Krystian Lukasz Wlodarczyk

Scalable stacked array piezoelectric deformable mirror for astronomy and laser processing applications

A prototype of a scalable and potentially low-cost stacked array piezoelectric deformable mirror (SA-PDM) with 35 active elements is presented in this paper. This prototype is characterized by a 2 μm maximum actuator stroke, a 1.4 μm mirror sag (measured for a 14 mm × 14 mm area of the unpowered SA-PDM), and a ±200 nm hysteresis error. The initial proof of concept experiments described here show that this mirror can be successfully used for shaping a high power laser beam in order to improve laser machining performance. Various beam shapes have been obtained with the SA-PDM and examples of laser machining with the shaped beams are presented.

Laser smoothing of binary gratings and multilevel etched structures in fused silica

We describe a promising approach to the processing of micro-optical components, where CO(2) laser irradiation in raster scan is used to generate localized surface melting of binary or multilevel structures on silica, fabricated by conventional reactive-ion etching. The technique is shown to provide well-controlled local smoothing of step features by viscous flow under surface tension forces, relaxing the scale length of etch steps controllably between 1 and 30 microm. Uniform treatment of extended areas is obtained by raster scanning with a power stabilized, Gaussian beam profile in the 0.5 to 1 mm diameter range. For step heights of 1 microm or less, the laser-induced relaxation is symmetric, giving softening of just the upper and lower corners at a threshold power of 4.7 W, extending to symmetric long scale relaxation at 7.9 W, with the upper limit set by the onset of significant vaporization. Some asymmetry of the relaxation is observed for 3 microm high steps. Also, undercut steps or troughs produced by photolithography and etching of a deep 64 level multistep surface are found to have a polarization-dependent distortion after laser smoothing. The laser reflow process may be useful for improving the diffraction efficiency by suppressing high orders in binary diffractive optical elements, or for converting multilevel etched structures in fused silica into smoothed refractive surfaces in, for example, custom microlens arrays.

Direct CO 2 laser-based generation of holographic structures on the surface of glass

A customized CO(2) laser micromachining system was used for the generation of phase holographic structures directly on the surface of fused silica (HPFS(®)7980 Corning) and Borofloat(®)33 (Schott AG) glass. This process used pulses of duration 10µs and nominal wavelength 10.59µm. The pulse energy delivered to the glass workpiece was controlled by an acousto-optic modulator. The laser-generated structures were optically smooth and crack free. We demonstrated their use as diffractive optical elements (DOEs), which could be exploited as anti-counterfeiting markings embedded into valuable glass-made components and products.

Generation of microstripe cylindrical and toroidal mirrors by localized laser evaporation of fused silica

We report a new technique for the rapid fabrication of microstripe cylindrical and toroidal mirrors with a high ratio (>10) of the two principal radii of curvature (RoC(1)/RoC(2)), and demonstrate their effectiveness as mode-selecting resonator mirrors for high-power planar waveguide lasers. In this process, the larger radius of curvature (RoC(1)) is determined by the planar or cylindrical shape of the fused silica substrate selected for laser processing, whilst the other (RoC(2)) is produced by controlled CO(2) laser-induced vaporization of the glass. The narrow stripe mirror aperture is achieved by applying a set of partially overlapped laser scans, with the incident laser power, the number of laser scans, and their spacing being used to control the curvature produced by laser evaporation. In this work, a 1 mm diameter laser spot is used to produce grooves of cylindrical/toroidal shape with 240 μm width and 16 mm length. After high reflectance coating, these grooves are found to provide excellent mode selectivity as resonator mirrors for a 150 μm core Yb:YAG planar waveguide laser, producing high brightness output at more than 300 W. The results show clearly that the laser-generated microstripe mirrors can improve the optical performance of high-power planar waveguide lasers when applied in a low-loss mode-selective resonator configuration.

400W Yb:YAG planar waveguide laser using novel unstable resonators

A planar waveguide laser consisting of a 13mm x 12mm x 150μm Yb:YAG core with 1mm high sapphire claddings is edge pumped using two 450W diode stacks with custom aberration correcting phase-plates. A plano-concave resonator gives 400W average power in a low-order transverse, multi-longitudinal mode beam with 75% slope efficiency, comparable to other thin disk and slab lasers. Transverse beam quality is improved through use of novel mode selective toroidal laser-cut resonator mirrors, whilst lateral beam quality is improved through the use of an unstable resonator. Uniform gain with an amplification of 3-4 per pass shows promise for amplifier operation.

