Detailed analysis of the working principle and application scenarios of fiber optic temperature sensors

Temperature is the physical quantity that measures the degree of heat and cold of an object. Many physical phenomena and chemical processes are carried out at a certain temperature. People's daily life is also closely related to temperature. With the rapid development of science and technology, more and higher requirements have been put forward for the measurement of temperature. The development of temperature sensors based on electrical signals, such as thermocouples, thermistors, pyroelectric detectors, etc., is very mature, but in the case of strong electromagnetic interference or flammable and explosive, based on Conventional temperature sensors for electrical signal measurement are greatly limited.

Optical fiber temperature sensing and measurement technology is one of the important development directions in the field of instrumentation. Due to its small size, light weight, flexibility, good electrical insulation, flexible bending, corrosion resistance, large measuring range and high sensitivity, the optical fiber can expand and improve the traditional sensor, especially the temperature sensor. Difficult to complete or even impossible tasks. Optical fiber sensing technology is used for temperature measurement. In addition to the above characteristics, compared with traditional temperature measuring instruments, it also has the characteristics of fast response, frequency bandwidth, explosion-proof, anti-combustion and anti-electromagnetic interference.

Fiber optic temperature sensor is a new type of temperature measurement technology developed in the 1970s. It transmits information based on optical signals and has the advantages of insulation, anti-electromagnetic interference and high voltage resistance. In foreign countries, fiber optic temperature sensors have developed rapidly, forming a variety of models, and have been applied to many fields, and achieved good results. Domestic research in this area is also in full swing. Many universities and research institutes have cooperated with the company to develop a variety of fiber optic temperature measurement systems for on-site applications.

Fiber Optic Temperature Sensor Features

Fiber optic temperature sensors have many advantages over traditional temperature sensors:

1. Light waves do not generate electromagnetic interference, nor are they afraid of electromagnetic interference;

2, easy to be received by various light detectors, can be easily photoelectric or electro-optical conversion;

3. Easy to match with highly developed modern electronic devices and computers;

4. The fiber has a wide operating frequency and a large dynamic range, and is a low-loss transmission line;

5, the fiber itself is not charged, small size, light weight, easy to bend, good radiation resistance, especially suitable for use in harsh environments such as flammable, explosive, space is strictly limited and strong electromagnetic interference.

Method for measuring temperature by optical fiber temperature measuring sensor

Fiber optic temperature sensors use fiber optics to measure temperature. There are two ways to achieve this:

The first is to use the characteristics of the measured surface radiant energy to change with temperature; the radiant energy is transmitted to the thermal element by the optical fiber, and converted into an electrical signal for recording and display. The uniqueness of this method is that it can be measured from a distance.

Another method is to use the characteristic that the phase of light transmitted in the optical fiber changes with the change of the temperature parameter. The change of the phase of the optical signal with temperature is caused by the change of the size and refractive index of the optical fiber material with temperature.

The basic principle of fiber optic sensors

A certain parameter of the optical wave is transmitted to change it, and then the known modulated optical signal is detected to obtain a measured value. When the measured physical quantity acts on the light wave transmitted in the fiber sensing head, the intensity changes, which is called the intensity modulation fiber sensor; when the result of the action changes the wavelength, phase or polarization state of the transmitted light, correspondingly It is called a wavelength, phase or polarization modulation type fiber sensor.

Principles of several fiber optic temperature sensors

Fiber optic temperature sensors can be divided into functional and transmission types according to their working principle. The functional fiber-optic temperature sensor performs temperature measurement by utilizing various characteristics of the fiber, f phase, polarization, intensity, etc., as a function of temperature. Although these sensors have the characteristics of "transfer" and "sense", they also increase the difficulty of sensitization and desensitization. The optical fiber of the transmission type optical fiber temperature sensor only serves as an optical signal transmission to avoid the complicated environment of the temperature measurement area, and the modulation function of the object to be measured is realized by other physical sensitive components. Such sensors have increased the complexity of the system due to the optical coupling problem between the optical fiber and the sensing head, and are sensitive to interference such as mechanical vibration.

Fiber Bragg Grating Temperature Sensor

Fiber Bragg Grating Temperature Sensing Technology focuses on Bmgg fiber sensing technology. The fiber grating temperature sensor is fabricated according to the principle that the wavelength of the Bragg fiber grating reflection wavelength changes with temperature to produce a "wavelength shift". In 1978, the Ottawa Communications Research Center of Canada first discovered the photosensitive effect of erbium-doped quartz fiber, and used the injection method to make the world's first fiber grating (FBG).

In addition to the many advantages of conventional fiber optic temperature sensors, fiber Bragg grating temperature sensors have some advantages over other fiber optic temperature sensors. The most important of these is that its sensing signal is wavelength modulated.

The advantage of this sensing mechanism is that the measurement signal is not affected by factors such as source fluctuation, fiber bending loss, connection loss and detector aging; it avoids the unclear phase measurement and the need for the inherent reference point in general interference sensors. It is convenient to use wavelength division multiplexing technology to serially connect multiple Bragg gratings in a single fiber for distributed measurement; it is easy to bury the material in its material for high resolution and wide range measurement.

