Optical fiber hydrophone array high-speed demodulation device based on spectrum interference technology

Abstract

The invention discloses an optical fiber hydrophone array high-speed demodulation device based on a spectrum interference technology, which obtains four-channel output with equal intermediate wavelength intervals by adopting a two-stage cascaded Mach-Zehnder interferometer structure, and can efficiently demultiplex a composite spectrum signal of a hydrophone array. And in cooperation with a high-speed digital orthogonal demodulation algorithm, synchronous and high-speed phase demodulation of all hydrophone units in a large-scale array is realized, and the demodulation speed is only limited by a system sampling rate. The cascade Mach-Zehnder interferometer high-speed demodulation technology and the array multiplexing technology are combined, the technical problems that the demodulation speed is low and the system is complex when a traditional demodulation scheme faces a large-scale array are effectively solved, and the cascade Mach-Zehnder interferometer array demodulation method is particularly suitable for application scenes where large-scale and real-time detection is conducted on high-speed and high-fidelity underwater acoustic signals.

Description

[0001] Project support statement The research work of the present application is supported by the Zhejiang Provincial Department of Education Scientific Research Project (Y202457133). TECHNICAL FIELD

[0002] The present application belongs to the technical field of optical sensing, and particularly relates to a high-speed demodulation device for fiber-optic hydrophone array based on wave spectrum interference technology. BACKGROUND

[0003] Fiber-optic hydrophones have become the core components of modern underwater acoustic detection systems due to their high sensitivity, resistance to electromagnetic interference, and ease of arraying. With the development of ocean monitoring, underwater safety, and resource exploration, higher performance is required for fiber-optic hydrophone systems. On the one hand, large-scale arrays with hundreds or even thousands of channels are needed to form a large detection aperture. On the other hand, high-frequency and transient underwater acoustic signals (such as sonar pulses and underwater explosion sounds) need to be accurately captured, which requires the demodulation system to have extremely high demodulation speed and working bandwidth.

[0004] Currently, the main technologies for realizing fiber-optic hydrophone array demodulation include PGC demodulation method and 3x3 coupler-based unbalanced interferometer demodulation method. Although these methods are mature in single-point or small-scale array applications, their inherent bottlenecks become increasingly prominent when applied to large-scale arrays. 1. Conflict between demodulation speed and channel number: Traditional PGC demodulation method requires modulation and sampling of the carrier, and its demodulation speed is limited by the carrier frequency and digital processing capability. In the construction of large-scale time-division multiplexing or wavelength-division multiplexing arrays, the system's scanning rate or data processing rate will become a bottleneck to maintain sufficient sampling rate for each channel, making it difficult to achieve high-speed and real-time demodulation of all channels, thereby losing high-frequency acoustic information.

[0005] 2. System complexity and high cost: Especially for wavelength-division multiplexing systems, each sensing channel usually needs to be equipped with an independent demodulation unit or use an expensive high-precision spectrometer, resulting in complex system structure, large volume, and linear cost growth with the number of channels, making it difficult to deploy on a large scale in practical engineering.

[0006] 3. Channel crosstalk problem: In densely multiplexed arrays, especially in time-division multiplexing systems with high scanning rates or wavelength-division multiplexing systems with narrow channel spacing, traditional demodulation methods are more sensitive to light source stability, phase noise, and signal processing algorithms, which can easily cause signal crosstalk between channels, reducing the positioning accuracy and signal-to-noise ratio of the array.

[0007] In addition, some demodulation schemes based on direct spectral detection, although simple in structure, are limited in demodulation speed by the spectral scanning rate of the spectrometer or the frame rate of the array camera, and are usually only suitable for the measurement of static or low-frequency signals, and cannot fully meet the needs of high-speed dynamic signal capture in underwater acoustic detection. SUMMARY

[0008] In view of the above, the present application provides a high-speed demodulation device for fiber-optic hydrophone array based on wave spectrum interference technology, which has the advantages of fast demodulation speed, wide working frequency band, and easy implementation.

