Fiber-optic tunable loop mirror using Berry's geometric phase

A method is presented for constructing a fiber-optic loop mirror whose reflectivity is tunable even though a fixed-ratio directional coupler is used. The tunability is polarization and wavelength independent and depends only on the geometry of the fiber-tuning element. This loop mirror can be used to transfer energy from one channel to another in a controllable way. We experimentally demonstrate the tunable reflectivity and present some experimental investigations.