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The Young and the Restless, PDA vs EDA, Photonic Soaps continued…

The Young and the Restless, PDA vs EDA, Photonic Soaps continued…
by Mitch Heins on 06-16-2016 at 7:00 am

 If you’ve followed my last article, The Guiding Light and Other Photonic Soaps, you read my comments about the use of waveguides to “guide the light” in photonic integrated circuits (PICs). This article continues the soap opera theme, this time with the Young and the Restless. My point here is that I am continually struck by the dichotomies between photonic and electronic design. The more these two domains look the same, the more they are different, even down to the engineers with whom I am now finding myself working (more on that later).

The place where I’ve found the dichotomy to be most profound is in the design automation tools used for both industries. In general, the strategy so far has been to try to make photonic design automation (PDA) look as much like electronic design automation (EDA) as possible, even to making the acronyms sound alike (PDA/EDA). The idea here is that eventually these two technologies will eventually merge and since we’ve got more than 3 decades of learning on the EDA side why not right? In fact, the American Institute for Manufacturing Integrated Photonics (AIM Photonics) has already created a work group called EPDA that is looking to do just that. Upon closer inspection of the challenges though, it may turn out that there are lot of reasons for why we might want to take another approach. A few examples might be handy at this point.

The first area of dichotomy is that if ever there was a technology that cried out for “automation” it is photonics. There are several degrees of freedom and dependencies in photonics that can make for a very rich and large solution space to be explored for any given design. In EDA manual interactive processes occur at manual interactive rates, making it difficult to use these processes to explore a large design space for an optimal solution. Why then does EDA seem to want to shoe-horn photonic design into a custom layout paradigm which is inherently manual and interactive? As the saying goes, when you have a hammer, everything looks like a nail. PDA companies seem to have a different approach.

A well-known design methodology in EDA is schematic driven layout (SDL). In electronic design we start with a schematic and spend a lot of time iterating the design between the schematic and simulation before we start to do a layout. The concept of SDL works because we have logical views that share parameters with physical views in a predictable way across foundry processes. In photonics this is not necessarily the case. The functionality of photonic components is highly dependent upon their layout, physical surroundings, temperature and variations in the fabrication process and materials being used. Adding to the complexity is that the interconnect between the photonic components are not merely conductors of light but are in fact active components of the circuit.  And… the coup de grace to all of this is the fact that photonic switching is usually done through evanescent coupling where physical components don’t actually touch each other and multiple wavelengths can be switched by a single component. In short we’ve broken several key assumptions for an EDA-based SDL flow and have set ourselves up for a schematic back annotation flow from hell. If you really want to twist your mind on something, think about the implications for layout versus schematic checks (LVS) in this scenario.

If I haven’t driven the point home enough yet, here is another dichotomy for our photonic soap opera. Unlike in EDA where most of the shapes are rectilinear with perpendicular angles, most photonic layout shapes are curvilinear in nature. Smooth curves, adiabatic tapers, spirals, Y-shaped splitters and joiners, grating couplers and circular resonance structures all of which can be drawn at any angle make for a very interesting exercise in a traditional EDA layout tools. Even if the tool has support for natively drawing a curved shape, it will eventually store that shape in the database as fractured discretized rectilinear polygons snapped to some design grid. This causes issues for later edits (like non-orthogonal rotations and re-connections) and physical design rule checking. See Silicon Photonics III, sections, and an excellent article in the IET Journals by members of MIT, University of Colorado and University of California, Berkley for more details on how PDA and EDA tools are trying to handle these issues.

So where does this leave us? In the end, we do need a design flow that will enable integrated electronic-photonic designs. Perhaps the “integration” or “assimilation” of PDA into EDA should not be our thrust, but instead we should be looking for convenient bridges that can be built between the two domains. There is good news in all of this and that’s where the “Young and the Restless” come into play. While most of the “not-so-young” EDA cast members were attending the Design Automation Conference last week, I was visiting photonic customers and I was amazed at the number of the “Young and the Restless” with whom I was meeting. Most of them are PhDs just out of school, highly educated and highly motivated professionals who are pushing to make integrated photonics a success. The best part about these young engineers is that they weren’t so tainted by 30+ years of ‘this is how we do EDA’. Instead they were tackling integrated photonics with a fresh new view. For a guy like me, who is still young at heart, it was refreshing to see the new enthusiasm and different thinking. It reminded me of me 30+ years ago when EDA was just beginning. Perhaps it’s time to let the PDA people do what they do best and look for ways to build bridges for them into EDA instead of trying to mold PDA into something it is not.

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