Abstract
A square flattop beam is a fundamental shape that is in high demand in various applications, such as ultra-high-power lasers, uniform surface processing and medical engineering. In this experiment, a new and simple scheme of the adaptive beam shaping system to generate a square flattop shape with high uniformity and edge steepness using virtual diagonal phase grating encoded on a spatial-light modulator and a 4f system is proposed. The grating vector kg is non-parallel to the normal vectors kx and ky of the objective beam profile to be extracted; thus, the residual and extracted components hit separately on the Fourier plane of the 4f system. Consequently, using a spatial-frequency filter passing components parallel to kx and ky, the residual components are blocked by the filter without loss of the high spatial-frequency domain of the extracted component. When the width of the filter was 1.0 mm, the edge of the shaped beam increased in height within 20 μm, which is less than 20% of that obtained with conventional vertical phase grating.
Highlights
Beam shape is key to realising a laser’s potential abilities, and various beam shaping techniques have been developed for different applications
Phase grating diffracts the residual components to extract the component regenerated on the image plane with a square flattop beam profile
We adopted virtual diagonal phase grating encoded on an spatial-light modulator (SLM) to utilize the high spatial-frequency (HSF) component of a square beam fully, and overcame the limitation in the edge steepness and resolution in the conventional adaptive beam shaping method
Summary
An experiment was conducted to demonstrate the concept. Figure 5a shows a beam profile with vertical phase grating as a function of the diameter of the SFF. Note that the points in the graphs reflect the pixels in the beam profiler It appears that the steepness is quite different for the vertical and diagonal phase gratings. This is due to the inclusion of a part of the residual components through the SFF. As is clearly shown in the upper left inset, the length is shorter than the period of the phase grating on the beam profiler (ΛD × M = 28.3 μm) This reflects utilization of the HSF component beyond the phase grating frequency, which is enabled by the proposed scheme for the first time. The beam shaping system is introduced at front-end subsystem in multiple amplification systems in the case of ultra-high-power laser facilities[9]
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