A magnetic field and temperature sensor with metallized fiber Bragg grating cascade FP structure

Abstract

The present invention discloses a magnetic field and temperature sensor of a metallized fiber grating cascade F-P structure, comprising a laser light source (1), an optical fiber circulator (2), a sensing probe (3), a first signal processing module (4), and a second signal processing module (5), wherein the sensing probe (3) comprises an optical fiber grating (3-1), an aluminum metal film (3-2), a magnetic fluid (3-3), a capillary glass tube (3-4), an epoxy resin (3-5), and a single-mode optical fiber (3-6); the optical fiber grating and the single-mode optical fiber are axially aligned and arranged at a micrometer interval in the capillary glass tube to form an F-P cavity; the F-P cavity filled with magnetic fluid utilizes the adjustable refractive index characteristics of the magnetic fluid to change the equivalent cavity length of the F-P cavity, thereby realizing magnetic field measurement; the metallized optical fiber grating utilizes the grating thermal expansion effect and the increased sensitivity of the metal film to change the grating period, thereby realizing temperature measurement; and the double parameter measurement of magnetic field and temperature is realized by measuring the drift of the reflection spectrum and the transmission spectrum.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing, and in particular relates to a magnetic field and temperature sensor of a metalized optical fiber grating cascade FP structure. Background Art

[0002] Optical fiber magnetic field sensors have outstanding advantages such as small size, corrosion resistance, strong anti-electromagnetic interference ability, convenience for distributed multi-point detection, and all-optical transmission. They have become a research hotspot in the field of magnetic field sensing. There are many types of magnetic field sensors based on optical fibers, such as magnetic field sensors based on fiber grating structures, magnetic field sensors based on FP interference structures, magnetic field sensors based on evanescent wave mechanisms, and magnetic field sensors based on surface plasmon resonance, etc., which can be applied to different occasions according to different sensing performances. Among them, the magnetic field sensor based on the FP interference structure combines the FP structure with magnetic sensitive materials, and realizes the measurement of the measured parameters by filling sensitive materials and using sensitive materials for packaging. The magnetic sensitive material used is generally a magnetic fluid, which is a new type of intelligent material composed of nanomagnetic particles, base liquid and surfactant, which has both liquid fluidity and solid strong magnetism. When a magnetic field is applied, the refractive index of the magnetic fluid changes, thereby changing the wavelength of the output light. By demodulating the change of the output light, the detection of the magnetic field is realized.

[0003] Fiber Bragg grating (FBG) is a passive device with the advantages of high sensitivity, anti-electromagnetic interference, corrosion resistance, no influence of light intensity fluctuation, good insulation, etc. It has been widely cited in civil engineering, water conservancy engineering, composite materials, medicine, electricity, aerospace and other fields, and has achieved remarkable research results. Since the main component of optical fiber is SiO2, it is small and brittle, and it is easy to be damaged in practical applications. In order to protect FBG from damage and improve the temperature sensitivity of fiber Bragg grating, we usually use chemical plating, electroplating and other methods to metallize the surface of bare grating.

[0004] The present invention combines a metallized fiber grating with an FP cavity filled with a magnetic fluid to form an optical fiber magnetic field sensor, and utilizes the adjustable magnetoresistance of the magnetic fluid and the thermal expansion characteristics of the metallized fiber grating to achieve dual-parameter simultaneous measurement of magnetic field and temperature. The sensor also has temperature self-compensation capability, thereby improving the accuracy of magnetic field measurement. Summary of the invention

