A Quasi-Distributed Sensing System Based on Fiber-Aided Interferometer and Vernier Sensitization

Abstract

This invention discloses a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization, relating to the field of fiber optic sensing technology. The system includes a data acquisition module, a sample parameter detection device, and a data processing module. The sample parameter detection device includes multiple FPI sensors. The data acquisition module is used to acquire the optical signal corresponding to the sample parameter, and 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, and the auxiliary interferometer includes a delay fiber of a preset length and two reflecting surfaces, and is mounted on the reference arm of the Mach-Zehnder interferometer. The data processing module performs first and second processing on the optical signal output from the data acquisition module to obtain the change in the sample parameter. This invention effectively overcomes the technical bottlenecks of traditional solutions, such as complex structure, difficult debugging, and low demodulation efficiency.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization. Background Technology

[0002] With the development of new-generation information technology and industrial intelligence, fiber optic sensors, due to their resistance to electromagnetic interference, small size, corrosion resistance, and long-distance transmission capabilities, are gradually becoming a key direction in advanced sensing technologies. Interferometric fiber optic sensors combined with frequency-modulated continuous wave (FMCW) technology, due to their high sensitivity, low noise, and multi-point decoupling capabilities, have become an important research direction in fiber optic sensing systems. 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 with time, and to resolve path length information through frequency differences. Optical frequency domain reflectometry (OFDR) is based on this principle. By analyzing the frequency domain characteristics of the reflected signal, it achieves spatial positioning and physical quantity measurement of multiple reflection points, possessing advantages such as 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 working principle involves an interference cavity formed between reflectors using optical fibers; changes in the interference spectrum are then monitored to invert the physical quantity. However, the sensitivity of FPI sensors is limited by the free spectral range (FSR) and cavity length, making it difficult to meet the high-precision demodulation requirements of weak signals.

[0004] To improve the sensitivity of interferometric fiber optic sensors, existing technologies have introduced the optical vernier effect. This involves constructing an interference structure with slightly different FSRs to generate periodic envelope modulation of its interference spectrum, thereby enhancing the response to minute wavelength shifts and improving sensitivity. However, this method suffers from problems such as complex structure, high manufacturing precision requirements, inconvenient adjustment, and difficulty in implementing distributed measurements. A solution is lacking that can achieve a high-sensitivity vernier enhancement effect while also simplifying the structure, providing flexible adjustment, and being suitable for distributed fiber optic sensing.

[0005] Currently, improving the sensitivity of FMCW interferometric systems, simplifying their structure, and enabling large-scale multi-parameter distributed measurements faces the following challenges: the physical vernier structure is complex and difficult to adjust, making it unscalable for 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 have poor real-time performance. Therefore, a new technology is urgently needed to leverage the frequency domain modulation advantages of FMCW systems, simulate auxiliary interferometry through mathematical algorithms, realize a virtual vernier effect, simplify the system structure, improve demodulation flexibility and sensitivity, and adapt to the future needs of distributed sensing and multi-parameter measurements. Summary of the Invention

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

[0007] A quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization includes a data acquisition module, a sample parameter detection device, and a data processing module. The sample parameter detection device is connected to the data acquisition module, and the data acquisition module is connected to the data processing module. The sample parameter detection device includes multiple FPI sensors.

[0008] The data acquisition module is used to acquire the optical signal corresponding to the parameter of the sample under test; 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, and the auxiliary interferometer includes a delay fiber of a preset length and two reflective surfaces, and the auxiliary interferometer is set on the reference arm of the Mach-Zehnder interferometer;

[0009] The data processing module is used to perform a first processing on the optical signal output by the data acquisition module to obtain the spectrum corresponding to the parameter of the sample under test; and to perform a second processing on the spectrum corresponding to the parameter of the sample under test to obtain the change in the parameter of the sample under test.

