(Invited) Performance and Reliability of a 25Gb/s Ge Waveguide Photodetector Integrated in a CMOS Process

The increasing demand for high data rate (>10Gbps) in LANs at distances spanning ~1 to ~1000m, and the intrinsic limitations of electrical interconnects in that range, have generated a large interest in optical active cables. These cables simplify the building of the interconnect fabric by offering a standard electrical interface, while providing the huge bandwidth and long reach of optics. Today, two technology approaches are competing in the active cable business: one is based on direct modulation of VCSELs operating at ~850nm and uses multimode fibers, while the second uses the third window of fibers (~1550nm) and is based on the monolithic integration of the main optical functions (modulation, splitting, detection) in a CMOS chip, along with the electronics. In this Author’s view, the second approach has several advantages, including: much longer reach, better signal integrity in the optical domain, and a potential for reducing the cost and increasing reliability by leveraging mature CMOS processes and by sharing a single light source across several channels. LuxG, Luxtera’s process integrating electronics and photonics on the same Si chip, is the world’s first incarnation of the second approach into an actual manufacturable and production-worthy CMOS technology (Freescale hip7_SOI). Luxtera has recently commercialized an optical active cable based on LuxG that transmits bidirectional data at 10Gbps on four parallel channels for an aggregate bandwidth of 80Gbps [OFC2010]. A key component of Luxtera’s cable is the high-speed receiver. This is based on an integrated waveguide Ge photodetector and a high impedance amplifier sharing the same Si substrate. The minuscule capacitance of the photodetector (due to the adopted waveguide configuration) and small parasitics allowed the design of an optimized amplifier resulting in a record sensitivity of ~-24dBm at 10Gbps, about 6dB better than typical discrete solutions at similar wavelengths and speed [ISSCC2010]. While the Ge waveguide photodetector is today used in a 10Gbps product, its speed makes it ready to be used in next generation optical transceivers operating at 25Gbps. In this paper, the performance and reliability of LuxG Ge waveguide photodetector will be reported and discussed.