ROTATING OPTICAL BEAMS: DIGITAL HOLOGRAPHIC GENERATION, PROPERTIES AND SELF-RECONSTRUCTION DYNAMICS

Abstract

This thesis reports on an experimental investigation of rotating optical beams. Superpositions of higher-order Bessel beams with rotating intensity profiles were generated using a digital holographic experimental technique and the propagation properties in free space were investigated. Phase masks of two annular rings, of equal width but different radii were digitally generated and imprinted on a spatial light modulator. The digital phase masks were illuminated with a linearly polarized He-Ne laser beam to obtain superpositions of higher-order Bessel beams of the same order but of opposite topological charges and of different orders but same or different topological charges depending on the number of times the azimuthal phase in one annular ring varied relative to the other ring. The superposition of beams with the same order but opposite topological charges resulted in fields with on average zero orbital angular momentum but which exhibited a rotation in their intensity profiles. On the other hand, the superposition of beams with unequal orders produced orbital angular momentum-carrying Helicon beams with rotating intensity profiles. The rotation rates of the generated superposition fields were measured for different orders and for various values of the difference between the wave-vectors of the superimposing beams. The experimental results showed that the intensity profiles rotated at constant rates, for example 30.14 rad/m, as the fields propagated and that the rotation rates varied linearly with the difference between the wave vectors of the superimposing beams and inversely as the order of the beams for zero orbital angular momentum beams and inversely as the difference between the orders of the beams for Helicon beams. In addition, the propagation of the rotating optical beams past total and partial obstructions of different geometries set on- or off-axis was investigated theoretically and experimentally. The experimental results showed that the rotation of the intensity profile does not affect the self-reconstruction of rotating Bessel beams. However, the position of the obstacle with respect to the propagation axis of the beam is important in determining the nature of the reconstruction process of the beams. For on-axis obstructions, the reconstructed beam had the form and orientation of the unobstructed beam but off-axis obstructions resulted in a beam which does not attain the exact form of the unobstructed beam.