A dual-mode resonant optical fiber microcavity air pressure measuring system and measuring method

Abstract

The application provides a dual-mode resonant optical fiber microcavity air pressure measurement system and a measurement method, relates to the technical field of optical fiber microcavity air pressure measurement, and specifically comprises a sensing probe preparation module, a dual-parameter sensitivity matrix calibration module, an optical fiber laser sensing system construction module, a real-time monitoring change module and a decoupling and output module. Through femtosecond laser etching combined with high-temperature tapering, an open microcavity is made on a single-mode optical fiber and a micro-nano taper region is formed, so that a sensing probe with two resonant valleys with different temperature and air pressure response characteristics is obtained. The sensitivity coefficient matrix is obtained by calibration, and the probe is embedded as a filter into a ring-shaped optical fiber laser cavity to construct an optical fiber laser whose laser wavelength is locked by the resonant valley. The laser wavelength drifts due to environmental changes, and the real-time synchronous measurement of temperature and air pressure is realized by using the inverse matrix decoupling, so that the application is suitable for high-precision and anti-interference monitoring in harsh industrial environments such as aircraft engines and power transformers.

Description

Technical Field

[0001] This invention relates to the field of fiber optic microcavity pressure measurement technology, specifically to a dual-mode resonant fiber optic microcavity pressure measurement system and method. Background Technology

[0002] Real-time status monitoring of high-end equipment such as aero-engines and large power transformers is crucial for their safe and stable operation. Taking aero-engines as an example, the dynamic and accurate measurement of air pressure inside the compressor section is key to assessing engine performance and preventing surge. In power transformers, changes in the pressure and temperature of dissolved gases in the oil are the core basis for diagnosing latent internal faults. While traditional electrical pressure sensors (such as piezoresistive and capacitive sensors) are widely used, they often face challenges such as electromagnetic interference, flammability risks, and insufficient long-term stability in these scenarios. In recent years, fiber optic sensing technology, with its inherent safety, resistance to electromagnetic interference, and corrosion resistance, has provided new approaches to measuring physical quantities in harsh environments. Among them, microstructure-based fiber optic sensors have become a research hotspot due to their high sensitivity and small size.

[0003] Currently, fiber optic sensing solutions for gas pressure can be mainly classified into fiber Bragg grating type, Fabry-Perot interferometry type, and Mach-Zehnder interferometry type. Fiber Bragg gratings typically have low sensitivity to gas pressure, and their response is highly dependent on temperature, requiring compensation using complex reference gratings or special packaging, resulting in complex structures and increased uncertainty. While intrinsic or extrinsic Fabry-Perot cavity pressure sensors offer high sensitivity, their manufacturing process is precise and expensive, and the absolute demodulation of the cavity length is susceptible to temperature disturbances, limiting demodulation accuracy in dynamic scenarios with wide temperature ranges and varying gas pressures. Fiber optic sensors based on the Mach-Zehnder interferometry principle, through the design of special structures (such as cones, misalignments, and microcavities), excite interference between the cladding mode and the fiber core fundamental mode, exhibiting sensitivity to changes in the environmental refractive index (related to gas density and pressure), demonstrating advantages such as relatively simple manufacturing and designable sensitivity. However, most existing fiber optic barometric pressure sensors with single-mode interferometric structures typically exhibit broadband modulation or a single resonant valley in their interference spectra. These sensors are not only equally sensitive to temperature and difficult to distinguish, but their broadband spectral monitoring also relies on expensive high-resolution spectrometers, making low-cost, high-resolution real-time dynamic measurements difficult. Directly integrating such sensors into a laser cavity, utilizing their narrowband filtering characteristics to convert spectral shifts into laser wavelength jumps, is an effective way to improve resolution and measurement speed. However, achieving both high-sensitivity response to barometric pressure and effective differentiation between temperature and barometric pressure within a miniaturized, integrated sensing probe, while ensuring stable integration with the laser system, remains a significant bottleneck for the practical engineering application of this technology. Summary of the Invention

[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:

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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 cone region is located at the axial center of the open microcavity, which is used to enhance the coupling efficiency between the core mode and the cladding mode, and to make the resonance valley depth of the interference spectrum greater than 10dB, thereby ensuring that at least two resonance valleys have contrast and detectability.

[0013] The interference spectrum generated by the sensing probe is formed by the interference between the fiber core fundamental mode and the excited multi-order cladding modes; of the two resonance valleys with different response characteristics to temperature and air pressure, one is the main resonance valley formed by the interference between the fundamental mode and the low-order cladding mode, and the other is the secondary resonance valley formed by the interference between the fundamental mode and the high-order cladding mode; the wavelength interval between the main resonance valley and the secondary resonance valley falls within the range of [20nm, 80nm].

[0014] Furthermore, calibrating the sensitivity coefficient matrix specifically includes:

[0015] Within a calibration chamber with temperature and 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 set of temperature calibration points as a variable temperature sequence; the pressure is changed at a fixed pressure step within the pressure measurement range to obtain a set of pressure calibration points as a variable pressure sequence; each temperature calibration point in the variable temperature sequence and each pressure calibration point in the variable pressure sequence are combined to obtain several calibration point groups;

[0016] In each calibration point group state, the sensing probe is illuminated with a broadband light source, and its output transmission interference spectrum is collected by a spectrum analyzer. The center wavelength of the pre-selected main resonance valley and the center wavelength of 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. Hereinafter, the main resonance valley is the main valley and the secondary resonance valley is the secondary valley.

[0017] Using a preset standard room temperature of 25℃ and a standard atmospheric pressure of 101.325 kPa as a reference, the wavelength drift of each calibration point group relative to the two resonance valleys under the reference condition is calculated using the following formula:

[0018] ;

[0019] in, This indicates the wavelength shift of the main valley under the q-state of the calibration point group; This indicates the center wavelength of the main valley in the q-state of the calibration point group; This indicates the center wavelength of the main valley under reference conditions, and q is the index of the calibration point group.

[0020] Similarly, the wavelength shift of the secondary valley in the q-state of the calibration point group is obtained, denoted as... ;

[0021] At the same time, record the corresponding temperature and air pressure changes:

[0022] ;

[0023] in, These represent the temperature change and air pressure change of the q-th calibration point group, respectively; , These represent the temperature and air pressure of q calibration point groups, respectively; , These represent the temperature and air pressure under reference conditions, respectively.

[0024] For data from all calibration point sets, the sensitivity coefficient matrix is ​​directly determined by solving the least-squares solution of the overdetermined equation system, as follows:

[0025] ;

[0026] ;

[0027] in, , These are the sensitivity coefficients of the main valley and the secondary valley to temperature, respectively. , These are the sensitivity coefficients of the main valley and the secondary valley to air pressure, respectively; thus, a sensitivity coefficient matrix is ​​constructed as follows: ;

[0028] The determinant of the sensitivity coefficient matrix is ​​calculated, and its absolute value is verified to be greater than a preset threshold to ensure that the sensitivity coefficient matrix is ​​invertible and can effectively distinguish between temperature and air pressure measurements. Subsequently, the sensitivity coefficient matrix and its inverse matrix are stored in the demodulation unit of the fiber laser.

[0029] Furthermore, the specific structure and connection relationship of the ring fiber laser resonator are as follows:

[0030] The output pigtail of the pump source is connected to the pump port of the wavelength division multiplexer; the common port of the wavelength division multiplexer is connected to one end of an erbium-doped fiber that serves as the gain medium; the other end of the erbium-doped fiber is connected to the input end of an optical isolator; the output end of the optical isolator is connected to the input end of a sensing probe; and the output end of the sensing probe is connected to the signal port of the wavelength division multiplexer, thereby forming a closed ring fiber laser resonant cavity.

