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Femtosecond Camera Takes Telecom System's Pulse

Photonics Spectra
Mar 1999
Daniel C. McCarthy, News Editor

The Optical Communications Research Group at the University of Bristol carries out research on optical components and subsystems for communications applications. As in most telecommunications research, part of the group's aim is to create practical systems that can manage bandwidths of one or more terabits. To this end, the university uses a femtosecond-resolution streak camera, the Imacon FS300 from DRS Hadland Ltd.
"At the moment, wavelength division multiplexing [WDM] is in vogue, and we carry out research in the area of WDM networking," said Richard Penty of the university's electrical and electronic engineering department. "But we feel that optical time division multiplexing still possesses a lot of potential."

The pulse source is key
Optical time division multiplexing uses a pulse train of very short pulses that are modulated and interleaved with other modulated pulse trains to generate higher aggregate data rates. The most critical component in this system, Penty explained, is the pulse source, and the most important parameters for this source are its pulse width and quality. Poor- quality pulses result in a lot of interference noise. The pulse width is important because shorter pulses enable more pulse trains to be interleaved, thus increasing the aggregate bandwidth of the link. It is the pulse width and quality of the pulse source that the Imacon FS300 measures.
The camera allows direct measurements of pulse shape and width down to 1-ps resolution. "We are aiming to achieve pulses of 100-fs width, which would allow [time division multiplexing] data rates in excess of 1 Tb/s. In our work to date, the original pulses are 20 ps long and are compressed to about 1 ps or less," Penty said.
He said the greatest benefit of Hadland's camera is real-time acquisition, which allows his group to make quick adjustments to the pulse generator. Previously, the researchers had taken measurements using a scanning autocorrelator that had an acquisition rate of two minutes.
Unlike the autocorrelator, Hadland's camera also permits much lower input powers to be used, removing the necessity for an erbium-doped fiber amplifier, Penty said.

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