Hollow core optical fiber and sensing system thereof

Abstract

The application discloses a hollow optical fiber and a sensing system thereof. The hollow optical fiber comprises, from inside to outside, an air core, a cladding and a coating layer. The inner wall surface of the air core is provided with a plurality of micrograting units with different periods and arranged alternately along the axial direction. The cladding comprises at least one solid core light guide wire. The sensing system comprises a dual-wavelength light source module, an optical circulator, a wavelength demultiplexing module, a photodetector module, a solid core wire detection unit, a signal processing module and the hollow optical fiber. The first port to the third port of the optical circulator are connected with the output end of the dual-wavelength light source module, the input end of the hollow optical fiber and the input end of the wavelength demultiplexing module respectively. The photodetector module is connected with the output end of the wavelength demultiplexing module and the input end of the signal processing module. The output end of the hollow optical fiber is connected with the input end of the solid core wire detection unit. The application can improve the measurement accuracy of the optical fiber sensing on temperature, strain and sound pressure, and can be widely applied in the field of optical fiber sensing technology.

Description

Technical Field

[0001] This application relates to the field of optical fiber sensing technology, and in particular to a hollow optical fiber and its sensing system. Background Technology

[0002] Distributed fiber optic sensing technology has a significant advantage: strong resistance to electromagnetic interference and the ability to perform continuous measurements over long distances. Therefore, it is increasingly used in fields such as structural health monitoring, oil and gas pipeline monitoring, and aerospace. However, existing systems face a challenge when simultaneously measuring multiple parameters: the optical responses of temperature, strain, and sound pressure are coupled within the optical fiber, making it difficult to precisely separate them on a single fiber, thus requiring improvement in measurement accuracy. Summary of the Invention

[0003] The main objective of this application is to propose a hollow optical fiber and its sensing system, which aims to improve the accuracy of optical fiber sensing in measuring temperature, strain, and sound pressure.

[0004] To achieve the above objectives, one aspect of this application proposes a hollow-core optical fiber, comprising, from the inside out, an air core, a cladding, and a coating. The inner wall surface of the air core has a plurality of micrograting units of different periods arranged alternately along the axial direction. The air core extends along the optical fiber axis. The cladding includes at least one anti-resonant capillary layer and at least one solid optical guide filament. The refractive index of the solid optical guide filament is higher than that of the cladding material. The air fiber core is used to transmit light waves; The cladding is used to confine the light field within the air core.

[0005] In some embodiments, the hollow fiber includes at least two micrograting units, the period of the first micrograting unit is in the range of 160 μm to 200 μm, the period of the second micrograting unit is in the range of 600 μm to 800 μm, the groove depth of the two micrograting units is in the range of 100 nm to 500 nm, and the groove width is in the range of 1 μm to 5 μm.

[0006] In some embodiments, the core diameter of the solid light guide filament is in the range of 6 μm to 12 μm, the numerical aperture is in the range of 0.12 to 0.20, and the minimum distance between the solid light guide filament and the air core needs to be greater than 10 μm.

[0007] To achieve the above objectives, another aspect of this application proposes a sensing system based on hollow-core optical fiber, including a dual-wavelength light source module, an optical circulator, a wavelength demultiplexing module, a photodetector module, a solid-core fiber detection unit, a signal processing module, and the aforementioned hollow-core optical fiber. The first to third ports of the optical circulator are respectively connected to the output of the dual-wavelength light source module, the input of the hollow-core optical fiber, and the input of the wavelength demultiplexing module. The photodetector module is connected to the output of the wavelength demultiplexing module and the input of the signal processing module. The output of the hollow-core optical fiber is connected to the input of the solid-core fiber detection unit. The dual-wavelength light source module is used to inject two probe light pulses of different wavelengths into the hollow optical fiber; The wavelength demultiplexing module is used to separate the light reflected from the optical circulator according to wavelength; The signal processing module is used to process the detection signals of the photodetector module and the solid wire detection unit to obtain multi-parameter values; the multi-parameter values ​​include temperature change, axial stress and sound pressure.

[0008] In some embodiments, the signal processing module performs the following processing steps: The wavelength offset of at least four reflection peak centers of the micrograting unit is extracted from the detection signal of the photodetector module; At least two changes in Rayleigh scattering spectrum shape parameters are extracted from the detection signal of the solid wire detection unit; A measurement vector is constructed based on the wavelength offset of the center of the reflection peak and the change in the shape parameter of the Rayleigh scattering spectrum, and the multi-parameter values ​​are calculated based on the measurement vector and the preset sensitivity matrix.

[0009] In some embodiments, the processing procedure of the signal processing module further includes: The standardized residuals of each measurement channel are calculated based on the measurement vector, the preset sensitivity matrix, and the multi-parameter values. Channels with standardized residuals greater than a preset threshold are marked as suspicious measurement channels. If the number of suspicious measurement channels is less than or equal to a first preset value, the suspicious measurement channels are removed, and the multi-parameter values ​​are recalculated based on the remaining measurement channels and the corresponding sensitivity submatrix.

