In today's digital age, demand for data processing and storage is increasing at an unprecedented rate. This spike is primarily due to the growth of emerging technologies such as cloud computing, big data analytics, artificial intelligence, and the Internet of Things (IoT). Hyperscale data centres, which are massive facilities housing thousands of servers, have emerged as the digital infrastructure's backbone. Significant advances in optical communication are required for data centres to scale computation and storage services in order to serve future bandwidth-intensive and compute-intensive applications.
What is Silicon Photonics?
Silicon Photonics is a cutting-edge technology that combines optical components including lasers, modulators, detectors, and waveguides with silicon-based semiconductor chips. It allows for the seamless integration of photonic and electronic functionalities on the same chip, resulting in substantial advances in data communication and processing. Silicon photonics has gained popularity because to its economic efficiency, high integration density, and energy efficiency.
Key components of Silicon Photonics
- Waveguides: Waveguides are key components in Silicon Photonics that guide and restrict light along a certain path. These structures are often built on a silicon substrate with materials like silicon dioxide (SiO2) or silicon nitride (Si3N4). It can be built to enable a variety of light propagation modes, including single-mode and multi-mode, depending on the application requirements. They are essential for routing optical signals between photonic components like modulators, detectors, and multiplexers/demultiplexers.
- Modulators: Modulators are devices used to modulate the properties of light, such as its intensity, phase, or polarization. In Silicon Photonics, modulators are often based on the electro-optic effect, where the refractive index of a material is modulated in response to an applied electrical signal. This modulation allows for the encoding of data onto optical signals, enabling high-speed communication and signal processing. Silicon Mach-Zehnder modulators (MZMs) and phase modulators are commonly used in Silicon Photonics for various applications, including optical interconnects and data transmission.
- Detectors: Detectors are components that detect and convert optical signals to electrical signals. Silicon Photonics detectors are often made from semiconductor materials such as germanium (Ge) or indium phosphide (InP), which are compatible with silicon processing processes. Photodetectors, such as p-i-n diodes or avalanche photodiodes (APDs), are widely employed for high-sensitivity and efficient optical signal detection. These detectors are critical for receiving and processing optical data in a variety of applications, including optical communication systems, sensing and imaging.
- Lasers: Lasers are sources of coherent light that produce optical radiation via stimulated emission. Lasers in Silicon Photonics are frequently made from compound semiconductor materials like indium phosphide (InP) or gallium arsenide (GaAs), which are integrated onto a silicon substrate. Silicon Photonics systems can incorporate a variety of lasers, including as distributed feedback (DFB) lasers, vertical-cavity surface-emitting lasers (VCSELs), and ring lasers, to provide reliable and efficient light sources for optical communication, sensing, and signal production.
- Multiplexers/Demultiplexers: Multiplexers and demultiplexers are components that combine or separate multiple optical signals of various wavelengths. These devices support wavelength division multiplexing (WDM), a method that allows numerous data streams to be delivered over a single optical cable at various wavelengths. Multiplexers and demultiplexers in Silicon Photonics are often built around wavelength-selective filters like arrays waveguide gratings (AWGs) or Mach-Zehnder interferometers (MZIs), which can efficiently and accurately route and separate optical signals. WDM technology increases the capacity and bandwidth of optical communication networks, allowing for higher data throughput and scalability.
Characteristics of Silicon Photonics
Silicon Photonics possesses several key characteristics that make it an attractive technology for various applications:
Integration: One of the primary advantages of Silicon Photonics is its compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication processes. This compatibility enables the integration of photonic and electronic components on the same silicon substrate, allowing for highly integrated systems-on-chip (SoCs). This integration can lead to smaller form factors, reduced power consumption, and lower manufacturing costs.
High-Speed Data Transmission: Silicon Photonics enables high-speed data transmission over optical fibers. By leveraging the inherent properties of light, such as its high bandwidth and low latency, Silicon Photonics can support data rates ranging from gigabits to terabits per second. This makes it well-suited for applications requiring high-speed communication, such as data centers, telecommunications networks, and high-performance computing systems.
Low Power Consumption: Compared to traditional electronic interconnects, Silicon Photonics offers the potential for lower power consumption, particularly over long distances. Optical signals experience less attenuation and dispersion compared to electrical signals, reducing the need for signal amplification and regeneration. Additionally, the integration of photonic components with CMOS electronics enables energy-efficient operation by leveraging existing power management techniques.
