Quasi-distributed sensing system based on fiber type auxiliary interferometer and vernier sensitization

Abstract

The invention discloses a quasi-distributed sensing system based on a fiber type auxiliary interferometer and vernier sensitization, and relates to the technical field of optical fiber sensing. The system comprises a data acquisition module, a to-be-detected sample parameter detection device and a data processing module, the to-be-detected sample parameter detection device comprises a plurality of FPI sensors; the data acquisition module is used for acquiring optical signals corresponding to parameters of a sample to be measured and comprises a tunable laser, a main interferometer, an auxiliary interferometer, a polarization beam splitter, a balanced photoelectric detector and a data acquisition card; the main interferometer is a Mach-Zehnder interferometer, the auxiliary interferometer comprises a section of delay optical fiber with a preset length and two reflecting surfaces, and the auxiliary interferometer is arranged on a reference arm of the Mach-Zehnder interferometer; and the data processing module is used for performing first processing and second processing on the optical signal output by the data acquisition module so as to obtain the variable quantity of the to-be-detected sample parameter. According to the invention, the technical bottlenecks of complex structure, difficult debugging, low demodulation efficiency and the like in the traditional scheme are effectively overcome.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitivity enhancement. Background Art

[0002] With the development of new-generation information technology and industrial intelligence, fiber optic sensors are becoming a key area of ​​advanced sensing technology due to their resistance to electromagnetic interference, small size, corrosion resistance, and long-distance transmission capabilities. Interferometric fiber optic sensors, combined with frequency-modulated continuous wave (FMCW) technology, have become an important research area in fiber optic sensing systems due to their high sensitivity, low noise, and multi-point decoupling capabilities. FMCW technology is widely used in radar, laser ranging, and fiber optic sensing. Its basic principle is to transmit a continuous wave whose frequency is linearly modulated over time and to resolve path length information through frequency differences. Optical frequency domain reflectometry (OFDR) builds on this principle and analyzes the frequency domain characteristics of the reflected signal to achieve spatial positioning and physical quantity measurement of multiple reflection points. It offers the advantages of long-distance, high-resolution imaging, and multi-point synchronous detection.

[0003] Fiber-optic Fabry-Perot (FPI) sensors are widely used for measuring refractive index, temperature, pressure, and salinity due to their simple structure, low cost, and high sensitivity. Their operating principle is that optical fibers form an interference cavity between reflectors, and the inversion of physical quantities is achieved by monitoring changes in the interference spectrum. However, the sensitivity of FPI sensors is limited by the free spectral range (FSR) and cavity length, making them difficult to meet the requirements for high-precision demodulation of weak signals.

[0004] To enhance the sensitivity of interferometric fiber optic sensors, existing technologies have introduced the optical vernier effect. By constructing interferometric structures with slightly different FSRs, the interference spectrum produces periodic envelope modulation, thereby enhancing the response to small wavelength shifts and improving sensitivity. However, this approach suffers from complex structures, high manufacturing precision requirements, inconvenient adjustment, and difficulty implementing distributed measurements. A solution that achieves a highly sensitive vernier-based sensitization effect while also simplifying the structure, providing flexible adjustment, and being suitable for distributed fiber optic sensing is lacking.

[0005] Currently, efforts to improve the sensitivity of FMCW interferometry systems, simplify their structure, and achieve large-scale, multi-parameter distributed measurement face the following challenges: the physical vernier structure is complex and difficult to adjust, making it difficult to scale to distributed sensing systems; traditional FPI sensors have limited sensitivity and cannot detect weak disturbances; multi-parameter coupling leads to high measurement uncertainty, and decoupling algorithms are complex and lack real-time performance. Therefore, a new technology is urgently needed that leverages the advantages of FMCW system frequency-domain modulation and uses mathematical algorithms to simulate auxiliary interferometry functions, achieve a virtual vernier effect, simplify the system structure, improve demodulation flexibility and sensitivity, and meet the future needs of distributed sensing and multi-parameter measurement. Summary of the Invention

[0006] In view of the above problems, the present invention proposes a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitivity enhancement.

[0007] A quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitivity enhancement includes a data acquisition module, a device for detecting sample parameters to be measured, and a data processing module. The device for detecting sample parameters to be measured is connected to the data acquisition module, and the data acquisition module is connected to the data processing module. The device for detecting sample parameters to be measured includes multiple FPI sensors.

[0008] The data acquisition module is used to collect optical signals corresponding to the sample parameters to be measured; the data acquisition module includes a tunable laser, a main interferometer, an auxiliary interferometer, a polarization beam splitter, a balanced photodetector, and a data acquisition card; the main interferometer is a Mach-Zehnder interferometer, the auxiliary interferometer includes a delay fiber of a preset length and two reflecting surfaces, and the auxiliary interferometer is arranged on the reference arm of the Mach-Zehnder interferometer;

[0009] The data processing module is used to perform a first process on the optical signal output by the data acquisition module to obtain a spectrum corresponding to the sample parameter to be measured; and perform a second process on the spectrum corresponding to the sample parameter to be measured to obtain a variation of the sample parameter to be measured.

