Abstract
Based on the analysis on the variable coupled-mode equation, we proposed an optimal profile of variably spun rate Q(z) = 0.5tan{arcsin[(z/L)sin(arctan2Q(L)]} to get better performance for all-fiber polarization transformer (AFPT), where Q(z) is normalized spun rate by beat length LB along the fiber length L. Then, in accordance with the intrinsic structure of AFPT, and in consideration of considering the phase-difference effect, we calculated the Jones vector from point to point along fiber to simulate the evolution of state of polarization (SOP) in AFPT with this optimal spun rate profile. For comparison, calculations are also made with other two kinds of spun rate profile (linear, cosine) often used in the AFPT fabricating process. The Influence of spun rate variation profile on the polarization transforming performance of AFPT is discussed and compared in detail. It is found that both in the case of linear-in/circular-out where the extinction ratio of output light is required as small as possible and in the case of circular-out/linear-in where the extinction ratio of output light is required as large as possible, the AFPT with optimal spun-rate profile presents best performance, and the linear spun-rate profile is the worst. The analysis and calculation results have referential values to the design, fabrication and evaluation of AFPT samples.
