Laser-machining experiment with an excimer laser and a kinoform

We have used a kinoform to increase the beam-power utilization in an excimer-laser-machining experiment. The kinoform creates the pattern to be machined. Thus less power is wasted on the blocking parts of a shadow mask. To achieve as smooth an intensity profile as possible, the kinoform was also used together with a microlens-array beam homogenizer. We discuss the intensity distributions of the patterns created by the kinoform with and without the beam homogenizer as well as the design of the kinoform and the homogenizer, with emphasis on the relation to the coherence properties of the laser beam.     

Small-feature-size fan-out kinoform etched in GaAs

A binary fan-out kinoform for focusing and splitting an incident beam into a 4 × 4 spot array, with a largest deflection angle of 28°, was designed, fabricated in GaAs, and evaluated. The kinoform was defined in resist with electron-beam lithography and etched into GaAs with chemically assisted ion-beam etching. Light at wavelength 0.98 μm from a single-mode fiber was used to illuminate the kinoform. The efficiency was measured to be 34%, and the uniformity error for the 4×4 spots was 29%. Although the typical feature size of the kinoform is only roughly two wavelengths, we found that the scalar theory of diffraction can be used. A first kinoform was designed with the customary Fresnel-diffraction theory, which was found to be too coarse, resulting in a fan-out exhibiting some distortion. A second kinoform was designed with the more rigorous Fresnel-Kirchhoff expression, and its fan-out shows no distortion.     

A Reflection Kinoform for Use with a CO2 Laser

The production of a binary reflection kinoform on stainless steel for use with a high-powered CO₂ laser is described. The kinoform focuses the laser beam into the letters LUT which may be burned onto a suitable material placed at its focus. A theoretical discussion of the reconstruction process from a binary kinoform is presented and the effects of phase quantization are considered.     

High-brightness mid-infrared photonic-crystal distributed-feedback lasers

The far-field emission and spectral characteristics of a photonic-crystal distributed-feedback (PCDFB) laser with an antimonide type-II ‘W’ active region were studied theoretically and experimentally. A 2nd-order grating was defined on a 2D rectangular lattice tilted by 20° relative to the facet normal, using optical lithography and dry etching. For pulsed optical pumping, the emission line centred on λ = 4.6–4.7 μm was up to an order of magnitude narrower (7–10 nm) than those of Fabry-Pérot and angled-grating DFB (α-DFB) lasers fabricated from the same wafer. The PCDFB beam quality was also substantially enhanced, for example, by a factor of 5 compared to the α-DFB at a pump-stripe width of 200 μm. Theoretical simulations based on a self-consistent time-domain Fourier-transform simulation confirm significant advantages of the PCDFB laser over an α-DFB geometry.     

Effect of beam waists on performance of the tunable fiber laser based on in-line two-taper Mach-Zehnder interferometer filter

A tunable fiber ring laser based on an in-line two-taper Mach-Zehnder interferometer (MZI) filter was realized, and the effect of beam waists of the tapers on performance of the laser was investigated with different beam waists of 70 μm, 49 μm, and 33 μm. Experimental results show that the tunable laser with MZI length of 1 m and beam waist of 49 μm can cover 16.1 nm with tuning steps of 0.07-0.5 nm, a bandwidth of 10 pm, and a side mode suppression ratio (SMSR) of 40-50 dB. Tuning range is not only determined by the number of the cladding modes but is also affected by the filter loss. Tuning step is determined by the differences of the effective refractive indexes between the cladding modes and the core mode. SMSR is determined by the balance between the extinction ratio of the filter and the cavity loss of the laser due to MZI filter.     

Determination of the longitudinal profile of a focused Nd:YAG Gaussian beam from second-harmonic generation in a thin KTP crystal

We present an original experiment describing a focused Gaussian laser beam by using second-harmonic generation (SHG) in a thin strip of a nonlinear optical material, in this case KTP doubling of an Nd:YAG laser at 1.064 μm. The dependence of the SHG efficiency on the fundamental beam radius allows the determination of beam parameters by harmonic-power measurements. The characterization of a telescope shows the good precision of this method in radii in the range of 10-100 μm. The average accuracy is 4% for the radius determination and 1.5% for the beam-waist localization.     

