Real-Time Control of Polarization in High-Aspect-Ratio Ultra-Short-Pulse Laser Micro-Machining

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.

In helical drilling the laser beam performs a circular movement with a defined diameter on the workpiece surface, working its way through the material on a helical path. A series of helical drilling tests were performed on 380 μm thick stainless steel samples. Circular beam paths with diameters of 65 μm were programmed on the laser scanning system. A pulse energy value of 75 μJ was used. The helical drilling tests produced tapered holes with an entrance opening diameter of typically around 110 μm. On the exit side, the shape and taper of the holes varied with polarisation, with a typical half-angle side-wall taper ranging between 4º and 5º. Optical micrographs of the exit holes for two polarisation modes are shown in figure 1.

Figure 1. (a) Exit hole resulting from helical drilling with linear polarisation. (b) Exit hole resulting from helical drilling with polarisation trepanning.

In the so-called polarization trepanning technique, the direction of linear polarization rotates synchronously to the beam motion around the hole so that it is always perpendicular to the side-wall. This was simulated by synchronising the liquid-crystal polarization rotator to the scanning system. The results in figure 1 show an improvement over drilling with static linear polarization but as only two orthogonal directions of polarization were allowed by the liquid-crystal device, the simulation to polarization trepanning was approximate. To our knowledge, this is the first time polarization trepanning has been achieved using a solid-state device for micro-machining. The liquid-crystal polarisation rotator is found to provide more flexibility than the wave-plate elements it replaces.