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

*Nonlinear Optics - Novel Results in Theory and Applications*

when the same total input of 4.1 mJ is made to shine on the crystal as two separate beams of 2.5 and 1.6 mJ from opposite directions by taking advantage of a cavity, a SH output of 0.325 mJ (refer to **Figure 10**) is generated at an efficiency of 7.93%; a clear advantage of ~48% in the SH conversion efficiency by going for non-uniform illumination. As discussed before this is attributed to the alternate high and low intensity regions seen by the crystal as a result of the interference of the forward

*Dependence of second harmonic output on the effective input pump energy in case of non-uniform illumination.*

In order to estimate the expected advantage of the situation when the crystal is non-uniformly illuminated over the case of uniform illumination, we used the

*Experimental SH conversion efficiency as a function of the total input to the crystal is shown for both uniform and non-uniform illumination cases. The % gain of SH conversion in case of non-uniform illumination over the uniform illumination case, defined as [(SHNUI-EFF−SHUI-EFF)/SHUI-EFF] × 100, is also shown here as a* 

and reverse beams travelling through the crystal in the latter case.

**100**

**Figure 11.**

*function of the overall input to the crystal.*

**Figure 10.**

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 of the non-uniform illumination case.

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 the forward and the reverse components in the present study.
