Low Loss Fiber Optic Directional Coupler

Four port single mode fiber optic directional couplers have been fabricated exhibiting coupler losses approaching 0.1% (.005 dB). A new experimental technique with an integrating sphere was used to measure these low losses. The couplers, which can be tuned for any coupling ratio, have coupled as much as 99.8% of the total input power in one fiber to another fiber. In this high coupling, low loss regime new polarization dependent effects were observed in the couplers.