A kind of cascaded hollow-core fiber interference sensor and its manufacturing process

Abstract

This invention discloses a cascaded hollow-core fiber interferometric sensor and its fabrication process, relating to the field of hollow-core fiber measurement technology. The cascaded hollow-core fiber interferometric sensor comprises, in sequence, a broadband light source (1), a first fiber optic jumper (2), a first single-mode fiber (3), a first fusion splice (4), a first anti-resonant Bragg hollow-core fiber (5), a second fusion splice (6), a second single-mode fiber (7), a third fusion splice (8), a second anti-resonant Bragg hollow-core fiber (9), a fourth fusion splice (10), a third single-mode fiber (11), a second fiber optic jumper (12), and a spectrometer (13). Compared with existing technologies, ① the integrated fiber Fabry-Perot interferometric structure and high-precision anti-resonance fiber temperature sensing structure provided by this invention enable large dynamic range measurement and temperature calibration. ② The anti-resonant hollow-core fiber provided by this invention enables miniaturized fiber optic temperature sensors.

Description

Technical Field

[0001] This invention relates to the field of hollow fiber measurement technology, and in particular to a cascaded hollow fiber interferometric sensor and its manufacturing process. Background Technology

[0002] Specialty optical fibers have made outstanding contributions to broadening the application range of optical fiber sensing. Especially in recent years, with the introduction of the anti-resonance effect, hollow-core specialty optical fibers have become an important platform for realizing large-scale, high-precision, and ultra-smart optical fiber sensors. Accurate temperature measurement is particularly important in engineering applications such as aerospace, high-power stable lasers, and high-precision CNC machine tools. Compared with ordinary electrical sensors, optical fiber temperature sensors are free from electromagnetic interference, highly corrosion-resistant, easy to manufacture, low-cost, have fast response, and high detection sensitivity, making them extremely versatile. Optical fiber interferometric temperature sensors are based on the principles of Fabry-Perot interferometry, Michelson interferometry, Mach-Zeder interferometry, and Saknaker interferometry. They utilize the photothermal effect caused by the influence of external temperature on the beam interference, which induces a phase change due to the optical path difference. Temperature changes are obtained by demodulating the characteristic spectrum.

[0003] Existing fiber optic interferometric temperature sensors can be broadly categorized into intrinsic and extrinsic temperature sensors. Extrinsic sensors typically obtain a smooth, vertical end face by cutting the optical fiber, and then combine a special non-fiber reflective film or cantilever beam with the fiber end to form a fiber optic sensing probe. Extrinsic sensors utilizing surface-fixed ultrathin polymer films can achieve ultra-wideband fiber optic acoustic sensing. Similarly, extrinsic Fabry-Perot cavities can be implemented using micro-nano fabrication techniques to achieve very low frequency acoustic wave detection. Furthermore, existing research indicates that extrinsic temperature sensors can be used for pressure sensing in high-temperature environments; however, under harsh conditions, machining, thin film, and cantilever beam structures can lead to reduced sensitivity or even failure. For intrinsic sensors, existing methods involve forming evanescent fields or intrinsic Fabry-Perot cavities through micro-nano fabrication, femtosecond laser direct writing, and 3D printing. These fabrication methods are complex or structurally unstable. Intrinsic fiber optic sensors based on hollow-core optical fibers are suitable for high-temperature environment measurements. A closed air cavity can be formed by cascading hollow-core optical fibers with single-mode optical fibers via arc fusion splicing, creating low-precision reflective surfaces at both ends. According to existing reports, there is a lack of in-depth discussion and research on the fiber optic interference mechanism and methods for high-temperature sensing based on this approach. Summary of the Invention

[0004] The purpose of this invention is to provide a cascaded hollow fiber interferometric sensor, namely a hollow fiber Fabry-Perot interferometric high-temperature sensor based on fine control.

[0005] The objective of this invention is achieved as follows:

[0006] I. Sensor Structure

[0007] Specifically, the sensor structure includes a broadband light source, a first fiber optic patch cord, a first single-mode fiber, a first fusion splice, a first anti-resonant Bragg hollow fiber, a second fusion splice, a second single-mode fiber, a third fusion splice, a second anti-resonant Bragg hollow fiber, a fourth fusion splice, a second single-mode fiber, a second fiber optic patch cord, and a spectrometer, connected in sequence.

