Optical fiber temperature sensor

Abstract

The utility model discloses an optical fiber temperature sensor, the temperature sensor is composed of an optical fiber and a rectangular through microgroove, the optical fiber comprises a first single-mode optical fiber, a multimode optical fiber and a second single-mode optical fiber, the rectangular through microgroove is arranged at one side of a multimode optical fiber core close to a central axis, and the rectangular through microgroove is filled with liquid. The refractive index of the multimode fiber core and the refractive index of the liquid in the rectangular through micro-groove can change along with the temperature change, and the refractive index of the multimode fiber core and the refractive index of the liquid in the rectangular through micro-groove change along with the temperature. The optical path difference of two parts of light passing through the multimode optical fiber and the liquid is changed, so that the interference spectrum is changed, and temperature measurement can be realized by detecting the transmission spectrum of the temperature sensor. The temperature sensitivity of the sensor can be improved, and the sensor has the advantages of simplicity in manufacturing, good repeatability, high sensitivity, compact structure and the like.

Description

Technical fields:

[0001] This utility model relates to the field of temperature sensing technology, and in particular to an optical fiber temperature sensor. Background technology:

[0002] Temperature anomalies are a common hazard to the safe operation of many industrial equipment. Accurate measurement is crucial for monitoring power system temperatures, detecting circuit abnormalities, and preventing equipment damage and accidents. Temperature sensors are essential detection devices, and fiber optic temperature sensors, in particular, offer advantages such as fast response, small size, light weight, resistance to electromagnetic interference, corrosion resistance, embedding in engineering structures, and easy cascading. They can be used for temperature measurement in complex electromagnetic environments with high current, high voltage, and strong electromagnetic interference, offering significant advantages over traditional electronic temperature sensors. Therefore, research on high-sensitivity fiber optic temperature sensors and the development of corresponding sensor signal demodulation systems are of great significance for ensuring industrial production safety and promoting rapid socio-economic development.

[0003] Fiber optic sensors use light as the carrier of environmental parameters and optical fibers as the sensing and transmission medium. They measure environmental parameters such as temperature, stress, and refractive index by detecting changes in the phase, polarization state, and intensity of light. In recent years, fiber optic temperature sensors have been widely used in temperature measurement research, achieving very high sensitivity. Currently, there are many types of fiber optic sensors on the market, but they all have certain drawbacks that limit their practical applications. For example, fiber optic interferometers, which use the interference principle in optical fibers to measure physical quantities, are sensitive to mechanical damage or strain. For instance, bending or stretching of the fiber can cause changes in the sensor's interference signal, requiring special attention to the fiber's arrangement and protection. Furthermore, the design and arrangement of fiber optic interferometers are relatively complex, requiring precise optical components and equipment, as well as careful fiber connection and debugging. Fiber optic Brillouin scattering sensors utilize the Brillouin scattering effect in optical fibers to measure temperature and strain, but their sensitivity and resolution are relatively low. Therefore, they may not be sensitive enough to minute changes in temperature or strain, and their output values ​​may drift due to the influence of ambient temperature. This requires temperature compensation or calibration to improve measurement accuracy. At the same time, fiber optic Brillouin scattering sensors may be subject to external interference in certain special environments, such as optical noise or vibration, which can affect the sensor's performance and measurement results. Fiber optic Raman scattering sensors use the Raman scattering effect in optical fibers to measure physical quantities such as temperature and pressure. However, the raw signals measured by fiber optic Raman scattering sensors usually require complex data processing and analysis to extract the required temperature or pressure information, which increases the complexity of system design and implementation. Utility model content:

[0004] To address the drawbacks of high cost and difficulty in mass production of many current high-precision temperature sensors, this invention proposes an optical fiber temperature sensor composed of an optical fiber and a rectangular through-groove. The optical fiber includes a first single-mode fiber, a multimode fiber, and a second single-mode fiber, with the first single-mode fiber core, the multimode fiber core, and the second single-mode fiber core respectively located at their central points. The first and second single-mode fiber cores are respectively connected to the two ends of the multimode fiber core. An optical fiber cladding covers the outer sides of the first single-mode fiber core, the multimode fiber core, and the second single-mode fiber core. The outer side of the optical fiber cladding is covered with an optical fiber coating to protect the internal structure of the fiber. The optical fiber cladding structure is a regular octagon; the rectangular through-groove is located on the side of the multimode fiber core near its central axis and is filled with liquid.

[0005] Preferably, the core diameter of the multimode optical fiber is 60-100 μm.

[0006] Preferably, the core diameters of the first and second single-mode optical fibers are the same, both being 9-20 μm.

