Laser etching of Thoria pellets for microstructural investigations

Metallographic investigation of the microstructure of sintered Thoria pellets necessitates appropriate surface preparation of these pellets. Conventional etching methods involving either chemical or thermal etching techniques being unsuitable for surface etching of irradiated Thoria fuel, laser-based surface treatment was envisaged as a potential alternative technique. Thoria pellets were subjected to laser surface treatment using a pulsed Nd:YAG laser.Our preliminary studies have successfully demonstrated laser etching of sintered Thoria pellets with good reproducibility, clearly revealing grain structures and well-defined grain boundaries. Detailed parametric investigations determining optimum laser parameters for the process, are presented. Our results on ultra-short laser-based etching of sintered Thoria pellets are also discussed.     

Fiber Bragg Grating Writing by Interferometric or Phase-Mask Methods Using High-Power Excimer Lasers

This paper reports the development of high-power excimer lasers with enhanced spatial and temporal coherence. These excimer lasers are applicable to writing fiber Bragg gratings by interferometric or phase-mask techniques. An excimer laser with a novel unstable resonator is analyzed with respect to its suitability to the production of passive fiber-optic components and in terms of production flexibility, efficiency, and reliability. A survey of applicability of this tool to short-and long-period fiber Bragg gratings is presented.     

Modern technology in artwork conservation: a laser-based approach for process control and evaluation

The present work includes a laser-based methodology for the cleaning of artworks, with emphasis on the preservation of their structural integrity and identity. Modern laser-based techniques and instrumentation offer new tools in the field of artwork and antiquities conservation, aiming to alleviate the traditionally applied methods from existing weaknesses. Although in several cases the use of lasers may give rise to superior results, there are still problems to be resolved in relation to the optimization of procedures for safeguarding from potential damage. Furthermore, several operational parameters have to be simultaneously controlled and the long-term effects induced by laser irradiation must be assessed in detail before a full exploitation of the new methods is established. The control of material removal during laser cleaning is achieved by using laser-induced breakdown spectroscopy (LIBS). This control relies on the collection of spectroscopic data by LIBS, which correspond to the in-depth compositional profile of the artifact. This technique may be combined with structural analysis by holographic interferometry. The latter is important for assessing structural changes, which may be induced during laser ablation. Selected examples of this type of applications in a carefully considered methodology are presented.     

High-power laser beam shaping by inseparable two-dimensional binary-phase gratings for surface modification of stamping dies

For surface modification of stamping dies, an inseparable two-dimensional binary-phase gratings is introduced to implement the wavefront transformation of high-power laser beams. The design and fabrication of the gratings are described in detail. Two-dimensional even sampling encoding scheme is adopted to overcome the limitations of conventional Dammann grating in the design of two-dimensional output patterns. High diffractive efficiency (>70%) can be achieved through the transformation of the Gaussian laser beam into several kinds of two-dimensional arrays in focal plan. The application of the binary-phase gratings in the laser surface modification of ductile iron is investigated, and the results show that the hardness and the wear resistance of the sample surface were improved significantly by using the binary-phase gratings.     

Oscillatory mode coupling and electrically strobed gratings in photorefractive quantum-well diodes

Oscillatory mode coupling between two coherent laser beams is produced when an interference pattern moves against a quasi-static electrically strobed grating in a photorefractive AlGaAs/GaAs multiple-quantum-well diode operated in the quantum-confined Stark geometry. The oscillation frequency is equal to the frequency difference between the two laser beams and provides a method to measure high-frequency Doppler shifts or large surface displacements for laser-based ultrasound. Combined photorefractive gains (normally forbidden by symmetry in the Stark geometry) and absorptive gains approach 1200cm(-1)during two-wave mixing using moving gratings.     

Fabrication of 550 nm gratings in fused silica by laser induced backside wet etching technique

A series of 550 nm spacing gratings were fabricated in fused silica by laser induced backside wet etching (LIBWE) method using the fourth harmonic of a Q-switched Nd:YAG laser (wavelength: λ = 266 nm; pulse duration: FWHM = 10 ns). During these experiments we used a traditional two-beam interference method: the spatially filtered laser beam was split into two parts, which were interfered at a certain incident angle (2θ = 28°) on the backside surface of the fused silica plate contacting with the liquid absorber (saturated solution of naphthalene-methyl-methacrylate c = 1.85 mol/dm³). We studied the dependence of the quality and the modulation depth of the prepared gratings on the applied laser fluence and the number of laser pulses. The surface of the etched gratings was characterized by atomic force microscope (AFM). The maximum modulation depth was found to be 180-200 nm. Our results proved that the LIBWE procedure is suitable for production of submicrometer sized structures in transparent materials.     

Optimization of near-field optical field of multi-layer dielectric gratings for pulse compressor

Multi-layer dielectric (MLD) gratings for pulse compressors in high-energy laser systems should provide high diffraction efficiency as well as high laser induced damage thresholds (LIDT). Nonuniform optical near-field distribution is one of the important factors to limit their damage resistant capabilities. Electric field distributions in the gratings and multi-layer film region are analyzed by using Fourier modal method. Optimization of peak electric field in the gratings ridge is performed with a merit function, including both diffraction efficiency and electric field enhancement when the top layer material is HfO₂ and SiO₂, respectively. A set of optimized gratings parameters is obtained for each structure, which reduce the peak electric field within the gratings ridge to being respective 1.39 and 1.84 times the value of incident light respectively. Finally, we also discuss the effects of gratings refractive index, gratings sidewall angle and incident angle on peak electric field in the gratings ridge.     

