Distribution Of Laser Beam Research Articles

Control of microstructure shape and morphology in femtosecond laser ablation of imprint rollers

With increasing demand for microstructure shape accuracy for MEMS and optoelectronic devices, controllability of shape and morphology in micro-fabrication has become increasingly crucial. In this paper, the effects of processing parameters on the shape and morphology of microstructures in femtosecond laser fabrication of imprint roller are explored. An optimized fabrication process is proposed to acquire high accuracy microstructures, in which a two-step inclination ablation process and optimal laser focus position are adopted. Adjusting and matching the processing parameters is a basic method to acquire well-defined shapes, but the ablation results indicate that the draft angle of microstructures can only be adjusted in a limited range due to the intensity distribution of laser beam. A two-step inclination ablation process is adopted to increase the draft angle. In the two-step inclination ablation process, the laser beam irradiates the target surface with an angle and the microstructure with a much steeper draft angle forms after the two-step fabrication. Laser focus position is explored as an important parameter affecting the morphology, and an optimal laser focus position is obtained to enhance the ablation quality. By matching the laser fluence and laser focus position, this morphology enhancement method can realize the high-quality ablation of microstructures with a wide range of dimensions without changing the focusing objective lens.

Influence of the power-density distribution of a laser beam on the wear resistance of frictional surfaces

Influence of the power-density distribution of a laser beam on the wear resistance of frictional surfaces

Three-Photon Absorption and Upconversion Fluorescence Properties of Series 1,3,4-Oxadiazole Derivatives

A newly synthesized 1,3,4-oxadiazole derivatives have been studied using a femtosecond Ti:sapphire laser system. The series molecules present strong three-photon absorption and frequency upconversion fluorescence at wavelengths from 1205nm to 1575nm. Furthermore, there is no proportional relationship between three-photon absorption cross sections and the chemical structure transformation from monomer, dimer to trimer. Effective charge-transfer distance by π-conjugation bonds may be the contributing factor. In the experimental design, the far-field intensity distribution of femtosecond laser beam has been taken into account. We give the optimized analytical solution of nonlinear transmission in a three-photon absorption (3PA) process when the incident beam has a Gaussian transverse spatial profile.

Laser-generated thermoelastic acoustic sources and Lamb waves in anisotropic plates

The effect of anisotropy and temperature on the dispersive Lamb wave generation and propagation in a transversely isotropic thin plate has been investigated. A quantitative numerical model for the laser-generated transient ultrasonic Lamb waves propagating along arbitrary directions is presented by using a finite-element method. All factors, such as spatial and time distributions of the incident laser beam, optical penetration, thermal diffusivity, thickness of the plate, and source–receiver distance, can be taken into account. The effects on the ultrasound waveform of the size of the optoacoustic source are investigated; in the limit of strong optical absorption, a subsurface thermal source gives rise to both vertical and lateral shear tensions. The lateral shear tension is equivalent to applying a shear dipole at the top face; the amplitude of the dipole is a function of material symmetry, contrary to the isotropic case, and the character and strength of the equivalent surface stress are a function of propagation direction. The specific results for the lower anti-symmetric and symmetric mode propagation in all planar directions are presented in the thermoelastic regime; the spatial dispersion (variation of the velocity with the direction of propagation) as well as the frequency dispersion is analyzed.

Measurement of the intensity profile of a Gaussian laser beam near its focus using an optical fiber

We have designed a new pinhole-scan system to measure the Gaussian intensity distribution of a He-Ne laser beam that passes through a series of lenses. Our detection device uses a single-mode optical fiber as the so-called pinhole. One side of the optical fiber is fixed on the top of a motorized translation stage to capture the laser light, and the other side is connected to an optical power meter. A digital oscilloscope is used to observe the detected light intensity. When the translation stage as well as the fiber tip is moved across the laser beam, the oscilloscope displays the scanned trace of the laser beam intensity. The evolution of the intensity distribution of a focused laser beam is measured and then fitted to distributions predicted from Gaussian optics. The measured beam waist size is compared with the result of Gaussian-beam q-parameter analysis. Using this simple system we can easily observe the transverse intensity profile of a Gaussian laser beam in the (1+1)-dimensional plane in the vicinity of the focus.

