Classification and Operation Principle of Optic Fiber Temperature Sensors

Optic fiber temperature sensor is a sensing device that analyze the spectrum transmitted by the optical fiber to obtain real-time temperature. Its operation bases on that for some materials, spectrum absorbed changes with temperature. These sensors mainly includes optic fibers, spectrum analyzer and transparent crystals, etc., and can be divided into distributed type and fiber optic fluorescence type.

Distributed optic fiber:
This type is often used in a system that detects distribution of temperature in a room. Its operation principle was first raised in 1981, and was developed for years into the technology we adopt today. This development is based on three researches about sensors: Rayleigh scattering, Brillouin scattering, and Raman scattering. Great achievements have been made on research about Rayleigh scattering and Brillouin scattering (OTDR), so in the future, more attention will be paid on research about new distributed type of optic fiber sensors based on Raman scattering.
Optic fiber fluorescence:
At present, most heated research is about optic fiber fluorescence temperature sensors, which uses fluorescent material’s feature of luminescence to detect the temperature of light area. When being stimulated by IR or UV light, this material will give out light whose parameters have positive connection with temperature, thus the temperature can be measured by testing fluorescence intensity.
Optic fiber temperature sensors have macro-molecule temperature-sensitive material that matches the refractive index of optic fiber coated on two welded optic fiber. This enables the light to be input from a fiber and then be output from another. Since that the refractive index of this new temperature-sensitive material is affected by temperature, the output luminous power has functional relationship with temperature. The physical essence of the sensor is that characteristic parameters of light-wave transmitted by optic fibers, like amplitude, phase, polarization state, wavelength and mode, are sensitive towards ambient factors like temperature, pressure and radiation, etc. It’s a kind of non-contact temperature measurement.
The operation principle of optic fiber temperature sensor is: in low temperature zone(under 400 ℃) where radioactive signal is weak, the system enables light emitting diode(LED) and the fluorescence temperature measurement system starts to work. The excitation light from LED goes through collecting lens and get coupling to the branch end of Y type optical fiber, and then couples to the temperature sensing probe through optical fiber. Being triggered by excitation light, the top of optical fiber sensing probe emits fluorescence light. This fluorescence signal is derived by optic fiber, emitted from the other branch of the optical fiber coupler, and received by photoelectric detector. The optical signal output by the photoelectric detector is amplified and processed by fluorescence signal processing system to calculate fluorescent lifetime, so as to obtain the measured temperature. In high temperature zone(over 400 ℃） where radiation signal is strong enough, the radiation temperature measurement system start to work and luminous diode is off. The radiation signal passes through sapphire optic fiber and get output from Y type fiber, and then be transferred into electric signal. The system measures the strength of the radiation signal to calculate the detected temperature.

ISWeek, the famous online mall of all kinds of sensors, has imported three types of optic fiber temperature sensors from Canada, all of which are of excellent quality and performance. These imported sensors are: Type FOT-L-BA and Type FOT-L-SD, which are ideal for temperature measurement in extreme environment; and Type FOT-HERO, which is specially designed for temperature measurement in electronic explosive environment. These sensors can be applied in various industries.
The top of optical fiber’s sensing probe, whose length is 8 to10mm, is Cr3+ ion-doped, so as to realize fluorescence emission when getting light excitation. Meanwhile the external surface of the top of the fiber is plated with blackbody chamber for radiation temperature measurement. (To satisfy blackbody chamber’s requirement of constant apparent radiance, the ratio between the length of blackbody chamber and diameter of the optical fiber is larger than 10.) In order to assure the performance of the whole system, it’s essential to avoid or reduce the mutual interference between fluorescent emission part and thermal radiation part.
It can be found out that the interference can be mainly presented as:

1. the background signal of fluorescent signal has influence on accuracy of fluorescence lifespan detection;
2. Surface plating of optical fiber has influence on strength of fluorescence;

3) Cr3+ ion-doping in optical fiber has influence on thermal radiation signal of blackbody chamber.
Many colleges and universities worldwide have researches on such sensors. For example, Hansung University in Korea found out that for 10cm of double-doping optical fiber, temperature index corresponding to the fluorescent strength of reflection at its 915nm point is 20℃~290℃; Study in Tsinghua University uses semi-conducting GaAs material to absorb light. Based on the principle that light changes with temperature, the study has developed an optical fiber fluorescent temperature sensor whose temperature range is 0℃~160℃.