Scientists at NSU have achieved laser generation for the first time in a miniature cylindrical microresonator with whispering gallery modes

Scientists have obtained laser generation based on a cylindrical microresonator supporting whispering gallery modes, made from tellurite glass doped with erbium.Faculty of PhysicsNovosibirsk State University, using a source of pumping, used an easily accessible broadband laser diode with a wavelength of 976 nm. This device operates in the spectral ranges of C and L, used in telecommunications. In a series of experiments, researchers confirmed the potential of cylindrical microresonators doped with erbium as compact, tunable, and narrowband laser sources for telecommunication and integrated photonic applications.The results of this study are presented by them in the article “Laser generation in an Er-doped tellurite cylindrical microresonator with effective radius variations,” published in the journal Optics Letters.

—The peculiarity of our study lies in the fact that we set ourselves the task of achieving laser generation in an almost cylindrical resonator of very small dimensions, with a diameter analogous to that of a human hair. Such resonators, which are essentially small segments of an optical fiber doped with ions of the rare-earth metal erbium. Besides the small size, allowing our resonator to be placed on various chips and detectors, and used for detecting certain atoms and molecules in the surrounding environment, our laser has other advantages. In particular, its almost cylindrical shape.Usually, such resonators have the shape of spheres, but the cylinder has its advantages — thanks to the cylindrical geometry, we have the ability to change the wavelengths of the generation of our laser. This makes cylindrical microresonators especially suitable for certain applications, unlike microspheres, microtoroids, or microrings. In particular, the cylindrical shape allows the implementation of several methods of laser tuning, which significantly expands the scope of its possible applications., — said a first-year graduate student of the Faculty of Physics of NSUNatalya Makarova.

In a cylindrical microresonator, light does not move linearly, but along a spiral, reflecting from the outer surface of the optical fiber wall. This phenomenon in acoustics is called the “whispering gallery effect,” which is a physical phenomenon where sound (or light) waves propagate along a concave curved surface over long distances, almost without losing their strength. Thanks to this, a laser in a cylindrical microresonator is very sensitive to any changes in the surrounding environment and can detect even small changes in air temperature or the presence of any gas.

In a series of experiments under the guidance of the senior research fellow of the Fiber Laser Laboratory of FFT NGSU, Candidate of Physical and Mathematical SciencesIlya VatnikIt turned out that laser excitation in a resonator of such small dimensions can possibly be produced not by a tunable narrowband laser pumping, which requires precise agreement of wavelength and polarization, but with the help of an inexpensive and widely available broadband laser diode, which was previously considered impossible. At the same time, the pumping radiation is introduced into the microresonator using a conical fiber about one micron in size, which is tens of times thinner than a human hair. As a result, selective excitation of various generation regimes is achieved, allowing tuning of the wavelength and switching of modes.

—In our case, the laser is excited using an easily accessible broadband laser diode with a wavelength of 976 nm and operates in two spectral ranges used in telecommunications. As the basis for the laser, we used active fiber specially created by our colleagues from the Institute of Applied Physics in Nizhny Novgorod, and now we have developed our own simple method for creating such active fibers. There is also the possibility to tune the wavelength and switch the laser operating mode between ranges.The laser demonstrates a generation mode with a narrow spectral line, which has been experimentally confirmed by us. The obtained results showed the potential of cylindrical microresonators doped with erbium as compact, tunable, and narrowband laser sources for telecommunications and integrated photonic applications., — explained Natalia Makarova.

It is important that the new cylindrical resonators emit laser generation approximately in the same range as the Internet operates — 1550–1610 nanometers. Sometimes this indicator reached 1530 nanometers. These are the optimal wavelengths for working in fiber optics. Thanks to the laser generation in this range, cylindrical microresonators can possibly be used in telecommunications and satellite navigation systems.

Another advantage of the given methodology is that in the cylindrical microresonator the laser demonstrates a generation mode with a narrow spectral line, which was confirmed in a series of experiments.

—The spectrum of our laser is very narrow, so it can detect single molecules, not just clusters. And not only detect them, it allows observing the movement of that very molecule, studying its rotation in any system. Why is this important? We assume that with the help of such a narrow laser it will be possible to detect single molecules of cancer cells. Nowadays, advanced technological methods are used in medical diagnostics to detect oncological diseases at early stages.It is not excluded that our laser will allow detection even earlier – when the very first, single molecules of cancer cells appear in the organism, which other equipment simply does not “see”.— said Natalya Makarova.