What is Silicon Photonics?

Silicon photonics (SiPh) is a platform for constructing photonic integrated circuits (PIC) for optical communication, high-speed data transfer, and photonic sensing devices. The semiconductor substrate material is a silicon-on-insulator (SOI) wafer. Standard semiconductor manufacturing processes are used to create components on a photonic layer from silicon (Si), which is transparent to infrared light. Silicon dioxide (SiO₂) or air is placed around the silicon to create a high refractive index difference, causing the light traveling in the components to travel through the circuit with very little loss.

On-chip photonic integrated circuits are very compact, use less power, and operate at higher speeds (over 100 Gb/s) than traditional photonics devices, transferring information faster and more efficiently than electrical circuits. These advantages, combined with the ability to use standard complementary metal-oxide-semiconductor (CMOS) manufacturing techniques, drive the rapid increase of chips featuring silicon photonics.

In the mid-1980s, researchers proposed placing circuits designed for photons on the same chip as circuits designed for electrons. Photonic circuits emit, modulate, switch, amplify, and detect light. Monolithic optoelectronic integrated silicon chips were uncommon until the development of the silicon modulator in 2005, and the linewidth available with new fabrication processes became thin enough to construct waveguides needed for photonic circuits.

Just as electronic chips are attached to external circuits with ball grid arrays or wires, light is transmitted in and out of photonic chips using optical fibers. Because of their higher frequency and greater bandwidth, these optical interconnects can transfer more data at higher speeds than electrical interconnects.

Lasers are then added to the circuit to create light, along with photodetectors in a receiver circuit to measure the energy, frequency, and other characteristics of photons. Electricity is delivered to or extracted from each device using traditional integrated circuit techniques. The electronics to convert the light signal to an electrical signal can be on the same chip or in separate electronics. These detectors and emitters are combined with on-chip devices to modulate (i.e., change a measurable characteristic), switch, and amplify light to carry out critical functions in optical networking, facilitate high-speed data transfer, or measure physical properties on the micro or macro scale.

Some of the more common photonic components used are:

1. Waveguides: the “wires” in a photonic circuit that photons travel along. A waveguide's cross-section, surface roughness, and bend radius can greatly impact light traveling inside the waveguide.
2. Optical modulators: components that modify the phase, amplitude, polarization, spacing, and diffraction characteristics of a beam of light to encode information into the beam
3. Light sources: semiconductor lasers of various types that create light in the circuit. Lasers are not silicon based but instead use III-V compound semiconductors. They can be external or built into the same chip as the photonic integrated circuit.
4. Receivers: photodectors that absorb photons and convert the information encoded in the light into an electrical signal
5. Optical Switches: devices that direct light using temperature, interaction with other light sources, and micro cavitation. Optical switches can be much faster than mechanical, micro-electromechanical (MEMS), or electrical switches.
6. Filters: A wide variety of components that use a range of light’s physical properties to pass through light at a desired frequency range. The frequency is set by geometry in passive filters and electrical input for active filters.
7. Couplers: devices that split or combine optical signals

The high performance of silicon photonics has resulted in the technology replacing electrical or electromechanical switching in communications and electrical interconnects in packages and across computer components, as well as controlling optical sensors such as lidar. The technology can augment existing solutions by providing a photonic step in an otherwise electronic system, such as single-chip transceivers for fiber optics and complex signal processing applications where photonics perform better.

Using light to carry information has significant advantages over electrons, radio waves, or microwaves. Light's higher frequency and its multiple modalities (frequency, amplitude, phase, etc.) allow for transmitting more information with low power consumption. These advantages are multiplied when the photonic devices are built into the same chip as the electronics needed for their operation, enabling the use of low-cost mass-production manufacturing processes.

Because the technology combines optical components with the electrical circuits on the same chip, optoelectronic devices can be packaged into a smaller form factor than separate optical and electrical solutions. Because light travels through the waveguides with very little loss and because of their microscopic size, silicon photonic devices consume less energy than electrical or stand-alone optical devices.

