High-sensitivity wide-range optical fiber temperature sensor and manufacturing method thereof

Abstract

The invention discloses a high-sensitivity wide-range optical fiber temperature sensor which comprises a capillary tube, PDMS filled in the capillary tube and a bubble cavity located in the PDMS, single-mode optical fibers are inserted into the two ends of the capillary tube respectively, one end of each single-mode optical fiber is inserted into the PDMS, the end face of one single-mode optical fiber extends to the bubble cavity, and the other end face of the other single-mode optical fiber extends to the bubble cavity. A distance L1 is reserved between the end face of the other single-mode optical fiber and the side wall of the bubble cavity, the two ends of the capillary tube are packaged through ultraviolet glue, and the outer walls of the single-mode optical fibers and the ends of the corresponding capillary tube are packaged into a whole through the ultraviolet glue. The optical fiber temperature sensor is very compact in structure, easy to manufacture, high in sensitivity, low in cost and wide in range, and is not influenced by external air pressure.

Description

Technical Field

[0001] The present invention relates to the technical field of sensor devices, and in particular to a high-sensitivity, large-range optical fiber temperature sensor and a manufacturing method thereof. Background Art

[0002] Accurate temperature measurement is crucial in fields such as atmospheric environmental monitoring, agricultural production, and chemical and pharmaceutical industries. Fiber optic sensors have been widely researched and applied due to their advantages, such as light weight, low cost, and resistance to electromagnetic fields. Fiber optic temperature sensors typically include fiber Bragg grating (FBG) and fiber interferometer types. Fiber interferometers, such as the Sagnac interferometer (SI), Mach-Zehnder interferometer (MZI), and Fabry–Perot interferometer (FPI), are very popular due to their ease of fabrication and low cost.

[0003] Another advantage of fiber optic interferometers is that they can be easily connected in series or parallel to form sensors based on the vernier effect. FPIs are particularly popular because of their small size and ease in forming regular spectra.

[0004] The interferometric spectrum of a fiber optic sensor based on the Vernier effect contains a periodic Vernier envelope. Compared to the periodic peak of a single interferometer, the Vernier envelope is more sensitive to small changes in the environment. Therefore, sensors based on the Vernier effect are very attractive in sensing measurements and are widely used for temperature, pressure, humidity, and strain. However, sensors based on the Vernier effect require a sensing unit and a reference unit.

[0005] These two units require a good combination of light intensity and free spectral range to form a vernier envelope, which increases the difficulty of production.

[0006] Furthermore, the need for two units increases the size of the sensor. In recent years, femtosecond lasers have been used in micro- and nanofabrication to achieve high integration of sensing and reference units. However, this approach increases production costs due to the high price of femtosecond lasers.

[0007] Another method to improve the sensitivity of fiber optic temperature sensors is to coat and wrap the outside of the sensing structure with temperature-sensitive materials. This method is generally used for MZI-type fiber optic sensors. This method can improve the sensitivity to a certain extent, but it is still limited. In addition, the uniform coating process is relatively complicated, and burying the fiber structure in the temperature-sensitive material has problems such as the temperature-sensitive material and the fiber structure not being tightly connected.

[0008] Polydimethylsiloxane (PDMS) is an odorless, flexible, colorless, non-toxic, and non-volatile material.

[0009] Within a wide response range of -55 to 220°C, the thermal expansion coefficient (TEC) of PDMS is 9.6×10-4 / °C, and the thermo-optic coefficient (TOC) is -4.66×10-4 / °C.

[0010] The high TEC and TOC of PDMS make it an excellent thermosensitive material. Although the dominance of TEC or TOC varies and overlaps in various PDMS-based fiber optic sensors, PDMS-based sensors still exhibit high sensitivity to temperature. Due to its advantages, PDMS has been widely used in fiber optic sensors.

[0011] Some MZI sensors, formed by coating or encapsulating the outer surface of microfibers with PDMS, are easy to fabricate but have low sensitivity, while others are sensitive enough but complex to produce. Using PDMS to form a FP cavity for a Vernier effect-based sensor effectively improves its sensitivity, but the manufacturing process inevitably increases the sensor's size.