Although fiber Bragg grating temperature sensors have many advantages, many factors need to be considered in the application: detection of small wavelength displacement; acquisition of wide spectrum and high power light source; improvement of wavelength resolution of photodetector; elimination of cross sensitivity; fiber grating The package; the reliability of the fiber grating; the lifetime of the fiber grating.

Fiber optic fluorescent temperature sensor

The fiber-optic fluorescent temperature sensor is a new type of temperature sensor that is currently active in research. The working mechanism of fluorescence temperature measurement is based on the basic physical phenomenon of photoluminescence.

Photoluminescence is a phenomenon of light emission, which is a phenomenon of luminescence when a material is excited by light in the ultraviolet, visible or infrared regions. The fluorescence parameters of the emission have a one-to-one correspondence with the temperature, and the desired temperature is obtained by detecting the fluorescence intensity or fluorescence lifetime.

The intensity-type fluorescent fiber sensor is affected by the microbending, coupling, scattering, and back reflection of the fiber, causing intensity disturbance, and it is difficult to achieve high precision. The fluorescence lifetime sensor can avoid the above shortcomings, so it is the main mode adopted, and the measurement of fluorescence lifetime is the key to the temperature measurement system. The University of Mississippi State uses a commercially available epoxy glue as the temperature indicator (PAHs). PAHs fluoresce when excited by ultraviolet light. The intensity of fluorescence decreases with increasing temperature around the epoxy. The sensor can monitor temperatures in the range of 20 °C to 100 °C.

Fiber-optic fluorescent temperature sensors have their own unique advantages over other fiber-optic temperature sensors: since the relationship between fluorescence lifetime and temperature is inherently intrinsic, independent of the intensity of the light, a self-aligning fiber-optic temperature sensor can be fabricated. . The general optical fiber temperature sensor based on light intensity detection needs to be calibrated frequently because the optical transmission characteristics of the system are often related to the transmission fiber and the fiber coupler. At present, foreign research mainly focuses on the selection of fluorescent sources. Mainly for the following aspects: sapphire and ruby ​​luminescence, rare earth luminescence and semiconductor absorption.

Interferometric fiber optic temperature sensor

The interference type fiber optic temperature sensor is a phase modulation type fiber sensor. It measures the temperature from outside using the interference fringes of some interferometers such as temperature-changing Mach-Zehnder interferometer, Fabry-Perot interferometer, and Sagnac interferometer. Samer K. AbiKaed Bev uses a long-period fiber grating to make a Mach-Zehnder interferometric fiber optic temperature sensor. Its temperature resolution is O. 7 ° C.

The interferometric fiber optic temperature sensor has a high temperature resolution: wide dynamic response: compact structure. The main work of studying the interference fiber temperature sensor is to reduce noise interference and signal demodulation.

Fiber optic temperature sensor based on bending loss

The fiber-optic temperature sensor based on the bending loss measures the temperature by using the principle that the refractive index difference between the silicon core and the plastic cladding changes with the temperature, the change in the aperture of the fiber, and the change in the local aperture caused by the sudden bending of the fiber. Ukrainian uses EBOC multi-mode step-clad plastic core fiber HCN ~ H, has made a fiber-optic temperature sensor based on bending loss, its temperature range is 30 ° C ~ 70 ° C, the sensitivity reaches O. 5 ° C.

Distributed fiber optic temperature sensor

The distributed optical fiber temperature measurement system is a sensor system for real-time measurement of spatial temperature field distribution. The distributed fiber optic sensor system was first proposed in 1981 by the University of Southampton, England. In 1983, Hartog of the United Kingdom conducted a distributed fiber optic temperature sensor principle experiment using the Raman spectroscopy effect of liquid fiber. In 1985, Dakin in the United Kingdom used an argon ion laser as a light source to conduct a temperature measurement experiment using a Raman spectral effect of a quartz fiber. In the same year, Hartog and Dakin independently used a semiconductor laser as a light source to develop a distributed optical fiber temperature sensor experimental device.

Distributed fiber optic temperature sensors are based on Rayleigh scattering, Brillouin scattering, and Raman scattering. The distributed fiber optic sensor began with the Rayleigh scattering system based on optical time-domain scattering fOTDR1. It has experienced 0TDR-based Raman scattering system and 0TDR-based Brillouin scattering system. The accuracy and range of temperature measurement are greatly improved. Optical frequency domain scattering (OFF) has also been proposed very early, but only recently, along with the deepening of Raman scattering and Brillouin scattering research, OFDR and their combination have shown its superiority. Distributed temperature fiber optic sensors based on 0TDR and OFDR have shown great advantages, so distributed temperature fiber optic sensors based on OTDR0FDR will still be a research hotspot, especially the new distributed fiber optic sensor based on OFDR will be a Important development direction.

Distributed fiber optic temperature sensors have advantages that are unmatched by other temperature sensors. It can continuously measure the temperature of the fiber along the line, the measurement distance is in the range of several kilometers, and the spatial positioning accuracy reaches the order of meters. It is capable of automatic measurement without interruption, and is especially suitable for direct use situations requiring a wide range of multi-point measurements.