[0008] II. Sensor Manufacturing Process:

[0009] ① By setting reasonable arc fusion power, discharge time and advance amount of the first and second fusion points through the fiber optic fusion splicer, adjusting the fineness of the reflective surface at the fusion point, and modulating the transmission spectrum, a Fabry-Perot interference comb spectrum is obtained.

[0010] ② The length of the first anti-resonant Bragg hollow fiber determines the free spectral range of the Fabry-Perot interferometer comb spectrum. By controlling its length within the range of 100-200 μm, a high extinction ratio of the comb spectrum is ensured, enabling the Fabry-Perot interferometer to achieve temperature calibration through transmission spectrum demodulation.

[0011] ③ By controlling the length of the second anti-resonant Bragg hollow fiber within the range of 1.8-2mm, the broadband light enters and generates an anti-resonance effect, which can obtain a resonant wavelength with a high extinction ratio and ensure the stability of temperature sensing over a wide range.

[0012] ④ The distance between the 3rd and 4th fusion splices is much greater than the distance between the 1st and 2nd fusion splices, ensuring that the 3rd and 4th fusion splices are in the same position. Due to the large optical path difference, the Fabry-Perot interference formed by the air core of the second anti-resonant Bragg hollow fiber is severely attenuated, thus avoiding interference with the transmission spectrum.

[0013] ⑤ The first anti-resonant Bragg hollow fiber and the second anti-resonant Bragg hollow fiber are cascaded with single-mode fibers to ensure that the incident light can acquire a detectable optical signal at the spectrometer receiver after passing through the sensor, thus realizing a complete transmission sensing structure.

[0014] Compared with the prior art, the present invention has the following advantages and positive effects:

[0015] ① The integrated fiber Fabry-Perot interferometer structure and the fiber temperature sensing structure with high precision anti-resonance effect provided by this invention can achieve large dynamic range measurement and temperature calibration because the comb spectrum and resonant wavelength formed by the two mechanisms with different precision have different sensitivities to temperature.

[0016] ②The anti-resonant hollow fiber provided by this invention can form an online Fabry-Perot interference and anti-resonance effect within a relatively short fiber length range, thereby enabling miniaturized fiber optic temperature sensors.

[0017] ③ The fiber optic temperature sensor with micro-size, large dynamic range sensing and temperature calibration functions provided by this invention has the advantages of simple structure, low price and easy manufacturing compared with traditional fiber optic interferometers that require couplers and fiber optic microstructures that require coating to enhance sensitivity, due to the use of a single online structure measurement.

[0018] ④ This invention summarizes a two-dimensional fiber interferometric sensing method with adjustable precision, which may provide an alternative approach in the field of hollow fiber microstructure sensing.

[0019] ⑤ Innovation points:

[0020] A. Propose an interferometric sensing method with adjustable precision;

[0021] B. Simultaneously realize temperature sensing applications of Fabry-Perot interferometry and anti-resonance effect;

[0022] C. Sensitivity at different temperatures in Fourier transform filtered demodulated transmission superimposed spectra;

[0023] D. It can guide various hollow fiber multi-parameter interferometric sensing applications. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of the present invention;

[0025] 1—Broadband light source;

[0026] 2—First fiber optic patch cord;

[0027] 3—First single-mode fiber;

[0028] 4—First weld point;

[0029] 5—First anti-resonant Bragg hollow fiber;

[0030] 6—Second weld point;

[0031] 7—Second single-mode fiber;

[0032] 8—Third weld point;

[0033] 9—Second anti-resonant Bragg hollow fiber;

[0034] 10—4th weld point;

[0035] 11—Third single-mode fiber;

[0036] 12—Second fiber optic patch cord;

[0037] 13—Spectrometer.

[0038] Figure 2This is a schematic diagram of the wide-range high-temperature sensing and temperature calibration functions of the present invention;

[0039] (a) is the process of obtaining a wide range of redshifts of the resonant wavelength under different high temperature conditions by performing fast Fourier filtering on a portion of the superimposed spectrum in the spectrometer in Embodiment 1 of the present invention.

[0040] (b) is the process of weak redshift of low-fineness Fabry-Perot interferometric comb spectrum obtained by fast Fourier filtering of part of the superimposed spectrum in the spectrometer in Embodiment 1 of the present invention under different high temperature conditions;

[0041] (c) is a linear fit to the resonant wavelength drift process;

[0042] (d) is a linear fit to the minimum point drift of the Fabry-Perot interferometric comb spectrum;

[0043] Figure 3 A schematic diagram of the superposition sensitivity obtained by partial superposition spectral filtering.