[0009] A high-speed demodulation device for fiber-optic hydrophone array based on wave spectrum interference technology, characterized in that it comprises an optical signal transmitting system, a fiber-optic hydrophone array, and a high-speed demodulation system; wherein the optical signal transmitting system is used to modulate a continuous broadband optical signal into an optical pulse sequence; the fiber-optic hydrophone array is used to load an underwater acoustic vibration signal onto the optical signal based on the pulse sequence; and the high-speed demodulation system is used to demodulate the interference optical signal of the fiber-optic hydrophone array, thereby obtaining an external underwater acoustic vibration signal. The present scheme modulates the optical signal into an optical pulse sequence by an optical modulation unit, loads an underwater acoustic vibration signal onto the optical signal by a fiber-optic hydrophone array, and detects the interference optical signal of the array system by a high-speed demodulation system composed of two cascaded Mach-Zehnder interferometers, thereby realizing high-speed demodulation of the target signal.

[0010] Further, the fiber-optic sensing system comprises: a broadband light source for generating low-coherence broadband laser; an optical modulation unit for modulating the broadband laser into an optical pulse sequence; a band-pass filter for band-pass filtering the modulated optical pulse sequence; an optical amplification unit for power amplifying the filtered optical pulse sequence; a circulator for inputting the amplified optical pulse sequence into the fiber-optic hydrophone array as a main optical path, and inputting the reflected spectrum of the hydrophone array into the high-speed demodulation system for demodulation; Further, the fiber-optic hydrophone array comprises: N one optical coupler for splitting the pulsed optical signal into two paths, one of which is input to the fiber-optic hydrophone to detect the external underwater acoustic vibration signal, and the other of which is input to the next optical coupler after being delayed by a delay optical fiber along the main optical path; N one delay optical fiber for connecting the adjacent two fiber-optic hydrophones in the main optical path with a certain time delay interval; N+1a plurality of fiber hydrophones: for converting the information of underwater acoustic vibration into the phase change of optical pulse through the hydrophone array element, each of the fiber hydrophone sensing units is a Michelson interference structure; Further, the first n The splitting ratio of the fiber coupler is configured as the ratio of the output optical power of the hydrophone branch to the main optical path , considering the double splitting loss of the round trip transmission, the specific splitting ratio should be determined by the recursive relationship , where n= 1, 2, …, N-1 , and the boundary condition is , to ensure that the amplitude of the interference signal of each hydrophone is approximately equal when it reaches the receiving end.

[0011] Further, the time delay introduced by the delay optical fiber between adjacent hydrophones in the fiber hydrophone array must be greater than the time domain width of the optical pulse injected by the optical modulation unit.

[0012] Further, the repetition period of the pulse injected by the optical modulation unit must be greater than the longest round trip time of the optical signal in the fiber hydrophone array, so as to ensure the effective isolation of the time division multiplexing channel in the time domain.

[0013] Further, the N+1 The fiber hydrophones are of the same structure and adopt the Michelson interference structure, including a fiber coupler, first and second fiber coils, an inner core shaft, an outer core shaft, and first and second Faraday rotating mirrors. The input optical signal is divided into two paths by the fiber coupler to form two fiber coils: one path is wound outside the outer core shaft as a sensing arm, and the other path is wound outside the inner core shaft as a reference arm. The other ends of the two paths of optical fibers are connected to the first and second Faraday rotating mirrors, respectively, and the reflected light returns to the fiber coupler. The inner core shaft and the outer core shaft are made of materials that can produce elastic deformation and are in the form of tubes, and air is filled in the annular gap between them to form an air back structure, thereby forming a Michelson interference type hydrophone.

[0014] Further, the high-speed demodulation system includes: A first Mach-Zehnder interferometer is used to configure the two arms with a fixed optical path difference, thereby decomposing the input optical signal into two output signals with the same free spectral range and the peaks and valleys of which are interlaced; Second and third Mach-Zehnder interferometers are used to connect their input ends to the two output signals of the first Mach-Zehnder interferometer, respectively, and are configured to align the center wavelength with the peak of the corresponding input signal to perform secondary interference, and each outputs two interference optical signals; first and second balanced photodetectors for receiving interference optical signals outputted by the second and third Mach-Zehnder interferometers respectively and converting them into differential electrical signals; a data acquisition card for acquiring the differential electrical signals and demodulating them through a digital signal processing unit integrated therein to restore external signals to be detected.

[0015] Further, the free spectral ranges of the second and third Mach-Zehnder interferometers are the same and twice that of the first Mach-Zehnder interferometer.