[0010] Furthermore, the main interferometer includes a first optical coupler, a first fiber circulator, a second fiber circulator, and a second optical coupler; the process of collecting the optical signal corresponding to the sample parameter to be measured in the data acquisition module includes: the light emitted by the tunable laser is divided into two parts after passing through the first optical coupler, one part of the light passes through the first fiber circulator and enters the reference arm, that is, enters the auxiliary interferometer, and the other part of the light passes through the second fiber circulator and enters the sample parameter detection device to be measured; the light signal reflected back by the sample parameter detection device to be measured passes through the second fiber circulator and interferes with the reference light signal of the reference arm of the main interferometer at the second optical coupler to form main interference light, and the two reflection surfaces of the auxiliary interferometer form auxiliary interference light; the main interference light and the auxiliary interference light are split by the polarization beam splitter after passing through the second optical coupler, and are then received by the balanced photodetector and enter the data acquisition card for collection.

[0011] Furthermore, the data processing module performs a first processing on the optical signal output by the data acquisition module to obtain the spectrum corresponding to the sample parameter to be measured, including: band-pass filtering the collected beat frequency signal to intercept the beat frequency information of the auxiliary interference light; extracting the instantaneous optical frequency sequence from the beat frequency information of the auxiliary interference light; arranging the instantaneous optical frequency sequence at equal intervals to obtain a linear optical frequency sequence; using the linear optical frequency sequence to interpolate the collected beat frequency signal; performing FFT operation on the obtained interpolation data to obtain corrected spatial domain information; performing band-pass filtering and IFFT operation on the corrected spatial domain information to obtain time domain information; and using Hilbert transform to extract the envelope of the time domain information to obtain the spectrum corresponding to the sample parameter to be measured.

[0012] Furthermore, the data processing module performs a second processing on the spectrum corresponding to the sample parameter to be measured to obtain the change in the sample parameter to be measured, including: superimposing the spectrum corresponding to the sample parameter to be measured and the interference spectrum corresponding to the preset virtual FPI interferometer cavity, and extracting the superimposed composite spectrum envelope; comparing the composite spectrum envelope with the reference composite spectrum envelope to obtain the change in the sample parameter to be measured; the reference composite spectrum envelope is the composite spectrum envelope after superimposing the spectrum corresponding to the sample parameter to be measured when it is unchanged and the interference spectrum of the preset virtual FPI interferometer cavity.

[0013] Furthermore, the interference spectrum corresponding to the preset virtual FPI interferometer cavity in the data processing module is constructed as follows:

[0014] Based on the virtual vernier effect, a virtual FPI interferometer cavity with a different cavity length from that of each FPI sensor is constructed. The calculation formula for the interference spectrum of the virtual FPI interferometer cavity is:

[0015]

[0016] Where I0 is the incident light intensity value; is the transmission coefficient of light in the virtual FPI interferometer cavity; r3 and r4 are the reflection coefficients of the two end faces of the virtual FPI interferometer cavity; n2 and L2 are the refractive index and cavity length of the virtual FPI interferometer cavity, respectively; Represents the wavelength in vacuum.

[0017] Furthermore, the sample parameters to be measured include temperature, salinity, pH value, and gas concentration.

[0018] Furthermore, the data processing module performs a second processing on the spectrum corresponding to the sample parameter to be measured, and further includes: determining the amplification factor of the virtual cursor effect between the FPI sensor and the virtual FPI interferometer cavity based on the free spectral range of the optical signal corresponding to the sample parameter to be measured and the free spectral range corresponding to the preset virtual FPI interferometer cavity.

[0019] Furthermore, the multiple FPI sensors are connected in series or in parallel.

[0020] Furthermore, the wavelength range of the laser emitted by the tunable laser is 1510nm-1640nm; and the splitting ratio of the first optical coupler is 3:7.

[0021] The beneficial technical effects of the present invention are:

[0022] In order to ensure the stable operation of the system and solve the nonlinear problems existing in the tuning process of the FMCW laser, the present invention proposes a quasi-distributed sensing system based on a fiber-type auxiliary interferometer and vernier sensitivity enhancement. Auxiliary interferometer arms and delayed fiber segments are introduced in the structural design to construct a simplified Michelson interference structure. The auxiliary arm embeds a delay fiber of precise length on the main line, and the delay fiber length is reasonably configured to ensure that its beat frequency signal not only meets the frequency uniformity requirements of resampling, but also is in the front part of the total power spectrum, thereby serving as a stable benchmark for time domain resampling and tuning nonlinear compensation, thereby further improving the demodulation accuracy and system stability; by constructing an adjustable virtual reference interferometer in the signal processing domain and superimposing it with the actual main interference spectrum, effective amplification and analytical optimization of weak interference responses are achieved without the need to physically construct an auxiliary cavity.

