An optical input/output (I/O) device is provided. The device includes a substrate including an upper trench; a waveguide disposed within the upper trench of the substrate; a photodetector disposed within the upper trench of the substrate and comprising a first end surface optically connected to an end surface of the waveguide; and a light-transmitting insulating layer interposed between the end surface of the waveguide and the first end surface of the photodetector.