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Chip-based device produces pulsed light for optical communications

Researchers from Swiss research facility, EPFL, and the Russian Quantum Centre claim to have made a chip-based device that can generate a laser signal with frequencies spaced in a comb-like fashion. The device could be used in telecommunications applications.

In general, light and water waves stretch out and dissipate as they move further and further away from their source. However, there is a type of wave that maintains its shape as it propagates: solitons.

The researchers say they have successfully produced optical solitons using a microresonator. The light is composed of a range of frequencies separated precisely, producing what physicists call a frequency comb, since it resembles the regular spacing between the teeth of a comb.

To generate the solitons, the researchers used microresonators. “These microscopic ring-shaped structures are made from very fine silicon nitride,” explained Tobias Kippenberg, the EPFL group leader. “They are capable of storing for a few nanoseconds the light of the laser to which they are coupled. This period of time is sufficient for the light to circumnavigate the ring thousands of times and to accumulate there, which strongly increases the intensity of the light”. The interaction between the microresonator and the light becomes non-linear. The laser, which is normally continuous by nature, is converted into ultra-short pulses, or solitons.

By adapting the parameters for manufacturing microresonators, the EPFL researchers have also managed to generate a so-called soliton Cherenkov radiation. This broadens the frequency spectrum meaning the comb contains a greater number of teeth. These results have set a new record for this type of structure. The frequencies generated extend over two thirds of an octave compared with the frequency of the laser.

“These results represent a promising advance for applications that require many widely spaced frequencies,” says Kippenberg. In the context of optical communications, one single laser would be enough to create a range of individual frequencies which could separately carry information.

Tom Austin-Morgan

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