Sensitivity Enhancement Factor for Gain-Assisted Cavity Enhanced Spectroscopy

Abstract

Fiber-based gain-assisted incoherent broadband cavity enhanced spectroscopy is a novel technique used to enhance spectroscopic measurement sensitivity. This technique overcome the drawbacks of the conventional cavity enhanced techniques, in the need for optical alignment and high reflectivity mirror as they are based on free-space Fabry-Perot cavity. However, to assess the value of this technique, the sensitivity enhancement factor over the direct path absorption spectroscopy should be calculated. In this work, an analytical, and experimental study for the sensitivity of the fiber-based incoherent gain-assisted ring enhanced spectroscopy techniques are demonstrated. Two fiber-based configurations are studied for different system parameters. The first configuration is a dual-coupler ring with the source outside the resonator and the gain medium inside the resonator. The broadband source is the amplified spontaneous emission (ASE) of a semiconductor optical amplifier (SOA). While the second configuration is single-coupler resonator with the gain medium acting as the broadband source. The optical absorption of acetylene (C 2 H 2 ) gas around 1540 nm wavelength has been measured. The output spectrum has been measured using an optical spectrum analyzer (OSA) of 70 pm resolution. The sensitivity is enhanced by a factor of 942 in the case of the single-coupler configuration, compared to the direct absorption of the gas cell. The study elaborates that intracavity (single-coupler) configuration provides SNR enhancement in thermal noise limited system. In contrast to conventional incoherent broadband techniques that don't provide any enhancement in case of thermal noise limited system.