**2.1 Experimental**

The experimental demonstration of this scheme was effected in the second harmonic generation of the 10 micron emission of a pulsed CO2 laser. A commercial uncoated 17 mm thick AgGaSe2 crystal served as the non-linear medium for this conversion process. A rise in the energy conversion efficiency by ~300% and even higher peak power conversion efficiency has been achieved by making the unconverted pump go through the crystal time and again. The increase in the effective length of the crystal should in principle, allow the performance of a thin crystal in such a cavity configuration to match that of a thick crystal in the conventional operation although at a lower level of optical flux, that in turn, precludes the possibility of its damage even in the pulsed operation.

The schematic of the experimental lay out is depicted in **Figure 2**. In the first set of experiments (**Figure 2a**), the pulsed emission of a commercial multi-atmosphere TE-CO2 laser was made use of to affect SHG in an uncoated AgGaSe2 crystal (crosssection 10 × 10 mm and length 17 mm). A plane master grating (150 lines/mm) and a concave (7 m ROC) 70%R ZnSe output coupler separated by 105 cm formed the passively stabilised pump laser cavity. For this experiment, the laser was operated on 10P (34) line for which the second harmonic phase matching occurred at an external angle of incidence of ~34<sup>ᵒ</sup> . Usage of an intra-cavity adjustable aperture A1 allowed the operation of the pump laser on the TEM00 mode. The energy incident on the crystal was controlled by varying the charging voltage of the laser. An external adjustable aperture 'A2' allowed maintaining the pump beam cross-section on the crystal entrance to ~4.5 mm diameter so as to ensure its clear passage through the non-linear medium. Monitoring of both the energy and the power of the incident pump pulse was possible by probing its Fresnel reflection off the incident face of the crystal. The energy and power profile of the generated SH beam were measured after blocking the unconverted pump beam that also emerged along with the SH beam through the crystal by means of a sapphire plate. The CO2 laser, by virtue of its multi-atmosphere operation, possessed inherently very high gain and thus emitted pulses of relatively short duration (FWHM ~110 nsec, **Figure 3**). In the present experiment, the maximum intensity was restricted to ~2.5 MW/cm2 .
