Analysis of mode coupling behavior in fused polarization-maintaining fiber couplers.

To achieve low excess loss fiber couplers, analysis on mode coupling behavior in fused fiber couplers has been carried out using a combination of a scalar wave finite element method and a step approximation method, with an emphasis on clarifying the excess loss mechanism in fused polarization-maintaining (PANDA) fiber couplers. Numerical analysis has indicated the following results: First, fused PANDA fiber couplers have a critical outer diameter in the biconical taper region,, at which mode couplings between the symmetric fundamental mode and the symmetric next higher-order mode take place. Second, PANDA fiber couplers in which the waist diameter in the biconical taper region is larger than the critical diameter have small excess loss, because the large waist diameter provides the small mode coupling, although too large a waist diameter does not allow power splitting as a coupler. This analysis shows that excess loss in PANDA fiber couplers largely depends on the outer diameter dimension of the biconical taper region and gives an optimal value of such diameter. This is in good agreement with the experimental results.