Scientists from the Faculty of Physics, the Lomonosov Moscow State University, under the guidance of Professor Michael Gorodetsky, in cooperation with colleagues from Swiss Federal Institute of Technology in Lausanne, Switzerland, under the leadership of Professor Tobias Kippenberg have devised a method, which allows to control the number of optical solitons in microresonators, that underlie modern Photonics. Photonics is a dynamically developing field of modern Physics. Microresonator is one of the basic structural elements of Photonics. Resonator is an integral part of almost all sophisticated optical and microwave devices. In fact, resonator turns out to be a circular light trap. Being trapped in it, photon goes round and is reflected from the sides. Microresonators are used nowadays for laser stabilization and also for optical filters.
In their research, the results of which are published in Nature Physics journal, the scientists have tried to solve the problem of stable optical pulses generation in resonators. In other words, to provide that every pulse (soliton), put into it, could exist for a long period of time. The second experiment aim was to create such conditions, in which the number of pulses-solitons, moving in a resonator, could be easily reduced to one. At the same time, outgoing emission spectrum has the appearance of a super-stable optical frequency comb, which could be used as a high-precision ruler for optical spectra.
Grigory Lichachev, a doctoral student at the Faculty of Physics, noted that "Pulses should live for a long period of time and it should be only one not several pulses. The explanation is simple: when there is only one pulse, it has the clearest spectrum - known as a comb - which could be easily used in different spheres, for instance, in spectroscopy."
Scientists have studied in their research the properties of two optical resonators. The first one was made out of optical crystal, magnesium fluorideMgF2; the second one - out of silicon nitrideSi3N4, on the chip base only 1 micron thick.
Laser was used for letting light into a resonator, the properties of pulses inside it were measured at output with the help of spectrometer.
The experiment has allowed to demonstrate the method that guarantees forming of one pulse, which propagates round in a resonator. In case of experiment success physicists expected to see a regular spectral comb which is the distinguishing character of soliton. Moreover, the article shows a new and very effective method worked out by scientists, which enables observing solitons' life in real-time. This became possible due to addition of weak phase modulation to input signal and further response registration to this disturbance. Such an approach opens up new possibilities for combs maintaining and stabilization.
Lichachev noted that "As experiment success one could consider the formation of guaranteed one-soliton regime. It's well-known that one soliton has cleaner spectrum, which is also easier to be measured."
The technique, worked out by the scientists, allows to actuate in a resonator an unknown large number of solitons and afterwards to sequentially reduce this number by unit unless there is only one pulse left. The scientists underline that the reduction of extra solitons one after another becomes possible only due to change of laser frequency, used for actuating the resonator.
Optical frequency comb is the foundation of laser-based precision spectroscopy technique, which was awarded with the Nobel Prize in Physics in 2005. In case physicists manage to generate separate stable solitons inside optical resonators, this could be used for different applied relevance tasks. Application spheres start from astronomy and finish in high-precision sensors, when it's necessary, for instance, to measure the spectrum of an unknown substance. Using two identical optical solitons and overlapping their optical frequency combs, scientists could measure optical frequencies, which could be hardly measured directly cause of their size (about 200 terahertz, what means the wave-length 1500 nm).
Lichachev clarifies that "The distance between two combs teeth is less than the distance between separate combs teeth. And if you take the difference between them you could measure low frequencies, which refer to microwave range and could be quite measured by modern electronics".
Potential application of this method is the measurement of gas composition with the help of spectroscopy technique in mid-infrared range. If you direct two optical solitons to the experimental gas through a common optical fiber, you could fix notches, connected with specific absorption lines, at output in their spectrum.
Usage of two solitons enables to measure frequencies in radio waves area not in optical range, where it proceeds too slowly with the help of present techniques. If it takes seconds to measure frequencies in optical spectrometer, then in microwave range measurement time is defined by electronics frequency and, consequently, comprises nanoseconds.