[0005] In view of the above-mentioned problems, the present invention proposes a design scheme of a magnetic field and temperature sensor of a metallized fiber Bragg grating cascade FP structure, which can realize dual parameter measurement of magnetic field and temperature.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] A magnetic field and temperature sensor of a metalized fiber Bragg grating cascade FP structure, comprising a laser light source (1), an optical fiber circulator (2), a sensing probe (3), a first signal processing module (4), and a second signal processing module (5), characterized in that: the sensing probe (3) is 8 to 10 mm long and comprises a fiber Bragg grating (3-1), an aluminum metal film (3-2), a magnetic fluid (3-3), a capillary glass tube (3-4), an epoxy resin (3-5), and a single-mode optical fiber (3-6); the aluminum metal film (3-2) is plated on the fiber Bragg grating (3-1) by magneto-sputtering, the coating thickness is 180 μm, and the coating length is 4 mm; the capillary glass tube (3-4) has an inner diameter of 195 μm and an outer diameter of 195 μm. The diameter is 1 mm and the length is 5 mm. The right end of the fiber grating (3-1) and the left end of the single-mode optical fiber (3-6) are axially aligned and then arranged at a micrometer interval in the capillary glass tube (3-4). The fiber grating end face and the single-mode optical fiber end face constitute an FP resonant cavity. The magnetic fluid (3-3) is filled into the capillary glass tube (5) by capillary action. Both ends of the capillary glass tube (3-4) are sealed and fixed with epoxy resin (3-5). The FP cavity filled with the magnetic fluid utilizes the adjustable refractive index characteristics of the magnetic fluid to change the equivalent cavity length of the FP cavity to achieve magnetic field measurement. The metallized fiber grating not only utilizes the grating thermal expansion effect and the increased sensitivity of the metal film to change the grating period to achieve temperature measurement, but also can perform temperature compensation for magnetic field measurement.

[0008] The end face of the fiber grating (3-1) and the end face of the single-mode optical fiber (3-6) form an FP resonance cavity with a cavity length of 45 to 85 μm.

[0009] The magnetic fluid (3-3) is a water-based magnetic fluid, which uses Fe3O4 nanoparticles as magnetic particles and linoleic acid as a surfactant.

[0010] When a broadband light source is transmitted in an FBG, mode coupling occurs and light that meets certain conditions will be reflected:

[0011] λ B =2n eff Λ

[0012] Where n eff is the effective refractive index of the fiber Bragg grating, Λ is the period of the fiber Bragg grating, λ B The FBG fiber cladding surface is coated with aluminum metal film. When the external temperature changes, the aluminum metal film expands and contracts, causing the Bragg fiber grating period to change, thereby causing the fiber grating center wavelength to move. The temperature change is calculated by detecting the drift of the fiber grating center wavelength.

[0013] Due to the thermal expansion of SiO2, the optical fiber cantilever beam in the capillary glass tube will be stretched by thermal expansion, thereby shortening the FP cavity length, and the capillary glass tube will also be expanded by thermal expansion to expand the cavity, thereby increasing the FP cavity length. Under the same conditions of optical fiber and capillary glass tube materials, the compression and elongation of the FP cavity are roughly equal, and the slight change in the FP geometric cavity length can be ignored. When interference occurs in the FP structure, the phase difference of adjacent coherent light is for:

[0014]

[0015] Where λ is the wavelength of the incident light, n is the refractive index of the medium between the two reflection surfaces, d is the length of the FP cavity, and δ is the optical path difference of adjacent light beams. When the cavity length d and the wavelength λ of the incident light are constant, the refractive index n of the medium between the two reflection surfaces and the phase difference There is a certain relationship:

[0016]

[0017]

[0018] When the cavity length d and the incident light wavelength λ are constant, k is a constant. When the refractive index of the medium changes, it can cause a change in the phase difference, thereby causing the interference spectrum or transmission spectrum to drift. The measured change is calculated by detecting the change in the spectrum.

[0019] When temperature and magnetic field act together, the change of magnetic field causes the change of refractive index of magnetic fluid, which brings about the change of phase difference and causes the drift of interference spectrum. The transmission spectrum will also drift. At the same time, the change of temperature will also affect the change of refractive index of magnetic fluid, which will also cause the spectrum drift. The change of measured magnetic field is calculated by detecting the drift of spectrum. However, metallized fiber Bragg grating does not respond to the change of external magnetic field, but only responds to the change of temperature. When the external temperature changes, due to the thermal expansion of fiber Bragg grating and the sensitivity of the metal aluminum film on the grating surface, the temperature change causes the fiber Bragg grating to stretch axially, and the fiber Bragg grating period changes, so that the central wavelength of the grating moves. The drift of the central wavelength of the fiber Bragg grating is detected to calculate the temperature change, and further temperature compensation is performed for the transmission spectrum output by the FP structure to reduce the influence of temperature on the magnetic field measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of a magnetic field and temperature sensor structure of a metallized fiber Bragg grating cascade FP structure according to the present invention;

[0021] Figure 2 This is a detection principle diagram of a magnetic field and temperature sensor with a metallized fiber Bragg grating cascade FP structure described in the present invention. DETAILED DESCRIPTION

[0022] The following will be combined Figure 1 The specific implementation modes of the present invention are further described.

