A fiber optic magnetic field and temperature sensing probe based on dual FP structure

Abstract

The present invention is an optical fiber magnetic field and temperature sensing probe based on a double F-P structure. The optical fiber comprises a laser light source (1), an optical fiber circulator (2), a sensing probe (3), and a signal processing module (4). The sensing probe (3) comprises a cladding (3-1), a first fiber core (3-2), a second fiber core (3-3), a first groove (3-4), a second groove (3-5), and a reflective membrane (3-6). The first groove (3-4) is filled with a magnetic fluid, and the second groove (3-5) is filled with glycerol. The reflective membrane and the two groove end faces form a double Fabry-Perot resonant cavity. By utilizing the magnetic field sensitivity of the magnetic fluid and the temperature sensitivity of glycerol, the refractive index of the magnetic fluid and glycerol will change due to changes in the magnetic field and temperature, thereby changing the equivalent cavity length of the F-P cavity, and ultimately causing a change in the phase difference between the two coherent lights. By detecting the change in the interference signal, dual parameter measurement of temperature and magnetic field can be achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing, and in particular relates to an optical fiber magnetic field and temperature sensing probe based on a double FP structure. Background Art

[0002] Magnetic fields or information related to magnetic fields exist in many places in nature and human social life. Magnetic sensors are devices that can convert various magnetic fields and their changing quantities into electrical signal outputs. Therefore, they are widely used in the tasks of detecting, collecting, storing, converting, and monitoring various magnetic fields and various information carried in magnetic fields. In recent years, with the rapid development of informatization, industrialization, transportation, electronic technology, etc., magnetic field sensors have been developed and applied more. As the price of optical fiber devices becomes cheaper and cheaper, the types become more and more numerous, and optical fiber communication technology becomes more and more mature, a sensor technology different from the traditional one, namely optical fiber sensing technology, was developed in the late 1970s. Combining magnetic field sensing technology with optical fiber sensing technology forms a new type of sensor, namely optical fiber magnetic field sensor, which inherits the advantages of both, with outstanding advantages such as small size, corrosion resistance, strong anti-electromagnetic interference ability, convenient distributed multi-point detection, all-optical transmission, etc., and has become a research hotspot in the field of magnetic field sensing. There are many kinds of magnetic field sensors based on optical fiber, such as magnetic field sensors based on fiber grating structure, magnetic field sensors based on FP interference structure, magnetic field sensors based on evanescent wave mechanism, and magnetic field sensors based on surface plasmon resonance, etc., which can be applied to different magnetic field combinations according to different sensing performances.

[0003] The fiber Fabry-Perot sensor is designed based on the FP interferometer. Since the fiber FP sensor itself is insensitive to external factors such as magnetic field and temperature, it is necessary to fill sensitive materials, use sensitive materials for packaging, or indirectly use mechanical mechanisms to measure the measured parameters, thereby realizing the corresponding sensor. The sensing principle of the fiber FP sensor can be realized in two ways: one is to fix the refractive index of the medium in the cavity and change the geometric cavity length. For example, strain, temperature, etc. make the cavity length of the FP resonant cavity larger or smaller, resulting in changes in the output reflection spectrum period. The periodic change of the demodulated output interference spectrum can achieve the purpose of detecting the measured quantity, and the output spectrum of the measured quantity is detected by using the change of the geometric cavity length; the second is to fix the geometric cavity length and change the refractive index of the medium in the cavity. For example, a liquid sensitive material with controllable refractive index is filled into the resonant cavity of the fiber FP to sense the external measured variable, such as temperature, magnetic field, etc. The change of the liquid refractive index brings about a change in the equivalent cavity length of the FP cavity, causing the output interference spectrum of the fiber FP sensor to have a corresponding wavelength drift, and the measurement purpose is achieved by detecting the spectrum drift.

