Optical Fiber Semi-Distributed Interferometer Assisted by an FBG for Thermorefractometry and Sweat Sensing

Abstract

Among optical fiber biosensors, the use of thermorefractometry is a promising approach for dual detection of refractive index (RI) and temperature with a single device, and it can find application in sweat analytes. We propose an optical fiber-based thermorefractometer made of a semi-distributed interferometer (SDI) supported by a fiber Bragg gratings (FBG), for the possible application of the detection of sweat biomarkers. The sensor is composed of two parts (SDI and FBG) integrated in a compact probe; the resulting spectrum is a composition of a periodic-like pattern throughout the whole spectrum (SDI) and a narrow-band reflection peak at the Bragg wavelength (FBG). The thermorefractometer detects the refractive index changes with a sensitivity of 75.752 dB/RIU, while the FBG detects thermal changes at 9.94 pm/°C with negligible cross-sensitivity. We demonstrate the use of the SDI/FBG probe for the detection of two sweat markers. We tested the performance of the sensor in artificial sweat with azelaic acid and suberic acid achieving a sensitivity of 0.629/% dB and -0.088 dB/%.