Low excess loss conditions of polarization-maintaining fiber couplers.

Loss characteristics and field intensity distributions of fused polarization-maintaining fiber couplers are measured. From the results, it is shown that excess loss spectra have a sinusoidal response. Further, the loss peaks of the usual polarization-maintaining and absorption-reducing (PANDA) fiber couplers with indexdepressed stress applying parts are larger than those of PANDA couplers with index-matched stress applying parts. It is also confirmed that low excess loss of <0.1 dB can be obtained for a large waist diameter of the fused region even in conventional PANDA fiber couplers.