Discrimination of chemicals via refractive index by EF-FLRD

Design and application of an evanescent field fiber loop ring-down (EF-FLRD) spectroscopy system for discrimination of chemicals via their refractive indices are presented. To our knowledge, this is the first system that utilizes visible light. The system employs a broadband laser source at 800 nm at 80 MHz whose pulses were selectively picked by a Pockels cell to eliminate overlap of the pulses in the cavity. Chemically etched fiber region was used as a sensing element and eight organic solvents were discriminated compared to the reference sample mainly due to their differences in refractive indices. The solvent dielectric constants cover a broad range from 2 (of decane) to 80 (of water) at 20 °C (dielectric constants are obtained from Solvent Polarities, http://murov.info/orgsolvents.htm#TABLE2 , 2019 ). Prior to the measurements, optimization of data collection protocols, etched sensing region geometry, and the sample compartment configuration was achieved. The results show that solutions with a refractive index unit difference of 0.0018 (acetone–ethanol couple) were able to be differentiated as the lowest difference and the detectable lowest loss was calculated to be 1.10 × 10 −5 dB. A single measurement takes less than 1 min (which is limited by the control system) with the lowest error of 0.37% (for acetone) and the highest error of 1.71% (for ethanol) showing real-time measurement possibility. Simplicity and unique design of the set-up is a promising first step in construction/utilization of EF-FLRD systems for trace chemical detection in the visible range. Graphic abstract