An interferometric low frequency fiber optic sensing system

Abstract

The application discloses an interferometric low-frequency optical fiber sensing system and relates to the technical field of optical fiber sensing. The application comprises the following steps: acquiring sensing interferometer and auxiliary interferometer phase signals; adding the sensing interferometer and auxiliary interferometer phase signals; performing time shift operation on the sensing interferometer and auxiliary interferometer phase signals; adding the time-shifted signals; subtracting the twice-added phase signals to eliminate the low-frequency drift noise of a laser, and obtaining the compensated external low-frequency disturbance signal. The application constructs an equal-arm-length interferometer in the algorithm through time translation interference, does not increase the system complexity, does not sacrifice the system detection real-time performance, does not limit the types and application scenarios of the sensing optical fiber, can suppress the low-frequency drift noise of a laser for the phase signal of a specified channel, improves the detection performance of the interferometric optical fiber sensing system on the low-frequency signal, has flexibility and diversity, and provides a new idea for the application of the interferometric optical fiber sensing in the fields of earthquake detection, underwater sound detection and the like.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing, and more specifically, relates to an interferometric low-frequency fiber optic sensing system. Background Technology

[0002] Interferometric fiber optic sensors, due to their unique characteristics such as high sensitivity, high frequency response bandwidth, resistance to electromagnetic interference, and long detection range, are widely used in perimeter security, structural health monitoring, pipeline monitoring, and geophysical exploration. In these applications, the acquisition of high-frequency signals is of paramount importance. In recent years, with the development of fields such as earthquake detection and underwater acoustic detection, interferometric fiber optic sensors have seen new development prospects, but higher demands are being placed on their low-frequency detection capabilities.

[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 low-pass filters. However, for low-frequency signals, a longer detection time is required to obtain the entire cycle of the low-frequency signal. In this case, laser low-frequency drift noise severely degrades the signal-to-noise ratio, causing the signal to be submerged and preventing effective detection.

[0004] To address the impact of low-frequency drift noise in lasers, patent CN116387954A discloses a frequency locking method based on a combination of optical feedback and PDH. This method adjusts the optical path between the laser and the optical resonant cavity by real-time acquisition of the error signal from optical feedback phase adjustment, thus achieving frequency locking. While this method can achieve frequency stability over a relatively long period, it requires additional temperature control and vibration isolation measures to prevent the resonant cavity and laser from being affected, increasing system complexity and cost, and hindering integration. Patent CN117251676A discloses a method that decomposes the disturbance signal into high-frequency and low-frequency components, denoises them separately, and then reconstructs the phase signal. This method requires multiple mode decompositions and the addition of Gaussian white noise of varying amplitudes a predetermined number of times, sacrificing system real-time performance. Patent CN117150228A discloses a method for obtaining low-frequency self-reference noise based on spatial deviation. However, low-frequency drift noise varies in different spatial channels, so this method can only suppress part of the low-frequency noise and cannot eliminate the differences in different spatial channels. The paper "Compensating for influence of laser-frequency-drift in phase-sensitive OTDR with twice differential method," Optics Express, 27(3): 3664-3671, 2019, proposes a quadratic differential method to compensate for laser frequency drift noise. This method uses the signal from the unperturbed channel as a reference and subtracts it from the signal from the perturbed channel to compensate for the influence of laser frequency noise. However, this method requires vibration isolation of the reference channel, and the reference channel must be as close as possible to the sensing channel, limiting its application scenarios. The paper "Ultralow-frequency vibration sensing in phase-sensitive OTDR using multiscale VMD," IEEE Sensors Journal, 23(24):30451-30461, 2023, proposes using multiscale variational mode decomposition to eliminate low-frequency laser drift noise in φ-OTDR systems. This method requires multiple decompositions and discriminations of the measured signal, and similarly, it can only process the data offline, sacrificing real-time performance.

