Yurii Loiko

Free-space optical polarization demultiplexing and multiplexing by means of conical refraction

This paper presents a novel method to demultiplex and multiplex a monochromatic input light beam into, in principle, an arbitrary number of polarization states by means of the conical refraction (CR) phenomenon. In CR, when a focused Gaussian beam passes along the optic axis of a biaxial crystal it its transformed into a light ring at the focal plane of the system (or ring plane). Angular amplitude masks forming a star burst like pattern with n (up to 12) opened circular sectors placed at the ring plane allow passing only some sectors of the ring. These sectors form the communication channels. A second biaxial crystal identical to the first one and placed after it with opposite orientation of its plane of optic axes multiplexes the selected channels into a single beam, similar to the input Gaussian one. Finally, a third biaxial crystal is used to decode the transmitted channels at the receiver stage. With this technique, an increase in one order of magnitude in the channel capacity for free space optical communications (FSOC) of a monochromatic input Gaussian beam at 640 nm for a free space propagation distance of 4m with controlled crosstalk is observed.

Multiple rings formation in cascaded conical refraction

When a light beam passes through a cascade of biaxial crystals (BCs) with aligned optic axes, the resulting transverse intensity pattern consists of multiple concentric rings. We provide a simple formulation for the pattern formation for both circularly and linearly polarized input beams, that could be applied for a cascade of an arbitrary number of BCs. We have experimentally investigated multiple ring formation with up to three cascade BCs, showing that the theoretical formulation is in full agreement with the experimental results.

Laser beams with conical refraction patterns

Laser beams with cone-refracted output from the plane mirror is demonstrated for the first time in lasers based on intracavity conical refraction (CR) phenomenon. Transverse profile of such lasers comprises a crescent ring of CR-like distribution, where any opposite points are of orthogonal linear polarizations. We confirm the existence of such mode of CR lasers by two observations. First, cascaded CR in reflection geometry has been demonstrated for first time and it provides experimental prove that a light beam passed along optic axis of a biaxial crystal, reflected back from a plane mirror and passed again through the crystal is restored. Second, CR cavity mode with CR-like pattern through the plane mirror is experimentally and theoretically demonstrated for the first time.

Type I and type II second harmonic generation of conically refracted beams

Type I and type II second harmonic generation (SHG) of a beam transformed by the conical refraction phenomenon are presented. We show that, for type I, the second harmonic intensity pattern is a light ring with a point of null intensity while, for type II, the light ring possesses two dark regions. Taking into account the different two-photon processes involved in SHG, we have derived analytical expressions for the resulting transverse intensity patterns that are in good agreement with the experimental data. Finally, we have investigated the spatial evolution of the second harmonic signals, showing that they behave as conically refracted beams.

Coherent injecting, extracting, and velocity filtering of neutral atoms in a ring trap via spatial adiabatic passage

Abstract We introduce a coherent technique to inject, extract, and velocity filter neutral atoms in a ring trap coupled via tunneling to two additional waveguides. By adiabatically following the transverse spatial dark state, the proposed technique allows for an efficient and robust velocity dependent atomic population transfer between the ring and the input/output waveguides. We have derived explicit conditions for the spatial adiabatic passage that depend on the atomic velocity along the input waveguide as well as on the initial population distribution among the transverse vibrational states. The validity of our proposal has been checked by numerical integration of the corresponding two-dimensional Schrödinger equation with state-of-the-art parameter values for 87Rb atoms and an optical dipole ring trap.

Fermionic transformation rules for spatially filtered light beams in conical refraction

In conical refraction, when a collimated light beam passes along the optic axis of a biaxial crystal it refracts conically giving rise to a characteristic conical refraction (CR) ring. At each point of the CR ring the light electric field is linearly polarized with the polarization plane rotating along the ring such that every two opposite points of the ring present orthogonal linear polarizations. With a pinhole we have spatially filtered a small part of the CR ring and experimentally reported that this filtered light does not yield a ring pattern when it refracts along the optic axis of a second biaxial crystal, called the CR-analyzer in what follows. Instead, after crossing the CR-analyzer the filtered beam splits into two beams with orthogonal linear polarizations that correspond to two opposite points of the otherwise expected CR ring. We have experimentally derived the transformation rules of the filtered beam. For a CR-analyzer rotated by an angle ω around the optic axis, the filtered beam splits in two beams with intensities following the fermionic transformation rule cos2 (ω / 2) , in contrast to the Malus law of cos 2ω followed by double refraction.

Formation of X-pulses in periodically gain/loss modulated materials

We propose a versatile and efficient technique for the formation of X-pulses in materials with a periodical gain/loss modulation on the wavelength scale. We show that in such materials the strong wave-vector anisotropy of amplification/attenuation of the Bloch modes enables the shaping of ultra-short light pulses around the edges of the first Brillouin zone. X-pulses generation is numerically demonstrated and the optimum conditions are derived; specific characteristics of X-pulses can be tailored by appropriate selection of the geometry and modulation depth.

Conical refraction multiplexing for free-space optical communications

Conical refraction (CR) is proposed to increase the channel capacity for free space optical communication applications. We present the first investigations of cascaded CR with a linearly polarized input beam and experimentally prove that two oppositely oriented consecutive identical biaxial crystals perform a forward-backward transformation of the incident light beam. This forward-backward transformation is reported for different input beams with Gaussian, elliptical and angularly modulated transverse intensity profiles and is the basis for our novel proposal on multiplexing and demultiplexing of optical beams. We present experimental proof of usefulness and perspective of the CR multiplexing technique by increasing in one order of magnitude the channel capacity at optical frequencies. The technique is applicable to any wavelength in optical and telecommunication bands. It can be also properly upgraded with the WDM technique.