Real-Time Demonstration of Homodyne Coherent Bidirectional Transmission for Next-Generation Data Center Interconnects

We experimentally propose and demonstrate homodyne coherent detection in a short-distance data center interconnect with bidirectional fiber transmission. The transmitter in the proposed system sends the modulated signal and a continuous wave (CW) tone originating from the same laser diode (LD) over the 2 lanes of a duplex fiber to the remote receiver for coherent detection. Thus, this system allows the use of an uncooled LD with a large linewidth and reduces the complexity of digital signal processing (DSP) compared to that in a classical coherent system. A successful real-time demonstration with 600-Gb/s DP-64QAM using uncooled large-linewidth DFB lasers is conducted. Automatic stabilization against polarization fluctuations of the transmitted tone is achieved by the proposed polarization-tracking integrated coherent receiver in compact silicon photonics (SiP), tracking up to 300 rad/s without a performance penalty. The phase noise caused by mismatch and the link loss budget limitations are discussed and analyzed. This study shows that the proposed system is a potentially attractive solution for future 800G and 1.6T intra-data-center optical interconnects.