Long-distance fiber optic interferometric sensing system based on synchronous differential closed-loop structure

Abstract

The present invention discloses a long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure, comprising: a light source module, a frequency modulation module, a long-distance transmission module, a reference light transmission module, a sensing light transmission module, a reference light photoelectric conversion module, a sensing light photoelectric conversion module, an intermediate frequency mixing module, a zero intermediate frequency mixing module, and a digital demodulation control module. The long-distance fiber optic interferometric sensing system based on the synchronous differential closed-loop structure of the present invention effectively realizes the common-mode suppression of carrier phase noise through the introduction of a synchronous signal pair and the combination of differential detection in long-distance high-precision measurement, thereby avoiding the limitation of carrier phase noise on the improvement of the system detection noise performance.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing, and in particular relates to a long-distance optical fiber interferometric sensing system based on a synchronous differential closed-loop structure. Background Art

[0002] Fiber optic sensing technology has been widely used in modern manufacturing, medical, military and other fields, and has become an indispensable and important technical means. Among them, heterodyne interferometry technology, as one of the core methods of fiber optic sensing, detects and demodulates optical signals with high precision based on the principle of interference, and can accurately measure parameters such as the amplitude, phase and frequency of light. With high sensitivity, high resolution and good reliability, this technology plays a key role in optical fiber communication, spectral analysis, biomedicine, materials science and industrial manufacturing. With the rapid growth of modern society's demand for ultra-long-distance and high-precision sensing, the importance of heterodyne interferometry technology has become increasingly prominent. Long-distance sensing is widely used in national defense security, energy monitoring, geological disaster warning, submarine communications and large-scale structural health monitoring. Its core lies in solving the challenges of signal attenuation and noise interference in long-distance transmission, and ensuring the stability and accuracy of the sensing system in complex environments. In this context, heterodyne interferometry technology has become a key solution to meet the needs of long-distance sensing with its excellent sensitivity and anti-interference ability. However, long-distance sensing puts forward higher requirements on detection performance, including the improvement of signal demodulation accuracy, effective suppression of noise and real-time response capability of the system, which poses new challenges to further optimization and innovation of technology.

[0003] In practical applications, the phase noise of the carrier signal and the noise optimization of the analog demodulation circuit are two key research issues that need to be solved urgently. First, in heterodyne interferometry technology, the phase noise of the carrier signal can usually be optimized by differential mixing cancellation. However, with the increase of transmission distance, the time correlation between the reference signal used for mixing and the signal to be measured gradually weakens, resulting in a significant decrease in the differential cancellation effect of the near-end phase noise. If the phase noise is not effectively cancelled, it will directly affect the accuracy of the demodulated signal. Especially in long-distance and high-precision measurements, further optimizing the carrier phase noise becomes a key direction to improve system performance. Direct optimization of carrier phase noise often relies on the use of a higher-performance, low-phase-noise reference crystal oscillator, which greatly limits the improvement of system detection performance. Secondly, in analog demodulation circuits, the traditional open-loop detection method has the problem of limited detection range and is difficult to adapt to the high dynamic range requirements of long-distance sensing. At the same time, in the low-frequency band, 1 / f noise has a significant impact on the accuracy of open-loop detection, which limits the performance of the system when detecting low-frequency signals. By introducing a closed-loop control mechanism, a wider detection bandwidth and higher detection accuracy can be achieved, effectively overcoming the limitations of 1 / f noise. However, closed-loop design needs to address noise optimization and system stability issues under high bandwidth conditions to ensure the reliability of the system in complex environments. Summary of the Invention

[0004] The present invention provides a long - distance fiber - optic interferometric sensing system based on a synchronous differential closed - loop structure to solve the above - mentioned technical problems, and specifically adopts the following technical solutions:

[0005] A long - distance fiber - optic interferometric sensing system based on a synchronous differential closed - loop structure, comprising: a light source module, a frequency modulation module, a long - distance transmission module, a reference light transmission module, a detection light transmission module, a reference light optoelectronic conversion module, a sensing light optoelectronic conversion module, an intermediate - frequency mixing module, a zero - intermediate - frequency mixing module, and a digital demodulation control module;

[0006] The light source module and the frequency modulation module form a light source end, the long - distance transmission module, the reference light transmission module, the detection light transmission module, the reference light optoelectronic conversion module, and the sensing light optoelectronic conversion module form a sensing end, and the intermediate - frequency mixing module, the zero - intermediate - frequency mixing module, and the digital demodulation control module form a demodulation end;

[0007] The light source module provides two optical signals. One of the optical signals passes through the frequency modulation module. After the frequency modulation module performs frequency - shift modulation on the optical signal, it is input into the subsequent long - distance transmission module. The other optical signal is directly transmitted into the subsequent long - distance transmission module;

