Temperature sensor
The temperature sensor uses a sensor optical fiber with a core and cladding to detect temperature based on test light intensity, addressing the cost and size issues of conventional sensors, offering accurate and compact temperature measurement.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- FURUKAWA ELECTRIC CO LTD
- Filing Date
- 2022-03-17
- Publication Date
- 2026-06-19
AI Technical Summary
Conventional temperature sensors using optical fibers require expensive and large devices like optical spectrum analyzers to detect wavelength shift, making them costly and less compact.
A temperature sensor comprising a light source, a sensor optical fiber with a core and cladding, and a photodetector, where the sensor optical fiber transmits test light with a loss of 0.3 dB/m or more, utilizing the refractive index difference between the core and cladding to detect temperature based on the intensity of received test light.
Provides an inexpensive and compact temperature sensor capable of accurate temperature detection without the need for expensive equipment, suitable for harsh environments and applications like gas tanks and oil wells.
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to a temperature sensor.
Background Art
[0002] Conventionally, a device for detecting temperature using an optical fiber has been known. For example, Patent Document 1 discloses this type of technology. Patent Document 1 describes a device including an optical fiber device that generates a first reflected light and a second reflected light when laser light is incident thereon, a light receiver that receives the interference light of the first reflected light and the second reflected light emitted from the optical fiber device, and an analyzer that analyzes the signal output from the light receiver.
Prior Art Documents
Patent Documents
[0003]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] However, in the device of Patent Document 1, the temperature is measured by detecting the output signal of the light receiver while changing the wavelength of the irradiated laser light and detecting the amount of wavelength shift. However, in order to detect this amount of wavelength shift, an expensive and large device such as an optical spectrum analyzer is required.
[0005] The present invention has been made in view of the above, and an object thereof is to provide an inexpensive and small temperature sensor.
Means for Solving the Problems
[0006] The present invention relates to a temperature sensor comprising a light source that outputs test light, a sensor optical fiber that receives the test light and transmits the test light with a loss of 0.3 dB / m or more at a temperature in the range of 20°C to 150°C, and a photodetector that receives the test light transmitted by the sensor optical fiber, wherein the temperature of the sensor optical fiber is detected based on the intensity of the test light received by the photodetector.
[0007] The sensor optical fiber has a core and a cladding formed on the outer circumference of the core, and as the temperature of the sensor optical fiber rises, the refractive index difference between the core and the cladding increases, and the confinement of the test light transmitted within the core becomes stronger, which may increase the intensity of the test light received by the photodetector.
[0008] The sensor optical fiber has a core and a cladding formed on the outer circumference of the core, and the diameter of the core may be 10 times or more the thickness of the cladding.
[0009] The sensor optical fiber has a core and a cladding formed on the outer circumference of the core, and a plurality of nanostructures are present near the interface between the core and the cladding, and the nanostructures may have a cross-sectional diameter of 100 nm or less in a cross section perpendicular to the longitudinal direction of the sensor optical fiber and be distributed in a region with a length of less than 1 m in the longitudinal direction. [Effects of the Invention]
[0010] According to the present invention, an inexpensive and compact temperature sensor can be provided. [Brief explanation of the drawing]
[0011] [Figure 1] This is a schematic diagram showing a temperature sensor related to one embodiment of the present invention. [Figure 2] This is a schematic diagram showing the structure of the sensor optical fiber of a temperature sensor according to one embodiment of the present invention. [Figure 3]This is a schematic diagram showing the changes in refractive index and light distribution as the temperature rises in the core and cladding of a temperature sensor according to one embodiment of the present invention. [Figure 4] This is a schematic diagram showing the relationship between transmission loss and optical distribution of a sensor optical fiber according to one embodiment of the present invention. [Figure 5] This is a schematic diagram illustrating the test method for optical output evaluation testing. [Figure 6] This figure shows the relationship between the temperature of the temperature sensor and the rate of increase in light output. [Figure 7] This is a schematic diagram showing the changes in refractive index and light distribution as the core temperature rises in a modified example of the sensor optical fiber of a temperature sensor according to one embodiment of the present invention. [Modes for carrying out the invention]
[0012] (Embodiment) The following describes a temperature sensor 1 according to an embodiment of the present invention. However, the present invention is not limited to the following embodiments. Furthermore, the figures referenced in the following description merely provide a schematic representation of the shape, size, and positional relationship to the extent that the contents of this disclosure can be understood. In other words, the present invention is not limited to the shapes, sizes, and positional relationships exemplified in the figures.