[0010] Further, the main interferometer includes a first optical coupler, a first fiber optic circulator, a second fiber optic circulator, and a second optical coupler; the process of acquiring the optical signal corresponding to the parameter of the sample under test in the data acquisition module includes: the light emitted by the tunable laser is split into two parts after passing through the first optical coupler; one part of the light passes through the first fiber optic circulator and enters the reference arm, i.e., enters the auxiliary interferometer; the other part of the light passes through the second fiber optic circulator and enters the parameter detection device of the sample under test; the optical signal reflected back from the parameter detection device of the sample under test interferes with the reference optical signal of the reference arm of the main interferometer at the second optical coupler to form the main interference light; the two reflecting surfaces of the auxiliary interferometer form the auxiliary interference light; the main interference light and the auxiliary interference light are split by a polarization beam splitter after passing through the second optical coupler, and then received by a balanced photodetector and entered into the data acquisition card for acquisition.

[0011] Further, 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 parameter of the sample under test, including: bandpass filtering the acquired beat frequency signal to extract 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 acquired beat frequency signal using the linear optical frequency sequence; performing FFT operation on the obtained interpolated data to obtain the corrected spatial domain information; performing bandpass 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 parameter of the sample under test.

[0012] Further, the data processing module performs a second processing on the spectrum corresponding to the parameter of the sample under test to obtain the change in the parameter of the sample under test, including: superimposing the spectrum corresponding to the parameter of the sample under test and the interference spectrum corresponding to the preset virtual FPI interference cavity, and extracting the superimposed composite spectral envelope; comparing the composite spectral envelope with the reference composite spectral envelope to obtain the change in the parameter of the sample under test; the reference composite spectral envelope is the composite spectral envelope obtained by superimposing the spectrum corresponding to the parameter of the sample under test when it has not changed and the interference spectrum of the preset virtual FPI interference cavity.

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

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

[0015]

[0016] In the formula, I0 is the incident light intensity; r1 is the transmission coefficient of light inside the virtual FPI interference cavity; r3 and r4 are the reflection coefficients of the two end faces of the virtual FPI interference cavity; n2 and L2 are the refractive index and cavity length of the virtual FPI interference cavity, respectively. This indicates the wavelength in a vacuum.

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

[0018] Furthermore, the second processing of the spectrum corresponding to the parameter of the sample under test in the data processing module also includes: 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 parameter of the sample under test 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 tunable laser emits a laser wavelength range of 1510nm-1640nm; the splitting ratio of the first optical coupler is 3:7.

[0021] The beneficial technical effects of this invention are:

[0022] To ensure stable system operation and address nonlinear issues during FMCW laser tuning, this invention proposes a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier-enhanced sensitivity. The structural design incorporates an auxiliary interferometer arm and a delay fiber segment, constructing a simplified Michelson interferometer structure. The auxiliary arm embeds a precisely-length delay fiber segment in the main path, and the optimized delay fiber length ensures that its beat frequency signal meets both the frequency uniformity requirements for resampling and is located in the early part of the total power spectrum. This serves as a stable reference for time-domain resampling and tuning nonlinearity compensation, further improving 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 for physically constructing an auxiliary cavity.

[0023] The system of this invention not only performs excellently in terms of sensing sensitivity, noise resistance and multi-point expansion capability, but also effectively overcomes the technical bottlenecks of traditional solutions such as complex structure, difficult debugging and low demodulation efficiency, providing a brand-new solution for high-sensitivity, multi-parameter, long-distance fiber optic sensing applications. Attached Figure Description

[0024] The above and other objects, features, and advantages of exemplary embodiments of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. Several embodiments of the invention are illustrated in the drawings by way of example and not limitation, wherein:

[0025] Figure 1 This is a schematic diagram of the structure of a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization as described in this 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 the original complex beat frequency signal and the spectrum after signal processing in an embodiment of the present invention;

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

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

[0030] Figure 6 This is an example of the superimposed interference spectrum envelope Ien-12 and its temperature response changes in this embodiment of the invention. Detailed Implementation

