Advances in dual-comb spectroscopy are expanding the range of molecules that can be measured quickly and accurately, according to a review published in Opto-Electronic Technology.
The review, led by Zhuoren Wan of East China Normal University in Shanghai, examined developments in the mid-infrared and terahertz regions. These parts of the electromagnetic spectrum contain information about molecular structure and behavior that may be useful in chemical analysis, environmental monitoring, and biomedical research.
Dual-comb spectroscopy uses two optical frequency combs, each containing a series of evenly spaced wavelengths. Their slightly different pulse rates create an interference signal that converts optical information into radio frequencies that can be measured electronically.
This approach can collect detailed spectra rapidly without the moving components used in some conventional instruments. It also provides high spectral resolution and precise frequency measurements, allowing laboratories to identify and quantify molecules from their characteristic absorption patterns.
The mid-infrared region, spanning approximately 2 to 20 µm, contains strong molecular vibrations that can provide detailed chemical fingerprints. The terahertz region, from approximately 0.1 to 10 THz, captures molecular rotations, weak intermolecular interactions, and collective movements within solid materials.
The authors compared several technologies for generating frequency combs. These included difference-frequency generation, optical parametric oscillators, quantum cascade lasers, electro-optic systems, and microresonators. Each approach offers a different balance of spectral range, sensitivity, speed, stability, output power, and instrument size.
Mid-infrared systems are being developed for trace-gas analysis, combustion studies, leak detection, and atmospheric monitoring. Terahertz systems may support analysis of gases, solids, and biological materials. Some platforms are also becoming smaller and more integrated, which could eventually make the technology suitable for use outside specialist optical laboratories.
The review also described indirect detection methods that convert molecular absorption into acoustic, thermal, or mechanical signals. These approaches may improve sensitivity where suitable infrared or terahertz detectors are limited. In one cited example, cavity-enhanced photoacoustic dual-comb spectroscopy detected acetylene at 0.6 parts per billion after 100 seconds of measurement.
For medical laboratories, the technique may eventually provide another way to study complex molecular fingerprints or monitor biomarkers. However, most biomedical uses remain at the research stage, and the review did not assess a clinically validated diagnostic test.
Important technical barriers remain. No single system provides continuous coverage across the full mid-infrared and terahertz range. Detector noise, cooling requirements, beam alignment, instrument complexity, and the need to maintain coherence between the two combs can also affect performance.
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