An optical sensing system and method using multicarrier probe pulses

Abstract

This invention provides an optical sensing system and method using multi-carrier probe pulses. It involves designing an intensity-modulated multi-carrier pulse sequence, using an arbitrary waveform generator to drive an acousto-optic modulator to modulate continuous light emitted from an ultra-narrow linewidth laser into pulsed optical signals with multiple subcarrier frequency components. The modulated pulse signals are then fed into the optical fiber under test. Beat frequency signals of the backscattered Rayleigh light signal and the local oscillator light signal in the optical fiber are acquired. The beat frequency signals are demodulated, and the probe data corresponding to the subcarriers are extracted and combined to obtain the sensing data, thus suppressing coherent fading. The beneficial effect of this invention is that it significantly reduces the incidence of interference fading.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to an optical sensing system and method using multi-carrier probe pulses. Background Technology

[0002] Unlike traditional discrete sensors, distributed fiber optic sensors can comprehensively monitor the environment surrounding the optical fiber. Among them, fiber optic distributed sensing technology based on backscattered Rayleigh (RBS) light waves has become a research hotspot both domestically and internationally due to its unique advantages such as ultra-high sensitivity, large measurement range, and strong environmental adaptability. It can utilize backscattered light in the sensing fiber to detect and analyze disturbances caused by external physical fields on the sensing fiber. In recent years, with the continuous deepening of research, many potential applications of distributed fiber optic sensing have been explored, and it has been deployed in many practical applications, such as oil and gas pipeline monitoring, earthquake monitoring, perimeter security, traffic and railway monitoring, and telecommunications fiber optic cable maintenance. In short, distributed fiber optic sensors are becoming an indispensable sensing technology, and their potential application market is enormous.

[0003] A phase-sensitive optical time-domain reflectometer (Φ-OTDR) based on refractive index (RBS) utilizes the interaction of photons with inherent defects within an optical fiber, typically fluctuations in the refractive index of glass. The Φ-OTDR uses a highly coherent, narrow-linewidth laser as its light source, sending laser pulses into the fiber and interrogating Rayleigh backscattered light from these inherent defects. When a disturbance occurs at a location in the fiber, the refractive index distribution at that point changes due to strain and elasto-optic effects. Consequently, the interference modes between the corresponding Rayleigh backscattered lights are affected, causing a significant change in the intensity of the Rayleigh backscattered light. By detecting this change in intensity, the disturbance can be located. Since refractive index fluctuations are random, there is no linear relationship between the intensity of the Rayleigh backscattered light and the fiber strain amplitude, meaning that the temporal information of the strain signal cannot be accurately measured using changes in Rayleigh backscattered light intensity. However, phase provides a linear response to strain, making complete reconstruction of the disturbance signal possible.

[0004] However, due to the random spatial distribution of the refractive index and the high coherence of the light source, RBS may suffer from coherent fading during phase demodulation. Specifically, this manifests as a sharp decrease in backscattered light intensity in certain regions, and the severely degraded signal-to-noise ratio cannot support correct phase demodulation, leading to positioning errors and distortion of the reconstructed signal. This phenomenon significantly reduces the reliability of the system.

[0005] The location of fading points has been shown to be related to the center frequency of the laser. Therefore, the effects of coherent fading can be suppressed by probing the fiber with optical pulses of different frequencies. Many schemes use multiple frequencies to eliminate interference attenuation. For example, multiple lasers or acousto-optic modulators (AOMs) can be used to generate optical signals with different center frequencies, but this undoubtedly increases the cost of experimental equipment. Fading suppression can also be achieved using bipolar coding, but the demodulation process is complex because the conversion between bipolar and unipolar signals requires multiple sets of pulses for encoding and decoding, which also sacrifices the system response bandwidth. Therefore, designing a simple and easily implemented multi-carrier detection scheme to improve the real-time performance of distributed fiber optic sensors and efficiently suppress interference fading is of great significance. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides an optical sensing system and method using multi-carrier probe pulses.

[0007] An optical sensing system using multi-carrier probe pulses includes: a laser, a first coupler, an acousto-optic modulator, an arbitrary waveform generator, an erbium-doped fiber amplifier, an optical filter, a circulator, a sensing fiber under test, a polarization controller, a second coupler, a balanced photodetector, a data acquisition card, and a digital signal processing module.