[0008] The long - distance transmission module, as the core medium for long - distance signal sensing, realizes the efficient transmission of optical signals from the light source to the sensing point. At the sensing point, the long - distance transmission module outputs four optical signals, which are divided into two pairs of optical signals and respectively input into the reference light transmission module and the sensing light transmission module;

[0009] The reference light transmission module is insensitive to the target signal and outputs a reference signal through the reference light optoelectronic conversion module ;

[0010] The sensing light transmission module converts the target signal to be detected into a phase change of the sensing light and outputs a phase - modulation signal through the sensing light optoelectronic conversion module ;

[0011] The reference signal and the phase - modulation signal are input into the intermediate - frequency mixing module for the first - stage mixing, and two intermediate - frequency signals and are output; The two intermediate - frequency signals and are then input into the zero - intermediate - frequency mixing module for the second - stage mixing, and baseband signals and are output; The baseband signals and Digital processing and phase information calculation are completed in the digital demodulation control module; the digital controller uses the calculated phase information for real-time closed-loop feedback regulation and dynamically configures the intermediate frequency mixing module to achieve negative feedback control.

[0012] Further, the light source module consists of a laser and a first coupler. The laser serves as the light source of the detection system, generating an optical signal with a narrow linewidth, and dividing it into two paths for output through the first coupler;

[0013] The frequency modulation module consists of a first crystal oscillator, a first phase-locked loop, and an acousto-optic modulator. The first crystal oscillator provides a first reference frequency signal, which generates a microwave signal with a fixed frequency after being frequency-multiplied by the first phase-locked loop and is input into the acousto-optic modulator. Under the action of the microwave signal, the acousto-optic modulator performs frequency shift modulation on the input optical signal. By changing the frequency of the microwave signal, precise control of the frequency of the output optical signal is achieved, and the acousto-optic modulator selects the negative first sideband.

[0014] Further, the long-distance transmission module consists of a first long optical fiber, a second long optical fiber, a second coupler, and a third coupler. The first long optical fiber and the second long optical fiber are encapsulated in the same cable to ensure precise matching of the lengths of the two optical fibers. The two optical signals output by the first long optical fiber and the second long optical fiber respectively pass through the second coupler and the third coupler to generate four optical signals for output. One optical signal output by the first coupler and one optical signal output by the second coupler form a pair of optical signals, while the other optical signal output by the first coupler and the other optical signal output by the second coupler form another pair of optical signals.

[0015] Further, the sensing optical transmission module consists of a first circulator, a second circulator, a first delay optical fiber, a first fiber mirror, a sensing probe, a second fiber mirror, and a fifth coupler. A pair of optical signals output by the long-distance transmission module are respectively input into the first circulator and the second circulator. One path input into the first circulator passes through the first delay optical fiber, is reflected by the first fiber mirror, and then passes through the first delay optical fiber and the first circulator again and is input into the fifth coupler; the other path input into the second circulator passes through the sensing probe, is reflected by the second fiber mirror, and then passes through the sensing probe and the second circulator again and is input into the fifth coupler. The two optical signals are combined and interfered in the fifth coupler for output. Among them, the sensing probe is the sensing sensitive unit. The change of the external physical quantity to be measured causes the change of the optical characteristics in the fiber of the sensitive unit, thereby changing the physical parameters such as the refractive index and length of the fiber, and causing the change of the parameters such as the phase and frequency of the sensing light;

[0016] The reference light transmission module consists of a second delay optical fiber, a third delay optical fiber, and a fourth coupler. Another pair of optical signals output by the long-distance transmission module respectively pass through the second delay optical fiber and the third delay optical fiber, and are output after beam combination interference in the fourth coupler.

[0017] Further, the sensing light photoelectric conversion module consists of a first photodiode, and the reference light photoelectric conversion module consists of a second photodiode. The optical signal output by the sensing light transmission module is input into the first photodiode to output a phase modulation signal , and the optical signal output by the reference light transmission module is input into the second photodiode to output a reference voltage signal . The parameter change information such as phase and frequency sensitively detected by the sensing probe is modulated on the phase modulation signal , and is not modulated on the reference voltage signal . The reference voltage signal and the phase modulation signal are then input into the demodulation end for phase demodulation.

[0018] Further, the optical fiber lengths of the first delay optical fiber, the second delay optical fiber, the third delay optical fiber, and the sensing probe are matched, so as to ensure that the reference signal and the sensing signal have a time synchronization characteristic when being input into the demodulation end for phase demodulation. The long-distance transmission module at the sensing end and the reference light transmission module introduce a transmission delay of on the reference signal , and the long-distance transmission module at the sensing end and the sensing light transmission module introduce a transmission delay of on the sensing signal . And are guaranteed to be consistent in design, ensuring a strong time correlation of the carrier signals of the reference signal and the sensing signal .