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

[0032] Traditional point-based fiber optic interferometers, while possessing high sensitivity, are difficult to reuse on a large scale, failing to meet the monitoring requirements of long distances and multiple nodes. Existing quasi-distributed sensing schemes, although capable of multi-point coverage, suffer from limited spectral changes under weak perturbations, resulting in low sensitivity and difficulty in achieving high-precision measurements of multiple parameters. Furthermore, existing systems typically rely on multiple independent demodulation channels, leading to complex structures, lengthy signal processing chains, and the nonlinear tuning characteristics of tunable lasers making the system prone to errors during beat frequency demodulation, affecting decoupling accuracy and system stability. To address these issues, this invention proposes a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier-enhanced sensitivity. By combining an optical vernier enhancement mechanism with interferometer compensation technology, it overcomes the technical bottlenecks of insufficient sensing sensitivity, complex structure, and difficulty in compensating for system nonlinear errors in existing quasi-distributed fiber optic sensing systems, significantly improving the system's sensitivity, structure, and scalability.

[0033] This invention introduces an auxiliary interferometer into the reference arm of the main interferometer. The designed auxiliary interferometer is a Michelson interferometer, consisting of two reflecting surfaces and a delay fiber (AOF). An optical path difference exists between the two reflecting surfaces, causing interference when they meet. This auxiliary interferometer generates a stable and structurally controllable beat frequency signal, serving as a nonlinear compensation reference signal. This ensures accurate extraction of interferometric cavity reflection information even with non-ideal sweep frequency characteristics of the tunable laser source. Subsequently, the instantaneous frequency trajectory of the auxiliary interferometric signal is extracted using Hilbert transform and phase expansion methods. Then, frequency domain reconstruction and dynamic resampling are performed on the main interferometer spectrum to achieve linearization compensation of 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" that is similar to but slightly different from the FPI optical structure to form a virtual vernier structure. By utilizing its high sensitivity to small 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. This effectively achieves vernier sensitization and can be applied to multi-parameter sensing measurements.

[0034] This invention proposes a quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization. The system includes a data acquisition module, a sample parameter detection device 11, and a data processing module 10. The sample parameter detection device 11 is connected to the data acquisition module, and the data acquisition module is connected to the data processing module 10. The sample parameter detection device 11 includes multiple FPI sensors.

[0035] The data acquisition module is used to acquire the optical signal corresponding to the parameter of the sample to be tested; 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, and the auxiliary interferometer includes a delay fiber 4 of a preset length and two strong reflective surfaces, and the auxiliary interferometer is set on the reference arm of the Mach-Zehnder interferometer;

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

[0037] The main interferometer includes a first optical coupler 2, a first fiber optic circulator 3, a second fiber optic circulator 5, and a second optical coupler 6. The process of acquiring the optical signal corresponding to the parameter of the sample under test in the data acquisition module includes: the light emitted by the tunable laser 1 is split into two parts after passing through the first optical coupler 2; one part of the light passes through the first fiber optic circulator 3 and enters the reference arm, i.e., the auxiliary interferometer; the other part of the light passes through the second fiber optic circulator 5 and enters the parameter detection device 11; the optical signal reflected back from the parameter detection device 11 passes through the second fiber optic circulator 5 and interferes with the reference optical signal of the reference arm of the main interferometer at the second optical coupler 6 to form the main interference light; the two strong reflective surfaces of the auxiliary interferometer form the 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 then received by the balanced photodetector 8 and entered into the data acquisition card 9 for acquisition.

[0038] 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 parameter of the sample under test, including: bandpass filtering the acquired beat frequency signal to extract 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 acquired beat frequency signal using the linear optical frequency sequence; performing FFT operation on the obtained interpolated data to obtain the corrected spatial domain information; performing bandpass 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 parameter of the sample under test.