[0008] The laser output is connected to the input of the first coupler. The output of the first coupler is connected to the first input of the acousto-optic modulator and the input of the polarization controller, respectively. The arbitrary waveform generator is connected to the second input of the acousto-optic modulator. The output of the acousto-optic modulator is connected to the input of the erbium-doped fiber amplifier. The output of the erbium-doped fiber amplifier is connected to the input of the optical filter. The output of the optical filter is connected to the first port of the circulator. The second port of the circulator is connected to the sensing fiber under test. The output of the polarization controller and the third port of the circulator are both connected to the input of the second coupler. The output of the coupler is connected to the input of the balanced photodetector. The output of the balanced photodetector is connected to the input of the data acquisition card. The output of the data acquisition card is connected to the digital signal processing module.

[0009] The laser outputs a highly coherent continuous optical signal, which is split into two paths by the first coupler. One path serves as the local oscillator signal, and the other path serves as the probe light that enters the acousto-optic modulator.

[0010] An arbitrary waveform generator generates intensity-modulated multi-carrier pulse electrical signals, which drive an acousto-optic modulator to modulate the probe optical signal into a multi-carrier pulse optical signal. The multi-carrier pulse optical signal passes sequentially through an erbium-doped fiber amplifier and an optical filter, and is injected into the optical fiber of the sensor under test through a circulator.

[0011] The backscattered Rayleigh light signal transmitted from the optical fiber under test returns through a circulator and beats with the local oscillator light signal. The beat signal enters the signal port of the balanced photodetector and is converted into an electrical signal. The acquisition card acquires data from all electrical signals, and the acquired digital signal data is demodulated by the digital signal processing module to extract the sensing signal.

[0012] Furthermore, in this optical sensing system, the arbitrary waveform generator needs to be clock-synchronized with the acquisition card, and the acquisition card needs to be provided with a trigger signal by the arbitrary waveform generator that is synchronized with the modulation signal driving the acousto-optic modulator.

[0013] Furthermore, the polarization controller is used to control the polarization of light, and the second coupler is used to mix the backscattered Rayleigh light and the local oscillator light signal before inputting them into the balanced photodetector.

[0014] Furthermore, the laser is an ultra-narrow linewidth laser.

[0015] Furthermore, the acousto-optic modulator is an analog acousto-optic modulator.

[0016] Furthermore, the laser, the first coupler, the acousto-optic modulator, the erbium-doped fiber amplifier, the optical filter, the circulator, the fiber optic sensor under test, the polarization controller, the second coupler, and the balanced photodetector are connected via single-mode fiber optic cable; the arbitrary waveform generator and the acousto-optic modulator are connected via coaxial cable, and the balanced photodetector, the acquisition card, and the digital signal processing module are also connected via coaxial cable.

[0017] A light sensing method using multi-carrier probe pulses, implemented based on the aforementioned light sensing system, mainly includes:

[0018] S1: The continuous light emitted by the laser is split into probe light and local oscillator light signals by the first coupler. A multi-carrier pulse sequence with intensity modulation is designed. The multi-carrier pulse sequence is input into an arbitrary waveform generator to generate a multi-carrier electrical signal to drive the acousto-optic modulator, thereby achieving intensity modulation of the probe light and obtaining a multi-carrier pulse light signal. The pulse light signal is then injected into the optical fiber of the sensor under test.

[0019] S2: Collect the back Rayleigh scattered light signal and the local oscillator light signal in the optical fiber under test and perform beat frequency analysis to obtain the beat frequency signal;

[0020] S3: Demodulate the beat frequency signal, extract and combine the detection data corresponding to the subcarrier to obtain the sensing data, and achieve the suppression of coherent fading.

