image: Forming a comb in a microresonator associated with an optical waveguide. The artistic version. view more
Credit: Mikhail Gorodetsky
A team from the Faculty of Physics of the Lomonosov Moscow State University together with the scientists from the Russian Quantum Center 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. The work was published in Optics Express journal.
A year ago a team of scientists led by Mikhail Gorodetsky, a professor of the Faculty of Physics, MSU and the Scientific irector of RQC, developed a method for controlling the number of solitons in the so-called optical microresonators. microrsonators are the basis of modern photonics - a science that specializes in optical signals. 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 (i.e. auto-focuses and turns into femtosecond-long pulses).
When using these resonators, scientists are especially interested in the so-called 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 (MgF2) or fused silica is associated with a number of harmful effects. These include the so-called combinational or 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 im microresonatorsand only apply some corrections to equations. In the new paper the team of 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 Professor 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.
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The study was conducted in collaboration with scientists from the Russian Quantum Center, ITMO University, and the University of Bath (UK).
Journal
Optics Express