Propagation and Transformation of Vortexes in Linear and Nonlinear Radio-Photon Systems

Abstract

The article is devoted to issues related to the propagation and transformation of vortexes in the optical range of frequency. Within the framework of the traditional and modified model of slowly varying envelope approximation (SVEA), the process of converting vortex beams of the optical domain into vortex beams of the terahertz radio range based on nonlinear generation of a difference frequency in a medium with a second-order susceptibility is considered. The modified SVEA splits a slowly varying amplitude into two factors, which makes it possible to more accurately describe the three-wave mixing process. The theoretical substantiation of the rule of vortex beams topological charges conversion is given—the topological charge of the output radio-vortex beam is equal to the difference between the topological charges of the input optical vortex beams. A numerical simulation model of the processes under consideration has been implemented and analyzed.

A Nonlinear Radio-photon Conversion Device

“…But according to the results of the simulation of the converter's parameters [1] and experimental results (see Section 2), the efficacy of this converter is very low. According to our study, this is due to: a) a very small coefficient of quadratic nonlinearity  (2) of the ferroelectric crystal used in [1], and b) due to radiation losses in the specified crystal because of divergence of the input light beam, which is associated with the absence of any wave-guiding structures in the scheme proposed in [1]. The negative impact of these factors can be significantly reduced if the material and structure for the nonlinear element are properly selected.…”

“…where 3 = |1 -2|. Some causes of this were emphasized in the introduction: a small value of  (2) and the absence of a guiding structure.…”

“…At the same time, the physical reasons for this low efficiency, generally speaking, have not been studied in detail, but only assumptions have been put forward, which, moreover, differ from work to work. The general assumptions of the authors who published the results of independent studies are as follows: firstly, existing ferroelectric crystals have a small value of  (2) , secondly, optical beams arriving at the input end of the crystal have a divergence, inverse proportional to the radiation wavelength. This leads to significant losses, moreover, one can list additional factors such as 1) the absence of a resonant propagation condition for the generated radio frequency; 2) violation of the few-mode propagation regime for the input optical radiation inside the crystal (this is in addition to the divergence).…”

“…Indeed, ferroelectric crystals, such as LiNbO3, LiTaO3, KNbO3, KTaO3, PbTiO3, and the like, have such advantages as [9]: 1) low dependence of light transmission on frequency in the visible and near-IR radiation; 2) low cost and high availability. But at the same time, they have the following disadvantages: 1) the value of the nonlinear susceptibility  (2) , defining the efficiency of nonlinear processes, is in the range  (0.5…45)10 -12 m/V, i.e. is very small in comparison with other quadratic non-linear materials that exist today; 2) these materials can't be made in the form of a waveguide or fiber-optic structure, which leads to beam divergence, violation of single-mode regime and resonant propagation modes.…”

“…Such tasks require both the use of new modulation formats and new physical layer technologies, which ensure, in particular, the "seamlessness" (absence of intermediate operations) of optical-radio conversion which is especially relevant in sub THz, THz systems where it is very difficult to generate high-frequency radio signals by electronic devices, and it also seems inefficient to decode a multichannel signal for subsequent conversion of its components [1][2][3][4]. Therefore, to ensure such seamlessness, in particular, for opticalradio conversion in two-frequency (or wavelengths λ: λ1, λ2) radio over fiber (RoF) networks transmitting vortex signals (carrying orbital angular momentum, OAM), a nonlinear converter was developed [1].…”