Millimeter-long fiber Fabry-Perot cavities

We present the realization of fiber Fabry-P\'{e}erot (FFP) micro-cavities with concave mirrors that can be operated at cavity lengths as large as 1.5 mm without significant deterioration of the finesse. This is achieved by using a laser dot machining technique to shape spherical mirrors with ultralow roughness and employing single-mode fibers with large mode area for good mode matching to the cavity. Additionally, in contrast to previous FFPs, these cavities can be used over an octave-spanning frequency range with adequate coatings. We also show directly that shape deviations caused by the fiber's index profile lead to a finesse decrease as observed in earlier attempts to build long FFP cavities, and show a way to overcome this problem. Beyond concave mirror structures, the novel multi-pulse laser fabrication technique further allows to enlarge the range of accessible structures, including asymmetric mirror profiles, convex shapes on fiber tips and on macroscopic fused silica substrates.