Dual-mode resonance optical fiber microcavity air pressure measuring system and measuring method

Abstract

The invention provides a dual-mode resonance optical fiber microcavity air pressure measurement system and method, and relates to the technical field of optical fiber microcavity air pressure measurement, and the system specifically comprises a sensing probe preparation module, a two-parameter sensitivity matrix calibration module, an optical fiber laser sensing system construction module, a real-time change monitoring module and a decoupling and output module. Femtosecond laser etching is combined with high-temperature tapering, an open type microcavity is manufactured on a single-mode optical fiber, a micro-nano cone area is formed, and the sensing probe with two resonance valleys with different temperature and air pressure response characteristics is obtained. A sensitivity coefficient matrix is obtained through calibration, a probe serves as a filter to be embedded into an annular optical fiber laser cavity, and an optical fiber laser with the laser wavelength locked by a resonance valley is constructed. Laser wavelength drift is caused by environmental change, real-time synchronous measurement of temperature and air pressure is realized by utilizing inverse matrix decoupling, and the method is suitable for high-precision and anti-interference monitoring of harsh industrial environments such as aero-engines and power transformers.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber microcavity gas pressure measurement, in particular to a dual-mode resonance optical fiber microcavity gas pressure measurement system and a measurement method. BACKGROUND

[0002] Real-time state monitoring of high-end equipment such as aircraft engines and large power transformers is crucial for their safe and stable operation. Taking an aircraft engine as an example, the dynamic and accurate measurement of the internal gas pressure of the compressor section is the key to evaluating the performance of the engine and preventing surges. In power transformers, the pressure and temperature changes of the dissolved gas in the oil are the core basis for diagnosing internal latent faults. Although traditional electrical gas pressure sensors (such as piezoresistive and capacitive) are widely used, they often face challenges such as electromagnetic interference, flammable risk, and insufficient long-term stability in the above-mentioned scenarios. In recent years, optical fiber sensing technology has provided a new approach to physical quantity measurement in harsh environments due to its inherent safety, electromagnetic interference resistance, and corrosion resistance. Among them, microstructure-based optical fiber sensors have become a research hotspot due to their high sensitivity and small size characteristics.

[0003] Currently, optical fiber sensing schemes for gas pressure can be mainly divided into fiber Bragg grating type, Fabry-Perot interference type, and Mach-Zehnder interference type. The sensitivity of fiber Bragg gratings to gas pressure is usually low, and their response is severely crossed with temperature, requiring complex reference gratings or special packaging for compensation, which is structurally complex and increases uncertainty. Although intrinsic or extrinsic Fabry-Perot cavity gas pressure sensors have high sensitivity, their manufacturing process is precise and expensive, and the absolute demodulation of the cavity length is easily disturbed by temperature, limiting the demodulation accuracy in wide temperature and variable pressure dynamic scenarios. Optical fiber sensors based on Mach-Zehnder interference principle are sensitive to changes in environmental refractive index (related to gas density and pressure) by designing special structures (such as taper, misalignment, microcavity, etc.) to excite cladding mode and core mode interference, exhibiting the advantages of relatively simple manufacturing and designable sensitivity. However, most existing single-mode interference structure optical fiber gas pressure sensors have a wide spectrum modulation or a single resonance valley, which is not only sensitive to temperature and difficult to distinguish, but also requires expensive high-resolution spectrum analyzers for wideband spectrum monitoring, making it difficult to achieve low-cost, high-resolution real-time dynamic measurement. Directly connecting such sensors to a laser cavity and using their narrowband filtering characteristics to convert spectral shifts into laser wavelength jumps is an effective way to improve resolution and measurement speed, but how to simultaneously achieve high sensitivity response to gas pressure and effective differentiation between temperature and gas pressure in a miniaturized and integrated sensing probe, while ensuring stable integration with the laser system, remains a significant bottleneck for the practical engineering application of this technology. SUMMARY

[0004] The purpose of this invention is to provide a dual-mode resonant fiber optic microcavity pressure measurement system and method to solve the problems mentioned in the background art.

[0005] A dual-mode resonant fiber optic microcavity barometric pressure measurement system, comprising: The sensor probe fabrication module is used to form an open microcavity penetrating the cladding and partially cutting into the core of a standard single-mode fiber by femtosecond laser etching. The open microcavity is then subjected to high-temperature tapering to form a micro / nano tapered region, thereby obtaining an open microtapered fiber-internal microcavity type Mach-Zehnder interferometer sensor probe. The interference spectrum of this sensor probe has at least two resonance valleys in the C+L band. The dual-parameter sensitivity matrix calibration module is used to place the sensor probe in a calibration chamber with independently controlled temperature and pressure, record its output interference spectrum under broadband light source illumination, select two resonance valleys with different response characteristics to temperature and pressure, monitor the drift of their center wavelength as the temperature and pressure of the calibration chamber change, and calibrate the sensitivity coefficient matrix of the sensor probe by linear fitting. The fiber laser sensing system construction module is used to connect the sensing probe as a wavelength selective filter into a prefabricated ring fiber laser resonant cavity. By utilizing its filtering characteristics to compete with the laser mode, a fiber laser whose output laser wavelength is determined by the position of the resonant valley is constructed. The real-time monitoring module is used to deploy a fiber laser system with integrated sensing probes at a calibrated monitoring point. When the ambient temperature and air pressure change, the interference spectrum of the sensing probes drifts, causing the output laser wavelength of the fiber laser to jump or drift continuously. The wavelength change is monitored and quantified in real time by a high-resolution optical wavelength meter. The decoupling and output module is used to substitute the wavelength change into the inverse matrix of the sensitivity coefficient matrix for calculation, and to decouple the temperature change and air pressure change of the monitoring point in real time.

[0006] Furthermore, the radial width range of the open microcavity is defined as follows: axial length range is And the etching depth is not less than the sum of the standard single-mode fiber cladding diameter and the depth cut into the fiber core, wherein the depth cut into the fiber core ranges from [value missing]. The open microcavity is clad and cut into the fiber core to achieve direct interaction between the fiber core's light guiding mode and the internal and external environment of the open microcavity. The micro / nano cone region formed by the high-temperature tapering has a cone waist diameter ranging from [value missing]. The range of values ​​for the cone length is: The micro-nano taper region is located at the axial center position of the open microcavity, used for enhancing the coupling efficiency of the core fundamental mode and the cladding mode, and making the resonance valley depth of the interference spectrum greater than 10 dB, so as to ensure that at least two resonance valleys have contrast and detectability; The interference spectrum generated by the sensing probe is formed by interference of the core fundamental mode and the excited multi-order cladding modes; among two resonance valleys with different temperature and gas pressure response characteristics, one is a main resonance valley formed by interference of the fundamental mode and the low-order cladding mode, and the other is a secondary resonance valley formed by interference of the fundamental mode and the high-order cladding mode; the wavelength interval of the main resonance valley and the secondary resonance valley falls within the interval [20 nm, 80 nm].

[0007] Further, the calibration of the sensitivity coefficient matrix specifically includes: In a calibration cavity with temperature and gas pressure control accuracy better than ±0.1℃ and ±0.01MPa, the temperature is changed at a fixed temperature step within the temperature measurement range to obtain a group of temperature calibration points as a temperature variation sequence; the gas pressure is changed at a fixed gas pressure step within the gas pressure measurement range to obtain a group of gas pressure calibration points as a pressure variation sequence; each temperature calibration point in the temperature variation sequence is combined with each gas pressure calibration point in the pressure variation sequence to obtain a plurality of calibration point groups; Under the state of each calibration point group, a broadband light source is used to irradiate the sensing probe, and a transmission interference spectrum output by the sensing probe is collected by a spectrum analyzer, and the center wavelengths of the preselected main resonance valley and the secondary resonance valley in the spectrum are recorded; the main resonance valley is calibrated as the i-th order resonance valley, and the secondary resonance valley is calibrated as the j-th order resonance valley, which are hereinafter referred to as the main valley and the secondary valley, respectively; A preset standard room temperature of 25℃ and a standard atmospheric pressure of 101.325Kpa are taken as a reference, and the wavelength drifts of the two resonance valleys of each calibration point group relative to the reference state are calculated, and the formula is as follows: ; Wherein, represents the wavelength drift of the main valley under the state of the q-th calibration point group; represents the center wavelength of the main valley under the state of the q-th calibration point group; represents the center wavelength of the main valley under the reference state, and q is the index of the calibration point group; Similarly, the wavelength drift of the secondary valley under the state of the q-th calibration point group is obtained, denoted as ; At the same time, the corresponding temperature change and gas pressure change are recorded: ; Wherein, respectively represent the temperature change and the gas pressure change of the q-th calibration point group; , respectively represent the temperature and the gas pressure of the q calibration point groups. , respectively represent the temperature and air pressure in the reference state; For all sets of calibration points, the sensitivity coefficient matrix is directly determined by solving the least square solution of the overdetermined equation group, which is as follows: ; ; wherein, , are the sensitivity coefficients of the main valley and the secondary valley to the temperature respectively; , are the sensitivity coefficients of the main valley and the secondary valley to the air pressure respectively; thus the sensitivity coefficient matrix is represented as ; The determinant value of the sensitivity coefficient matrix is calculated, and the absolute value of the determinant value is verified to be greater than a preset threshold value, so as to ensure that the sensitivity coefficient matrix is reversible and can realize effective temperature and air pressure distinguishing measurement; subsequently, the sensitivity coefficient matrix and its inverse matrix are stored in the demodulation unit of the fiber laser.

