[DGIST Series] Silicon Photonics: Data Transfer With Light

Semiconductor technologies usually feature electrons flowing around a silicon chip. However, there is a promising technology called silicon photonics that allows light to move with electrons on a semiconductor chip. Silicon photonics is the application of photonics¹ systems which use photons, or light, for faster data transmission both between and within microchips. As electrons and photons have strength in computation and telecommunications, respectively, their integration increases the data processing capability of semiconductor chips. This interaction can open up a host of possibilities that were considered impossible with the use of conventional semiconductors.

¹Photonics: The study of matters related to photons, which are the fundamental units of light.

Figure 1. Conceptual diagram of silicon photonics technology (Image credit: PhotonHub)

In this latest article in our series by faculty from South Korean university DGIST, Professor Sangyoon Han of the Robotics and Mechatronics Engineering department explains silicon photonics, its use in data centers and optical links, the manufacturing process of silicon photonics chips, and the technology’s future applications.

The Significance of Silicon Photonics in Data Centers

Although data centers and their significant role in operating a host of technologies may be a familiar concept to many people, it is less widely known that silicon photonics technology is essential to the running of these centers. Tens of thousands of servers in a data center are connected not by wires but by optical fibers that act as pathways for light, enabling high-bandwidth communication. Thus, the electrical signals generated in the servers need to be converted into light, or the conversion needs to happen the other way around in order for light and electrical signals to pass through the optical fibers. To facilitate this, optical transceivers—devices developed using silicon photonics technology—are applied.

Figure 2. A prototype of a silicon photonics optical transceiver (Image credit: IMEC)

Able to handle both light and electrical signals simultaneously on a single chip, silicon photonics technology is considered an ideal platform to form optical transceivers. Due to light’s low attenuation² and their parallel processing capability, optical transceivers enable communication between servers at speeds of up to 400 Gbps (gigabits per second). Considering that electrical wires can only achieve speeds of single-digit Gbps, it is clear that optical transceivers are essential for the efficient and speedy operation of data centers. Subsequently, around a million optical transceivers are used in a single large-scale data center.

²Attenuation: In the optical field, it is the rate at which the signal light decreases in intensity.

Use of Silicon Photonics in Optical Links

The application of silicon photonics spreads beyond data centers as it is currently being used in other areas which require high-bandwidth data transmission. Recently, there has been a growing trend in using optical waveguides instead of copper wires to connect chips that are located within a few centimeters of each other. This is where silicon photonics play a key role as there is active research into the use of silicon photonics-based optical links³ for interconnection between GPUs and between the cores in a multi-core CPU. These optical links are also being considered to connect CPUs and memory chips. As a prime example of such applications, Silicon Valley chip startup Ayar Labs is working with NVIDIA to develop technology that could embed optical links into a large-scale GPU system as shown in Figure 3.

³Optical link: An optical transmission channel designed to connect two end terminals or to be connected in series with other channels.

Figure 3. Example of applying optical links in chip-to-chip technology (Image credit: Ayar Labs)

How Silicon Photonics Chips Work

So, how exactly does a silicon photonics chip work? To make it easier to understand, it is helpful to compare the technology with electronic circuits. Although electronic circuits look complex, they are mostly made up of transistors and copper wires that connect the transistors. The functions of transistors and wires in an electronic circuit are parallel to the roles of optical modulators and waveguides in a silicon photonics chip, while the power supply of an electronic circuit corresponds to the role of a laser for such chips. Additionally, a photodetector is used to convert optical signals into electrical signals in a silicon photonics chip.

Figure 4. Conceptual diagram of the process of an optical modulator

Just as wires carry electrons, waveguides carry photons, or light. They work as channels that transmit light without any loss, similar to optical fibers. For silicon photonics chips, they use ultra-thin waveguides of less than 1 micrometer⁴(μm) in diameter that are manufactured using semiconductor processes. Furthermore, optical modulators alter the transmissivity of the waveguides to change the intensity of the light passing through the waveguides—this generates optical signals as shown in Figure 4.

⁴Micrometer (μm): 1 micrometer is one-millionth of a meter.

The Manufacturing Process of Silicon Photonics Chips

Figure 5. Silicon photonics chips fabricated on a 12-inch wafer (Image source: Nature)

The process of manufacturing silicon photonics chips is very similar to making electronic circuits through the CMOS process. Both processes use silicon as a base material, and for silicon photonics chips it is advantageous to be manufactured through the CMOS process rather than developing a whole new manufacturing process in terms of time, cost, and efficiency. In turn, the similarity of the processes makes it straightforward to produce silicon photonics chips on existing semiconductor production lines.

Moreover, the size of silicon photonics devices, which measure only a few micrometers, is optimal for production as they can be easily manufactured using nanometer⁵ processes. As a result, despite the relatively short history of silicon photonics, major foundries around the world have started to produce silicon photonics chips on 12-inch wafers. With more of these world-renowned foundries entering the silicon photonics business, it is foreseeable that the silicon photonics market will grow significantly going forward.

⁵Nanometer (nm): 1 nanometer is one-billionth of a meter. Therefore, 1μm = 1,000 nm.

Silicon Photonics in Autonomous Driving Sensors

Silicon photonics has not only made a significant impact in data transmission—an area where light holds an advantage—but is also being applied to a growing number of fields these days. For example, the technology is being utilized to achieve higher performance and miniaturization of autonomous driving sensors such as Light Detection and Ranging (LiDAR). Most LiDAR systems available in the market today are difficult to mass-produce at low costs because they are made by manually assembling components such as motors and lenses. But silicon photonics technology is expected to become a new solution to this as it enables these LiDAR systems to be manufactured with improved performance, energy efficiency, and lower costs. In practice, this technology will ultimately lead to a drastic fall in costs when adding an autonomous driving system to a car.

Future Applications of Silicon Photonics

Silicon photonics is also making it possible to develop new computing technologies beyond existing paradigms. Such next-generation computing technologies include: AI processors that use light’s ability of parallel processing to compute multiple AI inferences with a single physical device; quantum computing that transcends the limits of classical physics; and quantum cryptography communications that are physically impossible to wiretap.

With the emergence of advanced technologies such as large-scale AI models, the demand for improved computing power and data processing capabilities from hardware is greater than ever. As a result, the existing paradigm of electronic semiconductors will need to evolve to keep up with this strong demand, and this evolution can be materialized with silicon photonics technology. As the technology harnesses the power of light in semiconductors, physical limitations can ultimately be overcome so semiconductors can realize previously unobtainable applications. Looking ahead, it is also anticipated that the use of silicon photonics will result in colossal progress in computing and AI applications.

<Other articles from this series>

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[DGIST Series] The Role of Semiconductor Technologies in Future Robotics

[DGIST Series] The Technologies Handling the Growing Data Demands in Healthcare

[DGIST Series] AI-Powered Micro/Nanorobots to Revolutionize Medical Field

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