[0013] First, the configuration of the temperature sensor 1 according to this embodiment will be described with reference to Figure 1. Figure 1 is a schematic diagram showing the temperature sensor 1.
[0014] The temperature sensor 1 comprises a light source 10, a sensor optical fiber 20, a photodetector PD (Photo Diode) 30, and a detection unit processing device 40.
[0015] The light source 10 includes an LED 11 and a power supply 12 that supplies power to the LED 11. The LED 11 uses the power supplied from the power supply 12 to output test light to the sensor optical fiber 20. The wavelength of the test light output from the LED 11 is not particularly limited. In this embodiment, blue light with a wavelength of 470 nm is output from the LED 11 to the sensor optical fiber 20.
[0016] One end of the sensor optical fiber 20 is optically connected to the LED 11, and the other end is optically connected to the PD 30. The sensor optical fiber 20 transmits the test light input from the LED 11 to the PD 30 with a loss of 0.3 dB / m or more at any temperature in the range of 20°C to 150°C. By setting the transmission loss of the sensor optical fiber 20 to 0.3 dB / m or more at any temperature in the range of 20°C to 150°C, it becomes possible to detect the temperature based on the intensity of the test light. The principle of temperature detection by the temperature sensor 1 will be described later.
[0017] The configuration of the sensor optical fiber 20 will be described while referring to FIG. 2. FIG. 2 is a schematic diagram of the sensor optical fiber 20 used in the temperature sensor 1.
[0018] The sensor optical fiber 20 according to the present embodiment is made of a quartz-based material and has a core 21 and a cladding 22 formed on the outer periphery of the core 21.
[0019] The diameter of the core 21 is preferably 10 times or more the thickness of the cladding 22. This makes it easier for heat from the outside to be transmitted to the core 21. The refractive index of the core 21 is higher than the refractive index of the cladding. In the core 21 of the sensor optical fiber 20 of the present embodiment, germanium is doped at the center.
[0020] The sensor optical fiber 20 of the present embodiment is configured such that when its temperature rises, the refractive index difference between the core 21 and the cladding 22 increases.
[0021] In the sensor optical fiber 20, a plurality of nanostructures 23 exist near the interface between the core 21 and the cladding 22. The nanostructures 23 may exist throughout the entire radial direction of the cladding 22 or may exist partially. Each nanostructure 23 is, for example, a nanoparticle, a cylindrical tube, or a void, and may include at least two of nanoparticles, cylindrical tubes, and voids.
[0022] The PD30 receives the test light transmitted through the sensor optical fiber 20, converts the test light into a current signal corresponding to its intensity, and outputs it to the processing unit 40.
[0023] The processing unit 40 comprises a temperature-specification unit 41, a storage unit 42, and an output unit 43. The temperature-specification unit 41 is composed of a processor and corresponds to the central part of the computer that performs calculations and control necessary for the operation of the processing unit 40. The processor is, for example, a CPU (Central Processing Unit), MPU (Micro Processing Unit), SoC (System on a Chip), DSP (Digital Signal Processor), GPU (Graphics Processing Unit), ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or FPGA (Field-Programmable Gate Array). Alternatively, the processor may be a combination of several of these.
[0024] The temperature determination unit 41 acquires the intensity of the test light based on the current signal input from the PD 30. Then, the temperature determination unit 41 detects the temperature of the sensor optical fiber 20 based on the acquired intensity of the test light. For example, the temperature determination unit 41 may determine the temperature from the acquired intensity of the test light by referring to information that shows the relationship between the intensity of the test light received by the PD 30 and the temperature, which is predetermined for each sensor optical fiber 20. By detecting the temperature of the sensor optical fiber 20, the temperature of the object to be measured (gas, liquid, solid, etc.) in contact with the outer surface of the sensor optical fiber 20 can be determined. Specifically, the temperature determination unit 41 detects the average temperature of the region in which the sensor optical fiber 20 is in contact with the object to be measured. Then, the temperature determination unit 41 measures the difference between the temperature when the object to be measured is not in contact with the sensor optical fiber 20 and the temperature when the object to be measured is in contact with the sensor optical fiber 20, and calculates this difference as the temperature of the object to be measured.
