An optical fiber sensor based on double polymer microspheres and a preparation method thereof

Abstract

The present invention discloses an optical fiber sensor based on dual polymer microspheres and a preparation method thereof, which includes a tapered optical fiber and magnetic polymer microspheres and non-magnetic polymer microspheres that simultaneously cover the tapered structure region of the optical fiber; the magnetic polymer microspheres are polymer materials doped with magnetic nanoparticles, and the non-magnetic polymer microspheres are polymer materials without doping with magnetic nanoparticles. In the present invention, magnetic nanoparticles are doped in a polymer and integrated in the tapered structure region of the optical fiber to form magnetic polymer microspheres, and at the same time, the polymer is directly integrated on the tapered structure region of the optical fiber to form non-magnetic polymer microspheres. The thus obtained sensor simultaneously has excellent magnetic field and temperature response characteristics, can be used for highly sensitive detection of ambient magnetic fields, avoids the clustering of magnetic fluids, and can also filter out the influence of ambient temperature on magnetic field measurement, avoiding the crosstalk of temperature during the magnetic field measurement process.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic sensors, and particularly relates to a fiber optic sensor based on double polymer microspheres and a preparation method thereof. Background Art

[0002] As an important physical quantity, the magnetic field has been widely studied. In terms of magnetic field detection, traditional magnetic field sensors have disadvantages such as large volume, complex structure, and small dynamic range, and are slightly inferior to fiber optic magnetic field sensors in terms of being portable, sensitive, and fast. Fiber optic magnetic field sensors use optical signals as carriers to sense the measured signals, with high precision, corrosion resistance, and electromagnetic interference resistance, which can effectively avoid the deficiencies of electrical magnetic field sensors and are favored by researchers in this field. Among them, fiber optic magnetic field sensors are small in volume, high in sensitivity, can be monitored remotely, and can be made into sensing structures of required shapes through technologies such as laser micro-nano processing, chemical etching, and melting, thus attracting much attention. Currently, they have been widely applied in fields such as aerospace, offshore oil systems, power grid systems, road monitoring, rail transit, and food safety.

[0003] Since the fiber optic material itself is not sensitive to the environmental magnetic field, fiber optic magnetic field sensors are mainly prepared by integrating magnetostrictive materials and magneto-optical materials with optical fibers. Magnetic fluid is a new type of functional material. When there is an external magnetic field, the solid particles inside the magnetic fluid will aggregate and be oriented along the direction of the external magnetic field, thereby causing a change in the refractive index of the magnetic fluid. Therefore, technicians have developed fiber optic magnetic field sensors based on magnetic fluid, which provide a better solution in magnetic field detection. Although high-precision sensing of the magnetic field has been achieved by using the magneto-optical properties of magnetic fluid to modulate characteristic parameters such as the intensity, phase, and wavelength of optical signals, considering that magnetic fluid is prepared by mixing magnetic particles and carrier liquids, under the action of a strong magnetic field, the magnetic particles and the carrier liquid will separate and form cluster phenomena. The cluster phenomena of magnetic fluid will cause the performance of the device to change or even be damaged. Therefore, the application of sensors with such structures is currently subject to certain limitations. At the same time, since the spectral signals of fiber optic sensors are inevitably affected by the environmental temperature, the environmental temperature is an important factor affecting the measurement accuracy of fiber optic magnetic field sensors. Therefore, how to overcome the influence of environmental temperature in the magnetic field measurement of fiber optic sensors is also a key aspect that needs to be considered by technicians in this field. Summary of the Invention

[0004] Based on the above technical problems, the present invention provides an optical fiber sensor based on dual polymer microspheres and a preparation method thereof. By doping magnetic nanoparticles in a polymer and integrating them in the tapered structure region of an optical fiber to form magnetic polymer microspheres, and at the same time directly integrating the polymer on the tapered structure region of the optical fiber to form non-magnetic polymer microspheres, the resulting sensor simultaneously has excellent magnetic field and temperature response characteristics. It can be used for highly sensitive detection of environmental magnetic fields, avoid the clustering of magnetic fluids, and at the same time filter out the influence of environmental temperature on magnetic field measurement, avoiding temperature crosstalk during magnetic field measurement.

[0005] An optical fiber sensor based on dual polymer microspheres proposed by the present invention includes a tapered optical fiber and magnetic polymer microspheres and non-magnetic polymer microspheres that simultaneously cover the tapered structure region of the optical fiber.

[0006] The magnetic polymer microspheres are polymers doped with magnetic nanoparticles, and the non-magnetic polymer microspheres are polymers without doping magnetic nanoparticles.