[0004] In today's production applications and life, most optical sensors use single-core standard optical fibers. Fiber optic sensors made with single-core standard optical fibers as basic components can generally only measure a single variable, and there will be cross-sensitivity problems when measuring dual parameters, which greatly limits the performance improvement of fiber optic sensors in applications. Dual-core optical fibers contain two cores in the same cladding, and each core is an optical waveguide, that is, two single-core optical fibers are integrated in a dual-core optical fiber, which greatly improves the accuracy of the sensor. Summary of the invention

[0005] In response to the above-mentioned problems, the present invention proposes a design scheme for an optical fiber magnetic field and temperature sensing probe based on a dual FP structure. The dual parameter measurement of temperature and magnetic field is achieved by fixing the geometric cavity length of the FP resonant cavity and changing the refractive index of the sensitive medium in the cavity. This is because both the magnetic field and temperature will cause the refractive index of the magnetic fluid to change, while the glycerol liquid is only modulated by the temperature, and the change of the external magnetic field has almost no effect on the refractive index of glycerol. Therefore, the sensing probe can realize the simultaneous measurement of the dual parameters of magnetic field and temperature, and has the ability of temperature compensation, thereby improving the accuracy of magnetic field measurement.

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

[0007] like Figure 1 As shown, an optical fiber magnetic field and temperature sensing probe based on a dual FP structure comprises a laser light source (1), an optical fiber circulator (2), a sensing probe (3), and a signal processing module (4). The sensing probe (3) comprises a cladding (3-1), a first fiber core (3-2), a second fiber core (3-3), a first groove (3-4), a second groove (3-5), and a reflective film (3-6). The sensing probe (3) is characterized in that: the sensing probe (3) adopts an axially symmetric non-coaxial dual-core optical fiber, the diameter of the cladding (3-1) is 125 μm, the diameter of the first fiber core (3-2) is 8 μm, the diameter of the second fiber core (3-3) is 8 μm, and the distance between the two fiber cores is 62.5 μm The first groove (3-3) and the second groove (3-4) are of the same size and are obtained by etching the end face of the dual-core optical fiber by femtosecond laser. They are 30 μm long, 30 μm wide and 20 μm deep. The first groove (3-4) is filled with magnetic fluid and the second groove (3-5) is filled with glycerol. The filling method is syringe pressurization. The reflective diaphragm (3-6) is made by vacuum evaporation on polypropylene film with metal aluminum. The thickness of the diaphragm is 3 μm. The reflective diaphragm and the end face of the first groove (3-4) constitute a first FP cavity for measuring magnetic field. The reflective diaphragm and the end face of the second groove (3-5) constitute a second FP cavity for measuring temperature and performing temperature compensation for magnetic field measurement.

[0008] The magnetic fluid filled in the first groove (3-4) is a water-based magnetic fluid, with Fe3O4 nanoparticles as magnetic particles and linoleic acid as a surfactant.

[0009] like Figure 2 As shown, the sensing probe is based on an axisymmetric non-coaxial dual-core optical fiber with two cores of the same diameter. Two identical grooves are etched on the right end face of the dual-core optical fiber by a femtosecond laser. The magnetic fluid and glycerol are filled into the first groove and the second groove respectively by a syringe pressurization method, and then packaged with a reflective diaphragm. The reflective diaphragm and the two grooved end faces form a double Fabry-Perot resonant cavity. The FP cavity filled with magnetic fluid can measure the magnetic field by utilizing the adjustable refractive index characteristics of the magnetic fluid, and the FP cavity filled with glycerol can measure the temperature by utilizing the temperature-sensitive characteristics of glycerol, and temperature compensation is performed on the magnetic field measurement.

[0010] The sensor probe is based on the interference characteristics of the FP structure. When interference occurs, the phase difference of adjacent coherent light for:

[0011]

[0012] 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 FP cavity length, and δ is the optical path difference between adjacent light beams.

[0013] When the cavity length d and the incident light wavelength λ are constant, the refractive index n of the medium between the two reflection surfaces and the phase difference There is a certain relationship:

[0014]

[0015]

[0016] 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 the phase difference to change, thereby causing the interference spectrum to drift. The measured change is calculated by detecting the change in the spectrum.

