Using photonic filters for tuning dispersion-induced microwave transmission windows on radio over fiber schemes

In the RF domain, the frequency response of a radio over fiber system using a wide spectrum multi-longitudinal optical source and dispersive optical fiber exhibits band-pass microwave transmission windows. Such a response has been proposed for implementing RF filters and also for multiplexing analog or digital information when microwave carriers can be allocated at successive transmission windows. The position of the microwave bands depends on the optical spectrum and also on the length of the optical channel. The frequency response of the studied system can be modified either by changing the length of the optical channel or by modifying the optical spectrum. The first approach is not practical as it is not cost-effective to install more optical fiber length than required on an optical link. The second approach is more attractive and simple as the optical spectrum can be filtered by using optical delays. AA optical delay or its equivalent optical path-difference (OPD), acting as a photonic filter, can modify the optical spectrum by "channeling" it. Such a photonic filter is easily implemented by segments of polarization maintaining optical fibers (PMF). The tuning of position and number of the microwave transmission windows, when the optical spectrum of the multi-longitudinal source is filtered by optical delays, is reported in this paper.