Noise performance of a preamplifier for high-speed optical receiver front-ends

This paper reports on an optical receiver employing a p-i-n diode and a GaAs HBT monolithic microwave integrated circuit (MMIC) distributed preamplifier combination. The design is the first to have a photodiode mounted directly on the MMIC chip. The p-i-n preamplifier displays a measured average equivalent input noise current density of 24 pA/Hz. Good agreement is obtained between the predicted and measured noise performance. The monolithic eight-stage distributed amplifier is implemented using Nortels GaAs HBT (fT = 70 GHz) process, and makes use of a coplanar waveguide regime having a large input impedance optimized for noise performance and bandwidth. The p-i-n photodiode employed is an InGaAs vertically illuminated structure, also from Nortel. While the voltage gain of the amplifier displays a 3 dB bandwidth extending to nearly 40 GHz, the bandwidth of the complete optical receiver is found to be only 22 GHz. Packaging effects are believed to be responsible for this shrinkage.