Gas Sensing

Laser-based gas sensing has revolutionized how we detect and quantify gases in our environment or in industrial processes. By exploiting the interaction between laser light and the characteristic absorption features (often called molecular fingerprints) of gases, this technology provides highly sensitive, selective, and real-time measurements that far outperform conventional chemical or electrochemical sensors.

Every gas molecule has a unique optical fingerprint, absorbing light at specific wavelengths that correspond to transitions between its internal rotational-vibrational energy states. When a laser is tuned to one of these absorption lines, even trace concentrations of the gas can be detected with extraordinary precision. The technique is non-contact, calibration-free, and offers a dynamic range extending from parts-per-trillion (ppt) to percent levels.

The electromagnetic spectrum relevant to gas analysis spans from the ultraviolet (UV) to the terahertz (THz) range, with each region revealing distinct molecular information:

• UV / Visible Region (200–800 nm): Sensitive to electronic transitions. Useful for gases like ozone (O₃), nitrogen dioxide (NO₂), or sulfur dioxide (SO₂).

• Near-Infrared (NIR, 0.8–2.5 µm): Covers overtone and combination bands of molecular vibrations, ideal for in-situ, long-path applications like methane (CH₄), carbon dioxide (CO₂), and water vapor (H₂O).
• Mid-Infrared (MIR, 2.5–12 µm): Known as the molecular fingerprint region, where fundamental vibrational transitions occur. Here, absorption is strongest and most specific, perfect for detecting hydrocarbons, carbon monoxide (CO), nitric oxide (NO), ammonia (NH₃), and many volatile organic compounds (VOCs).
• Terahertz (THz, 0.1–10 THz): Provides access to low-energy rotational and torsional transitions. THz spectroscopy is particularly powerful for complex or large molecules, and for detecting gases through black smoke, which is opaque to visible or near-infrared light. See also THz Gas Analysis.

An application specialist reading a molecular spectrum in these regions can derive rich physical and chemical information, from concentration and pressure to temperature and molecular composition.

1. Tunable Diode Laser Absorption Spectroscopy (TDLAS)
   A narrow-linewidth tunable laser scans across a specific absorption feature of the gas. The resulting absorption profile allows precise quantification of its concentration.
   • Sensitivity: ppb–ppm
   • Response time: <1 s
   • Common lasers: DFB diode lasers <link DL-DFB >or widely tunable diode lasers <link CTL 1550> at 1.55 µm (CH₄, CO₂), 2.3 µm (CO), 1.5 µm (H₂O)
2. Wavelength Modulation Spectroscopy (WMS)
   Enhances the TDLAS principle by modulating the laser wavelength at high frequency and detecting higher harmonics, thus suppressing noise and improving the signal-to-noise ratio.
3. Cavity-Enhanced Absorption & Ring-Down Spectroscopy (CEAS / CRDS)
   The light is trapped between highly reflective mirrors, extending the optical path length to several kilometers. The decay time (ring-down) reveals the gas absorption coefficient, achieving ultra-high sensitivity down to parts-per-trillion levels.
4. Photoacoustic Spectroscopy (PAS)
   Converts absorbed laser energy into an acoustic signal. Compact and highly sensitive, this technique is used for multi-species analysis in confined environments such as medical breath sensing.
5. THz Spectroscopy
   THz radiation penetrates non-conductive materials and accesses molecular rotational spectra. It allows label-free, non-destructive identification of gases and complex mixtures, complementing IR analysis for the detection of large biomolecules or explosives .

This combination of speed, precision, and stability makes laser-based spectroscopy the method of choice in scientific and industrial environments where accuracy matters.

Laser-based gas sensing is already transforming multiple industries:

• Environmental Monitoring: CH₄ and CO₂ detection at 1.65 µm and 2.0 µm for greenhouse gas tracking.
• Industrial Process Control: CO and NO detection with lasers around 4.6 µm.
• Automotive & Aerospace: Exhaust gas analysis and combustion control.
• Medical Diagnostics: Breath analysis for biomarkers such as acetone (diabetes), nitric oxide (asthma) or ammonia (liver function)
• Safety & Leak Detection: CH₄, H₂S, and CO monitoring in refineries and mining.