[0039] The data processing module 10 performs a second processing on the spectrum corresponding to the parameter of the sample under test to obtain the change in the parameter of the sample under test, including: superimposing the spectrum corresponding to the parameter of the sample under test and the interference spectrum corresponding to the preset virtual FPI interference cavity, and extracting the superimposed composite spectral envelope; comparing the composite spectral envelope with the reference composite spectral envelope to obtain the change in the parameter of the sample under test; the reference composite spectral envelope is the composite spectral envelope obtained by superimposing the spectrum corresponding to the parameter of the sample under test when it has not changed and the interference spectrum of the preset virtual FPI interference cavity.

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

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

[0042]

[0043] In the formula, I0 is the incident light intensity; r1 is the transmission coefficient of light inside the virtual FPI interference cavity; r3 and r4 are the reflection coefficients of the two end faces of the virtual FPI interference cavity; n2 and L2 are the refractive index and cavity length of the virtual FPI interference cavity, respectively. This indicates the wavelength in a vacuum.

[0044] The second processing of the spectrum corresponding to the parameter of the sample under test in the data processing module 10 further includes: 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 parameter of the sample under test and the free spectral range corresponding to the preset virtual FPI interferometer cavity.

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

[0046] Compared with traditional OFDR sensing systems, the system structure of this invention is greatly simplified, resulting in lower cost and easier integration. The system design of this invention is described in detail below.

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

[0048] The second part is the sensing region structure, which is a Mach-Zehnder interferometer (MZI), i.e., the master 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 fiber optic couplers and circulators. The light output from the MZI sensing arm passes sequentially through multiple sensing units (FPIs) composed of multiple series structures. Each interferometer cavity in the multiple FPIs can correspond to the sensing function of a physical or chemical parameter. That is, each sensor open cavity can be filled with a sensitive material that responds to different physical parameters. Since the time delay τ of the beam reaching each FPI cavity is different, each reflecting end face will generate a sweep frequency signal with a specific frequency difference.

[0049] The third part is the data acquisition area. The echo signal light reflected back from each FPI sensor in the sensing area returns through the port of the fiber optic circulator and interferes with the reference light 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 is then split into p-polarized light and s-polarized light by the polarization beam splitter 7 (PBS) after passing through the coupler, and is received by the balanced photodetector 8 (BPD) and acquired by the data card (DAQ) respectively.

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

[0051] In the sensing system, the incident light emitted by the tunable laser 1 is split into two paths by a coupler. One part of the light enters an auxiliary interferometer, namely a Michelson interferometer, with two reflecting surfaces PR1 and PR2. Due to the optical path difference between PR1 and PR2, interference occurs when they meet, thus forming a beat frequency 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, and γ is the modulation rate of the light source. This is represented as a nonlinear phase noise term, where E0 is the amplitude of the swept-frequency photoelectric field, and r is the reflection coefficient of PR1. This indicates the splitting ratio of the reference arm.

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

[0055] (2)

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

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

[0058] (3)

[0059] As can be seen from formula (3), the frequency of the auxiliary beat frequency signal is... , where n is the effective refractive index of the optical fiber. This beat frequency information will be 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 sensing arm. The tunable laser 1 emits a swept beam that splits into a reference beam and a sensing beam, which propagate within the reference arm and sensing arm, respectively. The optical fields of the reference beam and the sensing beam can be represented as follows:

[0061] (4)

[0062] (5)

[0063] Where R(τ) Z τ is the reflection coefficient at point Z on the sensing arm. Z Let Z be the time delay between the reference arm and the Z-point. Considering that the reflection point of the sensing arm consists of a series of reflection points of a cascaded FPI array, the beat frequency signal after the reference light and the reflected light meet and interfere is:

[0064] (6)

[0065] Where, τ Zm and R(τ) Zm ) represent the time delay and reflection coefficient of the m-th reflection point, respectively. As can be seen from formula (6), there is a linear correspondence between the beat frequency signal frequency and the time delay between the reference light and the sensing arm.