However, the most significant advantage of silicon photonics is its use of existing CMOS manufacturing systems. Semiconductor manufacturers worldwide create more than one trillion chips of various kinds a year. Companies leverage the tools used to design, manufacture, package, and test semiconductor hardware for silicon photonics technology. Many semiconductor foundries are establishing standards for photonic integration designs, enabling faster, less expensive, and more robust new product development.

Even with the current state-of-the-art manufacturing techniques available in the silicon photonics ecosystem, many challenges remain in growing the technology's applications. Some result from fundamental physics, and manufacturing limitations create others. Companies and universities are conducting fundamental and practical research to understand and overcome these challenges.

Although the use of SOI wafers simplifies manufacturing, silicon limits the frequency of light used, and the material can not be used to make lasers and other needed components. Therefore, researchers are finding ways to include new materials such as silicon nitride (SiN) and indium phosphide (InP) to broaden the range of wavelengths. Other research focuses on integrating III-V materials like gallium arsenide (GaAs) into the manufacturing workflow to construct on-chip light sources.

Energy losses in photonic circuits are another challenge that designers must understand and control. Even something as simple as the bend radius of an optical waveguide must consider losses as a trade-off against compactness. Similarly, deciding which type of optical modulators or filters to use can be challenging. As people tackle the application of silicon photonics for sensing, they need to overcome limitations in sensitivity and miniaturization.

Although less expensive than other alternatives, the cost of devices that leverage SiPh is still too expensive for mass adoption. Currently, millions of chips are being made using the technology. Photonic components will need to scale to billions of chips a year to reduce costs and achieve widespread adoption in places like data centers and telecom networks.

The needs of the optical system must also be balanced with the requirements of the electronic side of optoelectronics. If the electronics are on the same chip, then the manufacturing approach must balance the needs of each type of signal processing. Or, if separate electronics are used, advanced semiconductor packaging techniques are usually used to connect the two. In both approaches, heat generation in the electronics can impact the photonics.

The compactness, relatively low cost, energy efficiency, and low latency of optical solutions created with silicon photonics make them applicable to a growing range of applications. The most common use is in high-speed data transfer over fiber optics. Companies like Intel are focusing on improving the capabilities of optical transceiver chips to make them available for a wider range of applications. Many of the chips are used in optical routers and signal processors that network computers.

Optical networking between nodes and optical interconnects within a given node can significantly increase the computation speed for high-performance applications like artificial intelligence, bitcoin mining, and digital twins. As the computation speed of processors increases, the bandwidth for data transfer becomes a bottleneck — one that can be relieved with silicon photonics.

Some emerging applications that are leveraging SiPh technology for photonic components include:

• Photonic sensors: SiPh sensors can measure small changes in refractive index caused when light passes through a sample. These changes can identify specific biomarkers in biological or environmental samples.
• Lidar: Light detection and ranging devices send out a pulse of light and measure the return time. Silicon photonics-enabled lidar solutions are more compact, use less power, and are less expensive to manufacture than systems constructed from discrete components.
• Quantum computing and networking: Quantum computers use photons to carry out calculations. The management of light in and between quantum computers benefits from the speed, accuracy, and low cost of photonic integrated circuits.

Since its introduction in the 1980s, the role of silicon photonics-based components has grown from supporting basic functionality for fiber optic communication for telecommunications. Over time, silicon photonics have found greater usage, moving into computing and now into optical sensors. Manufacturers are providing foundries for fabless companies, bringing the technology to a broader range of applications. With increased manufacturing capability and a growing set of applications, design and simulation tools have improved to support more complex and efficient designs.

The next generation of monolithic devices, which include electronic and photonic integrated circuits, will take advantage of hybrid manufacturing methods that introduce materials beyond silicon into the devices. They will also use more precise manufacturing methods, smaller feature sizes, and advanced semiconductor packaging methods. These changes will increase the bandwidth and speed of data transfer in datacom and high-performance computing applications and potentially revolutionize optical sensing technology.

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