At present, the focus of research on distributed optical fiber temperature sensors is to achieve simultaneous measurement of multiple physical parameters or chemical parameters on a single fiber: improve the detection capability of the signal receiving and processing system, improve the spatial resolution and measurement uncertainty of the system, and improve Measuring system measurement range, reducing measurement time, based on two-dimensional or multi-dimensional distributed fiber optic temperature sensor networks.

Distributed optical fiber sensing technology based on Brillouin scattering

Due to the existence of a certain form of vibration inside the medium molecule, the refractive index of the medium periodically fluctuates with time and space, thereby generating a spontaneous acoustic wave field. When the light is incident on the fiber medium, it is subjected to the acoustic wave field. When the optical phonon and the optical photon in the fiber are inelastically collided, Brillouin scattering is generated. In Brillouin scattering, the frequency of the scattered light has a frequency shift relative to the pump light, which is commonly referred to as the Brillouin frequency shift. The magnitude of the Brillouin frequency shift of the scattered light is directly related to the characteristics of the phonon of the fiber material. When the properties of the fiber material associated with the frequency of the scattered light are affected by temperature and strain, the Brillouin frequency shift will change. Therefore, distributed temperature strain measurement can be realized by measuring the amount of frequency shift of the backward Brillouin scattered light of the pulsed light.

The Brillouin scattering in the fiber is represented by the generation of the Stokes wave moving down relative to the incident pump wave frequency. Brillouin scattering can be regarded as the parametric interaction between the pump wave and the Stokes wave and the acoustic wave. . The Brillouin frequency shift produced by scattering is proportional to the speed of sound in the light:

In the formula,

It is found that the Brillouin power also changes with temperature and strain. Brillouin power increases linearly with increasing temperature and decreases linearly with increasing strain. Therefore, Brillouin power can also be expressed as:

among them,

Since the influence of strain on temperature on Brillouin scattered light power is much smaller, it is generally negligible, and it is considered that Brillouin scattered light power is only related to temperature. Therefore, it can be seen from the equations 3 and 4 that by detecting the optical power and frequency of the Brillouin scattered light, distribution information such as temperature and strain along the optical fiber can be obtained.

Distributed optical fiber sensor based on Brillouin optical frequency domain analysis (BOFDA) technology

BOFDA distributed optical fiber sensing technology is a new type of distributed optical fiber sensing technology proposed by D. Garus et al. in Germany in 1997.

BOFDA also uses Brillouin frequency shifting characteristics to achieve temperature/strain sensing, but its measured spatial localization is no longer a traditional wide-time domain reflection technique, but is achieved by obtaining the composite baseband transfer function of the fiber. Therefore, the light injected at both ends of the sensing fiber is continuous light of different frequencies, wherein the frequency difference between the detecting light and the pumping light is approximately equal to the Brillouin frequency shift component in the optical fiber.

Application of fiber optic temperature sensor

Since its inception, fiber optic temperature sensing has been used in power systems, construction, chemical, aerospace, medical, and marine development, and has achieved a large number of reliable application results.

1. Optical fiber temperature sensor has important applications in power system, surface temperature of power cable and temperature monitoring and monitoring in dense cable area; monitoring of heat-prone parts in high-voltage power distribution equipment; environmental temperature detection and fire alarm system of 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 of heating systems, steam pipes and oil pipelines of thermal power plants; geothermal power stations and indoors Equipment temperature monitoring of substations and so on.

2, fiber temperature sensing, especially fiber grating temperature sensor is easy to bury the material to high-resolution and wide-range measurement of its internal temperature, so it is widely used in buildings and bridges. Some developed countries such as the United States, Britain, Japan, Canada and Germany have long carried out research on bridge safety monitoring, and installed bridge safety monitoring and early warning systems on the main bridges to monitor the strain, temperature acceleration and displacement of the bridge. Safety indicators. In the summer of 1999, 120 fiber-optic grating temperature sensors were installed on a steel bridge on the Inter Cru Highway 10 in Las Cruces, New Mexico, creating the largest number of records on a single-seat bridge.

3. The aerospace industry is a place where sensors are used intensively. In order to monitor pressure, temperature, vibration, fuel level, landing gear status, position of the wing and rudder, etc., more than 100 sensors are required. The size and weight of the sensor become very important. Fiber optic sensors have few advantages compared to the advantages of small size and light weight.

4. The small size of the sensor is very meaningful in medical applications. The FBG sensor is the smallest sensor available today. Fiber Bragg Grating sensors provide internal measurements of human tissue functions with minimal invasive methods, providing accurate local information about temperature, pressure and acoustic fields. Fiber Bragg Grating sensors are quite rich in technology for human tissue. Research on fiber optic temperature sensors accounts for nearly 20% of all fiber optic sensor research.

In addition to external field verification, improvement and improvement of existing optical fiber temperature sensors, there are several development trends: Development of measurement techniques for measuring temperature distribution, from temperature measurement at a single point to on the optical fiber. Temperature distribution, and measurement of large-area surface temperature distribution; development of multi-function sensors including temperature measurement; development of large sensor arrays for full optical telemetry.