Temperature is a physical quantity that measures the degree of hotness or coldness of an object. Many physical phenomena and chemical processes occur at certain temperatures, and people's daily lives are also closely related to temperature. With the rapid development of science and technology, more and higher requirements have been placed on temperature measurement. Traditional temperature sensors based on electrical signals, such as thermocouples, thermistors, and pyroelectric detectors, have matured significantly. However, in environments with strong electromagnetic interference or flammable and explosive conditions, traditional temperature sensors based on electrical signal measurement are greatly limited.
Fiber optic temperature sensing and measurement technology is one of the important development directions in the field of instrumentation. Due to the characteristics of fiber optics, such as small size, light weight, flexibility, good electrical insulation, bendability, corrosion resistance, large measurement range, and high sensitivity, they can significantly enhance traditional sensors , especially temperature sensors, enabling them to perform tasks that are difficult or even impossible for traditional sensors. In addition to the above characteristics, fiber optic sensing technology used for temperature measurement also offers advantages over traditional temperature measuring instruments, including faster response, wider bandwidth, explosion-proof, flame-retardant, and resistance to electromagnetic interference.
Fiber optic temperature sensors are a novel temperature measurement technology developed in the 1970s. Based on optical signal transmission, they possess advantages such as insulation, resistance to electromagnetic interference, and high voltage resistance. Abroad, fiber optic temperature sensors have developed rapidly, resulting in various product models that have been applied in multiple fields with excellent results. Domestic research in this area is also booming, with numerous universities, research institutes, and companies collaborating to develop various fiber optic temperature measurement systems for field applications.
Features of fiber optic temperature sensors
Fiber optic temperature sensors have many advantages over traditional temperature sensors:
1. Light waves do not produce electromagnetic interference and are not afraid of electromagnetic interference;
2. It is easily received by various photodetectors and can be conveniently converted into photoelectric or electro-optical signals;
3. It is easily compatible with highly developed modern electronic devices and computers;
4. Optical fiber has a wide operating frequency range and a large dynamic range, making it a low-loss transmission line;
5. Optical fiber itself is non-electrical, small in size and light in weight, easy to bend, and has good radiation resistance, making it particularly suitable for use in harsh environments such as flammable, explosive, space-constrained, and strong electromagnetic interference environments.
Methods for measuring temperature using fiber optic temperature sensors
Fiber optic temperature sensors use optical fibers to measure temperature, and there are two methods to achieve this:
One method utilizes the characteristic that the radiant energy of the measured surface changes with temperature; the radiant energy is transmitted to a thermistor using optical fiber, and then converted into an electrical signal that can be recorded and displayed. The unique feature of this method is that it allows for long-distance measurement.
Another method utilizes the characteristic that the phase of light propagating in an optical fiber changes with temperature parameters. The change in the phase of the optical signal with temperature is caused by the change in the size and refractive index of the optical fiber material with temperature.
Basic principles of fiber optic sensors
A certain parameter of the transmitted light wave is changed accordingly, and then the known modulated light signal is detected to obtain the measured quantity. When the measured physical quantity acts on the light wave transmitted in the fiber optic sensing head, causing a change in its intensity, it is called an intensity-modulated fiber optic sensor; when the effect causes a change in the wavelength, phase, or polarization state of the transmitted light, it is called a wavelength, phase, or polarization-modulated fiber optic sensor, respectively.
The principles of several fiber optic temperature sensors
Fiber optic temperature sensors can be categorized into two types based on their working principle: functional and transmission types. Functional fiber optic temperature sensors utilize the characteristics of optical fiber (phase, polarization, intensity, etc.) that change with temperature to measure temperature. While these sensors combine transmission and sensing, they also increase the difficulty of enhancing and desensitizing. In transmission-type fiber optic temperature sensors, the optical fiber only serves to transmit the optical signal, avoiding the complex environment of the temperature measurement area. The modulation function of the measured object is achieved through other physically sensitive elements. These sensors suffer from optical coupling issues between the optical fiber and the sensing head, increasing system complexity and making them more sensitive to interference such as mechanical vibration.
