Broadband Terahertz Gas Spectroscopy Through Multimode Self-Mixing in a Quantum Cascade Laser

Abstract

We present a self-mixing terahertz-frequency gas spectroscopy technique using a multimode quantum cascade laser. A precision-micro-machined external waveguide module and a double-metal quantum cascade laser device are used increase the optical feedback and the laser's frequency tuning range. Methanol spectra are measured using two laser modes at 3.362 THz and 3.428 THz simultaneously, with more than 8 absorption peaks resolved over a 17-GHz bandwidth, which provide minimum detectable absorption coefficients of 2.7×10 -4 cm −1 and 4.9×10 −4 cm −1 , respectively. In contrast to all previous self-mixing spectroscopy, our multimode technique expands the sensing bandwidth significantly. This broadband spectroscopy technique can potentially be used for the identification and analysis of chemical, biological radiological and nuclear (CBRN) agents and explosives.

Terahertz microscopy using laser feedback interferometry based on a generalised phase-stepping algorithm

“…The experimentally simple form of the LFI scheme, combined with its coherent sensing capability, has motivated its use within a variety of class-A and class-B laser systems and across a wide range of applications spanning visible, infrared and terahertz (THz) regions of the spectrum. Such applications have included metrology 4 , coherent imaging [5][6][7] , materials analysis 8,9 , gas sensing and spectroscopy 10,11 , Doppler flow measurements 12 , three-dimensional imaging 13,14 , vibrometry 15 , and displacement sensing 16,17 . Furthermore, the SM effect has more generally been exploited for the measurement of fundamental laser parameters including the emission spectrum 18 , laser linewidth 19,20 and linewidth enhancement factor 21,22 .…”