A review on refractive index and temperature profile measurements using laser-based interferometric techniques

In this review we have discussed laser-based interferometric techniques for measurement of refractive index, temperature and temperature profile of burners with special emphasis on laser speckle techniques and Talbot interferometric technique.     

Surface plasmon scattering on polymer-bimetal layer covered fused silica gratings generated by laser induced backside wet etching

Large amplitude fused silica gratings are prepared by combining the UV laser induced backside wet etching technique (LIBWE) and the two-beam interference method. The periodic patterning of fused silica surfaces is realized by s-polarized fourth harmonic beams of a Nd:YAG laser, applying saturated solution of naphthalene in methyl-methacrylate as liquid absorber. Atomic force microscopy is utilized to analyze how the modulation amplitude of the grating can be controlled by the fluence and number of laser pulses. Three types of plasmonic structures are prepared by a bottom-up method, post-evaporating the fused silica gratings by gold-silver bimetal layers, spin-coating the metal structures by thin polycarbonate films, and irradiating the multilayers by UV laser. The effect of the bimetal and polymer-coated bimetal gratings on the surface plasmon resonance is investigated in a modified Kretschmann arrangement allowing polar and azimuthal angle scans. It is demonstrated experimentally that scattering on rotated gratings results in additional minima on the resonance curves of plasmons excited by second harmonic beam of a continuous Nd:YAG laser. The azimuthal angle dependence proves that these additional minima originate from back-scattering. The analogous reflectivity minima were obtained by scattering matrix method calculations realized taking modulation depths measured on bimetal gratings into account.     

Varied laser induced damage phenomena of gold coated gratings for pulse compression

In this paper, gold-coated gratings for pulse compression have been prepared and their laser damage experiments have been performed. Varied laser damage morphologies have been observed: when a 60 fs-pulsed laser with energy density slightly higher than the damage threshold was used, damage morphology with a characteristic of discrete distribution of small pits was appeared. These damage pits are linearly distributed at the junction of ridges and grooves. If the laser energy density is much higher than the damage threshold, the gold films was overall ablated and the grating structure disappeared. Besides, if the gold film has poor adhesion, it was peeled off. When a 450 ps-pulsed laser with energy density slightly higher than the damage threshold was used, part of grating ridges will be ablated and an obvious line exists between the ablated area and the unchanged area. In theory, the laser induced temperature field and stress field in gold-coated gratings were calculated based on the electromagnetic field using the finite element method. It is demonstrated that the temperature and thermal stress distribution characteristics are affected by the laser heating rate and the heat diffusion time (the calculated diffusion time ranges from 6 fs to 450 ps), which determines the laser damage characteristics. The possible damage drivers have electron hydrodynamic pressure, thermal ablation and thermal stress.     

Application of Bessel beams to 2D microfabrication

Fs laser-based two-photon polymerisation (2PP) has been widely reported as a means of directly writing three-dimensional nanoscale structures. Usually the voxel of a high numerical aperture microscope objective is scanned through the resin to build up the required model. In the case of high aspect ratio two-dimensional structures, such as cell scaffolds, repeated scanning is required to build up the height.The voxel shape can be substantially elongated by the inclusion of an axicon lens in the laser beam line. In this report we describe the use of a Bessel beam produced in the region beyond the focus of an objective lens when the beam has been modified in this way.A Ti:sapphire laser was used to write a range of 2D square cell structures in a Zr-loaded sol-gel system. The process was characterised, in terms of the dimensions of the polymerised Bessel region, for different processing conditions. Examples of the structures are also described.     

High-efficiency Q-switched erbium fiber laser using a Bragg grating-based modulator

In this work we analyze the behavior of an erbium-doped fiber laser which is based on a simple scheme. Excitation of the active medium is performed in the 980 nm pump band with a CW semiconductor laser source. Two fiber Bragg gratings acting as mirrors of the Fabry–Perot laser cavity were used. One of these gratings was mounted over a piezoelectric (PZT) element. By applying voltage pulses to the piezoelectric, the laser cavity was temporally modulated and Q-switched laser pulses up to 530 mW peak powers at 3 kHz were obtained. Typical laser emission of 2–3 μs temporal widths and 0.1 nm of optical bandwidth have been achieved when the system was operated at 18.5 kHz repetition rates. Different behaviors were observed depending on the pumping level of the active medium and on the amplitude and frequency of the signal applied on the PZT. Q-switched laser output, in the erbium spectral gain region, with high laser efficiency of energy conversion was generated. Pumping at 76 mW and operating the laser at 18.5 kHz, an efficiency of 26% was obtained.     