[0005] In summary, existing methods have limitations such as sacrificing system real-time performance, limiting application scenarios, and increasing system complexity and cost. There is an urgent need to propose a simple method for suppressing low-frequency drift noise of lasers in interferometric fiber optic sensing systems that does not sacrifice other performance characteristics. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for suppressing low-frequency drift noise of lasers in an interferometric low-frequency fiber optic sensing system. By constructing an auxiliary interferometer, the phase signal is time-shifted and data is combined. An equal-arm-length interferometer is constructed in the algorithm to eliminate low-frequency drift noise of the laser. This method aims to solve the problems of existing technologies sacrificing system real-time performance, limiting application scenarios, and increasing system complexity and cost when suppressing phase noise caused by low-frequency drift of the laser, thereby improving 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 optic interferometer, an auxiliary interferometer, a first detector, a second detector, a data acquisition card, and a processor; the sensing fiber optic interferometer comprises a second coupler, a first acousto-optic modulator, a circulator, a sensing fiber, and a third coupler, and the auxiliary interferometer comprises a fourth coupler, a second acousto-optic modulator, a delay fiber, and a fifth coupler; The light output from the light source is split into two paths by a first coupler. The first path enters the sensing fiber interferometer and is then split into a probe beam and a reference beam by a second coupler. The probe beam passes through a first acousto-optic modulator and a circulator before being injected into the sensing fiber. The backscattered Rayleigh light generated by the sensing fiber passes through a circulator before entering a third coupler. The reference beam directly enters the third coupler and beats with the backscattered Rayleigh light to generate a first beat frequency signal. The second path enters the auxiliary interferometer and is split into two beams by a fourth coupler. The first beam passes through a second acousto-optic modulator and a delay fiber before being injected into a fifth coupler. The second beam directly enters the fifth coupler and beats with the first beam to generate a second beat frequency signal. The first and second beat frequency signals are converted into a first electrical signal and a second electrical signal by a first detector and a second detector, respectively, and are then acquired by a data acquisition card. The processor is used to demodulate the phase of the first and second electrical signals to obtain the phase signals of the sensing fiber interferometer and the auxiliary interferometer. It then performs time shift on the phase signals of the sensing fiber interferometer and the auxiliary interferometer, and subtracts the sum of the time-shifted phase signals 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.

[0008] Preferably, the phase signals of the sensing fiber optic 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. Specifically, this includes: The phase signals of the sensing fiber optic interferometer and the auxiliary interferometer are as follows:

[0009]

[0010] in, The first fiber optic interferometer represents the sensing fiber optic interferometer. i The distance between the channel and the light source; This indicates that the light source output signal reaches the first sensor fiber optic interferometer. i Group delay of the channel; This represents the group delay of the light source output via the delay fiber after passing through an auxiliary interferometer; For the first sensing fiber optic interferometer i The group delay of the fiber length corresponding to the channel; with the time of probe light emission as the starting point. and These represent the arrival times of the optical signal at the [number]th [unit]. i The phase of the group delay at both ends of the channel; and In order to be in The phase of the two beams of light in the interferometer at constant time; Phase modulation caused by external disturbance signals; right A time delay is applied, the amount of which is the group delay corresponding to the delay fiber of the auxiliary interferometer. ,get ; right The time advance is performed, and the time advance amount is the first... i Group delay of the channel corresponding to the fiber length ,get ; The compensated external low-frequency disturbance signal is: .

[0011] Furthermore, the sensing fiber and the auxiliary interferometer fiber can be of arbitrary length, and the noise suppression method can achieve noise suppression for spatial channels of arbitrary length.

[0012] Optionally, the auxiliary interferometer can be any one of backscattering, reflection, or transmission type.

[0013] Optionally, the interferometric low-frequency fiber optic sensing system is any one of the following: phase-sensitive optical time-domain reflectometer (φ-OTDR), chirped pulse phase-sensitive optical time-domain reflectometer (CP-φOTDR), optical frequency-domain reflectometer (OFDR), phase optical frequency-domain reflectometer (φ-OFDR), coherent optical time-domain reflectometer (COTDR), interferometric acoustic wave sensor (DAS), and interferometric vibration sensor (DVS).

[0014] Optionally, the phase demodulation includes one or more of quadrature phase demodulation, Hilbert transform phase demodulation, 3×3 demodulation, or cross-correlation phase demodulation.

[0015] Optionally, the sensing optical fiber is one or more of the following: ordinary single-mode optical fiber, few-mode optical fiber, multi-mode optical fiber, multi-core optical fiber, scattering-enhancing optical fiber, fiber grating, special optical fiber such as bend-resistant fiber, and spirally wound acoustic-sensitive optical cable.

