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The present work reports on the first application of a ring-cavity-surface-emitting quantum-cascade laser (RCSE-QCL) for sensitive gas measurements. RCSE-QCLs are promising candidates for optical gas-sensing due to their single-mode, mode-hop-free and narrow-band emission characteristics along with their broad spectral coverage. The time resolved down-chirp of the RCSE-QCL in the 1227-1236 cm -1 (8.15-8.09 µm) spectral range was investigated using a step-scan FT-IR spectrometer (Bruker Vertex 80v) with 2 ns time and 0.1 cm -1 spectral resolution. The pulse repetition rate was set between 20 and 200 kHz and the laser device was cooled to 15-17°C. Employing 300 ns pulses a spectrum of ~1.5 cm -1 could be recorded. Under these laser operation conditions and a gas pressure of 1000 mbar a limit of detection (3σ) of 1.5 ppmv for hydrogen sulfide (H 2 S) in nitrogen was achieved using a 100 m Herriott cell and a thermoelectric cooled MCT detector for absorption measurements. Using 3 µs long pulses enabled to further extend the spectral bandwidth to 8.5 cm -1 . Based on this increased spectral coverage and employing reduced pressure conditions (50 mbar) multiple peaks of the target analyte H 2 S as well as methane (CH 4 ) could be examined within one single pulse.

Application of a ring cavity surface emitting quantum cascade laser (RCSE-QCL) on the measurement of H_2S in a CH_4 matrix for process analytics