[0007] In the present invention, by simultaneously integrating a polymer doped with magnetic nanoparticles and a polymer without doping magnetic nanoparticles in the tapered structure region of the optical fiber, an optical fiber sensor with magnetic polymer microspheres and non-magnetic polymer microspheres in the tapered structure region of the optical fiber is formed. On the one hand, due to the doping effect of magnetic nanoparticles, they can be relatively fixed in the polymer and are not easily clustered under the action of a magnetic field. Therefore, the resulting sensor has a highly sensitive magnetic field sensing effect. On the other hand, the non-magnetic polymer microspheres have a highly sensitive temperature sensing effect. Therefore, by using the non-magnetic polymer microspheres as a control, the influence of environmental temperature on magnetic field measurement is filtered out, avoiding temperature crosstalk during magnetic field measurement.

[0008] Preferably, the tapered structure region of the tapered optical fiber includes a waist region and two tapered transition regions located at both ends of the waist region.

[0009] Preferably, the cross-sectional diameter of the waist region is 0 - 25 μm, and the length of the tapered transition region is 0 - 15 mm.

[0010] Preferably, the length of the optical fiber between the magnetic polymer microspheres and the non-magnetic polymer microspheres is 0 - 1 mm.

[0011] Preferably, the magnetic polymer microspheres are obtained by mixing and curing magnetic nanoparticles and a polymer; preferably, the magnetic polymer microspheres are obtained by mixing and photo-curing magnetic fluid and photoresist.

[0012] Preferably, the non-magnetic polymer microspheres are obtained by curing a polymer; preferably, the non-magnetic polymer microspheres are obtained by photo-curing photoresist.

[0013] Preferably, the magnetic fluid is a colloidal solution containing Fe3O4 nanoparticles, a surfactant, and a solvent, and the photoresist is photoresist GM1070;

[0014] Preferably, the volume ratio of the magnetic fluid to the photoresist is 1:1 - 3.

[0015] Preferably, the fiber optic magnetic field sensor further includes a broadband light source and a spectrometer;

[0016] Preferably, the broadband light source and the spectrometer are respectively connected to the input end and the output end of the tapered optical fiber.

[0017] Preferably, the tapered optical fiber covered with both magnetic polymer microspheres and non-magnetic polymer microspheres in the tapered structure region is used to form interference light of the light entering the tapered optical fiber, and the interference light changes correspondingly with the change of the magnetic field or temperature.

[0018] The present invention also provides a preparation method of a fiber optic sensor based on dual polymer microspheres, including the following steps:

[0019] S1. Taper the optical fiber using the fused biconical taper technology to obtain a tapered optical fiber with a tapered structure region;

[0020] S2. Coat the tapered structure region of the optical fiber with a single polymer and a polymer mixed with magnetic nanoparticles respectively to form two microspheres, and after curing reaction, the fiber optic sensor based on dual polymer microspheres is obtained.

[0021] Preferably, in step S1, after fusing and splicing two optical fibers, heat the splicing region, and then stretch the optical fibers at both ends of the heated region to obtain a tapered optical fiber with a tapered structure region.

[0022] Preferably, in step S2, coat the tapered structure region of the optical fiber with a photoresist and a photoresist mixed with magnetic fluid respectively to form two microspheres, and after curing by ultraviolet light irradiation, the fiber optic sensor based on dual polymer microspheres is obtained;

[0023] Preferably, the spot size of the ultraviolet light is 5 - 7 mm, the wavelength is 365 nm, and the irradiation time is 1 - 3 min.

[0024] In the present invention, a polymer containing magnetic nanoparticles is coated on one side of the tapered structure region of an optical fiber to form a magnetic polymer microsphere, and a separate polymer is coated on the other side of the tapered structure region of the optical fiber to form a non-magnetic polymer microsphere. After curing, a magnetic polymer microsphere doped with magnetic nanoparticles and a non-magnetic polymer microsphere not doped with magnetic nanoparticles are respectively formed, thereby constituting a new Mach-Zehnder interferometer that simultaneously includes polymer double microspheres and a tapered optical fiber. During operation, when the incident light reaches the tapered structure region, it is transmitted in the optical fiber and the magnetic and non-magnetic polymer microspheres respectively and coupled on the other side of the taper to form a transmission interference spectrum. Under the action of a magnetic field, the magnetic polymer microsphere is magnetized by the external magnetic field, and the optical fiber taper structure integrated with the magnetic polymer microsphere deforms under the action of the external magnetic field. The stronger the magnetic field, the more obvious the deformation. At the same time, the refractive index of the magnetic polymer microsphere also changes with the magnetic field. The changes in refractive index and shape will both modulate the phase information of the optical fiber sensing structure, ultimately causing the movement of the interference spectrum. Due to the good thermo-optic and thermal expansion effects of the polymer microsphere, under the action of a temperature field, the polymer microsphere is affected by temperature, and its refractive index also changes with temperature, ultimately causing the movement of the interference spectrum. Therefore, the environmental temperature and magnetic field will modulate the phases of the two signals, thereby causing the transmission spectrum of this sensor to shift. The method of using a sensing matrix can achieve the simultaneous measurement of environmental temperature and magnetic field, and can also effectively avoid the crosstalk of temperature during the magnetic field measurement.