[0021] Furthermore, the generation of an intensity-modulated multi-carrier pulse optical signal sequence specifically includes:

[0022] First, a sequence consisting entirely of zeros is generated. Then, at symmetrical positions in this sequence, two conjugate complex numbers X+iY and X-iY are used, where X and Y are real numbers and i is the imaginary unit. This sequence is treated as a frequency domain sequence. An inverse Fourier transform is performed on this sequence to obtain the corresponding time series. In effect, this step generates a sine wave sequence with an initial phase. Then, an appropriate threshold is selected on the generated sine wave waveform for cropping to accommodate the modulation of the acousto-optic modulator. Frequency harmonics are generated during the cropping process, thus achieving multiple subcarriers. Cropping requires selecting an appropriate threshold, retaining the portion above the threshold, and setting a suitable DC value (DC) for the portion below the threshold, where the DC value is smaller than the original amplitude.

[0023]

[0024] Where amplitude represents the amplitude of the sine waveform before cutting, amplitude' represents the amplitude of the waveform after cutting, and threshold represents the set threshold value.

[0025] Furthermore, the sensor data is demodulated from the beat frequency electrical signal, specifically including:

[0026] First, a down-conversion operation is used to remove the frequency shift introduced by the acousto-optic modulator from the beat frequency signal;

[0027] Then, bandpass filtering is performed on the multicarrier signal moved to the base frequency in the frequency domain to filter out each subcarrier signal or select several subcarriers with higher energy to filter out.

[0028] The inverse Fourier transform of the filtered subcarrier signal in the frequency domain is performed to obtain the results of pulse detection at different frequencies;

[0029] These results are then vectorized using a method of rotating vector summation, and the combined vector is used as the sensing data.

[0030] The beneficial effects of the technical solution provided by this invention are as follows: Continuous light emitted by a laser is split into probe light and local oscillator light signals by a first coupler. A multi-carrier pulse sequence with intensity modulation is designed and input into an arbitrary waveform generator to generate a multi-carrier electrical signal to drive an acousto-optic modulator, thereby achieving intensity modulation of the probe light and obtaining a multi-carrier pulse light signal. This pulse light signal is injected into the optical fiber under test. Backscattered Rayleigh light signal and the local oscillator light signal in the optical fiber under test are collected and beat frequency is performed to obtain a beat frequency signal. The beat frequency signal is demodulated to obtain sensing data. The probe data corresponding to the subcarriers are extracted and combined to suppress coherent fading. In this scheme, the probe optical signal is converted into a multi-carrier pulse optical signal, and the resulting multiple bandwidth-controllable subcarriers can be easily filtered out to achieve equivalent detection of multiple frequencies. By applying the results of multiple frequencies, interference fading can be effectively suppressed. At the same time, this scheme uses specially designed pulses to achieve multi-frequency detection, which only requires simple intensity modulation. This can be done with just an analog acousto-optic modulator, instead of using multiple narrow-linewidth laser sources or multiple acousto-optic modulators. This does not significantly change the sensing platform and the experimental cost is low. In addition, this scheme still completes one detection with a single pulse instead of a pulse group of coded schemes. Obviously, this scheme is more efficient and simpler. Attached Figure Description

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings:

[0032] Figure 1 This is a diagram of a distributed optical sensing system based on multi-carrier pulses in an embodiment of the present invention.

[0033] Figure 2 This is a flowchart of an optical sensing method using multi-carrier probe pulses in an embodiment of the present invention.

[0034] Figure 3 This is a schematic diagram of intensity-modulated multicarrier pulse sequence data in an embodiment of the present invention.

[0035] Figure 4 This is a schematic diagram of the time and frequency domains of intensity-modulated multicarrier pulse optical signal sequence data in an embodiment of the present invention.

[0036] Figure 5 This is a schematic diagram of the frequency domain after down-conversion of the measured data of the beat frequency signal received by the distributed optical sensing system based on multi-carrier pulses in an embodiment of the present invention.

[0037] Figure 6 This is a schematic diagram of filtering to extract different subcarrier signals from a multi-carrier signal in an embodiment of the present invention.

[0038] Figure 7(a) is a schematic diagram of the phase-space-time demodulation from a single subcarrier signal in an embodiment of the present invention.

[0039] Figure 7 (b) is a schematic diagram of the phase-space-time obtained by demodulating the five subcarrier signals in an embodiment of the present invention. Detailed Implementation

[0040] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0041] Embodiments of the present invention provide an optical sensing system and method using multi-carrier probe pulses.

