By: O. J. Allegre, W. Perrie, K. Bauchert, G. Dearden, K. G. Watkins
Laser Group, School of Engineering, University of Liverpool, UK
Boulder Nonlinear Systems, Inc., Colorado
The past decade has seen the development of ultra-short pulse lasers, with processes based on femtosecond and picosecond pulse durations becoming increasingly widespread. Thanks to the ultra-short timescale on which laser energy is coupled to the material, high precision machining of metals has been achieved with very little thermal damage. Industrial applications include the very precise drilling of holes for fuel-injection nozzles in the automotive industry. Polarization plays a particularly important role in drilling high-aspect-ratio (depth/diameter) microscopic holes in metal. Drilling with a linear polarized laser beam produces distorted hole profiles due to the anisotropic reflectivity of linear polarization. This paper describes the use of a liquid-crystal polarization rotator developed by Boulder Nonlinear Systems, Inc. to improve drilling quality by removing the distortions associated with static linear polarization. This flexible device allows rapid switching of the linear polarization of a laser beam between two orthogonal directions during micro-drilling. As a proof of principle, helical drilling tests were performed on stainless steel, using a 775 nm, 200 femtosecond pulse laser.