High repetition rate femtosecond lasers are commonly used for fabricating laser-induced periodic surface structures (LIPSS) over large areas at high processing speeds. Industrially relevant metals, like steel, experience thermal modifications at repetition rates beyond several hundred kilohertz. In this work, we fabricate low spatial frequency LIPSS (LSFL) on steel, varying pulse repetition rates from 10 kHz to 2 MHz. The study characterizes laser-structured areas and redeposited debris using SEM and μ-Raman spectroscopy. A simple heat dissipation model identifies repetition rate ranges associated with thermal modifications. Morphological changes and debris impact functional wetting behavior, offering insights for optimizing parameters in high repetition rate femtosecond laser materials processing.
Reflectivity loss due to dust deposition over primary mirrors is a key issue for ground-based astronomical observatories. Suspended atmospheric dust falls over the optical surface affecting its performance and optical quality. Several in-situ and off-site techniques have been developed for eliminating dust particulates from optical surfaces, but all of them present important drawbacks like being time-consuming, expensive, contaminant or even involving dangerous operations in off-site approaches. Alternatively, a method based on laser-matter interaction and SENER’s experience in ground telescopes, has been tested and developed. Laser cleaning is a discipline that covers a large range of applications, from semiconductor physical decontamination to the removal of oxides from metallic surfaces. The wide catalogue of laser systems currently available, allows us to investigate and design the best laser source for astronomical mirror maintenance. This has allowed us to develop a system that performs a proper cleaning for astronomical mirrors, which is currently bridging the gap between the laboratory demonstrator and the telescope application. In this communication we present the development of a laser cleaning system for the removal of dust particulates and contaminants attached to telescope mirror surfaces. Several configurations have been tested in the laboratory, under different conditions, and cleaning was performed over aluminized Zerodur® flat samples, that were naturally contaminated in Atacama’s desert and Canary Islands, aiming at the definition, manufacturing, and testing of a demonstrator prototype. Results show a large improvement of reflectivity after cleaning and contribute to the design of a custom efficient laser cleaning system adapted to different observatories.
Nonlinear laser processing of dielectrics with ultrafast lasers has been extensively studied over the last years and successfully applied to the production of photonics and micro-fluidic devices. Still, problems related to the presence of strong optical nonlinearities make it difficult to optimize the spatial intensity distribution in the focal region (SIDFR) in some cases. Methods providing a rapid estimate of the latter, even approximately, can be of great help for optimizing processing strategies and in other applications conditioned by nonlinear propagation like spatial soliton shaping. We have developed a numerical method for estimating the SIDFR inside a dielectric material, considering nonlinear absorption, nonlinear refraction and spherical aberration for laser beams with arbitrarily shaped wavefront. It is based on a generalized adaptive fast-Fourier evolver and has been successfully tested for flat wavefronts in subsurface processing. In this work we demonstrate its applicability to complex wavefronts, like those that can be generated with spatial light modulators (SLM). For this purpose the beam wavefront is described using Zernike polynomials before being propagated inside the material for different depths, pulse parameters. The results obtained show that under certain conditions, nonlinearities can be not only controlled and pre-compensated but also exploited for producing tailored SIDFRs.
Strong ion migration in shown to enable the production of high refractive index contrast waveguides by fs-laser writing
in a commercial (Er,Yb)-doped phosphate based glass. Waveguide writing was performed using a high repetition rate fslaser
fibre amplifier operated at 500 kHz and the slit shaping technique. Based on measurements of the NA of
waveguides, the positive refractive index change (Δn) of the guiding region has been estimated to be ∼1-2 x10-2. The
compositional maps of the waveguides cross-sections performed by X-ray microanalysis evidenced a large increase of
the La local concentration in the guiding region up to ~25% (relative to the non-irradiated material). This large
enrichment in La was accompanied by the cross migration of K to a neighbouring low refractive index zone. The
refractive index of the La-phosphate glass increases linearly with the La2O3 content (Δn per mole fraction increase of
La2O3 ≈ 5x10-3) mainly because of the relative mass of the La3+ ions. The density increase without substantial
modification of the glass network was confirmed by space-resolved micro-Raman spectroscopy measurements showing
minor variations in the (PO2)sym vibration Raman band. These results provide evidence for the feasibility of adapting the glass composition for enabling laser-writing of high refractive index contrast structures via spatially selective
modification of the glass composition.
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