Focusing of Gaussian beam passed under small angle to optical axis of uniaxial crystal

We showed both experimentally and analytically, the effect of focusing of a Gaussian beam propagated under small angle phi with respect to the optical axis of a uniaxial crystal, on the generation of a bottle beam. At phi = 0 degrees two foci that correspond to ordinary and extraordinary parts of a beam form a closed 3D structure of a bottle beam. At this point, the beam, in the foci points, has radially and azimuthally aligned polarizations. Increasing the value of f leads to dramatic changes in the intensity and polarization structure of a bottle beam. Starting from the value of phi = +/- 2 degrees the closed 3D symmetric structure of a bottle beam breaks down. At phi = +/- 5 degrees both beams are focused at the same transverse plane, while its polarization evolves to x-and y-linear. With a further increase in angle f two foci 'switch' their spatial positions and move further away.