Laser Difference-Frequency Generation in the Mid-Infrared and Applications to High-Resolution Molecular Spectroscopy and Trace Gas Detection

摘要

We report on the development of a widely tunable continuous-wave (CW) laser spectrometer based on difference-frequency generation (DFG) in a nonlinear optical crystal, which operates in the mid-infrared (MIR) spectral region of 3-20 驴m with a Doppler linewidth limited spectral resolution. The state-of-the-art of CW DFG techniques as well as the applications of the DFG-based laser source to high-resolution molecular spectroscopy and trace gas spectroscopy will be presented. Recent advances in nonlinear optical materials and high power tunable laser technology have led to the development of continuous-wave (CW) coherent infrared radiation sources based on nonlinear difference-frequency generation (DFG). Broadly tunable CW laser-based infrared radiation can be generated from 3 to 20 驴m by difference-frequency mixing of visible or near infrared lasers in nonlinear optical crystals. The development of the laser-based infrared DFG sources has been motivated not only for high-resolution spectroscopy, but also for applications in trace gas spectroscopy such as environmental monitoring of greenhouse gases, toxic chemical species, as well as for medical diagnostics, because most molecular species exhibit strong rotational-vibrational absorption features with characteristic chemical bonds (such as C-H, C=O, C-Cl, N-H) in the infrared region. In the present work, two CW Ti:Sapphire lasers (899-29, Coherent Inc.) were employed as convenient versatile laboratory DFG pump sources.