[0012] In this invention, we draw on the advantages of FPI and PDMS, combine quartz capillaries and single-mode optical fibers, and invent a fiber optic temperature sensor with a particularly compact structure, easy production, no influence from air pressure, high sensitivity, low cost, and a large range. Summary of the Invention

[0013] The purpose of the present invention is to provide a high-sensitivity, large-range optical fiber temperature sensor and a method for manufacturing the same in response to the above situation. The sensor manufactured using the method of the present invention is not affected by external air pressure, has low cost, and has ultra-high sensitivity and a relatively large measurement range.

[0014] The specific solution of the present invention is: a high-sensitivity, large-range optical fiber temperature sensor, including a capillary, PDMS filled in the capillary, and a bubble cavity located in the PDMS. Single-mode optical fibers are respectively inserted at both ends of the capillary, and one end of each of the two single-mode optical fibers is inserted into the PDMS. The end face of one single-mode optical fiber extends into the bubble cavity, and the end face of the other single-mode optical fiber is spaced L1 from the side wall of the bubble cavity. Ultraviolet glue is used to encapsulate the two ends of the capillary, and the ultraviolet glue encapsulates the outer wall of the single-mode optical fiber and the corresponding capillary end into one.

[0015] Furthermore, the transverse diameter of the bubble cavity in the present invention is L2, the distance between the inner end faces of the two single-mode optical fibers is L3, and the size of L3 is equal to the sum of the sizes of L1 and L2.

[0016] Furthermore, the inner diameter and outer diameter of the capillary in the present invention are 150 μm and 300 μm, respectively.

[0017] Furthermore, the core inner diameter of the single-mode optical fiber in the present invention is 9 μm, and the cladding outer diameter is 125 μm.

[0018] A method for manufacturing a high-sensitivity, large-range optical fiber temperature sensor, the method being used to manufacture the above-mentioned optical fiber temperature sensor, comprises the following steps: The first step is to use wire strippers to remove the coating of the two single-mode optical fibers, clean the outer cladding, and use a cleaver to cut the single-mode optical fibers to obtain two flat single-mode optical fiber end faces; Step 2: Insert the two single-mode optical fiber end faces into the capillary tube facing each other. During this process, use a microscope to help adjust and observe the distance and alignment of the two single-mode optical fiber end faces to maintain an appropriate distance between the two single-mode optical fiber end faces. The inner and outer diameters of the capillary tube are 150 μm and 300 μm, respectively. The third step is to fill the liquid PDMS into the capillary through the capillary siphon effect. During this process, PDMS will fill the voids in the capillary and leave a bubble cavity. Step 4: Place the above structure in a drying oven and bake at 80°C for 8 hours. The PDMS is solidified, forming a stable PDMS and bubble cavity. The two are closely adjacent and highly integrated to form a compact temperature sensor. The fifth step is to drop UV glue on both ends of the capillary structure to form a packaging structure.

[0019] Furthermore, the PDMS in the present invention is prepared by thoroughly mixing Sylvard 184-A and Sylvard 184-B in a ratio of 5:1, and then storing the mixture in a refrigerator for 4.5 hours before use.

[0020] The present invention has the following beneficial effects: 1. Compared with other PDMS cavities directly connected to capillaries or single-mode optical fibers, the sensor structure of the present invention has a bubble cavity adjacent to the PDMS cavity. This makes it easier for the FPI1 cavity in the PDMS to expand during temperature changes due to the more easily deformed bubble cavity FPI2, thereby giving the sensor FPI1 cavity a very considerable temperature sensitivity K1.

[0021] 2. In this invention, because the length variations of the FPI2 and FPI1 cavities are always opposite, the resulting FPI3 cavity exhibits significantly less length variation than the FPI1 cavity, resulting in lower temperature sensitivity. The varying sensitivities of the three FPI cavities make this sensor suitable for a variety of applications requiring varying sensitivity and sensing range.

[0022] 3. The FPI spectrum is periodic, and the temperature sensing range is limited. Specifically, ΔT is equal to the ratio of the free spectral range (FSR) to the temperature sensitivity.

[0023] Within a range, there is a one-to-one correspondence between spectrum and temperature. If we only observe from the spectrum, when the temperature changes beyond this range, the spectrum drift will be greater than one FSR. In other words, a spectrum can correspond to multiple temperatures such as T1+ΔT, T1+2ΔT, T1+3ΔT, etc.

