25.1 Short-Reach Silicon Photonic Interconnects with Quantum Dot Mode Locked Laser Comb Sources

Replacing conventional electrical interconnects with optical counterparts at on-board and in-package length scales requires compact photonic integrated circuits (PICs) capable of scaling to aggregate capacities of 1Pbps with 0.1pJ/bit energy consumption at a range of ambient temperatures [1]. Photonic input-output can easily wavelength division multiplex (WDM) over enormous bandwidths without incurring any extra propagation loss; this allows staggering single line rates while using modest channel rates, which alleviates driver/receiver implementation. Silicon photonics is an attractive platform for this application due to its compact waveguide bends, its scalability and maturity, and its ability to serve as a substrate for other photonic materials that aid in the production of low-loss waveguides along with efficient photodetectors, modulators, and lasers [2]. Figure 25.1.1 illustrates the photonic architecture of a transceiver that serves as a milestone towards this goal. A 20-wavelength quantum dot mode-locked laser (QD-MLL) comb source has its outputs coupled to separate 1Tbps silicon photonic PICs. One technology, microring modulators (MRMs), has emerged from silicon photonics to produce extremely dense PICs. They are resonant based structures which can modulate a single wavelength out of summation of multiple wavelengths, which removes multiplexers from the system. Instead, the transmitter (TX) separates even and odd channels of the comb source to separate ring banks. Each bank has 10 MRM which operate at 26.4Gbps NRZ with a 5.6% forward error correction (FEC) overhead. The modulated odd and even comb lines are interleaved together, and a shallow 210MHz clock signal is encoded onto all channels for optical clock transmission via a variable optical attenuator (VOA). The 500G lanes then pass through quantum dot semiconductor optical amplifiers (QD-SOAs) before being polarization multiplexed onto a single 1T port for propagation over polarization-maintaining (PM) fiber. The receiver (RX) portion of the PIC polarization demultiplexes the two 500G lanes; a small fraction of the power of each channel is sent to a photodiode (PD) that detects the clock signal. The rest of the power is deinterleaved, and then each individual 25G channel is demultiplexed by a corresponding WDM ring resonator filter with a high-speed PD receiver on its drop port. The RF driver for the transceiver is flip chip bonded directly to the PIC to conserve energy. Additionally, the ring resonator filters in the PIC require circuitry to actively align themselves to the QD-MLL comb lines; 3D integration is used to reduce footprint.