[0044] (a) is to perform fast Fourier bandpass filtering on a portion of the superimposed spectra in the spectrometer in Embodiment 1 of the present invention to obtain a comb spectrum within the free spectral range 1, and to perform linear fitting on the large-scale redshift process of the minimum point of the comb spectrum under different high temperature conditions.

[0045] (b) is a control measurement result obtained by performing the same operation within the free spectral range 2.

[0046] Figure 4 This is a schematic diagram of the control group's high-temperature test to verify the intrinsic calibration function of the anti-resonant Bragg hollow fiber;

[0047] (a) shows the temperature response of the first anti-resonant Bragg hollow fiber made under the same conditions under different high temperature conditions. The intrinsic temperature sensitivity of the anti-resonant Bragg hollow fiber is obtained by linear fitting.

[0048] (b) is a comparison of the drift of the comb-shaped interference points in the control group under different temperature changes.

[0049] Figure 5 This is a schematic diagram of the structure of an anti-resonant Bragg hollow fiber under a microscope.

[0050] The horizontal axis represents the geometric dimensions of the fiber cross-section, and the vertical axis represents the corresponding refractive index distribution. Detailed Implementation

[0051] The following is a detailed description in conjunction with the accompanying drawings and embodiments.

[0052] I. Sensors

[0053] 1. Overall

[0054] like Figure 1 The present invention comprises a broadband light source 1, a first fiber optic patch cord 2, a first single-mode fiber 3, a first fusion splice 4, a first anti-resonant Bragg hollow fiber 5, a second fusion splice 6, a second single-mode fiber 7, a third fusion splice 8, a second anti-resonant Bragg hollow fiber 9, a fourth fusion splice 10, a third single-mode fiber 11, a second fiber optic patch cord 12, and a spectrometer 13, connected in sequence.

[0055] Specifically, the two ends of the first fiber optic patch cord 2 are connected to the output end of the broadband light source 1 and the head end of the first single-mode fiber 3, respectively; the tail end of the first single-mode fiber 3 is connected to the head end of the first anti-resonant Bragg hollow fiber 5; the tail end of the first anti-resonant Bragg hollow fiber 5 is connected to the head end of the second single-mode fiber 7; the tail end of the second single-mode fiber 7 is connected to the head end of the second anti-resonant Bragg hollow fiber 9; the tail end of the second anti-resonant Bragg hollow fiber 9 is connected to the head end of the third single-mode fiber 11; the tail end of the third single-mode fiber 11 is connected to the head end of the second fiber optic patch cord 12; the tail end of the second fiber optic patch cord 12 is connected to the input end of the spectrometer; the two connection points between the first anti-resonant Bragg hollow fiber 5 and the first single-mode fiber 3 and the second single-mode fiber 7 are the first fusion splice point 4 and the second fusion splice point 6, respectively; the two connection points between the second anti-resonant Bragg hollow fiber 9 and the second single-mode fiber 7 and the third single-mode fiber 11 are the third fusion splice point 8 and the fourth fusion splice point 10, respectively.

[0056] 2. Functional components

[0057] 1) Broadband light source 1

[0058] It is a supercontinuum white light source device that can provide broadband optical excitation in the visible to mid-infrared wavelength range. In this invention, it is used to provide near-infrared light to excite the interference and anti-resonance effects of fiber optic sensors. This invention uses a NKT Photonics supercontinuum white light laser (SuperK COMPACT Cost-effective white light laser) with a wavelength range of 450nm-2400nm.

[0059] 2) First fiber optic patch cord 2, second fiber optic patch cord 12

[0060] This is a device with a fiber optic connector on one end and a fusion splice connecting it to a single-mode fiber on the other. The fiber optic connector precisely aligns the two ends, maximizing the coupling of the light beam into the fiber core for transmission. This invention uses an FC-FC interface fiber optic patch cord, which is cut to obtain the first fiber optic patch cord 2 and the second fiber optic patch cord 12.

[0061] 3) First single-mode fiber 3, second single-mode fiber 7, third single-mode fiber 11

[0062] This is a commercially available communication optical fiber that only allows fundamental transverse mode transmission. Its function is to connect a series anti-resonant Bragg hollow fiber to transmit the input light and an interference-modulated broadband beam, which is then coupled with low loss to an optical fiber jumper for spectrometer analysis. This invention uses a commercially available single-mode optical fiber with a core diameter of 9 μm and a cladding outer diameter of 125 μm.

