Product Code: ICA11_M601

Ultra-Short Pulse Laser Micro-Machining of Metals with Radial and Azimuthal Polarization
Authors:
O.J. Allegre, University of Liverpool; Liverpool Great Britain
W. Perrie, University of Liverpool; -
S.P. Edwardson, University of Liverpool; -
G. Edwardson, University of Liverpool; -
K.G. Watkins, University of Liverpool; -
Presented at ICALEO 2011

Laser-material interactions are known to be strongly influenced by the polarization of the incident laser beam. In particular, polarization affects both the efficiency and the quality of ultra-short pulse micro-processes such as helical drilling and cutting. The potential benefits of using radial and azimuthal polarizations for laser machining have been studied theoretically for several years, leading to predictions that the use of a radially polarized beam could enhance the efficiency of the cutting process by more than 50% compared to circular polarization. However, producing these modes of polarization has been difficult until recently, making it complicated to verify these claims experimentally. Thanks to the latest technological developments in liquid-crystal devices such as spatial light modulators, it is now possible to produce such polarization modes in a relatively simple and cost effective manner.

In this work, a liquid-crystal spatial light modulator has been used to convert a linearly polarized femtosecond pulse laser beam to a radially, azimuthally and circularly polarized beam. The micro-processing properties of these modes of polarization have been investigated. Blind drilling, helical drilling and cutting tests have been carried out on metals such as stainless steel, copper and aluminium, generating micro-features of various depths and geometries. The resulting cutting profiles have been measured, demonstrating the effect polarization has on the process. Laser Induced Periodic Surface Structures (LIPSS) have also been investigated for the respective polarization modes.

These experiments did not confirm the 50% gain in efficiency when cutting metals with radial polarization, predicted by some of the early theoretical models. However, polarization emerged as a powerful tool for optimizing ultra-short pulse laser micro-processing. Depending on the material, the geometry and depth of the machined structures, radial or azimuthal polarizations produced the best results, reducing ellipticity, improving the edge quality of the exit apertures and reducing the processing time, compared with more traditional polarizations such as circular. Azimuthal polarization was more efficient at cutting higher aspect-ratio features, due to its high reflectivity which allows a larger proportion of the beam energy to be channeled to the bottom of the structure. Radial polarization was better for cutting lower aspect-ratio features. To our knowledge, this work is the first to investigate the effects of radial and azimuthal polarizations on femtosecond laser micro-processing of metals.