Millimeter-long Fiber Fabry-Perot cavities

TL;DR

This paper presents the development of long fiber Fabry-Perot cavities with high finesse and broad frequency range, achieved through advanced mirror shaping and mode matching techniques, overcoming previous limitations related to fiber shape deviations.

Contribution

The authors introduce a laser dot machining method for shaping spherical mirrors on fibers, enabling long cavities with high finesse and octave-spanning operation, and address shape deviation issues.

Findings

Cavities operate at lengths up to 1.5mm with high finesse.

Shape deviations from the fiber's index profile reduce finesse.

A method to mitigate shape deviation effects is demonstrated.

Abstract

We demonstrate fiber Fabry-Perot (FFP) cavities with concave mirrors that can be operated at cavity lengths as large as 1.5mm 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.