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

Abstract

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 (H₂S) 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 H₂S as well as methane (CH₄) could be examined within one single pulse.

Quantum cascade laser-based analyzer for hydrogen sulfide detection at sub-parts-per-million levels

“…This sensitivity is from 2 to 10 times better comparing to results reported for near-infrared systems that use QEPAS, integrated cavity output spectroscopy, WMS, or CLaDS. [2][3][4][5] It outperforms most previously demonstrated setups that also use QCLs in 7-to 8.2-μm range [10][11][12] and is comparable to a recently demonstrated system operating at 8.018 μm. 19 Modified two-point deviation analysis reveals that a basic WMS system with no active line-locking can achieve better than 400 ppbv accuracy over extended operation times spanning above 1000 s, which is an ideal solution for a cost-effective technology required for industrial safety monitoring applications.…”

“…Fortunately, even when larger than ambient concentrations of methane are expected, several other H 2 S lines with similar or even larger line strength are also available in the spectral region between 7 and 8.2 μm. With careful selection of target transition, reduced interference from other species can be obtained, 10,19 allowing for highly accurate open-path H 2 S detection at subppm levels with compact optical design and simple data processing.…”

Development and Integration of a CO Detection System Based on Wavelength Modulation Spectroscopy Using Near-Infrared DFB Laser

“…In order to avoid disasters and guarantee safety production in coal mines, CO concentration monitoring must be performed effectively in real time. Among the numerous gas sensing techniques, optical methods have been studied worldwide in recent years due to its advantages including high sensitivity, fast response time, nonintrusive nature, wide sensing range, and long lifespan [5][6][7]. Tunable diode laser absorption spectroscopy (TDLAS) using lasers as light sources is a powerful and widely used technique for gas sensing applications including environmental monitoring, industrial process control, chemical analysis, and combustion diagnostics [8][9][10][11][12].…”

Off-beam quartz-enhanced photoacoustic spectroscopy-based sensor for hydrogen sulfide trace gas detection using a mode-hop-free external cavity quantum cascade laser

“…The occupational exposure limit Technically viable setups for H 2 S trace gas measurements in the MIR spectral region using QCLs are often chosen to be based on absorbance measurements in multireflection Herriott cells [12] as the effective optical interaction pathlength will be increased up to several tens of meters. Recently, the first application of ring-cavitysurface-emitting quantum-cascade lasers (RCSE-QCLs) for sensitive H 2 S gas measurements was reported [13]. Together with phase-sensitive detection techniques, such as wavelength modulation spectroscopy (WMS) [14,15], the generally dominating 1/f electronic noise can be drastically minimized, and in general, high detection sensitivities of H 2 S can be achieved, enabling on-line monitoring of H 2 S and CH 4 content in a petrochemical process stream [16].…”