[0010] In some embodiments, the processing procedure of the signal processing module further includes: If the number of remaining measurement channels is less than the second preset value, the multi-parameter values ​​of the previous moment are maintained.

[0011] In some embodiments, the dual-wavelength light source mode includes a first narrow-linewidth laser, a second narrow-linewidth laser, a combiner, an electro-optic intensity modulator, and a timing controller. The input terminal of the combiner is connected to the output terminals of the first narrow-linewidth laser and the second narrow-linewidth laser. The input terminal of the electro-optic intensity modulator is connected to the output terminal of the combiner. The output terminal of the electro-optic intensity modulator is connected to the first port of the optical circulator. The output terminal of the timing controller is connected to the control terminal of the electro-optic intensity modulator.

[0012] In some embodiments, the timing controller is configured as follows: Within the first time window, a first electrical pulse is output to control the electro-optic intensity modulator to conduct light of the first wavelength, thereby forming a probe light pulse of the first wavelength; According to a preset time interval, a second electrical pulse is output within a second time window to control the electro-optic intensity modulator to conduct light of a second wavelength, forming a probe light pulse of a second wavelength; the preset time interval is greater than the sum of the round-trip time of the optical signal in the hollow optical fiber and the guard interval.

[0013] In some embodiments, the wavelength demultiplexing module includes an arrayed waveguide grating or a structure of at least one coarse wavelength division multiplexer cascaded together, and the channel isolation of the wavelength demultiplexing module is greater than 30dB.

[0014] The embodiments of this application include at least the following beneficial effects: This application provides a hollow optical fiber and its sensing system. On the inner wall of the air core of the hollow optical fiber, micro-grating units of different periods are alternately etched along the axial direction, and a solid light guide filament is embedded in the cladding region of the hollow optical fiber to assist in the realization of multi-parameter detection. The dual-wavelength light source module in the sensing system operates in a time-division multiplexing mode, sequentially emitting light pulses of two wavelengths into the hollow optical fiber. The receiving end uses a wavelength division multiplexer to separate the reflected light returned from the hollow optical fiber according to wavelength, and the detection signal of the photodetector module extracts the wavelength offset of at least four reflection peak centers. Based on this, combined with the changes in at least two spectral shape parameters of the Rayleigh scattering spectrum in the solid light guide filament, a multi-dimensional measurement vector is constructed to solve for the values ​​of multiple parameters such as temperature change, axial strain, and sound pressure. The multiple parameters do not interfere with each other, resulting in high accuracy. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the hollow optical fiber provided in the embodiments of this application; Figure 2 This is a schematic diagram of the structure of a sensing system based on hollow optical fiber provided in an embodiment of this application. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit it. In the following description, when referring to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with those of this application; they are merely examples of apparatuses and methods consistent with some aspects of the embodiments of this application as detailed in the appended claims.

[0017] It is understood that the terms “first,” “second,” etc., used in this application may be used herein to describe various concepts, but unless otherwise stated, these concepts are not limited by these terms. These terms are only used to distinguish one concept from another. For example, without departing from the scope of the embodiments of this application, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the words “if,” “when,” or “in response to a determination” as used herein may be interpreted as “when…” or “when…” or “in response to a determination.”

[0018] As used in this application, the terms "at least one", "multiple", "each", "any", etc., "at least one" includes one, two or more, "multiple" includes two or more, "each" refers to each of the corresponding multiples, and "any" refers to any one of the multiples.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing embodiments of this application only and is not intended to limit this application.

[0020] Before providing a detailed description of the embodiments of this application, some of the nouns and terms involved in the embodiments of this application will be explained first. The nouns and terms involved in the embodiments of this application are subject to the following interpretations.

[0021] In solid optical fibers, temperature and strain alter the refractive index and geometry through thermo-optic and elasto-optic effects, causing similar changes in parameters such as the phase and wavelength of light. While commonly used methods such as the dual-wavelength matrix method and the temperature reference grating method can decouple temperature and strain, they often require multiple independent sensing units or a complex compensation structure, and they cannot simultaneously measure sound pressure levels. To measure sound pressure levels simultaneously, an additional vibration sensor is needed, which not only makes the system more complex and costly but also easily introduces new cross-interference.

[0022] Hollow-core optical fibers offer an alternative approach to multi-parameter sensing. Because the light field is confined within the air core, the interaction between light and the silica material is weakened. While this reduces nonlinear effects, it also makes them less sensitive to temperature and strain. Introducing microgratings into hollow-core fibers either results in overly complex structures or limits the measurement to point-based dual-parameter measurements, failing to obtain the distribution information of all three parameters across the entire fiber.

