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What You Need to Know About Silicon Photonics

what is silicon photonics

By Twan Korthorst, Group Director of Photonic Solutions, Synopsys, and Kenneth Larsen, Product Marketing Director, Synopsys Silicon Realization Group

From Synopsys and Juniper Networks’ new silicon photonics venture OpenLight to Intel’s advancements in multiwavelength integrated photonics, silicon photonics is certainly having a moment in the spotlight. Manufacturing photonic circuits using CMOS technologies, also known as silicon photonics, not only offers the scale of semiconductor wafer-scale fabrication, it also enables advantages in new electronics applications using the properties of light in computation, communication, sensing, and imaging.

For these reasons, silicon photonics is increasingly being utilized in optical data communications, sensing, biomedical, automotive, virtual reality, and artificial intelligence (AI) applications. Until recently, the key challenge for silicon photonics has been the cost of adding discrete lasers acting as the “power supply” for the photonic circuits, which includes the manufacturing as well as the assembly and alignment of those lasers onto the photonic chip. Read on for a macro view of the silicon photonics industry, including the benefits of electronic integration, how the industry is accelerating the development of photonic IC designs across markets, why companies are interested in shifting to integrated lasers, and more.

What is Silicon Photonics?

Let’s start with the basics. We know that light can behave like a wave or a particle, and this behavior can be manipulated. The term “optics” refers to the study of light and often is used to talk about the light that is visible to the human eye (e.g., the light from a headlamp, light reflecting from a lens such as a magnifying glass, etc.). The term “photonics” means systems where light is being reflected or manipulated at a much smaller scale (think smaller than a few micrometers). Integrated photonics is when the photonic system is manufactured using semiconductor technology with wafers that are processed in a cleanroom facility. And if the manufacturing process that is used is very much like CMOS fabrication, that is when it is referred to as silicon photonics.

In other words, silicon photonics is a material platform from which photonic integrated circuits (PICs) can be made using silicon on insulator (SOI) wafers as the semiconductor substrate material. The technology is becoming much more popular and feasible than ever before, and there is an important reason why.

Initially, integrated photonics started using materials like doped silica glass, lithium niobate, or indium phosphide as the material surface, especially for telecom and long-haul datacom applications. However, the vast majority of the semiconductor industry uses silicon as the primary material to create integrated CMOS circuits, achieving very high yield and low cost. The physics of photonics makes it perfectly suitable to pattern and fabricate photonic devices and circuits using CMOS processes used on older silicon nodes. Using mature manufacturing processes has opened an economically viable path to mass production, and, consequently, integrated silicon photonics has taken off.

Today, silicon photonics has leveraged the mature CMOS manufacturing and design ecosystem that has proven to be very cost-effective at scale to start building integrated photonics systems.

The Main Advantages of Silicon Photonics

Now that the industry can efficiently manufacture PICs on silicon wafers, all the benefits that silicon photonics bring can begin to be leveraged in mainstream electronics. One of the key advantages of PICs is that they enable, extend, and increase data transmission. Historically, for longer distances, copper links reached the bandwidth versus energy consumption limit first. More recently, optical fiber connections are being used in datacenters for shorter and shorter connections in the network architecture. The latest trend is to move the optical connections even closer to the switch ASICs, by moving from a pluggable optical transceiver to an optical I/O chiplet that is in the same package as the switch. This reduces the distances for the high-speed electrical SerDes links, reducing the overall energy consumption for the I/O.

Silicon Photonics Applications

In addition to being used in data centers, silicon photonics can also be used for sensing, which is beneficial to a variety of different industries. For instance, optical sensing, the transmission of a signal and the receipt of a reflected or transmitted optical signal, can help determine the properties of the surrounding environment. This sensing activity is beneficial for health and biomedical applications such as diagnosis and analysis and consumer health wearable applications, as well as LiDAR for industrial automation and autonomous driving.

Solid-state LiDAR chips are gaining traction in the autonomous vehicle and industrial automation space. Instead of using radio frequency (RF) signals, LiDAR uses light that is reflected on surfaces to analyze and deliver critical information about what’s going on in the road and give input on how the car should react (e.g., what direction objects are moving in, where there might be obstacles, etc.). Of course, designing anything that will be used in the automotive industry comes with many safety regulations that need to be accounted for. As far as the widespread, high-volume consumer applications of LiDAR, augmented virtual reality has already been introduced in some smartphones. Another likely mass volume application of silicon photonics is human health measurements including heart rate, saturation, and hydration levels for wearables like smartwatches and in-body implanted medical devices.

As with any product development process, decisions about which technology would be best suited for a specific application need to be carefully considered and include factors such as cost, performance requirements, how long it will take to come to market, and pre-existing relationships with foundries and packaging providers.

The Shift to Integrated Lasers

Like a voltage supply in an electrical circuit, lasers are the power source for a silicon photonics circuit. Currently, it is impossible to manufacture a light source (or laser) into silicon due to the indirect bandgap of the material. That’s why materials such as indium phosphide are used to create semiconductor lasers for wavelengths used in telecommunications and data communications.

Companies like OpenLight have honed various techniques to integrate indium phosphide in silicon photonics chips to create integrated lasers, modulators, and detectors that drive the photonic circuit. This allows customers to reap the benefits of standard manufacturing processes and derive the many performance benefits of silicon photonics. Additionally, multiple lasers can be used at slightly different wavelengths in the same system to scale even further. In the past, hybrid-attached laser die have caused concerns around reliability, but integrated lasers increase reliability and open up the possibility of applications that require multiple lasers or amplification sections. However, designers should not ignore the thermal aspect, as lasers generate heat that needs to be considered when designing the circuit and packaging.

The industry of silicon photonics is just getting started due to the tremendous technical and economic value it brings. The closer the optical input/output (I/O) is to the core silicon (by means of 2.5/3D heterogenous integration), the less of a penalty there is for the communication, which makes it perfect for high-performance computing and artificial intelligence applications. Whatever the future of silicon photonics holds, we’re excited to be a part of the investments being made in the industry and the many innovations that will be enabled with this technology.

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