A tunable laser has a solid state laser medium with optical gain region and generates coherent radiation through a facet. A lens collects the coherent radiation and generates a collimated light beam. An external cavity includes a reflective surface and an optical filter, the reflective surface reflecting the collimated beam back to the lens and laser medium, the optical filter positioned between the reflective surface and the lens and having two surfaces and a thermally tunable optical transmission band within the optical gain region of the laser medium. The optical filter (1) transmits a predominant portion of the collimated beam at a desired wavelength of operation, and (2) specularly reflects a remaining portion of the collimated beam from each surface, the collimated beam being incident on the optical filter such that the reflected collimated beams propagate at a non-zero angle with respect to the incident collimated beam.