Transitioning To Photonics

Silicon photonics is undergoing a resurgence as traditional approaches for reducing power and heat become more difficult and expensive, opening the door to a whole new set of technological challenges and driving up demand for a skill set that is in short supply today.

From a technology standpoint, photonics is extremely complex. Signals drift, they are modulated with heat, and structures like interconnects and waveguides are very different from traditional electrical designs. From a skills standpoint, there is a shortage of expertise at all levels, and one that may be exacerbated by talent shortages in other parts of the chip industry. So there is a steep learning curve for everyone, and skills are not easily transferable from electrical to light. However, that doesn’t mean it’s impossible. For those who like a challenge, it’s well worth the effort.

“We have to think very carefully about professional backgrounds anytime we’re opening a position,” said Manish Mehta, vice president of marketing and operations in the Optical Systems Division at Broadcom. “There is limited commonality between designing electrical ICs and photonic ICs. Typically, industry design engineers are focused on one or the other. Design managers understand both types of designs and typically have advanced experience in one or the other. At senior levels, there is a deep level of experience and understanding in both.”

In photonic IC design, silicon photonic foundries typically provide process design kits (PDKs), to vendors, which either use the designs directly or customize them for use. “Although silicon photonics does utilize the same CMOS manufacturing processing for scale and low cost, the design between them is very different and requires different areas of expertise,” Mehta said.

The EDA industry is responding by incorporating more photonics parameters and methodologies into their tools. However, for EEs, it’s still not as simple as brushing up on college physics.

“There’s no Rosetta Stone between the two,” said Tom Daspit, product manager at Siemens Digital Industries Software. “Everybody wants to know what photonics is, and why it’s different.”

Simply put, photonics entails working with photons instead of electrons. From there, photonics gets so complicated that most people in the field have PhDs in physics. However, as the industry grows, more opportunities are opening for those without advanced degrees, due to an expanding roster of project roles.

Fig. 1: Differences in CMOS design (left side) vs. integrated photonics (right). Source: Siemens Digital Industries Software

One of the biggest differences is in the interconnects. “We have 1, or sometimes 2 basic layers in photonics, versus electronics where there can be 10 to 15 layers of interconnect,” said Mitch Heins, business development manager for photonic solutions at Synopsys. “The implication is that many layouts cannot be accomplished without having interconnects that cross each other. In electronics we do this by jumping to a different layer to avoid shorting connections together. But in photonics we can’t do that. However, the good news is that in photonics you can have crossings or intersections.”

When trying to explain the concept, Daspit tells his audiences to forget what they heard in Ghostbusters, the 1980s movie in which the heroes warn each other, “Don’t cross the streams” when they shoot their optical weapons. “In real-life photonics, you can cross the streams. When you do layout, optical channels can cross and it doesn’t create a short.”

The differences also can be categorized by where they fall on the EM spectrum. “CMOS electronics work with electromagnetic signal frequencies up to about 5GHz,” said Marc Swinnen, director of product marketing at Ansys. “The electrical component of the electromagnetic signal dominates the magnetic effects, and we call this simply ‘electronics.’”

Beyond about 5GHz, getting into radio frequency (RF) and millimeter-wave (mmWave) designs, the wires start behaving like antennas that radiate electromagnetic energy.

“Wires still act mostly like the wires we are familiar with from lower frequency electronics, but they leak a lot of energy as radio waves,” Swinnen said. “If you make your wire long enough, the entire signal energy will leak away into the air as radio waves. You can still use CMOS for some things, but these high-frequency chips often use specialized silicon process technologies like bipolar junction transistors (BJT) and silicon-germanium (SiGe) processes that are higher speed. On the design side, you now need to model all this electromagnetic leakage to get accurate predictions.”

Photonics comes next. “Beyond that, you hit the frequencies of electromagnetic signals that we start calling ‘light,’” Swinnen said. “It starts at infra-red frequencies and then builds through the optical spectrum (red, orange, yellow, etc.). They are useful because high frequencies allow you to transport much more information than low frequency signals. Transistors are not fast enough to process these very high frequency signals, so CMOS is not an option. These high-frequency electromagnetic signals (light) do not propagate through metals like lower frequency ‘electronic’ signals do, thus metals are opaque to light.”

