Data center links beyond 100 Gbit/s per wavelength

Increased traffic demands within and between data centers now necessitate low-cost and low-power systems with per-wavelength bit rates beyond what can be easily achieved using conventional on/off keying. We review spectrally efficient links based on direct detection, Stokes vector detection, coherent detection and differentially coherent detection for data center applications. We show that limited spectral efficiency and power margin will inhibit scaling of direct detection-compatible formats beyond 100 Gbit/s. Stokes vector receivers can provide higher spectral efficiency without requiring a local oscillator laser, but require power-hungry analog-to-digital converters (ADCs) and digital signal processing (DSP). Similarly, existing DSP-based coherent systems designed for long-haul transmission may be excessively complex and power-hungry for short-reach data center links. We present low-power DSP-free coherent and differentially coherent alternatives that avoid high-speed ADCs and DSP and achieve similar performance to their DSP-based counterparts in intra-data center links and dispersioncompensated inter-data center links.