[0008] Further, the specific composition and connection relationship of the ring-shaped fiber laser resonant cavity are as follows: The output tail fiber of the pump source is connected with the pump port of the wavelength division multiplexer; the common port of the wavelength division multiplexer is connected with one end of a section of erbium-doped fiber as a gain medium; the other end of the erbium-doped fiber is connected with the input end of the optical isolator; the output end of the optical isolator is connected with the input end of the sensing probe; the output end of the sensing probe is connected with the signal port of the wavelength division multiplexer, thereby forming a closed ring-shaped fiber laser resonant cavity; The process of constructing the fiber laser whose output laser wavelength is determined by the resonant valley position is as follows: The minimum value of the transmission spectrum of the sensing probe at the resonant valley is extracted, which is used as a wavelength selection filter to meet the threshold condition of laser oscillation, i.e. wherein, is the net gain of the erbium-doped fiber at the wavelength ; represents the transmission spectrum of the sensing probe; is the total transmission loss coefficient of all elements outside the sensing probe in the ring-shaped fiber laser resonant cavity; after the pump power exceeds the threshold value, the mode competition in the cavity limits the finally oscillated laser wavelength to the position with the relatively highest loss in the transmission spectrum of the sensing probe, i.e. the resonant valley, i.e. , thereby realizing that the output laser wavelength is determined by the resonant valley position of the sensing probe; To improve the resolution of the gas pressure sensing, the fiber laser is made to work in single longitudinal mode or few longitudinal modes by optimizing the parameters of the ring fiber laser cavity and using mode competition, which specifically satisfies the following conditions: The total cavity length of the ring fiber laser cavity is limited in the range of [10m, 50m], and the corresponding longitudinal mode interval is greater than the corresponding value of the 3dB bandwidth of the resonant valley in the transmission spectrum of the sensing probe as a filter window; first calculate the product of the effective refractive index of the optical fiber and the physical cavity length of the ring fiber laser cavity, and the ratio of the speed of light in vacuum to the product is the longitudinal mode interval; under this condition, only one or several longitudinal modes fall within the resonant valley filter window, and a narrow linewidth output is finally formed through mode competition, the linewidth of which is less than 1 / 10 of the corresponding value of the 3dB bandwidth of the resonant valley in the frequency domain, and the linewidth is less than 10MHz.

[0009] Further, the fiber laser is fixed at the monitoring point; the calibrated monitoring point includes but is not limited to the pressure measuring hole of the compressor section casing of an aero-engine; the sensing probe is fixed in the pressure measuring hole, and the open microcavity is directly exposed to the gas pressure environment of the engine flow channel; and the gas chamber sampling cavity of the on-line monitoring system of the dissolved gas in transformer oil; the sensing probe is sealed and installed inside the gas chamber for directly measuring the gas pressure of the outgassing gas; When the ambient temperature and gas pressure change, the spectral drift of the sensing probe will cause the output laser wavelength of the fiber laser to change in the following two modes: The first is a continuous tuning mode, when the environmental change causes the spectral drift of the sensing probe to be less than the interval of the adjacent longitudinal modes of the fiber laser, the output laser wavelength exhibits continuous wavelength tuning within a single resonant valley filter range; The second is a mode jump mode, when the environmental change causes the spectral drift of the sensing probe to be not less than the interval of the adjacent longitudinal modes of the laser, the output laser wavelength exhibits a jump between different longitudinal modes, and the new wavelength after the jump is still locked at the resonant valley after the drift; In the second mode, by recording the number of jumps of the laser wavelength and the interval of the longitudinal modes, and combining the effective cavity length and the group refractive index of the laser, the total drift of the resonant valley caused by the environmental change is roughly estimated, and the rough estimation formula is as follows: ; Wherein, represents the total drift of the resonant valley, N represents the number of forward or reverse jumps, represents the group refractive index of the fiber laser, represents the effective cavity length of the fiber laser.

[0010] Further, the wavelength change amount is quantified by using a static mapping method, which specifically includes: According to the center wavelengths of the main valley and the secondary valley measured in the reference state; in real-time monitoring, the absolute difference between the current output laser wavelength and the two center wavelengths measured in the reference state is calculated, denoted as and ; wherein, represents the current output laser wavelength, represents the center wavelength of the secondary valley in the reference state; If , it is determined that the current laser is locked in the main valley, and the wavelength variation of the main valley is directly calculated as ; wherein, represents the wavelength variation of the current main valley; At the same time, based on the linear response assumption, the wavelength variation of the secondary valley is preliminarily estimated as ; wherein, represents the wavelength variation of the current secondary valley; conversely, if it is determined that the laser is locked in the secondary valley, then is calculated, and is preliminarily estimated.

[0011] Further, when it is monitored that the output laser wavelength jumps in mode, or when the absolute value of the variation of the physical quantity calculated by the static mapping method in unit time is greater than a preset threshold, a dynamic iterative method is enabled for quantization, which includes the following steps: Set the iteration number to zero, and use the temperature and air pressure values calculated last time as the current estimated values, or use the reference state as the current estimated values if there is no historical calculation value; According to the current estimated values and the calibrated sensitivity coefficient matrix, the theoretical wavelength of the main valley is calculated, and the formula is as follows: ; wherein, represents the theoretical wavelength of the main valley; , respectively represent the temperature and air pressure in the current estimated values; similarly, the theoretical wavelength of the secondary valley is obtained, denoted as ; The real-time measured output laser wavelength is compared with the theoretical wavelengths of the main valley and the secondary valley respectively, and the one with the smallest difference is selected as the current locking valley, and the temporary wavelength variation is calculated as , where r represents the main valley or the secondary valley, and r is i or j; The temporary wavelength variation and the corresponding variation of the other valley not being the current locking valley form a vector, and the updated temperature and air pressure values are calculated using the inverse matrix of the sensitivity coefficient matrix; The absolute difference between the updated temperature and the current estimated temperature and the absolute difference between the updated air pressure and the current estimated air pressure are calculated, and if both absolute differences are less than a set threshold, the iteration is ended, and the temporary wavelength variation amount at the last iteration is output; otherwise, the updated temperature and air pressure values are taken as the current estimated values, and the iteration is continued.

[0012] Further, decoupling the temperature variation amount and the air pressure variation amount specifically comprises: Based on the wavelength variation amount of the two resonance valleys in real time, an input vector is formed, which is represented as ; The sensitivity coefficient matrix and its inverse matrix are called, and matrix multiplication operation is performed to obtain an output vector containing temperature and air pressure variation amount, and the formula is as follows: ; Wherein, The output vector containing temperature and air pressure variation amount is represented as , The temperature variation rate and the air pressure variation amount are represented as The inverse matrix of the sensitivity coefficient matrix is represented as Wherein, the definition and parameter meaning of the sensitivity coefficient matrix and the inverse matrix of the sensitivity coefficient matrix are as follows: ; ; Wherein, D is the determinant of the sensitivity coefficient matrix. The solved temperature variation amount and air pressure variation amount are added to the environmental values of the known monitoring points respectively to obtain real-time absolute air pressure value and real-time absolute temperature value.

[0013] The application further provides a dual-mode resonance optical fiber microcavity air pressure measurement method, which is executed by the dual-mode resonance optical fiber microcavity air pressure measurement system, and the specific steps comprise: Step 1: In the core and cladding area of a standard communication single-mode optical fiber, an open microcavity penetrating the cladding and partially cutting into the core is formed by femtosecond laser etching; the open microcavity is subjected to high-temperature tapering to form a micro-nano taper region, so as to obtain an open micro-tapered fiber microcavity type Mach-Zehnder interferometer sensing probe; the interference spectrum of the sensing probe has at least two resonance valleys in the C+L wave band; Step 2: The sensing probe is placed in a calibration cavity with independent temperature and air pressure control, and the output interference spectrum is recorded under the illumination of a wide-spectrum light source; two resonance valleys with different temperature and air pressure response characteristics are selected, the drift amount of the center wavelength with the change of the temperature and air pressure of the calibration cavity is monitored, and the sensitivity coefficient matrix of the sensing probe is calibrated through linear fitting. Step 3: The sensing probe is used as a wavelength selection filter and is connected to a pre-prepared ring-shaped optical fiber laser resonant cavity, the filter characteristic competes with the mode of the laser, and a fiber laser whose output wavelength is determined by the resonant valley position is constructed; Step 4: The fiber laser system integrated with the sensing probe is deployed at the calibrated monitoring point; when the ambient temperature and air pressure change, the interference spectrum of the sensing probe drifts, causing the output laser wavelength of the fiber laser to jump or continuously drift; the wavelength change is monitored and quantified in real time by a high-resolution optical wavelength meter; Step 5: The wavelength change is substituted into the inverse matrix of the sensitivity coefficient matrix for calculation, and the temperature change and air pressure change of the monitoring point are decoupled in real time.

[0014] In the above technical solution, the technical effects and advantages provided by the present application are as follows: The present application combines femtosecond laser and high-temperature taper to prepare an open micro-tapered fiber-in-cavity in a single-mode optical fiber, and the interference spectrum naturally forms at least two interference valleys with different resonant characteristics (sensitivity to temperature and air pressure) in the communication band. The probe is embedded as a filter in a ring-shaped fiber laser cavity to construct a sensing system whose laser wavelength is locked by the resonant valley. The system not only realizes high-resolution detection of air pressure changes by using the narrow line width characteristic of laser, but also constructs a reversible sensitivity matrix through the differential response of the double resonant valleys, and finally realizes real-time and synchronous differentiation measurement of temperature and air pressure dual parameters. The method is particularly suitable for harsh industrial scenes such as internal monitoring of aircraft engines, transformer oil and gas monitoring, etc. which have strong electromagnetic interference, require intrinsic safety and need multi-parameter synchronous perception. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The figure is a schematic diagram of the system structure of the present application; Figure 2 The figure is a schematic diagram of the relationship between the temperature change and the wavelength drift of the present application; Figure 3 The figure is a schematic diagram of the relationship between the air pressure change and the wavelength drift of the present application; Figure 4 The figure is a schematic diagram of the overall method flow of the present application. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical scheme and advantages of the present application clearer and more apparent, the present application is further described in detail below in combination with specific embodiments.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. As used in this application, the terms "first", "second", and the like, do not imply any order, quantity, or importance, but rather are used to distinguish one element from another. The terms "comprises", "comprising", "includes", "including" and the like can be used herein, and each means including but not limited to. The term "connected" and / or "coupled" can be used herein and each mean one or more of, but not limited to, mechanical, electrical, and / or magnetic connection and / or coupling between that which is connected and / or coupled together. The terms "upper" and "lower" are used only to represent relative positions, and when the absolute positions of the objects being described change, the relative positions can also change accordingly.

[0018] Embodiment: Please refer to Figures 1 to 3 The application provides a dual-mode resonant optical fiber microcavity gas pressure measurement system, comprising: A sensing probe preparation module is used to form an open microcavity penetrating the cladding and partially cutting into the core of a standard communication single-mode optical fiber through femtosecond laser etching in the core and cladding area of the optical fiber; the open microcavity is high-temperature tapered to form a micro-nano taper area, so as to obtain an open micro-tapered fiber-in-microcavity type Mach-Zehnder interferometer sensing probe; and the interference spectrum of the sensing probe has at least two resonant valleys in the C+L band.