[0025] The memory unit 42 is a storage area for various programs and data used by the temperature determination unit 41 for calculation and control processing, and can be composed of ROM, RAM, flash memory, a semiconductor drive (SSD), or hardware (HDD). The memory unit 42 stores, for example, information on the current signal input from PD30, time information on when the current signal was input, information on the intensity of the test light calculated from the current signal, information showing the relationship between the intensity of the test light received by PD30 and the temperature, and information on the temperature calculated by the temperature determination unit 41.
[0026] The output unit 43 consists of a display, speaker, etc., and outputs images and sound. The output unit 43 may be configured to display, for example, the temperature of the sensor optical fiber 20 calculated by the temperature determination unit 41 on the display.
[0027] Next, the principle of temperature detection using the temperature sensor 1 equipped with a sensor optical fiber 20 having a transmission loss of 0.3 dB / m or more will be explained with reference to Figure 3. Figure 3 is a schematic diagram showing the changes in the refractive index of the core 21 and cladding 22 of the sensor optical fiber 20 and the distribution of test light as the temperature rises. Figure 3(a) shows the sensor optical fiber 20 at a low temperature, and Figure 3(b) shows the sensor optical fiber 20 at a high temperature. The dashed line R in Figure 3 indicates the magnitude of the refractive index outside the sensor optical fiber 20 (the air layer covering the sensor optical fiber 20), the refractive index of the cladding 22, and the refractive index of the core 21. Note that in Figure 3, the higher the position, the larger the refractive index.
[0028] In the example shown in Figure 3, as the temperature of the sensor optical fiber 20 rises, the refractive index difference between the core 21 and the cladding 22 increases, and the optical confinement effect of the core 21 strengthens. As a result, as shown in Figure 3(b), the test light L distributed to the outer layer of the sensor optical fiber 20 becomes confined within the core 21, and the diffusion of the test light L to the outside is suppressed. In other words, as the temperature of the sensor optical fiber 20 rises, the intensity of the test light L received by the PD 30 increases. The temperature sensor 1 uses this relationship between temperature and the intensity of the test light L to detect the temperature of the sensor optical fiber 20.
[0029] Next, we will explain the effect of the transmission loss of the sensor optical fiber 20 on the detection sensitivity of the temperature sensor 1, referring to Figure 4.
[0030] Figure 4 is a schematic diagram showing the distribution of refractive indices of the core 21 and cladding 22 of sensor optical fibers 20 with different optical transmission losses and the test light L. Figure 4(a) shows sensor optical fiber 20 with low optical transmission loss, and Figure 4(b) shows sensor optical fiber 20 with high optical transmission loss. The dashed line R in Figure 4 shows the magnitude of the refractive index outside (in air), the refractive index of the cladding 22, and the refractive index of the core 21 of the sensor optical fiber 20. Note that the refractive index shown by the dashed line R is larger the higher it is located in Figure 4.
[0031] The sensor optical fibers 20 shown in Figures 4(a) and 4(b) have the same refractive index in their respective cores 21 and cladding 22, and the same temperature, differing only in their optical transmission loss. The sensor optical fiber 20 shown in Figure 4(b), which has a higher transmission loss than the sensor optical fiber 20 shown in Figure 4(a), exhibits greater leakage of the test light L to the outside. In other words, even with the same refractive index difference between the core 21 and cladding 22, the sensor optical fiber 20 with a higher transmission loss exhibits greater leakage of the test light to the outside.
[0032] Here, we will explain the change in the optical output of the sensor optical fiber 20 (the intensity of the test light received by PD30) due to temperature. The change in optical output due to temperature is given by the following equation (1).
[0033] Change in optical output due to temperature = Transmission loss due to the structure near the interface between core 21 and cladding 22 × Change in the power of the test light localized at the interface between core 21 and cladding 22 due to temperature... (1)
[0034] For example, the transmission loss of the sensor optical fiber 20 can be increased by providing a scattering structure near the interface between the core 21 and the cladding 22. Furthermore, the sensor optical fiber 20, whose optical transmission loss has been increased by the above-mentioned scattering structure, exhibits greater leakage of test light at low temperatures, and therefore the change in the light intensity distribution due to the confinement effect in the core 21 as the temperature rises is also greater. Thus, it can be seen that there is a synergistic effect on the change in transmission loss due to temperature.