[0019] Further, the intermediate frequency mixing module consists of a second crystal oscillator, a second phase-locked loop, a direct digital frequency synthesizer, a first intermediate frequency mixer, a second intermediate frequency mixer, a first low-pass filter, and a second low-pass filter. The second crystal oscillator provides two second reference frequency signals. One path generates a microwave signal with a fixed frequency after being frequency-multiplied by the second phase-locked loop , which is input into the first intermediate frequency mixer together with the reference voltage signal . The output stage of the first intermediate frequency mixer is cascaded with the first low-pass filter to filter out high-frequency components and output an intermediate frequency signal , another path of the second reference frequency signal is input to the direct digital frequency synthesizer, and after being frequency-multiplied inside the direct digital frequency synthesizer, it serves as the system main clock. Under the control of the digital controller, an output signal with dynamically adjusted phase-frequency is obtained. , together with the phase modulation signal is input to the second intermediate frequency mixer. The output stage of the second intermediate frequency mixer is cascaded with the second low-pass filter to filter out high-frequency components and output an intermediate frequency signal. ;

[0020] The zero intermediate frequency mixing module consists of an I / Q mixer, a third low-pass filter, an amplifier, and an analog-to-digital converter. The I / Q mixer performs quadrature mixing on the input intermediate frequency signal and , and through the third low-pass filter and the amplifier for analog signal conditioning, two baseband signals and are obtained. The analog baseband signals and pass through the analog-to-digital converter to obtain digitally quantized voltage information.

[0021] Further, the demodulation end adopts a differential closed-loop structure. Through two-stage mixing, the phase noise of the strong time-correlated carrier signal is suppressed. At the same time, based on the closed-loop characteristics, the detection dynamic range and noise performance of the system are optimized. The carrier phase noise introduced by the first crystal oscillator is common-mode noise for the reference signal and the sensing signal , and the carrier phase noise introduced by the second crystal oscillator is common-mode noise for the microwave signals and used for mixing. Through two-stage mixing, the carrier phase noise introduced by the first crystal oscillator and the second crystal oscillator is suppressed by the differential principle. In the first-stage intermediate frequency mixing link, the output of the direct digital frequency synthesizer is dynamically adjusted under the control of the controller, and the phase modulation information of the input sensing signal is reflected by the control quantity of the direct digital frequency synthesizer to achieve closed-loop phase demodulation.

[0022] Further, the digital demodulation control module consists of an arctangent demodulator and a controller. The arctangent demodulator performs phase calculation based on the digitally quantized voltage information and to obtain the tracking residual phase information. The digital controller uses the calculated phase information for real-time closed-loop feedback regulation and dynamically configures the direct digital frequency synthesizer in the intermediate frequency mixing module to achieve negative feedback control.

[0023] Further, the digital demodulation control module uses the arctangent demodulation method based on I / Q quadrature signals for phase calculation. The tangent demodulation method uses a four-quadrant arctangent demodulator, that is, an Arctangent demodulator with quadrant information, and the phase discrimination range is , and the third low-pass filters and amplifiers of the I-channel and Q-channel are kept consistent in circuit parameter design and layout design to ensure the orthogonality of the I / Q signals, thereby ensuring the phase demodulation performance.

[0024] In the long-distance fiber optic interferometric sensing system based on the synchronous differential closed-loop structure of the present application, in long-distance high-precision measurement, by introducing a synchronous signal pair and combining differential detection, the common-mode suppression of carrier phase noise is effectively achieved, thereby avoiding the limitation of carrier phase noise on the improvement of the system detection noise performance. On the one hand, the differential detection structure of the synchronous signal reduces the dependence on a high-performance and low-phase-noise reference crystal oscillator, which significantly reduces the system design cost; on the other hand, the reference crystal oscillator used in the optical sensing loop does not need to be homologous with the reference crystal oscillator used in the electrical demodulation loop, thereby effectively avoiding the signal demodulation drift problem caused by the difference in the drift characteristics of different crystal oscillators. This design ensures the performance stability of the system during long-term detection. In addition, the differential closed-loop detection structure dynamically adjusts the output phase of the feedback device, enabling the analog sampling loop to accurately track and lock the phase of the sensing signal, greatly reducing the phase fluctuation in the analog sampling loop, thereby reducing the requirement for the detection bandwidth. This optimization not only improves the detection bandwidth and detection linearity, but also can reduce the sampling rate and improve the detection noise performance by selecting higher-precision analog devices.

