An optical cavity can be utilized as an excellent transducer for highly sensitive gas detection with the application of photothermal spectroscopy, featuring the beneficial property of an ultra-low absorption volume within a rugged sensing element. We report the novel implementation of balanced detection in Fabry-Perot photothermal interferometry via two identical 1 mm-spaced cavities. That way, excess noise limiting the sensitivity of previous cavity-based photothermal sensors was effectively rejected close to the fundamental limit of shot noise. A quantum cascade laser served as mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced variations. The metrological qualities of the sensor were investigated by SO₂ detection. For the targeted absorption band centered at 1380.93 cm⁻¹, a 5 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 7.5 × 10⁻⁹ cm⁻¹ W Hz^−1/2. Additionally, the sensor showed excellent long-term stability, enabling integration times of a few thousand seconds.

Trace gas detection was performed by the principle of photothermal interferometry using a Fabry-Perot interferometer combined with wavelength modulation and second harmonic detection. The sensor employed a compact, low-volume gas cell in an overall robust set-up without the use of any moveable part. A quantum cascade laser was used as powerful mid-infrared excitation source to induce refractive index changes in the sample, whereas a near-infrared laser diode served as probe source to monitor the photo-induced variations. The functional principle of the selective sensor was investigated by detection of sulfur dioxide. For the targeted absorption band centered at 1379.78 cm⁻¹ a 1 σ minimum detection limit of about 1 parts per million by volume was achieved. The work demonstrates high potential for further sensor miniaturization down to a sample volume of only a few mm³. Limitations and possible improvements of the sensor regarding sensitivity are discussed.

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.

The development of a sensitive SO₂ QEPAS-based sensor platform employing a CW DFB-QCL will be reported. A detection sensitivity of 100 ppbv was achieved with a 1-sec averaging time for the 1380.94 cm⁻¹ SO₂ line.

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.