Remote Simultaneous Measurement of Liquid Temperature and Refractive Index Using Fiber-Optic Spontaneous Raman Scattering

In this paper, we propose and demonstrate a method for simultaneously measuring liquid temperature and refractive index by using spontaneous Raman scattered light in a multi-mode fiber under test, in which intensity ratio of the Raman anti-stokes, and stokes backscattered light from a fiber coil was used for liquid temperature measurement. Meanwhile, Fresnel reflection of the Raman stokes forward-scattered light from a fiber end was utilized for measuring liquid refractive index. Based upon a simply-designed sensing head consisting of the fiber coil and the fiber end, and using a 0.34-mW laser with 5-kHz frequency and 5-nm pulse width, proof-of-concept experiments show this method can achieve a temperature measurement accuracy of ±0.65 °C and a refractive index measurement accuracy up to 0.00015 RIU with a sensing distance as long as several kilometers. This type of sensor may have applications involving remote temperature and refractive index sensing such as warning of fires and explosions, remote monitoring of dangerous chemicals and so on.