quadratic differential method to compensate for laser frequency drift noise. The signal of the undisturbed channel is used as a reference and subtracted from the signal of the disturbed channel to compensate for the influence of laser frequency noise. This method requires the reference channel to be vibration-isolated and protected, and the reference channel must be as close to the sensing channel as possible, which limits the application scenarios. The literature Ultralow-frequency vibration sensing inphase-sensitive OTDR using multiscale VMD, IEEE Sensors Journal, 23(24): 30451-30461, 2023 proposed a multiscale variational mode decomposition method to The laser low-frequency drift noise in the system is eliminated. This method requires multiple decompositions and judgments of the measured signal. Similarly, the data can only be processed offline, sacrificing real-time performance.

[0005] In summary, the existing methods have limitations such as sacrificing the real-time performance of the system, limiting application scenarios, and increasing system complexity and cost. A method for suppressing low-frequency drift noise of the laser in an interferometric fiber optic sensing system with a simple structure and without sacrificing other performance is urgently needed. Summary of the Invention

[0006] In response to the defects of the existing technology, the purpose of the present invention is to provide a method for suppressing the low-frequency drift noise of the laser in an interferometric low-frequency fiber optic sensing system. By constructing an auxiliary interferometer, the phase signal is time-shifted and data combined, and an equal-arm-length interferometer is algorithmically constructed to eliminate the low-frequency drift noise of the laser. The purpose is to solve the problems of the existing technology that when suppressing the phase noise caused by the low-frequency drift of the laser, the real-time performance of the system is sacrificed, the application scenarios are limited, and the complexity and cost of the system are increased, so as to improve the low-frequency signal detection performance of the fiber optic sensing system.

[0007] To achieve the above objectives, the present invention provides an interferometric low-frequency fiber optic sensing system, comprising a light source, a first coupler, a sensing fiber interferometer, an auxiliary interferometer, a first detector, a second detector, a data acquisition card, and a processor; the sensing fiber interferometer comprises a second coupler, a first acousto-optic modulator, a circulator, a sensing fiber, and a third coupler; the auxiliary interferometer comprises a fourth coupler, a second acousto-optic modulator, a delay fiber, and a fifth coupler;

[0008] The light output by the light source is divided into two paths through a first coupler. After the first path enters the sensing fiber interferometer, it is divided into a detection light and a reference light through a second coupler. The detection light is injected into the sensing fiber through a first acousto-optic modulator and a circulator. The backscattered light generated by the sensing fiber enters the third coupler through the circulator. The reference light directly enters the third coupler to beat with the backscattered light and generate a first beat frequency signal. After the second path enters the auxiliary interferometer, it is divided into two beams through a fourth coupler. The first beam of light is injected into a fifth coupler after passing through a second acousto-optic modulator and a delay fiber. The second beam of light is directly injected into the fifth coupler to beat with the first beam of light and generate a second beat frequency signal. The first beat frequency signal and the second beat frequency signal are converted into a first electrical signal and a second electrical signal through a first detector and a second detector, respectively, and collected by a data acquisition card.

[0009] The processor is used to perform phase demodulation on the collected first electrical signal and the second electrical signal to obtain the phase signals of the sensing fiber interferometer and the auxiliary interferometer, and time shift the phase signals of the sensing fiber interferometer and the auxiliary interferometer, and then subtract the sum of the phase signals after time shift from the sum of the phase signals before time shift to eliminate the low-frequency drift noise of the laser and obtain the compensated external low-frequency disturbance signal.