[0066] Since the optical signal acquired by data acquisition card 9 is the original signal formed by the superposition of multiple FPI interference signals and auxiliary beat frequency signals, the interference spectrum is relatively complex. In order to obtain the interference spectrum envelope of multiple FPIs, the original signal is processed in data processing module 10. As an example, the original signal acquired by data acquisition card 9 is uploaded to the computing terminal, and the complex beat frequency signal is processed in Matlab software. That is, the light intensity spectrum analysis of the optical signal acquired by data acquisition card 9 includes: firstly, bandpass filtering is performed on the acquired original signal to extract the beat frequency information of the auxiliary interferometer. Since the auxiliary interferometer and multiple FPI sensors share the same data channel in this invention, its reflected echo and the beat frequency signal of the main sensor are acquired simultaneously in the time domain. According to the structural parameters of the auxiliary interferometer, its optical path delay is much smaller than that of each FPI sensor, so that its beat frequency signal frequency is the smallest and located in the frequency region at the forefront of the total power spectrum, which is easy to separate and identify. Then, instantaneous optical frequency sequences are extracted from the beat frequency information of the auxiliary interferometer. These sequences are then arranged at equal intervals to obtain a linear optical frequency sequence. This linear optical frequency sequence is used to interpolate the acquired original signal, achieving nonlinear frequency sweep compensation of the light source. Next, an FFT operation is performed on the acquired interpolated data to obtain the corrected spatial domain information. Then, bandpass filtering and an IFFT are applied to the corrected spatial domain information, i.e., the reflection peak of the FPI microcavity, to obtain its wavelength domain (time domain) information. The Hilbert transform is then used to extract the envelope of the wavelength domain information to obtain the interference spectrum of the FPI microcavity.

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

[0068] (7)

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

[0070] The virtual vernier effect refers to simulating the response characteristics of an equivalent interference cavity using digital signal processing technology. This simulation is then algorithmically superimposed or fitted to the measured interference spectrum, allowing for the simulation of the vernier interference effect without the need to construct a physical auxiliary cavity, thus enhancing the ability to perceive subtle changes in physical parameters. Therefore, this invention utilizes prior design parameters to construct a virtual FPI (VFPI) with an optical structure similar to an FPI but slightly different in its free spectral range (FSR) and periodicity. The VFPI design is based on the demodulated FPI structure in the system, but key optical parameters (such as cavity length, and possibly refractive index) are fine-tuned to ensure that its periodicity is not perfectly consistent with the FPI's FSR. The VFPI design parameters can be obtained through simulation to ensure that it achieves an optical response similar to the FPI in practical applications, while possessing unique periodic characteristics to adapt to specific measurement needs. This virtual FPI can be generated using a sine function, and the interference envelope spectrum of the VFPI cavity reflected light is shown in the figure. 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 light inside the VFPI cavity of the interferometer, r3 and r4 are the reflection coefficients of the two end faces of the VFPI cavity of the interferometer, 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 the envelope spectra of FPI1 and the virtual FPI are close but not equal, multiplying or weighting the interferometric spectra of the two interferometers in the signal domain will produce a virtual vernier effect between interferometers FPI1 and VFPI, constructing an interferometric envelope I with vernier effect characteristics. en The resulting composite spectrum is as follows:

[0074] (9)

[0075] Among them, I en M represents the amplitude of the envelope of the composite interference spectrum. en The amplification factor of the vernier effect between interferometers FPI1 and VFPI, i.e., the temperature sensitivity of the interference spectrum envelope, is M of a single interferometer FPI1. en times.

[0076] Taking temperature as an example, when the temperature of the sample to be tested increases from T0 to T... 0+1 At that time, by collecting the corresponding temperature T again 0+1The system receives and processes real-time signals, demodulates the FPI interferometric spectrum after temperature changes, and superimposes it with the VFPI interferometric envelope spectrum. Comparing the superimposed result with the superimposed envelope spectrum at temperature T0 reveals a significant drift in the superimposed interferometric envelope. This drift can serve as a sensitive indicator of temperature changes, thus achieving high-sensitivity temperature detection. The virtual vernier effect effectively amplifies the weak response of a single sensing cavity, improving the detection capability for minute parameter perturbations.

