Long-distance optical fiber interference type sensing system based on synchronous differential closed-loop structure

Abstract

The invention discloses a long-distance optical fiber interference type sensing system based on a synchronous differential closed-loop structure. The device comprises 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 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. According to the long-distance optical fiber interference type sensing system based on the synchronous differential closed-loop structure, in long-distance high-precision measurement, common-mode suppression of carrier phase noise is effectively achieved by introducing a synchronous signal pair and combining differential detection, and therefore the limitation of the carrier phase noise on improvement of the noise detection performance of the system is avoided.

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 optical fiber 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 optical fiber interferometric sensing system based on a synchronous differential closed-loop structure comprises: 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 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;

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

[0007] The light source module provides two optical signals, one of which passes through the frequency modulation module, the frequency modulation module performs frequency shift modulation on the optical signal and then inputs it into the subsequent long-distance transmission module, and the other optical signal is directly transmitted to the subsequent long-distance transmission module;

[0008] The long-distance transmission module is the core medium of 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 input into the reference optical transmission module and the sensing optical transmission module respectively;

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

[0011] Reference signal With phase modulated signal The intermediate frequency mixing module is input for the first stage mixing and outputs two intermediate frequency signals. and ; Two intermediate frequency signals and Then the signal is input into the zero intermediate frequency mixer module for second-stage mixing, and the baseband signal is output. and ; Baseband signal and The digital processing and phase information solution are completed in the digital demodulation control module; the digital controller uses the solved phase information to perform real-time closed-loop feedback adjustment, and dynamically configures the intermediate frequency mixing module to achieve negative feedback control.

[0012] Furthermore, the light source module is composed of a laser and a first coupler, wherein the laser serves as a light source of the detection system, generates an optical signal with a narrow line width, and divides the signal into two output paths through the first coupler;

[0013] 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 is multiplied by the first phase-locked loop to generate a microwave signal with a fixed frequency, 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, and achieves precise control of the frequency of the output optical signal by changing the frequency of the microwave signal. The acousto-optic modulator selects the negative first-order sideband.

[0014] Furthermore, 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 that the two optical fibers are accurately matched in length. The two optical signals output by the first long optical fiber and the second long optical fiber pass through the second coupler and the third coupler respectively to generate four optical signal outputs. 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.

[0015] Further, the sensing light transmission module is composed of a first circulator, a second circulator, a first delay optical fiber, a first fiber optic reflector, a sensing probe, a second fiber optic reflector 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, and after being reflected by the first fiber optic reflector, passes through the first delay optical fiber and the first circulator again to enter the fifth coupler; the other path input into the second circulator passes through the sensing probe, and after being reflected by the second fiber optic reflector, passes through the sensing probe and the second circulator again to enter the fifth coupler. The two paths of optical signals are output after beam combining interference in the fifth coupler, wherein the sensing probe is a sensing sensitive unit, and changes in the external measured physical quantity cause changes in the optical properties of the sensitive unit optical fiber, thereby changing physical parameters such as the refractive index and length of the optical fiber, causing changes in parameters such as the phase and frequency of the sensing light;

[0016] The reference light 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 pass through the second delay optical fiber and the third delay optical fiber respectively, and are output after beam combining interference in the fourth coupler.

[0017] Furthermore, the sensing light photoelectric conversion module is composed of a first photodiode, the reference light photoelectric conversion module is composed of a second photodiode, the optical signal output by the sensing light transmission module is input into the first photodiode, and the phase modulation signal is output. The optical signal output by the reference optical transmission module is input into the second photodiode, and a reference voltage signal is output. The phase, frequency and other parameter changes detected by the sensor probe are modulated on the phase modulation signal. , without modulation on the reference voltage signal , reference voltage signal and phase modulated signal Then it is input into the demodulation terminal for phase demodulation.

[0018] Furthermore, the first delay optical fiber, the second delay optical fiber, the third delay optical fiber and the sensor probe have matching optical fiber lengths, thereby ensuring that the reference signal and sensor signals The signal at the input demodulation end for phase demodulation has a time synchronization characteristic. The long-distance transmission module at the sensor end and the reference optical transmission module are synchronized with each other in the reference signal. The transmission delay introduced is The long-distance transmission module at the sensor end and the sensor light transmission module are The transmission delay introduced is , and Ensure consistency in design and reference signal and sensor signals The carrier signal has strong time correlation.