[0042] like Figure 1 As shown, an optical sensing system using multi-carrier probe pulses includes: a laser, a first coupler, an acousto-optic modulator (AOM), an arbitrary waveform generator (AWG), an erbium-doped fiber amplifier (EDFA), an optical filter (BPF), a circulator, a sensing fiber under test, a polarization controller, a second coupler, a balanced photodetector, a data acquisition card, and a digital signal processing module.

[0043] The laser output is connected to the input of the first coupler. The output of the first coupler is connected to the first input of the acousto-optic modulator and the input of the polarization controller. The arbitrary waveform generator is connected to the second input of the acousto-optic modulator. The output of the acousto-optic modulator is connected to the input of the erbium-doped fiber amplifier. The output of the erbium-doped fiber amplifier is connected to the input of the optical filter. The output of the optical filter is connected to the first port of the circulator. The second port of the circulator is connected to the sensing fiber under test. The output of the polarization controller and the third port of the circulator are both connected to the input of the second coupler. The coupler output is connected to the input of the balanced photodetector. The output of the balanced photodetector is connected to the input of the data acquisition card. The output of the data acquisition card is connected to the digital signal processing module.

[0044] The laser outputs a highly coherent continuous optical signal, which is split into two paths by a first coupler (90:10). One path, with 90% of the optical signal used as probe light, enters the acousto-optic modulator, while the other path, with 10% of the optical signal used as the local oscillator signal, is used. The polarization controller is used to control the polarization of the light. The second coupler (50:50) is used to mix the backscattered Rayleigh light and the local oscillator signal, and the two signals output by the second coupler are power-equalized before being input into a balanced photodetector.

[0045] An arbitrary waveform generator generates an intensity-modulated multi-carrier pulse electrical signal, which drives an acousto-optic modulator to modulate the probe optical signal into a multi-carrier pulse optical signal with multiple subcarrier frequency components. In this embodiment, the acousto-optic modulator has a frequency shift of 200 MHz. The multi-carrier pulse optical signal passes sequentially through an erbium-doped fiber amplifier and an optical filter, and is injected into the sensing fiber under test through a circulator. A piezoelectric ceramic is attached to the sensing fiber under test, and a signal is loaded onto the piezoelectric ceramic to simulate a disturbance scenario.

[0046] The backscattered Rayleigh light signal transmitted back from the optical fiber under test is returned through a circulator and beats with the local oscillator light signal to obtain a beat frequency signal. The beat frequency signal enters the signal port of a balanced photodetector with a bandwidth of 350MHz and is converted into an electrical signal. The acquisition card collects data from the electrical signal at a sampling rate of 1GSa / s. The acquired digital signal data is demodulated by the digital signal processing module on the computer to obtain the sensing signal.

[0047] In this optical sensing system, the arbitrary waveform generator needs to be clock-synchronized with the acquisition card, and the acquisition card needs to be provided with a trigger signal by the arbitrary waveform generator that is synchronized with the modulation signal driving the acousto-optic modulator.

[0048] The laser is an ultra-narrow linewidth laser. The acousto-optic modulator is an analog acousto-optic modulator (AOM).

[0049] Optical interfaces are connected via single-mode optical fibers, while electrical interfaces are connected via coaxial cables. Specifically, the laser, first coupler, acousto-optic modulator, erbium-doped fiber amplifier, optical filter, circulator, sensor fiber under test, polarization controller, second coupler, and balanced photodetector are connected via single-mode optical fibers. The arbitrary waveform generator and acousto-optic modulator are connected via coaxial cables, as are the balanced photodetector, acquisition card, and digital signal processing module.

[0050] Please refer to Figure 2 , Figure 2 This is a flowchart of an optical sensing method using multi-carrier probe pulses according to an embodiment of the present invention, specifically including:

[0051] S1: The laser emits continuous light, which is split by a 90:10 first coupler. One 10% continuous light is used as the local oscillator signal, and the other 90% continuous light is used as the probe light. The probe light is converted into a pulsed light signal that is driven into the sensing fiber. The local oscillator signal is used to beat the backscattered Rayleigh light returned from the sensing fiber. After multi-carrier pulse modulation, an intensity-modulated multi-carrier pulse sequence is designed. The multi-carrier pulse sequence is input into an arbitrary waveform generator to generate a multi-carrier electrical signal to drive an acousto-optic modulator. The probe light is converted into an intensity-modulated multi-carrier pulsed light signal with multiple subcarrier frequency components. The converted multi-carrier pulsed light signal is then driven into the sensing fiber under test.