[0031] The process of constructing a fiber laser whose output laser wavelength is determined by the location of the resonant valley is implemented as follows:

[0032] The minimum value of the transmission spectrum of the sensing probe at the resonance valley is extracted and used as a wavelength selective filter to meet the threshold condition for laser oscillation initiation. ,in, To erbium-doped fiber at wavelength Net gain at the location; This represents the transmission spectrum of the sensing probe; This represents the total transmission loss coefficient of all components outside the sensing probe within the ring fiber laser resonant cavity. When the pump power exceeds a threshold, intracavity mode competition limits the final oscillating laser wavelength to the position with the highest relative loss in the sensing probe's transmission spectrum, i.e., the resonance valley. Thus, the output laser wavelength is determined by the position of the resonant valley of the sensing probe;

[0033] To improve the resolution of the pressure sensing, the parameters of the ring fiber laser resonator and the mode competition are optimized to enable the fiber laser to operate in a single longitudinal mode or a few longitudinal modes, specifically satisfying the following conditions:

[0034] The total cavity length of the annular fiber laser resonator is limited to the range of [10m, 50m], and the corresponding longitudinal mode spacing is greater than the corresponding value of the 3dB bandwidth of the resonant valley, which serves as the filter window in the transmission spectrum of the sensing probe, in the frequency domain. First, the product of the effective refractive index of the fiber and the physical cavity length of the annular fiber laser resonator is calculated. The ratio of the speed of light in vacuum to this product is the longitudinal mode spacing. Under this condition, only one or more longitudinal modes fall within the resonant valley filter window, and a narrow linewidth output is finally formed through mode competition. Its 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.

[0035] Furthermore, 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 inside the pressure measuring hole, its open microcavity being directly exposed to the gas pressure environment of the engine flow channel; and the gas chamber sampling chamber of the online monitoring system for dissolved gases in power transformer oil; the sensing probe is sealed and installed inside the gas chamber for directly measuring the gas pressure of the evolved gas;

[0036] When ambient temperature and air 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:

[0037] The first type is the continuous tuning mode. When environmental changes cause the spectral shift of the sensing probe to be less than the interval between adjacent longitudinal modes of the fiber laser, the output laser wavelength exhibits continuous wavelength tuning within a single resonant valley filtering range.

[0038] The second type is the mode-hopping mode. When environmental changes cause the spectral drift of the sensor probe to be no less than the interval between 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.

[0039] In the second mode, by recording the number of laser wavelength jumps and the longitudinal mode spacing, and combining this with the effective cavity length and group refractive index of the laser, a coarse estimate is made of the total drift of the resonance valley caused by environmental changes. The coarse estimate formula is as follows:

[0040] ;

[0041] in, This represents the total drift of the resonance valley, and N represents the number of positive or negative transitions. The group refractive index of a fiber laser is represented by its group refractive index. This indicates the effective cavity length of the fiber laser.

[0042] Furthermore, quantifying the wavelength change using the static mapping method specifically includes:

[0043] Based on the center wavelengths of the main valley and secondary valley measured under reference conditions; during real-time monitoring, the absolute difference between the current output laser wavelength and the two center wavelengths measured under reference conditions is calculated and expressed as: and ;in, Indicates the current output laser wavelength. This indicates the center wavelength of the sub-valley under the reference condition;

[0044] like The laser is currently locked in the main valley, and the wavelength change in the main valley is directly calculated. ;in, This indicates the current wavelength change in the main valley;

[0045] Meanwhile, based on the linear response assumption, the wavelength change of the secondary valley is initially estimated to be... ;in, This represents the wavelength change at the current sub-valley; conversely, if it is determined that the laser is locked at the sub-valley, then calculate... And preliminary estimates .

[0046] Furthermore, when a mode jump in the output laser wavelength is detected, or when the absolute value of the change in a physical quantity calculated using the static mapping method exceeds a preset threshold per unit time, a dynamic iterative method is activated for quantization. This dynamic iterative method includes the following steps:

[0047] Set the number of iterations to zero, and use the most recently calculated temperature and pressure values ​​as the current estimates. If there are no historical values, use the baseline state as the current estimates.

[0048] Based on the current estimate and the calibrated sensitivity coefficient matrix, the theoretical wavelength of the main valley is calculated using the following formula:

[0049] ;

[0050] in, Indicates the theoretical wavelength of the main valley; , Let these represent the temperature and air pressure in the current estimates, respectively; similarly, obtain the theoretical wavelength of the sub-valley, denoted as... ;

[0051] The real-time measured output laser wavelength is compared with the theoretical wavelengths of the main valley and secondary valley, respectively. The one with the smallest difference is selected as the current locked valley, and the temporary wavelength change is calculated. Here, r represents the primary valley or secondary valley, and r is either i or j;

[0052] The temporary wavelength change is combined with another corresponding change that is not currently locked into a vector, and the updated temperature and pressure values ​​are calculated using the inverse of the sensitivity coefficient matrix.

[0053] Calculate 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. If both absolute differences are less than a set threshold, the iteration ends and the temporary wavelength change at the time of the last iteration is output. Otherwise, the updated temperature and air pressure values ​​are used as the current estimates and the iteration continues.

[0054] Furthermore, decoupling the temperature change from the pressure change specifically includes:

[0055] The input vector is constructed based on the real-time wavelength changes of the two resonance valleys, and is expressed as follows: ;

[0056] By calling the sensitivity coefficient matrix and its inverse matrix and performing matrix multiplication, an output vector containing temperature and pressure changes is obtained, as shown in the following formula:

[0057] ;

[0058] in, This represents an output vector that includes changes in temperature and air pressure. , These represent the rate of temperature change and the amount of air pressure change, respectively. The inverse matrix representing the sensitivity coefficient matrix;

[0059] The definitions and parameter meanings of the sensitivity coefficient matrix and its inverse are as follows:

[0060] ;

[0061] ;

[0062] Where D is the determinant of the sensitivity coefficient matrix;

[0063] The calculated temperature and pressure changes are added to the known environmental values ​​at the monitoring points to obtain the real-time absolute air pressure and real-time absolute temperature values.

[0064] This invention also provides a dual-mode resonant fiber optic microcavity pressure measurement method, which is executed by the aforementioned dual-mode resonant fiber optic microcavity pressure measurement system, and the specific steps include:

[0065] Step 1: In the core and cladding region of a standard single-mode fiber for communication, an open microcavity is formed by femtosecond laser etching, penetrating the cladding and partially cutting into the core; 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 sensing probe; the interference spectrum of this sensing probe has at least two resonance valleys in the C+L band;

[0066] Step 2: 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 its 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.

[0067] Step 3: Use the sensing probe as a wavelength selective filter and connect it to the prefabricated ring fiber laser resonant cavity. Utilize its filtering characteristics to compete with the laser mode and construct a fiber laser whose output laser wavelength is determined by the position of the resonant valley.

[0068] Step 4: Deploy the fiber laser system with integrated 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 drift continuously; monitor and quantify the wavelength change in real time using a high-resolution optical wavelength meter.

[0069] Step 5: Substitute the wavelength change into the inverse matrix of the sensitivity coefficient matrix for calculation, and decouple the temperature change and air pressure change at the monitoring point in real time.

[0070] The technical effects and advantages provided by the present invention in the above technical solution are as follows:

[0071] This invention combines femtosecond laser technology with high-temperature tapered fiber to fabricate an open micro-cavity within a single-mode fiber. The interference spectrum naturally forms at least two interference valleys with significantly different resonant characteristics (sensitivities to temperature and pressure) in the communication band. This probe is embedded as a filter within the ring fiber laser cavity, constructing a sensing system where the laser wavelength is locked by the resonant valleys. This system not only utilizes the narrow linewidth of the laser to achieve high-resolution detection of pressure changes but also constructs a reversible sensitivity matrix through the differential response of the two resonant valleys, ultimately achieving real-time, synchronous, and differentiated measurement of both temperature and pressure parameters. This method is particularly suitable for demanding industrial scenarios such as the interior of aero-engines and transformer oil and gas monitoring, where strong electromagnetic interference exists, intrinsic safety is required, and multi-parameter synchronous sensing is necessary. Attached Figure Description

[0072] Figure 1 This is a schematic diagram of the system structure of the present invention;

[0073] Figure 2 This is a schematic diagram illustrating the relationship between temperature change and wavelength shift in this invention.

[0074] Figure 3 This is a schematic diagram illustrating the relationship between the pressure change and wavelength shift in this invention.

[0075] Figure 4 This is a schematic diagram of the overall method flow of the present invention. Detailed Implementation

[0076] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0077] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0078] Example:

[0079] Please see Figures 1 to 3This invention provides a dual-mode resonant fiber optic microcavity barometric pressure measurement system, comprising:

[0080] The sensor probe fabrication module is used to form an open microcavity that penetrates the cladding and partially cuts 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 producing 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.

