We investigate in detail the origin of multiple resonance peaks
experimentally observed in an all-fiber acousto-optic tunable filter built
with a photonic crystal fiber having slightly deformed air-hole structure and
non-circular outer cladding. A model for the acousto-optic mode coupling in
the PCF is formulated taking into account both the acoustic and the optical
birefringence. All experimental results are in good agreement with
numerical calculations.