[0016] Optionally, the delay fiber of the auxiliary interferometer is one or more of the following: ordinary single-mode fiber, few-mode fiber, multimode fiber, multi-core fiber, scattering-enhancing fiber, fiber grating, bending-resistant fiber, etc.

[0017] Optionally, the delay operation in step S3 includes one or more of the following: cyclic shift delay, Fourier transform delay, and fractional delay filter delay.

[0018] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following beneficial effects: 1. This invention effectively suppresses the signal-to-noise ratio degradation of the interferometric fiber optic sensing system caused by low-frequency drift noise of the laser by delaying, leading, and combining the phase signal, thereby improving the system's detection performance for low-frequency signals.

[0019] 2. This invention achieves suppression of low-frequency drift noise of laser at the algorithm level. Compared with general interferometric fiber optic sensing systems, it only requires one path to be drawn from the light source as an auxiliary interferometer. The system structure is simple, the cost is low, and it has the ability to process signals in real time.

[0020] 3. This invention can adapt to various interferometric fiber optic sensing systems, has good adaptability, and can effectively suppress low-frequency drift noise of lasers regardless of the detection distance, providing a new approach for the application of interferometric fiber optic sensing in fields such as earthquake detection and underwater acoustic detection. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the interferometric low-frequency fiber optic sensing system provided by the present invention; Figure 2 This is a flowchart of the laser low-frequency drift suppression method in the interferometric low-frequency fiber optic sensing system of the present invention; Figure 3 This is a schematic diagram of the interferometric low-frequency fiber optic sensing system provided in an embodiment of the present invention; Figure 4 This is the original signal time-domain waveform provided in the embodiments of the present invention; Figure 5 This is the original signal power density spectrum provided in the embodiments of the present invention; Figure 6 This is the time-domain waveform of the signal after noise suppression processing provided in the embodiments of the present invention; Figure 7This is the power density spectrum of the signal after being processed by the noise suppression method provided in the embodiments of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be 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 illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] This invention proposes an interferometric low-frequency fiber optic sensing system, such as... Figure 1 The diagram shown is a system structure diagram of an embodiment of the fiber optic distributed acoustic wave sensing system of the present invention. The laser output light is split into a local oscillator beam and a probe beam via a 1:99 coupler OC1. The local oscillator beam is further split into two paths by a 1:1 coupler OC3, serving as the local oscillator beam for the auxiliary interferometer and the sensing interferometer, respectively. The probe beam is split into two paths by a 1:9 coupler OC2, injected into the auxiliary interferometer and the sensing interferometer, respectively. The probe beam injected into the auxiliary interferometer interferes with the local oscillator beam at a 1:1 coupler OC4 via a delay fiber and is received by a balanced photodetector BPD. The probe beam injected into the sensing interferometer is pulse-modulated by an acousto-optic modulator AOM, injected into the sensing fiber via a circulator, and its backscattered light exits from port 3 of the circulator, interferes with the local oscillator beam at a 1:1 coupler OC5, and is received by the BPD and converted into an electrical signal. Finally, the signals from the two interferometers are synchronously acquired by a data acquisition card DAQ. A vibration signal with a frequency of 0.01Hz is injected into the sensing fiber at a distance of 10km by a piezoelectric transducer PZT to simulate low-frequency external disturbances. The system has a spatial resolution of 10m.

[0024] This embodiment provides an interferometric low-frequency fiber optic sensing system for suppressing low-frequency drift noise in lasers. By time-shifting and combining the phase signals of the sensing fiber optic interferometer and the auxiliary interferometer, an interferometer of equal arm length is constructed algorithmically to eliminate low-frequency drift noise in the laser. Specifically, the system includes the following steps: S1: Acquire the interference signals from the sensing fiber optic interferometer and the auxiliary interferometer, and perform phase demodulation to obtain the original phase signal. , ; S2: Time shift the original phase signal; S3: Combine the original signal and the time-shifted signal; S4: Change Repeat steps S1-S3 to achieve low-frequency drift noise suppression of the laser in the specified channel.