[0010] Preferably, the phase signals of the sensing fiber interferometer and the auxiliary interferometer are time-shifted, and then the sum of the time-shifted phase signals is subtracted from the sum of the phase signals before the time shift to eliminate the low-frequency drift noise of the laser and obtain the compensated external low-frequency disturbance signal, which specifically includes:

[0011] The phase signals of the sensing fiber interferometer and the auxiliary interferometer are:

[0012]

[0013] Among them, z i represents the distance between the i-th channel of the sensing fiber interferometer and the light source; τ i represents the group delay of the light source output signal reaching the i-th channel of the sensing fiber interferometer; τ aux represents the group delay of the light source output after passing through the auxiliary interferometer delay fiber; Δt is the group delay of the fiber length corresponding to the i-th channel of the sensing fiber interferometer; the time starting point is the moment when the detection light is emitted. and They are respectively represented as the phases of the group delay when the optical signal reaches the front and rear ends of the i-th channel; and is at t+τ i The phase of the two beams of light that assist the interferometer at all times; Phase modulation caused by external disturbance signals;

[0014] right Perform time delay, the time delay amount is the group delay τ corresponding to the auxiliary interferometer delay fiber aux ,get

[0015] right Perform time advance, the time advance amount is the group delay Δt of the fiber length corresponding to the i-th channel, and obtain

[0016] The external low-frequency disturbance signal after compensation is:

[0017] Furthermore, the sensing optical fiber and the auxiliary interferometer optical fiber can be of any length, and the noise suppression method can achieve noise suppression for spatial channels of any length.

[0018] Optionally, the auxiliary interferometer is any one of a backscattering type, a reflective type, and a transmissive type.

[0019] Optionally, the interferometric low-frequency optical fiber sensing system is a phase-sensitive optical time domain reflectometer Chirped-Pulse Phase-Sensitive Optical Time Domain Reflectometry Optical Frequency Domain Reflectometry (OFDR), Phase Optical Frequency Domain Reflectometry Any one of coherent optical time domain reflectometry (COTDR), interferometric acoustic sensing (DAS), and interferometric vibration sensing (DVS).

[0020] Optionally, the phase demodulation includes one or more of quadrature phase demodulation, Hilbert transform phase demodulation, 3×3 demodulation or cross-correlation phase demodulation.

[0021] Optionally, the sensing optical fiber is one or more of ordinary single-mode optical fiber, few-mode optical fiber, multi-mode optical fiber, multi-core optical fiber, scattering-enhanced optical fiber, fiber Bragg grating, bend-resistant and other special optical fibers, and spirally wound acoustic-sensitive optical cables.

[0022] Optionally, the auxiliary interferometer delay fiber is one or more of ordinary single-mode fiber, few-mode fiber, multi-mode fiber, multi-core fiber, scattering-enhanced fiber, fiber Bragg grating, bend-resistant and other special fibers.

[0023] Optionally, the delay operation in step S3 includes one or more of cyclic shift delay, Fourier transform delay, and fractional-order delay filter delay.

[0024] Compared with the prior art, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0025] 1. The present invention can effectively suppress the deterioration of the signal-to-noise ratio of the interferometric fiber optic sensing system caused by the low-frequency drift noise of the laser by time delaying, advancing and combining the phase signal, thereby improving the system's detection performance for low-frequency signals.

[0026] 2. The present invention suppresses the low-frequency drift noise of the laser from the algorithm level. Compared with the general interferometric fiber optic sensing system, only one path needs to be drawn out from the light source as an auxiliary interferometer. The system has a simple structure, low cost, and the ability to process signals in real time.

[0027] 3. The present invention is adaptable to various interferometric fiber optic sensing systems and has good adaptability. It can effectively suppress the low-frequency drift noise of the laser regardless of the detection distance, providing new ideas for the application of interferometric fiber optic sensing in the fields of earthquake detection, underwater acoustic detection, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of the interferometric low-frequency optical fiber sensing system provided by the present invention;

[0029] Figure 2 This is a flow chart of a method for suppressing low-frequency drift of a laser in an interferometric low-frequency optical fiber sensing system of the present invention;

[0030] Figure 3 Schematic diagram of an interferometric low-frequency optical fiber sensing system provided by an embodiment of the present invention;

[0031] Figure 4 is the time domain waveform of the original signal provided by an embodiment of the present invention;

[0032] Figure 5 is the original signal power density spectrum provided by an embodiment of the present invention;

[0033] Figure 6 is the time domain waveform of the signal provided by the embodiment of the present invention after being processed by the noise suppression method;

