Dynamic fiber loop-mirror filter based on pump-induced saturable gain or saturable absorber gratings

Abstract

An all-fiber, optically-tunable, narrowband optical loop-mirror filter ( ) that makes use of a loop-mirror ( ). The optical filter ( ) includes a coupler ( ), including two input fiber ports ( ) and two output fiber ports ( ) that are connected together to form the loop-mirror ( ). A saturable medium ( ) is positioned in the loop-mirror ( ), and can be either a saturable absorber medium or a saturable gain medium. A potentially broadband optical input signal to be filtered and a pump are applied to the input ports ( ) of the coupler ( ). The counter propagating signals in the loop-mirror ( ) generated by the input signal and the pump create two standing wave interference patterns in the saturable medium ( ). Depending on whether the input signal and the pump are applied to the same input port ( ) or different input ports ( ) determines whether the two standing wave interference patterns are in phase with each other or &Xgr; radians out of phase with each other. If the saturable medium ( ) is a saturable gain medium, then the peaks of the pump standing wave interference pattern bleach the gain of the filter ( ) at the peak locations, preventing amplification of the input signal at these locations. Likewise, if the saturable medium ( ) is a saturable absorber medium, the peaks of the pump interference pattern bleach the absorption of the saturable medium ( ) at the peak locations, leaving rest of the saturable medium ( ) to provide saturation. Therefore, the filter ( ) can be either a bandpass filter or a notch filter.