[0025] Generally speaking, the present invention optimizes the performance of the traditional fiber optic interferometric sensing system, reduces the cost, enhances the long-term stability of the system, and improves the detection accuracy and bandwidth, and is applicable to high-precision long-distance fiber optic sensing applications. Specifically, on the basis of the single-channel sensing signal output, an additional synchronous reference signal output is constructed. The carriers of the reference signal and the sensing signal are generated by frequency doubling of the same crystal oscillator through a phase-locked loop, and the transmission optical paths of the reference signal and the sensing signal are kept highly consistent to ensure that the carriers of the synchronous reference signal and the sensing signal have strong time correlation. Therefore, the phase noise introduced by the carrier is manifested as common-mode noise on the reference signal and the sensing signal. The common-mode phase noise introduced by the carrier signal is effectively suppressed by the differential structure in the subsequent demodulation circuit, avoiding the effective detection of the low-frequency weak components in the target signal by the near-end phase noise of the carrier signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0027] Figure 1 It is a block diagram of the components of a long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure of the present invention;

[0028] Figure 2 It is a diagram of the optical path and circuit connection of an embodiment of a long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure of the present invention;

[0029] Figure 3 It is a frequency-domain simulation diagram of the phase detection result of an embodiment of the present invention;

[0030] A long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure, including a light source module 1, a frequency modulation module 2, a long-distance transmission module 3, a reference light transmission module 4, a sensing light transmission module 5, a reference light photoelectric conversion module 6, a sensing light photoelectric conversion module 7, an intermediate frequency mixing module 8, a zero intermediate frequency mixing module 9, and a digital demodulation control module 10;

[0031] Laser 11, first coupler 12, first crystal oscillator 21, first phase-locked loop 22, acousto-optic modulator 23, first long optical fiber 31, second long optical fiber 32, second coupler 33, third coupler 34, second delay optical fiber 41, third delay optical fiber 42, fourth coupler 43, first circulator 51, second circulator 52, first delay optical fiber 53, first fiber optic mirror 54, sensing probe 55, second fiber optic mirror 56, fifth coupler 57, second photodiode 61, first photodiode 71, second crystal oscillator 81, second phase-locked loop 82, direct digital frequency synthesizer 83, first intermediate frequency mixer 84, second intermediate frequency mixer 85, first low-pass filter 86, second low-pass filter 87, I / Q mixer 91, third low-pass filter 92, amplifier 93, analog-to-digital converter 94, arctangent demodulator 101, controller 102. Specific implementation manners

[0032] The following will describe in detail the embodiments of the present application. The examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals indicate the same or similar elements or elements with the same or similar functions from beginning to end. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present application, and should not be construed as a limitation of the present application.

[0033] As shown in the attachedFigure 1 As shown in the figure, a long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure includes: a light source module 1, a frequency modulation module 2, a long-distance transmission module 3, a reference light transmission module 4, a sensing light transmission module 5, a reference light photoelectric conversion module 6, a sensing light photoelectric conversion module 7, an intermediate frequency mixing module 8, a zero intermediate frequency mixing module 9, and a digital demodulation control module 10. The light source module 1 and the frequency modulation module 2 form a light source end, the long-distance transmission module 3, the reference light transmission module 4, the sensing light transmission module 5, the reference light photoelectric conversion module 6, and the sensing light photoelectric conversion module 7 form a sensing end, and the intermediate frequency mixing module 8, the zero intermediate frequency mixing module 9, and the digital demodulation control module 10 form a demodulation end.

[0034] The light source module 1 provides two optical signals for the entire fiber optic interferometric sensing system. One of the optical signals passes through the frequency modulation module 2. After the frequency modulation module 2 performs frequency shift modulation on the optical signal, it is input into the subsequent long-distance transmission module 3. The other optical signal is directly transmitted into the subsequent long-distance transmission module 3. The long-distance transmission module 3, as the core medium for long-distance signal sensing, realizes the efficient transmission of optical signals from the light source to the sensing point. At the sensing point, the long-distance transmission module 3 outputs four optical signals, which are divided into two pairs of optical signals and respectively input into the reference light transmission module 4 and the sensing light transmission module 5.

[0035] The reference light transmission module 4 is insensitive to the target signal and outputs a reference signal through the reference light photoelectric conversion module 6 . The sensing light transmission module 5 converts the target signal to be detected into a phase change of the sensing light and outputs a phase modulation signal through the sensing light photoelectric conversion module 7 . The reference signal and the phase modulation signal are input into the intermediate frequency mixing module 8 for the first-stage mixing, and two intermediate frequency signals and are output. These two intermediate frequency signals and are then input into the zero intermediate frequency mixing module 9 for the second-stage mixing, and baseband signals and are output. The analog baseband signals and complete digital processing and phase information calculation in the digital demodulation control module 10. The digital controller uses the calculated phase information for real-time closed-loop feedback regulation and dynamically configures the intermediate frequency mixing module to achieve negative feedback control. Since the reference voltage signal and the phase modulation signal The carrier waves are all provided by the frequency modulation module, and the demodulation end adopts a closed-loop detection structure. The entire system constitutes a closed-loop detection structure based on synchronous difference. Therefore, the proposed long-distance fiber optic interferometric sensing system based on the synchronous difference closed-loop structure not only improves the measurement accuracy and stability of the system, but also significantly enhances the long-distance sensing performance.

[0036] As Figure 2 shown is an exemplary optical path and circuit connection diagram of a long-distance fiber optic interferometric sensing system based on the synchronous difference closed-loop structure of the present invention.

