All-fiber acoustooptic filter with low-polarization sensitivity and no frequency shift

An acoustooptic tunable filter based on an all-fiber null coupler, exhibiting low polarization sensitivity and zero net frequency shift, is demonstrated. The polarization dependence has been eliminated by simultaneously applying two acoustic waves that independently satisfy the phase-matching condition for the two eigenpolarization states. A simple double-pass arrangement is employed to exactly cancel the net frequency shift and to provide an enhanced spectral sidelobe suppression of better than -17 dB.