[0019] Furthermore, the intermediate frequency mixing module is composed 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 which is multiplied by the second phase-locked loop to generate a microwave signal with a fixed frequency. , and the reference voltage signal The first intermediate frequency mixer outputs the first low-pass filter 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, and after being multiplied internally by the direct digital frequency synthesizer, it is used as the system master clock, and under the control of the digital controller, an output signal of dynamic phase-frequency adjustment is obtained. , with the phase modulated signal The second intermediate frequency mixer is input together, and the output of the second intermediate frequency mixer is cascaded with the second low-pass filter to filter out the high-frequency component and output an intermediate frequency signal ;

[0020] The zero intermediate frequency mixing module is composed of an I / Q mixer, a third low-pass filter, an amplifier and an analog-to-digital converter. The I / Q mixer processes the input intermediate frequency signal. and Perform orthogonal mixing, and perform analog signal conditioning through the third low-pass filter and amplifier to obtain two baseband signals and , analog baseband signal and The voltage information is digitally quantized through the analog-to-digital converter.

[0021] Furthermore, the demodulation end adopts a differential closed-loop structure, and through two-stage mixing, the phase noise suppression of the strongly time-correlated carrier signal is achieved. 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 relatively small for the reference signal. and sensor signals The carrier phase noise introduced by the second crystal oscillator is common mode noise for the microwave signal used for mixing. and It is common mode noise. Through two-stage mixing, the carrier phase noise introduced by the first crystal oscillator and the second crystal oscillator is suppressed through the differential principle. In the first intermediate frequency mixing link, the output of the direct digital frequency synthesizer is dynamically adjusted under the control of the controller, and the control amount of the direct digital frequency synthesizer reflects the input sensor signal. The phase modulation information is used to realize closed-loop phase demodulation.

[0022] Furthermore, the digital demodulation control module is composed of an inverse tangent demodulator and a controller. The inverse tangent demodulator is based on the digitally quantized voltage information. and Phase solution is performed to obtain tracking residual phase information. The digital controller uses the solved phase information to perform real-time closed-loop feedback adjustment and dynamically configures the direct digital frequency synthesizer in the intermediate frequency mixing module to achieve negative feedback control.

[0023] Furthermore, the digital demodulation control module adopts an inverse tangent demodulation method based on I / Q orthogonal signals for phase resolution. The tangent demodulation method adopts a four-quadrant inverse tangent demodulator, that is, an Arctangent demodulator with quadrant information, and the phase detection range is The third low-pass filter and amplifier of the I and Q paths are kept consistent during circuit parameter design and layout design to ensure the orthogonality of the I / Q signals, thereby ensuring the phase demodulation performance.

[0024] The long-distance fiber interferometric sensing system based on the synchronous differential closed-loop structure of the present application effectively realizes the common-mode suppression of carrier phase noise in long-distance high-precision measurement by introducing a synchronous signal pair and combining differential detection, thereby avoiding the limitation of carrier phase noise on the improvement of system detection noise performance. On the one hand, the differential detection structure of the synchronous signal reduces the dependence on the high-performance, 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 circuit does not need to be homologous to the reference crystal oscillator used in the electrical demodulation circuit, 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 in the long-term detection process. In addition, the differential closed-loop detection structure dynamically adjusts the output phase of the feedback device, so that the analog sampling loop can accurately track and lock the phase of the sensing signal, greatly reducing the phase fluctuation in the analog sampling loop, thereby reducing the demand for detection bandwidth. This optimization not only improves the detection bandwidth and improves the detection linearity, but also reduces the sampling rate, and improves the noise performance of the detection by selecting higher-precision analog devices.