[0037] The light source module 1 consists of a laser 11 and a first coupler 12. Among them, the laser 11 serves as the light source of the detection system, generates an optical signal with a narrow linewidth, and divides it into two paths for output through the first coupler 12. The frequency of the optical signal is expressed as .

[0038] The frequency modulation module 2 consists of a first crystal oscillator 21, a first phase-locked loop 22, and an acousto-optic modulator 23. The first crystal oscillator 21 provides a first reference frequency signal, which generates a microwave signal with a fixed frequency after being frequency-multiplied by the first phase-locked loop 22 , and inputs it into the acousto-optic modulator 23. Under the action of the microwave signal, the acousto-optic modulator 23 performs frequency shift modulation on the input optical signal, selects the negative first sideband, that is, the frequency of the optical signal output by the acousto-optic modulator 23 is .

[0039] The long-distance transmission module 3 consists of two long transmission optical fibers (the first long optical fiber 31 and the second long optical fiber 32), a second coupler 33, and a third coupler 34. The first long optical fiber 31 and the second long optical fiber 32 adopt high-quality optical fiber materials, have the characteristics of low loss and high transmission efficiency, and can transmit optical signals over a relatively long distance without significant attenuation, ensuring the effective transmission of signals over a long distance. At the same time, the first long optical fiber 31 and the second long optical fiber 32 are encapsulated in the same cable to ensure the precise matching of the lengths of the two optical fibers. This structural design effectively regards the environmental impact as a common-mode interference, thereby enhancing the anti-interference ability of the system and the stability of signal transmission. The two optical signals output by the long transmission optical fibers respectively pass through the second coupler 33 and the third coupler 34, generating four optical signals for output. Specifically, one optical signal output by the first coupler 12 and one optical signal output by the second coupler 33 form a pair of optical signals, while the other optical signal output by the first coupler 12 and the other optical signal output by the second coupler 33 form another pair of optical signals.

[0040] The sensing optical transmission module 5 consists of a first circulator 51, a second circulator 52, a first delay optical fiber 53, a first optical fiber mirror 54, a sensing probe 55, a second optical fiber mirror 56, and a fifth coupler 57. A pair of optical signals output by the long-distance transmission module 3 are respectively input into the first circulator 51 and the second circulator 52. One of the signals input into the first circulator 51 passes through the first delay optical fiber 53, is reflected by the first optical fiber mirror 54, and then passes through the first delay optical fiber 53 and the first circulator 51 again and is input into the fifth coupler 57; the other signal input into the second circulator 52 passes through the sensing probe 55, is reflected by the second optical fiber mirror 56, and then passes through the sensing probe 55 and the second circulator 52 again and is input into the fifth coupler 57. The two optical signals are combined and interfered in the fifth coupler 57 and then output. Among them, the sensing probe 55 is a sensing sensitive unit. The change of the external measured physical quantity causes the change of the optical characteristics in the fiber of the sensitive unit, thereby changing the physical parameters such as the refractive index and length of the optical fiber, causing the change of the parameters such as the phase and frequency of the sensing light, and introducing a phase modulation part into the optical signal .

[0041] The reference optical transmission module 4 consists of a second delay optical fiber 41, a third delay optical fiber 42, and a fourth coupler 43. Another pair of optical signals output by the long-distance transmission module 3 respectively pass through the second delay optical fiber 41 and the third delay optical fiber 42, and are combined and interfered in the fourth coupler 43 and then output

[0042] The sensing optical-to-electrical conversion module 7 consists of a first photodiode 71, and the reference optical-to-electrical conversion module 6 consists of a second photodiode 61. The optical signal output by the sensing optical transmission module 5 is input into the first photodiode 71 to convert the interfered signal into a microwave electrical signal and output a phase modulation signal . The optical signal output by the reference optical transmission module 4 is input into the second photodiode 61 to convert the interfered signal into a microwave electrical signal and output a reference voltage signal . The information of the change of parameters such as the phase and frequency sensed on the sensing probe 55 is modulated on the phase modulation signal , rather than on the reference voltage signal . The reference voltage signal and the phase modulation signal are then input into the demodulation end for phase demodulation. The optical fiber lengths of the first delay optical fiber 53, the second delay optical fiber 41, the third delay optical fiber 42, and the sensing probe 55 are matched, so as to ensure that the reference signal and the sensing signal have the time synchronization characteristic when they are input into the demodulation end for phase demodulation. Specifically, the transmission delays introduced by the long-distance transmission module at the sensing end and the reference optical transmission module on the reference signal are , the transmission delay introduced by the sensing end long-distance transmission module and the sensing optical transmission module in the sensing signal is . and are guaranteed to be consistent in design, so as to ensure the strong time correlation of the carrier signals of the reference signal and the sensing signal . The reference signal can be expressed as , and the sensing signal can be expressed as .

