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Demystifying Optical I/O: 12 Key Terms to Know

by | Jun 27, 2023

As Ayar Labs co-founder and chief architect, Vladimir Stojanovic noted in a recent blog post, there’s much excitement around recent advances in optical interconnects. However, there’s also a noticeable lack of clarity and resulting confusion around many key industry and technology terms. While optical I/O is an evolving technology, it is already making waves regarding market investment and ecosystem development.

With new technology comes a slew of new terms and technical concepts. Whether you’re looking to stay ahead of the curve in the world of optical I/O or just getting started, Ayar Labs has compiled a list of 12 key terms and concepts. You can also visit our glossary for more optical I/O terms to know.

1. Silicon Photonics: Silicon photonics is a technology process that combines optical components with silicon-based electronics to create integrated circuits (ICs) that transmit and process data using light. By overcoming the speed and bandwidth limitations of traditional copper networking, silicon photonics is ideal for high-speed data communication applications, such as data centers, artificial intelligence (AI) and machine learning (ML), high-performance computing (HPC), 6G/telecommunications, phased array sensor systems, and more. The technology uses traditional semiconductor fabrication techniques to create optical components on silicon wafers. Silicon photonics will continue to play a key role in enabling optical interconnects that deliver cost, power and latency benefits. This leads to our second industry term to know…

2. Optical Interconnects: A replacement for traditional copper interconnects that send electrical signals over metal wires, optical interconnects use light to transmit signals between different system components or parts of an IC. Specifically, optical interconnects can be used to generate, process, and manipulate light, transferring signals between and within microelectronic chips – for example, within a collection of advanced GPUs or between a processor and memory. Optical interconnects facilitate information input/output (I/O) and offer several advantages over copper-based interconnects, including higher bandwidth, lower power consumption, lower latency, and longer reach. Optical interconnects can either be integrated, containing transceiver access like ethernet, or pluggable (see below).

3. Pluggable Optics: Pluggable optics are interchangeable transceiver modules that connect different network components, such as switches, routers, and servers, to convert high-speed electrical signals into optical signals and vice versa. Pluggable optics are the most common form of data center optics used today due to their flexibility, scalability, and compatibility with various networking devices and standards. Roughly the size of a Pez™ dispenser or pack of gum, they can be used to easily insert and remove a fiber optic cable from a socket. A benefit of pluggable optics is that you can tailor the specs of the transceiver inside the module to the data rate and other network architecture requirements. While optical interconnects use light to transmit data inside a compute architecture, pluggable optics make it easy to transmit data between systems. However, they have struggled to break the 1 Gbps/$ barrier; it remains to be seen how they will significantly improve upon cost efficiency.

4. Co-Packaged Optics (CPO): Co-packaged optics is a technology in which optical components are integrated into a single package. CPO modules, which are a replacement strategy for pluggable optics, are best suited for large Ethernet network switches. This is because they can improve the performance and power efficiency of data centers. Manufacturing CPOs is more expensive and challenging than traditional optical modules, and their adoption may require changes to network infrastructure and design.

5. In-Package Optical I/O: In-package optical I/O, or simply optical I/O, is an optical interconnect integrated into the same package as compute chips (e.g., CPUs, GPUs, ASICs, or FPGAs). Optical I/O offers a chip-to-chip connectivity solution that uses light instead of electrical-based I/O in a single IC. Optical I/O relies on compact co-packaged complementary metal-oxide semiconductor (CMOS) chiplets (more on this term in a bit) for a system-on-a-chip (SoC) that is scalable in terms of fibers and connection throughputs. Optical I/O is designed to enable seamless communication in a distributed compute system across boards, racks, and compute rows. Ayar Labs’ TeraPHY™ chiplet is the first in-package optical I/O chiplet capable of offering high bandwidth density at the lowest energy cost per unit. TeraPHY chiplets can be combined with the SuperNova™ remote light source (we’ll cover this topic in the next section) to power Ayar Labs’ complete optical I/O solution.

To summarize, silicon photonics combines optical components with silicon-based electronics to create ICs that use light to transmit and process data. This technology is essential for creating optical interconnects, which use light to send signals between and within microelectronic chips. While pluggable optics and CPOs are optical modules connecting different network components, optical I/O is a chiplet-based optical interconnect packaged into a single IC.

These five industry terms provide valuable context for the following seven words and phrases that help demystify optical I/O.

