CN112710408A - Optical fiber Fabry-Perot temperature sensing head based on PDMS (polydimethylsiloxane) arc reflecting surface and preparation method thereof - Google Patents
Optical fiber Fabry-Perot temperature sensing head based on PDMS (polydimethylsiloxane) arc reflecting surface and preparation method thereof Download PDFInfo
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- G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
- G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
- G01D5/35309—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer
- G01D5/35312—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer using a Fabry Perot
Abstract
The invention discloses an optical fiber Fabry-Perot temperature sensing head based on a PDMS (polydimethylsiloxane) arc reflecting surface and a preparation method thereof, wherein the sensing head comprises a single-mode optical fiber (1) and a micro-tube (2) welded at the tail end of the single-mode optical fiber (1), PDMS (3) is filled in the micro-tube (2), a Fabry-Perot cavity is formed by the micro-tube (2) filled with polymer PDMS (3), and the tail end of the polymer PDMS (3) forms an arc reflecting surface. Compared with the prior art, the temperature sensitivity of the invention is improved by nearly 7.33 times compared with the unconfined temperature, the temperature sensitivity is controlled by controlling the length of PDMS or the radian of the end surface of the PDMS, the smaller the length of the filled polymer PDMS is, the larger the radian of the arc end surface is, and the higher the temperature sensitivity of the sensing head is; the sensing head has the advantages of small size, high sensitivity, simple structure, simplicity in manufacturing and the like.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a novel high-sensitivity optical fiber Fabry-Perot temperature sensing measurement system and method based on a PDMS (polydimethylsiloxane) arc reflecting surface.
Background
The rapid development of science and technology brings great convenience to production and life, but also brings serious environmental pollution such as global warming; in addition, temperature has been an indispensable factor in power systems, detection systems, and the like. The electric temperature sensing head cannot be used in environments with strong corrosion or electromagnetic interference due to the structure of the electric temperature sensing head. In order to solve these problems, in recent years, optical fiber temperature sensing heads based on different principles, such as a fiber grating type, a mare's interference type, a fabry-perot interference type, and the like, have been proposed. The optical fiber Fabry-Perot sensor is favored by students because of its small size, single-point measurement and relatively simple structure and demodulation. However, because the thermal expansion coefficients and the thermo-optic coefficients of air and quartz are limited, the sensitivity of the Fabry-Perot temperature sensing head based on the optical fiber structure is only about 10 pm/DEG C, for example, in 2013, LiuYi and the like in Harbour, a micro channel is formed in the optical fiber by using a method of inducing water breakdown by femtosecond laser, a columnar damaged area is formed in the quartz optical fiber by shock waves and high-speed jet flow generated in water, and the inner wall is smooth by using an arc discharge mode, so that a double Fabry-Perot cavity is formed. The temperature sensitivity of the temperature sensor head is 12.74 pm/DEG C (Liu Y, Qu S, Li Y. Single Microchannel high-temperature fiber sensor by femtocell resistive laser-induced water break [ J]Optics Letters,2013,38(3): 335-7.). The sensitivity of the single-mode optical fiber can be improved by using a temperature-sensitive polymer, and the initial method is to fill a temperature-sensitive material in an air cavity, such as a hollow optical fiber with an inner diameter of 50 μm in a small section welded at the end of the single-mode optical fiber by KUNJIAN CAO et al, in 2017, and then weld a hollow optical fiber with an inner diameter of 5 μm at the long end, and then drive alcohol into the hollow optical fiber by using a needle tube, and then taper the hollow optical fiber to seal the end. Alcohol has a large thermo-optic coefficient of 3.7 x 10-4The temperature sensitivity of the sensor head is 429 pm/deg.C (Kunjian, Cao, Yi, et al, compact fiber biosensor sensor based on a sensor-seal) at-5 deg.C to 30 deg.Ced liquid-filling structure[J]Optics Express 2017.). However, because of the limitation of the fiber walls, the thermal expansion of the material is limited, and the sensor head of the type described above can only use the thermo-optic coefficient of the material to improve the temperature sensitivity. In order to utilize the thermal expansion effect of the material, the polymer is plated on the end face of the optical fiber to form a Fabry-Perot cavity, but the sensing head has the defects that the polymer is easy to fall off, the interference contrast is low, and the temperature sensitivity is not too high. In 2016, Min Li et al, Hadamard, used a two-photon focusing technique to prepare a liquid-filled polymer microstructure on the end face of an optical fiber, the temperature-sensitive liquid was encapsulated by SU-8 photoresist at the end of a single-mode optical fiber, and the interference spectrum was red-shifted due to the expansion of the outer wall caused by the high thermal expansion coefficient of the liquid as the temperature increased, and the temperature sensitivity was 877 pm/deg.C (Li M, Liu Y, Gao R, et al].Sensors&The activators B Chemical,2016,233: 496-. Therefore, the present invention is a technical problem to be solved in the art to develop a high-contrast, high-sensitivity optical fiber temperature sensing head capable of protecting polymer from falling off.