Wavelength Division Multiplexing (WDM): Silicon Photonics supports WDM, a technique that allows multiple data streams to be transmitted simultaneously over a single optical fiber using different wavelengths of light. This enables increased data throughput and efficient utilization of optical infrastructure, making it possible to transmit and receive multiple channels of data over long distances with minimal interference.
Applications of Silicon Photonics in Hyperscale Data Centers
Optical Interconnects: Silicon Photonics' optical interconnects replace traditional copper-based links in data centers, revolutionizing the interconnection infrastructure. These optical networks provide more bandwidth, lower latency, and higher energy efficiency, allowing servers, switches, and storage systems to communicate more seamlessly.
High-Performance Computing (HPC): In HPC environments, where immense computational power is required for scientific simulations, weather forecasting, and other data-intensive tasks, Silicon Photonics plays a crucial role in enabling efficient data transmission and processing. By providing high-speed interconnects and low-latency communication, it accelerates the performance of HPC clusters and supercomputers.
Artificial Intelligence (AI) and Machine Learning (ML): Artificial intelligence (AI) and machine learning (ML) applications require massive volumes of data and computer resources. Silicon Photonics improves the productivity of AI and ML workflows by allowing for rapid and seamless data sharing between computing nodes and storage systems. This speeds up model training, inference, and data analytics, allowing enterprises to gain important insights from their data in real time.
Big Data Analytics: With the exponential expansion of data created by social media, IoT devices, and online transactions, big data analytics has become critical for firms looking to gain a competitive advantage. Silicon Photonics enables the quick transport of huge files throughout data centers, resulting in more efficient data processing and analysis. This improves the speed and accuracy of big data analytics applications, allowing businesses to gain meaningful insights and make data-driven decisions.
Cloud Computing: Cloud service companies rely on hyperscale data centers to supply on-demand computing capabilities to consumers worldwide. Silicon Photonics improves the performance and scalability of cloud computing infrastructure by allowing for faster data transfer and more efficient resource utilization. This leads to increased dependability, scalability, and cost-effectiveness for cloud services.
The Network Inside Hyperscale Data Centers
Networks within data centres are frequently based on Clos topologies (a type of non-blocking, multistage switching architecture that reduces the number of ports required), and hyperscale data centres will typically have tens of thousands of Ethernet switches interconnecting server racks via a leaf and spine network architecture.
A typical data center now has one or two 10GbE-based network interface controllers deployed at the server, which are aggregated to 40GbE at the top of rack (TOR) switch. Connections between server and TOR are typically done via direct attach copper (DAC) cables, which are the most cost efficient alternatives at these data rates for distances of a few metres.
However, the uplink from TOR to the next-tier switch is nearly always optical. Smaller data centres will likely use VCSEL-based transceivers over multimode fibres. These 40G transceivers combine four 10G lasers and can transmit for up to 300 metres. Higher tier switch interconnects (leaf to spine and above) typically need the usage of single mode fibres since the distances between the switches frequently surpass 300m.
Switching Trends
Today's TOR, leaf, and spine switches are typically 3.2Tb/s Ethernet switches in a 1RU chassis, with some 2RU systems offering 6.4Tb/s switch capacity. These switches have 25G SERDES and perform well with 100G QSFP28 transceivers. As switches progress from 3.2T/6.4T to 12.8T, line rates will increase to 50G SERDES with PAM4 modulation. At these data rates, new transceivers using 50G electrical I/O are required.
To enable 12.8T of switch capacity in a single RU, 400G transceivers in a form factor similar to the QSFP are required, and two MSAs have been established to address this: the QSFP-DD (DD stands for double density) and the OSFP (O means for octal). Both MSAs have 8 lanes of electrical I/O at 50G PAM4 and so can handle 400G optical interfaces. The problem will be determining which optical interface will be appropriate for 400G data centre connectivity.
The IEEE has standardized a DR4 interface, which is identical to 100G PSM4 but uses 100G PAM4 optical modulation instead of 25G NRZ on four parallel fibres. Using PAM4 modulation results in a drastically decreased link budget, higher power consumption due to additional ICs, and increased complexity, but it allows for the utilization of existing fibre infrastructure that has already been constructed for 100G PSM4. There is currently no uncooled option for duplex fibre, and MSAs are likely to arise to meet the demand for a low-cost, manufacturable duplex fibre solution.
