Broadband Terahertz Laser Absorption Spectroscopy with a Quantum-Cascade Vertical-External-Cavity Surface-Emitting Laser

A quantum-cascade vertical-external-cavity surface-emitting laser with a piezoelectrically variable cavity length is demonstrated as a tunable single-mode source for broadband laser absorption spectroscopy. The device is capable of coarse stepping between 2.56 and 2.79 THz, and continuous fine-tuning over 40 GHz is used to resolve several absorption features of hydrogen sulfide gas. The high output power of the laser allowed the use of a room-temperature pyroelectric detector. Stepped scans on the order of minutes produced a minimum detectable absorbance of 10 –2 with sub-GHz spectral resolution. Scans on the order of 1 s, limited by the detector bandwidth, can be achieved at the cost of resolution by switching to continuous frequency sweep operation. We discuss the interplay between the sensitivity, scan range, resolution and acquisition rate of the system, and what steps can be taken to improve them.