Disclosure of Invention
In order to overcome the defects of easy falling of polymer, poor interference contrast and low temperature sensitivity of the existing sensing head, the invention provides the optical fiber Fabry-Perot temperature sensing head based on the PDMS arc-shaped reflecting surface and the preparation method thereof.
The invention discloses an optical fiber Fabry-Perot temperature sensing head based on a PDMS (polydimethylsiloxane) arc reflecting surface, which comprises a single-mode optical fiber 1 and a micro-tube (2) welded at the tail end of the single-mode optical fiber 1, wherein polymer PDMS3 is filled in the micro-tube 2, the micro-tube 2 filled with polymer PDMS3 forms a Fabry-Perot cavity, and the tail end of the polymer PDMS3 forms an arc reflecting surface.
The invention discloses a preparation method of an optical fiber Fabry-Perot temperature sensing head based on a PDMS (polydimethylsiloxane) arc reflecting surface, which comprises the following steps of:
step 1, flattening the end faces of the single-mode optical fiber and the micro-tube, performing collapse-free fusion by using a manual fusion procedure of a fusion splicer, and then cutting by using a microscope to control the length of the micro-tube to be between 50 and 100 mu m;
and step 3: placing the sensing head prepared in the step 1 in a prepared polymer PDMS solution, and filling the polymer PDMS forwards along the inner wall of the micro-tube in an arc-shaped surface form by utilizing a capillary phenomenon; when the length of the polymer PDMS solution reaches an expected value, taking out the sensing head from the solution, standing for a period of time, and continuing the capillary phenomenon, wherein the polymer PDMS is continuously filled forwards along the inner wall of the microtube in the process and further filled along the end face of the single-mode optical fiber, so that an air bubble is formed in the solution; air in the solution is slowly exhausted, so that an arc reflecting surface is formed on the end face of the polymer PDMS; and after the air is exhausted, placing the prepared sensing head on a constant-temperature heating table for heating and curing.
The invention discloses a temperature measurement method realized by using an optical fiber Fabry-Perot temperature sensing head based on a PDMS (polydimethylsiloxane) arc reflecting surface, which comprises the following steps of:
fresnel reflection is generated on the interface of the single mode fiber and the polymer PDMS, transmitted light is continuously transmitted forwards in the polymer PDMS and is reflected again at the arc-shaped reflecting surface at the tail end of the polymer PDMS, two paths of reflected light can generate interference,
according to the relative light intensity I of two reflected lights when interference occurs1、I2Obtaining total relative reflected light intensity, and the expression is as follows:
wherein R is1、R2Respectively the reflectivities of two reflecting interfaces, eta isThe reflected light of the two interfaces is coupled into the single-mode fiber again, phi is 4 pi nL/lambda is the additional phase change of the light transmitted in the cavity, and n and L are the effective refractive index and the cavity length of the polymer Fabry-Perot cavity;
maximum and minimum light intensity I from interference spectrummax、IminObtaining interference contrast K of Fabry-Perot interference, wherein the expression is as follows:
when the interference phase difference satisfies
When the light intensity is maximum, m is the interference order corresponding to the interference peak, i.e. the wavelength lambda of the peak in the interference spectrummComprises the following steps:
the polymer PDMS is filled in a Fabry-Perot cavity formed in the microtube, the radial thermal expansion of the Fabry-Perot cavity is limited, the axial expansion of the Fabry-Perot cavity is known to be increased by the Poisson effect, the formula (3) is used for calculating the partial derivative of t, and the temperature sensitivity of the Fabry-Perot cavity is obtained, wherein the expression is as follows:
S＝λ0·(ε/n0+α+να) (4)
wherein α ═ Δ L/L0Delta t is the thermal expansion coefficient of the material, epsilon is delta n/delta t is the thermo-optic coefficient of the material, v is the Poisson coefficient of the material, and n is0、L0Is a temperature t0The initial refractive index and the cavity length of the Fabry-Perot cavity are measured, delta n and delta L are the refractive index and length change quantity of the Fabry-Perot cavity when the temperature is increased by delta t, and lambda is0The initial peak wavelength.