[0019] In this embodiment, first, a standard communication single-mode optical fiber (for example, the core diameter is about 8.2 , the cladding diameter is about 125 , and the coating layer diameter is about 250 ) is taken, the coating layer at one end of the optical fiber is removed using an optical fiber stripper and alcohol cotton, and a flat end face is prepared using an optical fiber cutting knife. Then, the optical fiber is fixed on a high-precision three-dimensional translation stage.

[0020] A femtosecond laser microprocessing system (for example, the central wavelength is 1030 nm, the pulse width is less than 300 fs, and the repetition frequency is 1 MHz) is used to etch the side surface of the optical fiber. By accurately controlling the laser energy, scanning speed, scanning times, and light spot overlap rate and other parameters, a rectangular open microcavity is etched in the cladding and core area of the optical fiber.

[0021] The radial width range of the open microcavity is . The width needs to be significantly greater than the core diameter (about 8.2 ) of the optical fiber, so as to ensure that the microcavity can completely cover and expose the transverse section of the core and provide sufficient space for the interaction between the core mode and the external environment. If the width is too small (less than 15 ), the disturbance to the core mode field is insufficient, the excited cladding mode energy is weak, and the interference contrast is low; if the width is too large (greater than 30 ), the mechanical strength of the optical fiber will be seriously weakened, and the optical fiber is easy to break in subsequent operations.

[0022] The axial length is in the range of 0.5-2.5 mm. The length determines the length difference of the interference arms. The longer the length, the denser the interference fringes (the smaller the free spectral range), which is beneficial to obtaining more resonance valleys within a limited spectral bandwidth. However, too long a length will make the microcavity structure fragile and may introduce high-order dispersion. The range is an optimization between ensuring sufficient interference fringe contrast and structural robustness.

[0023] The etching depth must ensure that the laser etching completely penetrates the fiber cladding and partially cuts into the fiber core. The etching depth is not less than the sum of the standard single-mode fiber cladding diameter and the depth of cutting into the fiber core. The depth of cutting into the fiber core is in the range of 3-8 This is the key to realizing the direct interaction of the fiber core guided mode with the internal and external environment of the open microcavity. Too shallow depth (such as less than 3 ) weakly disturbs the fundamental mode (LP01 mode) propagating in the fiber core, and cannot effectively couple the energy of the mode to the cladding mode or the radiation mode; too deep depth (such as greater than 8 ) will excessively destroy the waveguide structure of the fiber core, resulting in a dramatic increase in transmission loss, and even cause the optical fiber to break at the microcavity. The preferred cutting depth is about 0.7 to 2 times the fiber core radius (about 4.1 ).

[0024] The open microcavity is etched in the whole cladding and cuts into the fiber core to realize the direct interaction of the fiber core guided mode with the internal and external environment of the open microcavity. After etching, high-pressure nitrogen gas is used to blow the inside and surrounding area of the microcavity to remove the debris and attachments generated by etching. Subsequently, the morphology of the microcavity is observed under a microscope to confirm that the boundary is clear, the depth is uniform, and there is no crack or damage visible to the naked eye. This pretreatment step is crucial to ensure the subsequent optical performance and structural stability.

[0025] The optical fiber segment etched with the open microcavity is placed above the heating source (for example, hydrogen-oxygen flame or CO2 laser) of the fiber tapering system, and the heating point is accurately aligned with the axial center position of the microcavity. Heating tapering is performed while applying appropriate axial tension.

[0026] The micro-nano taper region formed by high-temperature tapering has a waist diameter in the range of 0.5-2.5 mm. In the tapering process, as the fiber is tapered, the core and cladding shrink synchronously, and the light field in the original core will gradually expand to the entire taper waist region, forming a strongly constrained mode field. When the taper waist diameter is in this range, it can not only ensure the effective expansion of the light field to enhance the coupling with the environment, but also maintain low transmission loss and basic waveguide characteristics. When the diameter is less than 3 , the waveguide structure is close to cutoff, and the loss increases sharply; when the diameter is greater than 8 , the constraint and enhancement effect on the mode field are not obvious.

[0027] The length of the taper region is in the range of . The length determines the transition slope of the taper. If the transition is too steep (the length is too short), high-order modes and non-adiabatic coupling will be introduced, resulting in stray interference and additional loss; if the transition is gentle (the length is moderate), it can be approximately regarded as an adiabatic taper, with high mode conversion efficiency, which can maximize the coupling of energy from the core fundamental mode to the specific cladding mode excited by the microcavity disturbance, thereby significantly improving the contrast of the interference spectrum. The length range is an empirical optimization value to ensure adiabatic tapering while maintaining compactness.

[0028] Annealing treatment after tapering: After the tapering is completed, the optical fiber is kept in the heating area for a short time (for example, 1-3 seconds) for in-situ annealing, and then the heat source is slowly removed for natural cooling. This treatment aims to eliminate the thermal stress formed in the quartz glass during the tapering process, improve the long-term stability of the micro-nano taper region, and prevent deformation or fracture due to stress release in subsequent use. The micro-nano taper region is located at the axial center position of the open microcavity, used to enhance the coupling efficiency of the core fundamental mode and the cladding mode, and make the resonance valley depth of the interference spectrum greater than 10 dB, thereby ensuring that at least two resonance valleys have contrast and detectability.

[0029] The sensing probe prepared by the above steps essentially constitutes an MZI based on asymmetric double-arm interference. One arm is the propagation path of the undisturbed core fundamental mode, and the other arm is the path of the cladding mode that is coupled to the cladding (or the interface between the microcavity and the cladding) after being disturbed by the open microcavity and propagates in the external region of the microcavity. The two lights re-couple and interfere in the core downstream of the microcavity, forming periodic interference fringes (resonance valleys) in the transmission spectrum.

[0030] Conditions and depth of the resonance valley: the interference spectrum has at least two resonance valleys in the C+L band (1530nm-1625nm). The introduction of the micro-nano tapered region greatly improves the excitation efficiency of specific cladding modes by enhancing the interaction between the mode field and the microcavity. This embodiment defines the resonance valley depth greater than 10dB as the qualified standard. The determination of this threshold (1dB) is based on the signal-to-noise ratio requirement of the actual demodulation system. Generally, the background noise of a commercial optical spectrum analyzer (OSA) or demodulator in the C / L band is about -70dBm to -80dBm. A resonance valley with a depth greater than 10dB (i.e. transmittance less than -10dB) has a high enough contrast relative to the background light intensity, which can be stably and accurately detected and tracked to avoid misjudgment caused by noise.

[0031] Among the multiple resonance valleys, two valleys with significantly different temperature and gas pressure response characteristics are selected as sensing signal carriers. The main resonance valley (main valley) usually corresponds to the resonance valley formed by the interference of the core fundamental mode (LP01) and the low-order cladding mode (such as LP11, LP02, etc.). Since the effective refractive index of the low-order cladding mode is closer to the core mode, its mode field is more distributed inside the cladding, and it is more directly affected by the change of external gas refractive index (i.e. gas pressure), so it has ultra-high sensitivity to gas refractive index and gas pressure change. The secondary resonance valley (secondary valley) usually corresponds to the resonance valley formed by the interference of the core fundamental mode (LP01) and the high-order cladding mode (such as LP31, LP12, etc.). The mode field of the high-order cladding mode is more extended to the outer edge of the cladding, and even part of the energy is in a radiation state, and its propagation constant (effective refractive index) is more affected by the thermo-optic effect and thermal expansion effect of the quartz material itself, so it has a significantly different sensitivity to temperature change than the main valley.

[0032] The wavelength interval of the main valley and the secondary valley should fall within the interval [20nm, 80nm]. The setting of this range mainly considers two aspects of balance: one is that the interval should be large enough to be clearly distinguished on the spectrometer under broadband light source illumination, and the ordinary wavelength meter or demodulation module can be used for simultaneous or time-sharing monitoring; the other is that the interval should not be too large, so as not to make the two valleys located at the edge or outside of the erbium-doped fiber gain spectrum (C+L band), affecting the gain uniformity and starting efficiency when constructing the fiber laser later. For example, one at 1545nm (main valley) and one at 1570nm (secondary valley), with an interval of 25nm, which is easy to distinguish, and both are in the high gain region of the erbium-doped fiber.

[0033] The main valley and the secondary valley have different effective refractive indices due to the different orders of the interference modes, resulting in different dependence of the effective refractive index on temperature (mainly through the material's thermal-optic and thermal expansion coefficients) and external medium refractive index / gas pressure. This differential response characteristic is mathematically represented by the coefficients and and the matrix determinant is not zero. This is exactly the core physical basis for the subsequent implementation of the temperature and gas pressure dual-parameter distinguishing measurement, that is, the construction and solution of the inverse problem of the reversible sensitivity coefficient matrix.

[0034] For some harsh environment applications (such as the presence of dust, oil or mechanical vibration), a thin (such as less than 1 ) hydrophobic and oleophobic and optically transparent protective layer (for example, fluorinated polymer or silica sol-gel coating) can be coated on the microcavity and cone region of the sensing probe. The protective layer needs to ensure that it does not affect the free diffusion of gas molecules to the microcavity surface, while preventing the permanent attachment of contaminants and maintaining the reversibility and long-term stability of the sensing response. The coating process can use physical vapor deposition or spin coating, and the specific parameters (such as thickness) need to be optimized through experiments to balance the protection and sensing sensitivity.

[0035] The dual-parameter sensitivity matrix calibration module is used to place the sensing probe in a calibration chamber with independent temperature and pressure control, record the output interference spectrum under wide spectrum light source illumination, select two resonant valleys with different temperature and pressure response characteristics, monitor the drift of the center wavelength with the change of the temperature and pressure of the calibration chamber, and calibrate the sensitivity coefficient matrix of the sensing probe through linear fitting.

[0036] In this embodiment, in order to ensure the accuracy of the calibration results, a calibration device with high controllability and accurate measurement of environmental parameters is needed, and the calibration of the sensitivity coefficient matrix specifically includes: A sealed cavity with independent temperature control module and pressure control module is used. The temperature control accuracy should be better than ±0.1℃, and the pressure control accuracy should be better than ±0.01MPa. Inside the cavity, a high-precision reference thermometer and a pressure gauge calibrated by metrological calibration should be provided to provide real environmental parameter readings, and their accuracy should be at least comparable to or higher than the cavity control accuracy. It is composed of a wideband light source (such as an ASE light source), an optical spectrum analyzer (OSA), and a data recording computer. The sensing probe is connected to the system through an optical fiber jumper. The wavelength resolution of the OSA should be set high enough (for example, ≤0.02 nm) to ensure that the small wavelength drift of the resonant valley can be accurately distinguished.

