Optical Fiber Specklegram Chemical Sensor Based on a Concatenated Multimode Fiber Structure

Abstract

An optical fiber specklegram chemical sensor based on a concatenated multimode-no-core-multimode structure is reported. The 30 mm-no-core section is immersed in the analyzed liquid so the number of excited modes is modulated by the surrounding refractive index value, affecting the interference of the several propagating modes and consequently the output specklegram, whose changes are quantified by the zero mean normalized cross-correlation coefficient. The sensor was evaluated on the measurement of ethanol-water solutions, resulting in 18.7 RIU $^{-1}$ sensitivity (5 × 10−4 resolution) for the 1.33 to 1.36 refractive index interval, besides 0.07 $^{\circ }$ C $^{-1}$ temperature sensitivity within the 26–32 $\,{}^{\circ }$ C range. Moreover, the system was also tested on liquid level measurements, yielding 0.05 mm $^{-1}$ sensitivity and 0.2 mm resolution. The results are comparable to previously reported multimodal interferometers based on more complicated interrogation schemes, indicating that the specklegram sensor is a feasible alternative for the sensitive and straightforward measurement of liquid samples.