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Chips give hope to all-optical memory

12 Oct 2005

Electronic control over the speed of light is demonstrated in three types of optoelectronic chip.

The development of all-optical buffers that can delay and temporarily store light pulses has been given a boost with the demonstration of three types of semiconductor device that can perform the task.

In two separate papers, scientists at the COM Research Center in Denmark have shown that two popular telecom components -- a quantum-dot semiconductor optical amplifier (SOA) and an electro-absorption modulator (EAM) -- can both slow the propagation of light pulses (Optics Express 13 8032 and Optics Express 13 8136).

At the same time, a team from University of California at Berkeley and Texas A&M University in the US has demonstrated that, when configured as an amplifier, a Vertical-Cavity Surface-Emitting Laser (VCSEL) can also perform the feat (Optics Express 13 7899).

All three approaches work by carefully controlling the electrical bias of the custom-built devices. The reports are significant because they suggest that a practical solution to controlling the timing of light pulses without having to convert them to the electrical domain is on the horizon. Such optical buffer chips could have important consequences for telecommunication networks, optical computing and optical phased-array antennae.

"The topic of slowing light down is very hot since it is both of fundamental interest and there are some exciting possible applications," Jesper Mørk, head of the nanophotonics group at COM, told Optics.org. "Semiconductors are interesting because of the possibility of small devices that can be fabricated using standard techniques and the possibility of integration with other functional elements."

Although there have many demonstrations of slowing the speed of light pulses in gases, laser crystals, photonic crystal waveguides and even ordinary optical fiber, all previous schemes have required relatively complex and bulky optical control set-ups. What’s more, they have all suffered from a limited bandwidth (megahertz or less) which makes them incompatible with the short data pulses found in today’s optical communications systems.

In contrast, the recent US and Danish experiments rely on small semiconductor chips that offer an electronically controllable delay and are compatible with high-data rates (gigahertz and terahertz).

In terms of performance, the EAM and VCSEL both operated at the telecoms wavelength of 1550 nm and pulse frequencies of 16.7 and 2.8 GHz respectively. The EAM slowed the light pulses down by up a factor of three while the VCSEL slowed light by a factor of 1 million to provide a time delay of up to 100 ps.

In comparison, COM’s SOA successfully processed 1260 nm pulses with a giant bandwidth of 2.6 THz (170 fs duration) but provided an optical delay of only 68 fs.

According to Mørk, there is still much work to be done before such semiconductor devices are ready for use as a temporary optical memory. "I think that before we see the buffer application there are still many challenges to overcome, in particular regarding the delay-bandwidth product," said Mørk. "Most investigations on fiber and semiconductor devices employ the effect of 'coherent population oscillations', a wave mixing effect for which it is a serious challenge to overcome this delay-bandwidth limitation."


Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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