"High-precision Generation of Parallel Long-axis Focused Beams for Fabrication of Diffraction Gratings" by Koichi Takahashi

Seminar by Koichi Takahashi, Center for Optical Research and Education (CORE), Université Utsonmiya, Japon

Abstract

The internal modification of materials (refractive index change and defect region) by ultrashort pulse laser processing is widely used for the fabrication of diffraction gratings, optical waveguides, photonic crystals, optical storage, and chemical etching guides. In the fabrication of diffraction gratings using the internal refractive index change by ultrashort pulse laser processing, it is necessary to realize high-throughput simultaneous parallel processing by parallelizing focused beams and to fabricate thick diffraction gratings with high diffraction efficiency by expanding the focal depth of the beam. The holographic laser processing method using computer-generated holograms (CGHs) displayed on a spatial light modulator (SLM) solves this problem by spatially shaping the laser beam [1]. In this study, we generate parallel long-axis beams by integrating a CGH for parallel beam generation with another CGH for long-axis focusing. Both CGHs are optimized using a novel approach, resulting in high-precision beam shaping within the processing optical system.

For high-precision generation, the CGH for array beam generation is updated so that the laser-induced emission intensity at each spot during laser processing is uniform. CGH is optimized while observing the processing situation, achieving high-precision processing.

In long-axis processing, a CGH with a uniform beam profile along the axial direction allows flexible control of the processing depth. The axial beam is generated using a three-dimensional weighted iterative optimization method based on the Weighted Gerchberg–Saxton (WGS) algorithm [2].

1) Y. Hayasaki, T. Sugimoto, A. Takita, and N. Nishida, “Variable holographic femtosecond laser processing by use of a spatial light modulator,” Appl. Phys. Lett. 87, 031101 (2005).

2)Nami Kuroo and Yoshio Hayasaki, "Design framework of a computer-generated hologram that performs volumetric beam shaping for advanced laser processing," Opt. Continuum 3, 1244-1253 (2024)