Engineers Create Laser to Slow Light

The prospect of moving from an Internet connection via a standard telephone line to broadband, resulting in faster networking, has long been considered the future of telecommunications. Now that reality is changing again.

UC Berkeley researchers have made a technological advance in the quest to slow down light for even more rapid communication networks applications. The team is led by principal investigator Connie J. Chang-Hasnain, a UC Berkeley professor of electrical engineering and computer sciences. Her colleagues include UC Berkeley electrical engineering and computer sciences graduate students Xiaoxue Zhao, Bala Pesala, and Phedon Palinginis.

"We sent a 2.8 gigahertz optical signal and measured the delay in my lab," Chang-Hasnain said. "This was a benchtop experiment."

Scientists showed that they could send light into the laser device and take the light back out by adjusting the delay. The device slows the speed of light more than one million-fold at room temperature.

The laser, according to Zhao, is an energy converter, which causes the interaction between electrons in a semiconductor with light. Electrical currents injected into the laser can act as an amplifier to pump up the signals of the light passing through it, decreasing the velocity of light as it does so.

Prior experiments have shown that light beams can be slowed through atomic vapor and solid-state crystal. However, semiconductors were used in this research because they were less elaborate and more cost-effective.

"Semiconductors are extremely small," Chang-Hasnain said. "Our device size is several orders of magnitude smaller than an atomic vapor cell or solid-state crystal."

According to Chang-Hasnain, semiconductors also have 1 million to 1 billion times broader bandwidth capacity and do not require the maintained state of extremely low temperature to operate.

In optoelectronics, adjusting the speed of light is partially in an effort to overcome a bottleneck in optical communications. Optical signals, when sped along fiber networks as part of Ethernet or wireless connections, are jammed when they hit a router. The router is responsible for directing light signals to their correct destinations by first converting them to slower moving electronic data, then switching them back to light, in a process known as optical-electronic-optical conversion.

"When you convert optical signals to electrical signals, it's like asking every passenger at an intersection to get out of the car, cross the street, and catch another car to their destination," she said.

Controlling the speed of light along those networks could eliminate the need for relatively inefficient conversions.

"So far we have succeeded in our goal to create a device with a large slowdown factor at high bandwidth," Pesala said. "You can think of it as slowing down each of the cars on a highway while increasing the number of available lanes."

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