MICHAEL LEBBY ELECTRO-OPTIC POLYMERS
It’s not often a potentially radical new technology comes on the market, but it seems that electro- optic polymers could be one. The concept of switching (or modulating) light using organic materials is not new, but in the drive for ever-higher bandwidth and low power consumption, the time could have come for this radical approach to optical switching (modulation). Optical Connections editor Peter Dykes spoke with Michael Lebby , CEO at Lightwave Logic, a company which has been working on the technology for a number of years and is now using it to develop applications for the communications industry. A REVOLUTION IN OPTICAL SWITCHING? ELECTRO-OPTIC POLYMERS
What are electro-optic polymers?
electro optic polymers because they knew about the bandwidth that comes with them, and so there was a lot of research funded by the likes of NSF and DARPA and American and European governments, and all the big multinationals had electro optic polymer programs. But in the late 80s or early 90s, somebody came up with WDM. But this meant you didn’t need the big bandwidths because you had different wavelengths going down the same pipe, so that took the excitement out of the use of polymers and a lot of the funding stopped. What really took off was Lithium Niobate and WDM. When I first joined Lightwave Logic in 2015, we wanted to make an external modulator from electro optic polymers. At the time, the requirements for polymers to get into telco were pretty strict, such as a 20-year lifetime, and extreme temperature ranges, which were all pretty difficult, however, data centres weren’t requiring those types of capabilities. The problem then, was the negative perception of the industry that they were not that reliable. What we’ve done at Lightwave Logic is focus on the materials and designed them to be robust, stable from light, temperature, and lifetime standpoints, and now we’re working on activation energies and proving reliability. Every customer has their own reliability requirements, but in general, they are not that far different, and so putting that data set together is where we are, and the negative perception has, for the most part, gone away. So I’m excited and I think we are progressing steadily. We’ve seen some demonstrations in prototyping, and suddenly, electro optic polymers are a candidate. Indium phosphide-based electro absorption modulators have been demonstrated at the level of performance, and I’ve seen some interesting work on
thin film Lithium Niobate (TFLN), even though it’s tricky to fabricate in silicon foundries.
PD
Electro-optic polymers is really a generic name for electro-optic chromophores, which is an
ML
So what does that all mean?
PD
organic material that has dipoles which can be aligned using an electric field, similar to liquid crystals. The same principle applies to electro-optic polymers, except it’s a solid material. However, in order to align the dipoles with a voltage, you have to heat the material up and soften it. When you take the field and the heat away, the dipoles remain in position. Then, when you apply electric voltage, you can make a small change to the optical refractive index. If you then put it into a Mach-Zender or slot type modulator structure, you can get more of the refractive index to be changed. The basic principle is switching or modulating light using an electric field, which is no different than semiconductor materials. However, the interesting thing about using organic materials is what is known as a velocity phase match, the phase matching of the material, and the velocity is pretty much aligned, and that allows extremely high bandwidths in the order of about a terahertz. This means you can design three dB electro optics, way over 100 gigahertz.
It means that electro-optic polymer-based devices can work fairly easily with those data rates
ML
that are being anticipated today using the standard encoding systems. Also, using organics over semiconductors means you don’t need a volt to drive the modulator, and now you have a device that can save power consumption. Indeed, you don’t have to have a driver because you can drive it directly from a DSP or an ASIC, because you’re within the CMOS capabilities. It’s also really small if you use a polymer slot modulator structure, then you have a very tiny form factor, and you now have a technique to speed up the natural silicon modulator that’s on a silicon photonics PIC. You can take this material, which is in powder form, add a solvent to it, put it in a dedicated spinner and spin it onto a 200-millimetre silicon wafer, meaning you can create polymer modulators using the silicon infrastructure on the wafer using standard fabrication techniques in a silicon foundry. This is an interesting attribute, because some of the competitive technologies, like TFLN or barium titanate (BTO) are complicated to fabricate in a silicon foundry. One key reason is that these materials are difficult to integrate into silicon because they’re significantly dissimilar to silicon semiconductors, and typically require wafer bonding. Fortunately, electro- optic polymers are an interesting new platform that has the performance that the industry is really looking for, and from that standpoint, I think it’s really exciting.
Is this a new technology?
PD
Electro optic polymers have been around for 30 to 40 years, when the telecom business was in the
ML
era of Ma Bell and Bell Labs, and we believed TDM was the answer to increased data rates. At that time, we looked at modulators, because we knew lasers were slow, so we looked at external modulators. People started looking at
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