[0063] 4) Welding point 1 (4), Welding point 2 (6), Welding point 3 (8), Welding point 4 (10)

[0064] This invention relates to a low-precision reflective surface formed at the arc discharge position during the linear cascading process of single-mode fiber and anti-resonant Bragg hollow fiber using a fiber fusion splicer (Fujikura 80s+). The precision of the splice point can be varied by controlling the splicing power, splicing time, and advance amount. Its function is to physically connect the single-mode fiber and the anti-resonant Bragg hollow fiber, and to form axially low-precision Fabry-Perot interference cavities on both sides of the anti-resonant Bragg hollow fiber. The incident light is partially reflected, and under a certain optical path difference, a two-beam interference is formed, modulating the input broadband light to form a comb-shaped interference spectrum and propagating it forward.

[0065] 5) First anti-resonant Bragg hollow fiber 5, second anti-resonant Bragg hollow fiber 9

[0066] It is a self-developed photonic crystal fiber, consisting of three layers: a cladding with periodically distributed bright rings as a high-refractive-index doped layer (Bragg reflection cladding structure), a dark gray low-refractive-index cladding, and a central white air core;

[0067] The beam is confined in the air fiber core by using a multi-Bracket reflection cladding structure based on the anti-resonance effect.

[0068] The Bragg reflector cladding structure is a multi-cladding structure formed by periodically high / low doping to create a periodic distribution of refractive index. This ensures high optical transmission efficiency while allowing light waves at the resonant wavelength to leak into the cladding for transmission. This achieves light-matter interaction while ensuring the complete transmission and detection of the optical signal after the interaction. Its function lies in the radial anti-resonance effect of the anti-resonant Bragg hollow fiber, which acts as a high-precision interferometer. Its resonant light energy responds well to changes in external temperature, while simultaneously exhibiting extremely low sensing signal loss, high transmission rate, and shorter demodulation sensitivity and response time for the output optical signal.

[0069] Specifically:

[0070] (1) Structure of anti-resonant Bragg hollow fiber:

[0071] It includes an air fiber core and a Bragg reflector cladding with a periodically distributed refractive index. The air core has a diameter of 32 μm, and the beam is mainly confined within the air fiber core for propagation. The cladding has an outer diameter of 125 μm, and its inner layer includes four refractive index step modulation distribution layers: the relative refractive index of the low-doped and outer cladding layers is 1.444, the relative refractive index of the high-doped layer is 1.454, the thickness of the high-doped layer is 1.06 μm, the thickness of the low-refractive index doped layer is 3.07 μm, and there are four high-refractive index doped layers. All the refractive indices are relative refractive indices at a wavelength of 1550 nm.

[0072] (2) The anti-resonant Bragg hollow fiber can be fabricated by the following process:

[0073] ① The periodically distributed Bragg reflector cladding preform is fabricated by chemical vapor deposition. First, a pure quartz tube (Heraeus, F300, Conamic, Shanghai, China) with an inner diameter of 25 μm and an outer diameter of 31 μm is fixed on a vapor deposition device; then, germanium-doped silicon and pure silicon are deposited on the inner wall surface of the quartz tube with thicknesses of 380 μm and 1000 μm, respectively; the above steps are repeated 4 times to obtain an optical fiber preform with an inner diameter of 13 mm and an outer diameter of 50 mm.

[0074] ② The anti-resonant Bragg hollow fiber is obtained by cutting and shaping the above-mentioned fiber preform and placing it in the fiber drawing tower. During the drawing process, the core diameter of the anti-resonant Bragg hollow fiber is controlled by a negative pressure gas pump, resulting in anti-resonant Bragg hollow fiber with core / cladding diameters of 32um and 125um.

[0075] 6) Spectrometer 13

[0076] This is an optical detection instrument that uses a spectroscopic element to disperse polychromatic broadband light and arranges the dispersed quasi-monochromatic light according to wavelength to obtain a spectrum. Its function is to process the input sensing light signal by dispersion, obtaining a superimposed transmission spectrum modulated by Fabry-Perot interference and anti-resonance effects; the actual spectral data is then demodulated using a fast Fourier bandpass filter to obtain the interference comb spectrum and anti-resonance spectrum, and the fitting analysis yields the temperature sensing result. This invention uses Anritsu product, model MS9740A, with a detection range of 0.6-1.75µm.

