Polarization-Independent All-Fiber Acousto-Optic Tunable Filter Using Torsional Acoustic Wave

The intrinsic polarization dependence of an all-fiber tunable wavelength filter based on polarization coupling by torsional acoustic wave is eliminated by employing a fiber loop configuration. In the fiber loop, two orthogonal polarizations split from input light propagate the single acousto-optic interaction region in opposite directions before being recombined to produce a polarization-independent filter spectrum. The polarization-dependent loss was estimated to be less than 0.1 and 0.3 dB within passband in bandpass and notch type filters, respectively.