**2.3 Conclusion**

**96** A thoughtful integration of a stable pump cavity with an unstable external Fabry-Perot cavity has resulted in remarkable enhancement in the SH conversion efficiency

**97**

**3.1 Experimental**

*Towards Enhancing the Efficiency of Nonlinear Optical Generation*

even in case of pulsed operation of the laser. Although coupled external resonant enhancement has found application for the non-linear conversion process in the cw operation of the pump laser over visible region, it has not gained popularity in the mid-infrared (MIR) region owing to the possibility of damage to the MIR crystals that are not only expensive but also scarce. An unstable cavity that has the intrinsic ability to limit the intra-cavity flux there by safe-guarding the crystal from optical damage even in case of pulsed operation has been shown to offer a practical solution to this problem. We note here that the performance of this scheme can be further improved by employing a crystal with its end faces broad band AR coated, appropriate choice of the cavity parameters and control of cavity lengths. Although feasibility of this scheme has been demonstrated in the case of SHG in the MIR region, the same should, in principle, be valid for application across the near IR, visible and the UV regions of the electromagnetic spectrum as well for both cw and pulse operations.

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

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.

doubling was found to occur for an external angle of incidence of ~36<sup>ᵒ</sup>

The experimental system utilised here is identical to the one used towards increasing the interaction length between the pump and the non-linear medium and the same would therefore, not be repeated here, and the reader may refer to **Figure 2** of Section 2.1 and its description therein instead. To be noted here that **Figure 2a** depicts the case of uniform illumination while **Figure 2b** represents the case of non-uniform illumination. The CO2 laser was tuned to the 10P(32) line giving rise to emission at 10.72 μm and consequently phase matching for frequency

section of the pump beam on the crystal entrance face was restricted to ~5.0 mm diameter that allowed its clear passage through the crystal. Although the crystal

. The cross-

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

**of the non-linear medium**
