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 firstname.lastname@example.org.
1. M. C. Cardakli et al, IEEE Phot. Tech. Lett. 14 (2) (February 2002).