'

Slowing light down to 245 meters per second!

Scientists at the University of California, Berkeley, have used a laser amplifier to slow light at room temperature at a speed slower than sound in air. This could lead to faster communication networks and change the world of optical memories and storage.

Several weeks ago, I told you about how Swiss researchers were able to control the speed of light both up and down. Today, I want to share with you the results obtained by other scientists at the University of California, Berkeley, which have used a laser amplifier to slow light at room temperature. With their device, they clocked the speed of light at 245 meters per second, slower than the speed of sound in air. This could lead to faster communication networks by eliminating the need for optical-electronic-optical (OEO) conversions. And the researchers want now to go even further, by almost 'freezing' light, which would completely change the world of optical memories and storage.

The research team, led by Connie J. Chang-Hasnain, UC Berkeley professor of electrical engineering and computer sciences, has created a device that uses a laser amplifier to slow the speed of light more than one million-fold. The researchers clocked the speed of light at 245 meters per second, or three-quarters the speed of sound in air. Moreover, the team did this at room temperature.

And things are moving fast (no puns intended!). Last year, Chang-Hasnain and her optoelectronics group were only able to reduce the speed of light to 6 miles per second, but at a temperature of 10 degrees Kelvin.

What is the reason for such a breakthrough for this Slow Light research project in less than one year? State-of-the-art lasers.

Below is a diagram showing the experiment setup with these new lasers (Credit: University of California, Berkeley).

The experiment setup to slow light

And here is the description of this setup.

A tunable laser provides the probe signal and is modulated using a Mach-Zehnder interferometer. The optical attenuator is used to adjust the signal power. PM OC and fibers are used to couple the light into the VCSEL cavity. Time domain measurements are carried out using a fast photo-receiver and oscilloscope. An EDFA is used to amplify the reflected signal. The optical spectra and power levels are monitored by OSA and power meter respectively.

[Note: Here are the meanings of some of the acronyms used in the diagram and its caption: VCSEL (vertical-cavity surface-emitting laser), EDFA (Erbium-doped fiber amplifier), OSA(optical spectrum analyzer), PM (polarization maintaining), OC (optical circulator).]

What can we expect from this experiment?

Adjusting the speed of light is part of an effort to overcome a bottleneck in optical communications. Optical signals speed along fiber networks, but are then jammed as they hit an intersection, or router. At these intersections, light signals are converted to slower moving electronic data to be directed to the correct path before being switched back to light, a process known as optical-electronic-optical (OEO) conversion.
"Controlling the speed of light along these networks could ultimately eliminate the need for these OEO conversions, which are both slow and costly to power," said Chang-Hasnain.

This experiment was recently described in Optics Express under the title "Tunable ultraslow light in vertical-cavity surface-emitting laser amplifier" (Vol. 13, No. 20, Pages 7899-7904, October 03, 2005). Here are two links to the abstract and the the full paper (PDF format, 6 pages, 361 KB) from which the above diagram was extracted.

Sources: University of California, Berkeley, October 3, 2005; and various web sites

You'll find related stories by following the links below.