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. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)

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 (M 2). 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. © 2010 SPIE.Conference Pape

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. © 2010 SPIE.Conference Pape