[0024] To address the issue of the small range of the FPI periodic spectrum, we extracted the phase of the main frequency components of the spatial Fourier (SF) spectrum of the reflection spectrum. A temperature change exceeding one range means that the phase change of the corresponding FPI cavity will be greater than 2π. Therefore, based on the trend of the phase change, when the phase suddenly changes at a certain point, it means that the temperature change has exceeded one range. Increasing or decreasing the phase by 2π and reconstructing the spectrum can obtain a spectrum with a spectral drift greater than one period.

[0025] Through this method, we obtained a high-sensitivity and large-range fiber optic temperature sensor.

[0026] 4. The production of the sensor of the present invention does not require the use of expensive equipment such as femtosecond lasers. The cost is relatively low, and a sensor with a compact structure and small size (100μm to 200μm) can be produced with simple equipment.

[0027] 5. The ultraviolet glue (UV glue) encapsulation at both ends of the sensor capillary of the present invention improves the robustness of the structure and shields the influence of changes in external air pressure, that is, solves the air pressure cross-sensitivity problem of general PDMS-based optical fiber temperature sensors. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the structure of the sensor of the present invention cut away in the main viewing direction; Figure 2 It is a schematic diagram of the structure of the sensor part after separation of the present invention.

[0029] In the figure: 1—single-mode optical fiber, 2—UV glue, 3—PDMS, 4—bubble cavity, 5—capillary. DETAILED DESCRIPTION

[0030] The technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. It is obvious that the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention. In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the drawings, or the directions or positional relationships in which the invented product is usually placed when in use. They are only for the convenience of describing the present invention or simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed or operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0031] In the description of the present invention, it should also be noted that, unless otherwise clearly specified and limited, the terms "setting", "installation" and "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal connection of two elements.

[0032] For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0033] See also Figure 1 、 Figure 2 The present invention is a high-sensitivity, large-range optical fiber temperature sensor, comprising a capillary 5, a PDMS cavity 3 filled in the capillary, and a bubble cavity 4 located in the PDMS. Single-mode optical fibers 1 are respectively inserted at both ends of the capillary, and one end of each of the two single-mode optical fibers is inserted into the PDMS. The end face of one single-mode optical fiber extends into the bubble cavity, and a distance L1 is left between the end face of the other single-mode optical fiber and the side wall of the bubble cavity. Ultraviolet glue 2 is used to encapsulate the two ends of the capillary, and the ultraviolet glue encapsulates the outer wall of the single-mode optical fiber and the corresponding capillary end into one.

[0034] Furthermore, the transverse diameter of the bubble cavity in the present invention is L2, the distance between the inner end faces of the two single-mode optical fibers is L3, and the size of L3 is equal to the sum of the sizes of L1 and L2.

[0035] Furthermore, the inner diameter and outer diameter of the capillary tube in the present invention are 150 μm and 300 μm respectively. Furthermore, the inner diameter of the core of the single-mode optical fiber in the present invention is 9 μm, and the outer diameter of the cladding is 125 μm.

[0036] A manufacturing method is also designed for the sensor of the present invention, namely, a manufacturing method for a high-sensitivity, large-range optical fiber temperature sensor. The manufacturing method is used to manufacture the above-mentioned optical fiber temperature sensor, and includes the following steps: The first step is to use wire strippers to remove the coating of the two single-mode optical fibers, clean the outer cladding, and use a cleaver to cut the single-mode optical fibers to obtain two flat single-mode optical fiber end faces; Step 2: Insert the two single-mode optical fiber end faces into the capillary tube facing each other. During this process, use a microscope to help adjust and observe the distance and alignment of the two single-mode optical fiber end faces to maintain an appropriate distance between the two single-mode optical fiber end faces. The inner and outer diameters of the capillary tube are 150 μm and 300 μm, respectively. The third step is to fill the liquid PDMS into the capillary through the capillary siphon effect. During this process, PDMS will fill the voids in the capillary and leave a bubble cavity. Step 4: Place the above structure in a drying oven and bake at 80°C for 8 hours. The PDMS is solidified, forming a stable PDMS and bubble cavity. The two are closely adjacent and highly integrated to form a compact temperature sensor. The fifth step is to drop UV glue on both ends of the capillary structure to form a packaging structure.

