Ultrafast holographic pulse burst laser surface modification of metals with orthogonal polarisations

An ultrafast laser system, combined with an optical delay line and two Spatial Light Modulators has allowed comparison of kHz surface laser processing with burst processing on stainless steel, Titanium alloy and Copper using pulselengths from 180 fs −10 ps. An optical delay line allows an intraburst frequency of 323 MHz at 5 kHz hence allowing independent and simultaneous micro-structuring with orthogonal polarisations. The finesse and development of Laser Induced Periodic Surface Structures (LIPSS) is clearly affected by bursts with orthogonal polarisations as well as pulselength dependent. Three pulse burst with 10 ps pulselength eliminates LIPSS on both stainless steel and Titanium, supporting surface melting. By spatially offsetting two 1 x 5 holographic arrays with orthogonal linear polarisations, we demonstrate simultaneous parallel writing of orthogonal LIPSS at 5 kHz and 323 MHz. By scanning overlapped arrays, LIPPS texturing with 5 kHz, double and triple pulse bursts is achieved. With the high pulse energy available, simultaneous LIPSS texturing of stainless steel employing two 16 spot orthogonally polarised arrays was produced. Finally, by tightly synchronising the applications of Computer Generated Holograms to two Spatial Light Modulators, 18 x 18 real time LIPSS encoded arrays were fabricated while binary surface encoding at 10 Hz is achieved using the contrast observed during simultaneous ablation with 5 kHz compared to pulse burst.