[0025] The specific working principle of the optical fiber sensor of the present invention is as follows:

[0026] The tapered optical fiber integrated with magnetic and non-magnetic polymer microspheres constitutes an optical fiber Mach-Zehnder interferometer sensor. The tapered optical fiber and the magnetic and non-magnetic polymer microspheres respectively serve as the two arms of the interferometer. The incident light is respectively coupled into the tapered optical fiber and the magnetic and non-magnetic polymer microspheres for transmission, and finally coupled into the optical fiber at the other end of the taper to interfere to obtain the interference spectrum of this structure. The intensity can be expressed as:

[0027]

[0028] In the formula, k1 and k2 are the coupling efficiencies of the magnetic polymer microsphere structure and the non-magnetic polymer microsphere structure, η is the transmission loss of the cladding, is the initial phase difference related to the polymer microsphere structure, I in is the input light intensity.

[0029] The incident light is divided into a core mode and a cladding mode at the magnetic polymer microsphere structure, and then recombined at the non-magnetic polymer microsphere; is the phase difference between the core mode and the cladding mode after transmitting a certain distance, which can be expressed as:

[0030]

[0031] where n core and n clad are the effective refractive indices of the core mode and the cladding mode, L is the interference length between the two, λ is the optical wave wavelength. When m = 0, 1, 2..., the minimum output optical intensity can be expressed by the formula:

[0032]

[0033] When the magnetic field in the environment changes, the magnetic polymer microspheres are magnetized, the refractive index of the polymer microspheres is modulated, and at the same time, the tapered optical fiber bends under the action of the magnetic field force, changing the effective interference length; similarly, due to the high thermo-optic and thermal expansion coefficients of the polymer, the change in the environmental temperature will also modulate the effective refractive index and the interference length of the sensor. The changes in the refractive index and the interference length cause the interference spectrum to drift. By monitoring the shift of the interference spectrum and using the method of the sensing matrix, the high-sensitivity simultaneous detection of the magnetic field intensity and temperature can be achieved. Description of the Drawings

[0034] Figure 1 is a structural schematic diagram of the preparation process of the optical fiber sensor described in the present invention;

[0035] Figure 2 is a structural schematic diagram of the optical fiber sensor described in the present invention;

[0036] Figure 3 is the interference spectrum diagram of the optical fiber sensor described in the present invention at 30°C. Detailed Embodiments

[0037] Next, the present invention will be described in detail through specific embodiments. However, it should be clearly stated that these embodiments are used for illustrative purposes only and are not construed as limiting the scope of the present invention.

[0038] Embodiment

[0039] This embodiment provides an optical fiber sensor based on dual polymer microspheres, which is prepared by the following method:

[0040] (1) Use an optical fiber pliers to remove the coating layer at the end of a multi-mode optical fiber (core diameter 100 μm, cladding diameter 125 μm), then use anhydrous ethanol to wipe the optical fiber with the coating layer removed cleanly. After cutting the end faces of the two optical fibers with the coating layer removed flat with an optical fiber cutter, place them in the card slot of an optical fiber fusion splicer (KL-300T). The structure is as Figure 1 (a) shown;

[0041] (2) Use the multimode fiber automatic fusion splicing program to splice the two ends of the optical fiber. After successful fusion splicing, perform manual discharge. The number of manual discharges is 4 times, and the manual discharge time is 800 ms. During the manual discharge process, apply an external force to the optical fiber to make it move uniformly to both sides, and prepare a tapered optical fiber with the fusion splice point as the center; the waist area of the tapered structure area of the tapered optical fiber is larger after the first pulling. After four more manual discharges, a tapered structure area with a smaller waist diameter and a length of 1.5 mm is finally made. The structure is as shown in Figure 1 (b);

