Introduction

Laser spectroscopy is a successful tool to detect specific gases or vapors with ultra-high sensitivity and fast time response. By using infrared-based spectroscopic techniques, such as cavity ring down spectroscopy, frequency modulation spectroscopy, or photoacoustic spectroscopy, molecular gases can be detected at and below the ppbv-level (part per billion volume, 1:10⁹). For ultimate performance, optical detection techniques have to operate in the so-called “molecular fingerprint region”, that is located in the mid-infrared (mid-IR) wavelength range between 2.5 and 25 µm. In this region, all of the important gas molecules possess dense series of strong, narrow-band absorption lines corresponding to rotational-vibrational transitions, and even larger organic molecules absorb light with maximum strength and reveal their chemical (and even nuclear) consistence through their highly specific response frequencies. By measuring the light absorption in a gas sample at a large number of different wavelengths (frequencies) in the fingerprint region, in principle, all compounds can be identified and their individual concentrations can be measured (Figure 1).

Figure 1: Typical absorption regions of some important gases. (Generated by SpectralCalc)

The light sources used in this spectral range can be cw lasers based on semiconductor technologies such as Quantum Cascade Lasers (QCLs) and Interband Cascade Lasers (ICLs), or nonlinear conversion of the near-infrared lasers by Optical Parametric Oscillation (OPO) and Difference Frequency Generation (DFG). Figure 2 shows a comparison of the wavelength coverage of these sources [1].

Figure 2: Comparison of the wavelength coverage of some available light sources in mid-infrared range.

Another option would be newly developed mid-infrared Supercontinuum Sources (SCs) that can provide an ultra-broad spectral coverage and high spectral brightness. Figure 3 shows a comparison of the wavelength coverage and spectral brightness of the SC light sources with thermal sources and QCLs [2].

Figure 3: Comparison of the wavelength coverage and spectral brightness of the SC light sources with thermal sources and QCLs.

The third option can be Optical Frequency Combs (OFCs) that can provide both a broad spectral coverage and a high spectral coverage mostly used in a Dual-Comb Spectroscopy technique. These sources in the mid-infrared region are usually based on QCLs (rather narrow spectral coverage, but compact) or nonlinear conversion of the near-infrared OFCs generated by mode-locked fiber lasers using OPOs and DFGs. Figure 4 shows a comparison of the wavelength coverage of the OFC sources with different technologies and different nonlinear crystals [3].

Figure 4: Comparison of the wavelength coverage of the OFC sources with (a) different technologies and (b) different nonlinear crystals.

References

1. https://qoe.ethz.ch/Tutorials/quantum_cascade_laser.html
2. Zorin, I., Gattinger, P., Ebner, A. & Brandstetter, M. Advances in mid-infrared spectroscopy enabled by supercontinuum laser sources. Opt. Express 30, 5222 (2022).
3. Schliesser, A., Picqué, N. & Hänsch, T. W. Mid-infrared frequency combs. Nature Photon 6, 440–449 (2012).