A fiber-optic temperature sensor based on lossy-mode resonance

Abstract

The application discloses a kind of fiber temperature sensors based on loss mode resonance, it is related to optical fiber sensing technical field, including D type optical fiber and composite film structure, D type optical fiber includes core and cladding, the middle of cladding is provided with D type area, flat area is provided on D type area, flat area is provided with composite film structure, composite film structure includes temperature sensitive layer and resonance layer, temperature sensitive layer is arranged between flat area and resonance layer, resonance layer is arranged in parallel on temperature sensitive layer, loss mode resonance occurs between resonance layer and optical fiber, temperature sensitive layer can reduce loss mode resonance bandwidth, realize the separation of TE polarization and TM polarization, reduce the crosstalk between polarization, so that the sensitivity of fiber temperature sensor is higher, measurement range is larger, response speed is faster, can realize to temperature Large-scale high sensitivity detection, effectively overcome the problem of limited measurement range of high sensitivity fiber temperature sensor.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a fiber optic temperature sensor based on loss mode resonance. Background Technology

[0002] Fiber optic sensors are a crucial class of passive fiber optic devices, possessing advantages such as high sensitivity, compact structure, corrosion resistance, and electromagnetic interference immunity, and have long been a research hotspot in the field of optical measurement. Among optical sensors, loss mode resonance, a newly discovered optical phenomenon, has been used to measure various physical parameters. The generation of loss mode resonance in optical fibers results from the coupling between the modes propagating in the fiber and the loss modes in the dielectric thin film, which depends on two conditions:

[0003] 1. There is overlap between the two mode fields;

[0004] Second, both modes satisfy the phase matching condition.

[0005] At phase-matched wavelengths, the modes propagating in the optical fiber resonantly couple with the loss modes in the thin film. Therefore, energy attenuation, or resonance trough, can be observed near the phase-matched wavelength in the transmission spectrum. The real part of the dielectric constant of the thin film used to generate loss mode resonance must be positive and its absolute value must be greater than the imaginary part of its own dielectric constant and the real part of the dielectric constant of the surrounding medium. Therefore, materials used to generate loss mode resonance are mainly metal oxides or polymers. Compared with surface plasmon resonance using metal thin films, the materials for generating loss mode resonance are more abundant and have greater applicability. Furthermore, while surface plasmon resonance wavelengths are mostly in the visible light band, loss mode resonance has a wider wavelength tunable range. By adjusting the film thickness or selecting appropriate film types, the operating wavelength can be located in the near-infrared band, broadening its application range. Currently, fiber optic sensors based on loss mode resonance have been applied in various fields. According to previous reports, sensors with larger measurement ranges often have lower sensitivity, while ultra-high sensitivity sensors typically have narrow measurement ranges. Temperature sensitivity and measurement range are mutually restrictive; therefore, expanding the measurement range of high-sensitivity temperature sensors is a practical need.

[0006] Therefore, an optical fiber temperature sensor based on loss mode resonance is provided to solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to provide a fiber optic temperature sensor based on loss mode resonance, which has higher sensitivity, a larger measurement range, and a faster response speed, enabling high-sensitivity detection of temperature over a large range, effectively overcoming the problem of limited measurement range of high-sensitivity fiber optic temperature sensors.

[0008] To achieve the above objectives, this invention provides an optical fiber temperature sensor based on loss mode resonance, comprising a D-shaped optical fiber and a composite film structure. The D-shaped optical fiber includes a core and a cladding, with a D-shaped region in the middle of the cladding. A flat region is formed on the D-shaped region, and the composite film structure is formed on the flat region. The composite film structure includes a temperature-sensitive layer and a resonant layer. The temperature-sensitive layer is disposed between the flat region and the resonant layer, and the resonant layer is disposed parallel to and above the temperature-sensitive layer. The temperature-sensitive layer reduces the loss mode resonance bandwidth, achieves separation of TE polarization and TM polarization, and reduces crosstalk between polarizations. The refractive index and thickness of the temperature-sensitive layer are affected by the external temperature.

[0009] Preferably, loss mode resonance occurs between the resonant layer and the fiber core, and the limiting loss CL of the loss mode resonance is expressed as:

[0010]

[0011] Where n eff λ is the effective refractive index of the mode, and λ is the wavelength of the transmitted light.

[0012] Preferably, the length of the D-shaped region is set to 20-30 mm, the length of the flat region is set to 10-20 mm, the diameter of the fiber core is set to 9 micrometers, the diameter of the cladding is set to 125 micrometers, and the thickness of the cladding disposed between the flat region and the fiber core is set to 1-3 micrometers.

