Hybrid laser processing for microfabrication of glass

Hybrid laser processing for the precision microfabrication of glass materials, in which the interaction of a conventional pulsed laser beam and a medium on the material surface leads to effective ablation and modification, is reviewed. A major role of the medium is to produce strong absorption of the conventional laser beam by the material. Simultaneous irradiation by a vacuum ultraviolet (VUV) laser beam that possesses an extremely small laser fluence and an ultraviolet (UV) laser greatly improves the ablation quality and modification efficiency for fused silica (VUV-UV multiwavelength excitation process). The metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials by the same laser beam, resulting in microstructuring, cutting, color marking, printing, and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion presented here includes the ablation mechanism of hybrid laser processing. Received: 18 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: ksugioka@postman.riken.go.jp     

New data processing for multichannel FIR laser interferometer

Usually, both the probing and reference signals received by LATGS detectors of FIR interferometer pass through hardware phase discriminator and the output phase difference--hence the electron line densities is collected for analysis and display with a computerized data acquisition system(DAS). In this paper, a new numerical method for computing the phase difference in software has been developed instead of hardware phase discriminator, the temporal resolution and stability is improved. An asymmetrical Abel inversion is applied to processing the data from a seven-channel FIR HCN laser interferometer and the space-time distributions of plasma electron density in the HT-6M tokamak are derived.     

Simulations of a graphene excitable laser for spike processing

We propose and simulate a novel excitable laser employing passively \(Q\) -switching with a graphene saturable absorber for spike processing. Our approach combines the picosecond processing and switching capabilities of both linear and nonlinear optical device technologies to integrate both analog and digital optical processing into a single hardware architecture capable of computation without the need for analog-to-digital conversion. We simulate the laser using the Yamada model—a three-dimensional dynamical system of rate equations—and show behavior that is typical of spiking processing algorithms simulated in small circuits of excitable lasers.     

Hilbert transform for processing of laser Doppler vibrometer signals

A first application of the digital Hilbert transform for processing of laser Doppler vibrometer signals is considered. The possibility of vibration amplitude measurement with a relation errors less than 1% in the amplitude interval 0.1-10Λ, where Λ is the fringe spacing, and the linear velocity measurement of the examined object has been proved. The analysis of the amplitude measurement errors is given. The applications of the method under review are discussed.     

Optothermal effects of laser modes in laser materials processing

All laser materials processing involves two fundamental phenomena: the conversion of optical energy into thermal energy, and the generation of thermal stresses in the laserprocessed materials. The distributions of the thermal energy and stresses in the substrate depend on the laser mode used for materials processing. A proper understanding of the temperature and stress distributions is essential for producing high-quality products using the laser technology. The solution of the complete thermoelastic problem is very complex and requires numerical methods. This paper presents simple analytic expressions for the temperature and thermal stress distributions in the substrate processed with TEM₀₀ and TEM _inf01 ^sup* mode laser beams. The temperature is found to be maximum at the point that encounters the maximum laser intensity. The thermal stress distributions are found to be different owing to the differences in the laser modes used for this study.     

高功率激光光束特性对激光加工的影响

决定光与物质相互作用的激光束的波长、入射角、偏振特性以及时间和空间特性是激光材料加工的主要光束特性。激光束的光束质量是其空间特性的量化反映。通过对两种不同激光加工系统输出激光光束质量进行测量和计算,根据多模激光束的聚焦理论,以及对激光深熔焊接实验结果的分析,研究了光束质量对深熔焊接焊缝成形的影响。结果表明,光束质量对聚焦光束的焦斑、聚焦角和焦深的影响不仅体现了激光源的可聚焦性,而且也标志了激光源的可加工能力,这是聚焦系统和焦点位置在选择过程中应该考虑的重要因素。     

