A compact sealed off metal-ceramic single-mode laser with an unstable-waveguide cavity has been made, which can use some active media (CO, carbon-dioxide molecules and Xe atoms). The output power per unit length is higher by a factor of 2...4 than in lasers with a round or square channel and a stable resonator.
V. G. Leont'ev,
E. F. Shishkanov,
"Outlook of unstable-waveguide-cavity usage in gas lasers", Proc. SPIE 2773, Laser Optics '95: Gas Lasers, (22 April 1996); doi: 10.1117/12.238523; https://doi.org/10.1117/12.238523