[0023] Furthermore, current grating-based fiber optic sensing systems generally lack fault-tolerant design. If a grating or wavelength channel experiences signal degradation due to damage or other reasons, the system's decoupling capability will be significantly reduced, or it may even fail entirely. This problem is particularly prominent in applications requiring long-term unattended operation and high reliability.

[0024] See Figure 1 , Figure 1 (a) shows a cross-sectional view of the inner wall surface of the air fiber core. Figure 1 (a) shows a cross-sectional view of a hollow optical fiber. This application provides a hollow optical fiber comprising, from the inside out, an air core 1-1, a cladding, and a coating 1-3. The inner wall surface of the air core 1-1 has several micro-grating units of different periods arranged alternately along the axial direction. The air core 1-1 extends along the optical fiber axis. The cladding includes at least one anti-resonant capillary 1-2-1 and at least one solid optical guide filament 1-2-2. The refractive index of the solid optical guide filament 1-2-2 is higher than that of the cladding material. Air fiber core 1-1, used for transmitting optical waves; Cladding is used to confine the optical field within the air core.

[0025] In one specific embodiment, an air core runs along the fiber axial direction for transmitting light waves. The cladding region consists of at least one layer of anti-resonant capillaries, confining the light field within the core. On the inner surface of the air core, two sets of micro-grating units with different periods are distributed axially at equal intervals, designated as the first period Λ1 and the second period Λ2. These two sets of micro-grating units are arranged alternately, each being a locally refractive index modulated structure, fabricated by femtosecond laser etching of grooves or deposition of a dielectric film. These micro-grating units are capable of reflecting incident light. Furthermore, at least one solid-core light guide filament with a refractive index higher than the surrounding cladding material is embedded in the fiber cladding region. The two sets of gratings are arranged alternately along the fiber length, and the spacing between adjacent grating units can be set according to the measurement resolution requirements. Each grating unit possesses complete reflection functionality, with its central reflection wavelength determined by the grating period and the effective refractive index of the fiber.

[0026] This hollow-core optical fiber exhibits independent optical responses to three physical quantities: temperature, axial strain, and sound pressure. Temperature changes primarily alter the refractive index of the gas within the air core, thus affecting the dispersion characteristics of the grating's reflection peak, manifesting as a significant change in the full width at half maximum (FWHM) of the reflection peak. Axial strain mainly alters the period Λ of the micrograting, causing a linear shift in the center wavelength of the reflection peak. Sound pressure causes periodic changes in the gas density within the air core due to external sound waves. These density fluctuations modulate the Rayleigh scattering intensity and spectral shape of the light transmitted through the solid-core fiber via evanescent field coupling or mechanical transmission. Therefore, by detecting the center wavelength shift of the micrograting's reflection peak, the change in FWHM, and the change in the shape parameter of the Rayleigh scattering spectrum of the solid-core fiber, information related to temperature, strain, and sound pressure can be obtained, respectively.

[0027] The relationship between the reflection peak wavelength and the micrograting period is as follows. When the probe light pulse is incident on the micrograting unit, the corresponding wavelength will be strongly reflected. For the long-period grating used in this invention, the relationship between the center wavelength λ of the reflection peak and the grating period Λ can be approximately expressed as:

[0028] Here, Δn0 represents the effective refractive index difference between the air core mode and the cladding mode, typically ranging from 0.01 to 0.03, ensuring that λ falls within the near-infrared communication window when Λ is several hundred micrometers. Since Δn0 varies slightly with wavelength, but the change is minimal, λ is essentially proportional to Λ.

[0029] In some embodiments, the hollow fiber includes at least two micrograting units, the first micrograting unit having a period ranging from 160 μm to 200 μm, the second micrograting unit having a period ranging from 600 μm to 800 μm, and the groove depth of the two micrograting units ranging from 100 nm to 500 nm and the groove width ranging from 1 μm to 5 μm.

[0030] It should be noted that the period, groove depth, and groove width of the micrograting units in the hollow fiber are determined according to the actual application, and this embodiment does not impose specific limitations. The dual-period micrograting is formed by etching a groove array on the inner wall of the hollow fiber using a femtosecond laser.

[0031] In some embodiments, the core diameter of the solid light guide filament is in the range of 6 μm to 12 μm, the numerical aperture is in the range of 0.12 to 0.20, and the minimum distance between the solid light guide filament and the air core needs to be greater than 10 μm.

[0032] It should be noted that the core diameter, numerical aperture, and minimum distance between the solid optical guide filament and the air core are determined based on the actual application, and this embodiment does not impose specific limitations. Solid optical guide filaments include, but are not limited to, germanium-doped single-mode optical fibers.

