Scalable data center communications : co-packaged optics and electro-optic frequency combs
Keywords: Electrical Engineering.

Optical communication links are the modern workhorse of the global internet. They can span tens of thousands of kilometers, such as in submarine long-haul cables, or just meters, as in many data center networks. Ever-increasing demand, as well as the rise of cloud computing, has placed significant strain on data centers and the optical networks that enable them. As global internet traffic doubles every two or three years, data center networks are required to be increasingly more efficient to keep up. These challenges have motivated a new class of solutions for data center switching that buck the traditional pluggable transceiver model by co-locating optics and electronics. This dissertation focuses on two distinct, yet related, topics in data center optical communications. In the first part, we analyze a new method of increasing network switching efficiency: co-packaging of optics and electronics on the same substrate. In the second part of this dissertation, we focus on an enabling optical source for data center communications: the resonator-enhanced electro-optic frequency comb generator. In Chapter 2, we analyze the requirements and challenges associated with co-packaging optics and electronics close to data center switches. We show that the high-loss, high-temperature environment is not suitable for traditional optical links and then propose a wavelength-division multiplexed temperature-independent link architecture compatible with co-packaging that uses coherent detection to overcome high loss. We demonstrate that coherent links, possibly using amplification to enable future technologies like optical switching, can scale easily past 10 Tb/s per fiber, while direct detection links, the current standard in data center communications, cannot scale past 1-2 Tb/s per fiber. We then experimentally validate the proposed link architecture by demonstrating an optical frequency comb-based 5.6 Tb/s coherent link that can tolerate temperature fluctuations of over 15 degrees Celsius. In Chapter 3, we investigate the light sources necessary to enable links between co-packaged optical interfaces near data center switches. The high-temperature, high-loss environment of co-packaging can reduce the performance and reliability of light sources. Thus, we propose several photonic architectures that rely on external and integrated light sources that can either be integrated near the switch or located externally and coupled onto the photonic circuit. We then analyze the link performance using modulation formats compatible with both direct and coherent detection to optimize these architectures for performance, reliability, and efficiency. We demonstrate that links based on direct detection may be able to scale to 51.2 Tb/s or 102.4 Tb/s switching, which is expected to be implemented in the next 5 years, but coherent links have link budgets 13-25 dB higher and may scale to switching bandwidths far beyond 102.4 Tb/s. In Chapter 4, we shift focus to multi-wavelength source technologies; in particular, we analyze optical frequency combs generated by electro-optic modulation. Electro-optic modulation, which results in comb formation, can be enhanced by the use of an optical resonator. We derive analytical expressions for the output comb power and noise properties and demonstrate high tolerance to detuning of the input laser frequency and the electrical modulation frequency. Working closely with the Laboratory for Nanoscale Optics at Harvard University, we fabricate a resonator-enhanced electro-optic frequency comb generator on a thin-film lithium niobate platform and demonstrate record span and flatness due to the low-loss integration. We experimentally demonstrate the flexibility of this comb generator platform by generating frequency combs of arbitrary width and combs driven by multiple modulation frequencies. In Chapter 5, we address one of the fundamental issues with single-resonator comb generators: low conversion efficiency. To understand the intra-resonator comb generation dynamics, we develop two methods of analyzing the comb generator output in the presence of frequency-dependent propagation, such as dispersion. We then propose a novel dual-resonator electro-optic comb generator and apply the previously developed models to predict the output power. We show that by optimizing the resonator length and coupling strength, comb generation with conversion efficiencies of over 60% are possible. We then present recent experimental results demonstrating the first integrated dual-resonator electro-optic frequency comb generator, with a conversion efficiency of 12%, a 40-fold improvement over comparable single-resonator generators. Finally, we analyze an inter-data center communications link based on these comb generators and demonstrate that the addition of another resonator can improve link optical signal-to-noise ratios by over 7 dB, enabling optical links with bandwidths over 20 Tb/s