Low-Power (1.5 pJ/b) Silicon Integrated 106 Gb/s PAM-4 Optical Transmitter

As next-generation data center optical interconnects aim for 0.8 Tb/s or 1.6 Tb/s, serial rates up to 106 GBd are expected. However doubling the bandwidth of current 53 GBd 4-level pulse-amplitude modulation (PAM-4) transmitters is very challenging, and perhaps unfeasible with compact, non-travelling wave, lumped modulators or lasers. In our previous work, we presented an integrated 4:1 optical serializer with electro-absorption modulators (EAMs) in each path. Transmitter (TX) functionality was shown up to 104 GBd non-return-to-zero (NRZ) On-Off Keying (OOK) or PAM-4. However the performance for PAM-4 was limited by the distortion introduced by the EAM non-linearity. We also presented a real-time, DSP-free 128 Gb/s PAM-4 link with a silicon photonic transmitter using binary driven EAMs in a Mach-Zehnder interferometer (MZI) configuration. By combining two of such half-rate (53 GBd) transmitters in an integrated 2:1-serializer, improved 106 GBd PAM-4 performance is expected without needing to compensate the inherent modulator non-linearity and without requiring faster modulators or drivers. In this paper, we present a Silicon integrated 53 GBd PAM-4 TX as a candidate for integration into 106 GBd PAM-4 2:1 serialized TX. The presented TX consists of two EAMs in an MZI configuration, wirebonded to a low-power 55 nm 4-channel SiGe BiCMOS driver, operating at 1.5 pJ/b (excluding laser). With a reference receiver (RX), transmission at or below the KP4-FEC threshold is shown beyond 1km standard single-mode fiber (SSMF) and up to 2 km non-zero dispersion-shifted fiber (NZ-DSF) at 1565 nm. Furthermore, the integrated TX was combined with an Si-integrated RX consisting of the same EAM component, wirebonded to a 55 nm SiGe BiCMOS transimpedance amplifier (TIA). Both TX and RX were wirebonded on an RF-PCB, with electrical connectivity through transmission lines and 6-inch 50 GHz multi-coax cable connectors. With this electrically connectorized all-EAM TX and RX, PAM-4 link operation is shown up to 40 GBd at 3.9 pJ/b (excluding laser), without using DSP or equalization.