We present an experimental technique to generate partially coherent vortex beams with an arbitrary azimuthal index using only a spatial light modulator. Our approach is based on digitally simulating the intrinsic randomness of broadband light passing through a spiral phase plate. We illustrate the versatility of the technique by generating partially coherent beams with different coherence lengths and orbital angular momentum content, without any moving optical device. Consequently, we study its cross-correlation function in a wavefront folding interferometer. The comparison with theoretical predictions yields excellent agreement.
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.
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 (<i>M<sup>2</sup></i>). The effect on the
SPGL is that the beam deterioration increases with rotation speed and temperature though the worst <i>M<sup>2</sup></i> measured at
speed 20 Hz and temperature 155 °C was ~3.5 which is fairly good.
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 (<i>M<sup>2</sup></i>). 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.
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 (M<sup>2</sup>) of the output beam.
Results are presented as a function of pipe wall temperature and pipe rotation speed.