[0052] S2: The beat frequency signal is obtained by collecting the back Rayleigh scattering signal and the local oscillator signal transmitted back from the optical fiber of the sensor under test.

[0053] Since the probe light itself is multi-carrier, with multiple subcarriers in the frequency domain, and the backscattered Rayleigh light signal generated by the probe light during its transmission through the optical fiber is also multi-carrier—because Rayleigh scattering is an elastic scattering process—the scattered light, like the probe light, is also multi-carrier, consisting of multiple subcarriers in the frequency domain. When the backscattered Rayleigh light and the local oscillator light beat, the electrical signal converted from the optical signal by the photodetector also has a multi-carrier structure in the frequency domain. Subsequent digital signal processing revolves around these multiple subcarrier signals.

[0054] Φ-OTDR is a method of obtaining information about disturbances in an optical fiber link by analyzing backscattered Rayleigh signals. Therefore, this invention requires the collection of backscattered Rayleigh signals using a coherent detection structure. This collection method is characterized by the need for the returned backscattered Rayleigh light signal to beat with the local oscillator signal. This beat signal contains information about the backscattered Rayleigh signal. By analyzing the beat signal, the characteristics of the backscattered Rayleigh signal can be determined, thereby obtaining information about disturbances in the optical fiber link. Backscattered Rayleigh scattering is generated by the probe light injected into the optical fiber during transmission.

[0055] S3: Further process the beat frequency signal, extract and combine the detection data corresponding to the subcarrier, demodulate the sensor data, and achieve suppression of coherent fading.

[0056] In this embodiment, converting the probe optical signal into a multi-carrier optical signal specifically includes: generating a multi-carrier pulse electrical signal using an intensity-modulated multi-carrier pulse optical signal; and driving an analog acousto-optic modulator with the multi-carrier pulse electrical signal to convert the probe optical signal into a multi-carrier pulse optical signal.

[0057] Before generating a multi-carrier pulse electrical signal using an intensity-modulated multi-carrier pulse optical signal, the process includes: first designing a single-frequency sine waveform, then performing threshold cutting and modification to generate an intensity-modulated multi-carrier pulse optical signal with controllable subcarrier bandwidth.

[0058] The process for generating intensity-modulated multi-carrier pulse optical signal sequences is as follows: Figure 3 As shown, it specifically includes:

[0059] First, a sequence consisting entirely of zeros is generated; in this example, a sequence of length 100 is generated. Then, two conjugate complex numbers X+iY and X-iY are used to replace the sequence at its symmetrical position, where X and Y are real numbers; in this example, X=1 and Y=1. This sequence is treated as a frequency domain sequence. An inverse Fourier transform is performed on this sequence to obtain the corresponding time series. In fact, this step generates a sine wave sequence with an initial phase. Then, an appropriate threshold is selected on the generated time-domain waveform for segmentation to accommodate the modulation of the acousto-optic modulator. The segmentation process can be viewed as a rectangular windowing operation, which generates frequency harmonics, thus achieving multiple subcarriers. Segmentation requires selecting an appropriate threshold, retaining the portion above the threshold, and setting a suitable DC value (DC) for the portion below the threshold, where the DC value is smaller than the original amplitude.

[0060]

[0061] Where amplitude represents the amplitude of the sine waveform before cutting, amplitude' represents the amplitude of the waveform after cutting, and threshold represents the set threshold value.

[0062] The sequence of length 100, i.e., 100 data points, generated using the above method is used as the modulation part. However, due to the characteristics of the φ-OTDR system, the light used for detection needs to be pulsed, meaning part contains optical signals and part does not. Therefore, the repetition frequency of the corresponding optical pulses needs to be padded with zeros according to the fiber length to form a complete pulse period. This ultimately forms multiple subcarriers in the frequency domain. By controlling the length of the original frequency domain sequence and the sampling rate of the final AWG, the bandwidth of each subcarrier in the generated multi-carrier signal can be controlled. The repetition frequency of the optical pulses is set to 40kHz, and the pulse width is 100ns. Combined with an AWG sampling rate of 1GSa / s, a bandwidth of 20MHz can be generated. The time and frequency domains of the designed pulses in this embodiment are as follows: Figure 4 As shown.

