**4. Conclusion**

In conclusion, we conceived the advantage in SH generation by a nonlinear crystal when it is illuminated with alternate high and low regions of intensity along its length as against the conventional case of its uniform illumination with the same average intensity. Exploitation of interference effect by placing the crystal inside a Fabry Perot cavity has allowed the imposition of such a non-uniform illumination condition on to the crystal along its conversion length. The decided advantage of the non-uniform illumination over uniform illumination has been experimentally established under conditions of equal intensity exposure in the two cases. We believe that this advantage was always present in intra-cavity or resonantly enhanced frequency doubling generation processes but stayed unrecognised as the motivation of these works was to enhance the conversion efficiency by increasing the effective interaction length of the crystal and the advantage gained was thus automatically attributed in totality to this. Carefully planned experiment here has allowed us to decouple the advantage due to interference (as seen in **Figure 11**) from the total advantage as recorded in the data of **Figure 10** that also included the gain due to increased interaction length. While we have achieved the spatial variation of intensity by the exploitation of interference effect, we do not rule out the

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**Author details**

provided the original work is properly cited.

\*Address all correspondence to: dhruba8biswas@gmail.com

Padma Nilaya J. and Dhruba J. Biswas\*

*Towards Enhancing the Efficiency of Non-Linear Optical Generation*

possibility of achieving the same effect by some other means, e.g., a train of ps or fs mode locked pulses will manifest as spatial intensity variations in the sub mm to sub-micron scale appropriate to derive this advantage in a crystal of finite length. Advantage can be derived from even chaotic pulse trains wherein the temporal oscillations occur in the similar time scales as above. However, it is to be noted that the restriction on the maximum period of the spatial variation of the intensity is imposed by the crystal thickness while there is no restriction on the minimum period. As a matter of fact smaller is the periodicity of bright and dark intensity regions, better will be the heat diffusion and thus will be preferred from the point

*DOI: http://dx.doi.org/10.5772/intechopen.80816*

of view of handling higher intensity.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai,
