Parallel laser micromachining based on diffractive optical elements with dispersion compensated femtosecond pulses

LIPSS-based functional surfaces produced by multi-beam nanostructuring with 2601 beams and real-time thermal processes measurement

A unique combination of the ultrashort high-energy pulsed laser system with exceptional beam quality and a novel Diffractive Optical Element (DOE) enables simultaneous production of 2601 spots organized in the square-shaped 1 × 1 mm matrix in less than 0.01 ms. By adjusting the laser and processing parameters each spot can contain Laser Induced Periodic Surface Structures (LIPSS, ripples), including high-spatial frequency LIPSS (HFSL) and low-spatial frequency LIPSS (LSFL). DOE placed before galvanometric scanner allows easy integration and stitching of the pattern over larger areas. In addition, the LIPSS formation was monitored for the first time using fast infrared radiometry for verification of real-time quality control possibilities. During the LIPSS fabrication, solidification plateaus were observed after each laser pulse, which enables process control by monitoring heat accumulation or plateau length using a new signal derivation approach. Analysis of solidification plateaus after each laser pulse enabled dynamic calibration of the measurement. Heat accumulation temperatures from 200 to 1000 °C were observed from measurement and compared to the theoretical model. The temperature measurements revealed interesting changes in the physics of the laser ablation process. Moreover, the highest throughput on the area of 40 × 40 mm reached 1910 cm²/min, which is the highest demonstrated throughput of LIPSS nanostructuring, to the best of our knowledge. Thus, showing great potential for the efficient production of LIPSS-based functional surfaces which can be used to improve surface mechanical, biological or optical properties.

Systematic study of laser ablation with GHz bursts of femtosecond pulses

We report on crater formation, line scribing and cavity milling experiments on Silicon, Copper, Aluminum and stainless steel with GHz bursts of femtosecond pulses. The intra-burst repetition rate has been varied between 0.88 and 3.52 GHz, the number of pulses per burst between 50 and 3200, the burst fluence between 8 and 80 J/cm². For these experiments, a 100-W femtosecond GHz-burst laser has been developed on an industrial laser basis, delivering a total burst energy up to 1 mJ at 100 kHz, with an adjustable number of pulses per burst. The results highlight the conditions to obtain high-ablation efficiency, show how to optimize the machining quality and point out the burst duration as the relevant parameter for femtosecond GHz machining.

Complex diffraction and dispersion effects in femtosecond laser writing of fiber Bragg gratings using the phase mask technique

The combined effect of chromatic dispersion and conical diffraction (i.e., off-plane diffraction) in femtosecond laser inscription of fiber Bragg gratings using the phase mask technique is characterized by measuring the light intensity distribution after the phase mask. As the distance from the mask and the observation point grows, chromatic dispersion and conical diffraction introduced by the mask gradually decrease the peak intensity inside the line-shaped focal volume of the cylindrical lens that is used to focus the femtosecond pulses inside the fiber. We also show that at a certain distance from the mask spherical aberration introduced by the plane-parallel mask substrate is cancelled out by conical diffraction and, at a different distance, chromatic aberration of the cylindrical lens is cancelled out by chromatic dispersion of the mask. These two independent cancellation effects lead to sharpening of the line-shaped focus and the consequent growth of peak light intensity inside it. The above phenomena become especially pronounced for tightly focused femtosecond laser beams and small-pitch phase masks, which, in turn, allows one to choose experimental conditions to inscribe Bragg gratings in polymer-coated non-sensitized 50 µm fibers.

All-optical, self-focused laser beam array for parallel laser surface processing

A single femtosecond laser beam can be focused to process material surfaces. However, the low processing efficiency by a single laser beam prevents broad applications of femtosecond laser surface processing. Here we have demonstrated an array of high-intensity femtosecond laser beams that self-focus in a piece of glass medium, enabling high-efficiency, millimeter scale parallel laser surface processing. Experimental and simulation results suggest that the laser fluence of each focused laser beamlet in the array is more than 1.2 J/cm², which is sufficient for micro- or nano-structure laser processing. To prove the concept, nanoscale structures on silicon surface are generated by the self-focused laser beam array, which may improve the efficiency of black silicon fabrication for photovoltaic devices.

Multi-watt, broadband second-harmonic-generation in MgO:PPSLT waveguides fabricated with femtosecond laser micromachining

We demonstrate optical waveguides fabricated in periodically poled MgO-doped stoichiometric lithium tantalate crystals using an fs-laser direct-write process. Two different waveguide architectures were developed: depressed cladding and stress-induced waveguides. Our strain-optic simulations confirmed the guiding mechanism for either case. We demonstrate designs optimized for low propagation loss (0.52 dB/cm) for both fundamental (1050 nm) and second-harmonic wavelengths (525 nm). Low-power CW second-harmonic-generation studies show normalized efficiencies comparable to that of annealed reverse-proton-exchange waveguides in lithium niobate. High-power studies demonstrate second-harmonic power levels up to 8.5 W in a single-pass configuration, using a 1-nm bandwidth CW IR fiber laser as a pump.

Generation of multifocal irradiance patterns by using complex Fresnel holograms

We experimentally demonstrate Fresnel holograms able to produce multifocal irradiance patterns with micrometric spatial resolution. These holograms are assessed from the coherent sum of multiple Fresnel lenses. The utilized encoded technique guarantees full control over the reconstructed irradiance patterns due to an optimal codification of the amplitude and phase information of the resulting complex field. From a practical point of view, a phase-only spatial light modulator is used in a couple of experiments addressed to obtain two- and three-dimensional distributions of focal points to excite both linear and non-linear optical phenomena.

Distortion-compensated multifocusing of ultrashort pulse beams using cascade optical system

We report a cascade optical system for multifocusing ultrashort pulse beams, particularly sub-50-fs pulses. System achromaticity is key to simultaneous compensation of the spatio-temporal pulse distortions. In this system, diffractive and refractive subsystems are optically coupled in cascade to correct chromatic aberrations, which are the primary cause of pulse distortion. We design a prototype system by applying achromatic conditions derived from an ABCD matrix analysis. The designed system is then evaluated using 20-fs pulses, by characterizing the transmitted pulses in terms of beam width and pulse duration; hence, the proposed distortion compensation scheme is validated. This pulse delivery system enables damage-free and high-throughput materials processing using ultrashort pulses.

Design and demonstration of fan-out elements generating an array of subdiffraction spots

We present a design method for diffractive fan-out elements generating an array of subdiffraction spots in a predetermined area using propagating light. A specific constraint is introduced to the Gerchberg-Saxton algorithm to control the dispersion of light energy and the phase of spots. In our demonstration, an element generating a 3 × 3 spot array achieves the spot size reduced to 70% comparing to the diffraction limited spot in simulation and about 80% in the experiment. Various layouts of subdiffraction spots can be made with a high signal to noise ratio using our design method. The relationship between the spot size and the power efficiency is also shown based on the simulation.