[0035] As shown in equation (1), the detection sensitivity of the temperature sensor 1 increases as the amount of change in optical output due to temperature increases. On the other hand, if the transmission loss is small and the optical output is high even at low temperatures, the amount of change in optical output due to temperature rise will also be small, and it is thought that sufficient detection sensitivity for the temperature sensor 1 to function will not be obtained. As shown in the results of the optical output evaluation test described later, temperature detection becomes possible by setting the transmission loss of the sensor optical fiber 20 to 0.3 dB / m or more.
[0036] Next, the configuration near the interface between the core 21 and the cladding 22, which affects the transmission loss of the sensor optical fiber 20, will be explained with reference to Figure 2.
[0037] Nanostructures 23, such as voids and metal particles, located near the interface between the core 21 and the cladding 22, have a cross-sectional diameter of 100 nm or less in a cross-section perpendicular to the longitudinal direction of the sensor optical fiber 20. When the cross-sectional diameter of the nanostructures 23 exceeds 100 nm, the area occupied by air in the case of voids and metal in the case of metal particles increases, resulting in a higher effective refractive index difference with the core 21 made of silica-based material, making light leakage or diffusion less likely. Therefore, by setting the cross-sectional diameter to 100 nm or less, sensitivity to temperature can be increased while suppressing excessive loss. It is desirable that the cross-sectional diameter be larger than the size of the silica molecule and at least 1 nm in size to avoid affecting light.
[0038] Furthermore, as shown in Figure 2, the multiple nanostructures 23 are distributed in a region of less than 1 m in length along the longitudinal direction of the sensor optical fiber 20. Here, being distributed in a region of less than 1 m in length means that the length of the region in which the nanostructures 23 exist continuously is less than 1 m. For example, in Figure 2, there are multiple regions in which the nanostructures 23 exist continuously, but all of them are less than 1 m in length along the longitudinal direction. This makes it possible to suppress excessive loss due to the nanostructures 23 and their impact on propagation characteristics.
[0039] A sensor optical fiber 20 having multiple nanostructures 23 can be manufactured, for example, by applying the optical fiber manufacturing method disclosed in Japanese Patent Publication No. 2013-511749. If the nanostructures 23 consist of fine particles in addition to voids, the sensor optical fiber 20 can be manufactured by, for example, mixing fine particles into the gap between the core matrix and the glass capillary cladding 22, and then drawing the optical fiber matrix. If the fine particles are made of a material with a melting point of 1500°C or higher, melting or deformation of the fine particles can be prevented even at high temperatures of around 1400°C, which is the heating temperature when drawing the silica-based optical fiber matrix. Suitable materials with high melting points include carbon, tantalum, molybdenum, chromium oxide, zirconium oxide, etc.
[0040] Next, we will explain the optical output evaluation test, which confirmed the relationship between the transmission loss of the sensor optical fiber 20, temperature, and optical output, with reference to Figures 5 and 6. Figure 5 is a schematic diagram showing the method of the optical output evaluation test. Note that the power supply 12 and processing unit 40 of the temperature sensor 1 are not shown in Figure 5.
[0041] As shown in Figure 5, in the optical output evaluation test, a light source 10 is connected to one end and a PD30 is connected to the other end. The sensor optical fiber, with approximately 50 cm of its coating removed in the longitudinal direction, is placed on a hot plate PH, and the intensity of the test light output from the sensor optical fiber to the PD30 is measured when the temperature is changed from approximately 30°C to approximately 130°C. Sensor optical fibers with transmission losses of 0.02 dB / m, 0.3 dB / m, and 3.0 dB / m were used. Each of the three sensor optical fibers has a core diameter of 43 μm, a cladding thickness of 63.5 μm, and an overall outer diameter of 170 μm.
[0042] Figure 6 is a graph showing the relationship between temperature and the rate of increase in optical output for sensor optical fibers with different transmission losses. The horizontal axis of Figure 6 represents the temperature of the hot plate (°C), and the vertical axis represents the rate of increase in optical output with respect to temperature (%), with the optical output at room temperature of 25°C as the reference. In Figure 6, the triangular plots represent the evaluation results for sensor optical fibers with a transmission loss of 0.02 dB / m, the circular plots represent the evaluation results for sensor optical fibers with a transmission loss of 0.3 dB / m, and the square plots represent the evaluation results for sensor optical fibers with a transmission loss of 3.0 dB / m.
