Moscow, Russia and Lausanne, Switzerland--In 2012, researchers from Moscow State University (MSU) and the École polytechnique fédérale de Lausanne (EFPL) demonstrated that the primary source of noise in microresonator based optical frequency combs (which are broad spectra composed of a large number of equidistant narrow emission lines) is related to nonlinear harmonic generation mechanisms rather than by fundamental physical limitations, and is thus in principle reducible. On December 22, 2013, a new paper in Nature Photonics describes how they have extended their results.1 Michael Gorodetsky, one of the coauthors of this paper, and professor of the Physical Faculty of MSU affiliated also in the Russian Quantum Centre in Skolkovo, says that the study contains at least three important results: scientists found a technique to generate stable femtosecond pulses, optical frequency combs, and microwave signals.
The physicists used a microresonator (in this case, a millimeter-scale magnesium fluoride disk whispering-gallery resonator) to convert continuous-wave (CW) laser emission into periodic femtosecond pulses. The best known conventional analogous devices are mode-locked lasers that generate femtosecond, high-intensity pulses. Applications of these lasers range from analysis of chemical reactions at ultrashort timescales to eye surgery.
"In mode-locked femtosecond lasers, complex optical devices, media, and special mirrors are normally used," says Gorodetsky. "However, we succeeded in obtaining stable pulses just in a passive optical resonator using its own nonlinearity." This ca lead to much smaller producers of femtosecond-pulse trains.
The short pulses generated in the microresonator are, in fact, optical solitons (a soliton is a stable, shape-conserving localized wave packet propagating in a nonlinear medium). "One can generate a single stable soliton circulating inside a microresonator," says Gorodetsky. "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."
These pulses have a duration of 100 to 200 fs, but the authors are sure that much briefer pulses are achievable. They suggest that their discovery allows to construct a new generation of compact, stable, and cheap optical pulse generators working in regimes unachievable with other techniques. In the spectral domain, these pulses produce optical frequency combs. Currently existing comb generators are much larger and more massive than the news setup.
In addition, as the scientists show, a signal generated by such a comb on photodetectors leads to a high-frequency microwave signal with very low phase noise level. Ultra-low-noise microwave generators are extremely important in modern technology; they are used in metrology, radiolocation, telecommunication hardware, including satellite communications. Authors note that their results are critical for such applications as broadband spectroscopy, telecommunications, and astronomy.
Together with EPFL's Tech-Transfer Office, the scientists have applied for a patent and hope that their discovery will soon prove itself in one of its many applications.
REFERENCE:
1. T. Herr et al., Nature Photonics (2013); doi: 10.1038/nphoton.2013.343