[0063] The beat frequency signals of the backscattered Rayleigh light signal and the local oscillator light signal generated by the designed optical pulse in the optical fiber under test are collected. A balanced photodetector is used to convert the beat frequency signals into electrical signals. The complex form of the electrical signals is then used... It means that, among them, The convolution operation is represented by I(t), the beat frequency signal is represented by ∝, the complex form of the electrical signal proportional to I(t) is represented by m(t), and the pulse modulation signal is represented by f(t). The system's impulse response records the interference information applied to the fiber under test (FUT).

[0064]

[0065] In the formula, N is the number of scattering events in the entire FUT, and τ i Let r(τ) be the round-trip time of the light wave from the near end to the i-th scatterer. i ) represents the Rayleigh scattering coefficient of the i-th scatterer, α represents the fiber attenuation, and v g ω is the speed of light in the fiber core. c Let ω be the angular frequency of the light source, and δ(t) be the Dirac function.

[0066] Mode The convolution operation in the diagram shows that the detected beat frequency signal I(t) is the response of the coherent system f(t) to the pulse modulation signal m(t), where f(t) contains the intrinsic information of the scattering particles in the optical fiber, and m(t) contains the shape and frequency information of the modulation signal. Due to the properties of convolution, the frequency domain expression of the beat frequency signal is I(ω) = M(ω)F(ω), where M(ω) represents the Fourier transform pair of m(t), i.e., the frequency domain form of m(t), and F(ω) represents the Fourier transform pair of f(t), i.e., the frequency domain form of f(t).

[0067] In this embodiment, sensor data is demodulated from the beat frequency electrical signal, specifically including:

[0068] First, the frequency shift introduced by the acousto-optic modulator in the beat frequency signal is removed by down-conversion. Since the acousto-optic modulator used in this embodiment has a 200MHz frequency shift, the acquired signal is multiplied by cos(2πft) and sin(2πft) respectively after receiving the signal, where f is 200MHz, to obtain the IQ components. Then, the in-phase component I and the quadrature component Q are added together as real and imaginary parts, and the backscattered light intensity is obtained by digital squaring. Performing a fast Fourier transform on this intensity yields the frequency domain diagram after spectral shifting, as shown below. Figure 5 As shown, the spectrum of the experimental data corresponds to the spectrum of the designed multi-carrier signal sequence data.

[0069] Then, the multi-carrier signal moved to the baseband is bandpass filtered in the frequency domain to filter out each subcarrier signal or select a few subcarriers with higher energy. In fact, the entire frequency band of the impulse response contains interference information, so each subcarrier can be extracted to independently recover the interference information. The five subcarriers with the highest energy are extracted using a 20MHz bandpass filter, as illustrated in the diagram below. Figure 6 As shown. Each subcarrier is numbered for ease of explanation. The center subcarrier with the highest energy is defined as the 0th-order subcarrier, denoted as subcarrier S0, and its frequency is denoted as f0. Subcarriers with frequencies higher than S0 are defined as S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S21, S31, S41, S12, S21, S31, S41 ...41, S31, S41, S41, S41, S +1 S +2 ..., the corresponding frequencies are denoted as f. +1 f +2 ...Subcarriers with frequencies lower than S0 are defined as S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S21, S32, S42, S83, S93, S103, S113, S21, S32, S42 ...3 -1 S -2 ..., the corresponding frequencies are denoted as f. -1 f -2 ...The dashed box in the figure represents the subcarrier extraction by the filter. Through the filtering extraction operation, equivalent detection results for five different frequency bands are obtained.

[0070] Performing an inverse Fourier transform on the filtered subcarrier signal in the frequency domain yields information collected by probe pulses at different frequencies. These signals have different amplitudes and fading points, which aligns with the principle of using different probe frequencies to eliminate coherent fading.

