S. V. Karpeev

Fibre sensors based on transverse mode selection

Use of diffractive optical elements (DOEs) in fibre sensors is very promising. This work is dedicated to the investigation of dependence of the transverse mode content in step-index fibres on the microbending value by means of diffractive optical elements. We describe the experimental set-up and methods of experimental data processing that afford precise measurements of mode power. The LP mode power dependencies on the fibre microbending value are obtained. The results obtained allow the designing of fibre sensors with advanced characteristics such as the dynamic range and accuracy.

Experimental demonstration of the generation of the longitudinal E-field component on the optical axis with high-numerical-aperture binary axicons illuminated by linearly and circularly polarized beams

We analytically and numerically show that by introducing asymmetry into axicon design it becomes possible to generate the longitudinal E-field component on the optical axis for linearly and circularly polarized incident beams. Binary axicons with high numerical aperture (NA) are fabricated in three configurations?axisymmetric and spiral, and bi-axicon?by electron beam lithography. Experimental measurements for the near-field diffraction of the most common and easy to implement incident beams?linearly and circularly polarized?are conducted. The experimental results agree with the theoretical analysis.

Strengthening the longitudinal component of the sharply focused electric field by means of higher-order laser beams

We study the sharp focusing of differently polarized low-order and high-order beams, including Bessel and Laguerre-Gaussian (LG) modes, to compare them using several criteria: the size of a light spot, the intensity ratio of the central peak and sidelobes, and the intensity of the longitudinal electric field component. The experiments performed using the near-field microscopy techniques are in general agreement with the results of the numerical simulation. We have validated the growth of the longitudinal component in the focus for high-order modes at moderate NA=0.6-0.8, and essential lower sidelobes of Bessel modes, in comparison with LG modes.

Generation of cylindrical vector beams of high orders using uniaxial crystals

The generation of cylindrical vector beams in birefringent crystals is studied analytically and experimentally in paraxial and non-paraxial regimes. At sharp focusing (in the non-paraxial case), two foci corresponding ordinary and extraordinary beams are formed along the crystal’s axis. There is the radially polarized distribution in one focus and the azimuthally polarized distribution in the other focus when the incident beam has the vortex phase of the first order and circular polarization of the opposite direction. The results are extended to the generation of higher-order radially and azimuthally polarized laser beams. The physical experiments with an Iceland spar crystal have been conducted.

Polarization converter for higher-order laser beams using a single binary diffractive optical element as beam splitter.

We propose a new approach to generating a pair of initial beams for a polarization converter that operates by summing up two opposite-sign circularly polarized beams. The conjugated pairs of vortex beams matched with laser modes are generated using binary diffractive optical elements (DOEs). The same binary element simultaneously serves two functions: a beam shaper and a beam splitter. Two proposed optical arrangements are compared in terms of alignment complexity and energy efficiency. The DOEs in question have been designed and fabricated. Natural experiments that demonstrate the generation of vector higher-order cylindrical beams have been conducted.

Grating-based optical scheme for the universal generation of inhomogeneously polarized laser beams

We propose and analyze a new optical system to transform linearly polarized laser modes into axially symmetric (radial or azimuthal) modes that show more promise in various applications, as well as generating various inhomogeneously polarized configurations. The system is based on the coherent composition of modal beams with phase diffraction gratings that allow the intermode phase shift to be varied without the need for auxiliary components. What makes the system simple and universal is the use of diffractive optical elements to generate required mode patterns with specific space orientation along with the simultaneous generation of different beams with different transverse mode content, all of which can subsequently be combined.

Experimental investigation of mode coupling in a multimode graded-index fiber caused by periodic microbends using computer-generated spatial filters

Abstract The results are described of an experimental investigation of mode coupling in a multimode graded-index fiber as a function of the periodic microbend amplitude. Computer-generated holograms were used as mode selecting spatial filters for measurements of the modal power distribution. The experimental data are in a good qualitative agreement with theoretical predictions. The results obtained may be helpful in the development of amplitude-type graded-index fiber sensors with high sensitivity.

Study of polarization properties of fiber-optics probes with use of a binary phase plate

We conduct a theoretical and experimental study of the distribution of the electric field components in the sharp focal domain when rotating a zone plate with a π-phase jump placed in the focused beam. Comparing the theoretical and experimental results for several kinds of near-field probes, an analysis of the polarization sensitivity of different types of metal-coated aperture probes is conducted. It is demonstrated that with increasing diameter of the non-metal-coated tip part there occurs an essential redistribution of sensitivity in favor of the transverse electric field components and an increase of the probes energy throughput.

Control of the formation of vortex Bessel beams in uniaxial crystals by varying the beam divergence

The transformation of zero-order Bessel beams into a second-order vortex Bessel beam in CaCO3 and LiNbO3 crystals is experimentally studied, and a possibility of controlling the beam transformation by changing the wavefront curvature of the illumi-nating beam is shown. A quasi-periodic nature of the Bessel beam transformation in a crystal while illuminating the diffraction axi-con by a convergent beam is observed

Transverse mode multiplexing by diffractive optical elements

An increase in the data-carrying abilities of modern communication systems is a most important scientific and technical challenge, requiring further studies of the physical effects involved. The possibility to increase in the data-carrying abilities of optical communication systems by waveguide transverse modes multiplexing by diffractive optical elements (DOEs) is considered.