[0077] It should be noted that although the above embodiments only use temperature measurement as an example, the present invention can be extended to multi-point, multi-parameter measurement. By filling or coating different sensitive materials in different cavities, and based on the phenomenon of changes in refractive index or cavity length, the spectrum can be calibrated through a comparison experiment of two interference envelope spectra shifts, and the shift amount can be extracted to solve for different responses to physicochemical parameters. The system achieves high-precision and high-sensitivity identification of physical parameters by analyzing the interference envelope drift. If multiple FPI sensor arrays are constructed and functional sensitive materials are integrated into the interference cavity structure through filling or surface coating processes, simultaneous measurement of multiple physicochemical parameters (such as temperature, salinity, pH, gas concentration, etc.) can be achieved.

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

[0079] Figure 3 This shows the original signal and its processed spectrum after complex beat frequency analysis. The original signal is as follows: Figure 3 As shown in (a), data processing is performed using MATLAB. First, the original signal undergoes a spatial domain transformation (such as...). Figure 3 In (b) of the paper, it was found that the original signal had severe spectral broadening. A bandpass filter was then applied to select the desired frequency band. Instantaneous photofrequency was extracted using an auxiliary beat frequency signal. The original signal was resampled using an equally spaced instantaneous photofrequency sequence to obtain calibration data. Spatial domain transformation was performed on the calibration data. Bandpass filtering and IFFT were then applied to the reflection peak of the FPI microcavity to obtain its wavelength domain (time domain) information, such as... Figure 3As shown in (c)-(e) in the figure, the interference spectrum of the FPI microcavity can be obtained by extracting the envelope of the wavelength domain information, as shown in the figure. Figure 3 As shown in (f), a sinusoidal FPI virtual cavity interferometer spectrum is constructed. The free spectral range of the virtual interferometer is very close to but not equal to that of the demodulated interferometer cavity, and can be cascaded to generate a virtual vernier effect, such as... Figures 4-6 As shown.

[0080] In summary, this invention solves the core technical challenges of insufficient sensitivity and complex structure in existing quasi-distributed fiber optic sensing systems. The proposed distributed 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 onto the reference arm of the main interferometer, greatly simplifying the system structure. Furthermore, it introduces a virtual vernier effect to amplify the drift response of the sensing interferometric spectrum, thereby significantly improving the sensing unit's ability to resolve weak parameter disturbances. This invention simplifies the sensing system structure, reduces manufacturing costs, and improves algorithm flexibility and system adjustability, making it particularly suitable for high-density, long-distance, and multi-parameter distributed fiber optic sensing networks. Especially in high-density sensing networks, it enables controllable sensitivity adjustment and multi-point collaborative enhancement detection, significantly reducing system demodulation complexity and engineering implementation costs while ensuring measurement accuracy.

[0081] This invention is applicable to large-scale, long-distance, and high-precision fiber optic sensing applications, and has broad prospects for promotion and engineering application value, especially in marine environmental monitoring, multi-parameter water quality detection, oil and gas resource transportation, bridge structural health monitoring, and smart city infrastructure sensing.