[0023] The present invention is a magnetic field and temperature sensor of a metallized fiber Bragg grating cascade FP structure that can detect magnetic fields. The specific implementation steps are as follows:

[0024] Step 1: Fiber Bragg Grating Metallization

[0025] Take a fiber Bragg grating of about 5mm in length, with an outer diameter of 125μm and an inner diameter of 8μm, remove the coating of the fiber Bragg grating and clean it with an ultrasonic cleaner, then use a high temperature resistant tape to stick the right end of the fiber cladding of the sensor probe, and the sticking part is 1mm long. Aluminum film is plated on the fiber Bragg grating by hysteresis sputtering method, and the coating is carried out in four times. After each coating, adjust the sample holder to rotate the sensor probe 90°, and finally achieve the purpose of uniform coating. The thickness of the aluminum metal film is 180μm, and the coating length is 4mm.

[0026] Step 2: Fiber end face pretreatment

[0027] Use a fiber optic cutter to cut the uncoated part of the fiber grating processed in step 1 flat, then take a piece of ordinary single-mode optical fiber, scrape off the coating at one end with wire strippers, use a fiber optic cutter to cut the end face of the single-mode optical fiber without the coating flat, and then use an ultrasonic cleaner to clean the fiber grating and single-mode optical fiber.

[0028] Step 3: Fill the capillary glass tube with magnetic fluid

[0029] First, clean the capillary glass tube with alcohol, and then drop magnetic fluid on any end of the capillary glass tube. The magnetic fluid will fill the entire tube cavity through capillary action, and then use alcohol to clean the residual magnetic fluid at the tube mouth.

[0030] Step 4: Insert the FBG into the capillary glass tube

[0031] Fix the filled capillary glass tube horizontally on the fixture between the two six-dimensional adjustment platforms, fix the FBG on the six-dimensional adjustment platform at the left end and roughly adjust the position of the FBG and the capillary glass tube. Then move the microscope to the left end of the capillary glass tube, slowly move the FBG, stop at a distance close to the left end of the capillary glass tube, and make fine adjustments. Then slowly insert the optical fiber into the tube mouth, and slowly push the FBG into the capillary glass tube, and finally seal and fix it with epoxy resin at the left end of the capillary glass tube.

[0032] Step 5: Insert the single-mode optical fiber into the capillary glass tube

[0033] Fix the processed capillary glass tube horizontally on the fixture between the two six-dimensional adjustment platforms, fix the FBG on the six-dimensional adjustment platform at the right end and roughly adjust the position of the FBG and the capillary glass tube. Then move the microscope to the right end of the capillary glass tube, slowly move the single-mode optical fiber, stop at a distance close to the right end of the capillary glass tube, and make fine adjustments. Then slowly insert the optical fiber into the tube mouth and push the optical fiber into the capillary glass tube. The magnetic fluid will overflow during the pushing process, and the pushing will stop at about 220μm.

[0034] Step 6: Construct the FP cavity structure

[0035] The sensor probe is connected to the laser light source, and the transmission spectrum of the FP cavity displayed on the spectrometer is observed. The cavity length of the FP cavity is calculated using the dual-wavelength demodulation method. The single-mode optical fiber is further slowly moved on the micro-adjustment platform for adjustment until the preset expected value is reached. The right end face of the capillary glass tube is then sealed and fixed with epoxy resin to complete the production of the FP cavity.

