Polarization independent (<0.01 dB) switching is achieved at 1.55 \mum in a strongly guiding, fused tapered fiber null coupler acoustooptic switch. An intermediate degree of fusion of the dissimilarly sized fibers is shown experimentally to have the form birefringence required to render the mode-splitting coupler polarization insensitive. The effect of the degree of fiber fusion on the acoustic wavelength of flexural waves is studied theoretically and experimentally, and precise control over the degree of fiber fusion is obtained for coupler waist cross-sectional dimensions as small as 6 \mum.
Selina G. Farwell, Michalis N. Zervas, and Richard I. Laming, "22 Fused Fiber Null Couplers with Asymmetric Waist Cross Sections for Polarization Independent (<0.01 dB) Switching," J. Lightwave Technol. 16, 1671- (1998)