New Model of Frequency Combs

Forming a comb in a microresonator associated with an optical waveguide. (Source: M. Gorodetsky)

A team from the Faculty of Physics of the Lomo­nosov Moscow State Univer­sity together with the scientists from the Russian Quantum Center RQC developed a new mathe­matical model that describes the process of soliton occur­rence in optical micro­resonators. After the physicists under­stand the existing effects and learn to predict new ones, they will be able to create high-precision devices and universal optical oscil­lators.

A year ago a team of scientists led by Mikhail Goro­detsky, scientific director of RQC, developed a method for control­ling the number of solitons in the optical micro­resonators. A resonator is a ring-shaped trap for light in which a photon grazingly reflects many times from the walls and moves in circles. Solitons are solitary loca­lized waves that appear in reso­nators if the refrac­tion index of a resonator’s building material is non-linear and is a certain function of the wave­length. In this case a laser beam, after making a number of rounds inside a reso­nator, splits into separate solitons.

When using these reso­nators, scientists are especially interested in the soliton optical combs – born in reso­nators having typical comb-shaped optical spectrum in which the distance between two adjacent peaks is equal to the inversed time the light requires to make the whole circle. Such combs may be used in solving a number of applied problems. The problem is that the occurrence of useful combs in resonator based on mag­nesium fluoride or fused silica is associated with a number of harmful effects. These include Raman scat­tering. It is caused by oscil­lations of separate mole­cules in a substance. After reaching the surface of such a substance, light is re­emitted with another wave­length.

The effect has a threshold, depending on the intensity of radia­tion and the compo­sition of the substance, and causes the destruc­tion of solitons and spectrum distor­tion. Scientists usually don’t dive deep into the nature of this effect when creating equations that describe effects in micro­resonators and only apply some correc­tions to equations. Now, the researchers studied the nature of this effect and developed new equations that describe the gene­ration of optical combs taking Raman scat­tering into account. The system of equations may be used for numerical simu­lation of the effects that occur in optical reso­nators.

“We used these equations to check the behavior of light in reso­nators with anomalous dispersion and obtained pre­viously known effects. Thus, we’ve tested our theory,” explained Goro­detsky. “After that we applied it to combs with normal dispersion that have plati­cons – pulses with plateau-shaped peaks of spectrum – instead of solitons.” The new model allowed the scientists to predict a number of pre­viously unknown effects. For example, in case of regular dispersion pulses are greatly distorted due to Raman scat­tering. They are destroyed, start to bifurcate, etc. The developed mathe­matical tools are important for the scientists to under­stand how to obtain optical combs in environ­ments with regular dispersion that is charac­teristics for the majority of substances around us. Further experi­ments are expected to prove the con­clusions on the example of platicons.

“Currently, there are only a few labs in the world that study soliton combs. Together with our Swiss colleagues we were the first to demonstrate them. Recently they have been widely used, in particular in high-accuracy spectro­scopy, to increase the speed of infor­mation exchange, in telecom networks, and in lidars,” explained Goro­detsky. “Some time ago German scientists used optical combs to accu­rately determine the shape of a moving bullet and managed to see how it changes due to air resis­tance.” Optical combs open for the scientists the prospects of deve­loping optical oscil­lators based on just one chip and emitting light with any preset frequency which is impos­sible for modern lasers and other gene­rators. Moreover, they may serve as a basis for pocket-type spectro­meters used to analyze the compo­sition of substances. Currently this task requires quite massive devices. (Source: LMSU)

Reference: A. V. Cherenkov et al.: Raman-Kerr frequency combs in microresonators with normal dispersion, Opt. Exp. 25, 031148 (2017); DOI: 10.1364/OE.25.031148

Link: Russian Quantum Center, Skolkovo, Russia

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