Effect of twist in 3 x 3 fused tapered couplers.
Equilateral 3 x 3 tapered couplers made from single-mode fiber are found to behave in a less-than-ideal manner. This behavior is due to twist and can be corrected by twisting the coupler after its fabrication. A small amount of twist causes only a redistribution of output power between the two nonlaunch fibers, whereas a larger twist can restore the original response but with a cyclic change of fiber identity. An analysis of local mode coupling between the two degenerate LP(11) modes in the waist of the coupler leads to equations that successfully account for the twist-tuning effect.
Birks
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