Vacuum electron acceleration and bunch compression by a flat-top laser beam

The field intensity distribution and phase velocity characteristics of a flat-top laser beam are analyzed and discussed. The dynamics of electron acceleration in this kind of beam are investigated using three-dimensional test particle simulations. Compared with the standard (i.e., TEM(00) mode) Gaussian beam, a flat-top laser beam has a stronger longitudinal electric field and a larger diffraction angle. These characteristics make it easier for electrons to be trapped and accelerated by the beam. With a flat-top shape, the laser beam is also applicable to the acceleration of low energy electron and bunch compression.

Optical properties of a new IR converter for laser beam analysis

This work presents the temporal and spacial resolution of a new infrared (IR) converter based on thermal radiation emission. Using this converter, it is possible to measure the intensity distribution of laser beams with a wavelength between 1 and 20 μm. For this purpose, the laser radiation (for example, 10.6 μm) is converted into a wavelength coverage of 800–1100 nm. In the actual converter thin metal foils provide the basis of this method. The metal foils are heated to a temperature of 600–800 K. The emitted radiation of the foils defers into the near-infrared (NIR) area, thus enabling detection by camera systems based on silicate. Additional heat input of the laser results in a local temperature increase, and then the increase in radiation intensity can be measured. Typical thicknesses of converter metal foils are <5μm. Foil materials with a low thermal conductivity, good absorption of the measured laser beams, and a high melting temperature are particularly suitable. These parameters are well shown by using stainless steels, such as INOX (stainless steel 1.4310 CrNi steel). Using this material, it is possible to gain a maximum spatial resolution of 250 μm and a temporal resolution of 12.5 Hz, by a measurement range from 1 to 100 W/cm2. The maximum measured intensity is 125 W/cm2.

간섭성 반스톡스 라만 산란 현미경 후방 신호지 방사패턴에 관한 이론계산 연구

높은 수치구경의 대물렌즈를 사용하는 간섭성 반스톡스 라만 산란 현미경(coherent anti-Stokes Raman scattering microscopy)에서 폴리스틸렌구에서 발생한 신호의 먼거리장 방사패턴에 대한 이론적 계산 연구를 수행하였다. 극초점 조건에서 입사 레이저 광의 전기장 분포를 계산하였고, CARS 신호 생성원인인 비선형 분극(헤르치안 쌍극자) 방사의 간섭성 합을 통하여 먼거리장 방사 패턴을 계산하였다. 폴리스틸렌구의 크기에 따른 후방 방사패턴을 계산하였고, 1100 nm 직경을 가진 폴리스틸렌구와 폴리스틸렌 구껍질의 방사패턴을 비교하였다. 또한, 극초점으로부터 폴리스틸렌구의 중심이 이동함에 따른 방사패턴의 변화를 보였다. We theoretically investigated the far-field radiation pattern of epi-signal from a polystyrene sphere in coherent anti-Stokes Raman scattering (CARS) microscopy with an objective lens of high numerical aperture. We calculated the field distribution of the incident laser beams under the tight-focusing condition and the far-field radiation pattern through coherent addition of radiation from the nonlinear polarizations (Hertzian dipoles) as the origin of CARS signal generation. The epi-radiation patterns for polystyrene spheres of different diameters are calculated, and the pattern of a sphere is also compared with that of a shell fer a diameter of 1100 nm. We finally show how the radiation pattern of the polystyrene sphere changes as the center of the sphere shifts from the focus of the beam.

Dependence of transverse and longitudinal resolutions on incident Gaussian beam widths in the illumination part of optical scanning microscopy

We studied both theoretically and experimentally the intensity distribution of a Gaussian laser beam when it was focused by an objective lens with its numerical aperture up to 0.95. Approximate formulas for full width at half-maximum (FWHM) of the intensity distribution at focus were derived for very large and very small initial beam waists with respect to the entrance pupil radius of the objective lens. In experiments, the energy flux through a 0.5 microm pinhole was measured for various pinhole positions. It was found in theoretical analysis and confirmed in experiments that the FWHMs at focus in the transverse and longitudinal directions do not increase much from the ultimate FWHMs until the input beam waist is reduced below half of the entrance pupil radius.

