Analysis and optimization of all-fiber polarization transformers

The influence of the spin rate profiles on the stability of polarization transformations of all-fiber polarization transformers (AFPTs), which behave as a broad-band all-fiber quarter-wave plate, is analyzed by the super-mode theory. Huang firstly proposed and demonstrated these AFPTs, and suggested qualitatively that to perform a stable polarization transformation, the variation of the spin rate from zero to fast of the AFPT should be slow. However a compact fiber structure is desirable for an optical current transformers (OCT) system, and bending or wrapping AFPTs will weaken their abilities to hold the output SOPs. A first-order analytical approximation describing the coupling between the two local eigen-modes in the AFPT is derived, and two more detail suggestions for the spin rate profiles design of AFPTs are proposed. A random orthogonal axial gradient method (ROAGM) is developed to optimize the spin rate profiles. Numerical simulations show that for an AFPT with a shorter length, the stable transformation can also be obtained by a carefully designed spin rate profile according the two suggestions.