A waveguide photodiode that can be used in 1.55-µm-band access receivers was designed and fabricated. We simulated the optical coupling characteristics, focusing especially on those related to the input spot size and for waveguide-layer structure. The results clearly showed that both high efficiency and high tolerance can be obtained in an optimized symmetric structure used with a hemispherically ended fiber. The fabricated device, optimized on the basis of the simulation, showed excellent characteristics such as an external quantum efficiency of more than 95% and a coupling tolerance of 6.5 µm in the vertical direction.

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