[0016] Further, the center wavelengths of the second and third Mach-Zehnder interferometers are accurately aligned with the center wavelength of the output of the first Mach-Zehnder interferometer to obtain four-channel interference outputs with equal optical frequency intervals.

[0017] Further, the bandwidth of the bandpass filter is consistent with the free spectral ranges of the second and third Mach-Zehnder interferometers.

[0018] Further, two of the four-channel interference outputs corresponding to adjacent center wavelengths have a constant phase difference.

[0019] Further, the data acquisition and processing unit comprises a phase difference compensation module, a division module, an inverse tangent module and a phase unwrapping module; and the processing procedure is as follows: sampling the differential signals of the two balanced photodetectors, performing trigonometric function compensation on one of the signals according to the constant phase difference, dividing the compensated signal by the other signal, performing inverse tangent operation to obtain an instantaneous phase, and then performing phase unwrapping to obtain the signal to be detected.

[0020] Further, when the center wavelengths of the cascaded Mach-Zehnder interferometers are shifted, the center wavelength shift is monitored to obtain a phase shift, trigonometric identities are used for phase compensation to eliminate the influence of center wavelength shift on the demodulation result.

[0021] The device of the application adopts a two-stage cascaded Mach-Zehnder interferometer structure to obtain four-channel outputs with equal intermediate wavelength intervals, and can efficiently demultiplex the composite spectrum signals of a hydrophone array. In combination with a high-speed digital quadrature demodulation algorithm, synchronous and high-speed phase demodulation of all hydrophone units in a large-scale array is realized, and the demodulation speed is only limited by the sampling rate of the system. The application combines the high-speed demodulation technology of the cascaded Mach-Zehnder interferometer with the array multiplexing technology, effectively solves the technical problems of slow demodulation speed and complex system of the traditional demodulation scheme when facing a large-scale array, and is particularly suitable for large-scale and real-time detection of high-speed and high-fidelity underwater acoustic signals. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1The schematic diagram of the specific structure of the device of the present application.

[0023] In the figure: 101 - broadband light source, 102 - light modulation unit, 103 - band-pass filter, 104 - optical amplification unit, 105 - circulator, 106 - fiber hydrophone array, 107 - first Mach-Zehnder interferometer, 108 - second Mach-Zehnder interferometer, 109 - third Mach-Zehnder interferometer, 110 - first balanced photodetector, 111 - second balanced photodetector, 112 - data acquisition card.

[0024] Figure 2 The timing diagram of the time division multiplexing fiber hydrophone array system used in the device of the present application.

[0025] Figure 3 The flowchart of the realization of data acquisition and processing of the device of the present application.

[0026] Figure 4 The schematic diagram of the realization principle of the high-speed demodulation algorithm of the device of the present application.

[0027] Figure 5 The schematic diagram of the structure of the fiber hydrophone used in the device of the present application.

[0028] In the figure: 501 - fiber coupler, 502 - first fiber coil, 503 - first fiber coil, 504 - outer core shaft, 505 - inner core shaft, 506 - first Faraday rotating mirror, 507 - second Faraday rotating mirror.

[0029] Figure 6 The output characteristics of the two-stage cascaded Mach-Zehnder interferometer in the device of the present application.

[0030] Figure 7 The simulation demodulation results of the high-speed demodulation algorithm provided by the device of the present application. (a) Sinusoidal wave demodulation result; (b) square wave demodulation result. DETAILED DESCRIPTION

[0031] In order to more specifically describe the present application, the technical solutions of the present application are described in detail below in combination with the drawings and specific embodiments.

[0032] As Figure 1 shown, the fiber vibration sensor high-speed demodulation device based on cascaded Mach-Zehnder interferometer of the present application includes a broadband light source 101, a light modulation unit 102, a band-pass filter 103, an optical amplification unit 104, a circulator 105, a fiber hydrophone array 106, a first Mach-Zehnder interferometer 107, a second Mach-Zehnder interferometer 108, a third Mach-Zehnder interferometer 109, a first balanced photodetector 110, a second balanced photodetector 111, and a data acquisition card 112.

