A new model of frequency combs in optical microresonators
A team from the Faculty of Physics of the Lomonosov Moscow State University, together with scientists from the Russian Quantum Center, have developed a new mathematical model that describes the process of soliton occurrence in optical microresonators. In the future, this could lead to universal optical oscillators and other advancements. The work was published in Optics Express.
In 2017, a team of scientists led by Mikhail Gorodetsky, a professor of the Faculty of Physics, MSU, developed a method for controlling the number of solitons in so-called optical microresonators. Microresonators 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 rebounds many times, moving in circles.
Solitons are solitary localized waves that appear in resonators if the refraction index of a resonator's building material is nonlinear 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"—created in resonators having a typical comb-shaped optical spectrum in which the distance between two adjacent peaks is equal to the inverse 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 a 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 re-emitted 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 deeply into the nature of this effect when creating equations that describe effects in microresonators, and 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, when regular dispersion pulses are greatly distorted due to Raman scattering—they are destroyed, start to bifurcate, etc. The new mathematical tools are important for scientists to understand how to obtain optical combs in environments with regular dispersion. 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. They are widely used, in particular in high-accuracy spectroscopy, to increase the speed of information exchange, in telecom networks, and in LIDAR," 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 offer 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 to analyze the composition of substances. Currently, this task requires quite massive devices.