[0025] Specifically, the original phase signals of the sensing fiber interferometer and the auxiliary interferometer in S1 are represented as follows:

[0026]

[0027] in, This indicates the length of the sensing fiber optic channel from the light source; This indicates that the light source output signal reaches the channel after passing through the sensing fiber optic interferometer. Group latency; This represents the group delay of the light source output via the delay fiber after passing through an auxiliary interferometer; For channel The group delay corresponding to the fiber length; taking the moment of probe pulse emission as the starting point, then and These represent the laser output optical signal arriving at the channel, respectively. The phase of the group delay at both ends; and In order to be in The phase of the two beams of light in the interferometer at constant time; Phase modulation caused by external disturbance signals.

[0028] Specifically, S2 includes: S21: The original phase signal of the sensing fiber optic interferometer A time delay is applied, the amount of which is the group delay corresponding to the delay fiber of the auxiliary interferometer. ,get ; S22: For the original phase signal of the auxiliary interferometer Perform time advance, where the time advance amount is the channel... Group delay corresponding to fiber length ,get .

[0029] Preferably, the method for suppressing low-frequency drift noise of a laser in an interferometric low-frequency fiber optic sensing system is characterized in that, in step S3, data is combined in the following manner:

[0030] Completely eliminate phase noise term caused by low-frequency drift of the modified laser channel Only retain external disturbance signals This improves the low-frequency signal detection capability of interferometric fiber optic sensing systems.

[0031] In this embodiment, as Figure 3As shown, A and B are two points in space along the axial direction of the sensing fiber, and their distances from the laser are denoted as . and And there are , Let be the distance between points A and B. Based on the principles of laser interferometry and phase demodulation, the phase at point A is acquired at time t. As shown in the following formula:

[0032] In the formula, The phase noise of the pulse emitted at time t. To detect light The phase noise of the local oscillator light interfering with the round trip of the optical fiber of a certain length, where Where n is the refractive index of the optical fiber and c is the speed of light in vacuum. Since the auxiliary interferometer and the sensing interferometer acquire data synchronously, their local oscillator phase noise is consistent. Therefore, the phase of the auxiliary interferometer at this time... for:

[0033] In the formula, To assist the interferometer in delaying the light propagation time introduced by the optical fiber, This represents the length of the delay fiber.

[0034] When the phase of point B is acquired, , To detect the time it takes for light to travel between points A and B, the phase at point B is... for:

[0035] Because the phases of points A and B are collected from the same pulse, that is... The constantly emitted light pulses therefore have a probe light phase noise that remains constant. , To detect light The phase noise of the local oscillator light interfering with the optical fiber after round trip. The vibration signal introduced for the piezoelectric transducer.

[0036] Therefore, the channel phase signal is obtained. for:

[0037] Correspondingly, the auxiliary interferometer signal is:

[0038] Figure 4 and Figure 5The time-domain waveform and power density spectrum of the channel phase signal are shown below. It can be seen that there is a large low-frequency drift, with a phase drift of 6231 rad within 1000s. From the power density spectrum, the 0.01Hz low-frequency signal is completely submerged by low-frequency drift noise, with a noise floor of up to 52.1dB at 0.01Hz.

[0039] The noise suppression method proposed in this invention is applied to the signal by adding the phase signal of the sensing fiber optic interferometer to the phase signal of the auxiliary interferometer.

[0040] It can be seen The phase noise term is eliminated, but it still exists. Therefore, further delay and lead operations are performed on the original phase signal: the phase signal of the sensing fiber optic interferometer is delayed. get Lead the phase signal of the auxiliary interferometer get And add the two together to get

[0041] It can be seen at this point that... and Having the same phase noise term, and The difference is obtained by subtraction:

[0042] It can be seen that at this time It no longer includes Only the item was retained. The term refers to the phase noise term caused by the low-frequency drift of the signal laser in the AB channel after processing by this method. Completely eliminated, retaining only external vibration signals. This improves the low-frequency signal detection capability of interferometric fiber optic sensing systems.

[0043] 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, the 0.01Hz low-frequency vibration signal is clearly visible, the signal-to-noise ratio is improved by 68.76dB, the system noise floor in the 0-10Hz frequency band is suppressed to -80dB, 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.

[0044] In summary, the laser low-frequency drift noise suppression method proposed in this invention, while ensuring system real-time performance and without increasing system complexity, can completely suppress phase noise caused by laser low-frequency drift, significantly improving the detection performance of interferometric fiber optic sensing systems for low-frequency signals. Furthermore, this invention can effectively suppress laser low-frequency drift noise regardless of the detection distance, providing a new approach for the application of interferometric fiber optic sensing in fields such as earthquake detection and underwater acoustic detection.