[0042] (3) Select a sample holder with a hollow structure in the middle, fix the tapered optical fiber with the tapered structure area prepared above on the sample holder, and control the tapered structure area to be above the hollow structure of the sample holder; respectively measure 0.5 mL of magnetic fluid (diacyl magnetic fluid MFS, Sichuan Hongbo New Materials Technology Co., Ltd.) and 1 mL of photoresist (SU-8, GM1070) with a disposable syringe, and place them in an ultrasonic container and shake them thoroughly for 20 min to make them mix evenly to obtain a magnetic mixture solution; then use a disposable syringe to measure 1 mL of photoresist (SU-8, GM1070) and place it in an ultrasonic container and shake it thoroughly for 20 min to obtain a non-magnetic mixture solution;

[0043] (4) Place the sample holder with the fixed tapered optical fiber under the lens of an optical microscope (XDC-10A-530HS), and apply the magnetic mixture solution and the non-magnetic mixture solution on both sides of the tapered structure area of the optical fiber respectively. The magnetic polymer microspheres and non-magnetic polymer microspheres formed in this way just wrap the tapered structure area of the tapered optical fiber. The structure is as shown in Figure 1 (c);

[0044] (5) Irradiate the two polymer microspheres on the tapered optical fiber with an ultraviolet light source (XP104). The spot size of the ultraviolet light is 6 mm, the wavelength is 365 nm, and the irradiation time is 2 min. The photoinitiator in the polymer material absorbs ultraviolet light to generate active acid, and the active acid acts as a catalyst to catalyze the cross-linking of polymer monomers. Finally, magnetic polymer microspheres and non-magnetic polymer microspheres are simultaneously integrated on the tapered structure area of the optical fiber to obtain the optical fiber sensor based on double polymer microspheres. The structure is as shown in Figure 2 ;

[0045] Refer to Figure 2 It can be seen that on the left side of the optical fiber sensor is a non-magnetic polymer microsphere, and on the right side is a magnetic polymer microsphere. The sizes of the two polymer microspheres are uniform and the surfaces are smooth. The length of the optical fiber between the two polymer microspheres is 0.2 mm, the axial length of each polymer microsphere along the optical fiber is 0.4 mm, and the longitudinal length is 0.2 mm; the magnetic polymer microsphere is darker in color than the polymer microsphere because it is doped with magnetic fluid material.

[0046] Put the above optical fiber sensor into a temperature control box (DHG-9031A, temperature range: RT + 10°C - 200°C), set the temperature of the temperature control box to 30°C, connect the two ends of the sensor to a broadband light source (YSL, SC-5-FC) and a spectrometer (YOKOGAWA AQ63700D, wavelength band: 600 - 1700 nm) respectively, scan and record the transmission spectrum line of the sensor, and obtain the interference spectrum signal of the sensor at this temperature, as Figure 3 shown. Refer to Figure 3 It can be seen that the optical fiber sensor has multiple characteristic peaks in the range of 1250 nm - 1650 nm, and the contrast of the interference peak at 1350 nm is the largest, nearly 12 dB.

[0047] Test of magnetic field sensing characteristics

[0048] Place the above optical fiber sensor in a magnetic field environment, keep the ambient temperature unchanged, gradually increase the magnetic field from low to high. In the magnetic field range of 5 mT - 35 mT, the interference spectrum drifts to the right, and there are multiple valleys in the interference spectrum that drift with the change of the magnetic field. Record the detailed information of its change with the magnetic field. As the magnetic field increases, the valleys all move towards the direction of longer wavelengths. Arbitrarily select two interference peaks, such as two interference peaks near 1250 nm and 1350 nm, fit the central wavelengths of these two interference peaks at different magnetic field intensities, and obtain the magnetic field sensitivities of the two valleys, with the sensitivities being 0.07727 nm / mT and 0.06745 nm / mT respectively.

[0049] Test of temperature sensing characteristics

[0050] When measuring temperature, place the sensor in the temperature control box. When the temperature is selected from 30°C to 60°C, control the temperature of the temperature control box to record data every 5°C. It is found that the valleys in the interference spectrum will have an obvious drift phenomenon within the measured temperature range. During the process of increasing temperature, the spectrum gradually drifts to the right. Select two interference peaks near 1250 nm and 1350 nm, monitor the detailed movement of their valleys, and linearly fit to give the temperature response characteristics, with the sensitivities being 0.22873 nm / °C and 0.16162 nm / °C respectively.