[0033] See Figure 2 This application provides a sensing system based on hollow-core optical fiber, including a dual-wavelength light source module, an optical circulator, a wavelength demultiplexing module, a photodetector module, a solid-core fiber detection unit, a signal processing module, and the aforementioned hollow-core optical fiber. The first to third ports of the optical circulator are respectively connected to the output of the dual-wavelength light source module, the input of the hollow-core optical fiber, and the input of the wavelength demultiplexing module. The photodetector module is connected to the output of the wavelength demultiplexing module and the input of the signal processing module. The output of the hollow-core optical fiber is connected to the input of the solid-core fiber detection unit. A dual-wavelength light source module is used to inject two probe light pulses of different wavelengths into the hollow fiber; A wavelength demultiplexing module is used to separate light reflected from the optical circulator according to wavelength; The signal processing module is used to process the detection signals from the photodetector module and the solid wire detection unit to obtain multi-parameter values, including temperature change, axial stress, and sound pressure.

[0034] The probe light pulses generated by the dual-wavelength light source module enter the hollow-core sensing fiber through ports ① and ② of the circulator, and the reflected light returns from the fiber via port ② to port ③. During system operation, the dual-wavelength light source emits λ pulses sequentially according to the time-division multiplexing sequence described above. a and λ b An optical pulse enters the sensing fiber via an optical circulator. The reflected light from each micro-grating unit in the fiber returns along its original path, passes through the circulator again, and enters the wavelength demultiplexing module at the receiver. This module separates the composite reflected light into multiple paths according to wavelength, each corresponding to a specific reflection peak. Each optical signal is converted into an electrical signal by an independent photodetector and sent to the signal processing module.

[0035] In one specific embodiment, the inner wall of the optical fiber of the present invention is alternately arranged with two gratings of different periods. When two probe lights of different wavelengths are used... a and λ b During excitation, each grating reflects two wavelengths. Therefore, at each grating location, four reflection peaks are obtained, with their center wavelengths denoted as λ. 1a : grating pair with period Λ1 λ a The reflection peak; λ 1b : grating pair with period Λ1 λ b The reflection peak; λ 2a : grating pair with period Λ2 λ a The reflection peak; λ 2b : grating pair with period Λ2 λ bThe reflection peaks. When the optical fiber is subjected to strain or temperature changes, the center wavelengths of these reflection peaks shift. The perception of sound pressure mainly relies on the changes in the shape parameters of the Rayleigh scattering spectrum in the solid core optical guide filament. Sound pressure causes periodic changes in the gas density within the air core, which modulates the scattering characteristics of the solid core filament through evanescent field coupling.

[0036] When an optical fiber is subjected to temperature or strain, the changes in the two grating periods are different, resulting in different wavelength response coefficients for the same temperature or strain change. Specifically, assuming the optical fiber material, temperature coefficient, and strain coefficient are the same, gratings with different periods exhibit different K0 values. T With K ε The ratio. The sensitivity coefficients corresponding to Λ1 and Λ2 satisfy K. T1 / K ε1 Not equal to K T2 / K ε2 This condition ensures that the wavelength shifts measured from the two gratings can form two independent linear equations, thus allowing temperature and strain to be solved independently without relying on grating bandwidth measurements, while also providing more independent equation sources for the construction of subsequent overdetermined equation sets.

[0037] At least one solid-core optical guide filament is embedded in the cladding region of a hollow-core optical fiber. The main function of this solid-core filament is to sense the sound pressure signal: external sound pressure causes periodic changes in the gas density within the air core, and this density fluctuation modulates the Rayleigh scattering signal of the light transmitted in the solid-core filament through evanescent field coupling or mechanical transmission. Unlike traditional methods that only measure the intensity of scattered light, this scheme simultaneously acquires the Rayleigh scattering spectral broadening information of the solid-core filament, including the full width at half maximum (FWHM) and the second moment. Utilizing the linear relationships between the spectral width and the sound pressure gradient, and between the spectral variance and the effective value of the sound pressure, two independent sound pressure correlation equations are constructed to form an overdetermined system of equations.

[0038] Compared to traditional methods that only extract scattering intensity, this invention further extracts two shape parameters of the scattering spectrum: full width at half maximum (FWHM) and second moment. These parameters are highly sensitive to sound pressure levels but less sensitive to cross-sensitivity to temperature and strain, thus facilitating accurate sound pressure extraction. The sensing signal of this invention originates from the reflected light of the micrograting, rather than the inherent backscattered light of the optical fiber. The intensity of the reflected light is much higher than that of the scattered light, facilitating high signal-to-noise ratio detection, and the center wavelength and bandwidth of the reflection spectrum can be accurately extracted.

[0039] In some embodiments, the signal processing module performs the following process: The wavelength offset of at least four reflection peak centers of the micrograting unit is extracted from the detection signal of the photodetector module; Extract the changes in at least two Rayleigh scattering spectrum shape parameters from the detection signal of the solid wire detector unit; A measurement vector is constructed based on the wavelength shift at the center of the reflection peak and the change in the shape parameter of the Rayleigh scattering spectrum. Multi-parameter values ​​are then calculated based on the measurement vector and the preset sensitivity matrix.

