Focussed efforts

InterComms talks to Dr. Gunther Vollrath, CEO of AIFOTEC GmbH, about recent developments in Fibre Grating Laser technology and the road ahead

Dr. Gunther Vollrath, born in 1964, began his involvement with semiconductor lasers at the Technical University of Braunschweig, Germany, where he completed his Dipl.-Ing. in the field of optoelectronic emitters. He then moved to the Research and Development Center of German Telekom in Darmstadt, working as a research fellow from the Technical University of Braunschweig. As a research fellow he was engaged in R&D work for integration of optoelectronic components for which he received his Dr.-Ing. Degree (Ph. D. in Electrical Engineering). Afterwards he worked as a consultant for the Research and Development Center of German Telekom in Darmstadt where together with co-workers, he developed a new hybrid lightemitting device with high performance and low costs, the so-called FGL.

Beneath his sound technical know-how and experience in the optoelectronics arena, Dr. Vollrath has lots of experience in setting up, building up and leading a company. Together with Claus-Georg Müller, he successfully co-founded AIFOtec AG in Munich, Germany, where he was acting CTO / COO. AIFOtec AG was later sold to Finisar Inc., Sunnyvale CA, USA. At Finisar Europe GmbH, he served as Director of Business Development and Product Marketing. In July 2002, together with Claus-Georg Müller, he established AIFOTEC GmbH as a management buyout from Finisar Europe GmbH.

Q: What is the state of the art in FGL's (Fibre Grating Laser) and what is AIFOTEC doing to push this further along?
A: Some time ago there was a lot of effort, especially in the R&D centres of the big Telecom carriers in Europe (Germany, Britain, Italy to name a few), in work on lasers with an external Bragg Grating cavity. But with their privatisation, work on this technology slowed. Nowadays there is a single US company manufacturing this type of laser. These ECL (Externally Cavity Lasers), which is the same principle as a FGL but with a Bragg Grating in the fibre.

The focus of AIFOTEC is to not only develop the FGL technology further but also develop a low cost manufacturing platform based on a Silicon motherboard in combination with a SSC-Laser Spot- Size-Converter. This gives us the opportunity to directly launch the light into the fibre without the need for any optical coupling elements e.g. lenses or lensed fibres. Also because free space optics are not involved, this eliminates the need for an expensive and bulky hermetic housing. So, to push this technology along we ar

e not only designing FGL lasers, but also a small footprint, low cost, high volume manufacturability, Silicon platform. The assembly is done by laser soldering, which means the parts are spot-soldered from the downside of the platform, which is a very reliable process. The alignment of the laser is done by image recognition, so the laser is not operating during the assembly. This gives us a very high throughput and control of accuracy.

Q: What advantages do FGL's have over EML's (Externally Modulated Lasers) and DFB's (Distributed Feedback Lasers)?
A: The main advantage of the EML over the DFB in terms of dynamic behaviour is the separation of the gain and modulator section. This gives a small line width and low dynamic chip because the Bragg Grating is not modulated by parasitic effects. By changing the carrier density and heat effects the refractive index rsp. peak wavelength of the laser is also modulated, caused by the modulating current. On the other side, the modulator in an EML also shows an absorption in the "off" state, which leads to rather low optical output powers. In the FGL setup, the gain section is modulated and the wavelength selection section, which is in fact the Bragg grating in the fibre, is completely insensitive of current induced effects and has a low wavelength shift with the temperature in comparison to the semiconductor material. So the FGL is a "best breed" approach and combines the advantages of the DFB with the advantages of the EML. In combination with the platform technology this gives a very competitive product to the market. Another advantage FGL's have over both types of lasers is the fast customer response and low inventory setup for DWDM applications. This is because the wavelength of the device is determined in the assembly process (last step in the manufacturing flow) instead of the chip manufacturing process (first step in the manufacturing flow).

Q: Are FGL's a one-for-one replacement or are there some roles in which EML will not be replaced by FGL's?
A: The FGL has a low chirp in comparison to a DFB, but in the EML the chirp can be lower or even negative. So the transmission span for an EML is larger than for an FGL. So for the time being there will be a role for the EML, especially for long haul transmission like 80km with 10 GbE.

