SEMICONDUCTOR WAVE-GUIDE DIRECTIONAL-COUPLERS FOR COHERENT RECEIVERS

Optical waveguide 3-dB couplers integrated on semiconductors have been studied, designed and fabricated, using both bulk and diluted multi-q uantum-well InGaAsP/InP-based materials, grown by MOCVD. The device st ructure is based on the two-mode interference (TMI) principle and is f ully compatible for integration with the optoelectronic and electronic components of a coherent receiver. Bulk material couplers provide an output balanced within 0.05 dB per nanometer and an excess loss of 1.4 dB, compared to a straight guide, while coupling loss to a tapered-le nsed single mode fibre is 4 dB. Improved coupling efficiency to single -mode fibres is achieved by use of moderately diluted multi-quantum-we ll waveguides, which include InGaAsP wells and InP barriers: coupling loss to a tapered-lensed single-mode fibre as low as 0.5 dB and excess loss of 1.8 dB are featured. Couplers fabricated with this waveguide structure have a balance sensitivity of 0.03 to 0.04 dB per nanometer. A moderately diluted multi-quantum-well 3-dB coupler has been permane ntly pigtailed and butt-coupled to a dual balanced PIN photoreceiver. This hybrid assembly was tested in a coherent transmission system at 1 55 and 622 Mbit s-1 showing sensitivities, for 10(-9) BER, of about -3 8.0 dBm and -28.8 dBm, respectively.