Computational model for operation of 2 μm co-doped Tm,Ho solid state lasers

A computational model for operation of co-doped Tm,Ho solid-state lasers is developed coupling (i) 8-level rate equations with (ii) TEM00 laser beam distribution, and (iii) complex heat dissipation model. Simulations done for Q-switched approximately 0.1 J giant pulse generation by Tm,Ho:YLF laser show that approximately 43% of the 785 nm light diode side-pumped energy is directly transformed into the heat inside the crystal, whereas approximately 45% is the spontaneously emitted radiation from (3)F(4), (5)I(7) , (3)H(4) and (3)H(5) levels. In water-cooled operation this radiation is absorbed inside the thermal boundary layer where the heat transfer is dominated by heat conduction. In high-power operation the resulting temperature increase is shown to lead to (i) significant decrease in giant pulse energy and (ii) thermal lensing.

Residual stress development due to thermal cycling of the particle-reinforced alloy EN AW-6061– experiment and simulation

Abstract The development of residual stresses and strains in the matrix material of the extruded aluminium alloy EN AW-6061-T6 both with no reinforcement and also reinforced with 15vol.% and 22vol.% α-alumina particles was investigated in thermal cycling tests and additionally by finite-element-simulation for the non-reinforced material. For this purpose cylindrical specimens were heated by a 3kW diode laser and cooled by a water quench. Residual stresses in the surface area of the laser beam and temperature distribution inside the specimen were measured. The results were used to validate the finite-element model for calculating stress and strain distributions inside the specimens.

Influence of measurement set-up of ground-based LiDAR for derivation of tree structure

The objectives of this research were to investigate (1) the influence of a geometric laser measurement pattern and (2) the shadow effect on the accuracy of a quantitative mathematical description of individual tree structure using a terrestrial laser system. The commercially available SICK Laser-Measurement-System-200 (LMS200, Sick, A.G.) was used in this study to obtain 3D structural datasets with high point densities. An artificial tree was constructed in an experimental set-up, thereby creating the possibility to arrange the structural key elements (leaves and branches) according to predetermined patterns. The multidirectional accuracy and quality of the laser measurement set-ups (lateral sideway, lateral bottom-up, hemispherical bottom-up) subsequently were compared based on the development of a mathematical description of each case. Vertical plant profiles were computed for the different experimental set-ups showing the direct influence of the vertical distribution pattern of the laser beams on the estimation of plant material in a certain height bin. The main measurement direction proved to have the biggest impact on the quality of the dataset, since the derived vertical profile followed the main trend of this laser beam distribution pattern. The use of probability-based statistics was necessary to correct the vertical profiles in order to obtain an acceptable estimation of the amount of plant material per height bin of 0.05 m.

Effect of pulse CO2 laser annealing on the crystallization of Er3+ doped Al2O3 thin film to a silica-on-silicon substrate

We analyze the effect of the laser annealing on the crystallization of Er3+ doped amorphous Al2O3 thin films deposited on the silica-on-silicon substrate. These thin films were prepared by the microwave electron cyclotron resonance plasma source and were annealed by pulse CO2 laser. The Gauss intensity distribution of laser beam was mended at 3×f position in the collimation system. Strong crystallization occurred in Al2O3 thin films and liquid phase crystallization was proposed to appear with the increase of laser powers. Photoluminescence intensity enhancement by a factor of 47 around 1.53μm has been realized by laser annealing process and Raman spectra study has suggested that Er3+ emission centers excited by Si nanocrystal (Si-nc) were formed in the Al2O3–SiO2 materials. The dual wavelength energy transfer mechanism between 800nm and 980nm is proposed in Er-doped Si-nc in Al2O3–SiO2 thin films.

