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How Do We Change the Color of a Beam of Light Carried in Fiber Cables in the Internet?

Hilmi Volkan Demir
Electrical Engineering
Stanford University
November 2002

Have you ever wondered why your transatlantic phone calls are now as cheap as five cents a minute? Or why can you chat with a friend of yours on the other end of the country on the phone or on the computer with almost no delay in your conversation? Or why is it faster than ever to download a file on the web, the World-Wide-Web that some people once used to call World-Wide-Wait? It is all because optical fiber communication, which is the backbone of the Internet, has become cheaper, better and faster--and it will get much cheaper, better and faster in future! In order to improve today's optical communication systems, we propose new devices that address some of the current problems in optical communication systems. We design, model, and build these devices; we also test them in communication systems.

In particular, we work on devices that take the signal that is carried in a beam of light going through an optical fiber and transfer that signal to another beam of light going through the same fiber. Using optical beams of different colors, our devices convert optical information encoded in one color beam to another. Such devices are useful, for example, in optical fiber communication systems that utilize several beams of different colors to transport information.

In optical fiber communication systems, it is typically cost-effective to transport information using multiple beams of light, each with a different color, through a single fiber. In such communication schemes where our devices can be used, each beam of light can be distinguished from the rest by its distinct color even though all of the light beams go through the same fiber simultaneously. Since each light beam carries a different set of information, the total amount of information communicated in a single fiber is increased as many times as the number of unique colors in the fiber. That way, the cost of the system is effectively reduced. However, it is difficult to manage such a system because it is often necessary to manipulate the colors of these light beams. For instance, two different beams of the same color may depart from two different points in a communication system and might later need to share the same fiber to arrive at the same destination. But two beams of the same color cannot use the same fiber--they will interfere with each other, and the information they carry will be lost. Hence, the color of one of these two beams has to be changed to a different color before they can share the same path. Therefore, in these communication systems, the inability to change the color of the light beams in the fiber poses a problem, and the devices that we are working on will remedy this problem.

Our devices combine a photodectector and a modulator in a new way. A photodetector detects an incoming optical signal and converts it to an outgoing electrical signal. A modulator, on the other hand, takes an incoming electrical signal and transfers it to an outgoing optical beam. Our devices integrate a photodetector and a modulator intimately into one compact device such that the photodetector part of the device detects an incoming light beam and drives the modulator part, which, in turn, transfers this incoming optical signal to an outgoing beam of a different color. This results in an outgoing beam of light that carries the same information as the ingoing light beam but in a different color. Furthermore, we can play with the other parameters of the outgoing beam such as its power level and its shape in time and space to improve the signal quality. Moreover, we can enable and disable these devices electrically, facilitating their control with electronics.

In the future, we plan to put together a group of these devices that will be able to handle multiple beams of different colors coming from one input fiber and simultaneously shuffle each of them into a different color before they go into the output fiber. With the use of such an array of our devices in an optical fiber communication system that employs multiple beams of different colors, we hope to solve the color management problem for high-speed information transfer in a cost-effective way.