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Integrated optical add/drop filter

Michael A team of MIT graduate students has recently conquered a major challenge of high-capacity optical communications, by inventing a device that can splice a single signal into or out of the many being transmitted together along a fiberoptic line.

The members of the team are all experienced scholars of Electrical Engineering, set to win their PHDs from MIT this year (2000). Michael Lim, age 28, born in Seoul, South Korea and raised in Duluth, Georgia, earned as BS from MIT (1993) and an MS from the University of Southern California (1995). M. Jalal Khan, age 27, from Karachi, Pakistan, earned both a BS (1994) and MS (1996) from MIT. Thomas Murphy, age 27, born in Arlington, Virginia and raised in nearby Falls Church, graduated with a double major in electrical engineering (BS) and Physics (BA) from Rice University in 1994.

JalalThe state of the art in optical communications employs wavelength division multiplexing (WDM), which is the transmission concurrently of many independent data channels, each at a different carrier frequency, along a single fiber. The different signals can be separated out at the receiving point. However, to achieve maximum efficiency in a network that has multiple nodes rather than a single transmission and receiving point, a method of splitting out (or splicing in) a specific signal or signals at a node, without disrupting the original bundle of signals, is required. Lim, Khan and Murphy have invented a wholly new and improved version of such an integrated "add/drop filter."

ThomasPreviously, integrated optical devices operated on lithographic principles used in semiconductors; but such devices are insufficiently subtle to deal with fiberoptic signals. Instead, the team built a filter that uses Bragg gratings to isolate WDM signals. Taking an unprecedented step in nanotechnology, the team built into a ridge waveguide a series of precisely positioned grooves, which act as narrow-band optical resonators, each excited by only one WDM signal. Once isolated, the signal's optical parameters can be matched with electrical parameters (and vice versa) using computer simulations.

Lim, Khan and Murphy have produced a device whose mechanism is complex; but its impact is easy to understand. By radically increasing the capacity and efficiency of WDM communications, their add/drop filter will improve every person's access to information, be it by cable television, telephone, or Internet. In addition, the filter's fabrication process has set the precedent for the large-scale manufacture of other complex integrated optical devices.

Michael Lim, M. Jalal Khan and Thomas Murphy succeeded where numerous industrial teams with greater numbers, experience and funding had failed to find a solution. In honor of their achievement, the team was recently awarded the first-ever Lemelson-MIT Team Prize for innovation in telecommunications and networking, sponsored by Unisphere Solutions, Inc. They plan to use the $30,000 prize money as a seed fund for further research and development in optical communications devices.

[April 2000]

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