Design of high performance dual detector GaAs IC receivers for coherent optical detection

Coherent optical communications systems can derive several benefits from using balanced dual detector receivers: local oscillator power is conserved, and excess intensity noise cancelled. The use of a custom GaAs IC transimpedance preamplifier to replace a previously used hybrid preamplifier can achieve the following advantages: higher gain within the receiver package with substantially improved gain flatness; greater phase linearity; superior noise performance through reduced device parasitics; and lower cost with greater reproducibility. To achieve high cancellation of local oscillator density noise with a balanced photodiode structure it is necessary to accurately match the coupled quantum efficiency of each photodiode and the fibre lengths between the fibre directional coupler and photodiodes. The authors investigate the interaction of these two parameters for heterodyne receivers at 622 Mbit/s and 2.5 Gbit/s and present results for an engineered solution of the coupling optics