by Sunny Bains
Researchers in Spain have extended a pulse replication technique to allow an entire set of WDM channels to be multiplied in a single fiber grating device. In numerical simulations, they have shown that pulse trains can be multiplied with little distortion and almost no energy loss. The team from the Polytechnic University of Madrid (UPM) says this technique can be used to produce optical time-division-multiplexed (OTDM) signals within the WDM channels, thus significantly increasing overall bandwidth. The devices could also be added to existing sources to produce ultrahigh-repitition-rate pulse streams.1
The proposed multichannel device (see figure) is based on a temporal version of the Talbot self-imaging effect. When a pulse travels through a medium with the right group-velocity dispersion—an appropriately designed linearly chirped fiber grating (LCFG), for example—the pulse sequence can be replicated a number of times. Small phase differences between the original and replicated trains mean that pulses interweave: the result being a multiplication of the density of pulses. Theoretically, this process involves no distortion or energy loss whatsoever.
The UPM team realized that, instead of performing this multiplication for each WDM channel individually, all of the required gratings could be built into a single device, with the grating structure required for each wavelength built up within just one piece of fiber. This works because each individual fiber grating device is inherently tuned to just one wavelength: only light in the appropriate band will interfere appropriately to produce the needed reflection. Because the wavebands are well-defined and discrete, hundreds of such gratings can be superimposed without any crosstalk.
In numerical simulations, the device designed by the UPM group performed well. However, because of high-frequency ripples and higher-order effects, the group-delay response was not perfect. As a result, a small amount of temporal broadening occured, as was a slight variation in the intensity of the different pulse streams. For more information, contact Miguel Muriel at email@example.com.Sunny Bains
1. José Azaña and Miguel A. Muriel, IEEE Phot. Tech. Lett. 13(12), 1358 (December 2001).