Discrimination of chemicals via refractive index by EF-FLRD
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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.
This study is financially supported by The Scientific and Technological Research Council of Turkey Grant No. 212T079 and funded in part by METU Scientific Research Project Coordinator.
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
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