Laser beam intensity profile transformation with a fabricated mirror

A nonaxisymmetric mirror is designed by the same method as a computer-generated hologram for laser beam intensity profile transformation and is fabricated by plasma chemical vaporization machining. We successfully transformed a circular Gaussian beam of a He-Ne laser into a rectangular uniform beam maintaining spatial coherence and using a nonaxisymmetric surface profile mirror. There are ripples in the intensity profile of the transformed rectangular beam. These ripples in the intensity profile result from small ripples on the mirror surface. These results show that we can perform coordinate transformation using these fabricated mirrors, which has so far been possible only by using computer-generated holograms.     

Size measurement of bubbles in a cavitation tunnel by digital in-line holography

Digital in-line holography (DIH) with a divergent beam is used to measure size and concentration of cavitation bubbles (6-100 μm) in hydrodynamic facilities. A sampling probe is directly inserted in the cavitation tunnel, and the holograms of the bubbles are recorded through a transparent test section specially designed for DIH measurements. The recording beam coming from a fiber-coupled laser diode illuminates the sample volume, and holograms are recorded by a CMOS camera. From each hologram, the sampling volume can be reconstructed slice by slice by applying a wavelet-based reconstruction method. Because of the geometry of the recording beam, a magnification ratio must be introduced for recovering the 3D location and size of each bubble. The method used for processing holograms recorded in such a configuration is presented. Then, statistical results obtained from 5000 holograms recorded under different pressures in the cavitation tunnel are compared and discussed.     

Theory of 1- N-way phase-locked resonators

A 1-N-way resonator based on beam splitting and beam combining effects in rectangular cross-sectional multimode waveguides was recently proposed. Such a resonator structure offers a valuable way in which N low-power laser elements may be combined in a coherent fashion. We examine the case of passive 1-N-way resonators. We develop a theory of these 1-N-way structures to show that there is only one possible mode of these resonators. The theory is used to give a scaling law for the design tolerances of the beam splitting and beam combining region of the resonator.     

On selective laser melting of ultra high carbon steel: Effect of scan speed and post heat treatment

Selective laser melting is a laser‐based additive manufacturing process applying layer manufacturing technology and is used to produce dense parts from metallic powders. The application of selective laser melting on carbon steels is still limited due to difficulties arising from carbon content. This experimental investigation aims at gaining an understanding of the application of the process on ultra high carbon steel, which is a special alloy with remarkable structural properties suitable for different industrial applications. The feedstock ultra high carbon steel (2.1% C) powder, 20 μm to 106 μm was prepared by water atomizing technique. This powder was used for the selective laser melting to build specimens 10×10×40 mm in dimensions. To decrease the thermal stresses during layer by layer building, laser scanning was done through 5×5 mm random island patterns while layer thickness was 30 μm. Laser beam diameter, maximum power output, layer thickness and scan speed range were 0.2 mm, 100 W, 30 μm and 50–200 mm/s respectively. The process was done inside high purity nitrogen environment, with less than 0.5% oxygen content. The results illustrate the influence of scan speed from 50 to 200 mm/s on product geometry and dimensions, surface roughness, internal porosity and cracks, microstructure and surface hardness. The effect of post heat treatment by heating and holding for one hour (annealing) at different temperatures of 700°C, 750°C, 950°C is studied. The results indicate that selective laser melting is able to produce near to 95% density of ultra high carbon steel parts with acceptable geometry and surface quality. Porosity cracks, and microstructure formed during the process could be controlled through proper selection of process parameters and post heat treatment. Industrial ultra high carbon steel products can be rapidly fabricated by selective laser melting.     