[0081] In this embodiment, firstly, a section of standard single-mode optical fiber for communication (e.g., with a core diameter of approximately 8.2 mm) is taken. The cladding diameter is approximately 125 mm. The coating diameter is approximately 250 mm. The coating layer at the end of the fiber is removed using fiber strippers and an alcohol swab, and a flat end face is prepared using a fiber optic cleaver. It is then fixed on a high-precision three-dimensional translation stage.

[0082] The sides of the optical fiber are etched using a femtosecond laser micromachining system (e.g., a center wavelength of 1030 nm, a pulse width of less than 300 fs, and a repetition rate of 1 MHz). By precisely controlling parameters such as laser energy, scanning speed, number of scans, and spot overlap rate, a rectangular open microcavity is etched in the cladding and core regions of the optical fiber.

[0083] The radial width range of the open microcavity is limited to: This width needs to be significantly larger than the fiber core diameter (approximately 8.2 mm). This ensures that the microcavity can completely cover and expose the transverse cross-section of the fiber core, providing sufficient space for the interaction between the fiber core's fundamental mode and the external environment. If the width is too small (less than 15...), If the core mode field is not sufficiently disturbed, the excited cladding mode energy is weak, resulting in low interference contrast; if the width is too large (greater than 30), the interference will be insufficient. If this is done, it will severely weaken the mechanical strength of the optical fiber, making it prone to breakage during subsequent operations.

[0084] The range of axial length values ​​is: This length determines the length difference of the interferometer arms. A longer length results in denser interference fringes (a smaller free spectral range), which is beneficial for obtaining more resonance valleys within a limited spectral bandwidth. However, excessive length can lead to a fragile microcavity structure and may introduce higher-order dispersion. The stated range represents an optimization between ensuring sufficient interference fringe contrast and structural robustness.

[0085] The etching depth must ensure that the laser etching completely penetrates the fiber cladding and partially cuts into the fiber core. The etching depth should not be less than the sum of the standard single-mode fiber cladding diameter and the depth cut into the fiber core. The depth cut into the fiber core can range from [value missing]. This is crucial for achieving direct interaction between the fiber core light guiding mode and the internal and external environments of the open microcavity. If the depth is too shallow (e.g., less than 3...),... The perturbation to the fundamental mode (LP01 mode) propagating within the fiber core is weak, and its energy cannot be effectively coupled to the cladding mode or radiation mode; if the depth is too deep (e.g., greater than 8), it will affect the fiber core. Excessive cutting depth will overly damage the waveguide structure of the fiber core, leading to a sharp increase in transmission loss and even causing the fiber to break at the microcavity. The preferred cutting depth is approximately 4.1 times the core radius. 0.7 to 2 times that of ) can achieve the best balance between effective coupling and acceptable loss.

[0086] The open microcavity is integrally clad and cut into the fiber core to achieve direct interaction between the fiber core's light-guiding mode and the internal and external environments of the open microcavity. After etching, high-pressure nitrogen gas is used to purge the interior and surrounding areas of the microcavity to remove etching debris and adhering substances. Subsequently, the morphology of the microcavity is observed under a microscope to confirm that its boundaries are clear, its depth is uniform, and there are no visible cracks or damage. This pretreatment step is crucial for ensuring subsequent optical performance and structural stability.

[0087] The fiber segment with an etched open microcavity is placed above the heating source (e.g., an oxyhydrogen flame or a CO2 laser) of the fiber tapering system, with the heating point precisely aligned with the axial center of the microcavity. Heating and tapering are then performed while applying appropriate axial tension.

[0088] The diameter of the waist of the micro / nano cone region formed by high-temperature tapering ranges from [value missing]. During the tapering process, as the fiber thins, the core and cladding shrink synchronously. The optical field in the original core gradually expands to the entire waist region, forming a strongly constrained mode field. This mode field's interaction with the microcavity edge is greatly enhanced when passing through the microcavity structure. A waist diameter within this range ensures effective optical field expansion to enhance coupling with the environment while maintaining low transmission loss and basic waveguide characteristics. A diameter less than 3... At this point, the waveguide structure approaches cutoff, and the loss increases sharply; when the diameter is greater than 8... At that time, the constraint and enhancement effects on the model field are not obvious.

[0089] The range of values ​​for the cone length is: This length determines the transition slope of the cone. An overly steep transition (too short a length) introduces higher-order modes and non-adiabatic coupling, resulting in stray interference and additional losses. A gentle transition (moderate length) can be approximated as an adiabatic tapering, exhibiting high mode conversion efficiency and maximizing the coupling of energy from the core fundamental mode to specific cladding modes excited by microcavity perturbations, thereby significantly improving the contrast of the interference spectrum. The stated length range is an empirically optimized value that ensures adiabatic tapering conditions while maintaining structural compactness.

[0090] Annealing after tapering: After tapering, the optical fiber is briefly held in the heated area (e.g., 1-3 seconds) for in-situ annealing, then the heat source is slowly removed to allow it to cool naturally. This process aims to eliminate thermal stress formed inside the quartz glass during tapering, improve the long-term stability of the micro / nano tapered region, and prevent deformation or breakage due to stress release during subsequent use. The micro / nano tapered region is located at the axial center of the open microcavity to enhance the coupling efficiency between the core mode and the cladding mode, and to ensure that the resonance valley depth of the interference spectrum is greater than 10 dB, thereby ensuring that at least two resonance valleys have contrast and detectability.

[0091] The sensing probe fabricated through the above steps essentially constitutes a micro-ZI based on asymmetric two-arm interference. One arm is the propagation path of the unperturbed fiber core fundamental mode, while the other arm is the cladding mode path, which, after being perturbed by the open microcavity, couples energy to the cladding (or the microcavity-cladding interface) and propagates in the external region of the microcavity. The two beams recouple and interfere in the fiber core downstream of the microcavity, forming periodic interference fringes (resonance valleys) in the transmission spectrum.

[0092] The formation 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 micro / nano cone regions enhances the interaction between the mode field and the microcavity, greatly improving the excitation efficiency of specific cladding modes. In this embodiment, a resonance valley depth greater than 10dB is defined as the acceptance standard. This threshold (1dB) is determined based on the signal-to-noise ratio requirements of the actual demodulation system. Typically, the background noise of commercial OSA or demodulators in the C / L band is around -70dBm to -80dBm. A resonance valley with a depth greater than 10dB (i.e., transmittance less than -10dB) has a sufficiently high contrast with the background light intensity, enabling stable and accurate detection and tracking of its center wavelength, avoiding misjudgments caused by noise.

[0093] Among multiple resonance valleys, two valleys with significantly different responses to temperature and pressure are selected as the sensing signal carriers. The main resonance valley (main valley) typically corresponds to the resonance valley formed by the interference of the core fundamental mode (LP01) and lower-order cladding modes (such as LP11, LP02, etc.). Since the equivalent refractive index of the lower-order cladding modes is closer to that of the core mode, their mode fields are more distributed inside the cladding, and they are more significantly affected by changes in the external gas refractive index (i.e., gas pressure), thus exhibiting ultra-high sensitivity to changes in gas refractive index and gas pressure. The secondary resonance valley (secondary valley) typically corresponds to the resonance valley formed by the interference of the core fundamental mode (LP01) and higher-order cladding modes (such as LP31, LP12, etc.). The mode fields of the higher-order cladding modes extend further to the outer edge of the cladding, and some energy is even in a radiative state. Their propagation constant (equivalent refractive index) is more affected by the thermo-optical effect and thermal expansion effect of the quartz material itself, thus exhibiting a significantly different sensitivity to temperature changes compared to the main valley.

[0094] The wavelength spacing between the primary and secondary valleys should fall within the range of [20nm, 80nm]. This range is set primarily to balance two aspects: firstly, the spacing should be large enough to allow for clear differentiation on a spectrometer under broadband illumination, and to enable simultaneous or time-division monitoring using a standard wavelength meter or demodulation module; secondly, the spacing should not be too large, lest the two valleys be located at or outside the edge of the erbium-doped fiber gain spectrum (C+L band), affecting the gain uniformity and start-up efficiency when constructing the fiber laser. For example, a 25nm spacing between a primary valley at 1545nm and a secondary valley at 1570nm is both easily distinguishable and within the high-gain region of the erbium-doped fiber.

[0095] The primary and secondary valleys, due to the different orders of their corresponding interference modes, exhibit different dependencies of their effective refractive index on temperature (primarily through the material's thermo-optical properties and thermal expansion coefficient) and on the refractive index / pressure of the external medium. Mathematically, these differentiated response characteristics are represented by the coefficients in the sensitivity coefficient matrix K calibrated in step 2. and And matrix determinant It is not zero. This is the core physical basis for the subsequent realization of the dual-parameter measurement of temperature and air pressure, that is, the construction and solution of the inverse problem of the reversible sensitivity coefficient matrix.