Compared with the prior art, the invention has the following positive effects:
1) the polymer PDMS is filled in the microtube, so that the protection effect can be achieved, the radial expansion of the polymer PDMS can be limited, the axial expansion of the polymer PDMS can be enhanced to a certain extent, and the temperature sensitivity of the polymer PDMS can be improved by 7.33 times;
2) a smooth cambered surface is formed at the tail end of the PDMS by using a special manufacturing method, and reflection loss is artificially introduced, so that the interference contrast is increased;
3) the temperature sensitivity is controlled by controlling the length of PDMS or the radian of the end surface of PDMS;
4) the sensing head has the advantages of small size, high sensitivity, simple structure, simplicity in manufacturing and the like.
Drawings
FIG. 1 is a schematic structural diagram of an optical fiber Fabry-Perot temperature sensing head based on a PDMS arc reflecting surface according to the present invention;
FIG. 2 is a schematic view of an embodiment of a measurement system for measuring temperature by using a PDMS-based Fabry-Perot temperature sensor head of the present invention;
FIG. 3 is a temperature response diagram of an optical fiber Fabry-Perot temperature sensing head based on a PDMS arc reflecting surface in the range of-20 ℃ to 70 ℃ in the invention; wherein, the length of PDMS is 31.25 μm, and the axial length of the end face radian is 37.35 μm;
fig. 4 is a graph showing the change of the wavelength of the trough of the fiber fabry-perot temperature sensing head based on the PDMS arc-shaped reflecting surface with temperature at a certain interference level, which shows that the temperature sensitivity is 2.825 nm/deg.c and the linearity is good.
Reference numerals: 1. the device comprises a single-mode optical fiber, 2, a micro-tube, 3, polymer PDMS, 4, a sensing element, 5, a broadband light source, 6, a spectrometer, 7, a circulator, 8 and an electric control constant temperature table.
Detailed Description
The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.
Fig. 1 is a schematic structural diagram of an optical fiber fabry-perot temperature sensing head based on a PDMS arc reflecting surface according to the present invention. The structure of the sensing head is composed of a single-mode optical fiber 1, a micro-tube 2 and a polymer PDMS 3. The sensing head 4 mainly comprises a single mode fiber 1, a micro-tube 2 and PDMS 3; a small section of micro-tube 2 is welded at the tail end of a single-mode optical fiber 1 to serve as a container, then polymer PDMS3 is filled in the micro-tube 2 by utilizing a capillary phenomenon, and an arc-shaped reflecting surface can be formed at the tail end of a PDMS solution by utilizing the preparation method provided by the invention. The polymer PDMS3 filled in the microtube 2 forms the Fabry-Perot cavity of the sensing head 4, and high-sensitivity temperature measurement can be realized due to the high thermal expansion coefficient of PDMS.
