Objective:

Review developments in dual-comb spectroscopy in the mid-infrared and terahertz regions and compare technologies used to generate frequency combs.

Approach:

• Technique: Two frequency combs with slightly different pulse rates produce an interference signal that translates optical information into electronically measurable radio frequencies.
• Scope: The review covered mid-infrared and terahertz applications, comb-generation technologies, indirect detection methods, and technical barriers.

Key Findings:

• Dual-comb spectroscopy can acquire detailed spectra rapidly, with high spectral resolution and precise frequency measurements, without moving components used in some conventional instruments.
• The review compared difference-frequency generation, optical parametric oscillators, quantum cascade lasers, electro-optic systems, and microresonators; these approaches differ in range, sensitivity, speed, stability, power, and size.
• Mid-infrared applications under development include trace-gas analysis, combustion studies, leak detection, and atmospheric monitoring. Terahertz systems may be used to analyze gases, solids, and biological materials.
• Indirect acoustic, thermal, or mechanical detection may help where suitable infrared or terahertz detectors are limited. A cited cavity-enhanced photoacoustic system detected acetylene at 0.6 parts per billion after 100 seconds.
• Biomedical applications remain largely at the research stage; the review did not assess a clinically validated diagnostic test.

Interpretation:

The review describes a versatile spectroscopy approach with applications across chemical and materials analysis, while noting that biomedical use is not yet clinically validated.

Limitations:

• No single system provides continuous coverage across the full mid-infrared and terahertz range.
• Detector noise, cooling requirements, beam alignment, instrument complexity, and maintaining coherence between combs can affect performance.

Conclusion:

Some platforms are becoming smaller and more integrated, but technical barriers remain.

Sources:

• Review published in Opto-Electronic Technology, led by Zhuoren Wan, East China Normal University