**2. Enhancing the SH conversion efficiency by increasing the interaction length between the pump and the non-linear medium**

By way of constructing a coupled plano-convex cavity external to the pump laser (**Figure 2**) that allowed to and fro passes of the unabsorbed pump through the crystal, we conceived a novel way to increase the effective interaction length between the non-linear medium and the pump beam [15]. An ideal situation demands that the coupling optics offers high transmission at the pump wavelength and high reflection too at the same wavelength to enable multiple passes through the crystal; a conflicting requirement indeed that is inherently taken care of in the above

## **Figure 2.**

*Schematic diagram of the experimental setup for second harmonic conversion of the emission of a CO2 laser in a AgGaSe2 crystal. G: Plane blazed grating, A1 and A2: Adjustable apertures, B1 and B2: ZnSe Brewster plates, M1: 70% R ZnSe concave mirror, D1 and D2: Energy/power detectors, M2: Dichroic mirror. (a) In case of single pass second harmonic generation, dichroic mirror M2 is absent. (b) In case of multi-pass second harmonic generation, dichroic mirror M2 in conjunction with pump laser output coupler M1 forms the unstable external cavity.*

scheme. As the output coupler of the pump laser itself functioned as the entrance mirror of the external cavity, its quality factor could be maintained high allowing at the same time, efficient transportation of the pump beam into it. Further, the intracavity photon flux could be maintained within acceptable level due to the unstable nature of the external cavity. This reduced the risk of optical flux induced crystal damage besides eliminating the possibility of feed back into the pump cavity.
