A pulse burst optical system has been developed, able to alter an energetic, ultrafast 10 ps, 5 kHz output pulse train to 323 MHz intra-burst frequency at the fundamental 5 kHz repetition rate. An optical delay line consisting of a beam-splitting polariser cube, mirrors, and waveplates transforms a high-energy pulse into a pulse burst, circulating around the delay line. Interestingly, the reflected first pulse and subsequent pulses from the delay line have orthogonal linear polarisations. This fact allows independent modulation of these pulses using two-phase-only Spatial Light Modulators (SLM) when their directors are also aligned orthogonally. With hybrid Computer Generated Holograms (CGH) addressed to the SLMs, we demonstrate simultaneous multi-spot periodic surface micro-structuring on stainless steel with orthogonal linear polarisations and cylindrical vector (CV) beams with Radial and Azimuthal polarisations. Burst processing produces a major change in resulting surface texture due to plasma absorption on the nanosecond time scale; hence the ablation rates on stainless steel with pulse bursts are always lower than 5 kHz processing. By synchronising the scan motion and CGH application, we show simultaneous independent multi-beam real-time processing with pulse bursts having orthogonal linear polarisations. This novel technique extends the flexibility of parallel beam surface micro-structuring with adaptive optics.
Wave-permeable materials with certain metallic film patterns are of importance for electromagnetic wave transportation. Laser processing showed obvious advantages when fabricating such materials. The article studied the etching effects of key laser parameters on Al-coated composite materials. It is found that (1) a balance of etching quality and efficiency could be obtained when the operating laser energy density is about five to eight times that of the ablation threshold; (2) the effects of the laser focal spot size and the pulse overlap are interrelated to decide machining precision; and (3) the laser scanning trajectory affects the appearance of the etched surface, possibly due to the fiber bundle braided direction. The findings could benefit similar wave-permeable materials laser processing.
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