Simultaneous machining of parallel grooves in SnO(2) thin films using a Nd:YAG laser and a kinoform

A kinoform is used to split the beam of a Nd:YAG laser into six beams. The laser beams are, by means of optical fibers and collimating-focusing optics, transferred to the surface to be machined. Thus multiple grooves can be machined simultaneously. For demonstration, 100-μm-wide grooves are simultaneously machined into a SnO(2) thin film deposited on a glass substrate. The resulting grooves are well isolated. This result shows that the technique could be used, for example, in solar cell manufacturing to increase the efficiency of laser scribing.     

Photothermal delivery of microscopic objects via convection flows induced by laser beam from fiber tip

We report a photothermal delivery of microscopic objects based on convection flows at the surface of water. The convection flows were induced by photothermal effect through a laser beam of 1.55 μm wavelength from a fiber tip. A 206 μm diameter oil drop was delivered forward and backward by changing the laser beam at a power of 28.5-40 mW. In addition, the delivery has been further demonstrated with a cluster of carbon and red blood cells at the laser powers of 14 and 20 mW, respectively.     

基于准分子激光微加工的光热微驱动器(英文)

提出了基于光热(PT)微膨胀原理的新型光热微驱动技术.设计了一种能将纵向光热膨胀转化成横向偏转的微驱动器.以AutoCAD设计图为基础,采用KrF准分子激光微加工系统,在单层高密度聚乙烯(HDPE)上加工出长1 500μm、宽250μm、厚40μm的开关式光热微驱动器.从微驱动器的扫描电子显微镜(SEM)图可以看出,微驱动器形状与AutoCAD设计值符合良好.光热微驱动实验采用脉冲频率可调的半导体激光器(4 mW,650 nm)作为驱动源.实验结果表明,在一定的脉冲频率范围(如0～17 Hz)内,光热微驱动器具有良好的静态和动态特性,其横向偏转量最大可达11μm,足以实现微开关功能.这种光热微驱动器可由激光束直接控制,具有原理新颖、结构简洁、体积小、易于加工制作等特点,在微纳米技术领域和微光机电系统(MOEMS)中具有广阔的应用前景.     

Plasmonics for Laser Beam Shaping

This paper reviews our recent work on laser beam shaping using plasmonics. We demonstrated that by integrating properly designed plasmonic structures onto the facet of semiconductor lasers, their divergence angle can be dramatically reduced by more than one orders of magnitude, down to a few degrees. A plasmonic collimator consisting of a slit aperture and an adjacent 1-D grating can collimate laser light in the laser polarization direction; a collimator consisting of a rectangular aperture and a concentric ring grating can reduce the beam divergence both perpendicular and parallel to the laser polarization direction, thus achieving collimation in the plane perpendicular to the laser beam. The devices integrated with plasmonic collimators preserve good room-temperature performance with output power comparable to that of the original unpatterned lasers. A collimator design for one wavelength can be scaled to adapt to other wavelengths ranging from the visible to the far-IR regimes. Plasmonic collimation offers a compact and integrated solution to the problem of laser beam collimation and may have a large impact on applications such as free-space optical communication, pointing, and light detection and ranging. This paper opens up major opportunities in wavefront engineering using plasmonic structures.      

Process parameter dependencies of continuous and pulsed laser modes on surface polishing of additive manufactured aluminium AlSi10Mg parts

Laser powder bed fusion is a well-established 3D printing technique for metal alloys, but exhibits a poor surface quality. Laser polishing provides the possibility of a fast contact-free and fully-automatable surface treatment. This paper deals with the experimental investigation of laser polishing of laser powder bed fusion parts made of aluminium AlSi10Mg. Laser polishing is done with a 4 kW solid state disc laser in combination with a multi-axis system and a one dimensional scanner optic. The laser is operated at continuous and pulsed operation mode. The parameter study reveals a high dependency of the achievable roughness on the laser beam intensity, the track and pulse overlap, the energy density and the number of polishing passes and polishing directions. Pulsed laser polishing mode with up to four passes from different directions revealed the lowest surface roughness of 0.14 μm Ra. With respect to the initial average surface roughness of Ra = 8.03 μm a reduction of the surface roughness of greater than 98 % could be achieved. Polishing with continuous laser radiation at one polishing pass resulted in Ra = 0.23 μm at an area rate of 20 cm²/min. Laser polishing using four passes achieved a further improvement up to Ra = 0.14 μm.     