The invention selects PDMS with high thermal expansion coefficient as Fabry-Perot cavity, with the temperature rising, PDMS will generate thermal expansion and its refractive index will reduce, but PDMS has larger thermal expansion coefficient, so the interference spectrum will generate red shift. The radial thermal expansion of PDMS is limited by the wall of the micro-tube, and the axial expansion of the PDMS is increased to a certain extent, so that the volume of the material is not changed, and the average volume expansion coefficient of the PDMS is three times of the average linear expansion coefficient, so that the temperature sensitivity of the sensor head can be improved by 7.33 times compared with the unconfined temperature sensitivity. The polymer PDMS is filled in the micro-tube by utilizing the capillary phenomenon, so that the polymer can be protected from falling off, and an arc reflecting surface is ingeniously formed on the end surface of the PDMS, thereby improving the interference contrast of the sensing head. The temperature sensitivity of the sensing head can be controlled by controlling the length of PDMS and the length of the arc-shaped end face of the PDMS, and experiments show that the smaller the length of the filled polymer PDMS is, the larger the radian of the arc-shaped end face is, and the higher the temperature sensitivity of the sensing head is.
The invention discloses a preparation method of a high-sensitivity optical fiber temperature sensing head based on a PDMS (polydimethylsiloxane) arc reflecting surface, which comprises the following steps of:
the first step is as follows: a single-mode optical fiber 1 having an outer diameter of 125 μm and an inner diameter of 9 μm and a micro-tube 2 having an inner diameter of 75 μm and an outer diameter of 125 μm were cut to have flat end faces with a cutter. And the cut single-mode optical fiber 1 and the micro-tube 2 are welded by using an optical fiber welding machine in a manual welding mode, so that the welding end face is a flat reflecting surface. Then, a microscope is utilized, the cutting knife and the welded optical fiber are placed under the microscope, and the welding surface of the optical fiber and the blade of the cutting knife can be seen on the microscope simultaneously by adjusting the focal length of the microscope. The position of the optical fiber is adjusted so that the cutting is performed at a distance of 50 μm to 100 μm from the fusion-spliced point.
The second step is that: preparing a PDMS solution 3, preparing a preset material and a cured material of PDMS3 according to a certain mass ratio, for example, 10:1, stirring for 20-30min by using a vortex stirrer to uniformly mix the solution, and performing ultrasonic treatment for 10-15min by using an ultrasonic cleaning machine in order to completely and uniformly mix the solution. Then, a centrifuge is used, the rotating speed is adjusted to 4000rad/min-5000rad/min, the time is about 8-10min, and the purpose of centrifugation is to remove gas mixed in the solution.
The third step: the sensor head prepared in step 1 is placed in the prepared PDMS solution 3, the PDMS solution 3 will be filled forward along the inner wall of the microtube 2 in the form of an arc surface due to capillary phenomenon, and the filling process is performed under a microscope, so that the filling length can be controlled, and the filling process is about 8-15 min. When the length of the PDMS solution 3 reaches an expected value, the sensing head is taken out of the solution and stands for a period of time, so that the capillary phenomenon is continued, in the process, the PDMS solution 3 is continuously filled forwards along the inner wall of the microtube 2 and further filled along the end face of the single-mode optical fiber 1, and an air bubble is formed in the solution at the moment. Because the intermolecular gap of the PDMS solution 3 is large, air inside the solution will be slowly exhausted, so that the end face of the PDMS solution 3 forms an arc-shaped reflecting surface. After the air is exhausted, the prepared sensor head is placed on a constant temperature heating table and heated for 4-6 hours at 100 ℃ to be solidified, and the schematic diagram of the final sensor head structure is shown in fig. 1.
As shown in fig. 2, an embodiment of a measurement system for measuring temperature by using the optical fiber temperature sensing head of the PDMS based curved reflecting surface of the present invention includes a sensing head 4, a broadband light source 5, a spectrometer 6, a circulator 7 and an electrically controlled thermostatic stage 8. The broadband light output by the broadband light source 5 is transmitted to the interface of the single-mode fiber 1 and the PDMS solution filled in the microtube 2 through the single-mode circulator 7, Fresnel reflection occurs, the transmitted light is continuously transmitted in the polymer PDMS3, reflection occurs again on the arc-shaped end face of the polymer PDMS3, and the two paths of reflected light are transmitted to the spectrometer 6 through the circulator 7 again. By adjusting the temperature of the electrically controlled thermostatic stage 8, the cavity length and the refractive index of the polymer PDMS3 are changed, so that the interference spectrum of the reflected light is shifted, and the temperature change can be obtained by demodulating the shift of the reflected light.