[0043] The intermediate frequency mixing module 8 is composed of a second crystal oscillator 81, a second phase-locked loop 82, a direct digital frequency synthesizer 83, a first intermediate frequency mixer 84, a second intermediate frequency mixer 85, a first low-pass filter 86 and a second low-pass filter 87. The second crystal oscillator 81 provides two paths of second reference frequency signals. One path is multiplied by the second phase-locked loop 82 to generate a microwave signal with a fixed frequency, which is input into the first intermediate frequency mixer 84 together with the reference voltage signal . The microwave signal generated by the second phase-locked loop 82 can be expressed as . The output of the first intermediate frequency mixer 84 is cascaded with the first low-pass filter 86 to filter out high-frequency components and output an intermediate frequency signal . The other path of the second reference frequency signal is input into the direct digital frequency synthesizer 83. After being multiplied inside the direct digital frequency synthesizer 83, it serves as the system main clock. Under the control of the digital controller, an output signal with phase-frequency dynamic adjustment is obtained, which is input into the second intermediate frequency mixer 85 together with the phase modulation signal , where represents the phase part dynamically adjusted by the direct digital frequency synthesizer 83. The output of the second intermediate frequency mixer 85 is cascaded with the second low-pass filter 87 to filter out high-frequency components and output an intermediate frequency signal , where the tracking residual phase part is defined as .

[0044] The zero intermediate frequency mixing module 9 is composed of an I / Q mixer 91, a third low-pass filter 92, an amplifier 93 and an analog-to-digital converter 94. The I / Q mixer 91 performs quadrature mixing on the input intermediate frequency signals and , and through the third low-pass filter 92 and the amplifier 93 for analog signal conditioning, two baseband signals and are obtained, which are respectively expressed as the real part and the imaginary part of the complex signal . The analog baseband signals and After passing through the analog-to-digital converter 94, the digitized and quantized voltage information is obtained. and , where represents the th sampling period, and the sampling rate is , corresponding to the moment .

[0045] The digital demodulation control module 10 consists of an arctangent demodulator 101 and a controller 102. The arctangent demodulator 101 performs phase calculation based on the digitized and quantized voltage information and to obtain the tracking residual phase information . The controller 102 uses the calculated phase information for real-time closed-loop feedback regulation and dynamically configures the direct digital frequency synthesizer 83 in the intermediate frequency mixing module to adjust the output phase of the direct digital frequency synthesizer 83 to track the phase modulation part caused by the target signal, realizing negative feedback control.

[0046] The intermediate frequency mixing module 8, the zero intermediate frequency mixing module 9, and the digital demodulation control module 10 form the demodulation end, constituting a differential closed-loop structure. Through two-stage mixing, the phase noise of the strongly time-correlated carrier signal is effectively suppressed. At the same time, based on the closed-loop characteristics, the detection dynamic range and noise performance of the demodulation system are optimized. Specifically, the carrier phase noise introduced by the first crystal oscillator 21 is common-mode noise for the reference signal and the sensing signal . The carrier phase noise introduced by the second crystal oscillator 81 is common-mode noise for the microwave signals and used for mixing. Therefore, through two-stage mixing, the carrier phase noise introduced by the first crystal oscillator 21 and the second crystal oscillator 81 can be effectively suppressed by the differential principle. At the same time, in the first-stage intermediate frequency mixing link, the output of the direct digital frequency synthesizer 83 is dynamically adjusted under the control of the controller 102, and the phase modulation information of the input sensing signal is reflected by the control quantity of the direct digital frequency synthesizer 83 to realize closed-loop phase demodulation, effectively expanding the detection range of the system and optimizing the detection accuracy and linearity.

[0047] The digital demodulation control module 10 uses the arctangent demodulation method based on I / Q orthogonal signals for phase calculation. Specifically, the I / Q orthogonal signals are in-phase / quadrature signals, and the arctangent demodulation method uses a four-quadrant arctangent demodulator 101, that is, an Arctangent demodulator with quadrant information, and the phase discrimination range is 。The third low-pass filters 92 and amplifiers 93 for the I-channel and Q-channel are kept consistent in circuit parameter design and layout design to ensure the orthogonality of the I / Q signals, thereby ensuring the phase demodulation performance.

[0048] The frequency-domain simulation diagram of the phase detection result of an embodiment of the long-distance fiber optic interferometric sensing system based on the synchronous differential closed-loop structure of the present invention is as Figure 3 shown, which shows the comparison of the frequency-domain performance of the present invention in long-distance target signal detection with that of the traditional single-ended open-loop detection structure. Among them, the phase modulation part caused by the target signal to be detected is represented as , that is, the modulation frequency is 171 Hz and the modulation depth is 1 radian. The phase spectrum curve obtained by the long-distance fiber optic interferometric sensing system based on the traditional single-ended open-loop structure is represented by a gray dashed line, and the phase spectrum curve obtained by the long-distance fiber optic interferometric sensing system based on the synchronous differential closed-loop structure of the present invention is represented by a black solid line. It can be seen that there are 171-Hz non-linear distortion harmonics in the result of target signal detection by the long-distance fiber optic interferometric sensing system based on the traditional single-ended open-loop structure, while there is only a clean 171-Hz spectral line in the result of target signal detection by the long-distance fiber optic interferometric sensing system based on the synchronous differential closed-loop structure of the present invention. At the same time, from the signal detection noise floor in the low-frequency band, it can be seen that the structure proposed by the present invention can construct a carrier phase noise condition with strong time correlation, and then realize the common-mode cancellation of the carrier phase noise through differential, with a lower detection noise floor and a higher phase detection signal-to-noise ratio, so as to ensure the weak signal detection performance of the phase modulation type advanced sensing system.