[0023] The system of the present invention not only excels in sensing sensitivity, noise resistance and multi-point expansion capability, but also effectively overcomes technical bottlenecks such as complex structure, difficult debugging and low demodulation efficiency in traditional solutions, providing a new solution path for high-sensitivity, multi-parameter and long-distance fiber optic sensing applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily apparent by reading the following detailed description with reference to the accompanying drawings, in which several embodiments of the present invention are shown by way of example and not limitation, in which:

[0025] Figure 1 This is a structural diagram of a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitivity enhancement according to the present invention;

[0026] Figure 2 This is an example diagram of the sensor head structure in an embodiment of the present invention;

[0027] Figure 3 This is an example of a complex beat frequency original signal and a spectrum after signal processing in an embodiment of the present invention;

[0028] Figure 4 is an example of the interference envelope spectrum of the interferometer FPI1 in an embodiment of the present invention;

[0029] Figure 5 is an example of the interference envelope spectrum of the virtual interferometer in an embodiment of the present invention;

[0030] Figure 6 This is an example of the superimposed interference spectrum envelope Ien-12 and its temperature response change in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The principles and spirit of the present invention will be described below with reference to several exemplary embodiments. It should be understood that these embodiments are provided solely to enable those skilled in the art to better understand and implement the present invention, and are not intended to limit the scope of the present invention in any way. Rather, these embodiments are provided to make this disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0032] Although traditional point-type fiber optic interferometer sensors have high sensitivity, they are difficult to reuse on a large scale and cannot meet the monitoring needs of long distances and multiple nodes. Although existing quasi-distributed sensing solutions have multi-point coverage capabilities, their spectral changes under weak disturbances are limited, resulting in low sensitivity and difficulty in achieving high-precision measurement of multiple parameters. In addition, existing systems usually rely on multiple independent demodulation channels, resulting in complex structures and lengthy signal processing chains. The nonlinear tuning characteristics of tunable lasers make the system prone to errors in beat frequency demodulation, affecting the decoupling accuracy and system stability. In response to the above problems, the present invention proposes a quasi-distributed sensing system based on fiber-assisted interferometers and vernier sensitivity enhancement. Combining the optical vernier enhancement mechanism with the auxiliary interferometer compensation technology, the system solves the technical bottlenecks of the existing quasi-distributed fiber optic sensing systems, such as insufficient sensing sensitivity, complex structure, and difficulty in compensating for system nonlinear errors, thereby significantly improving the sensitivity, structure, and scalability of the system.

[0033] The present invention introduces an auxiliary interferometer into the reference arm of the main interferometer. The designed auxiliary interferometer is a Michelson interferometer composed of two reflecting surfaces and a delay optical fiber (AOF). The two reflecting surfaces have an optical path difference, causing interference when they meet. This auxiliary interferometer generates a stable and structurally controllable beat frequency signal as a nonlinear compensation reference signal, ensuring that even with the nonideal frequency sweep characteristics of the tunable laser source, the interferometer cavity reflection information can still be accurately extracted. Subsequently, the instantaneous frequency trajectory of the auxiliary interferometer signal is extracted through Hilbert transform and phase unwrapping methods, and the main interferometer spectrum is then reconstructed in the frequency domain and dynamically resampled to achieve linear compensation processing for the beat frequency signal. This process not only improves demodulation accuracy and reduces systematic errors caused by laser nonlinearity, but also effectively avoids the drawbacks of traditional methods that require the independent construction of an auxiliary interferometer or complex interpolation algorithms. In addition, after processing the complex beat frequency signal on the software side, the system constructs a "virtual FPI" with a slightly different optical structure similar to the FPI to form a virtual cursor structure. By utilizing its high sensitivity to tiny spectral shifts, it significantly amplifies the weak optical path changes caused by external disturbances, thereby improving the detection sensitivity and resolution of parameters such as temperature, salinity, and gas concentration, effectively realizing cursor sensitivity enhancement, and can be applied to multi-parameter sensing measurements.