[0037] In order to fully characterize the response of the probe in the expected working range, the temperature and pressure need to be systematically changed, and the corresponding spectral data need to be recorded. According to the expected application scenario of the probe, its temperature measurement range and pressure measurement range are determined.

[0038] In the temperature measurement range, the temperature is changed at a fixed temperature step (such as 5℃ or 10℃), and a set of temperature calibration points are obtained as the temperature sequence, such as , the temperature sequence includes m temperature calibration points. Within the pressure measurement range, the pressure is changed with a fixed pressure step (e.g. 20 kPa or 50 kPa), and a set of pressure calibration points are obtained as the pressure sequence, such as , the pressure sequence includes n pressure calibration points. The full combination of the temperature sequence and each point in the pressure sequence is performed, thereby generating m x n calibration point groups (e.g. ; a is the temperature calibration point index, and b is the pressure calibration point index). For example, if m = 6 and n = 5, there are a total of 30 calibration point groups. Using the full factorial experimental design, the response data of the probe under different temperature and pressure combinations can be fully obtained, and the calibration error introduced by parameter coupling can be avoided, thereby providing sufficient and uniformly distributed data samples for subsequent linear fitting.

[0039] In each calibration point group state, the cavity is stabilized at the specified temperature and pressure of the group, and maintained for a long enough time (e.g. 10-15 minutes) to ensure that the sensing probe and the cavity environment reach complete thermal and force balance. A broadband light source is used to irradiate the sensing probe, and the transmitted interference spectrum output by the probe is collected by a spectrometer. The center wavelengths of two preselected resonance valleys (the main resonance valley and the secondary resonance valley) are extracted from the spectrum. The original spectrum data can be preprocessed to improve accuracy. The preprocessing operations can include but are not limited to: first, performing a sliding average filter on the spectrum data to suppress optical noise; second, performing a polynomial fitting (e.g. cubic spline interpolation) on the local spectrum interval containing the target resonance valley; and finally, accurately determining the center wavelength by finding the zero crossing point of the first derivative of the fitted curve or directly finding the local minimum value. The main resonance valley is labeled as the i-th order resonance valley, and the secondary resonance valley is labeled as the j-th order resonance valley, which are hereinafter referred to as the main valley and the secondary valley, respectively. At the same time, the actual temperature and pressure read by the reference sensor in the cavity are recorded. i is the index of the main resonance valley, and j is the index of the secondary resonance valley.

[0040] To eliminate the initial manufacturing deviation of the probe, all measurements are calculated relative to a common reference state. The pre-set standard room temperature of 25°C and the standard atmospheric pressure of 101.325 kPa are used as the reference. Before or after the start of the calibration test, a Yangpu is collected under the reference state, and the reference center wavelengths of the main valley and the secondary valley are recorded and . For each calibration point group, the wavelength drift of the two resonance valleys relative to the reference state is calculated, and the formula is as follows: ; wherein, represents the wavelength drift of the main valley in the state of the q-th calibration point group; represents the center wavelength of the main valley in the state of the q-th calibration point group; λ0 represents the central wavelength of the primary valley in the reference state, q is the index of the calibration point group.

[0041] Similarly, the wavelength shift of the secondary valley in the state of the calibration point group q is obtained, and the formula is: ; wherein, represents the wavelength shift of the secondary valley in the state of the calibration point group q; represents the central wavelength of the secondary valley in the state of the calibration point group q; represents the central wavelength of the secondary valley in the reference state.

[0042] At the same time, the corresponding temperature change and air pressure change are recorded: ; wherein, respectively represent the temperature change and air pressure change of the qth calibration point group; , respectively represent the temperature and air pressure of the q calibration point groups; , respectively represent the temperature and air pressure in the reference state.

[0043] Thus, m×n groups of corresponding calibration point groups are obtained.

[0044] Assuming that the response of the probe has good linearity in the calibration range, for each resonance valley, the relationship between the wavelength shift and the environmental change can be described as: wherein, represents the shift amount, and are the sensitivity coefficients to be solved.

[0045] For the data of m×n groups of calibration point groups, the linear equations of the primary valley and the secondary valley are substituted respectively, and the least square solution of the overdetermined equation group is solved to directly determine the sensitivity coefficient matrix, and the formula of the overdetermined equation group is as follows: ; ; wherein, , are the sensitivity coefficients of the primary valley and the secondary valley to the temperature respectively; , are the sensitivity coefficients of the primary valley and the secondary valley to the air pressure respectively. There are m×n equations for the primary valley i, and similarly, there are m×n equations for the secondary valley j. By solving the two overdetermined equations by using the least square method, the coefficients and which make the total error square sum minimum can be obtained respectively. This is more robust than solving only a small number of data points, and can effectively suppress the influence of random measurement noise.

[0046] The sensitivity coefficient matrix is thus constituted as This matrix completely describes the linear mapping relationship between the optical response of the dual-mode resonant sensing probe and the excitation of the temperature and pressure dual physical fields.

[0047] The determinant value of the sensitivity coefficient matrix is calculated, and the absolute value of the determinant value is verified to be greater than a preset threshold, so as to ensure that the sensitivity coefficient matrix is reversible and can realize effective temperature and pressure differentiation measurement. The sensitivity coefficient matrix K must be reversible, and the inverse matrix will be used in the subsequent steps to inversely solve the temperature and pressure changes from the wavelength change.

[0048] Reversibility verification: the determinant value of the matrix K is calculated ). The necessary and sufficient condition for the matrix to be reversible is that the determinant D is not zero. In order to ensure sufficient numerical stability and differentiation degree in actual calculation, it is required that the absolute value of D is greater than a preset threshold. The specific value of the threshold depends on the noise level of the measurement system and the required calculation accuracy. For example, the threshold can be set as If the calibration calculated is less than the threshold, it indicates that the main valley and the secondary valley are too similar in response to temperature and pressure, and the differentiation degree is insufficient, and it is necessary to return to step 1 to re-optimize the design of the sensing probe (such as adjusting the microcavity or the polarization beam splitting parameter) to increase the difference between the response characteristics of the two resonance valleys.

[0049] The sensitivity coefficient matrix K and its inverse matrix confirmed by the final calibration are stored in the non-volatile memory of the embedded demodulation unit (such as a microprocessor or an FPGA) of the fiber laser system together with the reference wavelengths and . These parameters constitute the identity features and calculation codes of the specific sensing probe and are the key data basis for completing real-time and accurate measurement.

[0050] The above calibration is based on the linear response assumption. In actual application, if significant nonlinearity is found in the response within a wide working range, the method can be extended by segment calibration. Specifically, the entire temperature and pressure range can be divided into several smaller sub-regions that can be approximately regarded as linear. In each sub-region, the above calibration step process is independently performed to obtain the local sensitivity coefficient matrix and reference value corresponding to the sub-region. In real-time calculation, the demodulation unit first determines the sub-region to which the current wavelength or historical calculation value belongs, and then calls the corresponding local matrix for operation. This ensures that high-precision measurement can be maintained within the entire working range.

[0051] In this embodiment, 36 calibration sets of data were collected, and the set reference conditions were a temperature of 25°C and an air pressure of 101.325 kPa. Under these reference conditions, the center wavelength of the main valley was 1545 nm and the center wavelength of the secondary valley was 1570 nm. The specific data are shown in the table below: Table 1: Sensitivity Calibration Data Schematic Table

[0052] Based on Table 1 above, Figure 2 and Figure 3 Perform analysis. For example... Figure 2 As shown, Figure 2 Plotting temperature change on the horizontal axis, the wavelength shift response curves of the primary and secondary valleys as a function of temperature were plotted. The graphs visually demonstrate the different sensitivities of the two-mode resonances to temperature, which is the physical basis for constructing a reversible sensitivity matrix in this method to achieve temperature-pressure decoupling. As shown in the figure, the slope of the secondary valley (approximately 0.07 nm / ℃) is significantly greater than that of the primary valley (approximately 0.025 nm / ℃), indicating that the secondary valley is more sensitive to temperature changes, while the primary valley's response is relatively weaker. This difference stems from the different orders of the cladding modes corresponding to the two resonant valleys: the propagation constant of the higher-order cladding mode (secondary valley) is more significantly affected by the thermo-optical effects of the material, thus its wavelength drift with temperature is more pronounced. This differentiated response characteristic ensures that the determinant of the sensitivity matrix is ​​not zero, allowing for the effective separation of temperature and pressure components from the mixed response through inverse matrix operations in subsequent steps. The slope mentioned here means that although each data point is discrete, such as one point for every 5°C change in temperature, when all points are plotted on a graph, it can be seen that the slope of each line is the same, roughly distributed along a straight line.

[0053] like Figure 3 As shown, Figure 3 Plotting the change in air pressure on the horizontal axis, the graph illustrates the response relationship between the wavelength shift of the primary and secondary resonant valleys and changes in air pressure. The image clearly reveals the different sensitivities of the two resonant valleys to air pressure, which is the key basis for achieving high-sensitivity air pressure measurement and temperature interference suppression in this method. As can be seen from the slope of the curves, the sensitivity of the primary valley to air pressure (approximately 0.008 nm / kPa) is significantly higher than that of the secondary valley (approximately 0.004 nm / kPa), indicating that the primary valley has a higher response capability to changes in gas refractive index and air pressure. This is because the primary valley is formed by low-order cladding mode interference, and its mode field is more concentrated within the fiber cladding and more susceptible to changes in the external gas refractive index. This differentiated air pressure response characteristic also demonstrates that the system can not only achieve high-sensitivity air pressure detection but also effectively distinguish the cross-influence of air pressure and temperature changes in subsequent decoupling operations, thereby improving the accuracy and reliability of air pressure measurement.

[0054] The fiber laser sensing system construction module is used for connecting the sensing probe as a wavelength selection filter into a prefabricated ring-shaped fiber laser resonant cavity, using the filtering characteristic to compete with the mode of the laser, and constructing the fiber laser whose output wavelength is determined by the resonant valley position.

[0055] In the embodiment, the specific structure and connection relationship of the ring-shaped fiber laser resonant cavity are as follows: The pump source is usually a semiconductor laser with a wavelength of 980 nm or 1480 nm, which provides energy for the whole laser system. The output tail fiber of the pump source is connected with the pump port of the wavelength division multiplexer through fiber fusion or connector.

[0056] The wavelength division multiplexer is a three-port passive device, and its core function is to combine and separate waves. Specifically, the pump port is used to efficiently introduce pump light; the common port is connected with the erbium-doped fiber, allowing pump light and generated laser signals to pass through together; and the signal port is used to guide the laser signal oscillating in the ring cavity to the sensing probe and form a closed loop.