[0007] The core diameter of an optical fiber directly affects the transmission characteristics and loss of a temperature sensor. Generally, a small core diameter leads to increased mode coupling loss, meaning the optical signal cannot be effectively coupled into the fiber due to mode mismatch, thus reducing transmission efficiency. Small-diameter multimode fibers experience significant signal attenuation during transmission, especially over long distances, where signal strength weakens rapidly. Small-diameter multimode fibers also increase dispersion, limiting bandwidth and transmission distance. Conversely, a large core diameter leads to mode mismatch, meaning the optical signal cannot be effectively confined within the core and easily leaks into the cladding, increasing transmission loss. Furthermore, large-diameter multimode fibers also increase dispersion, causing time broadening during transmission and further limiting bandwidth and distance. Additionally, bending or folding large-diameter multimode fibers causes greater optical loss, potentially leading to problems in fiber optic cabling and connections. Therefore, a suitable core diameter is necessary to achieve optimal temperature sensor performance.

[0008] The fiber cladding is composed of air-hole structures arranged and combined according to a certain pattern. The fiber cladding structure composed of air holes has the characteristics of high quality, large capacity, and low latency. Various media can be filled in the air holes, and the structure and parameters can be freely designed as needed.

[0009] Preferably, the fiber cladding is octagonal, achieving high birefringence and low loss. Simultaneously, the octagonal cladding shape guides light wave propagation, confining it within the core and reducing light loss. This fiber cladding structure effectively prevents mode mixing and light leakage, thereby improving the optical confinement characteristics of the fiber and making signal transmission more stable and reliable. Therefore, the sensor of this invention exhibits good linearity and repeatability.

[0010] Preferably, the duty cycle of the octagonal cladding of the optical fiber is 0.4-0.9. The optical fiber duty cycle is the ratio of the diameter of the air holes to the spacing between them. When an optical signal propagates in the optical fiber, a large duty cycle allows energy to be better confined within the core region, resulting in more significant advantages in sensing applications. Therefore, to achieve good sensing performance, the duty cycle range is 0.4-0.9.

[0011] Preferably, the cladding diameter of the optical fiber is 100-150 μm. A larger cladding diameter reduces inter-mode coupling and lowers loss; conversely, a smaller cladding diameter increases inter-mode coupling and correspondingly increases loss.

[0012] Preferably, the fiber cladding has 4-7 layers. Increasing the number of layers allows the fundamental mode to be more concentrated in the fiber core, reducing the fiber's confinement loss. Increasing the number of layers can result in lower loss, but it also leads to lower mechanical strength and more complex fabrication.

[0013] Preferably, the core-cladding refractive index difference of the multimode fiber is 0.005. The core-cladding refractive index difference is the difference in refractive index between the core and cladding of the fiber, and it affects the transmission speed and characteristics of the beam in the multimode fiber. A larger core-cladding refractive index difference results in stronger inter-mode interactions and slower transmission speed, but relatively lower transmission loss; conversely, a smaller refractive index difference results in faster transmission speed, but relatively higher transmission loss. Therefore, at this core-cladding refractive index value, optimal temperature sensor performance can be achieved.

[0014] The length and width of the rectangular through-groove can affect the heat exchange efficiency between the thermosensitive material and the environment, thus affecting the stability of the sensor. Preferably, the rectangular through-groove has a length of 250-300 μm and a width of 30-40 μm, with equal spacing between the two ends of the rectangular through-groove and the solder joints at both ends of the multimode fiber. If the length or width of the rectangular through-groove is too large, the heat capacity of the thermosensitive material increases, leading to a decrease in sensor stability. If the length or width is too small, the heat exchange efficiency is insufficient, which may also cause a decrease in sensor stability. Therefore, within this length and width range, optimal stability and accuracy of the temperature sensor can be achieved.

[0015] To improve the temperature sensitivity of the fiber optic temperature sensor, preferably, the liquid filled in the rectangular through-groove is a refractive index matching liquid with a refractive index of 1.300. The thermo-optic coefficient of this refractive index matching liquid is -0.000332 / ℃, which is two orders of magnitude higher than that of quartz (-0.0000086 / ℃).

[0016] The working principle of this invention, based on the multimode fiber Mach-Zehnder interferometry, is as follows: A rectangular through-groove parallel to the fiber axis is located on one side of the multimode fiber core, filled with a transparent liquid. Light emitted from the light source enters the multimode fiber core along the first single-mode fiber. When the light passes through the rectangular through-groove in the multimode fiber core, it is split into two parts: one part passes through the through-groove, while the other part continues to propagate within the multimode fiber core. Due to the difference in refractive index between the liquid and the fiber core, an optical path difference is generated as the two parts of light continue to propagate along the multimode fiber. After passing through the remaining multimode fiber core, the two parts are combined at the second solder joint and enter the second single-mode fiber, forming a Mach-Zehnder interference, which causes a change in the interference spectrum. Detecting the transmission spectrum of this temperature sensor allows for temperature measurement.

