Peculiarities of the Gaussian beam transformation in the optical scheme with an axicon and a biaxial crystal

Abstract

The transformation of a conical-type light beam by a biaxial crystal during propagation along one of its optical axes is herein investigated. The beam is formed by an axicon from the circularly polarized Gaussian input field. Depending on the position of the axicon either a Bessel beam or a combination of Bessel and conical beams falls on the crystal. The conversion coefficient from a zero-order Bessel beam to a first-order beam with phase dislocation is calculated. We show that if the angle of the beam cone and its diameter are large enough, then it is transformed into a field that is a first-order BesselGaussian beam with high accuracy. At the same time the conversion coefficient is close to 1. The case of a small cone angle of an incident Bessel beam is also investigated. In this case the efficiency of transformation significantly depends on the type of the spatial spectrum. At a small cone angle, the shape of the spatial spectrum is determined by the diameter of the incident Gaussian beam. Namely, as the diameter decreases, the beam spectrum changes from annular to close to Gaussian, passing through an intermediate form in the form of a superposition of these two profiles. The influence of the spatial spectrum is the conversion coefficient decreasing with a decrease of the of the ring component contribution to the spectrum. In this case, the conversion coefficient is always higher than for a scheme without an axicon when a Gaussian beam falls on the crystal. Consequently, the introduction of even a small conicity into the beam makes it possible to increase the field transformation coefficient. It can be implemented, for example, using a scheme with two axicons having close angles of a ray deflection. The results obtained are also of practical interest, in particular, for the development of conical-type laser emitters with a small cone angle for long-range reconnaissance and optical communication in free space.