[0033] In this embodiment, the light signal emitted by the wide spectrum light source 101 is modulated into a light pulse sequence by the light modulation unit 102, then filtered by the band-pass filter 103, amplified by the optical amplifier unit 104, and then enters the fiber hydrophone array 106 through the circulator 105. The fiber hydrophone responds to the external underwater acoustic vibration signal and modulates it onto the light phase. The interfered signal passes through the first Mach-Zehnder interferometer 107, and the cascaded second Mach-Zehnder interferometer 108 and third Mach-Zehnder interferometer 109 to obtain four-way output. The first balanced photodetector 108 and the second balanced photodetector 110 respectively receive the light signals of the second and third Mach-Zehnder interferometers and differentially amplify them into electrical signals, which are input to the data acquisition card 111 for the next sampling demodulation operation.

[0034] The fiber hydrophone array used by the device is a series of reflection type, the sensing element is a Michelson fiber acoustic sensor, and the reflecting end uses a Faraday mirror. A delay fiber is arranged between the adjacent channels of the main light path, and the pulse width and period are accurately designed. For each pulse injected into the array, a pulse train will be received at the receiving end, and the number of pulses is equal to the number of fiber hydrophones, N the number of fiber couplers and delay fibers, N+1 the number of fiber hydrophones. Each pulse corresponds to a sensing probe. The timing diagram of the pulse interference of each channel is shown in Figure 2 .

[0035] Specifically, the implementation process of data acquisition and processing is shown in Figure 3 , which includes: S1: The master system generates a synchronization pulse signal, and synchronizes the working timing of the data acquisition card by triggering the external clock of the data acquisition card.

[0036] S2: The data acquisition card is responsible for synchronously collecting the mixed time domain voltage waveforms output by the two balanced detectors.

[0037] S3: The collected differential interference signals are divided into time windows, and the width of the time window must be greater than the pulse width and strictly isolated in time from the time window of the adjacent channel. Then, the S301~S304 high-speed demodulation algorithm is used to analyze the data in each time window to obtain the phase state of each hydrophone signal at a specific time point.

[0038] S4: Extract the discrete phase points obtained by demodulation in each pulse period, classify them according to different detection channels, and then arrange these phase points according to their time sequence to form a time sequence.

[0039] S5: Perform the S305 unwrapping operation on the time sequence formed for each channel to recover the original signal.

[0040] In particular, the interference signal from the fiber hydrophone array can be expressed as

[0041] wherein, A is the DC component, B is the fringe contrast; is the refractive index of the fiber, is the wavelength of the broadband light source, is the initial arm length difference of the fiber hydrophone, is the fiber length change caused by the sensor responding to the external acoustic vibration signal. After passing through the cascaded Mach-Zehnder interferometers, the four-channel signal can be expressed as

[0042] wherein, the central wavelength of the first channel, i = 1, 2, 3 and 4. Wherein, i and are the outputs of the second Mach-Zehnder interferometer, and are the outputs of the third Mach-Zehnder interferometer. Let

[0043]

[0044] wherein, j = 2, 3 and 4; is the central wavelength of the broadband light source, since the central optical frequencies of the channels of the cascaded Mach-Zehnder interferometer are uniformly spaced, the central wavelength can be considered to be approximately uniformly spaced, and can be obtained

[0045]

[0046] The four-channel signal can be rewritten as

[0047] wherein, The value of depends on the central wavelength of the light source , the output wavelength interval of the cascaded Mach-Zehnder interferometer and the initial arm length difference of the hydrophone , and the first two are known quantities. For a specific fiber hydrophone, the initial arm length difference of the fiber hydrophone is determined, at this time is a constant.​

[0048] To obtain the external signal to be detected, according to one aspect of the present invention, an orthogonal demodulation algorithm for phase demodulation of an optical fiber sensor is provided, comprising: S301: Four optical signals with equal phase difference are output by using a cascaded Mach-Zehnder interferometer.

[0049] S302: By performing balanced detection on the output optical signals of the second and third Mach-Zehnder interferometers respectively, two differential signals can be obtained, which are defined as intermediate variables. and , can be represented as

[0050]

[0051] In the formula, It is a constant.

[0052] S303: Yes and By performing algebraic operations, we can obtain two orthogonal signals, which are defined as intermediate variables. ES and OS , can be represented as

[0053]

[0054] S304: By performing arctangent operation on the two obtained orthogonal signals, we can obtain...

[0055] S305: Finally, the external signal to be detected is obtained through phase decoupling.

[0056] Figure 4 A schematic diagram illustrating the implementation principle of the high-speed demodulation algorithm based on this four-channel circuit is shown.

