Jul 18 2017
Patterns of transverse distribution of optical radiation intensity in the output beam. (Credit: Moscow Institute of Physics and Technology)
Collaboration between researchers from the Moscow Institute of Physics and Technology (MIPT) and the Kotelnikov Institute of Radio Engineering and Electronics (IRE) of the Russian Academy of Sciences (RAS) and researchers from Finland has resulted in the development of an innovative kind of optical fiber with an exceptionally large core diameter as well as the ability to secure the coherent characteristics of light.
The study has been reported in a paper published in the Optics Express journal. The outcomes of the research look highly propitious for developing not only high-power pulsed fiber lasers and amplifiers but also polarization-sensitive sensors.
In the case of optical fiber applications, it is highly important to secure the characteristics of light. The two main parameters that usually have to be secured are the polarization of light and the distribution of light intensity along the cross section, where polarization of light indicates the directions of oscillation of the magnetic or electric field in a plane perpendicular to the direction of propagation of light wave. The scientists managed to accomplish both in their study.
Optical fiber research is one of the most rapidly developing fields of optics. Over the last decade, numerous technological solutions have been proposed and implemented. For instance, researchers and engineers at IRE RAS can now produce optical fiber of almost any diameter with arbitrary transverse structure. In the course of this study, a specific structure was formed in the optical fiber. It varies along two orthogonal axes, and its diameters change proportionally along the fiber. Individually, such solutions are already widely used, so it is critical to continue to work in this direction.
Vasily Ustimchik, Professor, MIPT
In general, an optical fiber is a very thin flexible strand of transparent plastic or glass. To the casual eye, it looks like a simple system. However, practically, a range of difficulties restrict its usage. Although the main difficulty is the attenuation of signal in fiber-optic lines, it has been resolved long back, thus enabling fiber-optic communications. Yet, communications are not the only field in which optical fibers can be used. At present, a highly common type of laser is reliant on fiber-optic technology. Similar to other lasers, a fiber laser includes an optical resonator which makes light to repeatedly travel to and fro. The geometrical parameters of the fiber resonator are such that they enable only restricted transverse patterns of distribution of light intensity in the output beam, namely, the transverse modes of the resonator. Naturally, researchers will aim to regulate the mode structure of the light, and in fact, in practice, engineers and scientists attempt to excite a classic fundamental mode that does not alter in the due course.
If single-mode operation has to be conserved, the fiber should have a core as well as a cladding, which are materials having distinctive refractive indices. Normally, the thickness of the fiber core (i.e. the inner part), through which radiation advances, must be under 10 μm.
If the optical power of the light passing through the fiber gets increased, more energy is attenuated, thus resulting in the modification of characteristics of the fiber. In particular, it results in uncontrolled changes in the refractive index of the fiber, consequently leading to parasitic nonlinear effects that causes the generation of additional spectral lines of emission and so forth, thereby restricting the strength of the transmitted optical signals. A prevalent solution to overcome the difficulty, also adopted by the authors, was to alter the core as well as outer diameters throughout the length of the fiber.
In case the expansion of the fiber happens in an adiabatic manner (i.e. very slowly), the amount of energy conveyed to other modes can be decreased to under 1%, even if the core diameter is nearly 100 μm. A core diameter of 100 μm is extremely large for single-mode fibers. Furthermore, if the core diameter is greater and changes throughout the fiber, the threshold for the occurrence of nonlinear effects increases.
In order to accomplish the second aim, that is, securing the state of polarization of light, the researchers made the cladding of the fiber anisotropic—the height and width of the inner cladding differ, or the cladding is elliptical, implying that the speed of propagation of light in different field oscillation directions also differs. In such a structure, the process of transfer of energy from one polarized mode to the other is nearly totally disrupted. In the research, the scientists have demonstrated that the geometric length of the path traversed by light via the fiber at which the oscillations of the two disparate polarizations are in anti-phase is reliant on the diameter of the fiber core: that is, it increases if the diameter is decreased, and vice versa. This length is also termed as the polarization beat length and is equal to one complete rotation of the linear polarization state in the fiber. Put differently, when a linearly polarized light is directed into a fiber, it will get once more linearly polarized after traversing exactly this distance. The capability to measure this parameter denotes that the polarization state in the fiber is secured.
To analyze the characteristics of light polarization in the fiber, the optical frequency-domain reflectometry technique was adopted, which involves directing an optical signal into the fiber and observing the backscattered signal. A large amount of information is included in the reflected signal. The technique is usually used to ascertain the location of impurities and defects in optical fibers. Yet, it can be also used to ascertain the coherence length as well as the spatial distribution of polarization beat length. Coherence reflectometry methods are largely used to observe the state of optical fibers. In contrast, the technique adopted in this research is significant in allowing data collection at a greater resolution of nearly 20 mm across the length of the fiber.
The fiber samples we obtained have demonstrated great results, indicating good prospects for further development of such technological solutions. They will find use not only in laser systems but also in optical fiber sensors, where the change of polarization characteristics is known in advance, since they are determined by external environmental factors, such as temperature, pressure, biological and other impurities. Besides, they have a number of advantages over semiconductor sensors. For example, they need no electrical power and are capable of carrying out distributed sensing, and that is not a complete list.
Professor Sergey Nikitov, Deputy Head, Section of Solid State Physics, Radiophysics and Applied Information Technologies, MIPT