A quantum cascade laser-based sensor for ambient air monitoring is presented and five gases, affecting the air quality, can be quantified. The light sources are selected to measure CO, NO, NO₂, N₂O and SO₂. The footprint of the measurement setup is designed to fit in two standard 19” rack (48 cm × 65 cm) with 4 height units (18 cm) whereas one is holding the optical components and the other one contains the electronics and data processing unit. The concentrations of the individual analytes are measured using 2f-Wavelength Modulation Spectroscopy (2f-WMS) and a commercially available multipass gas cell defines the optical path. In addition, CO can also be measured with a dispersion-based technique, which allows one to cover a wider concentration range than 2f-WMS. The performance of this prototype has been evaluated in the lab and detection limits in the range of 1ppbv have been achieved. Finally, the applicability of this prototype for ambient air monitoring is shown in a five-week measurement campaign in cooperation with the Municipal Department for Environmental Protection (MA 22) of Vienna, Austria.

We present a compact vertically emitting ring interband cascade laser (ICL) with low power consumption and the possibility for seamless integration into various CO2 sensing applications. Our devices exhibit desirable performance characteristics in battery-driven handheld devices, including room temperature (20 °C) threshold currents as low as 15 mA, small footprints, and stable single-mode emission, suitable for rapid isotope-resolved CO2 detection. Through epi-down bonding with sub-micron accuracy, we achieved robust integration of substrate-emitting ring ICLs, ensuring reliability and scalability that would be required for mass production. We present comprehensive experimental results validating the efficacy of our approach, including spectral analysis and CO2 sensing capabilities with limits of detection of 24 and 13 ppmv utilizing the 12CO2 P(60) and 13CO2 R(10) transitions in the ν3 fundamental band, respectively. The demonstrated devices hold great promise for a wide range of industrial applications, including environmental monitoring, process control, and atmospheric research, where compact low-power sensors are essential.

In this work, a highly sensitive, selective, and industrially compatible gas sensor prototype is presented. The sensor utilizes three distributed-feedback quantum cascade lasers (DFB-QCLs), employing wavelength modulation spectroscopy (WMS) for the detection of hydrogen sulfide (H₂S), methane (CH₄), methyl mercaptan (CH₃SH), and carbonyl sulfide (COS) in the spectral regions of 8.0 µm, 7.5 µm, and 4.9 µm, respectively. In addition, field-programmable gate array (FPGA) hardware is used for real-time signal generation, laser driving, signal processing, and handling industrial communication protocols. To comply with on-site safety standards, the QCL sensor prototype is housed in an industrial-grade enclosure and equipped with the necessary safety features to ensure certified operation under ATEX/IECEx regulations for hazardous and explosive environments. The system integrates an automated gas sampling and conditioning module, alongside a purge and pressurization system, with intrinsic safety electronic components, thereby enabling reliable explosion prevention and malfunction protection. Detection limits of approximately 0.3 ppmv for H₂S, 60 ppbv for CH₃SH, and 5 ppbv for COS are demonstrated. Noise-equivalent absorption sensitivity (NEAS) levels for H₂S, CH₃SH, and COS were determined to be 5.93 × 10⁻⁹, 4.65 × 10⁻⁹, and 5.24 × 10⁻¹⁰ cm⁻¹ Hz^−1/2. The suitability of the sensor prototype for simultaneous sulfur species monitoring is demonstrated in process streams of a hydrodesulphurization (HDS) and fluid catalytic cracking (FCC) unit at the project’s industrial partner, OMV AG.

Spectral beam combination of multiple single mode laser sources employing narrowband spectral filters which are arranged on the perimeter of regular polygons is demonstrated. With this simple geometric design, co-alignment and co-propagation of the individual laser beams can be reasonably achieved. Spectroscopic applicability is displayed by spatial filtering, mode-matching, and subsequent coupling of the combined beams into a 76 m astigmatic mirror multipass cell.

Heterodyne Phase Sensitive Dispersion Spectroscopy has emerged as a promising new approach to gas-phase chemical sensing. The most common implementation of this method uses direct modulation of the injection current of a quantum cascade laser. The modulation produces a three-tone optical signal which, when passed through the gas sample, allows the profile of the refractive index, from which the gas concentration can be derived, to be measured. However, the frequency of this modulation signal must be in the range of the linewidth of the target molecular resonance, which for many compounds of interest is in the few GHz at ambient pressure. This challenging requirement prevents the technique from operating in free space with good sensitivity. Nevertheless, in this article, we present a novel quantum cascade laser package that, together with a simple and reliable Heterodyne Phase Sensitive Dispersion Spectroscopy instrument design, demonstrates a very convincing solution to overcome the performance limitations of current systems at ambient pressure, hence enabling high performance gas analysis in free space. Various experiments targeting methane at 1311.5 cm ^-1 show excellent linearity over a concentration range of four orders of magnitude and a limit of detection which is well into the sub-ppm·m.