[0013] Preferably, the temperature-sensitive layer is a PDMS thin film with a thickness of 1-2 micrometers. The PDMS thin film is prepared by spin coating and is made of a material with a refractive index lower than that of pure quartz. The specific preparation steps are as follows:

[0014] Step 1: Mix the polymer and crosslinking agent at a weight ratio of 10:1 to obtain a PDMS solution;

[0015] Step 2: Mix the PDMS solution and volatile silicone oil at a weight ratio of 1:3 to obtain the diluted PDMS solution;

[0016] Step 3: Spin coat the diluted PDMS solution onto a flat area using a spin coater. Set the spin coater speed to 8000 rpm and the spin coat time to 4 minutes.

[0017] Step 4: The diluted PDMS solution is cured into a PDMS film by heating on a heating platform. The temperature of the heating platform is set to 110 degrees Celsius and the heating time is set to 1 hour.

[0018] Preferably, the resonant layer is a SnO2 thin film with a length of 10 mm and a thickness of 170-190 nm. The SnO2 thin film is prepared by radio frequency sputtering and is a dielectric material or polymer with a real refractive index that is positive and greater than the absolute value of the imaginary refractive index.

[0019] Preferably, if the thickness of the PDMS film is set to 1 micrometer, the response speed of the fiber optic temperature sensor is less than 200 milliseconds.

[0020] Therefore, the fiber optic temperature sensor based on loss mode resonance described above, as used in this invention, has the following beneficial effects:

[0021] (1) In this invention, a PDMS film is added between the D-type optical fiber and the SnO2 film to form a PDMS / SnO2 composite film structure. Temperature changes affect the refractive index and thickness of the PDMS film. The changes in these two parameters simultaneously affect the drift of the loss mode resonance wavelength. This expands the temperature measurement range and improves the temperature sensitivity, effectively overcoming the problem of the limited temperature measurement range of high-sensitivity optical fiber temperature sensors.

[0022] (2) The present invention adjusts the position of the loss mode resonant wavelength by adjusting the thickness of the PDMS film and the SnO2 film, making the working wavelength more flexible.

[0023] (3) The present invention adds a PDMS film between the D-type optical fiber and the SnO2 film, which effectively reduces the loss mode resonance bandwidth, realizes the separation of TE polarization and TM polarization, reduces crosstalk between polarizations, and improves the accuracy of temperature measurement.

[0024] (4) The present invention reduces the thickness of the PDMS film, increases the surface area to volume ratio of the PDMS film, improves the response speed of the fiber optic temperature sensor, and expands the application range of the fiber optic temperature sensor.

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of an optical fiber temperature sensor based on loss mode resonance according to the present invention.

[0027] Figure 2 This is a cross-sectional view of a D-type optical fiber for an optical fiber temperature sensor based on loss mode resonance according to the present invention.

[0028] Figure 3 This is a simulation diagram showing the variation of the loss mode resonant loss spectrum of the present invention with the SnO2 film thickness c;

[0029] Figure 4This is a simulation graph showing the variation of the loss mode resonance loss spectrum of the present invention with the thickness d of the PDMS film;

[0030] Figure 5 This is a simulation diagram showing the variation of the loss mode resonance loss spectrum of the present invention with the cladding thickness b between the flat region and the fiber core.

[0031] Figure 6 This is a simulation diagram showing the change of the resonant loss spectrum of the loss mode as a function of temperature in this invention.

[0032] Among them: 1. D-type optical fiber; 2. Fiber core; 3. PDMS thin film; 4. SnO2 thin film; 5. Flat region; 6. D-type region; 7. Cladding. Detailed Implementation

[0033] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0034] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0035] The terms "comprising" or "including" as used in this invention mean that the element preceding the term encompasses the element listed after the term, and do not exclude the possibility of encompassing other elements. Terms such as "inner," "outer," "upper," and "lower" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. When the absolute position of the described object changes, the relative positional relationship may also change accordingly. In this invention, unless otherwise explicitly specified and limited, the term "attached" and similar terms should be interpreted broadly. For example, it can refer to a fixed connection, a detachable connection, or an integral part; it can refer to a direct connection or an indirect connection through an intermediate medium; it can refer to the internal communication of two elements or the interaction relationship between two elements. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0036] Example

[0037] like Figure 1As shown, this invention provides an optical fiber temperature sensor based on loss mode resonance, comprising a D-type optical fiber 1 and a composite film structure. The D-type optical fiber 1 includes a core 2 and a cladding 7. A D-type region 6 is disposed in the middle of the cladding 7. A flat region 5 is disposed on the D-type region 6. The composite film structure is disposed on the flat region 5. The composite film structure includes a temperature-sensitive layer and a resonant layer. The temperature-sensitive layer is disposed between the flat region and the resonant layer, and the resonant layer is disposed parallel to the temperature-sensitive layer above it. The temperature-sensitive layer reduces the loss mode resonance bandwidth, realizes the separation of TE polarization and TM polarization, and reduces crosstalk between polarizations. The refractive index and thickness of the temperature-sensitive layer are affected by the external temperature.

