*Towards Enhancing the Efficiency of Non-Linear Optical Generation DOI: http://dx.doi.org/10.5772/intechopen.80816*

*Nonlinear Optics - Novel Results in Theory and Applications*

of the non-uniform illumination case.

data available from **Figures 8** and **10** in conjunction with the dependence of reverse input on forward input (**Figure 9**) for the reconstruction of the standing wave parameters. This is recorded in the **Table 1** above. It would be seen from this table that the advantage expected for the non-uniform illumination shows a definite reduction, although very marginal, with increasing input intensity. This reduction is because, with increasing intensity, ER/EF gradually reduces as is evident from **Figure 9** and discussed earlier. The experimentally measured advantage also recorded in **Figure 11** as a function of input intensity shows the same trend. The experimentally measured advantage of the non-uniform illumination, however, is seen to be considerably lower than the estimated value. This is due to the fact that a major fraction of the SH generated in the reverse direction escapes through the output coupler 'M1' of the pump laser. Usage of a coupler that offers high reflectivity at both fundamental and generated wavelengths will help square the full advantage

To be noted here that the enhancement in the second harmonic conversion efficiency achieved by way of placing the non-linear medium inside a cavity, basically comprises of two components arising out of: (i) increased effective length of interaction between the pump and the non-linear medium and, (ii) non-uniform illumination of the non-linear medium. The above study helps decouple these two components. In the above example where the input was maintained at 4.1 mJ for both uniform illumination (meaning EF = 4.1 mJ and ER = 0) and non-uniform illumination (meaning EF = 2.5 mJ and ER = 1.6 mJ), the added advantage arising out of increased interaction length has been annulled. Thus the enhancement in the SH conversion efficiency (viz., ~70%) is entirely attributable to the modulation of intensity arising out of interference of forward and reverse beams. In case of a non-uniform illumination with EF = 4.1 mJ, the corresponding ER = 2.6 mJ as evident from **Figure 9**. The SH output now is 0.85 mJ as against 0.22 mJ for uniform illumination and the advantage gained here comprises of both the above components. From the discussion above it is amply clear that the component of gain due to increase in interaction length between the pump beam and the non-linear medium is ~126%. The modest gain obtained due to non-uniform illumination of the active medium is attributable to the inequality of

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

the forward and the reverse components in the present study.

**102**

**4. Conclusion**

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 of view of handling higher intensity.
