As demand for internet applications, cloud computation, and AI continue to grow, data center networks must scale to support increased traffic flow. While current intensity modulation links are an economical solution today, coherent links are an increasingly attractive option to improve performance. Conventional coherent link technologies that are used today in >40km transmission are based on power-hungry and expensive digital signal processing, and cannot be easily scaled to the high volumes required for data centers. Coherent links must be optimized for low power consumption and cost for reaches <2 km to make them viable for intra-data center networks. The enlarged link loss budgets from such links will also enable optical switching networks, which can dynamically reconfigure the network for improved server utilization, leading to vast improvements in overall data center energy efficiency.

The focus of this work is to develop energy-efficient coherent links with large link budgets for intra-data center networks. Design methodologies and architectural trade-offs for short-reach coherent link optimization will be presented. Using these techniques, custom electronic and photonic integrated circuits for coherent transmitters and receivers were designed, fabricated, assembled, and characterized. The first-ever full coherent link operating in the O-band was demonstrated with 56 Gbaud QPSK transmission. An O-band coherent link with custom packaged integrated circuits for both the transmitter and receiver was also demonstrated with 224 Gbps DP-QPSK transmission below the HD-FEC threshold with <10 pJ/bit power consumption expected for circuits including integrated optical gain. These results shows that optimized O-band coherent links can support high data rates and link budgets with attractive energy efficiency for future intra-data center networks.

Wavelength-selective switches have been proposed for datacenter use to enhance datacenter scalability and to aid in meeting ever-increasing traffic demands and the resulting energy consumption. In silicon photonics, photonic integrated circuit (PIC) designers can take advantage of the high contrast between silicon and silicon dioxide---the latter of which acts as the cladding for the silicon nanowire waveguides---to design compact microring resonators with large free spectral ranges (FSRs). Furthermore, as commercial silicon photonics foundry offerings become more widely available, the ability to produce larger and more complicated PICs has become easier, as well as providing a clearer path towards large-scale manufacturability and adoption of such technologies. Thus, ring-based wavelength-selective switches are a particularly well-suited application for silicon photonics. Another major design consideration for PICs is low energy consumption.

A discussion of simulation results from a model for next generation energy efficient photonic links for data centers motivates and the two wavelength-selective switch designs that are presented in this thesis. The first design is an NxN crossbar switch with L ring pairs to route up to L wavelengths at each cross-point. The second design is an NxN ring-assisted Mach-Zehnder interferometer (RAMZI) switch with L ring pairs per switch element. In both designs, multiple ring pairs of differently sized rings were utilized to partition the FSR such that any one ring pair does not have to move far in the spectrum to complete the switching, thus saving in the power consumption.

Growing demands in high data capacity and low energy consumption have driven the development of high-performance optical interconnects for many commercial applications, such as links for long-haul, intra-/inter-datacenter, and 5G communication. Typically, the photonic devices used in these environments are optimized for operation at or above room temperature, however there is an existing and growing need for optimized photonic devices to operate in cryogenic and/or high-radiation environments. Applications of these optical interconnects range from control and readout from superconducting integrated circuits for quantum computing, to readout of tracking detectors in high-energy physics (HEP) particle accelerators, to readout of next-generation infrared (IR) focal plane array (FPA) detectors. Key to the success of these optical interconnects is the high-performance and ruggedization of the electro-optic modulator (EOM), typically implemented either as a remoted external device or as a directly modulated light source. This PhD dissertation addresses the challenges of optical interconnects for harsh environments in the following manner: (1) a novel and wavelength division multiplexed (WDM) scalable optical interconnect architecture has been developed—whereby the remoted EOM has been implemented as a microring resonator—and reduces both system complexity and energy consumption, (2) a high-speed optical link operating at cryogenic temperatures has been demonstrated—utilizing a silicon photonic based EOM—in addition to laser wavelength locking of the microring resonator, and (3) a semiconductor physics based model of the EOM has been developed to address the temperature dependent effects of the silicon p-n junction embedded in the microring resonator—based on the free-carrier plasma dispersion effect in doped silicon—to aid in the device design and optical interconnect link optimization for various harsh environment applications. Lastly, an outlook and suggested areas for future research—in the area of optimized silicon photonic devices for harsh environments—is given.

Data centers today account for about 2.5 % of the total electricity usage in the United States. The portion of that power that is consumed by the internal network is increasing dramatically, as server-to-server traffic grows over 23 % every year. As aresult, there is great demand for scalable power-efficient intra data center (IDC) optical communication links. IDC links today are based on intensity-modulation direct-detection modulation formats (IMDD). Thus far, the scaling of IMDD links has been achieved by increasing the number of signal amplitude levels, by moving to higher baud rates, and by adding multiple wavelengths or fibers. However, each of these scaling paths is severely constrained. Whereas IMDD systems only utilize the intensity of signal, coherent detection (CD) also leverages the two polarizations and two quadratures of the fiber optic channel, and thus is more scalable. CD also improves sensitivity by up to 20 dB by mixing the received signal with a strong local oscillator. CD has been employed in long-haul links for many years, yet these systems rely on power-hungry high-speed analog-to-digital converters (ADC) and digital signal processing (DSP) and thus may be unsuitable for IDC links. A compelling alternative to DSP-based CD is analog coherent detection (ACD), in which polarization demultiplexing, carrier recovery, and phase demodulation are performed using analog circuits. This work focuses on the co-design of analog circuits and opto-electric devices that enable low-power ACD-based IDC links. First, a discussion of a proposed next-generation ACD IDC link architecture is outlined as motivation for the low-power circuit designs. Three monolithic 50 Gb/s opto-electric receiver topologies in SiGe BiCMOS technology are analyzed and compared. Then, a 100 Gb/s QPSK hybrid receiver in 45 nm CMOS is described. The co-design of a driver 56 Gb/s and traveling-wave Mach-Zehnder modulator (TW-MZM) is also presented. Finally, the thesis describes a TW-MZM equalization technique based on a tunable termination mismatch.

High bandwidth optical links are required to support the continual increases in the demand for high-resolution video, high-performance computing, machine learning, cloud computing, the Internet of Things (IoT), 5G networks, and other applications. In particular, the bandwidth requirements for short reach inter- and intra-data center optical links are reaching the capability limits of the industry standard intensity modulation, direct detection (IMDD) links. Coherent modulation, while more complex for a single optical link, can operate at higher data rates and spectral efficiency by encoding multiple bits per symbol. The high sensitivity of coherent links also enables data centers to replace electrical switches with optical switches, reducing overall power consumption and hardware cost. O-band coherent receivers (RX) on indium phosphide (InP) and silicon photonics (SiP) platforms will be discussed in this dissertation. These receivers were designed for compatibility with an analog coherent link architecture using an optical phase-locked loop, but they can also be used in a traditional coherent link. Tradeoffs in design, performance, and manufacturability between material platforms will be discussed. The design, fabrication, and measurements from two generations of InP O-band coherent RX PICs will be described. Two generations of O-band coherent RX PICs fabricated using Intel’s SiP process (with and without integrated lasers) and a dual-mode coherent RX using Global Foundries 45SPCLO will also be shown. A sampled-grating distributed Bragg reflector (SG-DBR) laser designed for O-band coherent RXs using Intel’s SiP process will be presented. To demonstrate the RX PICs, they are packaged with high-speed electronic integrate circuits (EICs) and measured with multiple coherent modulation formats using digital signal processing (DSP) or analog electronics for data recovery. Architectures using reduced DSP or analog electronics both show promise for energy efficient, short reach coherent links.