[0025] In general, the present invention optimizes the performance of the traditional fiber optic interferometric sensing system, reduces costs, enhances the long-term stability of the system, and improves detection accuracy and bandwidth, and is suitable for high-precision long-distance fiber optic sensing applications. Specifically, on the basis of a 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 the same crystal oscillator through a phase-locked loop, and the transmission optical paths of the reference signal and the sensing signal remain highly consistent, ensuring that the carriers of the synchronous reference signal and the sensing signal have a strong time correlation, so that the phase noise introduced by the carrier is reflected 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 post-stage 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] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

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

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

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

[0030] A long-distance optical fiber interferometric sensing system based on a synchronous differential closed-loop structure, comprising a light source module 1, a frequency modulation module 2, a long-distance transmission module 3, a reference light transmission module 4, a detection 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 optical fiber reflector 54, sensor probe 55, second optical fiber reflector 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, inverse tangent demodulator 101, controller 102. DETAILED DESCRIPTION

[0032] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0033] As attached Figure 1 As shown, a long-distance optical fiber interferometric sensing system based on a synchronous differential closed-loop structure comprises: a light source module 1, a frequency modulation module 2, a long-distance transmission module 3, a reference light transmission module 4, a detection 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 constitute a light source end, the long-distance transmission module 3, the reference light transmission module 4, the detection light transmission module 5, the reference light photoelectric conversion module 6 and the sensing light photoelectric conversion module 7 constitute a sensing end, and the intermediate frequency mixing module 8, the zero intermediate frequency mixing module 9 and the digital demodulation control module 10 constitute a demodulation end.

[0034] The light source module 1 provides two optical signals for the entire fiber interferometric sensing system. One optical signal passes through the frequency modulation module 2, which performs frequency shift modulation on the optical signal and then inputs it into the subsequent long-distance transmission module 3. The other optical signal is directly transmitted to the subsequent long-distance transmission module 3. The long-distance transmission module 3 is the core medium for long-distance signal sensing, realizing 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 input into the reference optical transmission module 4 and the sensing optical transmission module 5 respectively.

[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. . Reference signal With phase modulated signal Input intermediate frequency mixer module 8 for first-stage mixing and output two intermediate frequency signals and These two intermediate frequency signals and Then the signal is input into the zero intermediate frequency mixer module 9 for the second stage mixing, and the baseband signal is output. and . Analog baseband signal and The digital processing and phase information solution are completed in the digital demodulation control module 10. The digital controller uses the solved phase information to perform real-time closed-loop feedback adjustment and dynamically configure the intermediate frequency mixing module to achieve negative feedback control. and phase modulated 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 differential. Therefore, the proposed long-distance fiber optic interferometric sensing system based on synchronous differential closed-loop structure not only improves the measurement accuracy and stability of the system, but also significantly enhances the long-distance sensing performance.

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

[0037] The light source module 1 is composed of a laser 11 and a first coupler 12. The laser 11 is used as the light source of the detection system to generate an optical signal with a narrow line width, and is divided into two outputs through the first coupler 12. The frequency of the optical signal is expressed as .

[0038] The frequency modulation module 2 is composed 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 is multiplied by the first phase-locked loop 22 to generate a microwave signal with a fixed frequency. , and input 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 and selects the negative first-order sideband, that is, the frequency of the optical signal output by the acousto-optic modulator 23 is .

[0039] The long-distance transmission module 3 is composed of two long transmission optical fibers (a first long optical fiber 31 and a 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 are made of high-quality optical fiber materials, and have the characteristics of low loss and high transmission efficiency. They can transmit optical signals over a 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 that the two optical fibers are accurately matched in length. This structural design effectively regards environmental influences as 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 fiber pass through the second coupler 33 and the third coupler 34 respectively to generate four optical signal outputs. 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, and 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 light transmission module 5 is composed of a first circulator 51, a second circulator 52, a first delay optical fiber 53, a first optical fiber reflector 54, a sensing probe 55, a second optical fiber reflector 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 path input into the first circulator 51 passes through the first delay optical fiber 53, is reflected by the first optical fiber reflector 54, and then passes through the first delay optical fiber 53 and the first circulator 51 again to enter the fifth coupler 57; the other path input into the second circulator 52 passes through the sensing probe 55, is reflected by the second optical fiber reflector 56, and then passes through the sensing probe 55 and the second circulator 52 again to enter the fifth coupler 57. The two optical signals are output after beam combining interference in the fifth coupler 57. The sensing probe 55 is a sensing sensitive unit. The change of the measured physical quantity in the outside world causes the optical characteristics of the optical fiber of the sensitive unit to change, thereby changing the physical parameters of the optical fiber such as the refractive index and length, causing the change of the sensing light phase, frequency and other parameters, and introducing a phase modulation part in the optical signal. .

