Multi-Tb/s-per-Fiber Coherent Co-Packaged Optical Interfaces for Data Center Switches

Co-packaging of optical and electronic interfaces inside data center switches has been proposed to reduce the system-level power consumption and support higher density, higher capacity optical links. However, co-locating electronics and optics increases optical losses and exposes lasers, (de)multiplexers and other components to temperature fluctuations. We propose a design for co-packaged interfaces using coherent detection and wide-passband (de)multiplexers to increase the total bit rate per fiber in the presence of demultiplexer center frequency shifts up to ±150 GHz. We compare direct and coherent detection systems within this architecture and demonstrate that unamplified coherent links can provide over 20 dB higher loss budget than direct detection links at 400 Gb/s per fiber. Moreover, coherent links can offer far higher bit rates per fiber. For example, in the presence of (de)multiplexer center frequency shifts of up to ±150 GHz, DP-QPSK links can scale to over 5 Tb/s per fiber, while four pulse amplitude modulation (PAM) links are optical bandwidth limited to ~400 Gb/s per fiber. We quantify signal path loss and optical signal-to-noise ratio budgets when optical amplifiers, limited to eye-safe output powers, are used to compensate for losses of fibers or optical switches. While the loss and SNR budgets of direct detection links may be improved by optical amplifiers, these systems cannot scale to high bit rates per fiber because large channel spacings are required to accommodate (de)multiplexer center frequency shifts. For example, amplified 4-PAM links can scale to less than 1.3 Tb/s per fiber, while amplified DP-QPSK links can scale to greater than 12.8 Tb/s per fiber.