Nanoscale laser processing and diagnostics

The article summarizes research activities of the Laser Thermal Laboratory on pulsed nanosecond and femtosecond laser-based processing of materials and diagnostics at the nanoscale using optical-near-field processing. Both apertureless and apertured near-field probes can deliver highly confined irradiation at sufficiently high intensities to impart morphological and structural changes in materials at the nanometric level. Processing examples include nanoscale selective subtractive (ablation), additive (chemical vapor deposition), crystallization, and electric, magnetic activation. In the context of nanoscale diagnostics, optical-near-field-ablation-induced plasma emission was utilized for chemical species analysis by laser-induced breakdown spectroscopy. Furthermore, optical-near-field irradiation greatly improved sensitivity and reliability of electrical conductance atomic force microscopy enabling characterization of electron tunneling through the oxide shell on silicon nanowires. Efficient in-situ monitoring greatly benefits optical-near-field processing. Due to close proximity of the probe tip with respect to the sample under processing, frequent degradation of the probe end occurs leading to unstable processing conditions. Optical-fiber-based probes have been coupled to a dual-beam (scanning electron microscopy and focused ion beam) system in order to achieve in-situ monitoring and probe repair.     

Excimer laser processing of embedded fibers

Novel aspects of the excimer laser etching of nanostructures in embedded optical fibers are discussed. Emphasis is put on the etching of a specified hole geometry and on excellent surface quality of the hole walls.On leave from: Institute of Quantum Electronics, ETH, CH-8093 Zürich, Switzerland     

Perspectives of laser processing and chemistry

Laser-induced material processing is reviewed with special emphasis on recent achievements mainly obtained by the Linz group. Among those are investigations using optical fiber tips for nanoscale photophysical etching, laser-induced surface patterning using self-assembled microspheres, the pulsed-laser deposition of thin films of high-temperature superconductors and the modification and cleaning of surfaces. Received: 7 February 2003 / Accepted: 6 March 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, E-mail: dieter.baeuerle@jku.at     

Signal processing of laser Doppler self-velocimeter

A frequency spectrum refinement and correction algorithm has been put forward to improve the accuracy of measurements, and the Cramer–Rao lower bound (CRLB) of Gaussian group Doppler signal was given based on the introduction of acceleration. Results of simulations and experiments showed that the Goertzel refinement algorithm could improve the resolution of the spectrum of the Doppler signal; the ratio correction algorithm made the results closer to the real value. The CRLBs of the estimated parameters were related to the sampling data, the signal to noise ratio (SNR) and the broadness of Gaussian group, and it can be decreased by increasing the length of the sampling data or improving the SNR; the gap between the variances of the measuring results and the CRLBs narrowed when the SNR of the signal was improved, and was almost eliminated when the SNR was higher than 6 dB.     

Signal processing method of phase correction for laser heterodyne interferometry

A novel signal processing method of movement direction identification and phase correction is presented for laser heterodyne interferometry. Based on the reference signal, four intervals with phase difference of 90° each other are set up. The real-time movement direction identification and the integer fringe counting are realized by detecting the times that the rising-edge of the measurement signal crosses the intervals. The phase correction approach is proposed in detail to solve the fraction phase compensation when the initial phase difference is not equal to the zero phase difference. Three experiments of the stability test, the nanometer and micrometer displacement tests on bi-directional movement were performed to demonstrate the usefulness and feasibility of the presented signal processing method.     

Development of ultra-short pulse VUV laser system for nanoscale processing

We have developed intense vacuum ultraviolet (VUV) radiation sources for advanced material processing, such as photochemical surface reactions and precise processing on a nanometer scale. We have constructed a new VUV laser system to generate sub-picosecond pulses at the wavelength of 126 nm. A seed VUV pulse was generated in Xe as the 7th harmonic of a 882-nm Ti:sapphire laser. The optimum conversion was achieved at the pressure of 1.2 Torr. The seed pulse will be amplified by the Ar₂^*\mathrm{Ar}_{2}^{*} media generated by an optical-field-induced ionization Ar plasma produced by the Ti:sapphire laser. We have obtained a gain coefficient of g=0.16 cm⁻¹. Our developing system will provide VUV ultra-short pulses with sub-μJ energy at a repetition rate of 1 kHz.     

Liquidly process in femtosecond laser processing

Nano-sized water-crown like structure in array was firstly generated on metallic thin film by interfering femtosecond laser processing. We named the structure as “nanocrown”. Ridges are standing on the edge of each ablated hole. The shapes of ridges are spike, nano-waterdrop and bead on column. The radius of the top of a spike was just 7 nm, which is far smaller than that of nanobump generated in the previous work. The self-rising in liquidly process result in the generation of mesoscopic nanostructure with the size between nanohorn or nanotube and micron structures processed by machining or lithography. This is a new surface modification technique in top-down technology.     