[0057] Considering the non-ideal output caused by insufficient light source flatness and the difficulty in perfectly aligning the center wavelengths of the two-stage Mach-Zehnder interferometer in actual fabrication (mainly manifested as inconsistent transmittance of each channel and center wavelength shift), this invention introduces corresponding correction steps into the digital demodulation algorithm. First, the signal is normalized by dividing by the measured transmittance of each channel to eliminate amplitude response differences. Second, for the non-ideal output of the second balanced detector due to wavelength shift, this signal can be characterized as a model containing a fixed phase deviation; accordingly, the algorithm incorporates a phase compensation function, which can effectively correct this deviation and ensure the accuracy of subsequent demodulation.

[0058] When there is an offset between channels 2 and 4, S2 needs to be corrected. In this case, the output of the second balanced detector can be expressed as...

[0059]

[0060]

[0061] In the formula, It is the center wavelength offset of channels 2 and 4. No. i The center wavelength of each channel can be obtained from the output spectrum. At this time, ES It can be corrected to the following formula

[0062] A schematic diagram of the fiber optic hydrophone used in the device of this invention is shown below. Figure 5 As shown. Each fiber optic hydrophone has an identical structure and employs a Michelson interferometer design, including a fiber optic coupler 501, a first fiber optic coil 502, a second fiber optic coil 503, an outer core 504, an inner core 505, a first Faraday rotator 506, and a second Faraday rotator 507. The input optical signal is split into two paths by the fiber optic coupler 501 to form the first fiber optic coil 502 and the second fiber optic coil 503: one path is wound around the outside of the outer core 504 as a sensing arm; the other path is wound around the outside of the inner core 505 as a reference arm. The other ends of the two optical fibers are connected to the first Faraday rotator 506 and the second Faraday rotator 507, respectively, and return to the fiber optic coupler 501 after reflection. The outer core 504 and the inner core 505 are both made of a material capable of elastic deformation and are tubular in shape. Air is filled into the annular gap in the middle to form an air-back structure, thus constituting a Michelson interferometer hydrophone.

[0063] The four-channel output characteristics of a cascaded Mach-Zehnder interferometer are as follows: Figure 6 As shown, the distribution of each channel in the spectrum is regular and approximately equally spaced. When the interference signal of the fiber optic hydrophone array is input into the interferometer, this characteristic is converted into a fixed phase difference between the output signals of adjacent channels, laying the foundation for subsequent quadrature demodulation.

[0064] In one embodiment, the demodulation results of a phase signal with a frequency of 1 kHz and an amplitude of 3 rad were simulated. The simulation results of the sine wave and square wave are shown in Figure 7(a) and Figure 7(b), respectively.

[0065] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A high-speed demodulation device for fiber optic hydrophone arrays based on spectral interferometry, characterized in that: The system comprises an optical signal transmission system, a fiber optic hydrophone array, and a high-speed demodulation system. The optical signal transmission system modulates a continuous broadband optical signal into a sequence of optical pulses. The fiber optic hydrophone array loads an underwater acoustic vibration signal onto the optical signal based on the pulse sequence. The high-speed demodulation system demodulates the interference optical signal from the fiber optic hydrophone array to obtain the external underwater acoustic vibration signal. This scheme modulates the optical signal into a sequence of optical pulses using an optical modulation unit, loads the underwater acoustic vibration signal onto the optical signal using a fiber optic hydrophone array, and uses a high-speed demodulation system composed of two cascaded Mach-Zehnder interferometers to detect the interference optical signal of the array system, thereby achieving high-speed demodulation of the target signal.

2. The demodulation device according to claim 1, characterized in that: The optical signal transmitting system includes: Broadband light source, used to generate low-coherence broadband lasers; Optical modulation unit: used to modulate broadband laser light into a sequence of optical pulses; A bandpass filter is used to bandpass filter a modulated optical pulse sequence. The optical amplification unit is used to amplify the power of the filtered optical pulse sequence; A circulator is used to input the amplified optical pulse sequence as the main optical path into the fiber optic hydrophone array, and to input the reflection spectrum of the hydrophone array into the high-speed demodulation system for demodulation.