[0077] 3. Working mechanism:

[0078] 1) The first fusion splice 4, the first anti-resonant Bragg hollow fiber 5, and the second fusion splice 6 are connected in sequence to form a finely adjustable two-dimensional fiber optic sensing structure; the broadband light source 1, the spectrometer 13, and the finely adjustable two-dimensional fiber optic sensing structure constitute a cascaded anti-resonance effect and a Fabry-Perot interferometer; among them, the Fabry-Perot interferometer and the anti-resonance effect exist in the axial and radial directions of the sensing structure, respectively. By performing fast Fourier filtering on the superimposed spectrum of the finely adjustable two-dimensional fiber optic sensing structure, and then using wavelength demodulation, a large dynamic range temperature measurement and temperature calibration can be achieved.

[0079] 2) The first single-mode fiber 3, the first anti-resonant Bragg hollow fiber 5, the second single-mode fiber 7, the second anti-resonant Bragg hollow fiber 9, and the third single-mode fiber 11 all use non-collapsed standard fusion splices. This fusion splicing method forms a vertical low-fineness reflective surface at the fusion splice, thereby forming a low-fineness Fabry-Perot reflector cavity composed of the first single-mode fiber 3, the first anti-resonant Bragg hollow fiber 5, and the second single-mode fiber 7. Therefore, the broadband light source 1 is reflected by the first fusion splice 4 and the second fusion splice 6 and interference is generated, so that the broadband light is modulated into a high extinction ratio Fabry-Perot interference comb spectrum, and is superimposed with the radial anti-resonance effect of the second anti-resonant Bragg hollow fiber 9 and transmitted to the third single-mode fiber 11.

[0080] 3) The different loss peaks of the second anti-resonant Bragg hollow fiber 9 are the result of the anti-resonance effect formed by the air holes, fiber cladding and external air forming a double air layer structure in the second anti-resonant Bragg hollow fiber 9. The principle of the anti-resonance effect is similar to that of a high-precision Fabry-Perot interferometer, which allows four dominant loss peaks to be displayed in the same window of the spectrometer 13. The resulting loss peaks are superimposed on the comb spectrum generated by the low-precision Fabry-Perot reflection cavity, thus displaying a transmission spectrum formed by the superposition of four loss peaks and the comb spectrum on the spectrometer 13.

[0081] 4) After the first single-mode fiber 3 and the first anti-resonant Bragg hollow fiber 5 are fused together, part of the light is reflected at the reflecting surface formed at the first fusion point 4. Due to the low precision of the fusion point, most of the light field enters the air core of the first anti-resonant Bragg hollow fiber 5 and is reflected at the reflecting surface formed at the second fusion point 6. The two reflected light parts meet the interference conditions on the return journey to form a low-precision Fabry-Perot interference. Due to the short length of the air core, the optical path difference of the reflected light is small, resulting in a low-precision Fabry-Perot interference with a high extinction ratio.

[0082] 5) Since the second anti-resonant Bragg hollow fiber 9 is a structure with two layers of air sandwiching the cladding, i.e., a three-layer structure of air-cladding-air, when light is transmitted to the air and cladding cross section, part of the light will be projected into the air, and the other part will be reflected back into the cladding; the light reflected back into the cladding and the light transmitted into the air form the anti-resonance effect; since the first anti-resonant Bragg hollow fiber 5 is relatively short and has not reached the critical length for the anti-resonance effect to occur, the anti-resonance effect only occurs in the second anti-resonant Bragg hollow fiber 9.

[0083] 6) The Fast Fourier Transform (FFT) method is used to filter out the Fabry-Perot interferometric comb spectrum and anti-resonance effect spectrum in the superimposed spectrum. Since the anti-resonance Bragg hollow fiber has a high photothermal coefficient of 5-cladding, that is, the anti-resonance effect is more sensitive to the external temperature, the high-order comb spectrum filtered by the FFT method can obtain higher temperature sensitivity, corresponding to temperature measurement with a large dynamic range.

[0084] 7) When the external temperature changes, the position of the transmission spectrum superimposed on the spectrometer 13 will change as a whole; the spectral data under different temperature conditions are subjected to fast Fourier filtering, and the wavelength demodulation method is used on the processed spectral data to obtain the temperature sensitivity of the two methods respectively; since the Fabry-Perot interference and anti-resonance effect have large differences in their sensitivity to temperature, they can be used for large dynamic range and temperature calibration measurement respectively.