[0017] When temperature and magnetic field act together, the interference spectrum generated by the first fiber core responds to both external magnetic field changes and temperature changes. Changes in the magnetic field cause changes in the refractive index of the magnetic fluid, which in turn causes changes in the phase difference and causes the interference spectrum to drift. At the same time, changes in temperature will also affect changes in the refractive index of the magnetic fluid, which will also cause the interference spectrum to drift. However, the interference peak generated in the second fiber core does not respond to changes in the external magnetic field, but only to changes in temperature. Temperature changes cause changes in the refractive index of glycerol, which in turn causes changes in the phase difference between reflected light 2 and incident light 2, and ultimately causes the interference spectrum generated by reflected light 2 and incident light 2 to drift. Therefore, the temperature change can be calculated by detecting the drift of the interference spectrum generated in the second optical fiber, and further temperature compensation can be performed for the interference spectrum generated by the first optical fiber to reduce the impact of temperature on magnetic field measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of the structure of an optical fiber magnetic field and temperature sensing probe based on a dual FP structure according to the present invention;

[0019] Figure 2 A cross-sectional view of an optical fiber magnetic field and temperature sensing probe based on a double FP structure according to the present invention;

[0020] Figure 3 This is a schematic diagram of the first signal detection optical path of an optical fiber magnetic field and temperature sensing probe based on a dual FP structure described in the present invention. DETAILED DESCRIPTION

[0021] The following will be combined Figure 1 , the specific implementation modes of the present invention are further described.

[0022] The present invention is an optical fiber magnetic field and temperature sensing probe based on a double FP structure that can detect magnetic fields. The specific implementation steps are as follows:

[0023] Step 1: Fiber end face pretreatment

[0024] Select a double-core optical fiber of appropriate length, take one end of the double-core optical fiber and scrape off the coating with wire strippers, then use a fiber optic cutter to cut the end face without the coating flat, and then use an ultrasonic cleaner to clean the optical fiber.

[0025] Step 2: Make a groove on the end face of the dual-core optical fiber

[0026] After observing the flatness of the end face under an electron microscope, fix it on the fiber fixture to ensure that it remains horizontal on the three-dimensional platform, keep the femtosecond laser stationary, adjust the three-dimensional platform so that the femtosecond laser is focused on the center of the first fiber core on the flattened fiber end face through the optical lens, and gradually adjust the position of the x,y two-dimensional micro-motion stage from the focal point contacting the fiber to allow the femtosecond laser focal spot to make relative displacement to control the width and length of the groove. When the laser scans and etches each layer in the two-dimensional platform, the objective lens drops 10μm in the z-axis direction accordingly and enters the second layer scanning. A total of 2 scans can achieve the first groove processing with a depth of 20μm. Then the femtosecond laser is focused on the center of the second fiber core on the fiber end face through the optical lens, and the above steps are repeated to complete the second groove etching.

[0027] Step 3: Clean with hydrofluoric acid

[0028] During the laser operation, fiber optic debris will burst out and adhere to the cavity and fiber, thus damaging the cavity end face, affecting the reflectivity, and also affecting the intensity of the interference spectrum and the accuracy of the contrast. First, use an ultrasonic cleaning device to remove most of the debris adhering to the cavity end face and the fiber surface, and then use 5% hydrofluoric acid for etching, and the etching time is controlled for 30 seconds. This can remove the very small amount of debris remaining on the cavity surface and modify the uneven grooves.

[0029] Step 3: Fill the first groove and the second groove with glycerin and magnetic fluid respectively

[0030] Place the dual-core optical fiber vertically with the groove end facing upward. Under an optical microscope, insert a syringe into any groove, fill the syringe with magnetic fluid, pressurize the syringe, inject the magnetic fluid into it, and obtain the first groove after filling (3-4). Then take another syringe and insert it into another groove, fill glycerin into the syringe, pressurize the syringe, inject glycerin into it, and obtain the second groove after filling (3-5). During the filling process, it cannot be done too fast to prevent bubbles from entering due to excessive squeezing and affecting the sensing effect.

[0031] Step 4: Encapsulate the grooved end face of the dual-core optical fiber with a polypropylene film coated with metal aluminum

[0032] The coating of the FP cavity is made of aluminum-plated polypropylene film. The appropriate amount of polypropylene is stretched into a pancake-shaped film with a diameter of 1mm by precision instruments. The surface of the film needs to be corona treated before evaporation to make it have good adhesion. During evaporation, the roll film is placed in a vacuum chamber, the evaporation boat is heated to 1300℃-1400℃ under vacuum, and then the high-purity aluminum wire is continuously sent to the evaporation boat. Adjust the unwinding speed, winding speed, wire feeding speed and evaporation amount, open the cooling source, so that the aluminum wire is continuously melted and evaporated on the evaporation boat, so that the moving film surface is cooled to form a 1μm thick metal aluminum film, place one end of the dual-core optical fiber with a groove vertically upward, apply epoxy resin glue to the unetched area on the end face of the optical fiber, and finally place the aluminum-plated polypropylene film directly above the dual-core optical fiber, control the dual-core optical fiber to move slowly upward until it presses against the film, and wait for the glue to solidify before cutting it off.