Q: Much of your work is proprietary and in-house. Do you think there is a role for international standards in the development of FGL's in the future?
A: Yes, I think we have to go this way, but maybe it is a long way. If you look at the module market, there were a lot of standards out there (300 pin MSA, XENPAK, XPAK, X2, XFP) and there are still different standards. In the components market the Japanese manufacturers are now starting to standardize 10 GbE TOSAs (Transmitter Optical Subassembly). So I think this is the only way to push the whole optical market towards lower prices and wider acceptance like FTTx. We have to go this way also with the FGL technology. I believe in the future we will have a standard called FGL TOSA.

Q: You've outlined a role for FGL's in the Metropolitan Area Networking (MAN) market. Are there further applications that would be equally relevant?
A: The MAN market is only the starting point for us because this is where we see the main bottlenecks for the time being. However, we also want to push the FGL technology towards high-end markets like WAN (Wide Area Networking) and low cost markets like SAN (Storage Area Networking) or LAN (Local Area Networking). When talking about the low cost LAN market you do not need the FGL performances. This is where our platform concept clicks in. For example, when we use our SSC-Laser in combination with a standard fibre without a Bragg Grating, we have a really low cost, small footprint FP (Fabry-Perot) laser.

Q: You say that bottlenecks in the MAN exist today. What are the current means to alleviate the problems they cause?
A: In the MAN market it is much more about money (or costs per transmitted byte) than in the WAN market. Nowadays the WAN guys, coming from the Telecom Market, are trying to sell their components cheap or trying to downsize their high-end modules. They however, have designed a Mercedes or a Porsche when you need a relatively cheap city (or Metropolitan Area) car for two persons. You have to design a small "footprint" and low cost car. On the other side the LAN guys are trying to scale up their products, but they are used to high throughput environments like in the electronics industry. However, fibreoptics are a completely different game. So what we need is a technology platform that is designed to fulfil the needs of these particular market demands which means relative high performance, but also low costs.

Q: What are the technical challenges that remain in FGL technology?
A: The FGL is in principle a DBR-Laser (Distributed Bragg Reflection), which means an external Bragg Grating. Therefore the phase shift of the gain chip and the Bragg Grating has to be kept constant. For this we use a patent pending technology, which uses the monitor current and a small heating wire on the gain element. In my opinion the main challenge is to develop a low cost production set-up in contrast to the old "clock-and-watch-maker" approach. I do not think anyone will pay for the FGL performance only, but a combination of performance and costs will make the race.

Q: What do you think about the future of fibre optic manufacturing?
A: "Clock-and-watch-maker" devices, in the market today, can be compared to the radio tube in the electronics industry: mechanical devices in a hermetic housing manufactured with too much handicraft. Then Bardeen, Brattain and Shockley built the first transistor. No hermetic housing, no mechanical alignments and a small footprint. And then the road goes down towards further integration. In the fibreoptics industry we are waiting for this first 'transistor'. Then there is integration on. Customers worry that if one laser dies, the whole array will have to be replaced. These same discussions occurred at the beginning of the electronics integration.

We have jettisoned today's requirement mindset. Currently qualification is done for an expected lifetime of 30 years and devices are tested in very harsh environments. Who really needs all this? Who needs 30 years lifetime? These rigorous standards come from the submarine and military applications where this makes sense. But you will not use a server or laptop for 30 years, so this is a ridiculous idea. The fiberoptics market will and has to move from a "hero device" market to a commodity market to bring the costs down and enable technological improvement.

Q: When talking about integration, do you have any thoughts about monolithic versus hybrid integration approaches?
A: Yes, for sure. You know, I wrote my PhD thesis about monolithic integration. It is a nice arena for R&D work, but in the real world you have to face many problems because you have to control a quaternary semiconductor like InGaAsP instead of only Silicon. The requirements are so different for a laser, a photodiode or an optical multiplexer / demultiplexer. This means you cannot duplicate one cell again and again like, for example, in a CMOS device. So in the near term I do not believe in monolithic integration schemes. Bringing together the best out of the different worlds - like the laser drive made of Silicon, the multiplexer made of Silica on Silicon, a Spot-Size-Converter Laser with or without Bragg Grating and integrating all this on a Silicon microbench - this is the next step in fibreoptics manufacturing.

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