[0096] For applications in harsh environments (such as those involving dust, oil, or mechanical vibration), an extremely thin layer (e.g., less than 1 mm thick) can be coated onto the microcavity and cone region of the sensing probe. A hydrophobic, oleophobic, and optically transparent protective layer (e.g., fluorinated polymers or silica sol-gel coatings) is required. This protective layer must ensure that it does not impede the free diffusion of gas molecules to the microcavity surface, while preventing permanent adhesion of contaminants and maintaining the reversibility and long-term stability of the sensing response. The coating process can employ physical vapor deposition or spin coating, and its specific parameters (such as thickness) need to be optimized experimentally to achieve a balance between protection and sensing sensitivity.

[0097] 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 with changes in temperature and pressure in the calibration chamber, and calibrate the sensitivity coefficient matrix of the sensor probe by linear fitting.

[0098] In this embodiment, to ensure the accuracy of the calibration results, a calibration device with highly controllable and precisely measurable environmental parameters is required. The calibration of the sensitivity coefficient matrix specifically includes:

[0099] A sealed chamber with independent temperature and pressure control modules is employed. Temperature control accuracy should be better than ±0.1℃, and pressure control accuracy should be better than ±0.01MPa. The chamber should contain calibrated high-precision reference thermometers and barometers to provide accurate environmental parameter readings; their accuracy should be at least equivalent to or higher than the chamber's control accuracy. The system comprises a broadband light source (such as an ASE light source), an optical spectral analyzer (OSA), and a data logging computer. Sensing probes are connected to the system via fiber optic patch cords. The OSA's wavelength resolution should be set sufficiently high (e.g., ≤0.02 nm) to ensure accurate resolution of minute wavelength shifts in resonance valleys.

[0100] To comprehensively characterize the probe's response within its expected operating range, it is necessary to systematically change the temperature and pressure and record the corresponding spectral data. Based on the probe's intended application scenario, its temperature and pressure measurement ranges are determined.

[0101] Within the temperature measurement range, the temperature is changed in fixed temperature steps (e.g., 5℃ or 10℃) to obtain a set of temperature calibration points as a variable temperature sequence, such as... The temperature variation sequence then includes m temperature calibration points. Within the pressure measurement range, the pressure is changed in fixed pressure steps (e.g., 20 kPa or 50 kPa) to obtain a set of pressure calibration points as the pressure variation sequence, such as... Therefore, the pressure variation sequence includes n pressure calibration points. By combining each point in the above temperature variation sequence with the pressure variation sequence, m×n calibration point groups are generated (e.g., ...). (where 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 a full factorial experimental design allows for the most comprehensive acquisition of probe response data under different temperature and pressure combinations, avoiding calibration errors introduced by parameter coupling, and providing sufficient and uniformly distributed data samples for subsequent linear fitting.

[0102] At each calibration point group, the cavity is stabilized at the specified temperature and pressure for a sufficient duration (e.g., 10-15 minutes) to ensure complete thermal and mechanical equilibrium between the sensing probe and the cavity environment. The sensing probe is illuminated with a broadband light source, and its output transmission interference spectrum is acquired using a spectral analyzer. The center wavelengths of two pre-selected resonance valleys (primary and secondary resonance valleys) are extracted from the spectrum. The raw spectral data can be preprocessed to improve accuracy. Preprocessing operations may include, but are not limited to: first, applying a moving average filter to the spectral data to suppress optical noise; second, performing polynomial fitting (e.g., cubic spline interpolation) on the local spectral 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 searching for a local minimum. The primary 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; hereinafter, the primary resonance valley is referred to as the primary valley, and the secondary resonance valley as the secondary valley. Simultaneously, the actual temperature and pressure read from the reference sensor within the cavity are recorded. i is the index of the primary resonance valley, and j is the index of the secondary resonance valley.

[0103] To eliminate initial manufacturing deviations of the probe, all measurements are calculated relative to a common reference condition. The preset standard room temperature is 25℃ and the standard atmospheric pressure is 101.325 kPa. Before or after the calibration test, a sample of Yangpu River must be taken under the reference condition to record the reference center wavelengths of the main valley and secondary valley. and For each calibration point group, calculate its wavelength shift relative to the two resonance valleys under the reference state, using the following formula:

[0104] ;

[0105] in, This indicates the wavelength shift of the main valley under the q-state of the calibration point group; This indicates the center wavelength of the main valley in the q-state of the calibration point group; This represents the center wavelength of the main valley under reference conditions, and q is the index of the calibration point group.

[0106] Similarly, the wavelength shift of the secondary valley in the q-state of the calibration point group can be obtained by formula:

[0107] ;

[0108] in, This indicates the wavelength shift of the sub-valley under the q-state of the calibration point group; This indicates the center wavelength of the sub-valley in the q-state of the calibration point group; This indicates the center wavelength of the sub-valley under the reference condition.

[0109] At the same time, record the corresponding temperature and air pressure changes:

[0110] ;

[0111] in, These represent the temperature change and air pressure change of the q-th calibration point group, respectively; , These represent the temperature and air pressure of q calibration point groups, respectively; , These represent the temperature and air pressure under the reference conditions, respectively.

[0112] Thus, the calibration point set corresponding to m×n groups was obtained.

[0113] Assuming the probe's response exhibits good linearity within its calibration range, the relationship between wavelength drift and environmental changes for each resonance valley can be described as follows: ,in, Indicates the amount of drift. and Let be the sensitivity coefficient to be determined.

[0114] For the data from the m×n set of calibration points, substitute them into the linear equations for the main valley and secondary valley, and directly determine the sensitivity coefficient matrix by solving the least squares solution of the overdetermined equation system. The formulas for the overdetermined equation system are as follows:

[0115] ;

[0116] ;

[0117] in, , These are the sensitivity coefficients of the main valley and the secondary valley to temperature, respectively. , These are the sensitivity coefficients of the main valley and the secondary valley to air pressure, respectively. There are m×n equations for the main valley i, and similarly, m×n equations for the secondary valley j. Solving these two overdetermined equations using the least squares method yields the coefficients that minimize the sum of squared total errors. and This is more robust than solving with only a small number of data points and can effectively suppress the effects of random measurement noise.

[0118] This constitutes the sensitivity coefficient matrix, which is expressed as follows: This matrix fully describes the linear mapping relationship between the optical response of the dual-mode resonance of the sensing probe and the thermo-baric dual-physics excitation.

[0119] Calculate the determinant of the sensitivity coefficient matrix and verify that its absolute value is greater than a preset threshold to ensure that the sensitivity coefficient matrix is ​​invertible and can effectively distinguish between temperature and air pressure measurements. The sensitivity coefficient matrix K must be invertible, and its inverse matrix... This will be used in subsequent steps to inversely solve for temperature and pressure changes from wavelength changes.

[0120] Invertibility verification: Calculate the determinant of matrix K. A matrix is ​​invertible if and only if its determinant D is not zero. To ensure sufficient numerical stability and discriminative power in practical solutions, the absolute value of D must be greater than a preset threshold. The specific value of this threshold depends on the noise level of the measurement system and the required solution accuracy. For example, the threshold can be set as follows: If the calibration calculation If the response is below this threshold, it indicates that the primary and secondary valleys are too similar in their response to temperature and pressure, and the differentiation is insufficient. It is necessary to return to step 1 to re-optimize the design of the sensor probe (such as adjusting the microcavity or tapered parameters) to increase the difference in the response characteristics of the two resonant valleys.

[0121] The final calibrated sensitivity coefficient matrix K and its inverse matrix are determined. Together with the reference wavelength and These parameters are stored together in the non-volatile memory of the embedded demodulation unit (such as a microprocessor or FPGA) of the fiber laser system. They constitute the identity characteristics and decoding code of this specific sensing probe, and are the key data foundation for achieving real-time, accurate measurements.

[0122] The above calibration is based on the linear response assumption. In practical applications, if significant nonlinearity is found in the response over a wide operating range, this method can be extended through piecewise calibration. Specifically, the entire temperature and pressure range can be divided into several smaller sub-regions that can be approximated as linear. Within each sub-region, the above calibration steps are executed independently to obtain the corresponding local sensitivity coefficient matrix and reference value. During real-time calculation, the demodulation unit first determines the sub-region based on the current wavelength or historical calculation values, and then calls the corresponding local matrix for calculation. This ensures high-precision measurement throughout the entire operating range.

