Cosmas Mafusire

Propagation of orbital angular momentum carrying beams through a perturbing medium

The orbital angular momentum of light has been suggested as a means of information transfer over free-space, yet the detected optical vortex is known to be sensitive to perturbation. Such effects have been studied theoretically, in particular through turbulence. Here we demonstrate a simple apparatus to introduce turbulence-like distortions to optical fields propagating over a long path. We create vortex beams and observe their propagation through a heated spinning pipe, known to mimic the two primary atmospheric aberrations, namely tip–tilt and defocus. We use a digital decomposition tool to modally resolve the distorted vortex beam into its azimuthal components to observe the impact of the medium on the detection of the encoded vortex charge. Such techniques are useful in studies of free-space optical communication with orbital angular momentum.

Optical aberrations in a spinning pipe gas lens

If a heated pipe is rotated about its axis, a density gradient is formed which results in the pipe acting as a graded index lens. In this study we revisit the concept of a spinning pipe gas lens and for the first time analyse both the wave propagation of optical fields through the lens, and determine the optical aberrations introduced by the lens to the laser beam. We show that such lenses are highly aberrated, thus having a deleterious effect on the laser beam quality.

Mean focal length of an aberrated lens.

We outline an approach for the calculation of the mean focal length of an aberrated lens and provide closed-form solutions that show that the focal length of the lens is dependent on the presence of defocus, x-astigmatism, and spherical aberration. The results are applicable to Gaussian beams in the presence of arbitrary-sized apertures. The theoretical results are confirmed experimentally, showing excellent agreement. As the final results are in algebraic form, the theory may readily be applied in the laboratory if the aberration coefficients of the lens are known.

Characterisation of a spinning pipe gas lens using a Shack-Hartmann wavefront sensor

A heated horizontal spinning pipe causes gases inside it to assume dynamics resulting in a graded index lens - a spinning pipe gas lens (SPGL). A CFD model is presented which shows that gas exchanges of the SPGL with the surroundings resulting in a near parabolic density distribution inside the pipe created by the combination of velocity and thermal boundary layers. Fluid dynamic instabilities near the wall of the pipe are thought to have an deleterious effect on the quality of the beam and its wavefront. Measurements of the wavefront of a propagating laser beam shows strong defocus and tilt as well as higher order aberrations, thereby reducing the beam quality factor (M2) of the output beam. Results are presented as a function of pipe wall temperature and pipe rotation speed.

Propagating aberrated light

We outline a theory for the calculation of the laser beam quality factor of an aberrated laser beam. We provide closed form equations which show that the beam quality factor of an aberrated Gaussian beam depends on all primary aberrations except tilt, defocus and x-astigmatism. The model is verified experimentally by implementing aberrations as digital holograms in the laboratory. We extend this concept to defining the mean focal length of an aberrated lens, and show how this definition may be of use in controlling thermal aberrations in laser resonators. Finally, we look at aberration correction and control using a combination of spatial light modulators and adaptive mirrors.

A Computational Fluid Dynamics Model of a Spinning Pipe Gas Lens

When a metal horizontal pipe is heated and spun along its axis, a graded refractive index distribution is generated which is can be used as a lens, thus its name, the spinning pipe gas lens (SPGL). Experimental results showed that though increase in rotation speed and/or temperature resulted in a stronger lens and removed distortions due to gravity, it also increased the size of higher order aberrations resulting in an increase in the beam quality factor (M2). A computational fluid dynamics (CFD) model was prepared to simulate the aerodynamics that show how it operates and, in the process shed some light on the optical results. The results of the model consist of velocity profiles and the resultant density data and profiles. At rest the cross-sectional density profile has a vertical symmetry due to gravity but becomes rotationally symmetric with a higher value of density at the core as rotation speed increases. The longitudinal density distribution is shown to be parabolic towards the ends but is fairly uniform at the centre. The velocity profiles show that this centre is the possible source of higher order aberrations which are responsible for the deterioration of beam quality.

Optical Aberrations in Gas Lenses

Gas lenses work on the basis that aerodynamic media can be used to generate a graded refractive index distribution which can be used to focus a laser beam. An example is a spinning pipe gas lens (SPGL). It is a steel pipe whose walls are heated to a preselected temperature and then rotated along the axis to any desired speed to generate a cooler core of incoming air. A laser beam propagating through these lenses is focussed in space. However, experimental observation has shown that distortions are generated in the beam. We provide a computational fluid dynamics (CFD) model of the lens and experimental results of the Zernike aberrations measured using a Shack-Hartmann wavefront sensor which show that the aerodynamic medium in the lens have a deleterious effect on laser beam quality (M2). The effect on the SPGL is that the beam deterioration increases with rotation speed and temperature though the worst M2 measured at speed 20 Hz and temperature 155 °C was ~3.5 which is fairly good.