Specialized materials are needed to conduct light, like quartz and other glass materials (e.g., fiber optics) that act like waveguides for the optical frequency signals. “Still, it is possible to have traditional electronic components interact, or even generate, these optical signals (e.g., LEDs, photo-sensitive resistors, etc.). So your optical frequency circuit can include some electronic components, as well. The result is a mix called photo-electronics, or photonics,” he noted.

Each of these frequency domains has its own materials that work at those frequencies and physical effects that dominate, and there is no clear dividing line between them. “They gradually transition from one to the next. Minor effects at one frequency become the dominant physics at other frequencies,” Swinnen added.

For the engineering team, this can mean shifting a mindset that’s been cultivated since the first year of undergraduate engineering courses. “After years of thinking about orthogonal structures, suddenly the world is full of curves,” noted Gilles Lamant, distinguished engineer at Cadence. “Light does not like corners. (It likes going straight.) Coupled with the fact that light wavelength needs ‘room’ to exist,  photonics typically will occupy a much larger dimensional space than CMOS design.”

Photonic layouts use curvilinear layouts to guide photonics from device to device.

“Electronics, especially in bleeding-edge process nodes, use what are called Manhattan connections (north, south, east, west),” said Synopsys’ Heins. “Photons don’t like to make hard left turns. The turns need to be gradual and smooth or the light will leak or escape from the waveguide connections. There are a host of issues that come up when assembling layout for curvilinear designs.”

Electrical signals typically have a single frequency, so there’s only one wavelength that goes into a conductor. By contrast, in photonics, every frequency in the EM spectrum is available, although current practice is confined to wavelength bands that produce the lowest losses. (There are research efforts to expand the range.)

“This is where photonics starts becoming challenging, because now I have all those different frequencies, and I need to separate them by ‘color’ (wavelength),” said Lamant. “One could also divide it by polarity, which means changing the angle of the fields within the light. That gives us yet another dimension for multiplying the number of streams of information one can carry in light. One can also choose to encode information in either the magnetic or the electric component of each of those wavelengths.” Encoding information on light can become very multi-dimensional.

There’s also the challenge of thermal issues but with an even more challenging twist, because temperature is very often how photonics circuits are tuned. “By changing by one Celsius degree the temperature around the device, you can actually change the characteristics of the medium and how light propagates through it significantly enough to change the phase of the optical signal,” said Lamant.

“The problem of thermal is orders of magnitude worse,” he observed. “We rarely use thermal to adjust an electrical circuit. We typically try to get rid of the heat to avoid side effects. The control we achieve in one spot is ‘parasitic’ in the spot next to it in photonics.”

That adds another dimension to chip design. “Because temperature changes can change the refractive index of the materials, and therefore the behavior of the photonic devices and chips, very often those chips are put at a constant temperature, or there are feedback loops in or around the chip to compensate for temperature changes,” said Twan Korthorst, group director of photonic solutions at Synopsys. “In addition, when you power up lasers, they become warm, which can become a heat source for the bigger system, affecting the whole IC. This is an important physical property that you need to take into account when you design and architect a photonic IC.”

On top of all that, there’s a huge assortment of photonics components and materials, said Michael Lebby, chairman and CEO of Lightwave Logic. “On the CMOS side, each transistor may have slightly different performance benefits, but they’re all transistors. When you get into optics, you’ve got photo detectors, waveguides, materials with different optical properties, as well as several types of lasers — dfb lasers, Fabry-Pérot lasers, DBL lasers, tunable lasers, external cavity lasers, and so on. Each library that’s created for an optical device will have a different set of parameters and specifications. It’s not impossible to master, but it’s a different scenario than a tweak on a transistor with f_max or F_tau that will give you 200 GHz instead of 100 GHz.”

Despite all of these interacting complexities, there are now roles for interested EEs without advanced physics degrees. Korthorst said, “Ten or fifteen years ago, you needed to have a PhD in photonics. Nowadays, if you’re have a master’s or a bachelor’s degree in IC design, the hurdle to move to designing a photonics integrated circuit (PIC) is way, way lower than it was before. And we actually can train you relatively easily to start designing PICs.”

