Dual-Polarization Optical Costas Loop for DSP-Free Homodyne Short-Reach Links

Here we report a dual-polarization, O-band optical phase-locked loop packaged in a footprint compatible with pluggable optics. Custom-designed analog Costas loop electronics and silicon photonic circuits, including a hybrid-integrated tunable laser serving as the local oscillator, were integrated into a functional receiver subsystem that was characterized as part of a full coherent link. The receiver frequency and phase locked loop had a bandwidth of 450 MHz and was capable of locking to frequency offsets $< $8 GHz and tracking the local oscillator's $\pm$1.5 GHz frequency drift, enabling DSP-free carrier-recovery. The entire homodyne coherent link achieved a maximum QPSK transmission rate of 112 Gb/s (56 Gbaud) for single polarization and 30 Gb/s (15 Gbaud) for dual-polarization operation. Bit error rates below hard decision forward error correction thresholds of $3.8\cdot 10^{-3}$ were achieved with 4.8 pJ/bit of transmitter and 8.8 pJ/bit of receiver power consumption, demonstrating the potential for a DSP-free solution for coherent links below 10km as an alternative to current IMDD schemes.