[0085] 4. Preferred Example

[0086] Preferably, the length of the first anti-resonant Bragg hollow fiber 5 is 180-200 μm to avoid radial anti-resonance effect; at the same time, the shorter Fabry-Perot reflector cavity has a shorter optical path, ensuring a higher Fabry-Perot interference spectrum extinction ratio.

[0087] Preferably, the length of the second anti-resonant Bragg hollow fiber 9 is 1800-2000 μm, which ensures that the anti-resonance effect in the comb spectrum has a high extinction ratio. In addition, the length of the second anti-resonant Bragg hollow fiber 9 is relatively long, which suppresses the interference formed by the third fusion point 8 and the fourth fusion point 10 from interfering with the comb spectrum corresponding to the low-fineness Fabry-Perot interference structure.

[0088] Preferably, the present invention provides an axial and radial interferometric sensing method, in which superimposed comb spectra are obtained by fast Fourier filtering in the transmission spectrum to obtain temperature sensing characteristics with different sensitivities. This method is applicable to the design of hollow fiber interferometric sensors based on precision.

[0089] 5. Specific examples

[0090] Specifically, in Example 1, the air hole diameter of the anti-resonant Bragg hollow fiber 5 is 32 μm, the fiber cladding diameter is 125 μm, and the thickness of the Bragg doped layer is 1.06 μm. The tail end of the first single-mode fiber 3 and the head end of the first anti-resonant Bragg hollow fiber 5, the tail end of the first anti-resonant Bragg hollow fiber 5 and the head end of the second single-mode fiber 7, the tail end of the second single-mode fiber 7 and the head end of the second anti-resonant Bragg hollow fiber 9, and the head end of the second anti-resonant Bragg hollow fiber 9 and the head end of the third single-mode fiber 11 are all spliced ​​using non-collapse standard fusion splicing. The broadband light source 1 and the head end of the first fiber patch cord 2, the tail end of the first fiber patch cord and the head end of the first single-mode fiber 3, the tail end of the third single-mode fiber 11 and the head end of the second fiber patch cord 12, and the tail end of the second fiber patch cord 12 and the spectrometer 13 are connected by an FC / APC connector through a flange.

[0091] 6. Example

[0092] The working principle of the above-mentioned micro-sized, adjustable precision two-dimensional fiber optic sensing structure for large dynamic range temperature measurement and temperature calibration is explained below with reference to Example 1.

[0093] The light emitted from the broadband light source 1 is transmitted to the first fusion splice 4 via the first fiber jumper 2 and the first single-mode fiber 3. Low-precision interference is formed at the first fusion splice 4 and the second fusion splice 6 on both sides of the first anti-resonant Bragg hollow fiber 5, constituting an axial low-precision Fabry-Perot interference cavity with the air core of the first anti-resonant Bragg hollow fiber 5. The light beam, after low-precision modulation, passes through the first anti-resonant Bragg hollow fiber 5, and is transmitted through the second single-mode fiber 7 into the second anti-resonant Bragg hollow fiber 9. Due to the mismatch between the core mode field radius of the second single-mode fiber 7 and the air core of the second anti-resonant Bragg hollow fiber 9, the transmitted light from the second single-mode fiber 7 enters the core and cladding of the second anti-resonant Bragg hollow fiber 9, thereby exciting the second anti-resonant Bragg hollow fiber. In the cladding mode of the second anti-resonant Bragg hollow fiber 9, some light rays from the air core and cladding are reflected back and forth by the periodically distributed refractive index layer unique to the second anti-resonant Bragg hollow fiber 9, forming an anti-resonance effect. When the back-and-forth reflected light meets the resonance condition, some wavelengths of light leak into the outside air, thus forming a resonant wavelength. After passing through the second anti-resonant Bragg hollow fiber 9, the transmitted light is again modulated by radial high-precision interference. The axial low-precision Fabry-Perot interference and the radial high-precision interference are superimposed and transmitted to the third single-mode fiber 11. The optical signal passes through the second fiber jumper 12 and finally enters the spectrometer 13. On the spectrometer 13, the interference spectrum of the axial low-precision Fabry-Perot interference cavity and the spectrum formed by the superposition of the resonant wavelength of the anti-resonance effect can be observed.

