Use of bending of a single SMS fiber structure for measurement of temperature sensing

Abstract

A simple temperature sensor based on a bent singlemode-multimode-singlemode (SMS) fiber structure fastened on a polymer base plate is proposed and experimentally investigated. The surrounding refractive index (RI) is higher than that of the silica fiber and RI changes with temperature will not lead to wavelength shift. This SMS fiber structure utilizes changing of temperature to control the curvature of SMS fiber which is induced by expanding of polymer base plate. The shifts of central wavelength are measured at temperature range from 59 to 82 ºC. The proposed temperature sensor offers sensitivity of 3.9 nm/°C, which is significantly higher than that of a normal straight SMS structure or a grating-based fiber structure. Keywords: Singlemode-multimode-singlemode fiber, fiber-optic sensor, displacement, temperature INTRODUCTIONOptical fiber sensors have many advantages such as immunity to electromagnetic interference, small size, the possibility of remote operation and high sensitivity. Recently, a number of optical fiber temperature sensors have been developed, such as fiber Bragg gratings (FBGs) [1, 2], long-period fiber gratings (LPFGs) [3], Fabry-Perot interferometer [4] and singlemode-multimode-singlemode (SMS) fiber structure [5]. FBGs for temperature sensing exhibit sensitivity of 0.01nm/°C and that for LPFGs is 0.1 nm/°C. A SMS fiber structure can also be used for temperature sensing (10pm/°C) and is much easier to fabricate than a grating-based fiber since it only involves cleaving and splicing processes that can be carried out in any fiber-optics laboratory, therefore a sensor based on a SMS fiber structure will be more economic than that based on a grating fiber. As a SMS sensor is sensitive to both strain (displacement) and temperature [6][7][8], it should also be possible to use a SMS fiber sensor for simultaneous measurement of displacement and temperature. However, the temperature sensitivity of such devices is always very low, as all fibers are made of silica with low thermo-optic and thermal expansion coefficients. In order to enhance the device temperature sensitivity, it is necessary to modify the materials and components in fiber sensor. Qiang Wu et al. [7] shows that a bent SMS fiber structure used as a temperature sensor has a much higher temperature sensitivity of 32 pm/°C.The underlying principle of SMS fiber sensor is multimode interference excited between modes in the multimode fiber (MMF) section, which can be influenced by external perturbation. So far, most SMS sensors is based on the surrounding RI lower than that of the multimode fiber core, measuring the wavelength shift to detect temperature or displacement. For the case that the surrounding RI is higher than that of the multimode fiber core [9], a new concept of leaky mode interference is presented. In that leaky structure, the variation of the output intensity with the surrounding RI is monotonic, providing the basis of RI sensing by intensity detection. It means that when RI is changing with temperature, we can measure the inten...