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The basic principle of distributed optical fiber temperature measurement and sensing process introduction

by:FOT     2020-07-06

the basic principle of distributed fiber optic equipment temperature measurement and sensing process description

the backward scattering field of distributed optical fiber temperature measurement system based on the principle can be divided into three types: based on Rayleigh scattering, based on Raman scattering and based on brillouin scattering. The current development of mature, and products used in engineering is distributed optical fiber temperature measurement system based on Raman scattering. Its main is based on optical fiber sensing principle of optical time-domain reflectometer ( OTDR) The principle and the backward Raman scattering of the fiber temperature effects.

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the basic principle of distributed optical fiber temperature measurement principle of distributed optical fiber temperature measurement system based on backward scattering can be divided into three types: based on Rayleigh scattering, based on Raman scattering and based on brillouin scattering. The current development of mature, and products used in engineering is distributed fiber optic equipment temperature measurement system based on Raman scattering. Its main is based on optical fiber sensing principle of optical time-domain reflectometer ( OTDR) The principle and the backward Raman scattering of the fiber temperature effects.

( A) Optical time domain reflection ( OTDR) Principle of

when laser pulse in a fiber optic transmission, due to the existing in the optical fiber refractive index of the microscopic non-uniformity, scattering is produced. In time domain, the backward scattering field of incident light after the return to the time needed for optical fiber incident side for t, laser pulse in optical fiber through the path of 2 l, for light in the optical fiber transmission speed of v, C is the speed of light in a vacuum, n for optical fiber refractive index. When the measured time t, is the distance from the light source can be obtained in L.

( 2) The backward Raman scattering of the fiber temperature effects

when a laser pulse from one end of the optical fiber into optical fiber, the optical pulse can spread along the fiber. By light pulses with optical fiber internal molecular inelastic collision and elastic collision, so the light pulses in the transmission of each point will produce reflection, there is a small part of the reflected light in reflection, its direction is opposite to the direction of the incident light ( Also known as backward) 。 After this to the intensity of the reflected light and the light of the temperature of the reflection point has certain correlation. The temperature of the reflection point ( That point fiber environment temperature) The higher, the intensity of the reflected light. Take advantage of this phenomenon, if can measure the intensity of the reflected light, the temperature of the reflection point can be calculated, this is the use of the basic principle of optical fiber measurement temperature.

if use formula to express: when the laser pulse propagation in optical fiber and optical fiber molecular interactions, Rayleigh scattering happens, brillouin scattering, Raman scattering, Raman scattering is due to the thermal vibration of optical fiber molecular and photon energy exchange and interaction taking place. If part of the light energy is converted into heat vibration, then will send a long light than the light wavelength, called stokes light; If part of the thermal vibration into light energy, would send a shorter than light wavelength of light, called anti stokes light. According to the theory of Raman scattering, under the condition of spontaneous Raman scattering, two beams of light intensity and temperature, the ratio of their R ( T) For:

( 1) Among them, and are respectively stokes light intensity and antistokes light intensity, h is Planck's constant, k is the boltzmann constant, T for juedui temperature. From ( 1) Can be seen in the type, R ( T) Only related to the temperature T. Therefore, we can use anti stokes and stokes light intensity ratios to realize the measurement of temperature.

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