[0094] 7. Sensing Results

[0095] Figure 2(a) is the redshift process of a portion of the spectrum in the 1535nm-1555nm band of the integrated spectrum, where the resonant wavelength changes from 1536.96nm to 1551.9nm at 113℃-698℃.

[0096] Figure 2 (b) shows the interference spectrum of the Fabry-Perot interferometer air cavity at 222℃-800℃;

[0097] Figure 2 (c) is to attach Figure 2 After reading the resonant wavelength values ​​(i.e., trough values) of each curve corresponding to different temperatures in (a), a linear fit was performed with different temperatures as the abscissa and the corresponding resonant wavelength values ​​as the ordinate. The slope of the straight line obtained after linear fitting is the temperature sensitivity, i.e., from... Figure 2 (c) yields a temperature sensitivity of 26.03 pm / ℃ for the anti-resonance effect;

[0098] Figure 2 (d) is to attach Figure 2 (b) After reading the interference trough values ​​of each curve corresponding to different temperatures, a linear fit was performed with different temperatures as the abscissa and the corresponding interference trough values ​​as the ordinate. The slope of the straight line obtained after linear fitting is the temperature sensitivity, i.e., from... Figure 2 (c) shows that the temperature sensitivity of the Fabry-Perot interferometer air cavity is 1.02 pm / ℃.

[0099] Figure 3 This is a partial spectrum in the 1545nm-1560nm band of the superimposed spectrum. The illustration shows the redshift process of the interference spectrum of the anti-resonance superimposed Fabry-Perot interferometer at 400℃-650℃. After reading the interference trough values ​​of each curve corresponding to different temperatures in the illustration, a linear fit is performed with different temperatures as the abscissa and the corresponding interference trough values ​​as the ordinate. The slope of the straight line obtained after fitting is the temperature sensitivity, that is, the temperature sensitivity of the superimposed spectrum at the interference trough is 27.46pm / ℃.

[0100] Figure 4 (a) is a sensing structure designed and fabricated with the same length as the first Bragg anti-resonant hollow fiber 5, the purpose of which is to verify the accuracy of the temperature sensitivity of the Fabry-Perot interferometer air cavity. The illustration shows the change of the interference valley value of the Fabry-Perot interferometer spectrum under high temperature changes and the physical diagram of the sensing structure. The interference valley values ​​of each curve corresponding to different temperatures are read. The different temperatures are used as the abscissa and the corresponding interference valley values ​​are used as the ordinate for linear fitting. The slope of the straight line obtained after fitting is the temperature sensitivity. The temperature sensitivity of this structure at the interference valley is 0.98 pm / ℃, which is basically consistent with the temperature sensitivity of the Fabry-Perot interferometer air cavity in Example 1.

[0101] Figure 4 (b) The interference spectrum of the interference structure under high temperature variation is described. The temperature calibration characteristics of the Fabry-Perot air cavity are verified by calibrating the relative positions of the four sets of interference troughs under various temperature conditions.

[0102] After performing fast Fourier filtering on the superimposed spectrum on spectrometer 13, the Fabry-Perot interferometric comb spectrum and anti-resonance spectrum in the superimposed spectrum were extracted. Figure 2 (c) The temperature sensitivity of the anti-resonance effect was found to be 26.03 pm / ℃. Figure 2 (d) The temperature sensitivity of the Fabry-Perot interferometer was found to be 1.02 pm / ℃; Figure 3 The temperature sensitivity of the two superimposed spectra is 27.46 pm / ℃, which proves that the temperature sensitivity obtained based on the two optical guide models in the superimposed spectrum can be obtained by demodulating the same transmission spectrum separately through fast Fourier filtering. This enables the above-mentioned micro-sized, finely adjustable two-dimensional fiber optic sensing structure to realize a large dynamic range temperature measurement and temperature calibration sensor.

[0103] In summary, the micro-sized, precision-adjustable two-dimensional fiber optic sensing structure and large dynamic range temperature measurement and calibration sensor provided by this invention can be implemented using the fast Fourier filter method.

