A Far Infrared Rooftop Resonator

Optically pumped far-infrared (FIR) lasers are known to suffer from power instabilities caused by reflected feedback of the pump beam by the FIR resonator [1-3]. To overcome this problem a self-isolating FIR cavity has been constructed using metallic rooftop mirrors as the reflecting elements. See Fig. 1. Cavity isolation is achieved when the incoming pump polarization is oriented at 45° with respect to the crease of the output-coupling rooftop. In this configuration the backward-going pump beam polarization is orthogonal to the input beam polarization. This backward-going feedback beam is then eliminated by a Brewster stack or polarizer. In this way the two cavities are decoupled. Further, since the crease of the input-coupling rooftop is oriented parallel (or perpendicular) to the crease of the output-coupling rooftop, the polarization reorientation described above is repeated. Hence, even if the pump beam makes several passes inside the FIR laser, the polarization of the backward-going pump beam remains orthogonal to that of the forward-going beam. Very significant increases in FIR output power stability have been achieved using this type of self-isolating resonator.