Investigation of the thermal and optical performance of a spatial light modulator with high average power picosecond laser exposure for materials processing applications

摘要

Spatial light modulators (SLM’s) addressed with Computer Generated Holograms (CGH’s) can create structured light fields on demand when an incident laser beam is diffracted by a phase CGH. The power handling limitations of these devices based on a liquid crystal layer has always been of some concern. With careful engineering of chip thermal management, we report the detailed optical phase and temperature response of a liquid cooled SLM exposed to picosecond laser powers up to P = 220W at 1064nm. This information is critical for determining device performance at high laser powers. SLM chip temperature rose linearly with incident laser exposure, increasing by only 5 deg. C at P = 220W incident power, measured with a thermal imaging camera. Thermal response time with continuous exposure was 1-2 seconds. The optical phase response with incident power approaches radians with average power up to P = 130W, hence the operational limit, while above this power, liquid crystal thickness variations limit phase response to just over radians. Modelling of the thermal and phase response with exposure is also presented, supporting experimental observations well. These remarkable performance characteristics show that liquid crystal based SLM technology is highly robust when efficiently cooled. High speed, multi-beam plasmonic surface micro-structuring at a rate R = 8cm2s-1 is achieved on polished metal surfaces at P = 25W exposure while diffractive, multi-beam surface ablation with average power P =100W on stainless steel is demonstrated with ablation rate of 4mm3min-1. However, above 130W, first order diffraction efficiency drops significantly in accord with the observed operational limit. Continuous exposure for a period of 45 minutes at a laser power of P = 160W did not result in any detectable drop in diffraction efficiency, confirmed afterwards by the efficient parallel beam processing at P = 100W. Hence, no permanent changes in SLM phase response characteristics have been detected. This research work will help to accelerate the use of liquid crystal Spatial light modulators for both scientific and ultra high throughput laser-materials micro-structuring applications.