[0041] The reference optical transmission module 4 is composed 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 pass through the second delay optical fiber 41 and the third delay optical fiber 42 respectively, and are output after beam combining interference in the fourth coupler 43.

[0042] The sensing light photoelectric conversion module 7 is composed of a first photodiode 71, and the reference light photoelectric conversion module 6 is composed of a second photodiode 61. The optical signal output by the sensing light transmission module 5 is input into the first photodiode 71, and the interference signal is converted into a microwave electrical signal, and a phase modulation signal is output. The optical signal output by the reference optical transmission module 4 is input to the second photodiode 61, which converts the interfered signal into a microwave electrical signal and outputs a reference voltage signal. The phase, frequency and other parameter change information detected by the sensor probe 55 is modulated on the phase modulation signal , without modulation on the reference voltage signal . Reference voltage signal and phase modulated signal The first delay optical fiber 53, the second delay optical fiber 41, the third delay optical fiber 42 and the sensor probe 55 have matching optical fiber lengths, thereby ensuring that the reference signal and sensor signals The signal has a time synchronization characteristic when it is phase demodulated at the input demodulation end. Specifically, the long-distance transmission module at the sensor end and the reference optical transmission module are synchronized at the reference signal. The transmission delay introduced is The long-distance transmission module at the sensor end and the sensor light transmission module are connected in the sensing signal The transmission delay introduced is . and Ensure consistency in design to ensure reference signal and sensor signals The carrier signal has strong time correlation. It can be expressed as , sensor signal It 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 second reference frequency signals, one of which is multiplied by the second phase-locked loop 82 to generate a microwave signal with a fixed frequency. , and 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 to the first low-pass filter 86 to filter out the high-frequency component and output an intermediate frequency signal. The other channel of the second reference frequency signal is input into the direct digital frequency synthesizer 83, and after being multiplied internally by the direct digital frequency synthesizer 83, it is used as the system main clock. Under the control of the digital controller, an output signal with dynamic phase and frequency adjustment is obtained. , with the phase modulated signal The two are input together into the second intermediate frequency mixer 85, wherein The second intermediate frequency mixer 85 outputs a cascaded 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 converts the input intermediate frequency signal and Orthogonal mixing is performed, and analog signal conditioning is performed through the third low-pass filter 92 and the amplifier 93 to obtain two baseband signals. and , respectively, as complex signals The real and imaginary parts of the analog baseband signal and After the analog-to-digital converter 94, the voltage information is quantized. and ,in Indicates sampling period, the sampling rate , corresponding to the time .

[0045] The digital demodulation control module 10 is composed of an inverse tangent demodulator 101 and a controller 102. The inverse tangent demodulator 101 is based on the digitally quantized voltage information. and Perform phase calculation to obtain tracking residual phase information The controller 102 uses the calculated phase information to perform real-time closed-loop feedback adjustment, 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. Tracking the phase modulation caused by the target signal , to achieve 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 a demodulation end, forming a differential closed-loop structure. Through two-stage mixing, the phase noise suppression of the strongly time-correlated carrier signal is effectively achieved. 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 and sensor signals The carrier phase noise introduced by the second crystal oscillator 81 is common mode noise for the microwave signal used for mixing. and It is common mode noise, so 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 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 control amount of the direct digital frequency synthesizer 83 reflects the input sensor signal The phase modulation information is used to realize closed-loop phase demodulation, which effectively expands the detection range of the system and optimizes the detection accuracy and linearity.

[0047] The digital demodulation control module 10 uses an inverse tangent demodulation method based on an I / Q quadrature signal to perform phase calculation. Specifically, the I / Q quadrature signal is an in-phase / quadrature signal. The inverse tangent demodulation method uses a four-quadrant inverse tangent demodulator 101, that is, an Arctangent demodulator with quadrant information. The phase detection range is The third low-pass filter 92 and the amplifier 93 of the I path and the Q path are consistent in circuit parameter design and layout design to ensure the orthogonality of the I / Q signal, thereby ensuring the phase demodulation performance.