[0034] Figure 7 It is the power density spectrum of the signal after being processed by the noise suppression method provided by the embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0036] The present invention proposes an interferometric low-frequency optical fiber sensing system, such as Figure 1 The figure shows the system architecture of an embodiment of the fiber-optic distributed acoustic wave sensing system according to the present invention. The laser output light is split into local oscillator light and probe light via a 1:99 coupler OC1. The local oscillator light is split into two paths by a 1:1 coupler OC3, serving as the local oscillator light for the auxiliary interferometer and the sensing interferometer, respectively. The probe light is split into two paths by a 1:9 coupler OC2, injected into the auxiliary interferometer and the sensing interferometer, respectively. The probe light injected into the auxiliary interferometer interferes with the local oscillator light at a 1:1 coupler OC4 via a delay fiber and is received by a balanced photodetector (BPD). The probe light injected into the sensing interferometer is pulse-modulated by an acousto-optic modulator (AOM) and injected into the sensing fiber via a circulator. The backscattered light is emitted from port 3 of the circulator, interferes with the local oscillator light at a 1:1 coupler OC5, and is received by the BPD and converted into an electrical signal. The signals from the two interferometers are then synchronously acquired by a data acquisition card (DAQ). A piezoelectric transducer (PZT) injects a vibration signal at a frequency of 0.01 Hz into the sensing fiber at a distance of 10 km, simulating low-frequency external disturbances. The system spatial resolution is 10m.

[0037] This embodiment provides an interferometric low-frequency fiber optic sensing system for suppressing low-frequency drift noise of a laser. By performing time shifting and data combination on the phase signals of a sensing fiber interferometer and an auxiliary interferometer, an interferometer with equal arm length is algorithmically constructed to eliminate the low-frequency drift noise of the laser. Specifically, the system includes the following steps:

[0038] S1: Obtain the interference signal of the sensing fiber interferometer and the auxiliary interferometer and perform phase demodulation to obtain the original phase signal

[0039] S2: Time shift the original phase signal;

[0040] S3: Combine the original signal and the time-shifted signal;

[0041] S4: Change z i , repeat steps S1-S3 to achieve the suppression of the laser low-frequency drift noise of the specified channel.

[0042] Specifically, the original phase signals of the sensing fiber interferometer and the auxiliary interferometer in S1 are expressed as:

[0043]

[0044] Among them, z i Indicates the length of the sensing fiber channel from the light source; τ i Indicates that the light source output signal reaches channel z through the sensing fiber interferometer i The group delay of aux represents the group delay of the light source output through the auxiliary interferometer delay fiber; Δt is the channel z i The group delay of the corresponding optical fiber length; taking the time when the detection pulse is emitted as the time starting point, then and They are respectively represented as the laser output optical signal reaching channel z i The phase of the group delay at the front and back ends; and is at t+τ i The phase of the two beams of light that assist the interferometer at all times; It is the phase modulation caused by the external disturbance signal.

[0045] Specifically, the S2 includes:

[0046] S21: Original phase signal of sensing fiber interferometer Perform time delay, the time delay amount is the group delay τ corresponding to the auxiliary interferometer delay fiber aux ,get

[0047] S22: The original phase signal of the auxiliary interferometer Perform time advance, the time advance amount is channel z i The group delay Δt corresponding to the fiber length is obtained

[0048] Preferably, the method for suppressing low-frequency drift noise of a laser in an interferometric low-frequency optical fiber sensing system is characterized in that S3 combines data in the following manner:

[0049]

[0050] Completely eliminate the phase noise caused by the low-frequency drift of the channel laser Only retain external disturbance signals Improve the low-frequency signal detection capability of interferometric fiber optic sensing systems.