A formula on phase velocity of waves and application

Phase velocity plays a key role in wave-matter interactions where phase matching is essential. Laser acceleration of electrons is a good example. We have derived an exact formula with two deductions of the phase velocity for a monochromatic wave field in homogeneous medium from the fundamental wave equation. The core of these formulae is that the phase velocity is sufficiently determined in terms of the wave amplitude with no explicit reference to the phase. We applied these formulae to make out the phase velocity distribution of a Gaussian laser beam and compared with the traditional method.

Propagation characteristics of laser beams with amplitude modulations and phase fluctuations in apertured fractional Fourier transforming system

The apertured fractional Fourier transforming system is introduced and applied to treat the propagation of Gaussian beams with amplitude modulations and phase fluctuations. Based on the treatment that a rectangular function can be expanded into an approximate sum of complex Gaussian functions with finite numbers, the analytical expressions for the mutual intensity distribution of laser beams with amplitude modulations and phase fluctuations passing through the apertured fractional Fourier transforming system are obtained. Some numerical simulations are illustrated for their propagation properties.

Multilevel Computer Generated Hologram on a Curved Surface Made of Copper for High Power CO2 Laser Beam Shaping

A multilevel computer generated hologram (CGH) was made on a parabolic copper mirror for laser beam shaping. The CGH consisting of eight height-levels with 0.94μm in step height has been fabricated with a laser-beam direct writing lithography and the mask-electroplating method. The CGH was designed for laser surface modification with a high-power CO2 laser. The Gaussian distribution of an incident laser beam was shaped into a Rectangle-Gaussian distribution on a focus plane of the parabolic mirror. The diffraction efficiency was 68%. When the shaped laser beam of a 1.5kW CO2 laser irradiated a steel surface, a uniform melting layer was obtained.

Focusing astigmatic Gaussian beams through optical systems with a high numerical aperture

We theoretically derive the electric field distribution of an astigmatic Gaussian laser beam after it is focused through a high-aperture objective. We show that astigmatism values that are hard to detect in the collimated laser beam can have a large effect after diffraction-limited focusing. Such astigmatic beams may be frequently encountered in fluorescence correlation measurements and in laser-scanning confocal microscopy. We present experimental measurements of the excitation intensity distribution measured by 3D scanning of single fluorescent molecules immobilized on a glass surface.

Intracavity coherent addition of single high-order modes

We report on efficient intracavity coherent addition of several single high-order mode distributions in a multichannel laser resonator. The phase locking and coherent addition is achieved by using an intracavity interferometric beam combiner. The principle, configuration, and experimental results with pulsed Nd:YAG Laguerre-Gaussian TEM01 and TEM02 laser beam distributions are presented. The results reveal more than 95% combining efficiency with a nearly pure high-order mode output beam distribution in both free-running and Q-switched operation.

Fast optical shuttering and wavelength selection in copper vapor lasers and copper bromide lasers by intracavity polychromatic acoustooptic modulation

We propose a fast effective intracavity technique for optical shuttering and wavelength selection in copper vapor lasers and copper bromide lasers by inserting a polychromatic acoustooptic modulator in a low-intensity section of a classical negative branch unstable resonator. Single-pulse or multiple-pulse operation in one- or two-wavelength emission was achieved in a CuBr laser system. The spatial distributions of the output laser beam as recorded by a CCD camera showed neither power losses nor worsening of the beam quality in comparison with the classical scheme without the modulator. The laser output was characterized with a high degree of linear polarization.

Aperture averaging for optimizing receiver design and system performance on free-space optical communication links

Feature Issue on Optical Wireless Communications (OWC) We have developed a flexible, empirical approach for optimizing the design of free-space optical communication links by using multiframe image analysis of received intensity scintillation patterns. This is a versatile way to perform aperture-averaging analysis. A high-performance digital camera with a frame-grabbing computer interface is used to capture received intensity distributions of a He-Ne laser beam propagating in weak and intermediate turbulence conditions. The aperture-averaging results demonstrate the expected reduction in intensity fluctuations due to increasing the receiver aperture diameter for various strengths of turbulence. Aperture averaging improves the bit error rate.