Linear optical feedback stabilized cavity ring down spectroscopy in the mid infrared range

This work describes a technique exploiting a linear optical cavity to frequency stabilize, by optical feedback, an 8.5 µm QCL laser and perform, at the same time, cavity ring down spectroscopy. An arbitrary frequency step resolution is achieved thanks to an active control of the cavity length using a 1.6 µm tunable pilot laser. A detectivity of 4 × 10^-10cm^-1 and a frequency resolution of 20 kHz are demonstrated, with a cavity finesse of 18800. A 3 GHz and a 4 MHz broad spectra are recorded with 8.5 MHz and 20 kHz step resolution, respectively. The first spectrum consists in a high-resolution overview showing two water lines that belong to H₂¹⁶O and H₂¹⁸O; the second one is a very high-resolution recording of a sharp Lamb dip structure observed at the top of the H₂¹⁶O line, illustrating the vast application fields this instrument opens.