Loop-mirror filters based on saturable-gain or -absorber gratings

We present a novel all-fiber narrow-band filter based on pump-induced satur able-gain or -absorber gratings in a loop mirror. Our design provides built -in interferometric phase alignment of the signal to the grating for optima l filtering. Notch or bandpass functionality is determined by the choice of gain or absorption and the input ports selected for the pump and signal. T he loop-mirror filter has potential bandwidths from the submegahertz to bey ond the gigahertz regimes, and one can tune it optically by changing the wa velength of the pump light that establishes the grating. Such filters have potential applications to wavelength-division-multiplexed optical networks and optical rf signal processing. (C) 1999 Optical Society of America.