[0123] 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:

[0124] Table 1: Sensitivity Calibration Data Schematic Table

[0125]

[0126] 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.

[0127] 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.

[0128] 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.

[0129] In this embodiment, the specific structure and connection relationship of the annular fiber laser resonator are as follows:

[0130] The pump source is typically a 980 nm or 1480 nm semiconductor laser, and its function is to provide power to the entire laser system. The output pigtail of the pump source is connected to the pump port of the wavelength division multiplexer via fiber optic splice or connector.

[0131] A wavelength division multiplexer (WDM) is a three-port passive device whose core function is wavelength multiplexing and wavelength division. Specifically, its pump port is used to efficiently introduce pump light; its common port is connected to erbium-doped fiber, allowing the pump light and the generated laser signal to pass through together; and its signal port is used to export the laser signal oscillating in the ring cavity to the sensing probe, forming a closed loop.

[0132] Erbium-doped fiber is used as the laser gain medium. Under the excitation of pump light, the erbium ions inside undergo population inversion, providing optical amplification in the 1550 nm band (C+L band) to compensate for various losses within the cavity, which is a necessary condition for realizing laser oscillation. Its other end is connected to the input end of an optical isolator.

[0133] An optical isolator ensures unidirectional transmission of optical signals within the ring cavity. It allows light to travel only from its input to its output, blocking light traveling in the opposite direction. This is crucial for the stable operation of a ring fiber laser resonator, preventing back-reflected light from forming unstable standing waves or self-pulses within the cavity, thus ensuring the stability of the laser output. Its output is connected to the input of the sensing probe.

[0134] The sensing probe is the core sensing element of this invention, serving a dual role as a wavelength selective filter and a sensing unit within this annular cavity. The resonance valleys in its transmission spectrum determine which wavelengths within the cavity have relatively low optical loss, thus enabling oscillation. Its output is connected to the signal port of a wavelength division multiplexer.

[0135] All of the above optical components are connected via standard single-mode fiber pigtails using low-loss fusion splicing technology or high-performance fiber optic connectors to form a fully fiber-based, closed ring fiber laser resonator.

[0136] In this implementation, before the sensing probe is connected to the ring cavity, its transmission spectrum is directly measured using a broadband light source (such as an ASE light source) and an OSA spectrometer. During this process, the broadband light source is first turned off, and the OSA's background noise spectrum is acquired. Then, the light source is turned on, and the total spectrum containing both signal and noise is acquired. The preprocessed transmission spectrum is the difference between the total spectrum and the background noise spectrum. To eliminate the influence of the light source's spectral shape and link loss, normalization 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 with a section of ordinary 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 as effective data for analyzing the resonance valley depth, bandwidth, and contrast.

[0137] The process of constructing a fiber laser whose output wavelength is determined by the location of the resonant valley is based on the fundamental principles of laser physics, and the specific implementation method is as follows:

[0138] The necessary condition for laser oscillation is that after light circulates once within the ring fiber laser resonator, its gain is not less than its loss. This condition is quantified as the threshold condition for laser oscillation initiation, i.e. ,in, To erbium-doped fiber at wavelength The net gain (i.e., amplification factor) at a given point is related to the pump power, the length of the erbium-doped fiber, and its characteristics. The pump power must exceed a certain threshold to achieve this. It is large enough in a specific frequency band. This represents the transmission spectrum of the sensing probe. At its resonance valley, The value is the smallest (i.e., the loss is the greatest). The total transmission loss coefficient (less than 1) represents all components (including fusion splices, connectors, wavelength division multiplexers, isolators, etc.) within the ring fiber laser resonator, excluding the sensing probe.

[0139] The pump power mentioned above must exceed a certain threshold. The preferred method for determining this threshold is through experimentation after the system is built. Specifically, under a reference environment (e.g., 25°C, standard atmospheric pressure), gradually increase the output power of the pump source while simultaneously monitoring the spectrum at the output of the ring cavity using a spectrometer (typically, a small amount of light can be extracted from the signal port of the wavelength division multiplexer via a 90:10 or 95:5 coupler for monitoring). 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 for laser oscillation initiation. In practical applications, to ensure stable laser operation, the pump power is typically set between 1.2 and 2 times the threshold power. This value ensures sufficient gain to overcome losses and stabilize oscillations while avoiding unnecessary nonlinear effects or thermal effects on the gain medium caused by excessively high pump power.

[0140] 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 refractive index of light at a length of... 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. .

[0141] 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:

[0142] 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 short-term stability of the laser in most industrial environments.

[0143] 3dB bandwidth of the resonant valley of the sensor probe ( For example, 0.2nm corresponds to a frequency domain bandwidth of ,in, It is an index that describes the width of the resonance valley in the transmission spectrum of a sensing probe on the wavelength coordinate axis (the horizontal axis is wavelength, and the unit is usually nanometers). Indicates the center wavelength; This refers to the width of the same resonant valley on a frequency coordinate axis (the horizontal axis is frequency, usually in Hertz or gigahertz). (For a wavelength of 1550 nm, a bandwidth of 0.2 nm corresponds to approximately 25 GHz). The optimization goal is to make the longitudinal mode spacing (on the order of MHz) much smaller than the frequency domain bandwidth of the filter. (The magnitude is on the order of GHz). Thus, within a filtering window spanning tens of GHz, tens of thousands of longitudinal modes are distributed. Through mode competition, ultimately only one or a few longitudinal modes with the highest gain win.

[0144] Under the above conditions, although a large number of longitudinal modes exist within the filtering window, due to the uniform broadening characteristics and gain saturation effect of erbium-doped fiber, the longitudinal mode that first reaches the threshold and begins to oscillate under continuous pumping will consume most of the inversion particle number, thereby suppressing the gain of other adjacent longitudinal modes and preventing them from reaching the threshold. This phenomenon is called mode competition under reduced spatial aperture effect. By finely adjusting the pump power and intracavity polarization state (e.g., selectively adding a polarization controller), the dominance of a single longitudinal mode can be promoted.

[0145] From the normalized transmission spectrum, the 3dB bandwidth and valley depth of the resonance valley can be accurately read. A valley depth greater than 10dB is a preset threshold in this invention. The rationale for this value is that a depth greater than 10dB means that the loss at that location is more than 10 times higher than in the passband region, providing sufficiently strong filtering and mode selection capabilities to ensure that the laser wavelength is effectively locked at the valley bottom, rather than oscillating unstablely on the valley's slope. Insufficient depth may lead to unstable laser modes or the presence of multiple laser wavelengths.

[0146] The linewidth of less than 10 MHz is a key indicator for evaluating the monochromaticity and frequency stability of a laser. Commercial narrow-linewidth fiber lasers can easily achieve linewidths of less than 10 kHz or even narrower by adding unpumped erbium-doped fiber as a saturable absorber (SESAM effect) or by using auxiliary mode selection methods such as ultra-long fiber gratings (ULFGs). For the system described in this invention, it mainly relies on the natural broadening of the gain medium and the filtering characteristics of the cavity. A linewidth of 10 MHz is a practical target; it is much smaller than the typical bandwidth of the resonance valley (e.g., 25 GHz), meaning that the laser frequency jitter is much smaller than the effective detection bandwidth of the sensing system and will not be a major factor limiting the sensing resolution. This value can be measured and verified using self-heterodyne or delayed self-heterodyne methods.

[0147] After the system is built, monitor the laser output using a high-resolution spectrometer (e.g., an OSA with a resolution of 0.02 nm or higher) or a wavelength meter. The main observation is whether the laser spectral lines are single and sharp. If multiple peaks appear, suppress side modes by fine-tuning the pump power and / or an intracavity polarization controller (if optional). A straightforward criterion for judging single-mode operation is that, on a high-resolution OSA, the laser linewidth is close to or reaches the instrument's resolution limit, and there are no obvious side modes in the background.

[0148] 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.

[0149] In this embodiment, the fiber laser system (typically with the pump source, demodulation unit, etc., placed in an environmentally controlled cabinet, and the sensing probe only extended to the monitoring point via an optical fiber) is fixed and installed at the calibrated monitoring point. The following are two typical but non-limiting embodiments.

