Optical fiber recirculating delay lines as controllable radio frequency notch filters

The authors analyze and demonstrate the use of optical fiber recirculating delay lines (RDLs) as electric filters at radio frequency (RF) frequencies. The electric filter is constructed from a single-mode fiber 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, it is possible to control the RF filter response by varying the optical parameters of the RDL. A matrix-based formulation is used to analyze the optical and RF characteristics pc birefringent RDLs and their interactions. 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 presence of amplitude modulation and fiber birefringence, however, the above-mentioned resonance splits into four subresonances in each free spectral range. The resonance conditions for these subresonances are controllable by adjusting the phase delay and/or the birefringence delay. Experimental confirmation of the results for RDLs with birefringent single-mode fiber excited by amplitude-modulated coherent fields is reported. The interplay of the optical and RF responses of the birefringent RDL has also been observed. The controllability of the coherent RDL RF filter is verified. It is demonstrated that the RF response of the system does indeed depend greatly upon the optical response of the RDL filter. Altering the optical path length and/or the birefringence delay alters the RF response of the filter.>