[0049] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any form. Any technical solutions obtained by using equivalent replacements or equivalent transformations fall within the protection scope of the present invention.

Claims

1. A long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure, characterized in that, Including: a light source module, a frequency modulation module, a long-distance transmission module, a reference light transmission module, a sensing light transmission module, a reference light optoelectronic conversion module, a sensing light optoelectronic conversion module, an intermediate frequency mixing module, a zero intermediate frequency mixing module, and a digital demodulation control module; The light source module and the frequency modulation module form a light source end, the long-distance transmission module, the reference light transmission module, the sensing light transmission module, the reference light optoelectronic conversion module, and the sensing light optoelectronic conversion module form a sensing end, and the intermediate frequency mixing module, the zero intermediate frequency mixing module, and the digital demodulation control module form a demodulation end; The light source module provides two optical signals. One of the optical signals passes through the frequency modulation module. After the frequency modulation module performs frequency shift modulation on the optical signal, it is input to the subsequent long-distance transmission module. The other optical signal is directly transmitted to the subsequent long-distance transmission module; The long-distance transmission module is the core medium for long-distance signal sensing, realizing the efficient transmission of optical signals from the light source to the sensing point. At the sensing point, the long-distance transmission module outputs four optical signals, which are divided into two pairs of optical signals and respectively input to the reference light transmission module and the sensing light transmission module; The reference optical signal transmission module is insensitive to the target signal and outputs a reference signal through the reference optical-to-electrical conversion module. ; The sensing optical transmission module converts the target signal to be detected into a phase change of the sensing light, and outputs a phase modulation signal through the sensing optical photoelectric conversion module ; Reference signal and the phase modulation signal are input into the intermediate frequency mixing module for the first-stage mixing, and two intermediate frequency signals are output and ; The two intermediate frequency signals and are then input into the zero intermediate frequency mixing module for the second-stage mixing, and baseband signals are output and ; The baseband signals and complete digital processing and phase information calculation in the digital demodulation control module; the digital demodulation control module uses the calculated phase information for real-time closed-loop feedback regulation and dynamically configures the intermediate frequency mixing module to achieve negative feedback control; The intermediate frequency mixing module consists of a second crystal oscillator, a second phase-locked loop, a direct digital frequency synthesizer, a first intermediate frequency mixer, a second intermediate frequency mixer, a first low-pass filter, and a second low-pass filter. The second crystal oscillator provides two second reference frequency signals. One of them generates a microwave signal with a fixed frequency after being multiplied in frequency by the second phase-locked loop , and is input into the first intermediate frequency mixer together with the reference signal . The output of the first intermediate frequency mixer is cascaded with the first low-pass filter to filter out high-frequency components and output an intermediate frequency signal . The other second reference frequency signal is input into the direct digital frequency synthesizer. After being multiplied in frequency inside the direct digital frequency synthesizer, it serves as the system main clock. Under the control of the digital demodulation control module, an output signal with dynamically adjusted phase-frequency is obtained , and is input into the second intermediate frequency mixer together with the phase modulation signal . The output of the second intermediate frequency mixer is cascaded with the second low-pass filter to filter out high-frequency components and output an intermediate frequency signal ; The zero-IF mixing module consists of an I / Q mixer, a third low-pass filter, an amplifier, and an analog-to-digital converter. The I / Q mixer performs quadrature mixing on the input intermediate-frequency signal and and performs analog signal conditioning through the third low-pass filter and the amplifier to obtain two baseband signals and . The analog baseband signals and pass through the analog-to-digital converter to obtain digitally quantized voltage information.

2. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 1, characterized in that The light source module is composed of a laser and a first coupler. The laser is used as the light source of the detection system to generate an optical signal with a narrow linewidth, and is divided into two outputs through the first coupler; The frequency modulation module is composed of a first crystal oscillator, a first phase-locked loop, and an acousto-optic modulator. The first crystal oscillator provides a first reference frequency signal, which generates a microwave signal with a fixed frequency after being frequency multiplied by the first phase-locked loop and is input to the acousto-optic modulator. Under the action of the microwave signal, the acousto-optic modulator performs frequency shift modulation on the input optical signal. By changing the frequency of the microwave signal, precise control of the frequency of the output optical signal is achieved. The acousto-optic modulator selects the negative first sideband.

3. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 2, characterized in that The long-distance transmission module is composed of a first long optical fiber, a second long optical fiber, a second coupler, and a third coupler. The first long optical fiber and the second long optical fiber are encapsulated in the same cable to ensure the precise matching of the lengths of the two optical fibers. The two optical signals output by the first long optical fiber and the second long optical fiber respectively pass through the second coupler and the third coupler to generate four optical signals for output. One optical signal output by the first coupler and one optical signal output by the second coupler form a pair of optical signals, and the other optical signal output by the first coupler and the other optical signal output by the second coupler form another pair of optical signals.

4. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 3, characterized in that The sensing optical transmission module is composed of a first circulator, a second circulator, a first delay optical fiber, a first fiber optic mirror, a sensing probe, a second fiber optic mirror, and a fifth coupler. A pair of optical signals output by the long-distance transmission module are respectively input into the first circulator and the second circulator. One path input into the first circulator passes through the first delay optical fiber, is reflected by the first fiber optic mirror, and then passes through the first delay optical fiber and the first circulator again and is input into the fifth coupler; the other path input into the second circulator passes through the sensing probe, is reflected by the second fiber optic mirror, and then passes through the sensing probe and the second circulator again and is input into the fifth coupler. The two optical signals are combined and interfered in the fifth coupler and then output. Among them, the sensing probe is a sensing sensitive unit. The change of the external physical quantity to be measured causes the change of the optical characteristics in the sensitive unit optical fiber, thereby changing the physical parameters such as the refractive index and length of the optical fiber, and causing the change of the sensing optical phase and frequency parameters; The reference optical transmission module is composed of a second delay optical fiber, a third delay optical fiber, and a fourth coupler. Another pair of optical signals output by the long-distance transmission module respectively pass through the second delay optical fiber and the third delay optical fiber, and are combined and interfered in the fourth coupler and then output.

5. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 4, characterized in that The sensing optical photoelectric conversion module is composed of a first photodiode, and the reference optical photoelectric conversion module is composed of a second photodiode. The optical signal output by the sensing optical transmission module is input into the first photodiode to output a phase modulation signal , and the optical signal output by the reference optical transmission module is input into the second photodiode to output a reference signal . The phase and frequency parameter change information sensed by the sensing probe is modulated on the phase modulation signal , but not on the reference signal . The reference signal and the phase modulation signal are then input into the demodulation end for phase demodulation.

6. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 5, characterized in that The optical fiber lengths of the first delay optical fiber, the second delay optical fiber, the third delay optical fiber and the sensing probe are matched, so as to ensure the reference signal and the phase modulation signal have time synchronization characteristics when entering the demodulation end for phase demodulation. The transmission delay introduced by the long-distance transmission module at the sensing end and the reference optical transmission module in the reference signal is , and the transmission delay introduced by the long-distance transmission module at the sensing end and the sensing optical transmission module in the phase modulation signal is . and are guaranteed to be consistent in design, ensuring strong time correlation of the carrier signals of the reference signal and the phase modulation signal .

7. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 5, characterized in that The demodulation end adopts a differential closed-loop structure. Through two-stage mixing, the phase noise of the strongly time-correlated carrier signal is suppressed. At the same time, based on the closed-loop characteristics, the detection dynamic range and noise performance of the system are optimized. The carrier phase noise introduced by the first crystal oscillator is common-mode noise for the reference signal and the phase modulation signal ; the carrier phase noise introduced by the second crystal oscillator is common-mode noise for the microwave signal and used for mixing. Through two-stage mixing, the carrier phase noise introduced by the first crystal oscillator and the second crystal oscillator is suppressed by the differential principle. In the first-stage intermediate-frequency mixing link, the output of the direct digital frequency synthesizer is dynamically adjusted under the control of the controller, and the phase modulation information of the input phase modulation signal is reflected by the control quantity of the direct digital frequency synthesizer, realizing closed-loop phase demodulation.

8. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 5, characterized in that The digital demodulation control module consists of an arctangent demodulator and a controller. The arctangent demodulator performs phase calculation based on the voltage information after digital quantization and to obtain the tracking residual phase information. The digital demodulation control module uses the calculated phase information to perform real-time closed-loop feedback adjustment and dynamically configure the direct digital frequency synthesizer in the intermediate frequency mixing module to achieve negative feedback control.

9. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 8, characterized in that The digital demodulation control module uses the arctangent demodulation method based on I / Q quadrature signals for phase calculation. The tangent demodulation method uses a four-quadrant arctangent demodulator, that is, an Arctangent demodulator with quadrant information. The phase discrimination range is (-2π, 2π]. The third low-pass filters and amplifiers of the I channel and the Q channel are kept consistent in circuit parameter design and layout design to ensure the orthogonality of the I / Q signals, thereby ensuring the phase demodulation performance.

Images