[0040] In one specific embodiment, two micro-grating arrays (Λ1 and Λ2, arranged alternately) with different periods are etched axially on the inner wall of a single hollow optical fiber, and two different wavelengths (λ) are sequentially injected into the fiber in a time-division multiplexing manner. a and λ b The detector pulse is used to detect the light source, and the reflected signal generated by the grating is used for detection. The wavelength positions (λ) of the four reflection peaks are measured. 1a , λ 1b , λ 2a , λ 2b The signal processing module first performs peak fitting on each reflection peak to accurately extract its center wavelength; simultaneously, it calculates the wavelength offset Δλ relative to the initial state. 1a , Δλ 1b , Δλ 2a , Δλ 2b For solid optical guides, the signal processing module also acquires their Rayleigh scattering spectrum and calculates the changes in spectral shape parameters, namely the change in full width at half maximum (FWHM) ΔB1 and the change in second moment ΔB2. Thus, a total of m measurements (m=6) are obtained, forming a measurement vector. Y .

[0041] Let the three parameters to be measured be the temperature change ΔT, the axial stress ε, and the sound pressure P. Through fiber optic structure analysis and calibration experiments, the sensitivity coefficients of each measurement quantity to the three parameters can be determined in advance.

[0042] Sensitivity matrix determined in advance through calibration experiments H These measurements were established in relation to the three parameters X = [ΔT, ε, P]. T Linear relationship between them:

[0043] in, ν To measure the noise, since this system of equations is overdetermined, the least squares method is used to find the optimal estimate of X:

[0044] In one specific embodiment Y The 6-dimensional measurement vector includes four reflection peak wavelength offsets and at least two Rayleigh scattering spectrum shape parameter offsets; X It is a three-parameter vector; H It is a 6×3 sensitivity matrix, with each element being a pre-calibrated coefficient; ν This is a 6-dimensional measurement noise vector.

[0045] In some embodiments, the signal processing module's processing procedure further includes: Based on the measurement vector, the preset sensitivity matrix, and the multi-parameter values, the standardized residuals of each measurement channel are calculated, and channels with standardized residuals greater than a preset threshold are marked as suspicious measurement channels. If the number of suspicious measurement channels is less than or equal to the first preset value, the suspicious measurement channels are removed, and the multi-parameter values ​​are recalculated based on the remaining measurement channels and the corresponding sensitivity submatrix.

[0046] The signal processing module calculates the residual vector. r :

[0047] The residual vector is used to evaluate whether the standardized residuals of each measurement channel exceed a preset threshold. If the standardized residual of a channel is significantly large, it is marked as a suspicious measurement channel. If the number of suspicious measurement channels does not exceed (m... 3) The signal processing module automatically removes these suspicious channels and uses the remaining measurement vector Y' and the corresponding H' submatrix to perform a least-squares solution again, which still yields valid three-parameter estimates. Finally, the ΔT, ε, and P calculated for all grating positions are arranged along the fiber length to obtain distributed three-parameter curves, which are displayed and stored in real time.

[0048] During long-term operation of the sensor, a grating unit may experience signal degradation due to contamination, mechanical damage, or fiber bending, or a wavelength channel may suffer a decrease in signal-to-noise ratio due to fluctuations in light source power or ambient temperature drift. Traditional decoupling methods lack the capability to handle such situations.

[0049] This solution identifies abnormal channels in real time by monitoring the residual contribution of each measurement channel: after standardizing the residual corresponding to each measurement, it is compared with its statistical threshold. If a measurement deviates from the expected range, it is marked as "suspicious." For example, when the ratio of the residual norm to the measurement vector norm, ||r|| / ||Y||, is less than the preset threshold ε, it is marked as "suspicious." th If the number of channels is within a certain range, the solution is considered reliable; otherwise, the solution is considered suspicious. When the number of suspicious channels does not exceed (m... 3) Even when these channels are removed, the system can still maintain normal operation by resolving the overdetermined equations. This fault-tolerant mechanism ensures the robustness of the system, making it particularly suitable for long-term unattended applications. The dual-wavelength architecture naturally provides wavelength-level redundancy: when a certain wavelength channel (such as λ) is removed, the system can still maintain normal operation by resolving the overdetermined equations. a When the light source or optical path of the system fails, the system can automatically downgrade to a single-wavelength mode, and can still maintain the three-parameter decoupling through the double grating (Λ1,Λ2) and solid wire channel.

[0050] In some embodiments, the signal processing module's processing procedure further includes: If the number of remaining measurement channels is less than the second preset value, maintain the multi-parameter values ​​from the previous moment.

[0051] When suspicious channels are removed, if the number of remaining channels m' is less than 3, the system will declare itself in an underdetermined state and maintain the valid output from the previous moment.

