20.1 Silicon photonics and on-chip optical interconnects

Silicon photonics combines optical and electronic components on a single chip, revolutionizing data communication. This game-changing tech uses silicon-on-insulator platforms to create compact, high-performance devices like waveguides, modulators, and photodetectors.

Photonic integrated circuits (PICs) take it further, packing multiple components onto one chip. This shrinks size, power use, and cost. Plus, silicon photonics plays nice with standard manufacturing processes, making it a promising solution for future computing and communication needs.

Silicon Photonics Components

Silicon-on-Insulator (SOI) Platform

• Silicon-on-insulator (SOI) consists of a thin layer of silicon on top of an insulating layer (typically silicon dioxide) on a silicon substrate
• SOI provides a high refractive index contrast between the silicon layer and the insulator, enabling tight confinement of light
• Enables the fabrication of compact and high-performance photonic devices
• SOI wafers are commercially available and compatible with standard CMOS manufacturing processes (300 mm wafers)

Waveguides and Optical Modulators

• Silicon waveguides guide light on the SOI platform with low loss and tight bends
• Typical waveguide dimensions are around 220 nm thick and 500 nm wide for single-mode operation at telecom wavelengths (1550 nm)
• Optical modulators control the phase, amplitude, or polarization of light in the waveguide
• Modulators can be based on the plasma dispersion effect, where the refractive index of silicon changes with the concentration of free carriers (electrons and holes)
• Examples of silicon modulators include Mach-Zehnder interferometers (MZIs) and microring resonators

Photodetectors and Optical Multiplexing

• Photodetectors convert optical signals into electrical currents
• Germanium (Ge) is often integrated on SOI for photodetection due to its strong absorption at telecom wavelengths
• Ge photodetectors can be fabricated using epitaxial growth or wafer bonding techniques
• Optical multiplexing combines multiple optical signals into a single waveguide using wavelength-division multiplexing (WDM) or mode-division multiplexing (MDM)
• WDM uses different wavelengths for each signal, while MDM uses different spatial modes in a multimode waveguide

Photonic Integrated Circuits

Integration and Fabrication

• Photonic integrated circuits (PICs) combine multiple photonic components on a single chip
• PICs enable complex optical systems with reduced size, power consumption, and cost compared to discrete components
• Silicon photonics PICs are fabricated using standard CMOS manufacturing processes, leveraging the existing infrastructure and economies of scale of the electronics industry
• Monolithic integration allows the integration of photonic and electronic components on the same chip

Optical Transceivers and CMOS Compatibility

• Optical transceivers convert electrical signals to optical signals (transmitters) and vice versa (receivers)
• Silicon photonics enables the integration of high-speed optical transceivers for data communication applications (data centers, high-performance computing)
• Transceivers typically include lasers, modulators, photodetectors, and driver/receiver electronics
• CMOS compatibility allows the integration of photonic devices with advanced electronic circuits (28 nm, 14 nm nodes) for improved performance and functionality

On-Chip Optical Communication

Optical Interconnects

• Optical interconnects use light for data communication between different parts of a chip or between chips
• On-chip optical interconnects can overcome the bandwidth and power limitations of electrical interconnects at high data rates (>10 Gbps)
• Optical interconnects consist of silicon waveguides, modulators, and photodetectors integrated with electronic circuits
• Examples include chip-to-chip interconnects, memory interfaces, and network-on-chip architectures

Optical Routing and Switching

• Optical routing directs light signals between different endpoints on a chip or between chips
• Optical switches control the path of light based on external control signals
• Examples of optical switches include MZI switches, microring switches, and MEMS-based switches
• Optical routing can be used for reconfigurable interconnects, programmable photonic circuits, and optical circuit switching in data centers