  By applying a high-reflectivity dielectric coating on both sides of a
commercial plano-convex lens, we produce a stable monolithic Fabry-Perot cavity
suitable for use as a narrow band filter in quantum optics experiments. The
resonant frequency is selected by means of thermal expansion. Owing to the long
term mechanical stability, no optical locking techniques are required. We
characterize the cavity performance as an optical filter, obtaining a 45 dB
suppression of unwanted modes while maintaining a transmission of 60%.
