**3.2 Results and discussion**

Towards finding the efficiency of the SHG process as a function of the pump energy for the conventional case of uniform illumination (**Figure 2a**), we gradually increased the input and monitored the corresponding SH energy and the dependence is as shown in **Figure 8**. The maximum SH energy conversion efficiency can be estimated from this figure as ~8.0%.

In the next set of experiments we subjected the crystal to alternate regions of high and low intensities along its length. This was readily possible by constructing a Fabry-Perot cavity comprising of the output coupler of the pump laser 'M1' (R ~80%@10.72 μm, T ~20%@5.36 μm) and a plane dichroic mirror 'M2' (R > 90%@10.72 μm, T > 90%@5.36 μm) located at the exit end of the crystal (refer to **Figure 2b**). The pump energy incident on the crystal, as measured by Detector D1, showed a dramatic increase as 'M2' was fine tuned to establish its parallelism with 'M1', resulting, in turn, in a corresponding improvement in the measured SH output. In effect, there are now two inputs to the crystal; (a) Forward Input: the actual input on the entrance face in the forward direction that comes directly from the pump laser and (b) Reverse Input: the pump, that stays unconverted after its passage through the crystal, gets reflected off 'M2' and shines on the exit face of the crystal from the opposite direction. When the cavity is perfectly aligned, the interference of these two components creates alternating nodal and anti-nodal intensity regions inside the cavity and partly contributes towards the observed dramatic enhancement of SH conversion by the crystal. At every instant, the reverse

**Figure 8.**

*Second harmonic output as a function of the input pump energy in the conventional operation wherein the crystal is uniformly illuminated by the pump beam along its conversion length.*

**99**

**Figure 9.**

*Dependence of the reverse input to the crystal as a function of the forward component.*

*Towards Enhancing the Efficiency of Nonlinear Optical Generation*

component, after traversing through the crystal, is reflected off M1 and falls in step with the pump photons emerging through it resulting in an effective increase in the energy incident on the entrance face of the crystal as measured by the detector D1. At this point, towards gaining a deeper insight into this process, we gradually varied the pump (forward) input and measured both, the corresponding reverse input and the generated SH. The difference in the energy measured by D1 with M1 aligned and misaligned gives the measure of the reverse input. **Figure 9** depicts the dependence of the reverse input on the forward input to the crystal while **Figure 10** shows the SH output as a function of the total effective input to the crystal which is now the sum total of the forward and the corresponding reverse components. It is apparent from **Figure 9** that the reverse input does not exactly bear a linear relationship with the forward input and this behaviour owes its origin to the square dependence of the SH output on the intensity of the input at the fundamental wavelength as is evident from **Figure 8**. The square dependence basically means that as the pump intensity rises, increasingly higher fraction of it gets converted into SH and thus less of it is left to constitute the reverse input to the crystal. This explains the observed departure from the linear dependence of the reverse input on the forward input to the crystal.

The increase in the effective input to the crystal in case of an aligned cavity due to addition of forward and reverse components leads to the generation of higher SH output as revealed in **Figure 10**. For instance, the maximum pump input of 6.5 mJ in case of uniform illumination (**Figure 8**) gets enhanced to 10.34 mJ (**Figure 10**) in the aligned cavity condition giving rise to almost 2.54 fold increase in the SH conversion efficiency. However a closer examination of **Figure 10**, in conjunction with **Figure 8**, reveals a wealth of information, hitherto unexplored, that constitutes the central theme of this study and is captured in the traces of **Figure 11**. It is clearly evident from this figure that SH output in case of non-uniform illumination of the crystal is significantly higher compared to the case of its uniform illumination even when the total input to the crystal is maintained the same. Let us consider a typical input of 4.1 mJ that in case of uniform illumination generates 0.22 mJ (refer to **Figure 8**) of SH at a conversion efficiency of ~5.36%. It can be readily estimated from **Figure 9** that this input of 4.1 mJ in case of non-uniform illumination comprises of a forward component of 2.5 mJ and a reverse component of 1.6 mJ. Thus,

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