A novel bidirectional optical coupling module for subscribers

To overcome drawbacks and limitations of planar lightwave circuit based modules for bidirectional communications, such as the demand for several chips and in consequence more packaging efforts, we have recently developed a novel optical coupling technique using our unique 155 Mbps bidirectional laser chip. Since the chip is structured with a pin-photodiode monolithically integrated on a laser diode´s waveguide, the optical coupling requires only the alignment of the chip with a fiber. To optically couple the laser diode and photodiode simultaneously with a single fiber, we have designed an unusual coupling structure using a fiber having a cleaved surface whose normal is 35° angled to the fiber core axis, and using an index-controlling medium with a refractive index of ∼1.3. The bidirectional chip is flip-chip bonded and the fiber is passively aligned using a V-groove on the same substrate of 2.5×1.3 mm² in size. Even with this extremely small and simple scheme for bidirectional optical coupling, we could obtain an optical output power of -7∼-10 dBm and a responsivity of <-30 dBm, which are satisfactory to the STM-1 level telecommunications specifications.