[0017] This invention utilizes multimode optical fiber to fabricate a sensor, which has the following advantages: (1) simple fabrication; (2) good repeatability; (3) high sensitivity; and (4) compact structure. Attached image description:

[0018] Figure 1 This is a structural diagram of a fiber optic temperature sensor;

[0019] Figure 2 This is a cross-sectional view of the second single-mode fiber end;

[0020] Figure 3 This is a diagram of fiber optic connections;

[0021] Figure 4 This is a diagram of a rectangular through-groove structure;

[0022] Figure 5 This relates the center wavelength of interference peak A to temperature.

[0023] Wherein, 1 is optical fiber, 11 is first single-mode optical fiber, 12 is multimode optical fiber, 13 is second single-mode optical fiber, 2 is optical fiber cladding, 21 is air hole, 3 is optical fiber coating, 4 is rectangular through-groove, 41 is liquid, 111 is first single-mode optical fiber core, 121 is multimode optical fiber core, and 131 is second single-mode optical fiber core. Detailed Implementation

[0024] To further explain the technical means adopted by this utility model to achieve its intended purpose, the following detailed description is provided in conjunction with the accompanying drawings and preferred embodiments:

[0025] Figure 1 The diagram shows the structure of an optical fiber temperature sensor. It consists of an optical fiber 1 and a rectangular through-groove 4. The optical fiber 1 comprises a first single-mode fiber 11, a multimode fiber 12, and a second single-mode fiber 13, with the first single-mode fiber core 111, the multimode fiber core 121, and the second single-mode fiber core 131 at their centers, respectively. The first single-mode fiber core 111 and the second single-mode fiber core 131 are connected to the two ends of the multimode fiber core 121, respectively. An optical fiber cladding 2 covers the outside of the first single-mode fiber core 111, the multimode fiber core 121, and the second single-mode fiber core 131. An optical fiber coating 3 covers the outside of the optical fiber cladding 2, protecting the internal structure of the optical fiber 1. The rectangular through-groove 4 is located on the side of the multimode fiber core 12 near its central axis and is filled with liquid 41.

[0026] The first single-mode fiber core 111 and the second single-mode fiber core 131 have the same diameter, both being 9μm.

[0027] The multimode fiber core 121 has a diameter of 62.5 μm. The diameter of the multimode fiber core 121 directly affects the transmission characteristics and loss of the temperature sensor. Generally, the smaller the core diameter, the more modes the beam has, and the more interacting modes there are, leading to increased beam transmission loss. Conversely, the larger the core diameter, the less interaction between modes, and the lower the transmission loss.

[0028] The diameter of the fiber cladding 2 also has a significant impact on the mode coupling and transmission performance of the fiber. The diameter of the fiber cladding 2 is 125 μm. The larger the diameter of the fiber cladding 2, the smaller the coupling between modes and the lower the loss; conversely, the smaller the diameter of the fiber cladding 2, the larger the coupling between modes and the higher the loss.

[0029] The core-cladding refractive index difference is the difference in refractive index between the core and cladding of an optical fiber, and it affects the propagation speed and characteristics of the beam in multimode fiber 12. The core-cladding refractive index difference of the multimode fiber core 121 is 0.005. A larger core-cladding refractive index difference results in stronger inter-mode interactions and slower propagation speed, but relatively lower transmission loss; conversely, a smaller refractive index difference results in faster propagation speed, but relatively higher transmission loss. Optimal temperature sensor performance can be achieved within this core-cladding refractive index difference range.

[0030] The length and width of the rectangular through-groove 4 affect the heat exchange efficiency between the thermosensitive material and the environment, thus impacting the sensor's stability. The rectangular through-groove 4 has a length of 291.8 μm and a width of 32.4 μm, with equal spacing between its two ends and the solder joints at both ends of the multimode fiber 12. If the length or width of the rectangular through-groove 4 is too large, the heat capacity of the thermosensitive material increases, leading to decreased sensor stability. If the length or width is too small, the heat exchange efficiency is insufficient, potentially causing a decrease in sensor stability. Optimal stability and accuracy of the temperature sensor can be achieved within these length and width dimensions.

[0031] Figure 2 This is a cross-sectional view of the second single-mode fiber end. The fiber cladding 2 is composed of air holes 21 arranged and combined according to a certain pattern. The fiber cladding 2 composed of air holes 21 has the characteristics of high quality, large capacity, and low latency. Various media can be filled in the air holes 21, and the structure and parameters can be freely designed as needed.

[0032] The fiber cladding 2 has a regular octagonal structure with periodically arranged air holes 21 inside. This structure achieves high birefringence and low loss, and the birefringence effect of the fiber is further improved by filling it with a thermosensitive liquid. Therefore, it exhibits good linearity and repeatability.