[0034] The present invention proposes a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitivity enhancement, which includes a data acquisition module, a sample parameter detection device 11 to be measured, and a data processing module 10. The sample parameter detection device 11 to be measured is connected to the data acquisition module, and the data acquisition module is connected to the data processing module 10; wherein, the sample parameter detection device 11 to be measured includes multiple FPI sensors;

[0035] The data acquisition module is used to collect optical signals corresponding to the sample parameters to be measured; the data acquisition module includes a tunable laser 1, a main interferometer, an auxiliary interferometer, a polarization beam splitter 7, a balanced photodetector 8, and a data acquisition card 9; the main interferometer is a Mach-Zehnder interferometer, the auxiliary interferometer includes a delay optical fiber 4 of a preset length and two strong reflective surfaces, and the auxiliary interferometer is arranged on the reference arm of the Mach-Zehnder interferometer;

[0036] The data processing module 10 is used to perform a first process on the optical signal output by the data acquisition module to obtain a spectrum corresponding to the sample parameter to be measured; and perform a second process on the spectrum corresponding to the sample parameter to be measured to obtain a variation of the sample parameter to be measured.

[0037] The main interferometer includes a first optical coupler 2, a first fiber circulator 3, a second fiber circulator 5, and a second optical coupler 6; the process of collecting the optical signal corresponding to the sample parameter to be measured in the data acquisition module includes: the light emitted by the tunable laser 1 is divided into two parts after passing through the first optical coupler 2, one part of the light passes through the first fiber circulator 3 and enters the reference arm, that is, enters the auxiliary interferometer, and the other part of the light passes through the second fiber circulator 5 and enters the sample parameter detection device 11 to be measured; the light signal reflected back by the sample parameter detection device 11 passes through the second fiber circulator 5 and interferes with the reference light signal of the reference arm of the main interferometer at the second optical coupler 6 to form main interference light, and the two strong reflection surfaces of the auxiliary interferometer form auxiliary interference light; the main interference light and the auxiliary interference light are split by the polarization beam splitter 7 after passing through the second optical coupler 6, and are then received by the balanced photodetector 8 and enter the data acquisition card 9 for collection.

[0038] The data processing module 10 performs a first processing on the optical signal output by the data acquisition module to obtain a spectrum corresponding to the sample parameter to be measured, including: band-pass filtering the collected beat frequency signal to intercept the beat frequency information of the auxiliary interference light; extracting an instantaneous optical frequency sequence from the beat frequency information of the auxiliary interference light; arranging the instantaneous optical frequency sequence at equal intervals to obtain a linear optical frequency sequence; using the linear optical frequency sequence to interpolate the collected beat frequency signal; performing FFT operation on the obtained interpolation data to obtain corrected spatial domain information; performing band-pass filtering and IFFT operation on the corrected spatial domain information to obtain time domain information; and using Hilbert transform to extract the envelope of the time domain information to obtain the spectrum corresponding to the sample parameter to be measured.

[0039] The data processing module 10 performs a second processing on the spectrum corresponding to the sample parameter to be measured to obtain the change in the sample parameter to be measured, including: superimposing the spectrum corresponding to the sample parameter to be measured and the interference spectrum corresponding to the preset virtual FPI interferometer cavity, and extracting the superimposed composite spectrum envelope; comparing the composite spectrum envelope with a reference composite spectrum envelope to obtain the change in the sample parameter to be measured; the reference composite spectrum envelope is the composite spectrum envelope after superimposing the spectrum corresponding to the sample parameter to be measured when it does not change and the interference spectrum of the preset virtual FPI interferometer cavity.

[0040] The interference spectrum corresponding to the preset virtual FPI interferometer cavity in the data processing module 10 is constructed as follows:

[0041] Based on the virtual vernier effect, a virtual FPI interferometer cavity with a different cavity length from that of each FPI sensor is constructed. The calculation formula for the interference spectrum of the virtual FPI interferometer cavity is:

[0042]

[0043] Where I0 is the incident light intensity value; is the transmission coefficient of light in the virtual FPI interferometer cavity; r3 and r4 are the reflection coefficients of the two end faces of the virtual FPI interferometer cavity; n2 and L2 are the refractive index and cavity length of the virtual FPI interferometer cavity, respectively; Represents the wavelength in vacuum.

[0044] The data processing module 10 performs a second processing on the spectrum corresponding to the sample parameter to be measured, further comprising: determining the amplification factor of the virtual vernier effect between the FPI sensor and the virtual FPI interferometer cavity based on the free spectral range of the optical signal corresponding to the sample parameter to be measured and the free spectral range corresponding to the preset virtual FPI interferometer cavity.

[0045] The sample parameters to be measured include temperature, salinity, pH, and gas concentration. The multiple FPI sensors are connected in series or in parallel. The laser wavelength emitted by the tunable laser 1 is in the range of 1510nm-1640nm; the splitting ratio of the first optical coupler 2 is 3:7.

[0046] Compared with the traditional OFDR sensing system, the system structure of the present invention is greatly simplified, which is not only low in cost but also easy to integrate. The system design of the present invention is described in detail below.

