The effects of femtosecond laser (wavelength 1035nm, pulse duration 400 fs, pulse repetition rate 1kHz) on the material removal threshold of superalloy was experimentally investigated, specifically the influence of pulse number irradiated on the threshold of superalloy. Irregular bulge structures were found at the bottom of ablation holes. Two sets of material removal thresholds are derived from the value of hole diameter and depth, which are 0.506 J/cm2 , 0.245 J/cm2 , 0.124 J/cm2 , 0.102 J/cm2 , 0.096 J/cm2 and 0.141 J/cm2 , 0.130 J/cm2 , 0.031 J/cm2 , 0.026 J/cm2 , 0.091 J/cm2 for diameter derivation and depth derivation, respectively, for 1, 10, 100, 500 and 1000 pulses. The deduced two sets of thresholds show similar behavior that the threshold sharply decreases with the pulse number and levels out as the pulse number exceeds 100. The phenomenon is due to incubation effects of multi-pulse irradiation on the surface of superalloy and the incubation coefficient for the thresholds calculated by diameter and depth is fitted to be ~0.72 and ~0.77.
Spiral phase plate (SPP) for generating vortex hollow beams has high efficiency in various applications. However, it is difficult to obtain an ideal spiral phase plate because of its continuous-varying helical phase and discontinued phase step. This paper describes the demonstration of continuous spiral phase plate using filter methods. The numerical simulations indicate that different filter method including spatial domain filter, frequency domain filter has unique impact on surface topography of SPP and optical vortex characteristics. The experimental results reveal that the spatial Gaussian filter method for smoothing SPP is suitable for Computer Controlled Optical Surfacing (CCOS) technique and obtains good optical properties.
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