[0036] Step 7: Clean the entire sensor probe

[0037] The light emitted by the laser light source is transmitted to the sensor probe through the fiber circulator. The metalized fiber grating reflects the light of a specific wavelength back to the circulator and is received by the first signal processing module. The light transmitted by the fiber grating enters the FP structure. The fiber grating end face and the single-mode fiber end face constitute the FP resonant cavity. The transmitted light is partially reflected at the first reflection surface, and the other part of the light continues to be transmitted to the second reflection surface of the FP cavity sensor and is partially reflected. The two beams of reflected light interfere, and the coherent light returns to the FBG and is transmitted to the circulator and received by the first signal processing module. The transmitted light through the FP structure is received by the second signal processing module. The change of the fiber grating reflection spectrum is manifested as the drift of the fiber grating center wavelength. The temperature change is calculated by detecting the drift of the center wavelength; the change of the free spectrum range of the FP resonant cavity is manifested as the drift of the wavelength in the transmission spectrum. The change of the magnetic field intensity is demodulated by detecting the wavelength drift.

[0038] The basic principle of the present invention is to realize dual parameter measurement through the FP structure of metalized fiber grating cascade filled with magnetic fluid. FBG is only modulated by temperature. The thermal expansion effect of FBG and the sensitivity of metal film are used. The temperature change causes FBG to be stretched axially, and the fiber grating period changes, so that the center wavelength of the grating moves. The drift of the center wavelength of the fiber grating is detected to calculate the temperature change. The fiber grating end face and the single-mode fiber end face constitute the FP resonant cavity, and the cavity is filled with magnetic fluid. The refractive index of the magnetic fluid is magneto-adjustable. When a magnetic field parallel to the direction of the transmitted light is applied, the refractive index of the magnetic fluid will change with the change of the magnetic field, thereby changing the equivalent cavity length of the FP cavity, and finally causing the change of the phase difference between the two coherent lights. By measuring the drift of the transmission wavelength, the magnetic field can be measured. The present invention has high sensitivity and can realize dual parameter measurement of magnetic field and temperature.

Claims

1. A magnetic field and temperature sensor of a metalized fiber Bragg grating cascade FP structure, comprising a broadband light source (1), a fiber circulator (2), a sensor probe (3), a first signal processing module (4), and a second signal processing module (5), characterized in that: The sensing probe (3) is 8 to 10 mm long and comprises an optical fiber grating (3-1), an aluminum metal film (3-2), a magnetic fluid (3-3), a capillary glass tube (3-4), an epoxy resin (3-5), and a single-mode optical fiber (3-6). The aluminum metal film (3-2) is plated on the optical fiber grating (3-1) by magneto-sputtering, and the coating thickness is 180 μm and the coating length is 4 mm. The inner diameter of the capillary glass tube (3-4) is 195μm, an outer diameter of 1mm, and a length of 5mm. The right end of the fiber grating (3-1) and the left end of the single-mode optical fiber (3-6) are axially aligned and arranged at a micrometer interval in the capillary glass tube (3-4). The end face of the fiber grating and the end face of the single-mode optical fiber form an FP resonant cavity. The magnetic fluid (3-3) is filled into the capillary glass tube (3-4) by capillary action. Both ends of the capillary glass tube (3-4) are sealed and fixed with epoxy resin (3-5). The FP cavity filled with magnetic fluid utilizes the adjustable refractive index characteristics of the magnetic fluid to change the equivalent cavity length of the FP cavity to achieve magnetic field measurement. The metallized fiber grating not only utilizes the thermal expansion of the grating and the sensitivity enhancement of the metal film to change the grating period to achieve temperature measurement, but also can perform temperature compensation for magnetic field measurement. The optical fiber in the capillary glass tube (3-4) will expand due to heat, causing stretching, thereby shortening the FP cavity length, but the capillary glass tube (3-4) will also expand due to heat, causing the cavity to expand, thereby increasing the FP cavity length. Under the same conditions of the optical fiber and the capillary glass tube materials, the compression and elongation of the FP cavity are approximately equal, and the slight change in the FP geometric cavity length is negligible. The end face of the fiber grating (3-1) and the end face of the single-mode optical fiber (3-6) constitute the FP resonant cavity with a cavity length of 45 to 85 μm. The magnetic fluid (3-3) is a water-based magnetic fluid, with Fe3O4 nanoparticles as magnetic particles and linoleic acid as a surfactant.

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