[0150] Example 1: Pressure Monitoring of Aircraft Engine Compressor Section

[0151] The sensing probe is securely mounted within the pressure measurement port of the engine casing. Crucially, the open microcavity region of the probe must be directly and unobstructed exposed to the high-speed gas environment of the engine flow path to ensure that gas pressure acts on the sensing region of the microcavity without loss. The seal between the probe and the pressure measurement port uses a high-temperature, pressure-resistant metal or ceramic seal to prevent gas leakage and isolate interference from non-measuring areas. The remainder of the laser is routed to the relatively stable engine nacelle area via armored optical fiber. In this application, drastic changes in engine operating conditions can lead to high-frequency pressure pulsations and vibrations. Therefore, the raw wavelength time-series data acquired from the wavelength meter needs to be low-pass digitally filtered (e.g., using a Butterworth filter with a cutoff frequency of 1 kHz) to filter out vibration-induced noise and extract low-frequency wavelength drift signals related to average air pressure and temperature.

[0152] Example 2: Dissolved Gas Pressure Monitoring in Power Transformer Oil

[0153] The sensing probe is sealed and installed inside the gas chamber (or gas collection chamber) of the transformer oil chromatography online monitoring system via a dedicated high-pressure sealing flange. The gas chamber is connected to the transformer oil through a permeable membrane to collect characteristic gases (such as H2, CH4, C2H2, etc.) precipitated from the oil. The probe directly measures the gas pressure within this sealed chamber. The laser pump and detection unit are located inside the main control cabinet.

[0154] This environment is relatively stable, but may exhibit slow temperature gradient changes. The acquired wavelength data needs to be processed using a moving average (e.g., employing a 60-second time window) to suppress random noise and improve the signal-to-noise ratio of single-point measurements. Simultaneously, the background temperature of the environment (which can be provided by a reference temperature sensor installed outside the air chamber) needs to be recorded and stored for verification or correction during subsequent calculations.

[0155] Since the operating wavelength of a fiber laser is determined by the resonant valley filtering characteristics of the sensing probe, the laser output wavelength will change accordingly when changes in ambient temperature and air pressure cause a resonant valley wavelength shift. The mode of this change depends on the relative magnitude of the resonant valley wavelength shift and the inherent longitudinal mode spacing of the laser.

[0156] When ambient temperature and air 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:

[0157] The first type is continuous tuning mode. When environmental changes are slow or minor, such that the spectral drift of the sensor probe within a single sampling / response cycle is less than the interval between adjacent longitudinal modes of the fiber laser, the laser does not need to change the longitudinal mode number of its oscillation. The output laser wavelength will continuously and smoothly change at the wavelength of that specific locked longitudinal mode as the resonance valley moves, exhibiting continuous wavelength tuning. This mode is suitable for high-resolution monitoring of slowly changing or minute parameter variations. For example, in transformer gas chamber monitoring, the slow accumulation of gas results in a very low rate of pressure change, and the laser wavelength typically operates in this mode.

[0158] The second type is the mode-hopping mode. When the environment changes drastically or significantly, the spectral drift of the sensor probe is no less than the interval between adjacent longitudinal modes of the laser. The original longitudinal mode suffers increased loss due to mismatch with the minimum resonance valley, no longer satisfying the lasing condition. At this time, another adjacent longitudinal mode that is currently more matched with the minimum resonance valley will win in the mode competition, and the output wavelength of the laser will undergo a discrete jump, with the jump amount being one or more longitudinal mode intervals. Despite the longitudinal mode jump, the laser wavelength is always anchored within the moved resonance valley filter window, that is, the output laser wavelength is approximately equal to the new resonance valley wavelength. This mode enables the system to track a wide range of parameter changes. For example, when an aero-engine suddenly accelerates from idle 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.

[0159] The output laser wavelength is monitored in real time using a high-resolution optical wavelength meter (accuracy better than 1 pm). The change is the difference between the current real-time output laser wavelength and the output laser wavelength under reference conditions.

[0160] For the first mode, the change directly reflects the drift of the resonance valley, and can be directly quantified.

[0161] In the second mode, simple changes in readings can result in discontinuous steps due to abrupt changes. To aid in determining the total resonance valley drift corresponding to the current laser wavelength and to assist subsequent calculation algorithms in determining the working range, this embodiment records the number of laser wavelength abrupt changes and the longitudinal mode interval, combined with the laser's effective cavity length and group refractive index, to coarsely estimate the total resonance valley drift caused by environmental changes. The coarse estimation formula is as follows:

[0162] ;

[0163] in, The total drift of the resonant valley is represented by N, which represents the number of forward or reverse transitions. This helps determine the resonant order and calculation range of the output laser wavelength. By monitoring the time series of the output laser wavelength, a transition is recorded when its change occurs abruptly close to the longitudinal mode interval. Forward transitions (wavelength side length) are represented by positive values ​​for N, and reverse transitions by negative values. A transition threshold can be set in the counting algorithm; the absolute value of the transition threshold ranges from 0.5 times the longitudinal mode interval to 1.5 times the longitudinal mode interval to distinguish between genuine mode transitions and noise. The group refractive index of the fiber laser is determined by the fiber type. For standard communication fiber SMF-28, it is typically 1.468 in the 1500nm band, a known material constant. Once the effective cavity length parameter of the fiber laser is determined, it can be stored in the memory chip of the demodulation unit for subsequent calculations.

[0164] This formula essentially multiplies the longitudinal mode interval by the number of transitions N to represent the total drift of the resonance valley. Its rationale lies in the fact that each transition corresponds to a shift of approximately one longitudinal mode interval in the resonance valley. Its main function is not to provide a final, highly accurate drift value, but rather to: quickly estimate the approximate drift range of the resonance valley after a large-scale transition; to provide an initial estimate or range verification for the dynamic iteration method in step 5, preventing the calculation process from losing the correct resonance order due to excessive transitions (i.e., determining whether the current laser is locked in the primary or secondary valley).

[0165] 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.

[0166] Due to the complexity of actual monitoring environments, raw data read directly from the wavelength meter may contain noise, jumps, or outliers. To ensure calculation accuracy, the raw wavelength data needs to be preprocessed before entering the quantization algorithm.

[0167] In this embodiment, the preprocessing operations include moving average filtering and outlier removal.

[0168] Moving average filtering uses a sliding time window of length Q to average the continuously acquired wavelength sequence in order to suppress high-frequency random noise. ,in, The sampling interval is defined by the value of Q. The length Q of the sliding window needs to be chosen to balance the smoothing effect and the response speed. Typically, the value of Q is chosen such that the time constant (Q*Δt) is much smaller than the characteristic change time of the measured environmental physical quantity (temperature, air pressure). For example, for monitoring relatively slow-changing transformer oil temperature, Q can be 5-20, corresponding to a time constant of several seconds to tens of seconds; for monitoring rapid pulsations in engine flow channel air pressure, Q can be 1-3 to ensure a fast response. m represents the loop index variable used to traverse the data points within the sliding window; m=0 represents the data at the current moment, and m=1 represents the data at the previous moment.

[0169] After filtering, a dynamic threshold can be set based on the statistical characteristics of recent data (such as mean and standard deviation). If the wavelength value of a sampling point deviates from the mean by more than 2 to 3 times the standard deviation, it is determined to be an outlier. In this case, the previous valid value can be used to replace the outlier, or the mean of the previous and next valid values ​​can be used to replace the outlier.

[0170] The preprocessed wavelength data is sent to the quantization module to determine the wavelength changes corresponding to the primary and secondary valleys. This embodiment provides two methods to adapt to different monitoring scenarios.

[0171] First, the wavelength change is quantified using the static mapping method (the default method). This method is suitable for situations where environmental changes are relatively gradual, and the laser output wavelength does not undergo mode hopping or the number of hopping cycles can be clearly recorded. Specifically, this includes:

[0172] Based on the center wavelengths of the main valley and secondary valley measured under reference conditions; during real-time monitoring, the absolute difference between the current output laser wavelength and the two center wavelengths measured under reference conditions is calculated and expressed as: and ;in, Indicates the current output laser wavelength. This indicates the center wavelength of the sub-valley under the reference condition.

[0173] like The laser is currently locked in the main valley, and the wavelength change in the main valley is directly calculated. ;in, This indicates the amount of wavelength change in the current main valley.

