All-fiber Fabry-Perot cavity enhanced spectroscopy of acetylene

Cavities are widely used in spectroscopy, for example to increase the absorption path and thus the detection sensitivity. With the advent of hollow core photonic crystal fibers (HCPCF) it became possible to realize long absorption paths by using long fibres and, moreover, providing high electric field strength due to the small core diameters. This paper demonstrates an all-fibre scanning Fabry-Perot interferometer with a Fabry-Perot cavity using a 7-cell HCPCF attached to a piezo-electrical actuator. Two 99 % reflective fibre connectors are positioned at both ends of the HCPCF which is fixed inside standard ceramic ferrules to prevent good light coupling. The finesse of the cavity is about 60 which is lower than the theoretical prediction given by the reflectivity of the mirrors. From theory the effective absorption length of such a fibre increases by a factor of 2F/pi.