**Abstract**

The chapter dwells on two novel approaches towards enhancing the efficiency of nonlinear optical generation. The former is to enable the unabsorbed pump beam to pass through the crystal repeatedly. Integration of an unstable cavity containing the crystal with the stable pump cavity made this possible. The Q of the unstable cavity could be maintained high as the output coupler of the pump laser, itself served as the entrance mirror of this cavity. The unstable nature of the cavity kept the crystal from being exposed to high flux while ensuring longer interaction length. Although this scheme demonstrated in mid-IR region its advantage should persist across UV, visible, and near-IR regions too. The enhancement of conversion efficiency is effected in the second scheme by way of illuminating the crystal with alternate high and low regions of intensity along its length as against the uniform illumination case maintaining the same average intensity as in the conventional operation. The advantage is attributed to the square dependence of the second harmonic on the intensity of the pump. A simple modification of the existing experimental setup involving integration of an additional optical element with the pump cavity allowed exploitation of interference effect to realise such a non-uniform illumination condition.

**Keywords:** non-linear optical conversion, unstable cavity, interference, CO2 laser, dichroic optics
