Optical aberrations in a gas lenses

Abstract

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. Examples of these are the spinning pipe gas lens (SPGL) and the flame lens (FL). The SPGL is a steel pipe which is heated to a preselected temperature and then rotated along the axis to any desired speed to generate a cooler core of incoming air. An FL is a short steel tube into which a strong pencil flame is injected and made to spiral so as to create a cooler core. The FL is presented on a proof of principle basis and therefore its operation cannot, as yet, be altered by changing any aspect of its dimensions. 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 experimental results of the Zernike aberrations measured using a Shack-Hartmann wavefront sensor which show that they 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 ºC was ~5 which is fairly good. The FL caused had an average M2 of ~10. Despite this, gas lenses have a high damage threshold and so have an important part to play in the development of optics for high power laser fields.