[0047] System structure such as Figure 1 As shown, the system mainly consists of four parts: the first part is the auxiliary interferometer optical path, which adopts a typical Michelson interferometer structure. Its role is not to directly participate in sensing, but to serve as the system's optical reference channel to compensate for the nonlinear tuning error generated by the frequency-modulated continuous-wave laser source during the frequency sweep process. A delay fiber 4 of precisely preset length is embedded in the auxiliary interferometer. The auxiliary interferometer is located in the reference arm of the main interferometer and consists of two reflective surfaces PR1 and PR2 and a delay fiber 4 to compensate for the nonlinear frequency sweep effect of the system. Setting a reasonable delay fiber length can ensure that the auxiliary beat signal appears in advance of the main beat signal, thus serving as a stable reference for time-domain resampling and tuning nonlinear compensation. This ensures that even if the tunable laser source has non-ideal frequency sweep characteristics, the interferometer cavity reflection information can still be accurately extracted. In other words, the generated beat signal has good time delay identification capability and meets the requirement that the auxiliary interference frequency is at the front end of the total power spectrum, facilitating subsequent resampling and processing.

[0048] The second part is the sensing area structure, which is a Mach-Zehnder interferometer (MZI), that is, the main interferometer. The upper branch is the reference arm, and the lower branch is the sensing arm. It is connected to the sensing end structure through a fiber optic coupler and a circulator: the light output from the MZI sensing arm passes through multiple sensing units FPI composed of a series structure in sequence. Each of the multiple FPI interferometer cavities can correspond to the sensing function of a physical or chemical parameter, that is, the open cavity of each sensor can be filled with sensitive materials that have a response effect on different physical parameters; due to the different time delay τ of the light beam reaching each FPI cavity, each reflection end face will generate a swept frequency signal with a specific frequency difference.

[0049] The third part is the data acquisition area. The echo signal light reflected by each FPI sensor in the sensing area returns through the port of the fiber circulator and superimposes and interferes with the reference light transmitted from the reference arm at the coupler, generating a beat frequency interference signal containing multi-point reflection information, namely the beat frequency signal of the auxiliary interferometer and the main interferometer; this beat frequency interference signal passes through the coupler and is split into p-polarized light and s-polarized light by the polarization beam splitter 7 (PBS), and is received by the balanced photodetector 8 (BPD) and collected by the data acquisition card (DAQ), respectively.

[0050] The fourth part is the data processing part. After the collected data is processed by this part, the interference spectrum information of the fiber microcavity is obtained. A matching sinusoidal virtual FPI interference spectrum is constructed in advance based on the FPI interference information. The actual FPI interference spectrum is superimposed on the preset sinusoidal virtual FPI interference spectrum to form a modulated virtual cursor effect on the envelope.

[0051] In the sensing system, the incident light from tunable laser 1 is split into two paths by a coupler. One portion of the light enters an auxiliary interferometer, the Michelson interferometer, with two reflective surfaces, PR1 and PR2. Due to the optical path difference between PR1 and PR2, interference occurs when they meet, forming a beat signal in the system. The reflected light field at PR1 can be expressed as:

[0052] (1)

[0053] Where f0 represents the starting frequency of the swept light, γ is the frequency modulation rate of the light source, It is expressed as a nonlinear phase noise term, E0 is the amplitude of the swept optical field, r is the reflection coefficient of PR1, Indicates the splitting ratio of the reference arm.

[0054] If the length of the delay fiber 4 is L, the reflected light field at PR2 can be expressed as:

[0055] (2)

[0056] in, is the time delay caused by the total length of the delay fiber 4, and r0 is the reflection coefficient of PR2.

[0057] The light intensity after the interference of the above two reflected lights can be expressed as:

[0058] (3)

[0059] From formula (3), we can see that the frequency of the auxiliary beat signal is , where n is the effective refractive index of the fiber. This beat frequency information is used to compensate for the nonlinear effects of the light source.

[0060] The main interferometer is a Mach-Zehnder interferometer, consisting of a reference arm and a sensor arm. The swept light emitted by the tunable laser 1 is divided into reference light and sensor light, which are transmitted in the reference arm and sensor arm respectively. The light fields of the reference light and sensor light can be expressed as:

[0061] (4)

[0062] (5)

[0063] Among them, R(τ Z ) is the reflection coefficient of point Z on the sensor arm, τ Z is the time delay between point Z and the reference arm. Considering that the reflection point of the sensor arm is composed of a series of reflection points of the FPI array, the beat frequency signal after the reference light and the reflected light meet and interfere is:

[0064] (6)

[0065] Among them, τ Zm and R(τ Zm ) represent the delay and reflection coefficient of the mth reflection point respectively. From formula (6), we can see that there is a linear correspondence between the beat signal frequency and the delay between the reference light and the sensor arm.