[0052] In some embodiments, the dual-wavelength light source mode includes a first narrow-linewidth laser, a second narrow-linewidth laser, a combiner, an electro-optic intensity modulator, and a timing controller. The input of the combiner is connected to the output of the first and second narrow-linewidth lasers, the input of the electro-optic intensity modulator is connected to the output of the combiner, the output of the electro-optic intensity modulator is connected to the first port of the optical circulator, and the output of the timing controller is connected to the control terminal of the electro-optic intensity modulator.

[0053] In some embodiments, the timing controller is configured as follows: Within the first time window, the first electrical pulse is output to control the electro-optic intensity modulator to conduct light of the first wavelength, thereby forming a probe light pulse of the first wavelength; According to a preset time interval, a second electrical pulse is output within a second time window to control the electro-optic intensity modulator to conduct light of the second wavelength, forming a probe light pulse of the second wavelength; the preset time interval is greater than the sum of the round-trip time of the optical signal in the hollow fiber and the guard interval.

[0054] To achieve independent detection of the four-wavelength reflection peaks, this invention employs a dual-wavelength injection scheme that combines time-division multiplexing (TDM) and wavelength-division multiplexing (WDM).

[0055] The light source module consists of two independent external cavity tunable narrow linewidth lasers (linewidth ≤ 10kHz), each outputting a wavelength λ. a and λ b The two lasers employ a common-cavity temperature control technique to lock the two wavelengths to the same reference cavity, ensuring long-term frequency stability. The two laser beams are combined using a 2×1 wavelength division multiplexer before entering the subsequent optical path.

[0056] In TDM mode, the system operates according to a fixed timing sequence: first, a λ is transmitted. a An optical pulse, upon entering the optical fiber, is reflected by various micro-grating elements along its path. The time it takes for the reflected light to return to the receiver corresponds one-to-one with the location of the grating. The system transmits a λ... a After the pulse, wait a sufficiently long time, at least equal to the time it takes for the light to travel one round trip in the optical fiber, plus a guard interval. This ensures that all reflected signals from the farthest end of the fiber are received before transmitting the next pulse. b Pulse. Similarly, λ bThe reflected signal of the pulse is also independently acquired within the subsequent time window. The preset time interval is greater than the sum of the round-trip time of the optical signal in the hollow fiber and the guard interval, thereby ensuring λ a and λ b The corresponding reflected signals will not overlap in the time domain.

[0057] In one specific embodiment, to prevent nonlinear interference caused by the simultaneous transmission of two wavelengths of light in the optical fiber, this invention employs a time-division multiplexing strategy with alternating pulse transmission. The specific timing design is as follows (taking an optical fiber length of L=10km as an example): Let the group velocity of light in a hollow optical fiber be... v :

[0058] Among them, the air core refractive index of hollow fiber is n≈1.0, and the fiber one-way transmission time is... t 0:

[0059] The round trip time is 2 t 0, approximately 66.7 μs. In t At time 0, the system emits a λ. a Optical pulses. The receiver acquires signals from λ within a 0–66.7 μs window. a All reflected signals. In (2) t At time 0+Δt, where Δt is the protection interval, the system transmits a λ b Optical pulse. The receiver acquires light from λ within a window of 66.7 - (133.4 μs + Δt). b All reflected signals.

[0060] This timing ensures complete separation of the reflected signals of the two wavelengths in the time domain, fundamentally eliminating nonlinear effects that may be caused by simultaneous transmission. The guard interval Δt can also compensate for minor disturbances in fiber length caused by changes in ambient temperature, ensuring the reliability of the timing.

[0061] Although the two wavelengths are staggered in the time domain, in order to further improve the wavelength isolation and signal-to-noise ratio of the system, a coarse wavelength division multiplexer (CWDM) is set after the optical circulator and before the photodetector at the receiving end. Its function is to separate the returned composite light according to wavelength to different output ports.

[0062] Specifically, the reflected light from the third port of the circulator first enters the demultiplexer. The demultiplexer has two output channels: the center wavelength of channel 1 is aligned with λ. a The center wavelength of channel 2 is aligned with λ b λ after wave splitting a Light enters photodetector 1, λ bLight enters photodetector 2. In this way, even if the two wavelength signals slightly overlap in the time domain for some reason, WDM can physically isolate them in the wavelength dimension, ensuring that the two signals do not interfere with each other.

[0063] In some embodiments, the wavelength demultiplexing module includes an arrayed waveguide grating or a structure of at least one coarse wavelength division multiplexer cascaded together, and the channel isolation of the wavelength demultiplexing module is greater than 30dB.

[0064] The overall decoupling process of the present invention is described below using a specific embodiment: Step S1: Calibration Stage. Under laboratory conditions, temperature, strain, and sound pressure are calibrated independently or in combination to determine the sensitivity matrix. H All elements.

[0065] Step S2: Initiate measurement. Following the TDM timing sequence described above, sequentially inject λ into the dual-period alternating micrograting hollow fiber. a and λ b Light pulses; synchronously acquire Rayleigh scattering spectra of solid optical guide filaments.