[0057] The erbium-doped fiber serves as a laser gain medium. Under the excitation of pump light, the erbium ions inside the fiber undergo population inversion, providing optical amplification in the 1550 nm waveband (C+L waveband) to compensate for various losses in the cavity, which is a necessary condition for realizing laser oscillation. The other end is connected with the input end of the optical isolator.

[0058] The function of the optical isolator is to ensure the one-way transmission of optical signals in the ring cavity. It only allows light to pass from the input end to the output end, and blocks the reverse transmission of light. This is the key to stabilizing the ring-shaped fiber laser resonant cavity laser, which can prevent back-reflected light from forming an unstable standing wave or self-pulse in the cavity, ensuring the stability of the laser output. The output end is connected with the input end of the sensing probe.

[0059] The sensing probe is the core sensing element of the application, which plays a dual role of wavelength selection filter and sensing unit in the ring cavity. The resonant valley in its transmission spectrum determines which wavelengths of light in the cavity have relatively low loss and can oscillate. The output end is connected with the signal port of the wavelength division multiplexer.

[0060] All the above optical elements are connected through standard single-mode fiber tails using low-loss fusion technology or high-performance fiber connectors to form a fully fiberized and closed ring-shaped fiber laser resonant cavity.

[0061] In this embodiment, before the sensing probe is connected to the ring cavity, the transmission spectrum of the sensing probe is directly measured using a broadband light source (such as an ASE light source) and an optical spectrum analyzer (OSA). In the modification process, first, the broadband light source is turned off, and the background noise spectrum of the OSA is collected; then the light source is turned on, and the total spectrum containing the signal and noise is collected. The preprocessed transmission spectrum is the difference between the total spectrum and the background noise spectrum. In order to eliminate the influence of the light source spectrum shape and the link loss, normalization processing is required. The sensing probe is connected to the measurement link, and the preprocessed transmission spectrum is measured. Then, the sensing probe is short-circuited (replaced by a normal optical fiber), and the reference spectrum is measured under the same conditions. The final normalized transmission spectrum is the ratio of the preprocessed transmission spectrum to the reference spectrum. This normalized transmission spectrum is used to analyze the effective data of the resonance valley depth, bandwidth, and contrast.

[0062] The process of constructing a fiber laser whose output laser wavelength is determined by the resonance valley position is based on the basic principles of laser physics, and the specific implementation is as follows: The necessary condition for laser oscillation is that the gain of light in the ring fiber laser cavity after one cycle is not less than the loss. This condition is quantified as the threshold condition for laser oscillation, that is, wherein, is the net gain (i.e. amplification factor) of the erbium-doped fiber at the wavelength It is related to the pump power, the length of the erbium-doped fiber, and the characteristics. The pump power must exceed a certain threshold value to make sufficiently large in a specific wavelength band. represents the transmission spectrum of the sensing probe. At its resonance valley, the value is the smallest (i.e. the loss is the largest). represents the total transmission loss coefficient (less than 1) of all elements in the ring fiber laser cavity except the sensing probe (including the fusion splice, connectors, wavelength division multiplexer, isolator, etc.).

[0063] The pump power mentioned above must exceed a certain threshold value, and the preferred determination method of the threshold value is as follows: after the system is built, the threshold value is determined by experiment. The specific operation is as follows: in the reference environment (such as 25°C, standard atmospheric pressure), gradually increase the output power of the pump source, and at the same time, monitor the spectrum of the output end of the ring cavity (usually a small amount of light can be introduced through a 90:10 or 95:5 coupler after the signal port of the wavelength division multiplexer for monitoring) spectrum. When one or more sharp spectral peaks with intensity much higher than the spontaneous emission background are observed at the wavelength corresponding to the resonance valley of the sensing probe, the corresponding pump power is the threshold power of laser oscillation. In practical applications, in order to ensure stable operation of the laser, the pump power is usually set to 1.2 to 2 times the threshold power. This value not only ensures that the laser has enough gain to overcome the loss and oscillate stably, but also avoids unnecessary nonlinear effects or thermal effects of the gain medium caused by excessive pump power.

[0064] After the pump power exceeds the threshold, the following conditions are met. Vibration can occur at various wavelengths (primarily distributed over a wide range near the resonance valley). However, the ring fiber laser resonator exhibits a series of discrete longitudinal modes with frequency spacing (free spectral range, FSR) of [missing information]. ,in, The longitudinal mode spacing represents the frequency domain, and c is the speed of light in vacuum. The group refractive index of a fiber laser is crucial, as the longitudinal mode spacing is determined by the optical path length, which is related to the group velocity. In estimation, the group refractive index is often considered as the effective refractive index. The denominator is the ratio of the optical path length of the light in a given length. The optical path length corresponding to the group delay propagating in an optical fiber; This represents the effective cavity length of the fiber laser. Ultimately, through mode competition (i.e., gain medium saturation), only one or a few modes reside at the deepest point of the resonance valley (i.e.,... The longitudinal mode (at its minimum point) with the highest net gain survives, forming a stable laser output. This process can be mathematically described as the laser output wavelength... By function The minimum point determines, i.e. .

[0065] To detect minute spectral shifts caused by environmental changes ( To convert the change in laser wavelength into a highly detectable quantity, the wavelength (or frequency) resolution of the system must be improved. This is achieved by optimizing the laser parameters to make it operate in a narrow linewidth state with a single or few longitudinal modes, specifically satisfying the following conditions: The total cavity length of the annular fiber laser resonator is limited to the range of [10m, 50m]. Longitudinal mode spacing. With cavity length Inversely proportional. When At that time, the corresponding frequency domain longitudinal mode spacing The range is approximately 4MHz (corresponding to a 50m cavity length) to 20MHz (corresponding to a 10m cavity length). Longer cavity lengths (e.g., 50m) result in smaller longitudinal mode spacing, which is beneficial for achieving single-mode operation, but makes the laser more sensitive to environmental vibrations and temperature fluctuations. Shorter cavity lengths (e.g., 10m) offer better stability, but the larger longitudinal mode spacing may require finer filtering or higher gain to achieve single-mode selection. Limiting the range within this range provides a sufficiently narrow longitudinal mode spacing while ensuring the short-term stability of the laser in most industrial environments.

[0066] 3dB bandwidth of the resonant valley of the sensor probe ( For example, 0.2nm corresponds to a frequency domain bandwidth of ,in, FWHM (Full Width at Half Maximum) of the transmission spectrum of the sensing probe, representing the width of the resonance dip in the wavelength coordinate (horizontal axis is wavelength, usually in nanometer); FWHM (Full Width at Half Maximum) of the transmission spectrum of the sensing probe, representing the width of the resonance dip in the wavelength coordinate (horizontal axis is wavelength, usually in nanometer); FWHM (Full Width at Half Maximum) of the transmission spectrum of the sensing probe, representing the width of the resonance dip in the wavelength coordinate (horizontal axis is wavelength, usually in nanometer); FWHM (Full Width at Half Maximum) of the transmission spectrum of the sensing probe, representing the width of the resonance dip in the wavelength coordinate (horizontal axis is wavelength, usually in nanometer);

[0067] Under the above conditions, although there are a large number of longitudinal modes in the filter window, due to the uniform broadening characteristics of the erbium-doped fiber and the gain saturation effect, under continuous pumping, the longitudinal mode that first reaches the threshold and starts to oscillate will consume most of the inverted population, thereby suppressing the gain of other adjacent longitudinal modes, so that they cannot reach the threshold. This phenomenon is called mode competition under the weakening of spatial hole burning effect. By fine-tuning the pump power and the polarization state in the cavity (for example, a polarization controller can be selectively added), the dominance of a single longitudinal mode can be promoted.

[0068] From the normalized transmission spectrum, the 3dB bandwidth and the depth of the resonance dip can be accurately read. A depth greater than 10dB is a preset threshold of the present application. The rationality of this value lies in the fact that a depth greater than 10dB means that the loss at this point is more than 10 times higher than in the passband region, providing sufficient filtering and mode selection capability to ensure that the laser wavelength is effectively locked at the dip, rather than unstable oscillation on the slope of the dip. Insufficient depth can lead to unstable laser mode or multiple laser wavelengths.

[0069] The line width less than 10MHz index is a key measure of laser monochromaticity and frequency stability. Commercial narrow line width fiber lasers or auxiliary mode selection means such as adding an unpumped erbium-doped fiber as a saturable absorber (SESAM effect) or an ultra-long fiber grating (ULFG) can easily achieve a line width of less than 10kHz or even narrower. For the system described in the present application, it mainly relies on the natural broadening of the gain medium and the filtering characteristics of the cavity. A line width of 10MHz is a practical target, which is much smaller than the typical bandwidth of the resonance dip (for example, 25GHz), meaning that the jitter of the laser frequency is much smaller than the effective detection bandwidth of the sensing system, and will not be the main factor limiting the sensing resolution. This value can be measured and verified by self-heterodyne or delay self-heterodyne method.

[0070] After the system is built, the laser output is monitored using a high resolution optical spectrum analyzer (e.g., OSA with a resolution of 0.02 nm or better) or a wavemeter. The main observation is whether the laser spectrum is single and sharp. If multiple peaks are observed, the side modes are suppressed by fine tuning the pump power and / or the intra-cavity polarization controller (if one is included). One intuitive criterion for judging the single longitudinal mode operation is that the laser spectrum is close to or reaches the resolution limit of the high resolution OSA, and there is no obvious side mode in the background.

[0071] The real-time monitoring change module is used to deploy the fiber laser system integrated with the sensing probe at the calibrated monitoring point; when the ambient temperature and air pressure change, the interference spectrum of the sensing probe drifts, causing the output laser wavelength of the fiber laser to jump or continuously drift; and the wavelength change amount is monitored and quantified in real time through a high-resolution optical wavemeter.

[0072] In this embodiment, the fiber laser system (usually the pump source, demodulation unit, etc. are placed in an environmentally controlled cabinet, and only the sensing probe is extended to the monitoring point through an extension fiber) is fixed and installed at the calibrated monitoring point. The following are two typical but non-limiting examples.

