Meeting High-Speed Demand with Optical Circuits

Estimated reading time: 7 minutes

With the growing demand for great amounts of data to move at higher rates, optical circuits may be the path forward. In an interview with I-Connect007 Publisher Barry Matties, Felix Betschon of vario-optics ag provides an update on the advancements in optical circuit boards, and explains the core advantage of this technology and what circuit designers must do to add optical circuits into their design disciplines.

Barry Matties: Why don't you talk a little bit about vario-optics—who you are and what you do.

Felix Betschon: We are a spin-off from Varioprint, a leading PCB manufacturer from Switzerland. We produce electro-optical circuit boards, a combination of electrical and optical circuits. We believe that's the really unique thing about us, and that's what also, in the near future, is going to provide benefits to a lot of different applications.

Matties: We're seeing growing demand in the server market for this technology.

Betschon: Yes, also for supercomputing or pretty much anywhere you have very high data rates.

Matties: Are you doing this in a commercial endeavor already?

Betschon: Yes, we understand ourselves as a manufacturer. Everything we do, also in terms of customer projects, should have a certain chance to eventually be a commercial product. So we are not interested in doing lab R&D; we really want to see an outcome at the end. We already have established products within sensor applications. That's another market, where we see a fit for our technology, in particular for highly integrated electro-optical sensors for different applications. The medical or chemical industries are where we can already see these products.

Matties: What's the main advantage between electrical and optical?

Betschon: It's not only the speed. The main motivation is power loss. We have reduced power losses compared to a purely electrical PCB; if it comes, for example, to 25 gigabits per second per lane, the estimation on the reduced losses is quite a significant number. If you have a throughput on a board in the five terabytes range, the losses are really significant. That's the main issue for a lot of our customers’ problems for which we can provide a solution.

The other thing is cost. We strongly believe that the solution based on the electro-optical circuit boards will save costs for high data rates and especially for complex routings. That's coming again from the fact that you can reduce power loss; you do not need so much effort for cooling, the EMC measures can be simplified as well as the board layout or buildup can be much easier, and you can also eliminate the expensive high-frequency dielectric layers in between. Those are the main reasons: power loss and cost.

Matties: So with regard to designing the board with the optics embedded, is this a new design discipline for circuit designers that they must learn?

Betschon: We have certain design rules, but they are not really difficult to understand. There are certain things like bend radius that you have to take care with. That's something which eventually would be implemented in the CAD tool.

But on the other hand, they can use standard CAD tools to design it. Therefore it is not really a big step. I would say that a person with a microwave background has already seen similar problems.

Matties: And what about the customers? How fast are they adopting this technology?

Betschon: It is now picking up. We have been working on it for 12 years. We always thought now is the time, but there are certain circumstances that have changed. In the past, our electro-optical circuit board lacked an established ecosystem. Suitable optical connectors and transceivers were not available. Now we have several collaborations with multiple connector suppliers. If it comes to a project now, we already have connector solutions that we can choose from, and that's a big difference.

One can see the ecosystem around this electro-optical circuit board is developing right now. That's the difference and it's a positive thing which leads to new opportunities, new interests and new customers.

Matties: When should a customer consider using this technology?

Betschon: As soon as they have very high data rate densities through the board.

Matties: So it's not just the rate, it's the amount?

Betschon: If it is only, let's say a single 50 gigabits per second link, from A to B on a board, then I would say our technology is too expensive. Our technology is based on photolithography. The cost in mask-lithography does not depend on the complexity of the structures. Therefore, the more complex the optical layout is, the more competitive our technology is.

Matties: It seems like this is really going to gain traction pretty quickly now with the demands of speed, the Internet of Things, and people trying to get more information quickly, as some of the driving factors behind this.

Betschon: Yes, in these cases, as well as 5G phones and the equipment needed to support it. Everything is moving towards higher speeds. The data must be available everywhere.

Matties: And instantly.

Betschon: Like the desert here in Phoenix, for example. Specific conditions don’t make it easier. But new solutions have to be found. On the other hand, you have the restrictions coming from the different governments for green computing, which puts a lot of pressure on many OEMs for increasing the speed but not increasing the power.

Matties: What is the manufacturing process like?

Betschon: We are using a UV-curable liquid polymer, which we deposit on the substrate. The substrate can be any PCB substrate, even flexible polyimide is a possibility. This polymer can be structured to waveguides, using mask or laser direct-writing systems. After removing the uncured material, we’re covering it with another polymer with a slightly different refractive index.  Light guiding happens in the same way as in glass fibers, using the refractive index difference.

Matties: Is there a space requirement or minimum for fine line type of work? How many beams can we transmit on a board?

Betschon: We can generate structures down to five micrometers for single-mode waveguides, and up to 500µm for multimode. We can realize an aspect ratio of 1:1, which is important for the pitch. For example, for a 50µm waveguide with a 100µm pitch can be realized. You don't have any cross-talk between them and with that a very high number of waveguides can be realized on a board. Multimode waveguides can be routed with a pitch of 100µm, and the pitch for singlemode waveguides is four times smaller. This, without significant crosstalk between the waveguides.

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