[0033] The fiber duty cycle is the ratio of the diameter of the air hole 21 to the spacing between the air holes 21. The diameter of the air hole 21 is d, the spacing between the air holes 21 is Λ, and the duty cycle is d / Λ. The duty cycle of the octagonal cladding of fiber 1 is 0.6. When optical signals propagate in the fiber, a large duty cycle allows energy to be better confined to the core region, resulting in more significant advantages in sensing applications.

[0034] The fiber cladding 2 has 5 layers. Increasing the number of layers allows the fundamental mode to be more concentrated in the fiber core, reducing confinement loss. Increasing the number of layers results in lower loss, but it also leads to lower mechanical strength and more complex fabrication.

[0035] Figure 3 This is a fiber optic connection diagram. The first single-mode fiber 11 and the second single-mode fiber 13 are respectively connected to the two ends of the multimode fiber 12.

[0036] Figure 4 It is a rectangular through-groove structure. To improve the temperature sensitivity of the fiber optic temperature sensor, the liquid 41 filled in the rectangular through-groove 4 is a refractive index matching liquid with a refractive index of 1.300. The thermo-optic coefficient of this refractive index matching liquid is -0.000332 / ℃, which is two orders of magnitude higher than that of quartz (-0.0000086 / ℃).

[0037] The sensor operates as follows: light emitted from the light source enters the multimode fiber core 121 along the first single-mode fiber core 111. When the light passes through the rectangular through-groove 4 in the multimode fiber core 121, it is split into two parts. One part passes through the rectangular through-groove 4, while the other part continues to propagate within the multimode fiber core 121. Due to the difference in refractive index between the liquid 41 and the fiber core 121, the two parts of light continue to propagate along the multimode fiber 12, generating an optical path difference and forming Mach-Zehnder interference, which causes a change in the interference spectrum. Detecting the transmission spectrum of this temperature sensor allows for temperature measurement, thus converting temperature changes into changes in optical signals. To test the temperature measurement performance of the fiber optic temperature sensor, the sensor is fixed to two fiber clamps and kept straight. The sensor and fiber clamps are placed in an environmental test chamber and connected to the light source and spectrometer to form a temperature measurement system. The environmental test chamber has a temperature control range of -20℃ to 160℃ and a temperature control accuracy of 0.1℃. During the experiment, humidity was kept constant (30% RH), and the temperature was gradually increased from 25℃ to 75℃, with wavelength changes recorded every 5℃. Each spectral recording was only performed after the temperature reached the set point and was maintained for 5 minutes to ensure the sensor and the experimental chamber temperature were synchronized. The fitting relationship between the center wavelength of interference peak A and temperature is as follows: Figure 5 As shown, the center wavelength of interference peak A gradually decreases with increasing temperature, the temperature sensitivity is -540.6 pm / ℃, and the linear fit is 0.9672.

Claims

1. An optical fiber temperature sensor, characterized by, The temperature sensor is composed of an optical fiber and a rectangular through micro groove; the optical fiber comprises a first single-mode optical fiber, a multi-mode optical fiber, and a second single-mode optical fiber, with a first single-mode optical fiber core, a multi-mode optical fiber core, and a second single-mode optical fiber core at the center, respectively; an optical fiber cladding is coated outside the first single-mode optical fiber core, the multi-mode optical fiber core, and the second single-mode optical fiber core; the optical fiber cladding is a regular octagonal structure; the rectangular through micro groove is on the side of the multi-mode optical fiber core close to the central axis, and the rectangular through micro groove is filled with a liquid.

2. The fiber optic temperature sensor of claim 1, wherein, The multi-mode optical fiber core has a diameter of 60-100 μm.

3. The fiber optic temperature sensor of claim 1, wherein, The first single-mode optical fiber core and the second single-mode optical fiber core have the same diameter, which is 9-20 μm.

4. The fiber optic temperature sensor of claim 1, wherein, The optical fiber cladding has a diameter of 100-150 μm.

5. The fiber optic temperature sensor of claim 1, wherein, The multi-mode optical fiber has a core-cladding refractive index difference of 0.

005.

6. The fiber optic temperature sensor of claim 1, wherein, The optical fiber cladding has a duty cycle of 0.4-0.

9.

7. The fiber optic temperature sensor of claim 1, wherein, The optical fiber cladding has a number of 4-7.

8. The fiber optic temperature sensor of claim 1, wherein, The rectangular through micro groove has a length of 250-300 μm and a width of 30-40 μm, and the distance between the two ends of the rectangular through micro groove and the two end welding points of the multi-mode optical fiber is equal.

9. The fiber optic temperature sensor of claim 1, wherein, The liquid filled in the rectangular through micro groove is a refractive index matching liquid with a refractive index of 1.300.

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