That said, an experienced PhD is still especially valued. “In photonics, people might use between 20% to 80% of a PDK,” said Korthorst. “But they need to complement it with their own devices to get to the full circuit they want to design. And that customization is where the domain expertise and domain knowledge are really critical.”

Fig. 2: It’s now possible for those without PhDs in physics to work comfortably in the field of photonics. Source: Synopsys

One niche that helps with transition is a background in layout. “I can train a person who knows how to do CMOS layout in how to implement photonic layout,” said Daspit. At the moment, he believes smaller companies will likely want engineers who can do both. Optical PhD designers are needed who can understand optical trade-offs, look at simulation results, and know what to do with them. However, at larger companies, the work is becoming more specialized, and thus there’s room for a non-PhD with a layout background.

And rather than being intimidated by the physicists, EDA veterans shouldn’t underestimate the value of their own experience. “We EDA people think everybody knows what a schematic-driven layout is. We think everybody knows what is DRC (design rule check) or LVS (layout vs. schematic). We think everybody knows how to do simulation and extraction. A photonics master’s student has no clue. Even an EE doing a photonics minor will probably only design one simple device, and not learn about EDA,” said Korthorst. “Getting a photonics PhD to help a customer design a PIC is a longer path than taking an EDA applications engineer, who already knows how to help a customer design an electric IC, and training them on the photonic-specific stuff.”

Another background that can prove useful is RF design. “Photonics is, in many ways, very similar to RF in terms of what you do as a designer,” said Lamant. “It’s about very high frequency design, so a lot of the concepts from RF design are applicable. For example, every waveguide is a device, similar to the transmission line in the RF world. You don’t just have a piece of electrical wire that carries the information in RF. And in both, you will use special tools to extract the characteristic (s-parameters). It is the same for integrated photonics.”

Nevertheless, he cautions that self-study won’t be enough, even for RF designers. “This is not an easy topic to learn. It’s a very challenging math class. Even though you use the same Maxwell’s equations, I don’t think you can learn it on your own,” he said. “I was lucky because I work with people who were willing to share knowledge and mentor me. That’s really important. It would be very, very challenging to do it on your own.”

Good mentors and on-the-job training are essential, many sources agreed. “You’ve got to have engineers that have actually fabbed designs and made products that are shipping in volume,” said Rajiv Pancholy, director of hyperscale strategy and products in Broadcom’s Optical Systems Division. “It’s not enough to say, ‘Hey, I did the design and the first silicon came back and it worked.’ You’ve got to be shipping hundreds of thousands of these. That is the type and the level of experience that is needed.”

Conclusion
As the field of photonics matures, both electronics and photonics skillsets will be required.

“You still have to design the electronic side of an optical modulator,” said Lightwave Logic’s Lebby. “If you design for high-speed optical performance, but you haven’t done a good job on the electrical, it’s not going to work very well. The two have to work in combination with each other. As part of our team, we have specialists who have optical simulation modeling design, but we also have RF engineers who specialize in electrical design. It’s a symbiotic relationship. You need both skills to do the job of creating super high speed solutions.”

Broadcom’s Mehta said it took that kind of partnership for one of the company’s latest projects. “We developed very high-density semiconductor packaging designs enabling wafer-scale processing and embracing the CMOS ecosystem. And we developed a passively aligned detachable fiber connector to facilitate soldered-down optical engines and enable fiber cable field replaceability and serviceability. To do this, we leveraged engineering teams with a high level of experience in both electrical IC and photonic IC design.”

Overall, the veterans are encouraging. “If people like to learn new things in an exciting field, then please come over and help us,” said Korthorst. “Don’t be afraid of the message, ‘It’s not yet mature,’ or, ‘It’s years behind CMOS.’ There are now tools, which 20 years ago, you could argue was not the case. There is a Wild West, but it’s not the Wild West 24 hours a day/7 days a week. There’s actually an enormously strong foundation and a lot of people active in this space whose work you can build upon. But you can also still make your own mark, because it is not yet all established.”

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