Fiber Bragg grating temperature sensor
Fiber Bragg grating temperature sensing technology mainly studies Bragg fiber optic sensing technology. Based on the principle that the reflected wavelength of a Bragg fiber grating changes with temperature, a fiber grating temperature sensor is fabricated. In 1978, the Ottawa Communications Research Centre in Canada first discovered the photosensitivity effect of germanium-doped silica fiber and used the injection method to fabricate the world's first fiber grating (FBG).
Besides possessing many advantages of ordinary fiber optic temperature sensors, fiber optic grating temperature sensors also have some aspects that are significantly superior to other fiber optic temperature sensors. The most important of these is that its sensing signal is wavelength modulated.
The advantages of this sensing mechanism are: the measurement signal is not affected by factors such as light source fluctuations, fiber bending loss, connection loss, and detector aging; it avoids the unclear phase measurement and the need for a fixed reference point in general interferometric sensors; it can easily use wavelength division multiplexing technology to connect multiple Bragg gratings in series in a single fiber for distributed measurement; and it can be easily embedded in materials to measure the internal temperature with high resolution and over a wide range.
Despite the many advantages of fiber Bragg grating temperature sensors, several factors still need to be considered in their application: detection of minute wavelength shifts; acquisition of broadband, high-power light sources; improvement of photodetector wavelength resolution; elimination of cross-sensitivity; fiber Bragg grating packaging; fiber Bragg grating reliability; and fiber Bragg grating lifespan.
Fiber optic fluorescence temperature sensor
Fiber optic fluorescence temperature sensors are a relatively new type of temperature sensor that is currently under active research. The working mechanism of fluorescence thermometry is based on the fundamental physical phenomenon of photoluminescence.
Photoluminescence is a light emission phenomenon, which occurs when a material is excited by light in the ultraviolet, visible, or infrared regions. The emitted fluorescence parameters have a one-to-one correspondence with temperature; the desired temperature is obtained by measuring the fluorescence intensity or fluorescence lifetime.
Intensity-based fluorescent fiber optic sensors are susceptible to intensity fluctuations due to fiber micro-bending, coupling, scattering, and back reflection, making it difficult to achieve high accuracy. Fluorescence lifetime sensors avoid these drawbacks and are therefore the primary method used; measuring fluorescence lifetime is crucial for temperature measurement systems. Mississippi State University uses a commercially available epoxy resin as a temperature indicator (PAH). PAHs fluoresce when excited by ultraviolet light, and the fluorescence intensity decreases as the temperature surrounding the epoxy resin increases. This sensor can monitor temperatures ranging from 20°C to 100°C.
Fiber optic fluorescence temperature sensors have unique advantages over other fiber optic temperature sensors: because the relationship between fluorescence lifetime and temperature is essentially intrinsic and independent of light intensity, self-calibrating fiber optic temperature sensors can be fabricated. In contrast, conventional fiber optic temperature sensors based on light intensity detection often require frequent calibration because the optical transmission characteristics of the system are often related to the transmission fiber and fiber coupler. Current international research mainly focuses on the selection of fluorescence sources, primarily including sapphire and ruby luminescence, rare-earth luminescence, and semiconductor absorption.
Interferometric fiber optic temperature sensor
An interferometric fiber optic temperature sensor is a phase-modulated fiber optic sensor. It measures external temperature by utilizing the changes in interference fringes observed by interferometers such as the Mach-Zehnder interferometer, Fabry-Perot interferometer, and Sagnac interferometer. Samer K. Abi Kaed Bev of the UK fabricated a Mach-Zehnder interferometric fiber optic temperature sensor using long-period fiber gratings, achieving a temperature resolution of 0.7°C.
Interferometric fiber optic temperature sensors offer high temperature resolution, wide dynamic response, and a compact structure. Research on interferometric fiber optic temperature sensors primarily focuses on reducing noise interference and signal demodulation.
