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.

Four-dimensional light shaping: manipulating ultrafast spatiotemporal foci in space and time

“…It is thus possible to generate a uniform "flat top" intensity extended line focus based upon the inherent pulse front tilt present in SSTF. This ability offers a further degree of freedom over conventional holographic spot arrays with conventional focusing, where there is a limitation on how closely spaced the spots may be [44]. If they are positioned closer than around 3 Airy units of each other, mutual interference causes significant distortion of the desired intensity profile, in effect resulting in non-uniformity in the illuminated area.…”

Method for holographic femtosecond laser parallel processing using digital blazed grating and the divergent spherical wave

“…Therefore, femtosecond laser holographic parallel processing based on SLM has attracted broad attention in research. [2][3][4][5][6] However, dynamic SLM holograms based on pixel structure still face many problems for achieve the target diffraction pattern, as the zero-order diffracted light and multiorder image reproduction caused by the characteristics of the SLM pixel structure have a large impact on the target image quality. A great number of studies aiming at addressing these issues was carried out by researchers in the holographic display fields all around the world.…”

Efficient speckle-free laser marking using a spatial light modulator

“…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][2][3][4][5][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/cm 2 , but it can be increased to approximately 10 W/cm 2 when a watercooled heat sink is mounted to the liquid crystal display [3].…”

“…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].…”

“…There are several benefits of the use of the optical system with the software-generated FZL over a conventional 6-f optical system [1][2][3][4][5]8] which requires the use of three lenses after the SLM together with a spatial filter to block (eliminate) the undiffracted zero-order beam at the processing plane. First of all, the optical setup is relatively short, being dependent on the focal length of FZL and the final focusing lens [11].…”