Broadening scheme solves slow light bandwidth issues

14 Mar 2006

Practical all-optical buffers and optical routers are now a step closer thanks to work being carried out in Switzerland and Japan.

A simple and inexpensive pump spectral broadening technique could be the key to producing high-speed all-optical buffers and optical routers, according to researchers in Switzerland and Japan. (Optics Express 14 1395)

Slow light team

The team, from the Ecole Polytechnique Fédérale de Lausanne (EPFL) and the University of Toyko, relies on stimulated Brillouin scattering (SBS). Up until now, the main limitation of this approach has been its restricted bandwidth of around 35 MHz in conventional singlemode optical fiber.

"We have improved the bandwidth of the interaction so as to transmit Gbps signals," Miguel Gonzalez Herraez told Optics.org. "Slow light setups have traditionally very limited bandwidth in comparison with the requirements of optical networks. But this result allows us to use Brillouin optical buffers in optical networks, since the bandwidth of the buffers can be made to match the bit-rate of the network."

To control the speed of light using SBS, a pump beam is used to amplify a probe beam at a slightly longer wavelength. This amplification leads to a delay in the probe signal.

In their experiment, the researchers use two conventional temperature and current-controlled DFB lasers to generate the pump and probe beams. Gonzalez Herraez and colleagues modulate the current of the pump laser to broaden its spectrum, which in turn broadens the Brillouin gain and improves the slow light bandwidth.

"With this method, we can achieve GHz-bandwidth slow light by simply adding noise to the electrical input of our laser," said Gonzalez Herraez. "In our experiments, we show the delaying of 2.7 ns pulses over slightly more than one pulse length. Our procedure is simple, inexpensive and uses off-the-shelf materials."

The team is now trying to control the speed of light without changing the amplitude of the signal. "A fractional delay of one pulse length currently requires an amplification of 30 dB, meaning that the signal is undesirably made 1000 times bigger," explained Gonzalez Herraez. "We are very close now to showing that you may have a group velocity control with no amplitude change. In our view, this would finally open the way to use these buffers in networks."