[0073] Example 1: Monitoring the pressure of the compressor section of an aero-engine The sensing probe is securely installed in the pressure measuring hole of the engine case. Crucially, the open microcavity region of the probe must be directly and unobstructed exposed to the high-speed gas environment of the engine flow channel to ensure that the gas pressure can act on the sensing region of the microcavity without loss. The seal between the probe and the pressure measuring hole uses a high-temperature pressure-resistant metal or ceramic seal to prevent gas leakage and isolate the interference from the non-measurement region. The rest of the laser is led to the relatively stable engine compartment area through an armored fiber. In this application, the engine operating conditions may change dramatically, which can cause high-frequency pressure pulsation and vibration. Therefore, the raw wavelength time series data obtained from the wavemeter needs to be low-pass digitally filtered (for example, using a Butterworth filter with a cutoff frequency of 1 kHz) to filter out the noise caused by vibration and extract the low-frequency wavelength drift signal related to the average gas pressure and temperature.

[0074] Example 2: Monitoring the gas pressure in the oil of a power transformer The sensing probe is sealed and installed inside the gas chamber (or gas collection chamber) of the transformer oil chromatography online monitoring system through a dedicated high-pressure sealing flange. The gas chamber is connected to the transformer oil through a permeation membrane to collect characteristic gases (such as H2, CH4, C2H2, etc.) separated from the oil. The probe directly measures the gas pressure in the sealed gas chamber. The pump and detection unit of the laser is placed in the main control cabinet.

[0075] This environment is relatively stable, but there can be slow temperature gradient changes. The collected wavelength data need to be smoothed (e.g. using a 60 second time window) to suppress random noise and improve the signal-to-noise ratio of individual measurements. At the same time, the ambient background temperature (provided by a reference temperature sensor mounted outside the gas cell) needs to be recorded and stored for later use in validating or correcting the results.

[0076] Since the operating wavelength of the fiber laser is determined by the resonance dip filtering characteristic of the sensing probe, when the resonance dip wavelength shifts due to changes in ambient temperature and pressure, the output laser wavelength will follow the change. The mode of change depends on the relative size of the resonance dip wavelength shift and the inherent longitudinal mode spacing of the laser.

[0077] When the ambient temperature and pressure change, the spectral shift of the sensing probe will cause the output laser wavelength of the fiber laser to change in the following two modes: The first is the continuous tuning mode. When the ambient changes are slow or small, so that the spectral shift of the sensing probe is less than the spacing between adjacent longitudinal modes of the fiber laser within a single sampling / response cycle, the laser does not need to change the number of its oscillating longitudinal modes. The output laser wavelength will continuously and smoothly change with the movement of the resonance dip, showing continuous wavelength tuning, at the wavelength of the specific longitudinal mode that is locked. This mode is suitable for high-resolution monitoring of slow or small changes in parameters. For example, in transformer gas cell monitoring, the slow accumulation of gas results in a very low rate of change in gas pressure, and the laser wavelength usually operates in this mode.

[0078] The second is the mode jump mode. When the environment changes dramatically or significantly, the spectral shift of the sensing probe is not less than the spacing between adjacent longitudinal modes of the laser. The original longitudinal mode causes the loss to increase due to mismatch with the resonance dip minimum, and no longer meets the lasing condition. At this time, another longitudinal mode that is currently more matched to the resonance dip minimum will win in the mode competition, and the output wavelength of the laser will have a discrete jump, with a jump of one or more longitudinal mode spacings. Although the longitudinal mode jumps, the laser wavelength is always anchored within the moved resonance dip filtering window, i.e. the output laser wavelength is approximately equal to the new resonance dip wavelength. This mode enables the system to track large-scale changes in parameters. For example, when an aero-engine suddenly accelerates from slow speed to maximum thrust, the compressor outlet pressure and temperature rise sharply, and the laser wavelength is likely to undergo a series of rapid mode jumps.

[0079] The output laser wavelength is monitored in real time by a high-resolution optical wavelength meter (with an accuracy better than 1 pm). The change is the difference between the current real-time output laser wavelength and the output laser wavelength in the reference state.

[0080] For the first mode, the change directly reflects the shift of the resonance dip and is directly quantified.

[0081] In the second mode, the simple change reading will produce discontinuous steps due to the jump. In order to assist in judging the total amount of resonance valley drift corresponding to the current laser wavelength, and assisting the subsequent algorithm in judging the working range, the embodiment records the number of jumps of the laser wavelength and the longitudinal mode interval, and combines the effective cavity length and the group refractive index of the laser to roughly estimate the total amount of resonance valley drift caused by environmental changes. The rough estimation formula is as follows: ; Wherein, represents the total amount of resonance valley drift, N represents the number of forward or reverse jumps, and is used to assist in judging the resonance order to which the output laser wavelength belongs and the settlement range; by monitoring the time sequence of the output laser wavelength, when the change amount thereof jumps by nearly the longitudinal mode interval, one jump is recorded. The forward jump (wavelength length) N takes a positive value, and the reverse jump N takes a negative value. A jump threshold can be set in the counting algorithm, that is, the absolute value of the jump threshold is in the range of 0.5 times the longitudinal mode interval and 1.5 times the longitudinal mode interval, so as to distinguish between real mode jumps and noise. The group refractive index of the fiber laser is determined by the type of optical fiber. For standard communication optical fiber SMF-28, the value at 1500 nm wavelength band is usually 1.468, which is a known material constant. The effective cavity length parameter of the fiber laser is once determined, and can be stored in the storage chip of the demodulation unit for subsequent all calculations.

[0082] The formula essentially multiplies the longitudinal mode interval by the number of jumps N to represent the total amount of resonance valley drift, which is reasonable because each jump corresponds to a resonance valley moving by about one longitudinal mode interval. Its main role is not to provide the final high-precision drift amount, but to: after a large range jump, quickly estimate the approximate range of the resonance valley drift; provide an initial estimate or range check for the dynamic iteration method in step 5, to prevent the solving process from losing the correct resonance order (i.e., judging whether the current laser is locked in the main valley or the secondary valley) due to too many jumps.

[0083] The decoupling and output module is used to calculate the temperature change and the air pressure change of the monitoring point by substituting the wavelength change into the inverse matrix of the sensitivity coefficient matrix.

[0084] Due to the complexity of the actual monitoring environment, the raw data directly read from the wavelength meter may contain noise, jumps or outliers. In order to ensure the accuracy of the calculation, the raw wavelength data needs to be preprocessed before entering the quantization algorithm.

[0085] The preprocessing operation in the embodiment includes sliding average filtering and outlier rejection.

[0086] Sliding average filtering is to use a sliding time window of length Q to average the continuously collected wavelength sequence to suppress high-frequency random noise. , wherein, is the sampling interval. The length Q of the sliding window needs to balance the smoothing effect and the response speed. Usually, the value of Q is selected such that the time constant (Q*Δt) is much smaller than the characteristic variation time of the environmental physical quantity (temperature, air pressure) to be measured. For example, for the monitoring of the relatively slow-changing temperature of transformer oil, Q can be 5-20, corresponding to a time constant of several seconds to tens of seconds; for the rapid pulsation monitoring of the air pressure in the engine flow channel, Q can be 1-3 to ensure rapid response. m represents a loop index variable for traversing the data points in the sliding window, m=0 represents the data at the current time, and m=1 represents the data at the previous time.

[0087] After filtering, a dynamic threshold based on the statistical characteristics (such as the mean and standard deviation) of the recent data can be set. If the wavelength value of a certain sampling point deviates from the mean value by more than 2-3 times the standard deviation, it is determined to be an abnormal value. At this time, the previous valid value can be used to replace the abnormal value, or the mean of the previous valid value and the next valid value can be used to replace the abnormal value.

[0088] The preprocessed wavelength data is sent to the quantification module to determine the wavelength change amount corresponding to the main valley and the secondary valley at the current time. Two methods are provided in this embodiment to adapt to different monitoring scenarios.

[0089] First, the static mapping method (default method) is used to quantify the wavelength change amount. This method is suitable for situations where the environment changes relatively gently, the output wavelength of the laser does not jump in mode or the number of jumps can be clearly recorded, and specifically includes: According to the center wavelengths of the main valley and the secondary valley measured in the reference state; during real-time monitoring, the absolute difference between the current output laser wavelength and the two center wavelengths measured in the reference state is calculated, represented as and ; wherein, represents the current output laser wavelength, represents the center wavelength of the secondary valley in the reference state.

[0090] If , it is determined that the current laser is locked in the main valley, and the wavelength change amount of the main valley is directly calculated as ; wherein, represents the wavelength change amount of the current main valley.

[0091] In order to obtain the secondary valley wavelength change for matrix solution, here is based on a key assumption: the wavelength drift trends of the two resonance valleys have a high degree of linear correlation when the sensing probe is subjected to the same environmental disturbance. Therefore, under the condition of locking the main valley, the wavelength change amount of the secondary valley can be estimated as: ; wherein, The wavelength variation of the current minimum. The essence of this estimation is that at any time, the wavelength interval of the two resonance minima is approximately kept constant. This assumption has high rationality when the environmental variation is small relative to the linear measurement range of the sensing probe (e.g., temperature variation less than 50°C, pressure variation less than 1 MPa), and its error can be verified by calibration experiments to be within an acceptable range.

[0092] Conversely, if it is determined that the laser is locked to the minimum, then is calculated, and the is preliminarily estimated.

[0093] Another dynamic iterative method (triggered activation method) is also provided in this embodiment, and the trigger condition is: When the demodulation system detects a step change in the wavelength of the laser that is not less than the interval between longitudinal modes, it is determined that mode jumping has occurred. At this time, the static mapping method simply uses Comparison with the fixed reference may fail because may have jumped to a position far from the original reference.

[0094] When the absolute value of the temperature variation rate or the pressure variation rate calculated using the static mapping method exceeds a preset threshold (e.g., temperature variation rate threshold of 10°C / s, pressure variation rate threshold of 0.1 MPa / s) within a unit time (e.g., 1 second), it indicates that the environment is changing dramatically, and the static linear estimation may be significantly biased, so a more accurate iterative algorithm needs to be activated.

[0095] The determination of the temperature variation rate threshold and the pressure variation rate threshold should be based on a large amount of historical data or analysis of typical working conditions. For example, in the test of an aero-engine compressor, the pressure variation rate threshold is set by analyzing the maximum gradient of the pressure pulsation; in the monitoring of a power transformer, the temperature variation rate threshold is set by analyzing the maximum rising rate of the oil temperature during a fault or inrush. The purpose is to switch algorithms in time before the error of the static method significantly increases.

