A team based at Purdue University has crafted a tiny, chip-sized ring resonator comb that generates shaped pulses.
(Top) A microring resonator that converts continuous laser light into numerous ultrashort pulses. (Bottom) A grooved structure that holds an optical fiber leading into the device.
Frequency combs generate discrete spectral lines at specific wavelengths—but they can also generate precise short pulses. A team based at Purdue University (Indiana, U.S.A.) crafted a tiny, chip-sized ring resonator comb that generates shaped pulses (Nature Photonics doi:10.1038/nphoton.2011.255).
Unlike most investigations of frequency combs, Andrew M. Weiner, a Purdue professor of electrical and computer engineering, and his colleagues have been studying the technology in the time domain. In their experiments, they sent light from a tunable diode laser through a silicon nitride ring and then through a pulse shaper to explore the coherence of the resulting pulse trains. Weiner’s former graduate student, Houxun Miao, now a scientist with the U.S. National Institute of Standards and Technology, fabricated the rings, which have diameters of 80 to 400 µm.
Weiner’s group showed that a frequency comb can produce precise pulse trains. The pulse shaper allows scientists to specify the relative phases of the comb lines. Stability of the phases is important. Combs with fluctuating phase alignment produce less periodic pulse trains, which the team dubbed “partially coherent” behavior.
“It’s proof that under some conditions you can make beautifully regularly repeating pulse trains, which open up time-domain applications, but also kind of a warning that it doesn’t always happen that way,” Weiner said. The researchers are still studying the behavior of partially coherent pulse trains.
According to Weiner, short, regular pulses, repeating at 100 GHz and above, are useful because they can measure very fast events and very high peak intensities. The technology could be useful in telecommunications signal processing and THz wave generation. Another potential application is in rapidly tunable microwave filters for wireless military communication.
Patricia Daukantas is a freelance science writers who specialize in optics and photonics.