[0043] In a typical sensor optical fiber with a transmission loss of 0.02 dB / m, the rate of increase in optical output was approximately 0% from approximately 30°C to approximately 130°C. This confirms that the optical output of the sensor optical fiber remains almost constant regardless of temperature. In contrast, in a sensor optical fiber with a transmission loss of 0.3 dB / m, the rate of increase in optical output increased with increasing temperature. From these results, it can be confirmed that the optical output of sensor optical fibers 20 with a transmission loss of 0.3 dB / m or higher increases monotonically with temperature.
[0044] In a sensor optical fiber with a transmission loss of 3.0 dB / m, the change in optical output with increasing temperature becomes even larger. For example, if the output light intensity is 1 mW, then 80 μW of that intensity changes with temperature from 30°C to 60°C, and the rate of change in output light intensity with respect to temperature is 2.67 μW / °C. For example, if a PD30 with a detection accuracy of 0.01 μW is used, a temperature change of approximately 0.01°C can be detected. From these results, it can be confirmed that increasing the transmission loss of the sensor optical fiber 20 improves the temperature detection sensitivity. Furthermore, the temperature sensor 1 according to this embodiment can more accurately detect temperatures in the range of 20°C to 150°C.
[0045] Next, in the above embodiment, the following modifications can be adopted for the configuration of the sensor optical fiber. Referring to the above description, the configuration of the sensor optical fiber 20A will be explained with reference to Figure 7.
[0046] Figure 7 is a schematic diagram showing the changes in the distribution of refractive index and test light L as the temperature of the core 21A of the sensor optical fiber 20A rises. Figure 7(a) shows the sensor optical fiber 20A at a low temperature, and Figure 4(b) shows the sensor optical fiber 20A at a high temperature. The dashed line R in Figure 4 shows the magnitude of the refractive index outside (in air) of the sensor optical fiber 20A and the refractive index of the core 21A. Note that the refractive index shown by the dashed line R is larger the higher it is located in Figure 4.
[0047] As shown in Figure 7, the sensor optical fiber 20A differs from the sensor optical fiber 20 mainly in that it does not have cladding 22 or nanostructures 23. The sensor optical fiber 20A is made of a silica-based material such as a silica rod and has a core 21A. As shown in Figure 7, the sensor optical fiber 20A is an air-clad fiber that transmits test light L due to the refractive index difference between the core 21A and the air layer surrounding the core 21A. The silica rod of the core 21A of the sensor optical fiber 20A has roughened sides and is set to have a high transmission loss.
[0048] In the example shown in Figure 7, as the temperature of the sensor optical fiber 20A rises, the refractive index of the core 21 increases, the refractive index difference between the core 21 and the air layer increases, and the optical confinement effect of the core 21 strengthens. As a result, as shown in Figure 7(b), the test light L distributed to the outer layer of the sensor optical fiber 20A can be confined within the core 21A, and leakage of the test light L is suppressed. In other words, as the temperature of the sensor optical fiber 20A rises, the intensity of the test light L received by the PD30 increases.
[0049] According to the embodiments described above, the following effects are achieved.
[0050] The temperature sensor 1 according to this embodiment includes a light source 10 that outputs test light, a sensor optical fiber 20 that receives the test light and transmits the test light at a temperature in the range of 20°C to 150°C with a loss of 0.3 dB / m or more, a PD 30 that receives the test light transmitted by the sensor optical fiber 20, and a processing device 40 that detects the temperature of the sensor optical fiber 20 based on the intensity of the test light received by the PD 30.
[0051] As a result, the optical transmission loss of the sensor optical fiber 20 is 0.3 dB / m or more, so the optical output of the sensor optical fiber 20 changes in accordance with the temperature, and the temperature of the sensor optical fiber 20 and the object in contact with the sensor optical fiber 20 can be detected from the intensity of the received test light. Compared to optical fiber temperature sensors that measure temperature from wavelength shift, an optical spectrum analyzer and the like are not required, and temperature can be detected in an inexpensive and compact configuration. In addition, because the temperature sensor 1 has a simple structure and a small diameter, it can be used even in harsh environments, making it promising for applications such as detecting temperature inside gas tanks and pipes, and detecting temperature inside oil wells.
