Photonic Integrated Circuits

Key Takeaways

• The history of photonics alongside electronics, and the need for integration.

• Advantages offered by photonic integrated circuits.

• The make-up of photonic integrated circuits and the need for future developments.

Photonic integrated circuitry offers tremendous benefits to data throughput, but integration with electronic ICs remains something of a bottleneck.

Light and electricity are different forms of the same phenomenon: electromagnetic radiation. The photoelectric effect states that it is the absorption and emission of photons that cause electrons to increase or decrease in energy, respectively. It should come as no surprise then that just as there are components that are governed by electricity, there are also those that directly rely on the transmission of frequencies in the visible and infrared sections of the electromagnetic spectrum.

Photonic integrated circuits are a promising yet nascent technology. Many of the hurdles faced by the current limits of electronic ICs are surmountable by photonics, but the field carries its challenges. 

Comparing Electronic and Photonic Systems

The Introduction and Evolution of Photonics

Arguably more than any other component in electronics, it is the continued development of the transistor that has continued to accelerate the pace of technology over the past century. Serving the dual purpose of a switch and amplifier, its ubiquitous nature has made it the most produced item in human history. However, the growth rate, as correctly predicted by Moore’s Law for the past five decades, is nearing its conclusion: transistor miniaturization is becoming boxed in by some significant physical realities. To counter this stagnation, the industry has turned its efforts to many emergent technologies, with photonics being an especially promising one. 

Optical devices are not entirely new, with devices like lasers and photodetectors as well as communication technology like fiber optics supporting electronic systems for the past several decades. Before integration, these optical components were large, heavy, and power-intensive compared to electronic ICs. For scalability reasons just like with discrete electronic components before integration, the industry has had to shrink optical components to avoid size, weight, and power bottlenecks. 

Photonic integrated circuits were introduced in the late 20th century, and have continued to make rapid improvements, albeit not at the speed of electronic components in the mid-20th century. Different design restrictions are unlikely to allow for photonic integrated circuits to ever reach the density of electronic ICs, but with this restriction in mind, there are still significant opportunities afforded to the adoption and continued maturation of photonic integrated circuit technology:

• Bandwidth over distance - The encoding of digital information from high/low electrical signals to pulses of light multiplexed across several frequencies. This allows for exceptionally high rates of data transfer, which when coupled with the extremely low loss rates of optical cables compared to electrical cabling, makes the medium highly suitable for industries like telecommunications.
• Implementability - Just like discrete electronic components before them, discrete optical components suffer from reduced performance due to size: optical components can drift and misalign over time with variance from vibration and temperature. Planning and maintenance of these delicate systems require a high technical ability. Meanwhile, photonic integrated circuits can bypass many of these issues due to tighter packaging.

It’s equally important to understand there are impediments to the continued growth of photonic integrated circuits. Photonic ICs produce much more heat than traditional ICs built out of electrical transistors, and as such, dissipative techniques demand greater attention. Packaging also encounters challenges, as photonic integrated circuits, generally speaking, have to interface with electrical ICs (the reverse is not necessarily true).

Material Requirements of Photonic Integrated Circuits

Unlike silicon in electronic ICs, it is a different element that dominates photonic integrated circuits: indium. Silicon still plays a role, however, and both semiconductors are used discretely and in tandem to take advantage of their distinct material properties:

• Indium phosphate (InP) - The biggest advantage InP has over silicon is arguably the most important for IC miniaturization: its nature as a direct bandgap semiconductor material. A direct bandgap material can radiatively recombine independent of any interaction with a phonon, while the same can not be said for indirect bandgap materials. This rapidly increases the rate of radiative recombination in the direct bandgap material. At the device level, this translates to InP transfer rates being a magnitude greater than standalone silicon.
• Silicon photonics (Si-Ph) - Silicon photonic integrated circuits boast numerous benefits: they are compatible with existing CMOS technology/infrastructure, the material exhibits much less temperature dependence than InP, and silicon wafer waveguides can support smaller bend radii. All of these factors contribute to the relative ease of silicon scaling and manufacturing with greater thermal reliability. Unfortunately, its aforementioned indirect bandgap property throttles its bandwidth, and the lack of direct laser integration with the die results in greater cost and packaging complications. 
• Hybrid silicon - Hybridized silicon melds the individual strength of InP and Si-Ph by placing InP elements on a Si-Ph base instead of less efficient methods of connecting the two materials. Hybrid silicon presents one potential avenue for cost and power reduction while retaining silicon’s scalability, but realizing these breakthroughs will require ongoing research and development in the field.

Market applications of photonic integrated circuits are necessary to meet the rising demand for large data transfer rates across several industries. As such, the design of devices that use or rely on these ICs will continue to increase in the coming years.

Cadence Products Shine a Light on Photonic Integrated Circuits

Devices powered by photonic integrated circuits are numerous, and evolutions in their production processes will drive improved performance as well as new applications. Optical and electrooptical system design is complex and intricate, and like other specialty electromagnetic spectrum wavebands, design teams need rigorous tools for simulation and modeling. Cadence AWR software offers a comprehensive solution to a wide range of circuit development challenges that can be readily consolidated into Cadence ECAD tools for a seamless transition to manufacturing.

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