Experimental investigation of the closest parallel pulses in holographic femtosecond laser processing

Abstract

In holographic femtosecond laser processing, diffractive parallel pulses are distorted by phase discontinuities and mutual interference between the neighborhoods in the reconstructed image of a Fourier computer-generated hologram when the interval is smaller than the beam diameter. We investigated holographic fabrication on a glass surface using parallel pulses with different intervals. We found the closest parallel pulses with sufficient separation to avoid mutual interference in holographic femtosecond laser processing. The minimum interval was 2.8 times larger than the diffracted beam diameter. The experimental results were also supported by a computer simulation. Our findings will be very useful in the design of holographic laser processing systems.

Appendix

The computer simulation for CGH reconstruction was performed as follows. The CGH was optimized by the ORA method [45]. The CGH was a Fourier hologram, and thus, the reconstruction calculation was performed with the fast Fourier transformation (FFT). The FFT calculation was performed on a graphic processing unit (GPU; GeForce GTX480) using the CUDA development toolkit (Ver. 4.0) produced by NVIDIA Corporation and the CUFFT library. All values were represented by floating-point data types because of the fast calculation. The computation region was 7168 ×7168 pixels, which was sufficiently large to prevent superposition between diffraction light from the CGH and undesired diffraction light derived from the periodic boundary conditions of the FFT. The CGH had 512 ×512 pixels, and the are outside of the CGH had phase values of zero. The pupil was located to coincide with the experimental setup shown in Fig. 2. The illumination inside the pupil and on the outside was represented by amplitudes of 1.0 and 0.0, respectively. The light source had a single wavelength of 800 nm, which was the only difference from the experimental conditions.