Translartion. Region: Russians Fedetion –
Source: Novosibirsk State University – Novosibirsk State University –
Research staff of the Laboratory of Fiber Lasers Faculty of Physics, Novosibirsk State University found that high precision is not required to produce a radius variation on the surface of an optical fiber. It is enough to take a regular piece of fiber, make a couple of notches on it, and automatically obtain a system in which it is already possible to generate an optical frequency comb with a low repetition rate. The results of their study were published in the journal Optics Letters (Soliton-comb solutions for fiber-based bottle microresonators, HTTPS: //d.org/10.1364/ul.544823)
— In our work, we show that cylindrical microresonators are a simple and reliable platform for generating optical frequency combs with a low repetition rate. If small variations in the radius are introduced into such a system, there is a set of axial modes with different spatial distributions along the fiber axis, while the spectral distance between them can be reduced to 100 MHz. In earlier works, a theoretical demonstration of an axial comb in a cylindrical microresonator with a parabolic radius variation was already carried out at a qualitative level. Our study reveals a counterintuitive fact: the axial soliton width weakly depends on the mode dispersion and can be observed, in principle, in a system with any form of radius variation. This significantly simplifies the fabrication of a system for generating optical frequency combs. Thus, any piece of optical fiber, regardless of its shape and surface roughness, can be used to generate combs, said Alena Kolesnikova, a junior researcher at the NSU Fiber Laser Laboratory.
An optical frequency comb is a signal spectrum that looks like a set of narrow spectral lines equidistant from each other with high accuracy. In essence, it is a frequency line. The signal itself, to which such a spectrum corresponds, is a sequence of pulses arriving at the measuring device with one frequency, which is exactly equal to the distance between the lines in the comb. Since the comb is a kind of frequency line, the main application is ultra-precise measurement of frequency and time. And this in turn opens up a wide range of applications in the fields of spectroscopy, optical clocks, GPS navigation, distance measurement in astronomy, and also has applications in telecommunications, etc.
There are two options for generating frequency combs: mode-locked lasers and microresonators. The first platform allows generating combs with a low repetition rate, i.e. with a small line pitch, but requires significant energy consumption and is relatively large. Microresonators, in turn, are small in size and require less power, but the distance between the lines is limited. To reduce it, it is necessary to increase the size of the resonator, but then it will require more power.
— As in any resonator, there are modes in microresonators — this is a stable distribution of the electromagnetic field, which is a consequence of the limitation of the space in which it exists. Depending on the shape of the microresonator, we obtain different spatial distributions of modes. Each mode has its own resonant frequency. In order for the generation of combs in microresonators to be possible, it is necessary for the system to have a set of modes whose resonant frequencies are equidistant, that is, equally spaced. It is the distance between the frequencies of the modes that determines the distance between the lines in the comb, — explained Alena Kolesnikova.
In previously known microresonators of spherical, ring or toroidal shape, the distance between the lines is about 10-1000 GHz. The best oscilloscopes at the moment allow direct measurement of frequencies up to 20 GHz, that is, without additional signal processing it is simply impossible to measure such frequencies.
In a cylindrical microresonator with a small radius variation, it is possible to generate a comb with a repetition rate of less than 10 GHz and with the possibility of reducing it to 100 MHz, while maintaining the micron dimensions of the platform. This became possible due to the fact that such a system has a set of axial modes (modes with a spatial distribution along the cylinder), which, due to the geometry of the cylinder itself, have a small distance between resonant frequencies.
— We have studied a cylindrical microresonator with a radius variation for the possibility of generating combs on a set of axial modes using the developed model. Such a microresonator can be made on the basis of a standard optical fiber, which is available in any laboratory that deals with fiber optics. To do this, it is enough to remove the plastic shell from the fiber and heat it with a CO2 laser. At the point of heating, the fiber will swell a little, that is, a small radius variation will occur. It is this radius variation that allows us to obtain a set of axial modes, since it will delay the radiation inside this area. Before us, such a system had already been studied for the possibility of generating combs. From the experience of generating combs in microspheres, rings, etc., it was believed that in order to make the comb as wide as possible, an almost perfectly equidistant spectrum of modes is necessary. For axial modes of a cylindrical microresonator, this is possible if we make a parabolic form of radius variation on its surface, which is actually a non-trivial experimental task and requires a good, precise algorithm for heating the fiber with a CO2 laser, said Alena Kolesnikova.
The laboratory scientists have shown that in fact almost any form of radius variation can be suitable for generating a comb in such a system. In this case, the width of the comb, all other parameters being equal, will not depend on the shape. They modeled two cases: microresonators with a parabolic shape and a rectangular form of radius variation, and obtained the generation of solitons, in the spectrum of which look like an optical frequency comb. In this case, the characteristics of solitons and combs are almost the same for both cases. They came to the conclusion that high precision in manufacturing the radius variation on the fiber surface is not required. You can take an ordinary piece of fiber, make a couple of notches on it (i.e. a rectangular form of radius variation), you can even mechanically and automatically obtain a system in which the generation of an optical frequency comb with a low repetition rate is already possible.
— It is also worth noting that the manufacturing process of other types of microresonators, spherical, toroidal, ring, etc., is also complex and requires high precision, while optical fiber is available and is a mass-produced product. As far as we know, no one has yet obtained optical combs in such a system, — explained Alena Kolesnikova.
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