The sensing head 4 is placed in an electrically controlled constant temperature table 8, and three ports of a circulator 7 are respectively connected with a broadband light source 5, a spectrometer 6 and the sensing head 4. Wherein, the light emitted by the broadband light source 5 is transmitted to the sensing head 4 through the circulator 7, Fresnel reflection occurs at the interface of the single-mode fiber 1 and the PDMS polymer 3, the transmitted light is transmitted forwards in the PDMS3, and reflection occurs again at the arc-shaped reflecting surface at the tail end of the transmitted light. The two reflected lights interfere and the interference light is transmitted to the spectrometer 6 by the circulator 7. Let the incident light be I0，I1、I2The relative light intensity of two beams of reflected light when interference occurs respectively, n and L are the effective refractive index and cavity length of the polymer Fabry-Perot cavity, R1、R2The reflectivities of the two reflecting interfaces, eta is the coupling efficiency of the reflected light of the second interface to the single-mode fiber, and phi is 4 pi nL/lambda is the additional phase change of the light transmission in the cavity. The total relative reflected light intensity is expressed as:
the expression of the interference contrast K of the Fabry-Perot interference is as follows:
wherein, Imax、IminThe maximum and minimum light intensities of the interference spectrum. When I is1≈I2Its interference contrast is maximal due to R1＜＜R2Therefore, the interference contrast of the sensor can be increased to some extent by reducing the coupling efficiency of the second reflected beam by introducing the arc-shaped reflecting surface.
When the interference phase difference satisfies
And then the interference light intensity is maximum, namely the wavelength of the peak in the interference spectrum is as follows:
due to the thermal expansion effect and the thermo-optic effect of the material, the cavity length L and the refractive index n of the Fabry-Perot cavity are changed. The aggregation Fabry-Perot cavity is filled in the microtube, the radial thermal expansion of the aggregation Fabry-Perot cavity is limited, the axial expansion of the Fabry-Perot cavity is increased by the Poisson effect, and the temperature sensitivity of the Fabry-Perot cavity can be obtained by calculating the bias of the equation (3) to t and is expressed as:
wherein α ═ Δ L/L0Δ t is the coefficient of thermal expansion of the material, and has a value of 9.6 × 10-4V, and the epsilon is delta n/delta t is the thermo-optic coefficient of the material; v is the Poisson coefficient of the material and has a value of 0.495 n0、L0Is a temperature t0The initial refractive index and the cavity length of the Fabry-Perot cavity are measured, delta n and delta L are the refractive index and length change quantity of the Fabry-Perot cavity when the temperature is increased by delta t, and lambda is0The initial peak wavelength.
The light source 1 adopts a broadband SLD light source and is used for providing a light source with a wavelength range of 1432-1632 nm;
the spectrometer 2 is used for collecting the reflection spectrum of the temperature sensing head in real time;
the circulator 2 is a single-mode 1550nm waveband circulator, transmits the reflection spectrum of the sensing head to a spectrometer, performs optical isolation and prevents the reflected light from returning to a light source;
the electric control constant temperature table 4 is a temperature control device, can provide the temperature range of minus 20 ℃ to 70 ℃, and the precision can reach plus or minus 0.01 ℃.
The sensing element 5 is a core sensing area of the temperature sensing head and mainly comprises a single-mode optical fiber 6, a micro-tube 7 and a PDMS solution 8.
The single-mode optical fiber 6 has the following model: 9 μm/125 μm for transmitting signal light.
The micro-tube 7 is a key element of the sensing head, and a polymerization Fabry-Perot cavity with an arc-shaped end face is formed by filling PDMS temperature sensitive materials in the micro-tube with the model of 75/125 μm.
