"Data Center Links Beyond 100 Gb/s per Wavelength"

Joseph M. Kahn, E. L. Ginzton Laboratory, Department of Electrical Engineering, Stanford University

August 24th (Thursday), 11:00am
Engineering Science Building (ESB), Room 2001

Increased traffic within and between data centers demands low-cost, low-power optical links with per-wavelength bit rates beyond those achievable using intensity modulation and direct detection. In this talk, we review spectrally efficient link designs based on direct detection, Stokes vector detection, coherent detection and differentially coherent detection. We show that poor spectral efficiency and optical power efficiency will inhibit the scaling of direct detection-based methods beyond 100 Gb/s. Stokes vector receivers can provide higher spectral efficiency without requiring a local oscillator laser, but require power-hungry analog-to-digital conversion (ADC) and digital signal processing (DSP). DSP-based coherent receivers developed for long-haul systems may be excessively complex and power-hungry for data center links. We propose low-power DSP-free coherent receivers for dual-polarization quadrature phase-shift keying. They avoid high-speed ADC and DSP, while achieving performance similar to their DSP-based counterparts in intra-data center links and dispersion-compensated inter-data center links. We propose a novel optical polarization de-multiplexing technique using three cascaded phase shifters driven by marker tone detection circuitry. We consider carrier recovery based on either optical or electrical phase-locked loops. We propose a novel multiplier-free phase detector based on XOR gates, which exhibits less than 0.5 dB power penalty relative to a conventional Costas loop phase detector. We describe efforts to demonstrate DSP-free coherent receivers in collaboration with industrial partners.

About Joseph M. Kahn:

photo of Joseph KahnJoseph M. Kahn received a Ph.D. in Physics from U.C. Berkeley in 1986. In 1987-1990, he was at AT&T Bell Laboratories. He demonstrated the first coherent (i.e., synchronous) optical detection using semiconductor lasers, and achieved record receiver sensitivities in multi-Gb/s fiber transmission systems. In 1990-2003, he was on the Electrical Engineering faculty at Berkeley. He demonstrated coherent detection of QPSK in 1992, and studied free-space optical communications, optical MEMS and MIMO wireless communication. In 2000, he and K.-P. Ho founded StrataLight Communications. Their 40 Gb/s transmission systems were deployed widely by AT&T, Deutsche Telekom, Comcast and other carriers. StrataLight was acquired by Opnext in 2009. Since 2003, Kahn has been a Professor of Electrical Engineering in the Ginzton Laboratory at Stanford University. His current research emphasizes low-power, high-capacity links for data centers; signal propagation, spatial multiplexing and imaging in multi-mode fibers; optimization of large-scale multi-wavelength networks; and free-space optical communications. Professor Kahn received the National Science Foundation Presidential Young Investigator Award in 1991. In 2000, he was elected a Fellow of the IEEE.

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