The conversion of data from optical to electrical and that from electrical to optical are key functions in optical communications. Obviously, optical data can go down optical fibers only in optical form, but processing it--for example, to remove accumulated noise or to add or drop data streams--requires digitized electrical pulses.
Until now, this conversion and reconversion process has been complicated and costly because of the many components needed to do the job. But new technology from Infinera Corp., a start-up in Sunnyvale, Calif., promises to make the transformations simple and inexpensive by combining more than 50 optical components--all the elements needed to convert as many as 10 optical data streams from light to electrons and back to light again--onto just two photonic ICs.
The two ICs that convert signals between the optical and electrical domains are the part of an optical networking system that slashes the cost of the conversion process by an order of magnitude. It is the first time such highly integrated photonic chips have ever been built. The new technology has the potential to reshape the communications industry.
Michael Howard, principal analyst and cofounder of Infonetics Research Inc., in San Jose, Calif., says he is a skeptic when it comes to the importance placed on some new technologies. He's been too many places where people said they had a disruptive technology when it was only incremental. Infinera is not one of them: "Infinera has developed a disruptive technology," Howard told IEEE Spectrum . "The breakthrough of having so many [photonic and electrical] devices on two chips gives it compactness and a set of capabilities that is not available anywhere else. It will force major carriers to look at this technology."
In optical communications systems, data starts out as electrical pulses and is encoded onto a laser beam by an optical modulator that pulses the output of a laser diode according to the pattern of 1s and 0s to be transmitted. The already high data rates of optical communications are made still higher by passing the outputs of several lasers of different wavelengths through a multiplexer, which combines them into a single beam that travels down an optical fiber.
At the end of the fiber, a demultiplexer separates the beam into its constituent wavelengths, and photodiodes convert the data riding on each wavelength back into an electrical signal. Once the processing of the data is complete, it is converted back to optical form to continue on its path.
Today this double conversion is very expensive because each element--each laser, multiplexer, demultiplexer, and photodiode--is in a separate package. And these packages are costly, because each requires precise alignment of an optical fiber with the processing element. So, for example, a laser diode costs between US $10 and $50, but the packaged part may cost more than $1000, says Jagdeep Singh, Infinera's chief executive officer. Add up the more than 50 optical parts needed, and the cost of the conversion process quickly goes through the roof.
For this reason, he says, communications companies avoid the conversion process wherever possible and instead amplify the optical signal itself, which has numerous drawbacks. Amplification is needed about every 80 kilometers to counteract the naturally occurring attenuation in a fiber and to bring the optical signal back up to detectable levels. A lot of fine-tuning is involved--of the total power through each amplifier as well as of the balance of power among the different wavelengths. And keeping the data in the optical domain prevents the carrier from filtering noise out of the signal, monitoring digital performance, or adding customers along the signal path.
Infinera's breakthrough lies in integrating all of the optical processing elements onto the two photonic ICs. Here's how the system works. Light from the optical fiber arrives at the receiver chip at the standard data rate of 10 gigabits per second for each of 10 wavelengths. A demultiplexer separates the light into separate wavelengths. Each wavelength travels through a semiconductor waveguide to a photodiode, which converts the data from optical pulses to electrical ones. The electrical digital data travels from the receiver chip to ordinary silicon CMOS circuitry for processing. Once processing is complete, the electrical data goes to the transmitter chip, where the reconversion process takes place.
On the transmitter chip are 10 laser diodes of different wavelengths and 10 laser modulators. Each modulator encodes the electrical data from a data stream onto one of the laser diode beams, converting it from an electrical to an optical signal. A multiplexer combines the 10 optical signals into a single beam for transmission through an optical fiber. Only two chip-to-optical-fiber alignments are required, one for each photonic package.
The minimization of these connections in Infinera's system is particularly important because it preserves signal strength, says Singh. At every connection, an optical signal loses up to half its power. In a conventional discrete system, such connections must occur many times. So on the receiver side, for example, the optical signal passes from the optical fiber to the demultiplexer chip, then back to an optical fiber and then onto the photodiode chip. That's three connections. The transmission side adds five more. And that doesn't include the fiber jumper cables needed if the components are on separate circuit boards.
The ability to put 10 laser diodes with different wavelengths onto the same chip is an essential element of Infinera's innovation. Needless to say, the company is not revealing how it does it. "But even more challenging than that," says Rick Dodd, Infinera's product marketing director, "was determining the specifications for the hundreds of parameters on the chip."
The company manufactures the photonic ICs, which are made out of indium phosphide, in its own wafer fab--designed and built specifically for this purpose. But it does not sell the ICs individually. It sells a complete system that contains the photonic chips along with the CMOS circuits needed to perform switching, routing, and other digital functions. Its first product is the Infinera DTN, a digital optical networking system with a bandwidth of 400 gigabits per second that can handle both Sonet and Ethernet transport, which together cover nearly all of today's network traffic.
The DTN should make it relatively inexpensive for communications companies to include more digital processing capability throughout the network, making it possible to pick up and drop off data streams at many more locations than are available in today's networks.
Infonetics's Howard suggests that Infinera's business model of selling systems rather than components could be one of its biggest challenges. "Because it is such a disruptive technology, they decided to keep the advantage to themselves by building the equipment that uses the technology." But now, to handle marketing successfully, "they're going to need a large partner like an Alcatel or a Lucent."
In today's market, Howard explains, carriers want a big company that has been in business for a long time to guarantee that if the little company goes out of business, the technology will still be available.
Infinera, which has operated in stealth mode since 2001, announced the DTN in May and plans to start carrier trials this summer. If all goes well, the company's technology may truly transform the way communications networks transmit and manipulate data.