[0051] For the optical fiber temperature and magnetic field sensing structure proposed by the present invention, the two interference peaks A and B in its interference spectrum have different responses to temperature and magnetic field, and the sensitivity matrix can be expressed as

[0052]

[0053] In the equation, ΔM and ΔT represent the change amounts of the magnetic field and temperature in the environment respectively. The unit of magnetic field intensity is mT, and the unit of temperature is °C. The change of the magnetic field in the environment is measured by a gaussmeter, and the change of temperature is realized by a temperature controller. ΔλA and ΔλB are the wavelength drifts of interference peaks A and B with the change of temperature and magnetic field, and the unit is pm. S MA and S TA are the magnetic field and temperature sensitivities of interference peak A respectively. S MB and S TB are the temperature and magnetic field sensitivities of interference peak B. Because the temperature and magnetic field sensitivities of the two peaks are different and do not form a linear proportional relationship, temperature and magnetic field simultaneous sensing can be realized.

[0054] As mentioned above, the above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. An optical fiber sensor based on dual polymer microspheres, characterized in that, It includes a tapered optical fiber, and magnetic polymer microspheres and non-magnetic polymer microspheres that simultaneously cover the tapered structure region of the optical fiber; The magnetic polymer microspheres are polymers doped with magnetic nanoparticles, and the non-magnetic polymer microspheres are polymers without doping with magnetic nanoparticles; The tapered structure region of the tapered optical fiber includes a waist region and two tapered transition regions located at both ends of the waist region.

2. The fiber optic sensor based on double polymer microspheres according to claim 1, characterized in that, The cross-sectional diameter of the waist region is 0 - 25 μm, and the length of the tapered transition region is 0 - 15 mm.

3. The fiber optic sensor based on double polymer microspheres according to claim 1 or 2, characterized in that, The length of the optical fiber between the magnetic polymer microspheres and the non-magnetic polymer microspheres is 0 - 1 mm.

4. The fiber optic sensor based on double polymer microspheres according to claim 1 or 2, characterized in that, The magnetic polymer microspheres are obtained by mixing magnetic nanoparticles and polymers and then curing; the non-magnetic polymer microspheres are obtained by curing polymers.

5. The fiber optic sensor based on double polymer microspheres according to claim 1 or 2, characterized in that, The magnetic polymer microspheres are obtained by mixing a magnetic fluid and a photoresist and then curing under light; the non-magnetic polymer microspheres are obtained by curing the photoresist under light.

6. The fiber optic sensor based on double polymer microspheres according to claim 5, characterized in that The magnetic fluid is a colloidal solution containing Fe3O4 nanoparticles, a surfactant, and a solvent, and the photoresist is photoresist GM1070.

7. The fiber optic sensor based on double polymer microspheres according to claim 5, characterized in that, The volume ratio of the magnetic fluid to the photoresist is 1:1 - 3.

8. The fiber optic sensor based on double polymer microspheres according to claim 1 or 2, characterized in that, The optical fiber sensor further includes a broadband light source and a spectrometer; The broadband light source and the spectrometer are respectively connected to the input end and the output end of the tapered optical fiber.

9. The optical fiber sensor based on double polymer microspheres according to claim 1 or 2, characterized in that, The tapered optical fiber with the tapered structure region simultaneously covered with magnetic polymer microspheres and non-magnetic polymer microspheres is used to form interference light of the light entering the tapered optical fiber, and the interference light changes correspondingly with the change of the magnetic field or temperature.

10. A method for preparing the optical fiber sensor based on double polymer microspheres according to any one of claims 1-9, characterized in that, It includes the following steps: S1. The optical fiber is tapered by using the fused biconical taper technique to obtain a tapered optical fiber with a tapered structure region; S2. A single polymer and a polymer containing magnetic nanoparticles are respectively coated on the tapered structure region of the optical fiber to form two microspheres, and after curing reaction, the optical fiber sensor based on double polymer microspheres is obtained.

11. The preparation method of the optical fiber sensor based on double polymer microspheres according to claim 10, wherein, In step S1, after fusing two optical fibers, the fused region is heated, and then the optical fibers at both ends of the heated region are stretched to obtain a tapered optical fiber with a tapered structure region.

12. The preparation method of the optical fiber sensor based on double polymer microspheres according to claim 10 or 11, characterized in that, In step S2, a photoresist and a photoresist containing a magnetic fluid are respectively coated on the tapered structure region of the optical fiber to form two microspheres, and after curing by ultraviolet light irradiation, the optical fiber sensor based on double polymer microspheres is obtained.

13. The preparation method of the optical fiber sensor based on double polymer microspheres according to claim 12, characterized in that, The spot size of the ultraviolet light is 5 - 7 mm, the wavelength is 365 nm, and the irradiation time is 1 - 3 min.

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