[0048] The frequency domain simulation diagram of the phase detection result of an embodiment of the long-distance optical fiber interferometric sensing system based on the synchronous differential closed-loop structure of the present invention is as follows: Figure 3 As shown in FIG. 1 , the frequency domain performance of the present invention in detecting long-distance target signals is compared with the frequency domain performance of the traditional single-ended open-loop detection structure. Among them, the phase modulation part caused by the target signal to be detected is expressed 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 interferometric sensing system based on the traditional single-ended open-loop structure is represented by a gray dotted line, and the phase spectrum curve obtained by the long-distance fiber 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 the result obtained by the target signal detection by the long-distance fiber interferometric sensing system based on the traditional single-ended open-loop structure has 171 Hz nonlinear distortion harmonics, while the result obtained by the target signal detection by the long-distance fiber interferometric sensing system based on the synchronous differential closed-loop structure of the present invention has only a clean 171 Hz spectrum line. At the same time, it can be seen from the signal detection noise floor of the low-frequency band 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, and has a lower detection noise floor and a higher phase detection signal-to-noise ratio, thereby ensuring the weak signal detection performance of the phase modulation 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, and any technical solution obtained by equivalent replacement or equivalent transformation falls 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: It includes: light source module, frequency modulation module, long-distance transmission module, reference light transmission module, detection light transmission module, reference light photoelectric conversion module, sensing light photoelectric conversion module, intermediate frequency mixing module, zero intermediate frequency mixing module and digital demodulation control module; The light source module and the frequency modulation module constitute a light source end, the long-distance transmission module, the reference light transmission module, the detection light transmission module, the reference light photoelectric conversion module and the sensing light photoelectric conversion module constitute a sensing end, and the intermediate frequency mixing module, the zero intermediate frequency mixing module and the digital demodulation control module constitute a demodulation end; The light source module provides two optical signals, one of which passes through the frequency modulation module, the frequency modulation module performs frequency shift modulation on the optical signal and then inputs it into the subsequent long-distance transmission module, and the other optical signal is directly transmitted to the subsequent long-distance transmission module; The long-distance transmission module serves as the core medium of long-distance signal sensing, realizing 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 input into the reference optical transmission module and the sensing optical transmission module respectively. The reference light transmission module is insensitive to the target signal, and outputs a reference signal through the reference light photoelectric conversion module. ; 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 photoelectric conversion module. ; Reference signal With phase modulated signal The intermediate frequency mixing module is input for the first stage mixing and outputs two intermediate frequency signals. and ; Two intermediate frequency signals and Then the signal is input into the zero intermediate frequency mixer module for second-stage mixing, and the baseband signal is output. and ; Baseband signal and The digital processing and phase information calculation are completed in the digital demodulation control module; The digital controller uses the calculated phase information to perform real-time closed-loop feedback regulation and dynamically configures the intermediate frequency mixing module to achieve negative feedback control.

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, wherein the laser is used as a light source of the detection system to generate an optical signal with a narrow line width, and is divided into two output paths 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 is multiplied by the first phase-locked loop to generate a microwave signal with a fixed frequency, 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, and achieves precise control of the frequency of the output optical signal by changing the frequency of the microwave signal. The acousto-optic modulator selects the negative first-order sideband.

3. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure as claimed in 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 that the two optical fibers are accurately matched in length. The two optical signals output by the first long optical fiber and the second long optical fiber pass through the second coupler and the third coupler respectively to generate four optical signal outputs. One optical signal output by the first coupler and one optical signal output by the second coupler form a pair of optical signals, and another optical signal output by the first coupler and another 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 as claimed in claim 3, characterized in that: The sensing light transmission module is composed of a first circulator, a second circulator, a first delay optical fiber, a first fiber optic reflector, a sensing probe, a second fiber optic reflector 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, and after being reflected by the first fiber optic reflector, passes through the first delay optical fiber and the first circulator again to enter the fifth coupler; the other path input into the second circulator passes through the sensing probe, and after being reflected by the second fiber optic reflector, passes through the sensing probe and the second circulator again to enter the fifth coupler. The two paths of optical signals are output after beam combining interference in the fifth coupler, wherein the sensing probe is a sensing sensitive unit, and changes in the external measured physical quantity cause changes in the optical characteristics of the sensitive unit optical fiber, thereby changing the refractive index and length physical parameters of the optical fiber, causing changes in the phase and frequency parameters of the sensing light; The reference light 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 pass through the second delay optical fiber and the third delay optical fiber respectively, and are output after beam combining interference in the fourth coupler.

5. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure as claimed in claim 4, characterized in that: The sensing light photoelectric conversion module is composed of a first photodiode, the reference light photoelectric conversion module is composed of a second photodiode, the optical signal output by the sensing light transmission module is input into the first photodiode, and a phase modulation signal is output. The optical signal output by the reference optical transmission module is input into the second photodiode, and a reference voltage signal is output. The phase, frequency and other parameter change information detected by the sensor probe is modulated on the phase modulation signal , without modulation on the reference voltage signal , reference voltage signal and phase modulated signal Then it is input into the demodulation terminal 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 sensor probe are matched, thereby ensuring that the reference signal and sensor signals The signal at the input demodulation end for phase demodulation has a time synchronization characteristic. The long-distance transmission module at the sensor end and the reference optical transmission module are synchronized with each other in the reference signal. The transmission delay introduced is The long-distance transmission module at the sensor end and the sensor light transmission module are The transmission delay introduced is , and Ensure consistency in design and reference signal and sensor signals The carrier signal has strong time correlation.

7. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure as claimed in claim 5, characterized in that: The intermediate frequency mixing module is composed 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 which is multiplied by the second phase-locked loop to generate a microwave signal with a fixed frequency. , and the reference voltage signal The first intermediate frequency mixer outputs the first low-pass filter 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, and after being multiplied internally by the direct digital frequency synthesizer, it is used as the system master clock, and under the control of the digital controller, an output signal of dynamic phase-frequency adjustment is obtained. , with the phase modulated signal The second intermediate frequency mixer is input together, and the output of the second intermediate frequency mixer is cascaded with the second low-pass filter to filter out the high-frequency component and output an intermediate frequency signal ; The zero intermediate frequency mixing module is composed of an I / Q mixer, a third low-pass filter, an amplifier and an analog-to-digital converter. The I / Q mixer processes the input intermediate frequency signal. and Perform orthogonal mixing, and perform analog signal conditioning through the third low-pass filter and amplifier to obtain two baseband signals and , analog baseband signal and The voltage information is digitally quantized through the analog-to-digital converter.

8. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 7, characterized in that: The demodulation end adopts a differential closed-loop structure, and through two-stage mixing, the phase noise suppression of the strongly time-correlated carrier signal is achieved. 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 relatively small for the reference signal. and sensor signals The carrier phase noise introduced by the second crystal oscillator is common mode noise for the microwave signal used for mixing. and It is common mode noise. Through two-stage mixing, the carrier phase noise introduced by the first crystal oscillator and the second crystal oscillator is suppressed through the differential principle. In the first intermediate frequency mixing link, the output of the direct digital frequency synthesizer is dynamically adjusted under the control of the controller, and the control amount of the direct digital frequency synthesizer reflects the input sensor signal. The phase modulation information is used to realize closed-loop phase demodulation.

9. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure as claimed in claim 7, characterized in that: The digital demodulation control module is composed of an inverse tangent demodulator and a controller. The inverse tangent demodulator is based on the digitally quantized voltage information. and Phase solution is performed to obtain tracking residual phase information. The digital controller uses the solved phase information to perform real-time closed-loop feedback adjustment and dynamically configures the direct digital frequency synthesizer in the intermediate frequency mixing module to achieve negative feedback control.

10. The long-distance fiber optic interferometric sensing system based on a synchronous differential closed-loop structure according to claim 9, characterized in that: The digital demodulation control module adopts the inverse tangent demodulation method based on I / Q orthogonal signal to perform phase calculation. The tangent demodulation method adopts a four-quadrant inverse tangent demodulator, that is, an Arctangent demodulator with quadrant information, and the phase detection range is The third low-pass filter and amplifier of the I and Q paths are kept consistent during circuit parameter design and layout design to ensure the orthogonality of the I / Q signals, thereby ensuring the phase demodulation performance.

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