Experimantal oberving of transformation Bessel beam spreading along axis of crystal during wavelength changes

Abstract

In paper decribe experimental observing transform bessel beam, formed by diffraction axicon in moment propagation through anisortopic birefringence crystal. This observation covers large range wavelength changes (from 520 nm to 534 nm). Theoretical explain effect is given.Keywords: laser with changing wavelength; diffraction axicon; birefringent crystal; bessel beams IntroductionIt is well-known the usage of anisotrophic elements to convert beams with the homogeneous polarization into cylindrical vector beams [1][2][3][4][5][6]. At the same time, it is necessary to implement the separation of the longitudinal modes along the optical axis of the system, which is parallel to the axis of the crystal. To improve convergence of the beams in the crystal, it is possible to use telescopic system or to form beams with high numerical aperture. Polarization and mode conversion during propagation along the axis of the crystal were considered for both Bessel and Gaussian beams [7][8][9][10][11][12][13][14][15][16].It has been shown in the studies [17,18] that during the propagation along the crystal axis neuraxial Bessel beams have other properties than Gaussian beams, namely, experiencing a uniform periodic change of intensity. In this case, the Bessel beam of zero order and second-order are periodically converted from one to another [7-9, 17, 18]. The oscillation period is directly proportional to the wavelength of the laser radiation and inversely proportional to the square of the spatial frequency of the laser beam and the difference of the dielectric capacitivity, which is corresponding to the ordinary and extraordinary rays. This dependence allows control occurring transformation in the crystal due to changes of the characteristics in either Bessel beam or crystal. In particular, the spatial frequency of the beam depends on the numerical aperture of the axicon [19][20][21][22] which shapes the beam, also it is possible to adjust characteristics of the beam by changing the beam divergence [23]. To change the parameters of the crystal, it can be heated [24] or effected by electro-optic [25]. However, the most convenient way of adjustment is to change the wavelength of the laser radiation which has a direct linear relationship from the period of transformation [26].It was experimentally demonstrated the ability [26] to manage the transformation of the Bessel beam at the output of the CaCO3 crystal by changing the wavelength of the radiation illuminating the diffractive axicon. It was achieved almost complete transformation of the Bessel beam of zero order beam to the second order using the axicon period of 2 µm and the wavelength at Δλ =1.5 of the initial value of λ = 637.5. The variation of the wavelength within a small range was achieved by changing the temperature of the laser. In contrast to this method, the usage of a laser with variable wavelength provides a wide range of Δλ, and therefore the possibility of achieving complete conversion using the axicon with a large period, i.e., a smaller numerical apertur...