Polarization-coupling all-fiber acousto-optic tunable filter insensitive to fiber bend and physical contact.

We show that an all-fiber acousto-optic tunable filter based on polarization mode coupling using torsional acoustic wave is immune to the fiber bend and physical contact in the acousto-optic interaction region. We also propose and demonstrate a novel strain-free and size-reduced tunable filter with a 4-m-long fiber acousto-optic interaction region looped into a 5- cm-diameter coil.