  We demonstrate the first optically pumped sub-300 nm UV laser structures
grown by plasma-assisted molecular beam epitaxy on single-crystal bulk AlN. The
edge-emitting laser structures fabricated with the AlN/AlGaN heterostructures
exhibit multi-mode emission with peak gain at ~284 nm. Having the goal of
electrically injected, continuous wave deep-UV AlGaN laser diodes in mind, with
its intrinsic material challenges of achieving sufficient optical gain, the
optical cavity loss of a laser diode should be minimized. We derive an
expression to quantify the effect of mirror imperfections, including slant and
surface roughness on the optical mirror loss of a Fabry-P\'erot cavity. It is
found that the optical imperfection loss is a superlinear function of the RMS
roughness and slant angle of the facets, and also scales as the inverse
wavelength squared of the principal lasing mode. This highlights the importance
of device processing optimization as Fabry-P\'erot cavities couple to lower
wavelengths.