[0038] The fabrication method for D-type optical fiber 1 is fiber side polishing technology. During the polishing process, an optical power meter is used to measure the transmission loss of the side-polished fiber. Polishing is stopped when the power meter attenuates by -5dB at a wavelength of 1550nm. Figure 2 As shown, at this time, the thickness of the flat region 5 of the D-type fiber 1 is 1 micrometer from the remaining thickness of the fiber core 2, the length of the flat region 5 is 16 millimeters, the length of the D-type region 6 is set to 20-30 millimeters, the diameter of the fiber core 2 is set to 9 micrometers, and the diameter of the cladding 7 is set to 125 micrometers.

[0039] The temperature-sensitive layer is set as PDMS film 3, the thickness d of PDMS film 3 is set to 1 micrometer, the PDMS film 3 is prepared by spin coating, and PDMS film 3 is set as a material with a refractive index less than pure quartz. The specific preparation steps are as follows:

[0040] Step 1: Mix the polymer and crosslinking agent at a weight ratio of 10:1 to obtain a PDMS solution;

[0041] Step 2: Mix the PDMS solution and volatile silicone oil at a weight ratio of 1:3 to obtain the diluted PDMS solution;

[0042] Step 3: Spin coat the diluted PDMS solution onto a flat area using a spin coater. Set the spin coater speed to 8000 rpm and the spin coat time to 4 minutes.

[0043] Step 4: The diluted PDMS solution is cured into a PDMS film 3 by heating on a heating platform. The temperature of the heating platform is set to 110 degrees Celsius and the heating time is set to 1 hour.

[0044] The resonant layer is a SnO2 thin film 4 with a length of 10 mm and a thickness of 185 nm. The SnO2 thin film 4 is prepared by radio frequency sputtering. It is a dielectric material or polymer with a positive real part of refractive index greater than the absolute value of its imaginary part. At room temperature, its refractive index is 1.39–1.41, and its thermo-optical coefficient and coefficient of thermal expansion are -4.5 × 10⁻⁶.-4 / ℃ and 340ppm / ℃.

[0045] If the thickness of PDMS film 3 is set to 1 micrometer, the response speed of the fiber optic temperature sensor is less than 200 milliseconds. PDMS film 3 is a transparent elastic polymer with a lower refractive index than SnO2 film 4. The evanescent field can pass through PDMS film 3 and enter SnO2 film 4 to generate loss mode resonance.

[0046] Loss mode resonance occurs between SnO2 film 4 and fiber core 2. The confining loss CL of loss mode resonance is expressed as:

[0047]

[0048] Where n eff λ is the effective refractive index of the mode, and λ is the wavelength of the transmitted light. Because the thermal expansion coefficient of PDMS film 3 is high, when the temperature rises, PDMS film 3 will thermally expand, resulting in an increase in the thickness of PDMS film 3, which in turn leads to a decrease in CL. The shift in the center wavelength of the loss mode resonance peak in the transmission spectrum is due to the change in the refractive index and thickness of PDMS film 3 with temperature, which causes a change in the conditions for generating loss mode resonance.

[0049] The thicknesses of the PDMS film 3 and SnO2 film 4 mentioned above were selected with the help of finite element method simulation. The thickness c of SnO2 film 4, the thickness d of PDMS film 3, the thickness b of cladding 7 between flat region 5 and fiber core 2, and the effect of temperature-varying PDMS refractive index on the loss mode resonant wavelength shift were simulated using finite element method simulation software.

[0050] like Figure 3 As shown, with the increase of the thickness c of the SnO2 film 4, the loss mode resonance gradually redshifts, moving towards longer wavelengths, and the limiting loss at the peak continues to increase.

[0051] like Figure 4 As shown, when the thickness d of the PDMS film 3 is not less than 2 micrometers, the loss peak decreases significantly until it disappears. If the PDMS film 3 is too thick, the evanescent field cannot enter the SnO2 film 4. When the thickness d of the PDMS film 3 is 1 micrometer, the loss peak can be clearly seen. In addition, the change in the thickness d of the PDMS film 3 will cause a slight blue shift in the resonance of the entire loss mode, which will increase the temperature measurement range and sensitivity of the sensor to a certain extent.

[0052] like Figure 5 As shown, the smaller the remaining thickness b in the flat area, the greater the loss value.