[0082] While the spirit and principles of the invention have been described with reference to several specific embodiments, it should be understood that the invention is not limited to the disclosed specific embodiments, and the division of aspects does not imply that features in these aspects cannot be combined for benefit; such division is merely for ease of description. The 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 a fiber-assisted interferometer and vernier sensitization, characterized in that, It includes a data acquisition module, a sample parameter detection device (11), and a data processing module (10). The sample parameter detection device (11) is connected to the data acquisition module, and the data acquisition module is connected to the data processing module (10). The sample parameter detection device (11) includes multiple FPI sensors. The data acquisition module is used to acquire the optical signal corresponding to the parameter of the sample to be tested; 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 fiber (4) of a preset length and two reflective surfaces, and the auxiliary interferometer is set on the reference arm of the Mach-Zehnder interferometer; The data processing module (10) is used to perform a first processing on the optical signal output by the data acquisition module to obtain the spectrum corresponding to the parameter of the sample under test, including: bandpass filtering the acquired beat frequency signal to extract 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 acquired beat frequency signal using the linear optical frequency sequence; performing FFT operation on the obtained interpolated data to obtain the corrected spatial domain information; performing bandpass filtering and IFFT operation on the corrected spatial domain information to obtain time domain information; and extracting time domain information using Hilbert transform. The envelope of the information is obtained by acquiring the spectrum corresponding to the parameter of the sample under test. The spectrum corresponding to the parameter of the sample under test is then subjected to a second processing to obtain the change in the parameter of the sample under test. This includes: superimposing the spectrum corresponding to the parameter of the sample under test with the interference spectrum corresponding to a preset virtual FPI interferometer cavity, and extracting the superimposed composite spectral envelope; comparing the composite spectral envelope with a reference composite spectral envelope to obtain the change in the parameter of the sample under test; the reference composite spectral envelope is the composite spectral envelope obtained by superimposing the spectrum corresponding to the parameter of the sample under test when it has not changed with the interference spectrum of the preset virtual FPI interferometer cavity; wherein, the interference spectrum corresponding to the preset virtual FPI interferometer cavity is constructed as follows: Based on the virtual vernier effect, a virtual FPI interference cavity with a cavity length different from that of each FPI sensor is constructed. The formula for calculating the interference spectrum of the virtual FPI interference cavity is as follows: ； In the formula, I 0 This represents the incident light intensity. The transmission coefficient of light within the virtual FPI interference cavity; r 3 , r 4 The reflection coefficients are the two end faces of the virtual FPI interference cavity; n 2 , L 2 These represent the refractive index and cavity length of the virtual FPI interference cavity, respectively. This represents the wavelength in a vacuum.

2. The quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization as described in claim 1, characterized in that, The main interferometer includes a first optical coupler (2), a first fiber optic circulator (3), a second fiber optic circulator (5), and a second optical coupler (6). The process of acquiring the optical signal corresponding to the parameter of the sample under test in the data acquisition module includes: the light emitted by the tunable laser (1) is split into two parts after passing through the first optical coupler (2). One part of the light passes through the first fiber optic circulator (3) and enters the reference arm, i.e., the auxiliary interferometer. The other part of the light passes through the second fiber optic circulator (5) and enters the parameter detection device (11) of the sample under test. The optical signal reflected back from the parameter detection device (11) of the sample under test interferes with the reference optical signal of the reference arm of the main interferometer at the second optical coupler (6) after passing through the second fiber optic circulator (5) to form the main interference light. The two reflective surfaces of the auxiliary interferometer form the 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 then received by the balanced photodetector (8) and entered into the data acquisition card (9) for acquisition.

3. The quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization as described in claim 1, characterized in that, The parameters of the sample to be tested include temperature, salinity, pH value, and gas concentration.

4. The quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization as described in claim 1, characterized in that, The data processing module (10) further includes performing a second processing on the spectrum corresponding to the parameter of the sample under test: determining the amplification factor of the virtual vernier effect between the FPI sensor and the virtual FPI interference cavity based on the free spectral range of the optical signal corresponding to the parameter of the sample under test and the free spectral range corresponding to the preset virtual FPI interference cavity.

5. A quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization as described in claim 1, characterized in that, The multiple FPI sensors are connected in series or in parallel.

6. A quasi-distributed sensing system based on a fiber-assisted interferometer and vernier sensitization as described in claim 2, characterized in that, The tunable laser (1) emits a laser wavelength range of 1510nm-1640nm; the first optical coupler (2) has a splitting ratio of 3:7.

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