Doppler-limited molecular spectroscopy by difference-frequency mixing*

A cw difference-frequency spectrometer has been developed for the 2.2 to 4.2 μm infrared region by mixing argon and tunable dye lasers in LiNbO3. A resolution of 5 × 10−4 cm−1 with continuous scans up to 1 cm−1 and a scan precision better than 2 × 10−3 cm−1 have been achieved. Tunable ir powers of ~1 μW have been obtained, enabling sensitive absorption measurements and real-time spectroscopy. The range, resolution, speed, and sensitivity of this difference-frequency technique are demonstrated by absorption traces of water vapor, ammonia, methane, and nitrous oxide.