NARROW-BAND ACOUSTOOPTIC TUNABLE FILTERING IN A 2-MODE FIBER

We demonstrate an optical filter that utilizes acousto-optic coupling between the spatial modes that propagate in a two-mode elliptical-core fiber. The optical bandwidth at 1090 nm is 0.85 nm, which is in agree ment with predictions based on measurement of differential group delay between the modes. The filter is slightly polarization dependent, wit h a 0.28-nm wavelength difference between the passband peaks for the t wo polarization eigenstates. The optical insertion loss is dominated b y 2-4-dB bending loss in nonoptimized mode strippers, and the coupling loss is negligible. Full acousto-optic mode conversion was achieved a t an 8-mW electrical input power to the acoustic transducer.