Demonstration of two-point velocity measurement using diffraction grating elements for integrated multi-point differential laser Doppler velocimeter

We have proposed a configuration of an integrated multi-point differential laser Doppler velocimeter (LDV) using arrayed waveguide gratings (AWGs) as grating elements. This paper demonstrates two-point velocity measurement using the proposed configuration with diffraction grating elements. An experiment was conducted using a free-space optical setup with bulk diffraction gratings instead of AWGs. The experimental results indicate that velocities at different positions can be measured using the proposed configuration. The measured separation of the two measurement positions was about 20.5 mm, about 11% of the working distance.     

Optical investigation of dynamics of phenomena of laser-based lithotripsy

Using time-resolved high-speed shadowgraphy, the dynamics of phenomena due to laser-based lithotripsy is studied. Collapsing mechanism of bubble formed therein is investigated. In order to study the mechanism, the optically implemented mathematical morphology is applied. The study of the shape of the plasma and the collapsing region of the bubble of fluid that we are studying can possibly be used for practical application for laser-based lithotripsy.     

The applicability of a material-treatment laser pulse in non-destructive evaluations

A practical optodynamic study was performed to determine the usability of different lengths of laser pulses for the generation of ultrasonic transients in a solid material. The aim of the study was to evaluate the possibility of a dual use for a laser pulse-for laser material processing, on the one hand, and for the ultrasonic wave generation on the other-with both processes being combined on the same production line. The propagation of the laser-generated ultrasonic waves is evaluated by detecting and measuring with a PID-controlled stabilized interferometer. Thus, both systems provided the basic tools, the generation and detection of ultrasonic waves, for an ultrasonic, laser-based, non-destructive material evaluation. The ultrasonic transients generated by 'classical' nanosecond laser pulses were compared with the transients generated by industrial laser pulses with a duration of a few tenths of a microsecond. The experimental results are compared with the results of a time-of-flight analysis that also involved part of a mode-conversion analysis for both regimes in a layered material structure. The differences between the two waveforms were assessed in terms of their visibility, wavelength and resolution. The limit values were calculated and estimated for the laser-pulse parameters, when such pulses are intended for use in an ultrasonic, laser-based, non-destructive evaluation. The possibility of using an industrial marking laser for laser ultrasound generation is thus demonstrated.     

Investigation on infrared laser absorption spectroscopy measurement of acetylene trace quantities

A laser-based infrared spectrometer was developed for use in high-resolution spectroscopic analysis of trace gases in the atmosphere. Continuous wave, broadly tunable coherent infrared radiation was generated from 8 to 19 μm in a gallium selenide crystal by laser difference-frequency mixing. Measurements of acetylene trace concentrations using laser absorption spectroscopy are reported. The measurements have been performed by using the P(21), Q(11), and R(9) lines of the ν₅ band, respectively, in order to investigate optimal detection conditions: a trade-off choice between higher line absorption strength for sensitive detection and better spectral discrimination from lines overlapping for open path trace-gas monitoring applications. Minimum detectable concentrations are compared under different experimental conditions. The ν₅ R(9) line seems to be close to optimum in terms of absorption strength and freedom from spectral interference for spectroscopic detection of acetylene trace concentration at atmospheric pressure.     

Directional discrimination for fiber-optic non-mechanical scanning laser Doppler velocimeter using single transmission path

We propose a fiber-optic non-mechanical scanning laser Doppler velocimeter (LDV) with directional discrimination of the velocity component using a single transmission path. The LDV uses a polarization maintaining fiber (PMF) as the transmission path between the main body and probe. Two beams with different frequencies induced by acousto-optic modulators (AOMs) are transmitted to the probe as two orthogonal polarization modes of the PMF. The measurement position is axially scanned using gratings in the probe and the change in wavelength. The functions of the scan and directional discrimination are demonstrated experimentally.     

High-efficiency, high-dispersion diffraction gratings based on total internal reflection

We report a new class of high-dispersion immersed diffraction gratings for which the reflective nature of the diffraction is provided by the phenomenon of total internal reflection (TIR) regardless of grating tooth shape. Thus, the component can be fabricated from a single dielectric material and requires no metallic or dielectric film layers for high reflection diffraction efficiency. With the absence of metallic absorption, diffraction efficiencies of these TIR gratings can reach more than 99% for 15-20-nm spectral bandwidths, making them suitable for many laser-based technologies.     

Fast microstructuring of silica glasses surface by NIR laser radiation

The glass surface microstructuring technology using laser radiation with NIR wavelength (λ=1.064 μm) was revealed in this work. Glass plates were placed on the cellular graphite surface. Focused laser radiation passed through the glass plate and interacted with cellular graphite. The radiation heated the graphite surface and thus the high temperature influenced the back side of the glass plate. After consecutive laser scans, having certain periods and interruptions of laser radiation, the microstructures with depth ~0.5 μm were formed. Besides, in this work we suggested the method to calculate optical characteristics of formed elements. It was experimentally shown that these microstructures could be used to form phase diffraction gratings (PDGs) and random phase plates (RPPs). We experimentally demonstrated the possibility of these elements being used as RPPs which are suitable for multimode laser radiation homogenization and as PDGs which are suitable for laser simultaneous processing of metal films.