[0066] Step S3: Feature Extraction. For each grating position, the center wavelength shift of the four-wavelength reflection peaks is demodulated through WDM demultiplexing and photoelectric conversion at the receiver; for the solid silk Rayleigh scattering spectrum, the changes in at least two spectral shape parameters are extracted.

[0067] Step S4: Construction and solution of the overdetermined system of equations. Substitute the m measurements (m=6) obtained in step S3 into the measurement model, and use the least squares method to solve for the optimal estimates of ΔT, ε, and P.

[0068] Step S5: Reliability Assessment and Fault Diagnosis. Calculate the solution residuals to determine if each measurement channel is in normal condition. If the residuals exceed a preset threshold, mark the abnormal channel and notify the user.

[0069] Step S6: Fault-Tolerant Dimensionality Reduction. Based on the labeling of abnormal channels, eliminate abnormal measurements, reconstruct the dimensionality-reduced measurement model, and iteratively solve until the residuals converge or the number of channels is insufficient. Output the final three-parameter measurement values ​​and their confidence labels.

[0070] Step S7: Repeat steps S3 to S6 along the entire optical fiber to obtain the distributed three-parameter measurement results.

[0071] Compared with the prior art, the present invention has the following beneficial effects: 1. Synchronous Distributed Measurement of Three Parameters: Information on three physical quantities—temperature, strain, and sound pressure—is simultaneously acquired on a single dual-period micro-grating hollow fiber. The combination of dual wavelengths and dual gratings provides four independent grating channel equations. Combined with Rayleigh scattering spectrum analysis of the solid fiber, complete decoupling of the three parameters is achieved, and the sensitivity matrices among the three parameters are non-singular, fundamentally eliminating the problem of cross-sensitivity.

[0072] 2. High-precision decoupling: Wavelength measurement rather than bandwidth measurement is used as the main decoupling method. The dual-wavelength dual-grating scheme decouples temperature and strain by using the difference in strain / temperature sensitivity of different period gratings at different wavelengths. It does not rely on the low-precision bandwidth measurement, which significantly improves the measurement accuracy.

[0073] 3. Overdetermined redundancy and self-diagnosis: By constructing measurement vectors to perform overdetermined measurements on three unknowns, not only is the noise immunity improved, but the reliability of the solution can also be evaluated in real time through residual analysis, providing a basis for system self-diagnosis.

[0074] 4. Active Fault Tolerance: The automatic dimensionality reduction mechanism of this invention enables the system to maintain sensing functionality even when some channels fail, preventing the entire system from collapsing due to a single point of failure, thus significantly improving the system's robustness and reliability. The wavelength-level redundancy provided by the dual-wavelength architecture further enhances fault tolerance.

[0075] 5. Intrinsically safe and suitable for long distances: There are no electrical signals on the entire optical fiber, making it suitable for flammable and explosive environments; hollow optical fiber has low transmission loss (typical value <5dB / km), and combined with the periodic arrangement of micro-gratings, it can achieve a spatial resolution of ten meters over a distance of several kilometers.

[0076] 6. Multiple anti-interference design: The combination of time division multiplexing (TDM), wavelength division multiplexing (WDM) and polarization isolation ensures that the dual-wavelength signals do not interfere with each other when transmitted in optical fiber, resulting in high system stability.

[0077] 7. High scalability: The dimension of the overdetermined equation system can be extended to a higher level according to actual needs. By introducing more wavelengths or more spectral features such as skewness and higher-order moments, noise can be further suppressed, accuracy can be improved, and the range of measurement parameters can be expanded.

[0078] The embodiments described in this application are for the purpose of more clearly illustrating the technical solutions of the embodiments of this application, and do not constitute a limitation on the technical solutions provided by the embodiments of this application. As those skilled in the art will know, with the evolution of technology and the emergence of new application scenarios, the technical solutions provided by the embodiments of this application are also applicable to similar technical problems.

[0079] Those skilled in the art will understand that the technical solutions shown in the figures do not constitute a limitation on the embodiments of this application, and may include more or fewer steps than shown, or combine certain steps, or different steps.

[0080] Those skilled in the art will understand that all or some of the steps in the methods disclosed above, as well as the functional modules / units in the systems and devices, can be implemented as software, firmware, hardware, or suitable combinations thereof.

[0081] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0082] It should be understood that in this application, "at least one (item)" means one or more, and "more than" means two or more. "And / or" is used to describe the relationship between related objects, indicating that three relationships can exist. For example, "A and / or B" can represent three cases: only A exists, only B exists, and both A and B exist simultaneously, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one (item) of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one (item) of a, b, or c can represent: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, and c can be single or multiple.

[0083] The preferred embodiments of the present application have been described above with reference to the accompanying drawings, but this does not limit the scope of the claims of the present application. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and substance of the embodiments of the present application shall be within the scope of the claims of the present application.