[0033] The first signal detection optical path schematic diagram is as follows Figure 3 As shown, the light emitted by the laser light source (1) is transmitted to the first reflection surface of the FP sensor probe (3) through the optical fiber circulator (2) and partially reflected, and the other part of the light continues to be transmitted to the second reflection surface of the FP cavity sensor and partially reflected, and the two beams of reflected light interfere at the optical fiber circulator, and the output signal is received and processed by the signal processing module (4). The components required for the second signal detection optical path are the same as those described above.

[0034] The basic principle of the present invention is that the reflective film and two grooved end faces form a double Fabry-Perot resonant cavity, and the groove depth is the cavity length of the FP cavity. The modulated light emitted by the laser light source enters the optical fiber through the coupler and is incident on the Fabry-Perot cavity. After being reflected by the optical fiber end face and the reflective film in the Fabry-Perot cavity, it returns along the original path and meets each other to generate interference, and is finally received by the photoelectric detector. By utilizing the magnetic field sensitivity characteristics of the magnetic fluid and the temperature sensitivity characteristics of glycerol, the refractive indexes of glycerol and the magnetic fluid change due to the changes in the magnetic field and temperature, thereby changing the equivalent cavity length of the FP cavity, and finally causing the change in the phase difference between the two coherent lights. By detecting the change in the interference signal, the change in the magnetic field and temperature can be calculated.

Claims

1. An optical fiber magnetic field and temperature sensing probe based on a double FP structure, comprising a laser light source (1), an optical fiber circulator (2), a sensing probe (3), and a signal processing module (4), wherein the sensing probe (3) comprises a cladding (3-1), a first fiber core (3-2), a second fiber core (3-3), a first groove (3-4), a second groove (3-5), and a reflective film (3-6), characterized in that: The sensing probe (3) adopts an axially symmetric non-coaxial dual-core optical fiber, the diameter of the dual-core optical fiber cladding (3-1) is 125μm, the diameter of the first fiber core (3-2) is 8μm, the diameter of the second fiber core (3-3) is 8μm, the distance between the two fiber cores is 62.5μm, the first groove (3-4) and the second groove (3-5) are of the same size, and are obtained by etching the end face of the dual-core optical fiber with a femtosecond laser, with a length of 30μm, a width of 30μm, and a depth of 20μm. The first groove (3-4) is filled with magnetic fluid, and the second groove (3-5) is filled with glycerol, and the filling method adopts a syringe pressurization method. The reflective diaphragm (3-6) is made by vacuum evaporation on a polypropylene film by plating metal aluminum, and has a thickness of 3μm. The reflective diaphragm and the two grooved end faces form a double Fabry-Perot resonant cavity. The FP cavity filled with magnetic fluid can realize the measurement of the magnetic field by utilizing the adjustable refractive index characteristics of the magnetic fluid, and the FP cavity filled with glycerol can realize the measurement of the magnetic field by utilizing the adjustable refractive index characteristics of the magnetic fluid. The temperature sensitive property of glycerol can be used to measure temperature and perform temperature compensation on magnetic field measurement. The preparation of the reflective film (3-6) and the packaging of the sensor probe include the following steps: (a) firstly stretching an appropriate amount of polypropylene into a pancake-shaped film with a diameter of 1 mm, and performing corona treatment on the surface of the polypropylene film to make it have good adhesion; (b) then placing the rolled polypropylene film in a vacuum chamber for evaporation to form a 1 μm thick metal aluminum film on the surface of the film; (c) then placing one end of the double-core optical fiber with the groove facing upward vertically, and applying epoxy resin glue on the unetched area of ​​the optical fiber end face; (d) finally placing the aluminum-plated polypropylene film above the double-core optical fiber, controlling the double-core optical fiber to move slowly upward until it presses against the film, and cutting it after the glue is solidified. The magnetic fluid filled in the first groove (3-4) is a water-based magnetic fluid, with Fe3O4 nanoparticles as magnetic particles and linoleic acid as a surfactant.

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