[0066] Because the optical signal collected by the data acquisition card 9 is a raw signal formed by the superposition of multiple FPI interference signals and auxiliary beat signals, the interference spectrum is relatively complex. To obtain the interference spectrum envelope of multiple FPIs, the raw signal is processed in the data processing module 10. As an example, the raw signal collected by the data acquisition card 9 is uploaded to a computing terminal, and the complex beat signal is processed using Matlab software. Specifically, the light intensity of the optical signal collected by the data acquisition card 9 is subjected to spectral analysis, including: first, bandpass filtering the collected raw signal to intercept the beat information of the auxiliary interferometer. Because the auxiliary interferometer in the present invention shares the same data channel with multiple FPI sensors, its reflected echo and the main sensor beat signal are collected simultaneously in the time domain. According to the structural parameter settings of the auxiliary interferometer, its optical path delay is much smaller than that of each FPI sensor, resulting in the lowest frequency of its beat signal, located at the front end frequency region of the total power spectrum, and easy to separate and identify. Next, the instantaneous optical frequency sequence is extracted from the beat frequency information of the auxiliary interferometer. This instantaneous optical frequency sequence is arranged at equal intervals to obtain a linear optical frequency sequence. This linear optical frequency sequence is used to interpolate the original signal collected to achieve nonlinear frequency sweep compensation of the light source. The interpolated data is then subjected to an FFT operation to obtain the corrected spatial domain information. This corrected spatial domain information, i.e., the reflection peak of the FPI microcavity, is then bandpass filtered and subjected to an IFFT to obtain its wavelength domain (time domain) information. The Hilbert transform is used to extract the envelope of this wavelength domain information to obtain the interference spectrum of the FPI microcavity.

[0067] The FPI interferometer response spectrum contains optical path difference interference information related to the cavity length and refractive index. Taking the interferometer FPI1 as an example, the interference envelope spectrum of the light reflected from the FPI1 cavity can be expressed as follows:

[0068] (7)

[0069] Among them, I0 is the incident light intensity value, η1 is the transmission coefficient of the light in the interferometer FPI1 cavity, r1 and r2 are the reflection coefficients of the two end faces of the interferometer FPI1 cavity, n1 and L1 are the refractive index and cavity length of the FPI1 cavity respectively, Represents the wavelength in vacuum.

[0070] The virtual vernier effect refers to the simulation of an equivalent interferometer cavity response characteristic through digital signal processing technology, and the algorithm-level superposition or fitting with the measured interference spectrum, thereby simulating the vernier interference effect without the need to build a physical auxiliary cavity, thereby enhancing the perception of tiny changes in physical parameters. Therefore, the present invention uses a priori design parameters to construct a virtual FPI (VFPI) that is similar to the FPI optical structure but slightly different in the free spectral range (FSR) and periodic characteristics. The design of the VFPI is based on the structure of the FPI demodulated in the system, but fine-tunes key optical parameters (such as cavity length, which can also include refractive index) to ensure that it has a periodicity that is not completely consistent with the free spectral range (FSR) of the FPI. The design parameters of the VFPI can be obtained through simulation to ensure that in actual applications, an optical response similar to that of the FPI can be achieved, but at the same time, it has unique periodic characteristics to meet specific measurement requirements. The virtual FPI can be generated by a sine function, and the interference envelope spectrum of the light reflected from the VFPI cavity is and free spectral range It can be expressed as follows:

[0071] (8)

[0072] Where I0 is the incident light intensity, η2 is the transmission coefficient of the light in the interferometer VFPI cavity, r3 and r4 are the reflection coefficients of the two end faces of the interferometer VFPI cavity, and n2 and L2 are the refractive index and cavity length of the VFPI cavity, respectively.

[0073] Taking interferometer FPI1 as an example, since the free spectral ranges of FPI1 and virtual FPI envelope spectra are close but not equal, the two interferometer interference spectra are multiplied or weighted superimposed in the signal domain, and a virtual vernier effect will be generated between interferometer FPI1 and VFPI, constructing an interference envelope I with vernier effect characteristics. en The composite spectrum is formed as follows:

[0074] (9)

[0075] Among them, I en is the composite interference spectrum envelope amplitude; M en is the magnification of the vernier effect between interferometers FPI1 and VFPI, that is, the temperature sensitivity of the interference spectrum envelope is M of a single interferometer FPI1. en times.

[0076] Taking the temperature of the sample to be tested as an example, when the temperature of the sample to be tested increases from T0 to T 0+1 When the corresponding temperature T is collected again 0+1The real-time signal is processed and demodulated to produce the FPI interference spectrum after temperature changes. This spectrum is then superimposed with the VFPI interference envelope spectrum. Comparing the superimposed result with the corresponding envelope spectrum at temperature T0 reveals a significant drift in the superimposed interference spectrum envelope. This envelope drift can be used as a sensitive indicator of temperature changes, enabling highly sensitive temperature detection. The virtual vernier effect effectively amplifies the weak response of a single sensing cavity, improving the ability to detect small parameter perturbations.

