Doughnut-Shaped and Top Hat Solar Laser Beams Numerical Analysis

Aside from the industry-standard Gaussian intensity profile, top hat and non-conventional laser beam shapes, such as doughnut-shaped profile, are ever more required. The top hat laser beam profile is well-known for uniformly irradiating the target material, significantly reducing the heat-affected zones, typical of Gaussian laser irradiation, whereas the doughnut-shaped laser beam has attracted much interest for its use in trapping particles at the nanoscale and improving mechanical performance during laser-based 3D metal printing. Solar-pumped lasers can be a cost-effective and more sustainable alternative to accomplish these useful laser beam distributions. The sunlight was collected and concentrated by six primary Fresnel lenses, six folding mirror collectors, further compressed with six secondary fused silica concentrators, and symmetrically distributed by six twisted light guides around a 5.5 mm diameter, 35 mm length Nd:YAG rod inside a cylindrical cavity. A top hat laser beam profile (Mx2 = 1.25, My2 = 1.00) was computed through both ZEMAX® and LASCAD® analysis, with 9.4 W/m2 TEM00 mode laser power collection and 0.99% solar-to-TEM00 mode power conversion efficiencies. By using a 5.8 mm laser rod diameter, a doughnut-shaped solar laser beam profile (Mx2 = 1.90, My2 = 1.00) was observed. The 9.8 W/m2 TEM00 mode laser power collection and 1.03% solar-to-TEM00 mode power conversion efficiencies were also attained, corresponding to an increase of 2.2 and 1.9 times, respectively, compared to the state-of-the-art experimental records. As far as we know, the first numerical simulation of doughnut-shaped and top hat solar laser beam profiles is reported here, significantly contributing to the understanding of the formation of such beam profiles.

Novel Approach to Improve the Optical Performance by Machining Process Without Surface Finishing

With the increase in dimensions of optical elements in addition to ever rising demand for aspherical optics, the millimeter-scale periodic waviness that is naturally produced by machining (such as diamond turning) process in precision optical engineering has been one of the most crucial issues in the development of high surface quality optical elements. Even an extremely small waviness can affect the laser beam profile significantly through interference caused by Bragg scattering. This paper presents a novel method for improving a laser beam profile by utilizing the characteristics of Bragg scattering without requiring established final surface finishing processes such as optical polishing. By engraving an artificial periodic structure with a period of a few hundred microns, the Bragg scattering angle that influences the formation of interference fringes in the laser beam profile was drastically enlarged. Consequently, the quality of the beam profile was improved at a propagation distance where the 0th and 1st (− 1st) order beam modes are spatially separated, only by diamond turning machining without the surface finishing process. In addition, this approach represents an important contribution to green technology, which seeks energy saving and waste reduction in the optical surface manufacturing process.