Claims

1. A hollow-core optical fiber, characterized in that, The optical fiber comprises, from the inside out, an air core, a cladding, and a coating. The inner wall surface of the air core has several micro-grating units arranged alternately with different periods distributed along the axial direction. The air core extends along the optical fiber axis. The cladding includes at least one anti-resonant capillary and at least one solid optical guide filament. The refractive index of the solid optical guide filament is higher than that of the cladding material. The air fiber core is used to transmit light waves; The cladding is used to confine the light field within the air core.

2. The hollow-core optical fiber according to claim 1, characterized in that, The hollow fiber includes at least two micrograting units. The period of the first micrograting unit ranges from 160 μm to 200 μm, and the period of the second micrograting unit ranges from 600 μm to 800 μm. The groove depth of the two micrograting units ranges from 100 nm to 500 nm, and the groove width ranges from 1 μm to 5 μm.

3. The hollow-core optical fiber according to claim 1, characterized in that, The solid optical guide filament has a core diameter ranging from 6μm to 12μm and a numerical aperture ranging from 0.12 to 0.

20. The minimum distance between the solid optical guide filament and the air core must be greater than 10μm.

4. A sensing system based on hollow optical fiber, characterized in that, The system includes a dual-wavelength light source module, an optical circulator, a wavelength demultiplexing module, a photodetector module, a solid-core fiber detection unit, a signal processing module, and a hollow-core optical fiber as described in any one of claims 1-3. The first to third ports of the optical circulator are respectively connected to the output of the dual-wavelength light source module, the input of the hollow-core optical fiber, and the input of the wavelength demultiplexing module. The photodetector module is connected to the output of the wavelength demultiplexing module and the input of the signal processing module. The output of the hollow-core optical fiber is connected to the input of the solid-core fiber detection unit. The dual-wavelength light source module is used to inject two probe light pulses of different wavelengths into the hollow optical fiber; The wavelength demultiplexing module is used to separate the light reflected from the optical circulator according to wavelength; The signal processing module is used to process the detection signals of the photodetector module and the solid wire detection unit to obtain multi-parameter values; the multi-parameter values ​​include temperature change, axial stress and sound pressure.

5. The sensing system according to claim 4, characterized in that, The signal processing module performs the following steps: The wavelength offset of at least four reflection peak centers of the micrograting unit is extracted from the detection signal of the photodetector module; At least two changes in Rayleigh scattering spectrum shape parameters are extracted from the detection signal of the solid wire detection unit; A measurement vector is constructed based on the wavelength offset of the center of the reflection peak and the change in the shape parameter of the Rayleigh scattering spectrum, and the multi-parameter values ​​are calculated based on the measurement vector and the preset sensitivity matrix.

6. The sensing system according to claim 5, characterized in that, The processing procedure of the signal processing module also includes: The standardized residuals of each measurement channel are calculated based on the measurement vector, the preset sensitivity matrix, and the multi-parameter values. Channels with standardized residuals greater than a preset threshold are marked as suspicious measurement channels. If the number of suspicious measurement channels is less than or equal to a first preset value, the suspicious measurement channels are removed, and the multi-parameter values ​​are recalculated based on the remaining measurement channels and the corresponding sensitivity submatrix.

7. The sensing system according to claim 6, characterized in that, The processing procedure of the signal processing module also includes: If the number of remaining measurement channels is less than the second preset value, the multi-parameter values ​​of the previous moment are maintained.

8. The sensing system according to claim 4, characterized in that, The dual-wavelength light source mode includes a first narrow-linewidth laser, a second narrow-linewidth laser, a combiner, an electro-optic intensity modulator, and a timing controller. The input terminal of the combiner is connected to the output terminals of the first narrow-linewidth laser and the second narrow-linewidth laser. The input terminal of the electro-optic intensity modulator is connected to the output terminal of the combiner. The output terminal of the electro-optic intensity modulator is connected to the first port of the optical circulator. The output terminal of the timing controller is connected to the control terminal of the electro-optic intensity modulator.

9. The sensing system according to claim 8, characterized in that, The timing controller is configured as follows: Within the first time window, a first electrical pulse is output to control the electro-optic intensity modulator to conduct light of the first wavelength, thereby forming a probe light pulse of the first wavelength; According to a preset time interval, a second electrical pulse is output within a second time window to control the electro-optic intensity modulator to conduct light of a second wavelength, forming a probe light pulse of a second wavelength; the preset time interval is greater than the sum of the round-trip time of the optical signal in the hollow optical fiber and the guard interval.

10. The sensing system according to claim 4, characterized in that, The wavelength demultiplexing module includes an arrayed waveguide grating or a structure of at least one coarse wavelength division multiplexer cascaded together, and the channel isolation of the wavelength demultiplexing module is greater than 30dB.

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