Efficient speckle-free laser marking using a spatial light modulator
Krystian L. Wlodarczyk • Jarno J. J. Kaakkunen •
Pasi Vahimaa • Duncan P. Hand
Received: 26 September 2013 / Accepted: 29 November 2013 / Published online: 12 December 2013
 The Author(s) 2013. This article is published with open access at Springerlink.com

Abstract An approach for laser marking surfaces using a liquid–crystal-based spatial light modulator (LC-SLM) for beam patterning and manipulation is presented, designed to avoid the speckle interference problem which is a typical drawback of current SLM-based laser marking processes. In our approach, the LC-SLM is used to generate complex twodimensional micropatterns (e.g., 20 * 20 datamatrices) with overall dimensions of < 320 by 320 um. The micropatterns are generated in a series of 16 steps, using a Fresnel zone lens (FZL) combined with a computer-generated hologram (CGH); for each step the whole kinoform (FZL + CGH) is spatially shifted off-axis by a different amount of pixels to build-up the required pattern. In comparison with other SLM-based laser marking approaches already reported in the literature, our method not only eliminates (or at least significantly reduces) unwanted speckle interference but also reduces the laser power required for marking

1 Introduction
Liquid–crystal-based spatial light modulators (LC-SLMs) are electrically programmable devices which provide the ability to modify both phase and amplitude of linearly polarized light. The high spatial resolution of LC-SLMs (typically more than a half million pixels) coupled with their relatively high optical damage threshold and ease of programming mean that they have started to be used with commercially available short-pulsed (picosecond and nanosecond) lasers to generate complex beam shapes for effectively parallel processing of various materials [1–6], in contrast to the relatively time-consuming sequential approach of a scanning laser beam. An average laser power handling capability of commercially-available SLMs in the visible spectral range is approximately 2 W/cm², but it can be increased to approximately 10 W/cm² when a watercooled heat sink is mounted to the liquid crystal display[3].

To generate an appropriate beam pattern with an LCSLM, the device is typically used as a diffractive optical
element (DOE). In this approach, the LC-SLM unfortunately produces unwanted speckles that affect the quality of the laser marking area. As explained in [5], speckles result from (a) the pixilated (digital) character of the SLM display that introduces phase discontinuity to a computer-generated hologram (CGH) and (b) mutual interference between the neighbouring beams in the reconstructed image plane of a CGH when they are very close to each other. One of the methods to overcome the speckle problem is to use a series of periodically-shifted CGHs, as reported by Golan and Shoham [7]. Although this approach allows us to reduce the speckles and improve the quality of the laser-making area, as shown by Parry et al. [8], it seems to be ineffective when
very small-scale marks are required, i.e., less than 30 * 30 um, as shown in ‘‘Experimental results’’ below.The other potential solution to generate a micropattern without speckles can be the use of a random laser with low
spatial coherence, as reported recently by Redding et al. [9]. However, this approach requires an unconventional laser source and a physical mask for the image projection, which makes the process inflexible and more complicated.
In this paper, we present a novel SLM-based laser marking method which allows complex two-dimensional
micropatterns, e.g., datamatrices, to be produced without speckles, using relatively low laser power levels that do not risk damaging the SLM display. This is a sequential–parallel approach, sequentially using arrays of diffractive beams (beamlets), where each array machines/marks an array of subpixels in parallel. A Fresnel zone lens (FZL) in combination with a CGH is used to repeatedly move the array of diffractive beams across the workpiece to sequentially generate the subpixel arrays that eventually merge into a designed micropattern. Although steering the laser beam with a FZL written on to the SLM display has been already reported by Davis et al. [10], this is the first time that this approach has been reported with an array of laser spots. The FZL has the important added benefit of defocusing the zero-order beam at the workpiece, thereby
preventing its unwanted damage [11].
To demonstrate efficient operation of our sequential–parallel approach, we performed an experiment in which a 230 um square checkerboard pattern was produced by using: (a) the basic approach without speckle reduction, (b) the speckle reduction technique introduced by Golan and Shoham [7], and (c) our novel laser marking method. We also demonstrate an alternative approach to our method (d), in which a series of 16 different CGHs sequentially generate the array of diffractive beams at the processing plane. Finally, we demonstrate a possible application of our laser-marking approach for secure data coding of small and valuable metal parts.