Incoherent Gain-Assisted Ring Enhanced Gas Absorption Spectroscopy

A novel incoherent gain-assisted ring enhanced spectroscopy technique is presented in overcoming the need for precise optical alignment and high-reflectivity dielectric mirrors. A gain medium pumped below the lasing threshold is used to improve the ring finesse and, thus the interaction length. Two configurations are considered and compared. The first configuration is dual coupler ring with the source outside the ring and the gain medium inside the ring, while the second configuration is single coupler resonator with the gain medium acting as the source. The configurations are studied analytically and experimentally for different system parameters. The experimental results are obtained for a coupling ratio of 90/10. The broadband source is the amplified spontaneous emission of a semiconductor optical amplifier (SOA), while another SOA is used as the gain medium. Acetylene (C
₂
H
₂
) gas absorption is measured around 1540-nm wavelength. The output spectrum of the ring is measured using a 70-pm resolution. The interaction length is found to be enhanced by a factor of 9.35 and 44 in case of the dual coupler and a single coupler resonator, respectively, compared with the direct absorption of the gas cell.