Portland, Ore. Optical flip-flops could enable a new breed of all-optical chips, from communications switches to full-fledged optical computers, according to the Cobra Research Institute at Eindhoven (Netherlands) University of Technology. Optical flip-flops using light, instead of electrons, to store, process and move data around could eliminate the need for expensive optical-to-electronic-to-optical conversions.
Today most semiconductor lasers are on discrete chips, but the Cobra Research Institute claims that the answer may be its ring lasers, fabricated in indium gallium arsenide phosphide.
According to the researchers, ring lasers could enable optical signals' use not only for communications but also for memory and processing, thanks to a configuration that enables bistable states an optical flip-flop. "Two ring lasers are required to form the bistable system," said Cobra researcher Martin Hill. He performed the work with Meint Smit, head of the Opto-Electronic Devices group at the Eindhoven university, and Harm Dorren, for whom Hill works at the Electro-Optical Communications group at Cobra.
For memory applications, the ring laser flip-flops could encode one state as a 1 and the other as a 0. The group claims that logic functions could also be fabricated with the scheme, qualifying ring lasers as a potential building block for optical computers that could process, as well as store, information with photons.
"Our long-term goal is digital processing of optical telecommunications data in optics," said Hill. "For this, optical logic functions need to be densely integrated and interconnected in planar optics; to have low operating and switching power, so many can be used on one chip; and to switch at high speed."
To prove the point, Hill and colleagues fabricated side-by-side 16-micron-diameter ring lasers from InGaAsP material, enabling them to operate at the popular 1.55-micron communications wavelength. With an interlaser ridge waveguide 2 microns long, the entire two-ring flip-flop measured about 18 x 40 microns.
When tested as a bistable optical memory device, the ring lasers stored binary information indefinitely and switched binary states in less than 20 picoseconds.
Microring lasers are thinner versions of the ordinary vertical-cavity emitting laser. Their lasing path is constructed to be just half their wavelength confining their oscillations to a two-dimensional ring. Now Hill and colleagues have further demonstrated it is possible to switch the direction in which the light is traveling around each microring.
When one is lasing in the clockwise direction, for instance, the other coupled microring is forced to lase in the opposite, counterclockwise direction in a resonant-amplification mode, thereby creating a bistable system that can store information indefinitely.
"One laser is, at any one time, the master," said Hill. "Since the system is symmetric, it is possible for either of the two to be the master."
To create their optical flip-flop from the bistable device, the researchers injected a 13-picosecond laser pulse in the opposite direction along the connecting waveguide. That caused the microrings to switch roles so that the second ring laser became the master and the first the slave. The result was a bistable optical flip-flop that could represent binary ones and zeros.
Now the researchers are cooperating with like-minded labs to demonstrate that arrays of optical flip-flops can be fabricated and interconnected on all-optical chips. To ease integration, Hill and colleagues hope to shrink their microring lasers down from 16 microns to less than one micron.