[0096] Set the number of iterations to zero k = 0, and use the most recently calculated temperature and pressure values as the current estimates. If there are no historical calculation values, use the reference state as the current estimates; According to the current estimates and the calibrated sensitivity coefficient matrix, the theoretical wavelength of the main minimum is calculated, and the formula is as follows: ; wherein represents the theoretical wavelength of the main minimum in the kth iteration; , respectively represent the temperature and pressure in the kth iteration estimate; similarly, wherein represents the theoretical wavelength of the minimum in the kth iteration. k represents the iteration index.

[0097] The real-time measured output laser wavelength is compared with the theoretical wavelengths of the main valley and the secondary valley, and the one with the smallest difference is selected as the current locked valley; if the main valley is selected as the current locked valley, the temporary wavelength change is calculated as follows: ,in, This represents the wavelength change at the moment of the main valley in the k-th iteration. Here, the k-th iteration is assumed to be at the current time t, where t represents the time variable. Subsequent iterations will be indexed using iterations.

[0098] Another method for estimating the change is consistent with the idea of ​​the static mapping method, but it uses the theoretical wavelength difference based on the current estimate. If the main valley is considered as the locked valley, then the wavelength change of the secondary valley is calculated. ; in, This represents the wavelength change during the temporary valley in the k-th iteration.

[0099] Will and The vector is composed, and the new environmental estimate is calculated using the inverse matrix of the sensitivity coefficients: ; in, These represent the temperature estimate and air pressure estimate in the (k+1)th iteration after the update, respectively. In the k-th iteration and The vector formed by these vectors.

[0100] Calculate the absolute difference between the updated temperature estimate in the (k+1)th iteration and the current temperature estimate in the kth iteration, as well as the absolute difference between the updated air pressure estimate in the (k+1)th iteration and the current air pressure estimate in the kth iteration. If both absolute differences are less than the set convergence threshold, the iteration ends, and the temporary wavelength change at the end of the last iteration is output. Otherwise, the updated temperature and air pressure values ​​are used as the current estimates, and the iteration continues. The convergence threshold for temperature is set to 0.01℃, and the convergence threshold for air pressure is set to 0.001MPa. The number of iterations is usually small, generally 2-5 iterations are sufficient for convergence. The convergence threshold should be set slightly higher than the final required temperature and air pressure measurement resolution of the system to ensure that the accuracy meets the requirements when the iteration stops, while avoiding unnecessary iteration loops.

[0101] In this embodiment, decoupling the temperature change and the air pressure change specifically includes: The input vector is constructed based on the real-time wavelength changes of the two resonance valleys, and is expressed as follows: ; The sensitivity coefficient matrix and its inverse matrix are called, a matrix multiplication operation is performed, and an output vector containing temperature and pressure change amounts is obtained, as follows: ; wherein, represents an output vector containing temperature and pressure change amounts; , represent a temperature change rate and a pressure change amount, respectively; represents an inverse matrix of the sensitivity coefficient matrix; wherein, the definition and parameter meaning of the sensitivity coefficient matrix and the inverse matrix of the sensitivity coefficient matrix are as follows: ; ; wherein, D is a determinant of the sensitivity coefficient matrix; the absolute value of D must be large enough (much larger than zero), which is a mathematical basis for realizing effective differentiation of two parameters.

[0102] The solved temperature change amount and pressure change amount are added to the known environmental values of the monitoring point, respectively, to obtain real-time absolute pressure values and real-time absolute temperature values. The environmental values herein refer to the field values measured by independent high-precision thermometers and barometers at the monitoring point when the sensing probe is deployed, which are used to convert relative measurement into absolute measurement. The environmental values herein are not necessarily the same as the aforementioned reference values.

[0103] Please refer to Figure 4 , the application further provides a dual-mode resonance optical fiber microcavity gas pressure measurement method, which is executed by the above-mentioned dual-mode resonance optical fiber microcavity gas pressure measurement system, and the specific steps include: Step 1: an open microcavity is formed by femtosecond laser etching through the cladding and partially into the core in the core and cladding area of a standard communication single-mode optical fiber; a micro-nano taper region is formed by high-temperature tapering of the open microcavity, so as to obtain an open micro-tapered fiber microcavity type Mach-Zehnder interferometer sensing probe; the interference spectrum of the sensing probe has at least two resonance valleys in the C+L wave band; Step 2: the sensing probe is placed in an independently controlled temperature and pressure calibration cavity, and the output interference spectrum is recorded under the illumination of a wide-spectrum light source; two resonance valleys with different temperature and pressure response characteristics are selected, the drift amount of the center wavelength with the temperature and pressure of the calibration cavity is monitored, and the sensitivity coefficient matrix of the sensing probe is calibrated through linear fitting; Step 3: the sensing probe is connected as a wavelength selection filter into a pre-prepared ring fiber laser resonant cavity, and a fiber laser is constructed by using the filtering characteristics of the sensing probe and the mode competition of the laser, wherein the output laser wavelength is determined by the resonance valley position. Step 4: The optical fiber laser system integrated with the sensing probe is deployed at the calibration monitoring point; when the ambient temperature and air pressure change, the interference spectrum of the sensing probe drifts, causing the output laser wavelength of the optical fiber laser to jump or continuously drift; the wavelength change is monitored and quantified in real time by a high-resolution optical wavelength meter; Step 5: The wavelength change is substituted into the inverse matrix of the sensitivity coefficient matrix for calculation to decouple the temperature change and air pressure change of the monitoring point in real time.

[0104] The above formulas are all dimensionless numerical calculations, the formulas are obtained by software simulation of a large amount of data to obtain a formula of the nearest real situation, and the preset parameters in the formula are set by a person skilled in the art according to the actual situation.

[0105] The above embodiments can be realized wholly or partially by software, hardware, firmware or any combination thereof. When realized by software, the above embodiments can be realized in the form of a computer program product wholly or partially. Those skilled in the art can realize that the units and algorithm steps of the examples described in connection with the embodiments disclosed herein can be realized by electronic hardware or a combination of computer software and electronic hardware. Whether the functions are realized by hardware or software methods depends on the specific application and design constraints of the technical solutions.

[0106] The units described as separate components can or can not be physically separated, and the components shown as units can or can not be physical units, which can be located in one place or distributed on multiple network units. Part or all of the units can be selected according to actual needs to achieve the purpose of the embodiments.

[0107] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed in the present application, which should be covered within the protection scope of the present application.

Claims

1. A dual-mode resonant fiber-microcavity gas pressure measurement system, characterized in that, The method comprises the following steps: a sensing probe preparation module is used to form an open microcavity by femtosecond laser etching through the cladding layer and partially into the core of the standard communication single-mode optical fiber in the core and cladding area of the standard communication single-mode optical fiber; a high-temperature tapering is performed on the open microcavity to form a micro-nano taper region to obtain an open micro-tapered fiber-in-microcavity type Mach-Zehnder interferometer sensing probe; the interference spectrum of the sensing probe has at least two resonance valleys in the C+L wave band; a double-parameter sensitivity matrix calibration module is used to place the sensing probe in a calibration cavity with independent temperature and air pressure control, record the output interference spectrum under the illumination of a wide-spectrum light source, select two resonance valleys with different temperature and air pressure response characteristics, monitor the drift amount of the center wavelength with the change of the temperature and air pressure of the calibration cavity, and calibrate the sensitivity coefficient matrix of the sensing probe through linear fitting; an optical fiber laser sensing system construction module is used to connect the sensing probe as a wavelength selection filter into a pre-prepared ring optical fiber laser resonant cavity, utilize the filtering characteristics and mode competition of the laser to construct an optical fiber laser whose output laser wavelength is determined by the resonant valley position; a real-time monitoring change module is used to deploy the optical fiber laser system integrated with the sensing probe at the monitoring point; when the environmental temperature and air pressure change, the interference spectrum of the sensing probe drifts, resulting in the output laser wavelength of the optical fiber laser jumping or continuously drifting; the wavelength change amount is monitored and quantified in real time through a high-resolution optical wavelength meter; a decoupling and output module is used to calculate the temperature change amount and air pressure change amount of the monitoring point by substituting the wavelength change amount into the inverse matrix of the sensitivity coefficient matrix.

2. The dual-mode resonant fiber-microcavity gas pressure measurement system of claim 1, wherein, The radial width of the open microcavity ranges from The axial length ranges from The etching depth is not less than the sum of the cladding diameter of the standard single-mode optical fiber and the depth of the cut-in core, and the depth of the cut-in core ranges from The open microcavity entirely clads and cuts in the core to realize the direct interaction between the core light-guiding mode and the internal and external environment of the open microcavity. The micro-nano taper region formed by the high-temperature tapering has a taper waist diameter in a range of , and a length in a range of ; the micro-nano taper region is located at an axial center position of the open microcavity, for enhancing the coupling efficiency of the core mode and the cladding mode, and making the resonance valley depth of the interference spectrum greater than 10 dB, so as to ensure that at least two resonance valleys have contrast and detectability; The interference spectrum generated by the sensing probe is formed by the interference between the core fundamental mode and the excited multi-order cladding modes; among the two resonance valleys with different temperature and air pressure response characteristics, one is a main resonance valley formed by the interference between the fundamental mode and the low-order cladding mode, and the other is a secondary resonance valley formed by the interference between the fundamental mode and the high-order cladding mode; the wavelength interval of the main resonance valley and the secondary resonance valley falls within the interval [20nm, 80nm].