[0051] In this embodiment, if Figure 3 As shown, A and B are two points on the axial space of the sensing fiber, and their distance from the laser is z A and z B , and there is z B =z A +Δz, Δz is the distance between points A and B. According to the principle of laser interference and phase demodulation, the phase of point A is collected at time t. As shown in the following formula:

[0052]

[0053] Where, is the phase noise of the pulse emitted at time t, z is the detection light A The phase noise of the local oscillator light after the optical fiber with the length of the round trip is n is the refractive index of the optical fiber, and c is the speed of light in a vacuum. Since the auxiliary interferometer and the sensor interferometer are synchronously collected, the local oscillator phase noise of the two is consistent. Therefore, at this time, the auxiliary interferometer phase for:

[0054]

[0055] Where, The light propagation time introduced by the auxiliary interferometer delay fiber, z aux is the delay fiber length.

[0056] When the phase of point B is collected at t+Δt, To detect the time it takes for light to travel between points AB, the phase of point B is for:

[0057]

[0058] Because the phases of points A and B are collected from the same pulse, i.e. the light pulse emitted at time t, the phase noise of the detected light is still z is the detection light B The phase noise of the local oscillator light after the long optical fiber goes back and forth, Vibration signal introduced by the piezoelectric transducer.

[0059] It follows that the channel phase signal for:

[0060]

[0061] Correspondingly, the auxiliary interferometer signal is:

[0062]

[0063] Figure 4 and Figure 5 The time domain waveform and power density spectrum of the channel phase signal, respectively, show a large low-frequency drift, with a phase drift of 6231 rad within 1000 s. The power density spectrum shows that the 0.01 Hz low-frequency signal is completely overwhelmed by the low-frequency drift noise, with the noise floor at 0.01 Hz reaching 52.1 dB.

[0064] The noise suppression method proposed in the present invention is applied to the signal, and the phase signal of the sensing fiber interferometer is added to the phase signal of the auxiliary interferometer to obtain

[0065]

[0066] It can be seen is eliminated, but the laser drift noise term still exists. Therefore, further, the original phase signal is delayed and advanced: the phase signal of the sensing fiber interferometer is delayed by τ aux get The phase signal of the auxiliary interferometer is advanced by Δt to obtain And add the two together to get

[0067]

[0068] Now we can see that Φ1(t) and Φ2(t) have the same phase noise term. Difference between Φ1(t) and Φ2(t) yields:

[0069]

[0070] It can be seen that at this time Φ(t) no longer contains Item, only the The term is the phase noise term caused by the low-frequency drift of the signal laser in the AB channel after being processed by this method. is completely eliminated, leaving only the external vibration signal Improve the low-frequency signal detection capability of interferometric fiber optic sensing systems.

[0071] The time domain waveform and power density spectrum of the vibration signal after processing by this method are as follows: Figure 6 and Figure 7 As shown in the figure, the 0.01Hz low-frequency vibration signal is clearly visible, the signal-to-noise ratio is improved by 68.76dB, the system noise floor is suppressed to -80dB in the 0-10Hz frequency band, and the noise floor at 0.01Hz reaches -105.6dB, which greatly improves the low-frequency detection performance of the fiber optic distributed acoustic wave sensing system.

[0072] In summary, the proposed method for suppressing low-frequency laser drift noise in an interferometric low-frequency fiber optic sensing system can completely suppress phase noise caused by low-frequency laser drift while ensuring real-time performance and minimizing system complexity. This significantly improves the detection performance of the interferometric fiber optic sensing system for low-frequency signals. Furthermore, the present invention effectively suppresses low-frequency laser drift noise regardless of detection distance, providing new insights for the application of interferometric fiber optic sensing in fields such as seismic and underwater acoustic detection.

