Infrared Laser Frequency Combs for Multispecies Gas Detection

We discuss two methods for mid-infrared frequency comb generation in quadratic nonlinear materials. The first one of these methods is based on half-harmonic generation in a degenerate femtosecond optical parametric oscillator, which is synchronously pumped with a near-infrared laser frequency comb. The second method uses a continuous-wave-pumped optical parametric oscillator. In this approach, the quadratic nonlinearity is used not only for frequency down conversion, but also for the actual frequency comb generation from a continuous-wave pump laser field. Both methods can be used to produce coherent broadband mid-infrared light that is ideal for real-time spectroscopic detection of small traces of biomarkers, toxics, and other interesting molecular compounds. In particular, these methods provide access to the 3–5 μm molecular fingerprint region, which contains strong fundamental vibrational stretching bands of hydrocarbons.