[0077] It should be noted that while the above embodiments use temperature measurement as an example, the present invention can be extended to multi-point, multi-parameter measurements. By filling or coating different cavities with different sensitive materials and comparing the refractive index or cavity length, the spectra are calibrated using two interferometer envelope shift comparison experiments. Extracting the shifts allows for the determination of different responses to physical and chemical parameters. By analyzing the interferometer envelope drift, the system achieves high-precision and high-sensitivity identification of physical parameters. By constructing multiple FPI sensor arrays and integrating functional sensitive materials into the interferometer cavity structure through filling or surface coating processes, simultaneous measurement of multiple physical and chemical parameters (such as temperature, salinity, pH, and gas concentration) can be achieved.

[0078] As an example, the FPI interferometer structure in the system is as follows Figure 2 As shown, the sensor head fabrication steps are as follows: The interferometer (FPI) is constructed by sequentially fusion-splicing a single-mode fiber (SMF) with a core diameter of 8-10 microns and a hollow-core fiber (HCF) filled with PDMS at one end. (The SMF has a diameter of 125 microns and a core diameter of 8-10 microns; the HCF has an outer diameter of 125 microns and an inner diameter of 50 microns.) The desired length is cut and sealed at one end. A fiber fusion splicer is used to perform low-power fusion splicing of the SMF and HCF, creating a flat reflective surface on one side. Liquid PDMS (polydimethylsiloxane) is then filled into the HCF end through capillary action. PDMS has an extremely high thermo-optic coefficient and thermal expansion coefficient, making it sensitive to temperature changes. The PDMS is then heated and cured to form a stable closed cavity. The refractive index and length of this closed cavity change with temperature, thus achieving temperature sensitivity.

[0079] Figure 3 The original signal of the complex beat frequency and the spectrum after signal processing. Figure 3 As shown in (a), data processing is performed through Matlab. First, the original signal is transformed into the spatial domain (such as Figure 3 (b) in the figure), it is found that the original signal has serious spectrum broadening. Then a bandpass filter is applied to filter out the required frequency band, and the instantaneous optical frequency is extracted using the auxiliary beat signal. The original signal is resampled using an equally spaced instantaneous optical frequency sequence to obtain calibration data. The calibration data is converted into the spatial domain, and the reflection peak of the FPI microcavity is bandpass filtered and IFFT is performed to obtain its wavelength domain (time domain) information, as shown in the following figure. Figure 3As shown in (c)-(e) in FIG, the interference spectrum of the FPI microcavity can be obtained by extracting the envelope of the wavelength domain information, as shown in FIG. Figure 3 As shown in (f) in the figure, a sinusoidal FPI virtual cavity interference spectrum is constructed. The free spectral range of the virtual interferometer is very close to but not equal to the demodulated interferometer cavity, and a virtual vernier effect can be generated by cascading, as shown in Figures 4-6 shown.

[0080] In summary, the present invention solves the core technical problems of insufficient sensitivity and complex structure in existing quasi-distributed fiber optic sensing systems. The proposed distributed sensitivity enhancement mechanism not only retains the inherent high-precision spatial positioning advantage of frequency-modulated continuous-wave fiber optic sensing technology, but also integrates the auxiliary interferometer into the reference arm of the main interferometer, greatly simplifying the structure of the system; a virtual cursor effect is introduced to amplify the drift response of the sensing interference spectrum, thereby greatly improving the sensing unit's ability to resolve weak parameter disturbances. The present invention simplifies the sensing system structure, reduces manufacturing costs, improves algorithm flexibility and system adjustment capabilities, and is particularly suitable for high-density, long-distance, and multi-parameter distributed fiber optic sensing networks; in particular, controllable sensitivity adjustment and multi-point collaborative sensitivity enhancement detection can be achieved in high-density sensing networks, significantly reducing system demodulation complexity and engineering implementation costs while ensuring measurement accuracy.

[0081] The present invention is suitable for large-scale, long-distance, high-precision fiber optic sensing application scenarios, and has broad promotion prospects and engineering application value, especially in the fields of marine environment monitoring, multi-parameter water quality detection, oil and gas resource transportation, bridge structure health monitoring, and smart city infrastructure perception.