3. The dual-mode resonant fiber-microcavity gas pressure measurement system of claim 2, wherein, The calibration of the sensitivity coefficient matrix specifically comprises: in the calibration cavity with temperature and air pressure control accuracy better than ±0.1℃ and ±0.01MPa, the temperature is changed at a fixed temperature step within the temperature measurement range to obtain a group of temperature calibration points as a temperature change sequence; the air pressure is changed at a fixed air pressure step within the air pressure measurement range to obtain a group of air pressure calibration points as a pressure change sequence; each temperature calibration point in the temperature change sequence is combined with each air pressure calibration point in the pressure change sequence to obtain a plurality of calibration point groups; under the state of each calibration point group, a broadband light source is used to irradiate the sensing probe, and a spectrum analyzer is used to collect the transmitted interference spectrum output by the sensing probe to record the center wavelengths of the preselected main resonance valley and secondary resonance valley in the spectrum; the main resonance valley is calibrated as the i-th resonance valley, and the secondary resonance valley is calibrated as the j-th resonance valley, which are hereinafter referred to as the main valley and the secondary valley, respectively; a preset standard room temperature of 25℃ and a standard atmospheric pressure of 101.325Kpa are taken as the reference, and the wavelength drift amounts of the two resonance valleys of each calibration point group relative to the reference state are calculated, and the formula is as follows: ； wherein, represents a wavelength shift amount of the main valley in the state of the calibration point group q; represents a center wavelength of the main valley in the state of the calibration point group q; represents a center wavelength of the main valley in the reference state, q is a calibration point group index; Similarly, the wavelength drift amount of the secondary valley in the state of the calibration point group q is obtained, denoted as ; At the same time, record the corresponding temperature change and pressure change: ； wherein, respectively represent a temperature change amount and a gas pressure change amount of the qth calibration point group; , respectively represent a temperature and a gas pressure of the qth calibration point group; , respectively represent a temperature and a gas pressure in a reference state; For all calibration point groups, the sensitivity coefficient matrix is directly determined by solving the least square solution of the overdetermined equation group, and the overdetermined equation group is as follows: ； ； wherein , are the sensitivity coefficients of the primary and secondary valleys to temperature, respectively; , are the sensitivity coefficients of the primary and secondary valleys to air pressure, respectively; the sensitivity coefficient matrix is represented as ; The determinant value of the sensitivity coefficient matrix is calculated, and the absolute value of the determinant value is verified to be greater than a preset threshold value, so as to ensure that the sensitivity coefficient matrix is reversible and can realize effective temperature and pressure discrimination measurement; subsequently, the sensitivity coefficient matrix and its inverse matrix are stored in the demodulation unit of the fiber laser.

4. The dual-mode resonant fiber-microcavity gas pressure measurement system of claim 1, wherein, The specific structure and connection relationship of the ring-shaped fiber laser resonant cavity are as follows: The output tail fiber of the pump source is connected with the pump port of the wavelength division multiplexer; one end of a section of erbium-doped fiber serving as a gain medium is connected with the common port of the wavelength division multiplexer; the other end of the erbium-doped fiber is connected with the input end of the optical isolator; the output end of the optical isolator is connected with the input end of the sensing probe; the output end of the sensing probe is connected with the signal port of the wavelength division multiplexer, thereby forming a closed ring-shaped fiber laser resonant cavity; The process of constructing the fiber laser whose output laser wavelength is determined by the resonant valley position is as follows: The minimum of the transmission spectrum of the sensing probe at the resonance valley is extracted as a wavelength selective filter to satisfy the threshold condition for laser oscillation, i.e. wherein, Gnet is the net gain of the Erbium-doped fiber at the wavelength λ0; T(λ) represents the transmission spectrum of the sensing probe; is the total transmission loss coefficient of all elements inside the ring fiber laser cavity except the sensing probe; after the pump power exceeds the threshold, the mode competition in the cavity makes the final oscillation of the laser wavelength be limited at the position of the relative highest loss in the transmission spectrum of the sensing probe, i.e. the resonance valley, i.e. so that the output laser wavelength is determined by the resonance valley position of the sensing probe; In order to improve the resolution of the pressure sensor, the fiber laser is made to work in a single longitudinal mode or a few longitudinal modes by optimizing the parameters of the ring-shaped fiber laser resonant cavity and using mode competition, and the following conditions are met: The total cavity length of the ring-shaped fiber laser resonant cavity is limited in the range of [10m, 50m], and the corresponding longitudinal mode interval is greater than the corresponding value of the 3dB bandwidth of the resonant valley in the transmission spectrum of the sensing probe in the frequency domain; the product of the effective refractive index of the fiber and the physical cavity length of the ring-shaped fiber laser resonant cavity is calculated first, and the ratio of the speed of light in vacuum to the product is the longitudinal mode interval; under this condition, only one or several longitudinal modes fall within the resonant valley filter window, and a narrow linewidth output is finally formed through mode competition, and the linewidth is less than 1 / 10 of the corresponding value of the 3dB bandwidth of the resonant valley in the frequency domain, and the linewidth is less than 10MHz.

5. The dual-mode resonant fiber-microcavity gas pressure measurement system of claim 1, wherein, The fiber laser is fixed at the monitoring point; the calibration monitoring point includes but is not limited to the pressure measuring hole of the compressor section of the aero-engine; the sensing probe is fixed in the pressure measuring hole, and the open microcavity is directly exposed to the gas pressure environment of the engine flow channel; and the gas chamber sampling cavity of the power transformer oil dissolved gas online monitoring system; the sensing probe is sealed and installed inside the gas chamber for directly measuring the gas pressure of the precipitated gas; When the ambient temperature and pressure change, the spectral drift of the sensing probe will cause the output laser wavelength of the fiber laser to change in the following two modes: The first is continuous tuning mode, when the environmental change causes the spectral drift of the sensing probe to be less than the adjacent longitudinal mode interval of the fiber laser, the output laser wavelength shows continuous wavelength tuning within a single resonant valley filter range; The second is mode jump mode, when the environmental change causes the spectral drift of the sensing probe to be not less than the adjacent longitudinal mode interval of the laser, the output laser wavelength shows jump between different longitudinal modes, and the new wavelength after jump is still locked at the drifted resonant valley. In the second mode, the total drift of the resonance valley caused by the environmental change is coarsely estimated by recording the number of mode jumps of the laser wavelength and the longitudinal mode interval, combining the effective cavity length and the group refractive index of the laser, and the coarse estimation formula is as follows: ； wherein, represents the total shift of the resonance valley, N represents the number of forward or reverse jumps, represents the group refractive index of the fiber laser, represents the effective cavity length of the fiber laser.

6. The dual-mode resonant fiber-microcavity gas pressure measurement system of claim 3, wherein, The wavelength change is quantified by the static mapping method, which specifically includes: According to the center wavelengths of the main valley and the secondary valley measured in the reference state; in real-time monitoring, the absolute difference between the current output laser wavelength and the two center wavelengths measured in the reference state is calculated, denoted as and ; wherein, the current output laser wavelength is denoted as the center wavelength of the secondary valley in the reference state is denoted as If , it is determined that the current laser is locked in the main valley, and the wavelength variation of the main valley is directly calculated as ; wherein, represents the wavelength variation of the current main valley; Meanwhile, based on the linear response assumption, the preliminary estimation of the wavelength variation of the secondary valley is ; wherein, represents the wavelength variation of the current secondary valley; otherwise, if it is determined that the laser is locked in the secondary valley, the calculation of is performed, and the preliminary estimation of is performed.

7. The dual-mode resonant fiber-microcavity gas pressure measurement system of claim 6, wherein, When the output laser wavelength is monitored to jump in mode, or when the absolute value of the change of the physical quantity calculated by the static mapping method in unit time is greater than the preset threshold, the dynamic iteration method is enabled for quantization, which includes the following steps: Set the iteration number to zero, and use the last calculated temperature and pressure values as the current estimated values, or use the reference state as the current estimated values if there is no historical calculation value; According to the current estimated value and the calibrated sensitivity coefficient matrix, the theoretical wavelength of the main valley is calculated, and the formula is as follows: ； wherein, represents the theoretical wavelength of the main valley; , respectively represent the temperature and the air pressure in the current estimate; similarly, the theoretical wavelength of the secondary valley is obtained, denoted by ; The real-time measured output laser wavelength is compared with the theoretical wavelength of the main valley and the secondary valley respectively, one with the minimum difference is selected as the current locking valley, and the temporary wavelength change is calculated as wherein r represents the main valley or the secondary valley, and r is i or j; The temporary wavelength change and the corresponding change of another valley not being the current locking valley form a vector, and the updated temperature and pressure values are calculated using the inverse matrix of the sensitivity coefficient matrix; The absolute difference between the updated temperature and the current estimated temperature, and the absolute difference between the updated pressure and the current estimated pressure are calculated, and if both absolute differences are less than the set threshold, the iteration is ended, and the temporary wavelength change at the last iteration is output; otherwise, the updated temperature and pressure values are used as the current estimated values, and the iteration is continued.

8. The dual-mode resonant fiber-microcavity gas pressure measurement system of claim 6, wherein, The temperature change and the pressure change are decoupled, which specifically includes: Based on the wavelength variation quantity in real time of two resonance valleys, an input vector is constituted, which is expressed as ; The sensitivity coefficient matrix and its inverse matrix are called, and matrix multiplication operation is performed to obtain an output vector containing temperature and pressure changes, and the formula is as follows: ； wherein, represents an output vector including a temperature and a pressure change amount; , respectively represent a temperature change rate and a pressure change amount; represents an inverse matrix of a sensitivity coefficient matrix; Wherein, the definition and parameter meaning of the sensitivity coefficient matrix and the inverse matrix of the sensitivity coefficient matrix are as follows: ； ； Wherein, D is the determinant of the sensitivity coefficient matrix; The calculated temperature change and pressure change are added to the known environmental values of the monitoring points to obtain the real-time absolute pressure value and the real-time absolute temperature value.

9. A dual-mode resonant fiber microcavity gas pressure measurement method, characterized in that, The double-mode resonance fiber microcavity gas pressure measurement method is performed by the double-mode resonance fiber microcavity gas pressure measurement system of any one of claims 1-8, and the specific steps include: Step 1: In the core and cladding area of a standard communication single-mode optical fiber, an open microcavity is formed by femtosecond laser etching through the cladding and partially into the core; a micro-nano taper region is formed by high-temperature tapering of the open microcavity to obtain an open micro-taper fiber-in-microcavity Mach-Zehnder interferometer sensing probe; the interference spectrum of the sensing probe has at least two resonance valleys in the C+L band; Step 2: Place the sensing probe in a calibration cavity with independent temperature and pressure control, record its output interference spectrum under illumination of a wide-spectrum light source, select two resonance valleys with different temperature and pressure response characteristics, monitor the drift of the center wavelength with the change of the temperature and pressure of the calibration cavity, and calibrate the sensitivity coefficient matrix of the sensing probe by linear fitting; Step 3: The sensing probe is connected to a pre-prepared ring fiber laser resonant cavity as a wavelength selection filter, and a fiber laser is constructed by using the filtering characteristics of the sensing probe and the mode competition of the laser. Step 4: The optical fiber laser system integrated with the sensing probe is deployed at the monitoring point of calibration; when the ambient temperature and air pressure change, the interference spectrum of the sensing probe drifts, causing the output laser wavelength of the optical fiber laser to jump or continuously drift; the wavelength change is monitored and quantified in real time by a high-resolution optical wavelength meter; Step 5: The wavelength change is substituted into the inverse matrix of the sensitivity coefficient matrix for calculation, and the temperature change and air pressure change of the monitoring point are decoupled in real time.

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