The PDMS solution 8 is abbreviated as dimethylsiloxane in english, and is a temperature-sensitive material with a thermal expansion coefficient TEC of 9.6 × 10-4/° c, thermo-optic coefficient TOC ═ 5 × 10-4/℃。
Claims (4)
1. The optical fiber Fabry-Perot temperature sensing head based on the PDMS arc-shaped reflecting surface is characterized by comprising a single-mode optical fiber (1) and a micro-tube (2) welded to the tail end of the single-mode optical fiber (1), wherein PDMS (3) is filled in the micro-tube (2), a Fabry-Perot cavity is formed in the micro-tube (2) filled with polymer PDMS, and an arc-shaped reflecting surface is formed at the tail end of the polymer PDMS (3).
2. The fiber optic Fabry-Perot temperature sensing head based on PDMS curved reflecting surface of claim 1, wherein the smaller the length of the filled polymer PDMS, the larger the radian of the curved reflecting surface, and the higher the measurement sensitivity of the sensing head.
3. A preparation method of an optical fiber Fabry-Perot temperature sensing head based on a PDMS arc reflecting surface is characterized by comprising the following steps:
step 1, flattening the end faces of the single-mode optical fiber and the micro-tube, performing collapse-free fusion by using a manual fusion procedure of a fusion splicer, and then cutting by using a microscope to control the length of the micro-tube to be between 50 and 100 mu m;
step 2, mixing the preset polymer PDMS with a curing agent, and fully and uniformly stirring the mixture by using a stirrer, a centrifugal machine and the like and removing air in the mixture;
and step 3: placing the sensing head prepared in the step 1 in a prepared polymer PDMS solution, and filling the polymer PDMS forwards along the inner wall of the micro-tube in an arc-shaped surface form by utilizing a capillary phenomenon; when the length of the polymer PDMS solution reaches an expected value, taking out the sensing head from the solution, standing for a period of time, and continuing the capillary phenomenon, wherein the polymer PDMS is continuously filled forwards along the inner wall of the microtube in the process and further filled along the end face of the single-mode optical fiber, so that an air bubble is formed in the solution; air in the solution is slowly exhausted, so that an arc reflecting surface is formed on the end face of the polymer PDMS; and after the air is exhausted, placing the prepared sensing head on a constant-temperature heating table for heating and curing.
4. The temperature measurement method implemented by the optical fiber Fabry-Perot temperature sensing head based on the PDMS curved reflecting surface of claim 1, wherein the method comprises the following steps:
fresnel reflection is generated on the interface of the single mode fiber and the polymer PDMS, transmitted light is continuously transmitted forwards in the polymer PDMS and is reflected again at the arc-shaped reflecting surface at the tail end of the polymer PDMS, two paths of reflected light can generate interference,
according to the relative light intensity I of two reflected lights when interference occurs1、I2Obtaining total relative reflected light intensity, and the expression is as follows:
wherein R is1、R2The reflectivity of two reflecting interfaces is respectively, eta is the coupling efficiency of the reflected light of the second interface to be coupled into the single-mode optical fiber again, phi is 4 pi nL/lambda is the additional phase change of the transmission of the light in the cavity, and n and L are the effective refractive index and the cavity length of the polymer Fabry-Perot cavity;
maximum and minimum light intensity I from interference spectrummax、IminObtaining interference contrast K of Fabry-Perot interference, wherein the expression is as follows:
when the interference phase difference satisfies
When the light intensity is maximum, m is the interference order corresponding to the interference peak, i.e. the wavelength lambda of the peak in the interference spectrummComprises the following steps:
the polymer PDMS is filled in a Fabry-Perot cavity formed in the microtube, the radial thermal expansion of the Fabry-Perot cavity is limited, the axial expansion of the Fabry-Perot cavity is known to be increased by the Poisson effect, the formula (3) is used for calculating the partial derivative of t, and the temperature sensitivity of the Fabry-Perot cavity is obtained, wherein the expression is as follows:
S＝λ0·(ε/n0+α+να) (4)
wherein α ═ Δ L/L0Delta t is the thermal expansion coefficient of the material, epsilon is delta n/delta t is the thermo-optic coefficient of the material, v is the Poisson coefficient of the material, and n is0、L0Is a temperature t0The initial refractive index and the cavity length of the Fabry-Perot cavity are measured, delta n and delta L are the refractive index and length change quantity of the Fabry-Perot cavity when the temperature is increased by delta t, and lambda is0The initial peak wavelength.
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