We present a numerical and experimental demonstration of a waveguide regime in a broad band spectral range for the hollow core microstructured optical fibers (HC MOFs) made of silica with a negative curvature of the core boundary. It is shown that HC MOFs with the cladding consisting only of one row of silica capillaries allows to guide light from the near to mid infrared despite of high material losses of silica in this spectral region. Such result can be obtained by a special arrangement of cladding capillaries which leads to a change in the sign of the core boundary curvature. The change in the sign of the core boundary curvature leads to a loss of simplicity of boundary conditions for core modes and to “localization” and limitation of their interaction with the cladding material in space. Such HC MOFs made of different materials can be potential candidates for solving problem of ultra high power transmission including transmission of CO and CO₂ laser radiation.

Revolver optical fibers (RF) are special type of hollow-core optical fibers with negative curvature of the core-cladding boundary and with cladding that is formed by a one ring layer of capillaries. The physical mechanisms contributing to the waveguiding parameters of RFs are discussed. The optical properties and possible applications of RFs are reviewed. Special attention is paid to the mid-IR hydrogen Raman lasers that are based on RFs and generating in the wavelength region from 2.9 to 4.4 μm.

In hydrodynamics, vortex generation upon the transition from smooth laminar flows to turbulence is generally accompanied by increased dissipation. However, vortices in the plane can provide transport barriers and decrease losses, as it happens in numerous geophysical, astrophysical flows and in tokamaks. Photon interactions with matter can affect light transport in ways resembling fluid dynamics. Here, we demonstrate significant impact of light vortex formation in micro-structured optical fibres on the energy dissipation. We show possibility of vortex formation in both solid core and hollow core fibres on the zero energy flow lines in the cladding. Through intensive numerical modelling using different independent approaches, we discovered a correlation between appearance of vortices and reduction of light leakage by three orders of magnitude, effectively improving wave guiding. This new effect potentially might have strong impact on numerous practical applications of micro-structured fibres. For instance, a strong light localization based on the same principle can also be achieved in the negative curvature hollow core fibres.

Coherent vortex structures are fascinating physical objects that are widespread in nature: from large scale atmospheric phenomena, such as tornadoes and the Great Red Spot of Jupiter to microscopic size topological defects in quantum physics and optics. Unlike classical vortex dynamics in fluids, optical vortices feature new interesting properties. For instance, novel discrete optical vortices can be generated in photonic lattices, leading to new physics. In nonlinear optical media, vortices can be treated as solitons with nontrivial characteristics currently studied under the emerging field of topological photonics. Parallel to theoretical advances, new areas of the engineering applications based on light vortices have emerged. Examples include the possibility of carrying information coded in the vortex orbital angular momentum, understood as a spatial-division-multiplexing scheme, to the creation of optical tweezers for efficient manipulation of small objects. This report presents an overview highlighting some of the recent advances in the field of optical vortices with special attention on discrete vortex systems and related numerical methods for modeling propagation in multi-core fibers.

Structured light, in particular light possessing orbital angular momentum (OAM), has been actively studied in recent decades. Helical Bragg grating (HBG) is a reflecting optical element, which predicted to be able to convert the OAM of light in waveguides and fibers. However, the HBG has not been demonstrated experimentally yet. Here we report the first experimental manifestation of HBG created in the form of a waveguide with depressed-index cladding. Few-mode channel helical-depressed-cladding waveguides (HDCWs) have been written in a YAG crystal by a femtosecond laser beam. The HDCW pitches were comparable to near-IR wavelengths. The spectral and polarization characteristics of the transmitted and reflected light have been studied. It was shown that HDCWs behave like HBGs providing narrowband resonance coupling of the counter propagating modes, the OAM of the modes differing by the Bragg order or, what is the same, the topological charge of the helical cladding structure. Adjusting the HDCW parameters allows one to excite a reflected wave at the predetermined wavelength and with the predetermined OAM change with respect to the light coupled to the waveguide. Thus, for the first time, the HBGs suitable for the generation and filtration of the vortex light have been experimentally demonstrated. The experimental results obtained agree well with the coupled mode theory calculations.

Standard vortex beams carrying different OAM (optical angular momentum) modes can provide independent communication channels for information transmission. However, they are unable to reach the capacity limit of a communication channel due to a rapid divergence of the beams with high values of the OAM order. The solution can be found by using multi-vortex geometric beams.