Optical fiber Fabry-Perot cavities and recirculating delay lines as tunable microwave filters

The radio frequency or microwave responses of single-mode fiber Fabry-Perot cavities and recirculating delay lines are studied. It is shown that fiber Fabry-Perot cavities may be used as bandpass filters and the recirculating delay lines as bandstop filters at radio or microwave frequencies. These electric filters are tunable by electrooptic effects since the bandpass or bandstop characteristic changes when the cavity or the delay line length is varied by a few tenths of an optical wavelength. The ultimate limit on speed of tuning is set by the coherence time of the light source. The dependence of the filter responses on the spectral purity of light is studied. Examples are given to illustrate the tunability of these electric filters. The availability of various components required to build the tunable RF or microwave filters is reviewed