Design of a tunable mid-IR OPO source for DIAL detection of trace gases

Abstract

The design of an efficient optical parametric oscillator (OPO) tunable in the range of 2.3 to 4.5 micron, as LIDAR source, for sensitive, selective and rapid remote detection and measurement of atmospheric trace gases at ranges upto 5 km is described. Potassium Titanyl Arsenate (KTA) a nonlinear crystal, having good transmission from 350 to 5000 nm is proved to be most suitable for this application. Tuning is achieved by angle tuning in critical phase matching of type II in X-Z plane, by changing the propagation direction from about 41 to 49 degrees with the Z-axis, when pumped with Nd:YAG laser at 1064nm. The expected linewidth of KTA OPO in the absence of any wavelength selective device is found to vary from 10 to 110 cm^-1 which reduces to 0.1 cm^-1 on introduction of an intracavity grating at grazing incidence. Using a 600 l/mm grating, with the tuning mirror fixed on a commercially available fast rotation stage, random tuning to any desired wavelength can be achieved in 40 ms. Threshold of optical parametric oscillation is found to vary from 2 to 3 J/cm² as the idler wavelength varies from 2.3 to 4.5 micron. With this limitation, the pump energy requirement varies from 450 mJ to 600 mJ for 20 mJ output energy at different wavelengths in singly resonant oscillator configuration. Tuning arrangement for rapid tuning of output to lambda_on and lambda_off wavelengths of different chemical species for DIAL detection is described.