From the Internet to LiDAR: How Optical Semiconductors Are Changing the World

The importance of silicon-based semiconductors that manipulate electrons for digital electronics and computing is well known. What is perhaps less well known is the role of optical semiconductors in powering the modern world. Optical technology obviously plays a critical role in communications, enabling tens of terabits per second to be transmitted over the submarine and long-haul terrestrial fibers that provide the backbone for the internet, while also facilitating high-speed metro, mobile backhaul, and broadband access networks. It enables the hyperscale data centers from Google, Meta, Amazon, and Microsoft to deliver scalable communications between servers and switches. It even has a role to play in the future of space communications.

Better Gadgets, Healthier Lives

Optical technology also powers the 3D sensing technology enabling facial recognition and enhanced photography in the latest smartphones. Optical sensors in smartwatches provide the ability to monitor heart rate, blood oxygen, and skin temperature, with noninvasive blood glucose monitoring a potential addition for the future. Additional medical applications include lasers for surgery (e.g., eye surgeries like LASIK), photo therapy for skin rejuvenation, and the treatment of many conditions, including acne and even cancer.

Enabling a Safer World

Furthermore, optical technology is enhancing automobile safety and reliability while providing a pathway to automation with 3D sensing, including short-range and long-range LiDAR. In-cabin 3D sensing enables occupancy and driver monitoring, as well as gesture-based control for navigation, communications, and infotainment. Defense applications for optical technology include range finders, target designators, submarine communications, laser X-rays for bomb detection, and battle simulation/training. Industrial applications include the use of lasers for cutting, welding, and soldering parts made from metal, glass, and other materials.

Optics and Electronics: A Symbiotic Relationship

In fact, there is a symbiotic relationship between electronic semiconductors and optical technology. Optical technology is a key enabler for semiconductor manufacturing, with a variety of lasers used for process stages including lithography, die cutting, and wafer inspection. On the other hand, elemental semiconductors, such as  silicon, and compound semiconductors, such as indium phosphide, can be used to create optical components that generate, detect, and/or manipulate photons rather than electrons. And as we will discuss later, older silicon foundries are often used to manufacture silicon photonics.

Photonic Integrated Circuits

With silicon, commonly referred to as “silicon photonics,” and indium phosphide, hundreds of previously discrete photonic components can be integrated into a single photonic integrated circuit (PIC). As is the case with conventional electronics, manufacturing one PIC is far more cost-effective than manufacturing individual optical components and then integrating and packaging them. PICs also have a dramatic impact on footprint, enabling the miniaturization of optical devices. Power consumption is also reduced, while performance can be improved due to minimized optical coupling losses when connecting optical functions with waveguides inside the PIC, as opposed to coupling optics between discrete components. And equipment failures are reduced, as these coupling optics are eliminated as a source of failure.

As an example, photonic integration has taken us from coherent 100 Gb/s DWDM transponders consuming approximately 200 watts to compact digital coherent pluggables with power of under 20 watts, which are now capable of delivering 400 Gb/s of coherent optical capacity to 1,000+ km over a DWDM network. High-performance embedded optics such as Infinera’s ICE6 can now deliver 800 Gb/s to 1,000+ km and 400 Gb/s over practically any distance. Another key communications application for PICs has been in 100G, 200G, and 400G Ethernet pluggable transceivers inside data centers.

Indium Phosphide and Silicon Photonics: Different Advantages

Both silicon photonics and indium phosphide offer different advantages when it comes to PICs. Indium phosphide can provide laser and optical amplification functions at DWDM frequencies. In contrast, silicon is an indirect bandgap semiconductor, meaning excited electrons produce heat, not light. Silicon photonics therefore typically requires external DWDM lasers and amplifiers. That said, there has been and continues to be a lot of R&D focus on overcoming these limitations by heterogeneously integrating light-emitting materials, such as indium phosphide, into silicon PICs. This heterogeneous integration, however, requires a specialized silicon foundry line and is not currently supported as a standard offering by silicon foundries. Indium phosphide also has an inherently superior modulation effect, which is especially important for the highest-performance segment of the DWDM transceiver market, where 800 Gb/s DSPs using 7-nm CMOS technology and, typically, indium phosphide photonics is the current state of the art. In addition, indium phosphide can detect DWDM light, while silicon photonics requires the integration of germanium (Ge) for this function.

At present, several silicon foundries offer physical design kits (PDKs) for silicon photonics, enabling customers to design PICs that can be manufactured on legacy (~90 nm-45 nm) production lines and then be used with external light sources and amplifiers. This lowers the barriers to entry for vendors that want to manufacture silicon photonics PICs but limits the design to using only the PDK functions. For example, if a customer wants to use a higher-speed modulator than in the PDK, they will incur all process development and engineering costs for the foundry to develop this device. However, in simpler applications, silicon photonics may have a cost advantage for very high-volume applications (i.e., millions of units per year).

Indium Phosphide Manufacturing: A Scarce and Valuable Resource

Indium phosphide manufacturing is a scarce resource and a source of differentiation and competitive advantage for the few vendors that have this expertise and have made the investment in InP manufacturing facilities. Given the impact of recent supply chain disruptions and the current level of geopolitical risk, governments around the world are now recognizing the strategic importance of domestic fabs through legislation and financial support, with the $52B CHIPS Act that was passed by U.S. Congress in July 2022 and the proposed €43B European Chips Act being prominent examples.

Additional Optical Semiconductors

Additional compound semiconductors with optical applications include gallium arsenide (GaAs) and lithium niobate (LiNbO3). Key gallium arsenide applications include vertical-cavity surface-emitting lasers (VCSELs) and photodetectors for short-reach optics over multi-mode fiber at 850 nm, as well as VCSEL arrays for 3D sensing in devices such as smartphones, smartwatches, and VR headsets, and automotive applications such as in-cabin sensing and LiDAR. Additional GaAs applications include gas sensing, biomedical sensing, and computer mice. Lithium niobate applications include high-speed coherent modulators, with thin-film lithium niobate an emerging technology that promises to overcome some of the previous disadvantages of lithium niobite modulators.

Photonic Integrated Circuits Beyond Communications

Beyond optical communications, PICs are being used to drive down cost, size, and power for applications such as automotive LiDAR, optical coherent tomography, biosensing (with silicon nitride PICs) for healthcare, and agricultural sensing. Potential future applications include photonic processors for artificial intelligence and quantum computing.

So, to summarize, optical semiconductors are changing the world just as much as their electronic cousin, with both technologies benefiting from each other while sharing opto-electronic semiconductor materials.