Ultrafast laser beam shaping for material processing at imaging plane by geometric masks using a spatial light modulator

https://doi.org/10.1016/j.optlaseng.2015.02.004Get rights and content

Highlights

  • An ultrafast laser beam shaping technique for material processing is demonstrated.

  • Arbitrary beam intensity shapes can be created using a spatial light modulator.

  • The beam shape is obtained at near field by geometric masks.

  • The shape reconstructed at imaging plane has a size comparable to the beam waist.

  • The machined footprint on a metallic sample has the corresponding beam shape.

Abstract

We have demonstrated an original ultrafast laser beam shaping technique for material processing using a spatial light modulator (SLM). Complicated and time-consuming diffraction far-field phase hologram calculations based on Fourier transformations are avoided, while simple and direct geometric masks are used to shape the incident beam at diffraction near-field. Various beam intensity shapes, such as square, triangle, ring and star, are obtained and then reconstructed at the imaging plane of an f-theta lens. The size of the shaped beam is approximately 20 µm, which is comparable to the beam waist at the focal plane. A polished stainless steel sample is machined by the shaped beam at the imaging plane. The shape of the ablation footprint well matches the beam shape.

Introduction

In the last two decades, ultrafast lasers have been widely used for high precision and high quality material micro-processing. Materials ranging from metals [1], [2], semiconductors [3], dielectrics [4], [5], [6] to biological tissues [7], [8] can be processed by ultrafast laser pulses with a very small heat affected zone around the irradiated area. In recent years, the price of commercial ultrafast lasers has decreased rapidly and the laser system becomes more and more compact. Nowadays, some type of ultrafast laser systems, such as high repetition rate picosecond fibre laser systems, have been increasingly employed by manufacturing industries.

Due to the well defined ablation threshold, one of the characteristics of ultrafast laser material processing is that the shape of the processed area is very close to the laser beam׳s intensity distribution. This has motivated some efforts in the field of ultrafast laser beam shaping. From the use of amplitude mask projection and diffractive optical elements (DOEs) [9] to deformable mirrors [10], different technique has been attempted to shape ultrafast laser beams for various applications. Multiple annular beams were generated at focal plane by us recently for ultrafast laser micro-drilling with diffractive axicon phases using a spatial light modulator (SLM) [11]. Sanner et al. successfully obtained top-hat, doughnut, square, and triangle beam shapes at focal plane by programmable wave-front modulations using a nonpixelated optically addressed light valve [12], [13]. However, to produce a desired shape at focal plane (i.e. far field), the phase modulation to the incident laser beam is complicated. Although algorithms based on time-consuming iterative calculations, such as Gerchberg and Saxton [14], [15], [16], were attempted to solve the issue, the accuracy was still not perfect due to the complexity nature of light diffraction.

In this paper, we demonstrate an original ultrafast laser beam shaping technique for material processing using a spatial light modulator (SLM). Complicated and time-consuming diffraction far-field phase hologram calculations based on Fourier transformations are avoided, while simple and direct geometric masks are used to shape the incident beam at diffraction near-field. Arbitrary beam intensity shapes can be easily obtained and then reconstructed at the imaging plane of an f-theta lens. The size of the shaped beam is approximately 20 µm, which is comparable to the beam waist at the focal plane. A polished stainless steel sample is machined by the shaped beam at the imaging plane. The shape of the ablation footprint well matches the beam shape.

Section snippets

Experimental setup

A schematic of the experimental setup is shown in Fig. 1. A laser beam output (beam diameter: Dia. ≈1 mm, pulse duration: tp=20 ps, wavelength: λ =1064 nm, and repetition rate: R=200 kHz) from a picosecond fibre laser system (Finanium) is passed through a half wave plate used for adjusting the linear polarisation direction, a beam expander (M≈×5), and illuminated on a reflective SLM (Holoeye LC-R 2500), oriented at <10° angle of incidence. A pick-off (1%) beamsplitting mirror, placed after the SLM,

Results

Fig. 3 shows two geometric masks, square and triangle, applied to the SLM at A and the correspondent beam profiles observed at A′. As shown, the beam was successfully shaped to the desired geometries near the SLM surface.

Fig. 4 demonstrates micrographs of a series of drilling footprints on a polished stainless steel sample. The sample was machined at different heights on the Aerotech stage from the focal plane H to the image plane A″. The input laser pulse energy Ep was approximately 5 µJ,

Shaping quality

Fig. 6 shows the beam profiles observed by the CCD and the footprints machined on a stainless steel sample when changing the size of geometric mask (i.e. the square size). The input laser beam diameter was measured ≈6 mm. As shown in Fig. 5, the shaping quality is affected by the size of the geometry displayed on the SLM at A. When applying a larger sized square (e.g. 5 mm×5 mm or 4 mm×4 mm), the machined footprint at image plane did not have a good square shape. However, when applying a smaller

Conclusions and future work

An original ultrafast laser beam shaping technique for material processing using a spatial light modulator (SLM) was demonstrated in this paper. Simple and direct geometric masks are used to shape the input beam at diffraction near-field. Arbitrary intensity shapes can be obtained and then reconstructed at the imaging plane of a focusing lens. The size of the shaped beam (~20 µm) is comparable to the beam waist at the focal plane. A polished stainless steel sample was machined by the shaped beam

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