Development of electronics and communication requires hardware capable of increasingly larger precision, ergonomics and throughput. For communication and GPS navigation satellites, it is of great importance to reduce the payload mass as well as to ensure signal stability. Last year, researchers from Moscow State University (MSU), together with their Swiss colleagues, performed a study aimed at certain improvements in this direction. In a paper published in Nature Photonics, the scientists reported that the primary source of noise in microresonator-based optical frequency combs (broad spectra composed of a large number of equidistant narrow emission lines) is related to non-linear harmonic generation mechanisms rather than by fundamental physical limitations and is, in principle, reducible.
On December 22, a new publication in Nature Photonics reports extended results. Michael Gorodetsky, one of the co-authors of this paper, 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 (duration of the order of 10-15 seconds) pulses, optical combs and microwave signals.
Physicists used a microresonator (in this particular case, a millimeter-scale magnesium fluorite disk was used, where whispering-gallery electromagnetic oscillations may be excited, propagating along the circumference of the the resonator) to convert continuous laser emission into periodic pulses of extremely short duration. The best known analogous devices are mode-locked lasers that generate high-intensity femtosecond pulses. Applications of these lasers range from analysis of chemical reactions at ultra-short timescales to eye surgery.
"In mode-locked femtosecond lasers, complex optical devices, media and special mirrors are normally used. However, we succeeded in obtaining stable pulses just in a passive optical resonator using its own non-linearity," Gorodetsky says. This will allow future reductions in the size of the device.
The short pulses generated in the microresonator are, in fact, what are known as optical solitons (a soliton is a stable, shape-conserving, localized wave packet propagating in a non-linear medium like a quasiparticle; an example of a soliton existing in nature is a tsunami wave). "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 explains.
Such pulses last for 100-200 femtoseconds, but the authors are sure that much shorter solitons are achievable. They suggest that their discovery allows construction of a new generation of compact, stable and cheap optical pulse generators working in regimes unachievable with other techniques. In the spectral domain, these pulses correspond to the so-called optical frequency "combs" that revolutionized metrology and spectroscopy and brought to those who developed the method a Nobel Prize in physics in 2005 ( American John Hall and German Theodor Haensch received the Prize "for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique"). Currently existing comb generators are much larger and more massive.
At the same time, as the scientists show, a signal generated by such a comb on a photodetector produces 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, NS telecommunication hardware, including satellite communications. The authors note that their results are critical for such applications as broadband spectroscopy, telecommunications, and astronomy.
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Herr T, Brasch V, Jost JD, Wang CY, Kondratiev NM, Gorodetsky ML and T. J. Kippenberg. 2013 Temporal solitons in optical microresonators. Nature Photonics, 22 December 2013. www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2013.343.html