[0053] like Figure 6As shown, the refractive index at different temperatures can be calculated based on the thermo-optical coefficient of PDMS. The temperature simulation range is from 10 degrees Celsius to 70 degrees Celsius, with a step size of 10 degrees Celsius. When the temperature increases, the resonant wavelength of the loss mode shifts to blue, and the loss gradually decreases. The temperature sensitivity obtained from the simulation results is approximately -1.861 nm / ℃. Due to the high coefficient of thermal expansion of PDMS film 3, its thickness d increases with increasing temperature, leading to a decrease in loss. In addition, PDMS film 3 has a high thermo-optical coefficient, and its refractive index decreases with increasing temperature, thereby causing a change in the wavelength of the loss mode resonance, thus enabling the measurement of the external temperature.

[0054] Therefore, this invention employs a fiber optic temperature sensor based on loss mode resonance (DMR). A PDMS thin film is added between a D-type fiber and a SnO2 thin film to form a PDMS / SnO2 composite thin film structure. Temperature changes affect the refractive index and thickness of the PDMS thin film, and changes in these two parameters simultaneously affect the drift of the loss mode resonance wavelength. This expands the temperature measurement range while improving temperature sensitivity, effectively overcoming the problem of limited temperature measurement range in high-sensitivity fiber optic temperature sensors. It also effectively reduces the loss mode resonance bandwidth, achieving separation of TE and TM polarization, reducing crosstalk between polarizations, and improving the accuracy of temperature measurement. By adjusting the thickness of the PDMS and SnO2 thin films, the position of the loss mode resonance wavelength is adjusted, making the operating wavelength more flexible. Reducing the thickness of the PDMS thin film increases the surface area to volume ratio, improving the response speed of the fiber optic temperature sensor and expanding its application range.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A fiber optic temperature sensor based on loss mode resonance, characterized in that: The structure includes D-type optical fiber and a composite film structure. The D-type optical fiber consists of a core and a cladding. A D-type region is set in the middle of the cladding, and a flat region is set on the D-type region. A composite film structure is set on the flat region. The composite film structure includes a temperature-sensitive layer and a resonant layer. The temperature-sensitive layer is set between the flat region and the resonant layer, and the resonant layer is set parallel to the temperature-sensitive layer above it. The temperature-sensitive layer reduces the resonant bandwidth of the loss mode, realizes the separation of TE polarization and TM polarization, and reduces crosstalk between polarizations. The refractive index and thickness of the temperature-sensitive layer are affected by the external temperature. The length of the D-type region is set to 20-30 mm, the length of the flat region is set to 10-20 mm, the diameter of the fiber core is set to 9 micrometers, the diameter of the cladding is set to 125 micrometers, and the thickness of the cladding between the flat region and the fiber core is set to 1-3 micrometers. The temperature-sensitive layer is a PDMS thin film with a thickness of 1-2 micrometers. The PDMS thin film is made of a material with a refractive index lower than that of pure quartz. The resonant layer is set as a SnO2 thin film, the length of the SnO2 thin film is set to 10 mm, the thickness of the SnO2 thin film is set to 170-190 nm, and the SnO2 thin film is set as a dielectric material or polymer with a positive real part of refractive index that is greater than the absolute value of the imaginary part.

2. A fiber optic temperature sensor based on loss mode resonance according to claim 1, characterized in that: Loss-mode resonance occurs between the resonant layer and the fiber core, and the loss-mode resonance limits the loss. CL Represented as: ； in For the effective refractive index of the mode, For transmitting light wavelength.

3. A fiber optic temperature sensor based on loss mode resonance according to claim 1, characterized in that: The PDMS thin film was prepared by spin coating, and the specific preparation steps are as follows: Step 1: Mix the polymer and crosslinking agent at a weight ratio of 10:1 to obtain a PDMS solution; Step 2: Mix the PDMS solution and volatile silicone oil at a weight ratio of 1:3 to obtain the diluted PDMS solution; Step 3: Spin coat the diluted PDMS solution onto a flat area using a spin coater. Set the spin coater speed to 8000 rpm and the spin coat time to 4 minutes. Step 4: The diluted PDMS solution is cured into a PDMS film by heating on a heating platform. The temperature of the heating platform is set to 110 degrees Celsius and the heating time is set to 1 hour.

4. A fiber optic temperature sensor based on loss mode resonance according to claim 1, characterized in that: The SnO2 thin film was prepared by radio frequency sputtering.

5. A fiber optic temperature sensor based on loss mode resonance according to claim 3, characterized in that: If the thickness of the PDMS film is set to 1 micrometer, the response speed of the fiber optic temperature sensor is less than 200 milliseconds.

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