*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*

of 105 cm long cavity, is seen. Upon comparison with the typical temporal profile of the emission of the pump laser (**Figure 3**), it becomes obvious that the integration of the pump laser with this external unstable cavity does in no way bring in any new feature in its temporal profile or alter the beat period. This clearly implies that the coupling of the external cavity with the pump cavity has no effect on the dynamics of the pump laser. A comparison of the time expanded second harmonic temporal profile (upper trace of **Figure 7b**) with that of the temporal profile of the pump (lower trace of **Figure 7b**) readily establishes their phase and amplitude synchroni-

*(a) Temporal profiles of the pump (lower trace) and the cavity enhanced SH (upper trace). Mode beating is reflected in the SH emission also. (b) 7 ns beat period indicates operation of the pump laser on two* 

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

sation: a signature of the instantaneous nature of the SHG process.

*longitudinal modes (lower trace), the same is also reflected in the SH pulse (upper trace).*

**96**

**2.3 Conclusion**

**Figure 7.**

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.
