Difference-frequency generation in PPLN and water vapor detection in air

The continuously tunable laser source has been realized in a periodically poled LiNO₃ crystal based on difference frequency generation and quasi-phase-matching technique. The pump laser is an 1 W tunable Ti: Sapphire laser with a tunable region from 770 to 870 nm. The signal laser is an 1 W diode-pumped monolithic Nd:YAG laser. When the grating period is 20 μm and the temperature is tuned between room temperature and 200°C, the generated wavelength of idler laser is around 2.8 μm with the general power of 1-2 μW. The direct absorption spectra of (001←000) band of water in laboratory air were measured based on the laser source. The concentration of water vapor in the laboratory air was estimated with an absorption optical path of 8.5 cm in open air.