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 lambdaon and lambdaoff wavelengths of different chemical species for DIAL detection is described.
The strength of backscattered signals for a Differential Absorption Lidar (DIAL) system operating in the mid infrared
band is estimated for the cases of (i) back scattering from topographic targets and (ii) Mie scattering from the aerosols.
The estimation is based on the following input parameters : (a) the energy of the lidar transmitted pulses is 20 mJ with
pulse duration of 10 nanoseconds and (b) the receiver system consists of a telescope with 500 mm aperture with a field
of view of 0.33 mrad.
The inherent noise of the detector, background noise due to solar radiation and the noise due to thermal radiation from
the objects within the field of view of the receiver, are separately estimated to obtain the total noise in the detection
For the case of topographic targets, the reflectivity is taken as 10%. It is shown that atmospheric trace gases like
methane with (path integrated) concentration upto 0.5 ppm-km, can be detected without difficulty upto ranges of 5 km.
For the Mie scattering from the aerosols, typical meteorological visibility of 10 km is assumed. The case for methane
detection is studied. It is shown that during day time, range resolved measurements of trace chemicals can made upto 3
km range, which can be extended to 5 km during night time.