Fiber optic temperature sensor based on bending loss
Fiber optic temperature sensors based on bending loss utilize the principle that changes in the fiber aperture caused by temperature variations in the refractive index difference between the silicon core and the plastic cladding, as well as changes in the local aperture caused by sudden bending of the fiber, to measure temperature. Ukraine has developed a bending-loss-based fiber optic temperature sensor using EBOC-manufactured multimode step-index plastic-clad silicon core fiber HCN-H, with a temperature measurement range of -30℃ to 70℃ and a sensitivity of 0.5℃.
Distributed fiber optic temperature sensor
A distributed fiber optic temperature measurement system is a sensor system used for real-time measurement of the temperature field distribution in space. The concept of a distributed fiber optic sensor system was first proposed by the University of Southampton in the UK in 1981. In 1983, Hartog in the UK conducted a principle experiment on a distributed fiber optic temperature sensor using the Raman spectroscopy effect of liquid optical fiber. In 1985, Dakin in the UK conducted a temperature measurement experiment in the laboratory using an argon-ion laser as a light source and a distributed fiber optic temperature sensor based on the Raman spectroscopy effect of silica optical fiber. In the same year, Hartog and Dakin independently developed experimental devices for distributed fiber optic temperature sensors using semiconductor lasers as light sources.
Distributed fiber optic temperature sensors are based on three types of distributed temperature sensors: Rayleigh scattering, Brillouin scattering, and Raman scattering. Distributed fiber optic sensors originated from the initial Rayleigh scattering system based on optical time-domain scattering (fOTDR), and have evolved through Raman scattering systems based on OTDR and Brillouin scattering systems based on OTDR, significantly improving temperature measurement accuracy and range. Optical frequency-domain scattering (fOFDR) was also proposed early on, but only recently, with the deepening research on Raman and Brillouin scattering, has the combination of OFDR and these technologies demonstrated its superiority. Distributed fiber optic temperature sensors based on OTDR and OFDR have already shown significant advantages, therefore, distributed fiber optic temperature sensors based on OTDR and OFDR will remain a research hotspot, especially new distributed fiber optic sensors based on OFDR, which will be an important direction for development.
Distributed fiber optic temperature sensors offer unparalleled advantages over other temperature sensors. They can continuously measure temperature along the fiber optic cable over distances spanning several kilometers, with spatial positioning accuracy on the order of meters. Capable of uninterrupted automatic measurement, they are particularly suitable for applications requiring large-scale, multi-point measurements.
Current research on distributed fiber optic temperature sensors focuses on: achieving simultaneous measurement of multiple physical or chemical parameters on a single fiber; improving the detection capability of signal receiving and processing systems; increasing the spatial resolution and measurement uncertainty of the system; expanding the measurement range of the measurement system; reducing measurement time; and developing two-dimensional or multi-dimensional distributed fiber optic temperature sensor networks.
Distributed fiber optic sensing technology based on Brillouin scattering
Because of the vibrations within the molecules of a medium, the refractive index of the medium fluctuates periodically with time and space, thus generating a spontaneous acoustic wave field. When light is directionally incident on the fiber medium, it is affected by this acoustic wave field, causing inelastic collisions between optical phonons and optical photons in the fiber, resulting in Brillouin scattering. In Brillouin scattering, the frequency of the scattered light has a frequency shift relative to the pump light; this frequency shift is commonly referred to as the Brillouin shift. The magnitude of the Brillouin shift is directly related to the phonon properties of the fiber material. When the fiber material properties related to the scattered light frequency are affected by temperature and strain, the magnitude of the Brillouin shift will change. Therefore, distributed temperature and strain measurement can be achieved by measuring the frequency shift of the backscattered Brillouin light from a pulsed light.
Brillouin scattering in optical fibers manifests as the generation of Stokes waves with a frequency shifted downward relative to the incident pump wave. Brillouin scattering can be viewed as a parametric interaction between the pump wave, Stokes wave, and sound wave. The Brillouin frequency shift caused by scattering is proportional to the speed of sound in the light ray.
