A new technique that stabilizes femtosecond (fs) pulses generated in a microresonator could improve devices dependent on such pulses for applications in telecommunications, broadband spectroscopy and astronomy, among others.

Last year, a team from Lomonosov Moscow State University (MSU) and from EFPL in Switzerland published a paper in Nature Photonics stating that the primary source of noise in microresonator-based optical frequency combs is related to nonlinear harmonic generation mechanisms, rather than the fundamental physical limitations of the devices. That means that, in principle, the noise can be reduced.

Signal stability is of great importance for making communications and GPS-navigation satellites, for example, more precise and with higher throughput.

In a new paper this week in Nature Photonics (doi: 10.1038/nphoton.2013.343) , the team extends its work, describing how it found a technique to generate stable femtosecond pulses, optical combs and microwave signals.

The researchers used a microresonator (a millimeter-scale magnesium fluoride disk), then propagated an optical whispering-gallery mode along the circumference of the resonator by exciting electromagnetic oscillations. This converted continuous-wave laser emission into a train of femtosecond pulses, analogous to those from mode-locked femtosecond lasers.

"In mode-locked femtosecond lasers, complex optical devices, media and special mirrors are normally used. However, we succeeded in obtaining stable pulses just in passive optical resonator using its own nonlinearity," said Michael Gorodetsky, professor of the Physical Faculty of MSU, and who also is affiliated with the Russian Quantum Centre in Skolkovo. That could drastically reduce the size of such devices in the future.

The short pulses are known as optical solitons. A soliton is a stable, shape-conserving localized wave packet propagating in a nonlinear medium like a quasiparticle.

"One can generate a single stable soliton circulating inside a microresonator. In the output optical fiber, one can obtain a periodic series of pulses with a period corresponding to a round-trip time of the soliton," Gorodetsky said.

Such pulses last for only 100 to 200 fs, but the researchers believe that much shorter solitons are achievable. They suggest that their discovery allows for the construction of a new generation of compact, stable and inexpensive optical pulse generators working in regimes unachievable with other techniques. Their results are critical for such applications as broadband spectroscopy, telecommunications and astronomy.

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