The right components make all the difference

A discussion with Steve Alexander, senior vice president and chief technology officer of Ciena Corp.

Conard Holton, editor-in-chief
WDM Solutions

First in a new series that will feature in-depth interviews with leaders in WDM technology.

About Mr. Alexander. Steve Alexander is senior vice president and chief technology officer of Ciena Corp. (Linthicum, MD). Before joining Ciena in 1994, he was employed at MIT Lincoln Laboratory, where he worked on a WDM testbed, high-capacity intersatellite optical communication systems, optoelectronic device characterization, and high-speed coherent detection systems. He received both his BS and MS degrees in electrical engineering from the Georgia Institute of Technology. He can be reached at 410-865-8544 or [email protected].

WDM Solutions: Ciena was one of the first manufacturers to develop and deploy WDM systems. Can you tell us about the initial component technologies you chose and why?

Alexander: The strength we had at the time—going back to 1994—is that we had a clean sheet of paper to start with. We didn`t have what you might call religious belief on the technology front. What we wanted to do was pick technologies that really allowed us to achieve a dramatic improvement in capacity.

When you looked at the type of filter technologies that were available then, they spanned the gamut from thin-film filters through fiber Fabry-Perot heterodyne techniques and fiber Bragg gratings. The one that would get us to where we needed to go to in terms of cost, complexity, and manufacturability was really the fiber Bragg grating, and we`ve been very happy with choice.

In addition, we realized that if we were really going to be pushing channel spacings to the very fine structures they are at today, with 50 GHz shipping commercially, then we would need frequency-locked lasers. So we basically built that into the product from the beginning. And time has proven us right since everybody else has had to add frequency locking. By the way, next year we will be offering 25 GHz and by the end of next year, 12.5 GHz channel spacing.

The last piece was the use of gain- flattened erbium-doped fiber amplifiers (EDFAs). In the 1994 timeframe, you couldn`t go out and buy a WDM amplifier of any kind. The systems that we were going to be competing against were two-, four-, and eight-channel systems, and we were looking at producing a product that would scale upwards into the hundreds of channels and occupy significantly more optical bandwidth than the store-bought amplifiers, and so we crafted our own amplifier architectures in the way we built the pumps and the flattening schemes to make them very amenable for use in WDM systems.

WDM Solutions: Did you make these amplifiers yourself?

Alexander: We bought the parts on the merchant marketplace-the pump lasers, the couplers, and that sort of thing-and we actually assembled our own amplifiers basically to our own design.

WDM Solutions: I know that you are moving to closer channel spacing in your fiber Bragg gratings. How are you able to move to closer channel spacing?

Alexander: In fact, the real enabling technology is having a very good understanding of the laser stability issues-how do you frequency lock the laser and make it stay put over tens of years in the field? Another piece of it is obviously the optical-selective element. What is it that is going carve up that spectrum into such a fine spacing? And again this is where the fiber Bragg grating really shows its strength. The last piece is a good understanding of how densely packed signals propagate over fibers.

WDM Solutions: So what markets is Ciena in?

Alexander: We span all the way from ultralong-haul intercity transport down to metropolitan access and edge access. The product lines go from the port speeds of OC-3 and OC-12, up to OC-192. So it is a diverse product line and includes both transport and switching capability.

WDM Solutions: How are you approaching ultralong haul-with Raman amplification, FEC?

Alexander: All the ultralong-haul approaches really have their origin in what people have done for the submarine lightwave case and when you look at what people did there, it was good applications of the available technologies. Forward error correction (FEC) is clearly a piece of that. Understanding the nonlinearities and then either compensating for them by changing launch power or by modifying fiber loss and gain through the use of Raman amplification in conjunction with erbium amplification-these are all known tricks that come out of the submarine world, where for years they have been solving the problem of how do I get my light to go 5000 km, and applying it into the terrestrial marketplace.

WDM Solutions: Turning to the manufacturing process, what components do you make in-house and what do you subcontract at this point?

Alexander: We continue to manufacture all of our own fiber Bragg gratings. There was just no commercial market for them at the start, so we built our own and now we have what we believe is the world-leading technology for producing the kind of gratings we need for our systems. We have the capability in-house to do some thin-film filter manufacturing for certain key applications. We subcontract out an awful lot of the remaining pieces of the system. We have a lot of parts built to our specs by the suppliers you would expect, and then we integrate them into systems here.

WDM Solutions: Can you tell us a little bit about how you work with your suppliers?

Alexander: Closely. What we try to do is have a relationship with them where we understand where they are moving with their in-house technologies and what they have, not only on the plate today but what is coming over the next months and years. In many cases, we can help guide that development along the lines of where we see the systems business evolving. We will frequently form a strategic alliance with people in order to bring the correct tech- nology to the marketplace.

WDM Solutions: What do you see as the most important next-generation technologies that you are hoping to incorporate or already have incorporated into your systems?

Alexander: I think we are just now beginning to see the first examples of photonic integrated circuits. If you go back to the revolution that occurred in electronics, people were able to go from discrete transistors to much higher-functionality integrated circuits. Some of that same promise is there in the photonics world as well, but we are just at the beginning of that curve. So in the past, where we might have had to purchase four, five, or six individual components and string them together to build a subsystem, we are now able to purchase that entire subsystem.

WDM Solutions: You are already able to do this?

Alexander: We are in certain cases. Some simple examples-the ability to have a laser and a electroabsorption modulator in the same package, in what people call the integrated-laser module approach. That is the first baby step of a photonic integrated circuit. Similarly, if you have things like integrated attenuators with couplers, taps, or waveguides, we can start to combine multiple functions. I think this capability has a tremendous economic advantage as well as advantages in things like footprint, size, density, and power consumption.

WDM Solutions: What about all-optical switches?

Alexander: In terms of simple all-optical switching-the 1 × 2, 2 × 2 switches- we actually support these today. We`ve got a customer that has a route where the fibers are buried on either side of a highway. The fibers are subject to relatively frequent damage from car accidents or fires, for example. In this case, we support optical switching at every hut, and the customer can basically redirect signals from one side of the road to the other if an event takes out one of the fibers.

What we see coming is the next generation of those switches in terms of 16 × 16, 32 × 32 switches, and also 3-D MEMS that can get you up to much higher switching numbers. The issue with all the MEMS approaches right now is that they really haven`t proven in yet. There haven`t been enough of the fabrics built and operated for people to get a handle on manufacturing yield and what they actually can do.

WDM Solutions: So you are not committing yourself to one switching technology?

Alexander: No. What we have done is design our CoreDirector product-which is essentially a big switch-to have three layers in it. The bottom layer is the one shipping today that has the OEO (optical-electronic-optical) layer; the middle layer is a wavelength selective fabric; and the top layer is a pure fiber switch. The OEO fabric can emulate all the other functions until the all-optical fabrics prove their worth. We expect to be offering classic all-optical switching functions next year.

WDM Solutions: Do you think we`ll see 40-Gbit/s per channel [OC-768] systems coming soon?

Testing a Ciena channel shelf.