High-energy hybrid Raman optical parametric amplifier eye safe laser source

A high-energy eye safe laser source at 1.54 μm is demonstrated experimentally by using a hybrid system of stimulated Raman scattering and optical parametric amplification pumped by a single 1.06-μm Nd:YAG laser source. This system overcomes some of the technical problems that occur in conventional eye safe lasers, such as optical breakdown and thermal blooming in the Raman laser, and thermal conduction problems in the erbium-doped glass solid-state laser that limit the repetition rate when high-energy output is sought. Thus this hybrid design provides a simple system that could provide a high pulse energy output (> 50 mJ) at a repetition rate of greater than 10 Hz.     

Infrared laser ablation sample transfer for MALDI imaging

An infrared laser was used to ablate material from tissue sections under ambient conditions for direct collection on a matrix assisted laser desorption ionization (MALDI) target. A 10 μm thick tissue sample was placed on a microscope slide and was mounted tissue-side down between 70 and 450 μm from a second microscope slide. The two slides were mounted on a translation stage, and the tissue was scanned in two dimensions under a focused mid-infrared (IR) laser beam to transfer material to the target slide via ablation. After the material was transferred to the target slide, it was analyzed using MALDI imaging using a tandem time-of-flight mass spectrometer. Images were obtained from peptide standards for initial optimization of the system and from mouse brain tissue sections using deposition either onto a matrix precoated target or with matrix addition after sample transfer and compared with those from standard MALDI mass spectrometry imaging. The spatial resolution of the transferred material is approximately 400 μm. Laser ablation sample transfer provides several new capabilities not possible with conventional MALDI imaging including (1) ambient sampling for MALDI imaging, (2) area to spot concentration of ablated material, (3) collection of material for multiple imaging analyses, and (4) direct collection onto nanostructure assisted laser desorption ionization (NALDI) targets without blotting or ultrathin sections.     

The delphi silicon strip microvertex detector

The silicon strip microvertex detector for the DELPHI experiment at LEP is presented. It consists of two cylindrical layers with a total of 165 888 strips. The design parameters of the final project are described.The microstrip counters have a pitch of 16.6 μm, and are read out every 50 μm using the capacitive charge division method. The electronics used is the Microplex chip, an NMOS integrated circuit, which provides 128 channels of low noise charge sensitive amplifiers with multiplexed analog output. Results of signal-to-noise ratio from beam tests on prototype detectors are given and discussed.     

High-performance solid-state dye laser based on peryleneorange-doped polycom glass

Abstract

The new laser dye, perylene orange, doped into polycom glass (sol–gel glass–poly(methyl methacrylate) composite) has been developed into a tunable gain material with high performance. A sample polished to 0.02 μm smoothness and λ/5 flatness, over the central region, was excited in an optical cavity with a second harmonic Q-switched Nd-doped yttrium aluminium garnet laser as a pump source. The solid-state dye laser had a threshold of 53 μJ, a slope efficiency of 72% and a normalized photostability of 50 GJ mol^?1. The laser damage thresholds of various host materials were obtained and the damage threshold of polycom glass was found to be 26 J cm^?2. The beam profile of a He[sbnd]Ne laser transmitted through samples of host material showed that the optical homogeneity of polycom glass is approaching that of pure polymers.     

Extra-axial stigmatic imagery with a thick planar kinoform

A kinoform lens and its substrate are usually treated as two distinct components with a common interface. Thick kinoform lenses treat the relief profile and the substrate as an integrated component. An analytical approach for the design of higher-order thick kinoform lenses for stigmatic imaging between extra-axial conjugate points is presented. An illustrative example is given.