3. The demodulation device according to claim 1, characterized in that: The fiber optic hydrophone array includes: N One fiber optic coupler: used to split the pulsed light signal into two paths. One path is input to the fiber optic hydrophone to detect the external underwater acoustic vibration signal, and the other path is input to the next fiber optic coupler after being delayed along the main optical path through the delay fiber. N One delay fiber: used to connect two adjacent fiber optic hydrophones in series in the main optical path with a certain time delay interval; N+1 Each fiber optic hydrophone is used to convert underwater acoustic vibration information into optical phase changes of optical pulses passing through the hydrophone array elements. Each fiber optic hydrophone sensing unit is a Michelson interferometer structure.

4. The demodulation device according to claim 3, characterized in that: The first n The splitting ratio of the fiber couplers is configured such that the ratio of the output optical power of the hydrophone branch to that of the main optical path is... Considering the double splitting loss during round-trip transmission, the specific splitting ratio should be determined through a recursive relationship. Confirmed, among which n= 1, 2, ..., N-1 And the boundary conditions are This ensures that the amplitude of the interference signal from each hydrophone is approximately equal when it reaches the receiving end.

5. The demodulation device according to claim 3, characterized in that: The time delay introduced by the delay fiber pair of the optical fiber between adjacent hydrophones in the optical fiber hydrophone array must be greater than the time domain width of the optical pulse injected by the optical modulation unit.

6. The demodulation device according to claim 3, characterized in that: The repetition period of the pulse injected by the optical modulation unit must be greater than the longest round-trip time of the optical signal in the fiber optic hydrophone array, so as to ensure effective isolation of the time-division multiplexed channel in the time domain.

7. The demodulation device according to claim 3, characterized in that: The N+1 Each fiber optic hydrophone has an identical structure and employs a Michelson interferometer configuration, including a fiber optic coupler, first and second fiber optic coils, an inner core, an outer core, and first and second Faraday rotators. The input optical signal is split into two paths by the fiber optic coupler, forming two fiber optic coils: one coil is wound around the outside of the outer core as a sensing arm; the other coil is wound around the outside of the inner core as a reference arm. The other ends of the two fibers are connected to the first and second Faraday rotators, respectively, and the signal is reflected back to the fiber optic coupler. Both the inner and outer cores are made of a material capable of elastic deformation and are tubular in shape. Air is filled into the annular gap in the middle to form an air-back structure, thus constituting a Michelson interferometer hydrophone.

8. The demodulation device according to claim 1, characterized in that: The high-speed demodulation system includes: The first Mach-Zehnder interferometer is used to configure the two arms with a fixed optical path difference, thereby decomposing the input optical signal into two output signals with the same free spectral range and interlacing peaks and troughs; The second and third Mach-Zehnder interferometers are used to connect their input ends to the two outputs of the first Mach-Zehnder interferometer, respectively, and are configured to align their center wavelengths with the peaks of the corresponding input signals to perform secondary interference, and each outputs two interference light signals. The first and second balanced photodetectors are used to receive the interference light signals output by the second and third Mach-Zehnder interferometers, respectively, and convert them into differential electrical signals; The data acquisition card is used to acquire the differential electrical signal and demodulate it through its internally integrated digital signal processing unit to restore the external signal to be detected.

9. The demodulation apparatus according to claim 8, characterized in that: The second and third Mach-Zehnder interferometers have the same free spectral range, and both are twice that of the first Mach-Zehnder interferometer.

10. The demodulation device according to claim 8, characterized in that: The center wavelengths of the second and third Mach-Zehnder interferometers are precisely aligned with the center wavelength of the corresponding output of the first Mach-Zehnder interferometer to obtain four equally spaced optical frequency interference outputs; The bandwidth of the bandpass filter is consistent with the free spectral range of the second and third Mach-Zehnder interferometers; In the four-way interference output, the two outputs corresponding to adjacent center wavelengths have a constant phase difference; The data acquisition and processing unit includes a phase difference compensation module, a division module, an arctangent module, and a phase unwinding module. Its processing flow is as follows: sample the differential signals of two balanced photodetectors, perform trigonometric function compensation on one of the signals according to the constant phase difference, divide the compensated signal with the other signal and obtain the instantaneous phase by arctangent operation, and then perform phase unwinding to obtain the measured quantity. When the center wavelength of the cascaded Mach-Zehnder interferometer shifts, the center wavelength shift is obtained by monitoring the output spectrum and the phase shift is calculated. The phase is then compensated using trigonometric identities to eliminate the influence of the center wavelength drift on the demodulation results.

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