[0104] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A cascaded hollow fiber interferometer sensor, characterized in that: The system includes, in sequence, a broadband light source (1), a first fiber optic patch cord (2), a first single-mode fiber (3), a first fusion splice (4), a first anti-resonant Bragg hollow fiber (5), a second fusion splice (6), a second single-mode fiber (7), a third fusion splice (8), a second anti-resonant Bragg hollow fiber (9), a fourth fusion splice (10), a third single-mode fiber (11), a second fiber optic patch cord (12), and a spectrometer (13). The aforementioned anti-resonant Bragg hollow fiber is a self-developed photonic crystal fiber, comprising three layers: a cladding layer with periodically distributed bright rings representing a high refractive index doped layer, a dark gray cladding layer representing a low refractive index cladding layer, and a central white air core. Structure of anti-resonant Bragg hollow fiber: It includes an air fiber core and a Bragg reflector cladding with a periodically distributed refractive index; the air fiber core has a diameter of 32 μm, and the beam is mainly confined to the air fiber core for propagation; the outer diameter of the cladding is 125 μm, and its inner layer includes 4 layers of refractive index step modulation distribution: the relative refractive index of the low-doped and outer cladding layers is 1.444, the relative refractive index of the high-doped layer is 1.06 μm, the thickness of the high-doped layer is 1.06 μm, the thickness of the low-refractive index doped layer is 3.07 μm, and the number of high-refractive index doped layers is 4, and the refractive indexes are all relative refractive indices at a wavelength of 1550 nm; The length of the first anti-resonant Bragg hollow fiber (5) determines the free spectral range of the Fabry-Perot interference comb spectrum. By controlling its length within the range of 100-200um, a high extinction ratio of the comb spectrum is guaranteed, so that the Fabry-Perot interference can achieve temperature calibration through transmission spectrum demodulation. By controlling the length of the second anti-resonant Bragg hollow fiber (9) within the range of 1.8-2mm, the broadband light enters and generates an anti-resonance effect, which can obtain a resonant wavelength with a high extinction ratio and ensure the stability of temperature sensing over a wide range. The first single-mode fiber (3), the first anti-resonant Bragg hollow fiber (5), the second single-mode fiber (7), the second anti-resonant Bragg hollow fiber (9), and the third single-mode fiber (11) all use non-collapsed standard fusion splices. This fusion splicing method forms a vertical low-fineness reflective surface at the fusion splice, so that the first single-mode fiber (3), the first anti-resonant Bragg hollow fiber (5), and the second single-mode fiber (7) form a low-fineness Fabry-Perot reflector cavity. The broadband light source (1) is reflected by the first fusion splice (4) and the second fusion splice (6) and generates interference, so that the broadband light is modulated into a high extinction ratio Fabry-Perot interference comb spectrum, and is superimposed with the radial anti-resonance effect of the second anti-resonant Bragg hollow fiber (9) and transmitted to the third single-mode fiber (11).

2. The cascaded hollow fiber interferometer sensor according to claim 1, characterized in that: Anti-resonant Bragg hollow fiber is fabricated using the following process: ① The periodically distributed Bragg reflector cladding preform is fabricated by chemical vapor deposition. First, a pure quartz tube with an inner diameter of 25 μm and an outer diameter of 31 μm is fixed on a vapor deposition device. Then, germanium-doped silicon and pure silicon are deposited on the inner wall surface of the quartz tube with thicknesses of 380 μm and 1000 μm, respectively. The above steps are repeated 4 times to obtain an optical fiber preform with an inner diameter of 13 mm and an outer diameter of 50 mm. ② The anti-resonant Bragg hollow fiber is obtained by cutting and shaping the above-mentioned fiber preform and placing it in the fiber drawing tower. During the drawing process, the core diameter of the anti-resonant Bragg hollow fiber is controlled by a negative pressure gas pump, resulting in anti-resonant Bragg hollow fiber with core / cladding diameters of 32um and 125um.

3. A cascaded hollow fiber interferometer sensor according to claim 1 or 2, characterized in that... The manufacturing process includes: ①The first fusion splice (4) and the second fusion splice (6) are connected by setting reasonable arc fusion power, discharge time and advance amount through an optical fiber fusion splicer, adjusting the fineness of the reflective surface at the fusion splice point, modulating the transmission spectrum, and obtaining the Fabry-Perot interference comb spectrum; ②The distance between the third weld point (8) and the fourth weld point (10) is much greater than the distance between the first weld point (4) and the second weld point (6); ③ The first anti-resonant Bragg hollow fiber (5) and the second anti-resonant Bragg hollow fiber (9) are cascaded by single-mode fiber to ensure that the incident light can obtain a detectable light signal at the receiver of the spectrometer (13) after passing through the sensor, thus realizing a complete transmission sensing structure.

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