[0082] Although the spirit and principles of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the specific embodiments disclosed, and the division into various aspects does not mean that the features of these aspects cannot be combined to benefit. Such division is only for the convenience of expression. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement, characterized in that: The device comprises a data acquisition module, a sample parameter detection device (11) to be tested, and a data processing module (10), wherein the sample parameter detection device (11) to be tested is connected to the data acquisition module, and the data acquisition module is connected to the data processing module (10); wherein the sample parameter detection device (11) to be tested comprises a plurality of FPI sensors; The data acquisition module is used to acquire optical signals corresponding to the sample parameters to be measured; the data acquisition module includes a tunable laser (1), a main interferometer, an auxiliary interferometer, a polarization beam splitter (7), a balanced photodetector (8), and a data acquisition card (9); the main interferometer is a Mach-Zehnder interferometer, the auxiliary interferometer includes a delay optical fiber (4) of a preset length and two reflection surfaces, and the auxiliary interferometer is arranged on a reference arm of the Mach-Zehnder interferometer; The data processing module (10) is used to perform a first process on the optical signal output by the data acquisition module to obtain a spectrum corresponding to the sample parameter to be measured; and to perform a second process on the spectrum corresponding to the sample parameter to be measured to obtain a variation of the sample parameter to be measured.

2. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 1, characterized in that: The main interferometer comprises a first optical coupler (2), a first optical fiber circulator (3), a second optical fiber circulator (5), and a second optical coupler (6); the process of collecting the optical signal corresponding to the sample parameter to be measured in the data acquisition module comprises: the light emitted by the tunable laser (1) is divided into two parts after passing through the first optical coupler (2), one part of the light passes through the first optical fiber circulator (3) and enters the reference arm, that is, enters the auxiliary interferometer, and the other part of the light passes through the second optical fiber circulator (5) and enters the sample parameter detection device to be measured (11); the optical signal reflected by the sample parameter detection device to be measured (11) passes through the second optical fiber circulator (5) and interferes with the reference light signal of the reference arm of the main interferometer at the second optical coupler (6) to form main interference light, and the two reflection surfaces of the auxiliary interferometer form auxiliary interference light; the main interference light and the auxiliary interference light pass through the second optical coupler (6) and are split by the polarization beam splitter (7), and then are received by the balanced photoelectric detector (8) and enter the data acquisition card (9) for collection.

3. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 2, characterized in that: The data processing module (10) performs a first processing on the optical signal output by the data acquisition module to obtain the spectrum corresponding to the sample parameter to be measured, including: band-pass filtering the collected beat frequency signal to intercept the beat frequency information of the auxiliary interference light; extracting the instantaneous optical frequency sequence from the beat frequency information of the auxiliary interference light; arranging the instantaneous optical frequency sequence at equal intervals to obtain a linear optical frequency sequence; interpolating the collected beat frequency signal using the linear optical frequency sequence; performing FFT operation on the obtained interpolation data to obtain corrected spatial domain information; performing band-pass filtering and IFFT operation on the corrected spatial domain information to obtain time domain information; and extracting the envelope of the time domain information using Hilbert transform to obtain the spectrum corresponding to the sample parameter to be measured.

4. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 3, characterized in that: The data processing module (10) performs a second processing on the spectrum corresponding to the sample parameter to be measured to obtain the variation of the sample parameter to be measured, including: superimposing the spectrum corresponding to the sample parameter to be measured and the interference spectrum corresponding to the preset virtual FPI interferometer cavity, and extracting the composite spectrum envelope after superposition; comparing the composite spectrum envelope with a reference composite spectrum envelope to obtain the variation of the sample parameter to be measured; the reference composite spectrum envelope is the composite spectrum envelope after superposition of the spectrum corresponding to the sample parameter to be measured when it is unchanged and the interference spectrum of the preset virtual FPI interferometer cavity.

5. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 4, characterized in that: The interference spectrum corresponding to the preset virtual FPI interferometer cavity in the data processing module (10) is constructed as follows: Based on the virtual vernier effect, a virtual FPI interferometer cavity with a different cavity length from that of each FPI sensor is constructed. The calculation formula for the interference spectrum of the virtual FPI interferometer cavity is: ； Where I0 is the incident light intensity value; is the transmission coefficient of light in the virtual FPI interferometer cavity; r3 and r4 are the reflection coefficients of the two end faces of the virtual FPI interferometer cavity; n2 and L2 are the refractive index and cavity length of the virtual FPI interferometer cavity, respectively; Represents the wavelength in vacuum.

6. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 1, characterized in that: The sample parameters to be measured include temperature, salinity, pH value, and gas concentration.

7. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 5, characterized in that: The data processing module (10) performs a second processing on the spectrum corresponding to the sample parameter to be measured, further comprising: determining the amplification factor of the virtual cursor effect between the FPI sensor and the virtual FPI interferometer cavity according to the free spectrum range of the optical signal corresponding to the sample parameter to be measured and the free spectrum range corresponding to the preset virtual FPI interferometer cavity.

8. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 1, characterized in that: The multiple FPI sensors are connected in series or in parallel.

9. The quasi-distributed sensing system based on fiber-assisted interferometer and vernier sensitivity enhancement according to claim 2, characterized in that: The wavelength range of the laser light emitted by the tunable laser (1) is 1510 nm-1640 nm; the splitting ratio of the first optical coupler (2) is 3:7.

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