[0174] To obtain the sub-valley wavelength variation used for matrix solution This is based on a key assumption: when the sensing probe is subjected to the same environmental disturbance, the wavelength drift trends of the two resonant valleys exhibit a high degree of linear correlation. Therefore, with the primary valley locked, the wavelength change of the secondary valley can be estimated as follows: ;in, This represents the wavelength change of the current valley. The essence of this estimate is that, at any given moment, the wavelength interval between the two resonant valleys approximately remains constant relative to their reference interval. This assumption is highly reasonable when environmental changes are relatively small compared to the linear measurement range of the sensing probe (e.g., temperature changes less than 50°C, air pressure changes less than 1 MPa), and its error can be verified to be within an acceptable range through calibration experiments.

[0175] Conversely, if it is determined that the laser is locked in the secondary valley, then calculate... And preliminary estimates .

[0176] This embodiment also provides another dynamic iteration method (triggering activation method), the triggering condition being:

[0177] When the demodulation system detects a step change in the laser wavelength of at least one longitudinal mode interval, it determines that a mode hopping has occurred. At this point, the static mapping method simply uses... Comparison with a fixed reference may fail because It may have jumped to a position far from the original reference.

[0178] When the absolute value of the rate of change of temperature or the rate of change of air pressure calculated by the static mapping method exceeds a preset threshold (e.g., the threshold for the rate of change of temperature is 10℃ / s, and the threshold for the rate of change of air pressure is 0.1MPa / s) within a unit time (e.g., 1 second), it indicates that the environment has changed drastically, the static linear estimation may be too biased, and a more accurate iterative algorithm needs to be used.

[0179] The determination of temperature and pressure change rate thresholds should be based on the analysis of extensive historical data or typical operating conditions. For example, in aero-engine compressor testing, the pressure change rate threshold is set by analyzing the maximum gradient of pressure pulsations; in power transformer monitoring, the temperature change rate threshold is set by analyzing the maximum rate of oil temperature rise during faults or surges. The aim is to switch algorithms promptly before the static method error increases significantly.

[0180] Set the iteration count to zero (k=0), and use the most recently calculated temperature and pressure values ​​as the current estimates. If there are no historical values, use the baseline state as the current estimate.

[0181] Based on the current estimate and the calibrated sensitivity coefficient matrix, the theoretical wavelength of the main valley is calculated using the following formula:

[0182] ;

[0183] in, This represents the theoretical wavelength of the main valley in the k-th iteration; , Let these represent the temperature and air pressure in the estimated value of the k-th iteration, respectively; similarly, ,in, This represents the theoretical wavelength of the second valley in the k-th iteration. k represents the iteration index.

[0184] 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.

[0185] 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.

[0186] ;

[0187] in, This represents the wavelength change during the temporary valley in the k-th iteration.

[0188] Will and The vector is composed, and the new environmental estimate is calculated using the inverse matrix of the sensitivity coefficients:

[0189] ;

[0190] 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.

[0191] 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.

[0192] In this embodiment, decoupling the temperature change and the air pressure change specifically includes:

[0193] The input vector is constructed based on the real-time wavelength changes of the two resonance valleys, and is expressed as follows: ;

[0194] By calling the sensitivity coefficient matrix and its inverse matrix and performing matrix multiplication, an output vector containing temperature and pressure changes is obtained, as shown in the following formula:

[0195] ;

[0196] in, This represents an output vector that includes changes in temperature and air pressure. , These represent the rate of temperature change and the amount of air pressure change, respectively. The inverse matrix representing the sensitivity coefficient matrix;

[0197] The definitions and parameter meanings of the sensitivity coefficient matrix and its inverse are as follows:

[0198] ;

[0199] ;

[0200] Where D is the determinant of the sensitivity coefficient matrix; the absolute value of D must be large enough (much greater than zero), which is the mathematical basis for achieving effective differentiation between the two parameters.

[0201] The calculated temperature and pressure changes are added to the known environmental values ​​at the monitoring points to obtain the real-time absolute air pressure and real-time absolute temperature values. Here, the environmental values ​​refer to the on-site values ​​measured at the monitoring points using independent high-precision thermometers and barometers when the sensor probes are deployed, used to convert relative measurements into absolute measurements. These environmental values ​​may not be the same as the previously set reference values.

[0202] Please see Figure 4 The present invention also provides a dual-mode resonant fiber optic microcavity pressure measurement method, which is executed by the aforementioned dual-mode resonant fiber optic microcavity pressure measurement system, and the specific steps include:

[0203] Step 1: In the core and cladding region of a standard single-mode fiber for communication, an open microcavity is formed by femtosecond laser etching, penetrating the cladding and partially cutting into the core; 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 sensing probe; the interference spectrum of this sensing probe has at least two resonance valleys in the C+L band;

[0204] Step 2: 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 its 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.

[0205] Step 3: Use the sensing probe as a wavelength selective filter and connect it to the prefabricated ring fiber laser resonant cavity. Utilize its filtering characteristics to compete with the laser mode and construct a fiber laser whose output laser wavelength is determined by the position of the resonant valley.

[0206] Step 4: Deploy the fiber laser system with integrated 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 drift continuously; monitor and quantify the wavelength change in real time using a high-resolution optical wavelength meter.

[0207] Step 5: Substitute the wavelength change into the inverse matrix of the sensitivity coefficient matrix for calculation, and decouple the temperature change and air pressure change at the monitoring point in real time.

[0208] The above formulas are all dimensionless calculations. The formulas are derived from software simulations based on a large amount of collected data to obtain the most recent real-world results. The preset parameters in the formulas are set by those skilled in the art according to the actual situation.

[0209] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented in software, the above embodiments can be implemented, in whole or in part, as a computer program product. Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution.

[0210] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment, depending on actual needs.

[0211] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application.

Claims

1. A dual-mode resonant fiber optic microcavity barometric pressure measurement system, characterized in that, include: The sensor probe fabrication module is used to form an open microcavity that penetrates the cladding and partially cuts into the core of a standard single-mode fiber by femtosecond laser etching in the core and cladding regions of the fiber. A high-temperature tapering process was used to form a micro-nano tapered region in an open micro-tapered fiber-internal microcavity to fabricate an open micro-tapered fiber-internal microcavity 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.

2. The fiber optic microcavity barometric pressure measurement system according to claim 1, characterized in that, The radial width range of the open microcavity is limited to: 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 cone region is located at the axial center of the open microcavity, which is used to enhance the coupling efficiency between the core mode and the cladding mode, and to make the resonance valley depth of the interference spectrum greater than 10dB, thereby ensuring 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 fiber core fundamental mode and the excited multi-order cladding modes; of the two resonance valleys with different response characteristics to temperature and air pressure, one is the main resonance valley formed by the interference between the fundamental mode and the low-order cladding mode, and the other is the secondary resonance valley formed by the interference between the fundamental mode and the high-order cladding mode; the wavelength interval between the main resonance valley and the secondary resonance valley falls within the range of [20nm, 80nm].

3. The fiber optic microcavity barometric pressure measurement system according to claim 2, characterized in that, The calibration of the sensitivity coefficient matrix specifically includes: Within a calibration chamber with temperature and 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 set of temperature calibration points as a variable temperature sequence; the pressure is changed at a fixed pressure step within the pressure measurement range to obtain a set of pressure calibration points as a variable pressure sequence; each temperature calibration point in the variable temperature sequence and each pressure calibration point in the variable pressure sequence are combined to obtain several calibration point groups; In each calibration point group state, the sensing probe is illuminated with a broadband light source, and its output transmission interference spectrum is collected by a spectrum analyzer. The center wavelength of the pre-selected main resonance valley and the center wavelength of 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. Hereinafter, the main resonance valley is the main valley and the secondary resonance valley is the secondary valley. Using a preset standard room temperature of 25℃ and a standard atmospheric pressure of 101.325 kPa as a reference, the wavelength drift of each calibration point group relative to the two resonance valleys under the reference condition is calculated using the following formula: ； in, This indicates the wavelength shift of the main valley under the q-state of the calibration point group; This indicates the center wavelength of the main valley in the q-state of the calibration point group; This indicates the center wavelength of the main valley under reference conditions, and q is the index of the calibration point group. Similarly, the wavelength shift of the secondary valley in the q-state of the calibration point group is obtained, denoted as... ; At the same time, record the corresponding temperature and air pressure changes: ； in, These represent the temperature change and air pressure change of the q-th calibration point group, respectively; , These represent the temperature and air pressure of q calibration point groups, respectively; , These represent the temperature and air pressure under reference conditions, respectively. For data from all calibration point sets, the sensitivity coefficient matrix is ​​directly determined by solving the least-squares solution of the overdetermined equation system, as follows: ； ； in, , These are the sensitivity coefficients of the main valley and the secondary valley to temperature, respectively. , These are the sensitivity coefficients of the main valley and the secondary valley to air pressure, respectively; thus, a sensitivity coefficient matrix is ​​constructed as follows: ; The determinant of the sensitivity coefficient matrix is ​​calculated, and its absolute value is verified to be greater than a preset threshold to ensure that the sensitivity coefficient matrix is ​​invertible and can effectively distinguish between temperature and air pressure measurements. Subsequently, the sensitivity coefficient matrix and its inverse matrix are stored in the demodulation unit of the fiber laser.