The impact of graphite coating and wavelength on picosecond laser machining of optical glasses

This paper presents the impact of a graphite coating and wavelength on picosecond laser processing of fused silica (HPFS®7980 Corning) and Borofloat®33 glass (Schott AG). Within this work, the surface damage threshold, single-pulse ablation depth, and multi-pulse removal rate of ‘as-manufactured’ glasses have been determined for three different wavelengths: 343, 515 and 1030 nm, and compared with the results obtained for the same glasses when coated with a thin layer of graphite prior to laser treatment. The experimental results show that the surface damage threshold of graphite-coated glasses is significantly lower (up to 35 times) than that of ‘as-manufactured’ glasses, whereas the single-pulse ablation depth is the highest at the laser wavelength of 343 nm. Moreover, it has been found that fused silica and Borofloat®33 glass can be machined with better control of the ablation depth when its surface is coated with graphite. This is attributed to the increased absorption of the workpiece at wavelengths at which this material is normally transparent.This paper presents the impact of a graphite coating and wavelength on picosecond laser processing of fused silica (HPFS®7980 Corning) and Borofloat®33 glass (Schott AG). Within this work, the surface damage threshold, single-pulse ablation depth, and multi-pulse removal rate of ‘as-manufactured’ glasses have been determined for three different wavelengths: 343, 515 and 1030 nm, and compared with the results obtained for the same glasses when coated with a thin layer of graphite prior to laser treatment. The experimental results show that the surface damage threshold of graphite-coated glasses is significantly lower (up to 35 times) than that of ‘as-manufactured’ glasses, whereas the single-pulse ablation depth is the highest at the laser wavelength of 343 nm. Moreover, it has been found that fused silica and Borofloat®33 glass can be machined with better control of the ablation depth when its surface is coated with graphite. This is attributed to the increased absorption of the workpiece at wavelengths a...

Mode-selective toroidal mirrors for unstable resonator planar waveguide and thin slab solid-state lasers

An effective method for efficient power extraction from planar waveguide or thin slab solid-state lasers is provided by the hybrid configuration, which combines an unstable lateral resonator with a stable or waveguide resonator in the transverse direction. A two-mirror configuration is preferred for ease of alignment and simplicity and previously we have used spherical optics [1], but transverse mode selection has relied on the use of an intra-cavity slit. More general 2-mirror resonators require at least one toroidal mirror surface, with large curvature (RL ∼ 0.2m to ≫1m) in the lateral direction and small curvature (RT ∼ 15 to 50mm) in the transverse direction. For a large core height planar waveguide as in [1], RT and the mirror distance can be chosen to obtain mode selectivity by low-loss coupling of the fundamental mode into the core, combined with spread of higher order modes into the cladding. However, additional selectivity may be available if the transverse size of the toroidal mirror is restricted to form a slit-shaped mirror. Such mirrors are difficult to fabricate by conventional means.

Tamper-proof markings for the identification and traceability of high-value metal goods

A customized UV nanosecond pulsed laser system has been developed for the fast generation of tamper-proof security markings on the surface of metals, such as stainless steel, nickel, brass, and nickel-chromium (Inconel) alloys. The markings in the form of reflective phase holographic structures are generated using a laser microsculpting process that involves laser-induced local melting and vaporization of the metal surface. The holographic structures are formed from an array of optically-smooth craters whose depth can be controlled with ± 25nm accuracy. In contrast to conventional security markings, e.g., engraved serial numbers, etched part numbers and embossed polymer holographic stickers, which are only attached to the metal products as an adhesive tape, the phase holographic structures are robust to local damage (e.g. scratches) and resistant to tampering because they are generated directly on the metal surface. This paper describes a novel laser-based process for security marking of high-value metal goods, investigates the optical performance of the holographic structures, and demonstrates their application to watches.

Shaping the surface of Borofloat 33 glass with ultrashort laser pulses and a spatial light modulator

We demonstrate an application of a liquid-crystal-based spatial light modulator (LC-SLM) for the parallel generation of optically smooth structured surfaces on Borofloat 33 glass. In this work, the picosecond laser beam intensity profile of wavelength 515 nm is spatially altered by a LC-SLM, and then delivered to the workpiece in order to generate surface deformations whose shape corresponds to the image generated by the LC display. To ensure that localized melting occurs without ablation, the glass surface is covered by a thin layer of graphite prior to laser treatment to provide increased linear absorption of the laser light. After laser treatment the residual graphite layer is removed using methanol and the whole sample is annealed for 1 h at a temperature of 560 °C, making the laser-induced surface deformations optically smooth.

Rapid Laser Manufacturing of Microfluidic Devices from Glass Substrates

Conventional manufacturing of microfluidic devices from glass substrates is a complex, multi-step process that involves different fabrication techniques and tools. Hence, it is time-consuming and expensive, in particular for the prototyping of microfluidic devices in low quantities. This article describes a laser-based process that enables the rapid manufacturing of enclosed micro-structures by laser micromachining and microwelding of two 1.1-mm-thick borosilicate glass plates. The fabrication process was carried out only with a picosecond laser (Trumpf TruMicro 5×50) that was used for: (a) the generation of microfluidic patterns on glass, (b) the drilling of inlet/outlet ports into the material, and (c) the bonding of two glass plates together in order to enclose the laser-generated microstructures. Using this manufacturing approach, a fully-functional microfluidic device can be fabricated in less than two hours. Initial fluid flow experiments proved that the laser-generated microstructures are completely sealed; thus, they show a potential use in many industrial and scientific areas. This includes geological and petroleum engineering research, where such microfluidic devices can be used to investigate single-phase and multi-phase flow of various fluids (such as brine, oil, and CO2) in porous media.