A stable fiber-based Fabry-Perot cavity

We report the development of a fiber-based, tunable optical cavity with open access. The cavity is of the Fabry-Perot type and is formed with miniature spherical mirrors positioned on the end of single- or multi-mode optical fibers by a transfer technique which involves lifting a high-quality mirror from a smooth convex substrate, either a ball lens or micro-lens. The cavities typically have a finesse of $\sim 1,000$ and a mode volume of 600 $\mu$m$^3$. We demonstrate the detection of small ensembles of cold Rb atoms guided through such a cavity on an atom chip.