4. The fiber optic microcavity pressure measurement system with dual-mode resonance according to claim 1, characterized in that, The specific structure and connection relationship of the annular fiber laser resonator are as follows: The output pigtail of the pump source is connected to the pump port of the wavelength division multiplexer; the common port of the wavelength division multiplexer is connected to one end of an erbium-doped fiber that serves as the gain medium; the other end of the erbium-doped fiber is connected to the input end of an optical isolator; the output end of the optical isolator is connected to the input end of a sensing probe; and the output end of the sensing probe is connected to the signal port of the wavelength division multiplexer, thereby forming a closed ring fiber laser resonant cavity. The process of constructing a fiber laser whose output laser wavelength is determined by the location of the resonant valley is implemented as follows: The minimum value of the transmission spectrum of the sensing probe at the resonance valley is extracted and used as a wavelength selective filter to meet the threshold condition for laser oscillation initiation. ,in, To erbium-doped fiber at wavelength Net gain at the location; This represents the transmission spectrum of the sensing probe; This represents the total transmission loss coefficient of all components outside the sensing probe within the ring fiber laser resonant cavity. When the pump power exceeds a threshold, intracavity mode competition limits the final oscillating laser wavelength to the position with the highest relative loss in the sensing probe's transmission spectrum, i.e., the resonance valley. Thus, the output laser wavelength is determined by the position of the resonant valley of the sensing probe; To improve the resolution of the pressure sensing, the parameters of the ring fiber laser resonator and the mode competition are optimized to enable the fiber laser to operate in a single longitudinal mode or a 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], and the corresponding longitudinal mode spacing is greater than the corresponding value of the 3dB bandwidth of the resonant valley, which serves as the filter window in the transmission spectrum of the sensing probe, in the frequency domain. First, the product of the effective refractive index of the fiber and the physical cavity length of the annular fiber laser resonator is calculated. The ratio of the speed of light in vacuum to this product is the longitudinal mode spacing. Under this condition, only one or more longitudinal modes fall within the resonant valley filter window, and a narrow linewidth output is finally formed through mode competition. Its 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 fiber optic microcavity barometric pressure measurement system according to claim 1, characterized in that, A fiber laser is fixed at a 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 inside the pressure measuring hole, and its open microcavity is directly exposed to the gas pressure environment of the engine flow channel; and inside the gas chamber sampling chamber of the online monitoring system for dissolved gases in power transformer oil; the sensing probe is sealed and installed inside the gas chamber for directly measuring the gas pressure of the evolved gas; When ambient temperature and air 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 type is the continuous tuning mode. When environmental changes cause the spectral shift of the sensing probe to be less than the interval between adjacent longitudinal modes of the fiber laser, the output laser wavelength exhibits continuous wavelength tuning within a single resonant valley filtering range. The second type is the mode-hopping mode. When environmental changes cause the spectral drift of the sensor probe to be no less than the interval between 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 laser wavelength jumps and the longitudinal mode spacing, and combining this with the effective cavity length and group refractive index of the laser, a coarse estimate is made of the total drift of the resonance valley caused by environmental changes. The coarse estimate formula is as follows: ； in, This represents the total drift of the resonance valley, and N represents the number of positive or negative transitions. The group refractive index of a fiber laser is represented by its group refractive index. This indicates the effective cavity length of the fiber laser.

6. The fiber optic microcavity barometric pressure measurement system according to claim 3, characterized in that, The quantification of the wavelength change using the static mapping method specifically includes: Based on the center wavelengths of the main valley and secondary valley measured under reference conditions; during real-time monitoring, the absolute difference between the current output laser wavelength and the two center wavelengths measured under reference conditions is calculated and expressed as: and ;in, Indicates the current output laser wavelength. This indicates the center wavelength of the sub-valley under the reference condition; like The laser is currently locked in the main valley, and the wavelength change in the main valley is directly calculated. ;in, This indicates the current wavelength change in the main valley; Meanwhile, based on the linear response assumption, the wavelength change of the secondary valley is initially estimated to be... ;in, This represents the wavelength change at the current sub-valley; conversely, if it is determined that the laser is locked at the sub-valley, then calculate... And preliminary estimates .

7. The fiber optic microcavity barometric pressure measurement system according to claim 6, characterized in that, When a mode jump in the output laser wavelength is detected, or when the absolute value of the change in a physical quantity calculated by the static mapping method per unit time exceeds a preset threshold, the dynamic iteration method is activated for quantization. The dynamic iteration method includes the following steps: Set the number of iterations to zero, and use the most recently calculated temperature and pressure values ​​as the current estimates. If there are no historical values, use the baseline state as the current estimates. Based on the current estimate and the calibrated sensitivity coefficient matrix, the theoretical wavelength of the main valley is calculated using the following formula: ； in, Indicates the theoretical wavelength of the main valley; , Let these represent the temperature and air pressure in the current estimates, respectively; similarly, obtain the theoretical wavelength of the sub-valley, denoted as... ; The real-time measured output laser wavelength is compared with the theoretical wavelengths of the main valley and secondary valley, respectively. The one with the smallest difference is selected as the current locked valley, and the temporary wavelength change is calculated. Here, r represents the primary valley or secondary valley, and r is either i or j; The temporary wavelength change is combined with another corresponding change that is not currently locked into a vector, and the updated temperature and pressure values ​​are calculated using the inverse of the sensitivity coefficient matrix. Calculate 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. If both absolute differences are less than a set threshold, the iteration ends and the temporary wavelength change at the time of the last iteration is output. Otherwise, the updated temperature and air pressure values ​​are used as the current estimates and the iteration continues.

8. The fiber optic microcavity barometric pressure measurement system according to claim 6, characterized in that, Decoupling the temperature change from the 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: ; By calling the sensitivity coefficient matrix and its inverse matrix and performing matrix multiplication, an output vector containing temperature and pressure changes is obtained, as shown in the following formula: ； in, This represents an output vector that includes changes in temperature and air pressure. , These represent the rate of temperature change and the amount of air pressure change, respectively. The inverse matrix representing the sensitivity coefficient matrix; The definitions and parameter meanings of the sensitivity coefficient matrix and its inverse are as follows: ； ； Where D is the determinant of the sensitivity coefficient matrix; The calculated temperature and pressure changes are added to the known environmental values ​​at the monitoring points to obtain the real-time absolute air pressure and real-time absolute temperature values.

9. A method for measuring air pressure in a fiber optic microcavity with dual-mode resonance, characterized in that, The dual-mode resonant fiber optic microcavity barometric pressure measurement method is executed by the dual-mode resonant fiber optic microcavity barometric pressure measurement system according to any one of claims 1-8, and the specific steps include: Step 1: In the core and cladding region of a standard single-mode fiber for communication, an open microcavity is formed by femtosecond laser etching, penetrating the cladding and partially cutting into the core; 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 sensing probe; the interference spectrum of this sensing probe has at least two resonance valleys in the C+L band; Step 2: 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 its 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. Step 3: Use the sensing probe as a wavelength selective filter and connect it to the prefabricated ring fiber laser resonant cavity. Utilize its filtering characteristics to compete with the laser mode and construct a fiber laser whose output laser wavelength is determined by the position of the resonant valley. Step 4: Deploy the fiber laser system with integrated 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 drift continuously; monitor and quantify the wavelength change in real time using a high-resolution optical wavelength meter. Step 5: Substitute the wavelength change into the inverse matrix of the sensitivity coefficient matrix for calculation, and decouple the temperature change and air pressure change at the monitoring point in real time.

Images