Exploitation of an external unstable multi-pass cavity to enhance the second harmonic conversion efficiency

Attempts to overcome the low conversion efficiency often encountered in second harmonic generation process by increasing the interaction length through usage of a number of crystals in tandem or in parallel or a single crystal in the intra-cavity or external resonant enhanced configuration have been made although with mixed success. We report here on the gainful exploitation of an unstable cavity to greatly enhance nonlinear conversion efficiency by integrating it with the stable pump cavity. This integration allowed transport of the pump radiation into the external cavity that at the same time exhibited high 'Q', prerequisite for multi-pass of the unabsorbed pump through the crystal. The unstable cavity not only ensured longer interaction length in the crystal without exposing it to high optical flux thereby safeguarding the crystal even in the pulsed operation but also eliminated the feedback problem. This scheme is demonstrated in the mid-IR region by frequency doubling of pulsed CO2 laser emission but its advantage should persist across UV, visible, and near-IR regions too.