**3. Enhancing the SH conversion efficiency by non-uniform illumination of the non-linear medium**

As explained in the introduction to this chapter, if a non-linear crystal can in some way be subjected to alternate high and low regions of pump intensity along its conversion length the conversion efficiency can be shown to increase 100% as against the case of conventional uniform illumination maintaining the same average intensity. We provide experimental validation of this hypothesis wherein a significant enhancement in the SH conversion efficiency has been achieved by subjecting the crystal to non-uniform illumination. Such a situation could be realised by shining the crystal from both ends as against the conventional operation of illuminating it from one end. This was readily possible by placing the crystal inside a Fabry Perot cavity wherein the interference of the forward and the reverse beams creates a periodic intensity modulation along its length. The coherent input beam was derived from the emission of a high pressure CO2 laser while an AgGaSe2 crystal was made use of to affect its frequency doubling. Subjecting the crystal to alternate high and low intensity of coherent pump radiation requires placing it inside a high 'Q' cavity that, at the same time, should allow significant transport of the pump energy into it. As in the previous case, integration of the pump laser cavity with the external Fabry-Perot cavity allowed efficient transport of the pump beam into the crystal while at the same time maintaining high Q of the external cavity at the pump wavelength. The only work that we came across and that explicitly connects SHG with cavity interference, albeit with a totally different central theme, is of Wu and Kimble [19] wherein two fundamental beams generate one or two SH coherent beams under non-collinear phase matched condition and the focus has been to study the phase dependence of the pump and the generated waves.