[0071] In this embodiment, to reduce fading points while preserving phase information, a rotating vector sum method is used to combine these signals. Rotating vector sum is an effective multi-frequency combination method for eliminating fading points. These complex vector results are vector-synthesized using the rotating vector sum method, and the combined vector is used as the sensing data.

[0072] Specifically, the detection results corresponding to each frequency (assuming m subcarrier frequencies are extracted from multiple carriers) are first grouped according to the trigger period, that is, the first detection result corresponding to the first pulse at time t1, the second detection result corresponding to the second optical pulse at time t2, ..., the nth detection result corresponding to the nth pulse at time tn. Through grouping, the Rayleigh signal can be obtained: multiple signals of frequency 1. Multiple signals at frequency 2 Multiple signals at frequency m Here, the detection result at a certain position z obtained by detecting at frequency k and time t1 is taken. As a reference for the n detection results at frequency k, the ideal complex vector representation of the l-th detection tl is given by l∈[1,n]:

[0073]

[0074] in, and Let z be the magnitude and phase angle of the Rayleigh signal at position z at frequency k and time t1. This refers to the phase change between two detections caused by external vibrations.

[0075] The reason complex numbers cannot be directly added is due to the existence of the initial phase angle. To remove its influence, the normalized conjugate of the Rayleigh signal obtained from the first detection at time t1 can be used as a reference, i.e. The Rayleigh signal was obtained by multiplying the signal by each subsequent detection. Obtain the Rayleigh signal after rotation When processing multiple subcarrier signals using the rotating vector summation method, the complex vector is rotated so that the vectors of reference time t1 for different subcarriers at each fiber position z lie on the same phase axis. Because the phase changes caused by disturbances at different optical frequencies are considered equal, the orientation of the rotated vector is the same each time, resulting in a zero-angle phase difference between the new complex signals. This rotational summation of the complex signals increases the vector magnitude, mitigating coherent fading while still preserving the phase changes caused by vibrations.

[0076] like Figure 7 (a) shows the phase results obtained from a single subcarrier probe. It can be seen that outside the perturbation point set in the experiment, there are still many phase demodulation errors caused by coherent fading. For example... Figure 7 (b) shows the phase result obtained using the multi-carrier pulse detection proposed in this invention. In the embodiment, the phase demodulation result after rotating and combining the five subcarriers with the highest energy was used. It can be seen that the final phase is flat in the unperturbed areas and fluctuates in the perturbed areas due to the sinusoidal signal we applied. This indicates that coherent fading is effectively suppressed and demodulation errors caused by fading are eliminated.

[0077] The beneficial effects of this invention are as follows: Continuous light emitted by the laser is split into probe light and local oscillator light signals by a first coupler. A multi-carrier pulse sequence with intensity modulation is designed and input into an arbitrary waveform generator to generate a multi-carrier electrical signal to drive an acousto-optic modulator, thereby achieving intensity modulation of the probe light and obtaining a multi-carrier pulse light signal. This pulse light signal is injected into the sensing fiber under test. Backscattered Rayleigh light signal and the local oscillator light signal in the sensing fiber under test are collected and beat frequency is performed to obtain a beat frequency signal. The beat frequency signal is demodulated to obtain sensing data. The probe data corresponding to the subcarriers are extracted and combined to suppress coherent fading. In this scheme, the probe optical signal is converted into a multi-carrier pulse optical signal, and the resulting multiple bandwidth-controllable subcarriers can be easily filtered out to achieve equivalent detection of multiple frequencies. By applying the results of multiple frequencies, interference fading can be effectively suppressed. At the same time, this scheme uses specially designed pulses to achieve multi-frequency detection, which only requires simple intensity modulation. This can be done with just an analog acousto-optic modulator, instead of using multiple narrow-linewidth laser sources or multiple acousto-optic modulators. This does not significantly change the sensing platform and the experimental cost is low. In addition, this scheme still completes one detection with a single pulse instead of a pulse group of coded schemes. Obviously, this scheme is more efficient and simpler.

