All-optical wavelength conversion/time-slot interchange (TSI) performed


by Sunny Bains

A new multistage network has been demonstrated to change both the optical-time-division multiplexing (OTDM) time slots and wavelength of a 2.5 Gbit/s signal without optoelectronic conversion. Created by researchers at the University of Southern California (Los Angeles, CA), Stanford University, (Stanford, CA), and Lucent Technologies (Holmdel, NJ), it works by exploiting difference-frequency generation (DFG) and tunable fiber Bragg gratings (FBGs). Though the ability to perform interchange is currently limited, the system could allow an extra degree of flexibility for future high-density networks.

To perform wavelength conversion, researchers chose to use DFG in a periodically-poled lithium niobate waveguide.1 This is a very fast conversion method relying on interaction between a signal and high-power pump beam inside the nonlinear crystal. The necessity for the pump beam to be present allows selectivity in which signal pulses are converted.

For instance, the USC/Stanford/Lucent team chose to supply a pump made up of rectangular pulses that would initially only address, and wavelength-convert the odd-numbered time slots (see figure). A highly wavelength-selective FBG was then tuned such that the new wavelength would experience a preset optical delay, here equal to two time slots. Meanwhile, the original signal continues to propagate, and the wavelength transform repeated: this time selecting only the even-numbered time slots for conversion. When the two signals were recombined, the signal data was the same, but the wavelength had changed and the time-slots had been shuffled.

The system works well, and most problems with performance should be easily solved. Researchers say, for instance, that with some optimization, they could effectively eliminate crosstalk: this currently introduces an increase in bit-error rate and thus a power penalty. Their conversion efficiency, currently at -16 dB, could also be greatly improved using known techniques. However, allowing an arbitrarily complex time-slot interchange may not be easy, particularly for signals with a high number of slots. To do this properly, each would require it's own conversion stage.

For more information, contact Alan Willner at

1. M. C. Cardakli et al, IEEE Phot. Tech. Lett. 14 (2) (February 2002).

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