[0052] In the temperature sensor 1 according to this embodiment, the sensor optical fiber 20 has a core 21 and a cladding 22 formed on the outer circumference of the core 21. When the temperature of the sensor optical fiber 20 rises, the refractive index difference between the core 21 and the cladding 22 increases, and the confinement of the test light transmitted within the core 21 becomes stronger, thereby increasing the intensity of the test light received by the PD 30.
[0053] This allows for accurate detection of the temperature of the sensor optical fiber 20 and the object in contact with the sensor optical fiber 20, based on the intensity of the received test light.
[0054] In the temperature sensor 1 according to this embodiment, the sensor optical fiber 20 has a core 21 and a cladding 22 formed on the outer circumference of the core 21, and the diameter of the core 21 is 10 times or more the thickness of the cladding 22.
[0055] As a result, the cladding 22 is thinner relative to the core 21, which increases the amount of test light that seeps out from the outer layer of the sensor optical fiber 20, especially at low temperatures. In addition, external heat is more easily transferred to the interface between the core 21 and the cladding 22, which improves the detection sensitivity to external temperature.
[0056] In the temperature sensor 1 according to this embodiment, the sensor optical fiber 20 has a core 21 and a cladding 22 formed on the outer circumference of the core 21, and a plurality of nanostructures 23 are present near the interface between the core 21 and the cladding 22, and the nanostructures 23 have a cross-sectional diameter of 100 nm or less in a cross section perpendicular to the longitudinal direction of the sensor optical fiber 20 and are distributed in a region with a length of less than 1 m in the longitudinal direction.
[0057] This improves the transmission loss of the sensor optical fiber 20, thereby improving the sensitivity of the temperature sensor 1.
[0058] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above and can be modified as appropriate.
[0059] In the above embodiment, the temperature sensor 1 was equipped with sensor optical fibers 20 and 20A, but instead of sensor optical fibers 20 and 20A, it may be configured to be equipped with plastic fibers in which light is diffused due to the irregularities on the sides of the optical fibers.
[0060] In the above embodiment, the diameter of the core 21 of the sensor optical fiber 20 was 10 times or more the thickness of the cladding 22, but it may be less than 10 times. Also, the thickness of the cladding 22 may be greater than the diameter of the core 21.
[0061] In the above embodiment, the sensor optical fiber 20 had a configuration in which multiple nanostructures 23 existed near the interface between the core 21 and the cladding 22, but it may also have a configuration in which multiple nanostructures 23 do not exist.
[0062] In the above embodiment, the sensor optical fiber 20 was configured such that the refractive index difference between the core 21 and the cladding 22 increased as the temperature of the sensor optical fiber 20 rose. However, the sensor optical fiber 20 may also be configured such that the refractive index difference between the core 21 and the cladding 22 decreases as the temperature rises. By changing the glass composition of the sensor optical fiber or the additives to the core, a sensor optical fiber 20 can be manufactured in which the refractive index difference between the core and the cladding decreases as the temperature rises. [Explanation of Symbols]
[0063] 1. Temperature sensor 10 light source 20 Sensor optical fibers 30 PD (photo receiver) 40 Processing unit (detection unit)
Claims
1. A light source that emits test light, A sensor optical fiber to which the aforementioned test light is input and which transmits the test light with a loss of 0.3 dB / m or more at a temperature within the range of 20°C to 150°C, A light receiver that receives the test light transmitted by the sensor optical fiber, The system includes a detection unit that detects the temperature of the sensor optical fiber based on the intensity of the test light received by the light receiver, The aforementioned sensor optical fiber has a core and a cladding formed on the outer circumference of the core. The lower the temperature, the greater the leakage of the test light transmitted through the core into the cladding side of the sensor optical fiber. A temperature sensor in which, as the temperature of the sensor optical fiber rises, the refractive index difference between the core and the cladding increases, and the confinement of the test light transmitted within the core becomes stronger, causing the intensity of the test light received by the photodetector to increase monotonically in proportion to the rise in temperature of the sensor optical fiber.
2. The temperature sensor according to claim 1, wherein the diameter of the core is 10 times or more the thickness of the cladding.
3. Multiple nanostructures are present near the interface between the core and the cladding, The temperature sensor according to claim 1 or 2, wherein the nanostructure has a cross-sectional diameter of 100 nm or less in a cross-section perpendicular to the longitudinal direction of the sensor optical fiber, and is distributed in a length region of less than 1 m in the longitudinal direction.