Performance of an optical stand-off control system for laser materials processing

This paper presents an experimental investigation into a high-bandwidth optical range sensor for laser materials processing stand-off control. Radiation from a low-power laser beam is focused onto a workpiece surface and light reflected from the surface is collected through a main lens and directed into an imaging lens which focuses the signal to two positions after being split by a beam splitter. The irradiances of the two beams are detected by photodiodes placed behind pinhole apertures positioned fore and aft of the two focal positions. A differential amplifier is used to generate an output signal that determines the magnitude and direction of any workpiece displacement. The system facilitates a measuring range of ±6 mm. A set of of experiments are performed and results are analysed for different setup configurations. The approximate range of instrument linearity is ±1 mm for the 75-mm focal length main less and ±2 mm for the 120-mm lens; in this linear range the optimal accuracy resolution is 1 μm. The system's effectiveness is controlling the stand-off distance of a laser cutting machine, and hence cut quality, is assessed.     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).     

Design and fabrication of diffractive elements for laser material processing applications

Diffractive optical elements have an important role to play in the provision of process adapted beam shaping for laser-materials processing applications. The design techniques and dominant methods of fabrication are described for intra-cavity and external beam shaping diffractive elements fabricated in a variety of dielectric and metallic materials for use with a wide range of laser sources. Theoretical and experimental examples are given in each section.     

Ultrafast laser processing of drug particles in water for pharmaceutical discovery

The laser fragmentation technique has been extensively used to produce inorganic nanoparticles, but its practice on organic materials, especially on drugs, is less common. Here, we briefly review the recent advances in laser micro-/nanonization of organic materials and the rationale of using laser fragmentation for drug discovery. We present our case studies of two drug models: fenofibrate and naproxen. Both drugs were fragmented in water with femtosecond (fs) laser and characterized in terms of particle size distribution and physicochemical properties. Effects of fs laser fragmentation were also compared with nanosecond (ns) laser fragmentation and with conventional media milling technique. Fs laser was more suitable to produce sub-micron size drug particles than ns laser, but degradation of drugs after nanonization was also more pronounced than micronization. Physicochemical transformations such as oxidation, hydration and amorphisation might occur during the laser–material interactions. Laser nanonization showed improved dissolution kinetics, similar to media milling. Unlike the conventional milling techniques, laser fragmentation enabled the treatment of minute amount (as small as several milligrams) of drugs with high efficiency, thus is a useful tool for particle size reduction during the early phases of drug discovery.     

Practical uses of femtosecond laser micro-materials processing

We describe several approaches to basic femtosecond machining and materials processing that should lead to practical applications. Included are results on high-throughput deep hole drilling in glasses in ambient air, and precision high-speed micron-scale surface modification of composite materials and chalcogenide glasses. Ablation of soda-lime silicate glass and PbO lead-silicate is studied under three different sets of exposure conditions, for which both the wavelength and pulse duration are varied. Ablation rates are measured below and above the air ionization threshold. The differences observed are explained in terms of self-channeling in the ablated hole. Fabrication of practical devices such as waveguides and gratings is demonstrated in chalcogenide glass. Received: 11 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-407/8233-570, E-mail: mrichard@mail.ucf.edu     

Optical methods of laser radar field processing

The book describes modern methods of receiving laser signals, viz., by interferometry, by nonlinear optics, by laser amplification, and by heterodyne mixing. The theory of these methods is considered as applied to the problem of laser radars. Investigated in detail are features such as the resolving power of interferometry and nonlinear-optics method, the efficiency of parametric conversion of laser signals, the sensitivity of laser amplifiers, and problems of optimal heterodyne detection. Methods of speckle interferometry and adaptive optics are described. The book is aimed at a large circle of scientists and engineers active in the problem of recording of laser signals, as well as to students in advanced courses of the corresponding specialties.N. D. Ustinov — General Editor.Translated from Metody Obrabotki Opticheskikh Polei v Lazernoi Lokatsii, pp. 3–269, 1983.