[0078] 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, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An optical sensing method using multicarrier probe pulses, the optical sensing method being implemented based on an optical sensing system using multicarrier probe pulses, the optical sensing system comprising: laser, first coupler, acousto-optic modulator, arbitrary waveform generator, doped fiber amplifier, optical filter, circulator, to-be-measured sensing optical fiber, polarization controller, second coupler, balanced photodetector, acquisition card and digital signal processing module; The laser output end is connected with the input end of the first coupler, the output end of the first coupler is connected with the first input end of the acousto-optic modulator and the input end of the polarization controller respectively, the arbitrary waveform generator is connected with the second input end of the acousto-optic modulator, the output end of the acousto-optic modulator is connected with the input end of the doped fiber amplifier, the output end of the doped fiber amplifier is connected with the input end of the optical filter, the output end of the optical filter is connected with the first port of the circulator, the second port of the circulator is connected with the to-be-measured sensing optical fiber, the output end of the polarization controller and the third port of the circulator are connected with the input end of the second coupler, the output end of the coupler is connected with the input end of the balanced photodetector, the output end of the balanced photodetector is connected with the input end of the acquisition card, and the output end of the acquisition card is connected with the digital signal processing module; The laser outputs a continuous light signal with high coherence, and the continuous light signal is divided into two paths by the first coupler, one path is used as a local light signal, and the other path is used as a probe light entering the acousto-optic modulator; The arbitrary waveform generator generates a multi-carrier pulse electrical signal with intensity modulation, and drives the acousto-optic modulator to modulate the probe light signal into a multi-carrier pulse light signal, and the multi-carrier pulse light signal sequentially passes through the doped fiber amplifier and the optical filter, and is injected into the to-be-measured sensing optical fiber through the circulator; The backscattering Rayleigh scattering light signal returned by the to-be-measured sensing optical fiber returns through the circulator and beats with the local light signal, the beat signal enters the signal port of the balanced photodetector and is converted into an electrical signal, the acquisition card collects all the electrical signals, and the digital signal data collected by the acquisition card is demodulated by the digital signal processing module to obtain a sensing signal; The method comprises the following steps: S1: The continuous light emitted by the laser is divided into a probe light and a local light signal by the first coupler, a multi-carrier pulse sequence with intensity modulation is designed, the multi-carrier pulse sequence is input into the arbitrary waveform generator to generate a multi-carrier electrical signal to drive the acousto-optic modulator, intensity modulation of the probe light is realized, a multi-carrier pulse light signal is obtained, and the pulse light signal is injected into the to-be-measured sensing optical fiber; S2: The backscattering Rayleigh scattering light signal in the to-be-measured sensing optical fiber and the local light signal are beat to obtain a beat signal; S3: The beat signal is demodulated, the probe data corresponding to the subcarriers are extracted and combined, sensing data is obtained, and suppression of coherent fading is realized; The multi-carrier pulse sequence with intensity modulation comprises the following steps: First, a sequence of all zeros is generated, then two conjugate complex numbers X+iY and X-iY are used to replace the symmetric positions of the sequence, X and Y are real numbers, and i is the imaginary unit. The sequence is taken as a frequency domain sequence, and inverse Fourier transform is performed on the sequence to obtain the time sequence corresponding to the frequency domain sequence. In fact, this step generates a sine wave sequence with an initial phase. Then, a suitable threshold is selected for cutting on the generated sine wave to adapt to the modulation of the acousto-optic modulator. In the cutting process, frequency harmonics are generated, thereby obtaining multiple subcarriers. The cutting needs to select a suitable threshold, retain the part above the threshold, and set a suitable direct current DC below the threshold, which is smaller than the original amplitude: Wherein, amplitude represents the amplitude of the sine wave before cutting, amplitude' represents the amplitude of the waveform after cutting, and threshold represents the set threshold.

2. An optical sensing method using multicarrier probe pulses as claimed in claim 1, characterized in that: The sensing data is demodulated from the beat frequency signal, specifically including: First, use the down-conversion operation to remove the frequency shift introduced by the acousto-optic modulator in the beat frequency signal; Then, band-pass filter the multi-carrier signal moved to the base frequency in the frequency domain, filter out each sub-carrier signal or select several sub-carriers with larger energy; Inverse Fourier transform is performed on the filtered sub-carrier signal to obtain the results of different frequency pulse detection; These results are vector synthesized by rotating the vector sum, and the combined vector is taken as the sensing data.

Images