In the formula,
Experiments have shown that the Brillouin power also varies with temperature and strain; it increases linearly with increasing temperature and decreases linearly with increasing strain. Therefore, the Brillouin power can also be expressed as:
in,
Since strain has a much smaller effect on the Brillouin scattered light power than temperature, it can generally be ignored, and the Brillouin scattered light power is considered to be only related to temperature. Therefore, as shown in equations 3 and 4, the distribution information of temperature, strain, etc. along the optical fiber can be obtained by detecting the optical power and frequency of the Brillouin scattered light.
Distributed fiber optic sensor based on Brillouin optical frequency domain analysis (BOFDA) technology
BOFDA distributed fiber optic sensing technology is a novel distributed fiber optic sensing technology proposed by D. Garus et al. in Germany in 1997.
BOFDA also utilizes the Brillouin frequency shift characteristic to achieve temperature/strain sensing, but its measured spatial positioning is no longer the traditional wide-time-domain reflectometry technique. Instead, it achieves this by obtaining the composite baseband transfer function of the optical fiber. Therefore, the light injected into both ends of the sensing fiber is continuous light with different frequencies, where the frequency difference between the probe light and the pump light is approximately equal to the Brillouin frequency shift component in the fiber.
Applications of fiber optic temperature sensors
Since its inception, fiber optic temperature sensing has been mainly used in fields such as power systems, construction, chemical industry, aerospace, medical care and even marine development, and has achieved a large number of reliable application results.
1. Fiber optic temperature sensors have important applications in power systems, including monitoring the surface temperature of power cables and the temperature in densely packed cable areas; monitoring easily heated parts in high-voltage power distribution equipment; environmental temperature detection and fire alarm systems in power plants and substations; temperature distribution measurement, thermal protection, and fault diagnosis of various large and medium-sized generators, transformers, and motors; temperature and fault point detection in heating systems, steam pipelines, and oil pipelines of thermal power plants; and equipment temperature monitoring in geothermal power plants and indoor enclosed substations, etc.
2. Fiber optic temperature sensors, especially fiber optic grating temperature sensors, are easily embedded in materials to measure internal temperatures with high resolution and over a wide range, making them widely used in construction and bridges. Developed countries such as the United States, the United Kingdom, Japan, Canada, and Germany have long been engaged in bridge safety monitoring research and have installed bridge safety monitoring and early warning systems on major bridges to monitor key safety indicators such as strain, temperature acceleration, and displacement. In the summer of 1999, 120 fiber optic grating temperature sensors were installed on a steel structure bridge on the Las Cruces 10 Interstate Highway in New Mexico, setting a record for the largest number of such sensors used on a single bridge.
3. The aerospace industry is a sensor-intensive field. An aircraft needs over 100 sensors to monitor pressure, temperature, vibration, fuel level, landing gear status, wing and rudder positions, etc., making sensor size and weight extremely important. Fiber optic sensors, in terms of their small size and light weight, are virtually unmatched by any other type of sensor.
4. The small size of sensors is highly significant in medical applications, and fiber Bragg grating sensors are currently the smallest sensors achievable. Fiber Bragg grating sensors can perform internal measurements of human tissue function with minimal invasiveness, providing precise local information on temperature, pressure, and sound wave fields. The technology of fiber Bragg grating sensors for human tissue applications is quite extensive. Research on fiber optic temperature sensors accounts for nearly 20% of all fiber optic sensor research.
In addition to field verification, improvement and enhancement of existing devices, the research on fiber optic temperature sensors currently has the following development trends: vigorously developing measurement technology for measuring temperature distribution, that is, from measuring the temperature of a single point to measuring the temperature distribution along the fiber optic cable and the temperature distribution of a large area surface; developing multifunctional sensors that include temperature measurement; and developing large-scale sensor arrays to achieve all-optical remote sensing.