[0037] Furthermore, the PDMS described in the present invention is prepared by thoroughly mixing Sylvard 184-A and Sylvard 184-B in a ratio of 5:1 and storing in a refrigerator for 4.5 hours before use. The PDMS described in the present invention is polydimethylsiloxane (PDMS), which is an odorless, elastic, colorless, non-toxic, and non-volatile material.

[0038] The working principle of the sensor made according to the above method is as follows: exist Figure 1 The structure contains three dielectric contact surfaces, namely three reflective surfaces: M1 of SMP (single-mode fiber)-PDMS, M2 of PDMS-air (bubble cavity), and M3 of air (bubble cavity)-SMF (single-mode fiber on the other side). These three reflective surfaces form a PDMS cavity - FPI1 with a length of L1, a bubble cavity - FPI2 with a length of L2, and a hybrid cavity - FPI3 with a length of L1+L2. The reflectivity of multiple reflections on each surface in the FP cavity is low, so the multiple reflections can be ignored, and the reflection interference spectrum of the sensor can be determined by the resonant modes of the three FP cavities. The total reflected light field intensity E of the sensor structure is r for: (1) in, E0 and E r are the input and reflected light field intensities, respectively. r 0 、r 1 and r 2 are the reflection coefficients of the M1, M2 and M3 reflective surfaces respectively. 1 and α 2 are the transmission losses of the M1 and M2 reflectors respectively. The phases corresponding to FPI1, FPI2 and FPI3 are: (2) The total intensity of the reflected light I r for: (3) The resonance trough positions corresponding to FPI1, FPI2 and FPI3 are: (4) PDMS has significant thermo-optic effect (TOC) and thermal expansion effect (TEC), and temperature changes will cause n PDMS There are significant changes in L1 and L2. An increase in L1 directly leads to a decrease in L2. The effective optical path length of L3 changes with changes in L1 and L2. All of these factors will cause changes in the interference spectrum. Ignoring the change in the refractive index of air with temperature, the temperature sensitivity of FPI1, FPI2, and FPI3 is: (5) The free spectral range (FSR) is the wavelength difference between two adjacent troughs in the spectrum. The FSRs of FPI1, FPI2, and FPI3 are approximately: (6) Then, the relationship between the trough wavelength shift and phase change of FPI1, FPI2 and FPI3 can be expressed as: (7) From this perspective, the temperature sensitivities of FPI1, FPI2, and FPI3 can be deduced as follows: (8) Recording reflection spectra at each temperature reveals a complex aggregate spectrum for the three FPI cavities. A fast Fourier transform (FFT) is performed on the reflection spectra to obtain the corresponding spatial Fourier (SF) spectra. The SF components corresponding to the three FPI cavities are then identified. The phase values ​​of these three primary SF components are then extracted, and the corresponding FPI cavity spectra can be constructed from these phases. By fitting the FPI cavity phase change with temperature, the temperature sensitivity of the FPI cavity phase change can be directly determined.

[0039] On the other hand, the phase change can be converted into a wavelength shift, and the temperature sensitivity of the wavelength can be obtained by fitting the wavelength shift with temperature.

[0040] Compared with other PDMS cavities directly connected to capillaries or single-mode optical fibers, the bubble cavity in the sensor of the present invention is adjacent to the PDMS cavity. This makes it easier for the FPI1 cavity in the PDMS to deform during temperature changes due to the more easily deformed characteristic of the bubble cavity FPI2, thereby making the FPI1 cavity in the sensor have a very considerable temperature sensitivity K1.

[0041] In this invention, because the length variations of the FPI2 and FPI1 cavities are always opposite, the resulting FPI3 cavity exhibits significantly less length variation than the FPI1 cavity, resulting in FPI3 having lower temperature sensitivity. The varying sensitivities of the three FPI cavities make this sensor suitable for a variety of applications requiring varying sensitivity and sensing range.

[0042] The spectrum of FPI is periodic, and the temperature sensing range is limited, specifically, ΔT is equal to the ratio of the free spectral range (FSR) to the temperature sensitivity.

