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

Abstract

recommended by the European Agency for Safety and Health at Work (EU-OSHA) is 5 ppmv [2]. The permissible exposure limit value for H 2 S is 10 ppmv, the Immediately Dangerous to Life and Health (IDLH) level is 300 ppmv and lethal concentrations are in the range of 2000 ppmv [2]. In practice, concentrations ranging from sub-ppmv levels at low pressures to several per cents at atmospheric conditions need to be monitored. Despite a variety of online monitoring options for gaseous H 2 S, its reliable quantitative and selective determination still remains challenging in the field of chemical sensors [3][4][5].In the field of laser spectroscopy, the constant improvement of quantum cascade lasers (QCLs) has led to their application as reliable sources of coherent light ranging from the mid-infrared (MIR) to the terahertz spectral region for sensitive detection of molecular species on their fundamental vibrational, respectively, rotational bands [6][7][8][9]. Due to their tailorable emission wavelength, high output power, compactness, narrow spectral linewidth, and wavelength tuneability, QCLs are optimal choices for spectroscopic applications. In addition, optical resonator designs are constantly improved over the years with the distributed feedback (DFB) [10] and the external cavity (EC) [11] approach being the most prominent ones. A general aim with respect to the ongoing development of QCLs for sensing applications is to reduce the line width of the emitted radiation to a minimum while achieving a spectral coverage as large as possible. So far, EC-QCLs offer the largest tuning range which, depending on the employed gain medium, may cover up to several hundreds of wavenumbers. The external cavity design facilitates broadband spectral tuning by an external diffraction grating, while the selection of the emission wavelength takes place by changing the grating angle relative to the QCL chip.Abstract Hydrogen sulfide (H 2 S) trace gas detection based on off-beam quartz-enhanced photoacoustic spectroscopy using a continuous wave (CW), mode-hop-free external cavity (EC) quantum cascade laser tunable from 1310 to 1210 cm −1 was performed. A 1σ minimum detection limit of 492 parts per billion by volume (ppbv) using a 1 s lock-in time constant was obtained by targeting the line centered at 1234.58 cm . This value corresponds to a normalized noise equivalent absorption coefficient for H 2 S of 3.05 × 10 −9 W cm −1 Hz −1/2 .

Review of Recent Advances in QEPAS-Based Trace Gas Sensing

“…The high Q-factor (~100,000 in a vacuum and~10,000 in a standard atmosphere pressure) and narrow resonance frequency band (<1 Hz) of QTF improve the QEPAS selectivity and immunity to environmental acoustic noise [13][14][15][16][17][18][19]. Due to the merits of high selectivity and sensitivity, low cost, compactness, and a large dynamic range, QEPAS sensors have been widely applied in gas detection for atmospheric monitoring [20][21][22][23][24][25][26][27], chemical analysis [28][29][30][31][32], biomedical diagnostics [33][34][35][36], and trace gas sensing [37][38][39][40][41][42][43][44]. Different QEPAS sensor architectures were developed to meet the requirements of a large number of applications.…”

High-Power DFB Diode Laser-Based CO-QEPAS Sensor: Optimization and Performance

“…QEPAS eliminates the limitation of a gas cell caused by sound resonance conditions. The quartz tuning fork (QTF) can be placed in the near-field area of the excitation laser beam [ 8 , 9 , 10 , 11 ]. Therefore, the sealing of the gas is not necessary in this technique, and is only used to separate the gas sample from the surroundings in order to control its pressure.…”

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.…”

“…11 Another QEPAS setup with an external cavity QCL was presented and detection limit of 492 ppbv (1-s integration time, 118 mW of optical power) was achieved. 12 In this work, we present a setup that uses a distributed feedback (DFB) QCL source near 7.2 μm to target the H 2 S transition at ∼1389.3 cm −1 and achieve the desired sub-ppm detection limit using a multipass cell-based WMS spectrometer arrangement. A system design and its performance evaluation are demonstrated in this work.…”