Fiber fabry-perot optical cavities using expanded-core fibers with concave form

A new type of fiber Fabry-Perot interferometer with expanded cores was proposed. The form of the fiber ends was concave curve. It has simple structure and wide wavelength range, and the free spectral range of the device was suitably controllable. We have obtained the high performance of the fiber Fabry-Perot cavity by concave processing of a expanded core fiber end.