Low-loss all-fiber acousto-optic tunable filter.

We report significant advances in the development of simple all-fiber, tunable acousto-optic spectral filters based on null couplers. The performance agrees well with theoretical predictions and is attributed to improved control of the coupler uniformity. Furthermore, using a double-pass arrangement, we demonstrate filter bandwidth reduction and improved sidelobe suppression of -20 dB . The double-pass configuration is also shown to double the frequency shifts that are obtainable from such devices.