[0043] Within a range, there is a one-to-one correspondence between spectrum and temperature. If we only observe from the spectrum, when the temperature changes beyond this range, the spectrum drift will be greater than one FSR. In other words, a spectrum can correspond to multiple temperatures such as T1+ΔT, T1+2ΔT, T1+3ΔT, etc.

[0044] To address the issue of the small range of the FPI periodic spectrum, we extracted the phase of the main SF component of the spatial spectrum of the reflection spectrum. A temperature change exceeding one range means that the phase change of the corresponding FPI cavity will be greater than 2π. Therefore, based on the trend of the phase change, when the phase suddenly changes at a certain point, it means that the temperature change has exceeded one range. Increasing or decreasing the phase by 2π and then reconstructing the spectrum can obtain a spectrum with a spectral drift greater than one period.

[0045] Through this method, we obtained a high-sensitivity and large-range fiber optic temperature sensor.

[0046] The production of the sensor of the present invention does not require expensive equipment such as a femtosecond laser, and the cost is relatively low. Simple equipment can produce a sensor with a compact structure and small size (100 μm to 200 μm).

[0047] The ultraviolet glue (UV glue) packaging at both ends of the sensor capillary of the present invention improves the robustness of the structure and shields the influence of external air pressure changes, thereby solving the air pressure cross-sensitivity problem of general PDMS sensors.

Claims

1. A high-sensitivity, large-range optical fiber temperature sensor, characterized in that: The invention comprises a capillary tube, PDMS filled in the capillary tube, and a bubble cavity located in the PDMS. Single-mode optical fibers are respectively inserted at both ends of the capillary tube. One end of each of the two single-mode optical fibers is inserted into the PDMS. The end face of one single-mode optical fiber extends into the bubble cavity, and a distance L1 is left between the end face of the other single-mode optical fiber and the side wall of the bubble cavity. Ultraviolet glue is used to encapsulate the two ends of the capillary tube, and the ultraviolet glue encapsulates the outer wall of the single-mode optical fiber and the corresponding capillary tube end into one.

2. The high-sensitivity, large-range optical fiber temperature sensor according to claim 1, characterized in that: The transverse diameter of the bubble cavity is L2, and the distance between the inner end faces of the two single-mode optical fibers is L3. The size of L3 is equal to the sum of the sizes of L1 and L2.

3. The high-sensitivity, large-range optical fiber temperature sensor according to claim 1, characterized in that: The inner diameter and outer diameter of the capillary are 150 μm and 300 μm respectively.

4. The high-sensitivity, large-range optical fiber temperature sensor according to claim 1, characterized in that: The core inner diameter of the single-mode optical fiber is 9 μm, and the cladding outer diameter is 125 μm.

5. A method for manufacturing a high-sensitivity, large-range optical fiber temperature sensor, the method being used to manufacture the optical fiber temperature sensor according to claims 1 to 4, characterized in that: The following steps are included: The first step is to use wire strippers to remove the coating of the two single-mode optical fibers, clean the outer cladding, and use a cleaver to cut the single-mode optical fibers to obtain two flat single-mode optical fiber end faces; Step 2: Insert the two single-mode optical fiber end faces into the capillary tube facing each other. During this process, use a microscope to help adjust and observe the distance and alignment of the two single-mode optical fiber end faces to maintain an appropriate distance between the two single-mode optical fiber end faces. The inner and outer diameters of the capillary tube are 150 μm and 300 μm, respectively. The third step is to fill the liquid PDMS into the capillary through the capillary siphon effect. During this process, PDMS will fill the voids in the capillary and leave a bubble cavity. Step 4: Place the above structure in a drying oven and bake at 80°C for 8 hours. The PDMS is solidified, forming a stable PDMS and bubble cavity. The two are closely adjacent and highly integrated to form a compact temperature sensor. The fifth step is to drop UV glue on both ends of the capillary structure to form a packaging structure.

6. The method for manufacturing a high-sensitivity, large-range optical fiber temperature sensor according to claim 1, characterized in that: The PDMS was prepared by thoroughly mixing Sylvard 184-A and Sylvard 184-B at a ratio of 5:1, and the mixture was kept in a refrigerator for 4.5 hours before use.

Images