6. Microring Resonators: A microring resonator is a photonic device consisting of a looped arrangement of mirrors and other elements. An efficient way to guide light-carrying photonic signals in extremely small spaces (think nanometers), microring resonators loop light waves back on themselves to guide the light in a circle until it reaches a targeted wavelength. Microring resonators are essential to photonic ICs (we’ll get to them in a moment) and play a key role in enabling high-speed data transmission and processing using light.

7. Wavelength Division Multiplexing: Wavelength division multiplexing (WDM) makes it possible to efficiently transmit multiple optical signals in a single fiber using different light wavelengths (or colors). This makes it possible to join several optical signals together and transmit them more efficiently. A transceiver multiplexer joins signals on one side while the demultiplexer in the receiver splits them apart. Another way to think of WDM: it’s like having multiple lanes on a highway, but each lane carries a different color of light. This approach allows transmitting more data over the same fiber, increasing throughput.

8. Chiplet: Smaller subsets of ICs that offer a modular alternative to pluggable optics, chiplets can be combined (like Lego™ blocks) to create a larger, more complex chip. This approach provides advantages over a traditional SoC. With small footprints, low power usage, and high throughputs, chiplets form the backbone of modern optical I/O solutions. The TeraPHY chiplet is an example of an in-package optical I/O chiplet that uses silicon photonics and standard CMOS manufacturing to provide 5x higher data rates versus electrical I/O. The TeraPHY chiplet is the first optical interconnect to be UCIe (Universal Chiplet Interconnect Express) compatible, a new standard for high-bandwidth, low-latency, power-efficient, and cost-effective connectivity between chiplets.

9. Lambda: Lambda refers to the wavelength of light — the distance between two peaks of a wave — used in optical communication systems. Lambda is a commonly used unit of measurement for optical signals and is typically described in nanometers. It represents the length of one complete cycle of the light wave. Using different lambda values allows for WDM and other advanced optical communication techniques. Ayar Labs currently uses eight lambdas (wavelengths) per fiber, each supporting 32 Gbps; the CW-WDM MSA (Continuous-Wave Wavelength Division Multiplexing Multi-Source Agreement) roadmap already has specifications for 8, 16 and 32 lambdas per fiber. Every time the number of lambdas per fiber is doubled, the bandwidth is also doubled.

10. Remote (Disaggregated) Lasers: A remote laser (also known as a disaggregated or external laser) is an optical transceiver that separates the laser from the optical module, allowing for better system design and manufacturing flexibility.  A remote laser source can be placed up to hundreds of meters away from the optical I/O chiplet, ultimately saving space and cost. In addition to offering platform flexibility, remote lasers can be serviced or replaced separately from the rest of the system and operate at lower power levels and generate less heat, reducing overall power consumption. The SuperNova light source is the first optical source that meets the CW-WDM MSA specification. Remote lasers can be used in a wide range of applications like high-speed I/O, optical computing, AI and machine learning, and high-density co-packaged optics.

11. Photonic Integrated Circuit (PIC): A photonic integrated circuit (PIC) is an electronic circuit that uses optical components, such as waveguides, modulators, and lasers, all on a single chip. In comparison, an electronic integrated circuit (EIC) uses only electronic devices, like transistors and capacitors, on a single chip. In its simplest terms, PIC is often used to denote a photonic chip containing photonic components that manipulate and transmit light. PICs transmit data at high speeds, using light instead of electricity, resulting in faster data transmission speeds, lower power consumption, and higher bandwidth. Ayar Labs takes the concept one step further with its TeraPHY in-package optical I/O chiplet, which integrates a PIC and an EIC in an electro-optical transceiver. And that brings us to our final definition…

12. Electro-Optical Transceiver: An electro-optical transceiver converts electronic data into an optical signal and vice versa. Ayar Labs’ TeraPHY optical I/O chiplet is a fully integrated electro-optical transceiver. The SuperNova remote light source provides optical energy to the TeraPHY chiplet, resulting in a groundbreaking solution that offers significant cost, power, and latency benefits.

The silicon photonics and optical interconnects field is rapidly evolving and has the potential to revolutionize various industries, including data centers, AI/ML, HPC, 6G/telecom, and phased array sensory systems. Knowing the differences between technologies like pluggable optics, CPO, and optical I/O can help organizations decide which solutions best suit their needs. By staying current on these industry terms, you can stay ahead of the curve and take advantage of the latest technological advances in this rapidly evolving field.

Check out our glossary for more optical I/O terms and definitions.

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