OPTICAL-FIBER RECIRCULATING DELAY-LINES AS CONTROLLABLE RADIO-FREQUENCY NOTCH FILTERS

In this work, we analyze and demonstrate the use of optical fiber reci rculating delay lines (RDL) as electric filters at radio frequency (RF ) frequencies. The electric filter is constructed from a single-mode f iber and a single-mode fiber directional coupler and is excited by an RF modulated coherent optical source. By taking advantage of a unique interaction between RF and optical effects within the RDL cavity, we a re able to control the RF filter response by varying the optical param eters of the RDL. A matrix-based formulation is used to analyze the op tical and RF characteristics of birefringent RDL's and their interacti ons. Several interesting features are noted. For example, there is one resonance in each free spectral range of the RDL if the optical field is not amplitude-modulated and the RDL is not birefringent. In the pr esence of amplitude modulation and fiber birefringence, however, the a bove-mentioned resonance splits into four subresonances in each free s pectral range. The resonance conditions for these subresonances are co ntrollable by adjusting the phase delay and/or the birefringence delay . We also report the experimental confirmation of the results for RDL' s with birefringent single-mode fiber excited by amplitude-modulated c oherent fields. The interplay of the optical and RF responses of the b irefringent RDL has also been observed. The controllability of the coh erent RDL RF filter is verified. Specifically, we demonstrated that th e RF response of the system does indeed depend greatly upon the optica l response of the RDL filter. Altering the optical path length and/or the birefringence delay alters the RF response of the filter.