[0045] Those skilled in the art will readily understand 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 within the scope of protection of the present invention.

Claims

1. An interferometric low-frequency fiber optic sensing system, characterized in that, include: The system comprises a light source, a first coupler, a sensing fiber optic interferometer, an auxiliary interferometer, a first detector, a second detector, a data acquisition card, and a processor; the sensing fiber optic 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 from the light source is split into two paths by the first coupler. The first path enters the sensing fiber interferometer and is split into probe light and reference light by the second coupler. The probe light is injected into the sensing fiber through the first acousto-optic modulator and circulator. The backscattered Rayleigh light generated by the sensing fiber enters the third coupler through the circulator. The reference light directly enters the third coupler and beats with the backscattered Rayleigh light to generate the first beat frequency signal. After the second beam 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 injected into the fifth coupler. The second beam is directly injected into the fifth coupler and beats with the first beam to 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 by the first detector and the second detector, respectively, and are synchronously acquired by the data acquisition card. The processor is used to demodulate the acquired first and second electrical signals to obtain the phase signals of the sensing fiber optic interferometer and the auxiliary interferometer. It then performs a time shift on the phase signals of the sensing fiber optic interferometer and the auxiliary interferometer, and subtracts the sum of the time-shifted phase signals from the sum of the phase signals before the time shift to eliminate low-frequency drift noise from the laser, obtaining the compensated external low-frequency disturbance signal and the phase signal of the sensing fiber optic interferometer. Phase signal of the auxiliary interferometer They are respectively: in, The first fiber optic interferometer represents the sensing fiber optic interferometer. i The distance between the channel and the light source; This indicates that the light source output signal reaches the first sensor fiber optic interferometer. i Group delay of the channel; This represents the group delay of the light source output via the delay fiber after passing through an auxiliary interferometer; For the first sensing fiber optic interferometer i The group delay of the fiber length corresponding to the channel; with the time of probe light emission as the starting point. and These represent the arrival times of the optical signal at the [number]th [unit]. i The phase of the group delay at both ends of the channel; and In order to be in The phase of the two beams of light in the interferometer at constant time; Phase modulation caused by external disturbance signals; right A time delay is applied, the amount of which is the group delay corresponding to the delay fiber of the auxiliary interferometer. ,get ; right Perform time advance, the time advance amount is the first i Group delay of the channel corresponding to the fiber length ,get ; Compensated external low-frequency disturbance signal for: .

2. The interferometric low-frequency fiber optic sensing system according to claim 1, characterized in that, The auxiliary interferometer can be any one of the following: backscattering type, reflection type, or transmission type.

3. The interferometric low-frequency fiber optic sensing system according to claim 1, characterized in that, The sensing fiber optic interferometer is any one of the following: phase-sensitive optical time-domain reflectometer, chirped pulse phase-sensitive optical time-domain reflectometer, optical frequency-domain reflectometer, phase optical frequency-domain reflectometer, coherent optical time-domain reflectometer, interferometric acoustic wave sensor, and interferometric vibration sensor.

4. The interferometric low-frequency fiber optic sensing system according to claim 1, characterized in that, The phase demodulation includes one or more of the following: quadrature phase demodulation, Hilbert transform phase demodulation, 3×3 demodulation, and cross-correlation phase demodulation.

5. The interferometric low-frequency fiber optic sensing system according to claim 1, characterized in that, The sensing optical fiber is one or more of the following: ordinary single-mode optical fiber, few-mode optical fiber, multimode optical fiber, multi-core optical fiber, scattering-enhancing optical fiber, fiber grating, bend-resistant optical fiber, and spirally wound acoustic-sensitive optical cable.

6. The interferometric low-frequency fiber optic sensing system according to claim 1, characterized in that, The delay fiber of the auxiliary interferometer is one or more of the following: ordinary single-mode fiber, few-mode fiber, multimode fiber, multi-core fiber, scattering-enhancing fiber, fiber grating, and bending-resistant fiber.

7. The interferometric low-frequency fiber optic sensing system according to claim 1, characterized in that, The time delay includes one or more of the following: cyclic shift delay, Fourier transform delay, and fractional delay filter delay.

Images