[0073] 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. An interferometric low-frequency optical fiber sensing system, characterized in that: include: A light source, a first coupler, a sensing fiber interferometer, an auxiliary interferometer, a first detector, a second detector, a data acquisition card, and a processor; the sensing fiber interferometer includes a second coupler, a first acousto-optic modulator, a circulator, a sensing fiber, and a third coupler; the auxiliary interferometer includes a fourth coupler, a second acousto-optic modulator, a delay fiber, and a fifth coupler; The light output by the light source is divided into two paths through a first coupler. After the first path enters the sensing fiber interferometer, it is divided into detection light and reference light through a second coupler. The detection light is injected into the sensing fiber through a first acousto-optic modulator and a circulator. The backscattered Rayleigh light generated by the sensing fiber enters a third coupler through the circulator. The reference light directly enters the third coupler to beat with the backscattered Rayleigh light and generate a first beat frequency signal. After the second path enters the auxiliary interferometer, it is split into two beams by the fourth coupler. The first beam passes through the second acousto-optic modulator and the delay fiber and is then injected into the fifth coupler. The second beam is directly injected into the fifth coupler to beat with the first beam and generate a second beat signal. The first beat signal and the second beat signal are converted into a first electrical signal and a second electrical signal by the first detector and the second detector, respectively, and are collected by the data acquisition card. The processor is used to perform phase demodulation on the collected first electrical signal and the second electrical signal to obtain the phase signals of the sensing fiber interferometer and the auxiliary interferometer, and time shift the phase signals of the sensing fiber interferometer and the auxiliary interferometer, and then subtract the sum of the phase signals after time shift from the sum of the phase signals before time shift to eliminate the low-frequency drift noise of the laser and obtain the compensated external low-frequency disturbance signal.

2. The interferometric low-frequency optical fiber sensing system according to claim 1, characterized in that: The phase signals of the sensing fiber interferometer and the auxiliary interferometer are time-shifted, and then the sum of the phase signals after the time shift is subtracted from the sum of the phase signals before the time shift to eliminate the low-frequency drift noise of the laser and obtain the compensated external low-frequency disturbance signal, which specifically includes: The phase signals of the sensing fiber interferometer and the auxiliary interferometer are: Among them, z i represents the distance between the i-th channel of the sensing fiber interferometer and the light source; τ i represents the group delay of the light source output signal reaching the i-th channel of the sensing fiber interferometer; τ aux represents the group delay of the light source output after passing through the auxiliary interferometer delay fiber; Δt is the group delay of the fiber length corresponding to the i-th channel of the sensing fiber interferometer; the time starting point is the moment when the detection light is emitted. and They are respectively represented as the phases of the group delay when the optical signal reaches the front and rear ends of the i-th channel; and is at t+τ i The phase of the two beams of light that assist the interferometer at all times; Phase modulation caused by external disturbance signals; right Perform time delay, the time delay amount is the group delay τ corresponding to the auxiliary interferometer delay fiber aux ,get right Perform time advance, the time advance amount is the group delay Δt of the fiber length corresponding to the i-th channel, and obtain The external low-frequency disturbance signal after compensation is:

3. The interferometric low-frequency optical fiber sensing system according to claim 1, wherein: The auxiliary interferometer is any one of a backscattering type, a reflection type, and a transmission type.

4. The interferometric low-frequency optical fiber sensing system according to claim 1, wherein: The sensing fiber interferometer is any one of a phase-sensitive optical time domain reflectometer, a chirped pulse phase-sensitive optical time domain reflectometer, an optical frequency domain reflectometer, a phase optical frequency domain reflectometer, a coherent optical time domain reflectometer, an interferometric acoustic wave sensor, and an interferometric vibration sensor.

5. The interferometric low-frequency optical fiber sensing system according to claim 2, wherein: The phase demodulation includes one or more of quadrature phase demodulation, Hilbert transform phase demodulation, 3×3 demodulation or cross-correlation phase demodulation.

6. The interferometric low-frequency optical fiber sensing system according to claim 1, characterized in that: The sensing optical fiber is one or more of common single-mode optical fiber, few-mode optical fiber, multi-mode optical fiber, multi-core optical fiber, scattering-enhanced optical fiber, fiber Bragg grating, bend-resistant special optical fiber, and spirally wound acoustic sensitive optical cable.

7. The interferometric low-frequency optical fiber sensing system according to claim 1, wherein: The auxiliary interferometer delay fiber is one or more of common single-mode fiber, few-mode fiber, multi-mode fiber, multi-core fiber, scattering-enhanced fiber, fiber Bragg grating, bend-resistant and other special fibers.

8. The interferometric low-frequency optical fiber sensing system according to claim 2, wherein: The delay includes one or more of cyclic shift delay, Fourier transform delay, and fractional order delay filter delay.

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