Optical-feedback cavity-enhanced absorption spectroscopy in a linear cavity: model and experiments

Abstract

Optical-feedback cavity-enhanced absorption spectroscopy is a highly sensitive trace gas sensing technique that relies on feedback from a resonant intracavity field to successively lock the laser to the cavity as the wavelength is scanned across a molecular absorption with a comb of resonant frequencies. V-shaped optical cavities have been favoured in the past in order to avoid additional feedback fields from non-resonant reflections that potentially suppress the locking to the resonant cavity frequency. A model of the laser–cavity coupling demonstrates, however, that the laser can stably lock to a resonant linear cavity, within certain constraints on the relative intensity of the two feedback sources. By mode mismatching the field into the linear cavity, we have shown that it is theoretically and practically possible to spatially filter out the unwanted non-resonant component in order for the resonant field to dominate the feedback competition at the laser. A 5.3 $\upmu \hbox {m}$ cw quantum cascade laser scanning across a $\hbox {CO}_2$ absorption feature demonstrated stable locking to achieve a minimum detectable absorption coefficient of $2.7\,\times \,10^{-9}\,\hbox {cm}^{-1}$ for 1-s averaging. Detailed investigations of feedback effects on the laser output verified the validity of our theoretical models.