Flat-top Beam Profile Research Articles

Studies on laser ablation of silicon using near IR picosecond and deep UV nanosecond lasers

Micromachining and other processing of different materials using lasers have been the focus of a large number of investigations in recent years. In this study, we investigate the interactions of two different lasers with crystalline silicon in air. The first laser is a solid state Ti:sapphire laser that has a pulse width of ∼110 ps and emits radiation of a wavelength in the near infrared (IR) region of 785 nm. The second laser is an excimer laser that emits radiation in the deep ultraviolet region (DUV) at a wavelength of 193 nm and has a pulse width of ∼ 4 ns. The excimer laser has a flat-top beam profile while the picosecond laser has a Gaussian beam profile. A series of craters was produced on a crystalline silicon wafer at different fluence, repetition rate, and number of pulses, and the resultant craters were characterized offline for surface morphology and ablation rate using optical and scanning electron microscopy. At low fluence, ablation with the excimer laser produced clean craters with minimal ejection of molten material. Increasing the fluence increased the amount of molten material deposited around the ablation crater, as well as the crater depth. Particles of different shapes and sizes with distributions depending on the number of laser pulses and laser fluence were observed around the ablation craters. Ablation rate was found to be dependent on the repetition rate of the excimer laser. Craters produced by picosecond laser pulses showed surface morphology and ablation rate differences from those of craters produced by excimer laser pulses. The differences in crater morphology, ablation rate, and ablation threshold observed using the two different types of laser can be attributed to differences in the properties of the two laser beams.

Experimentally-validated computational model for temperature evolution within laser heated fiber-reinforced polymer matrix composites

Comprehensive experimental and computational studies have been conducted to accurately measure and predict the temperature evolution of polymer matrix composite material subjected to laser heating. Plain-woven 16-ply composites with T650 carbon fibers in a bismaleimide matrix were irradiated with a continuous wave fiber laser (1.0692 μm wavelength) with a 3.0 cm diameter flat-top beam profile. The spatial and temporal evolution of temperature for the front (laser-exposed side) and back (unexposed) surfaces of the composite specimens were monitored and recorded with calibrated infrared cameras. Additionally, internal temperature evolution was measured with thermocouples embedded in the midplane of the composite specimens. A nonlinear transient finite element (FE) analysis, based on conductive, convective, and radiative heat transfer, was conducted using ANSYS to predict the temperature history during heating (laser exposure) and cooling (after exposure). For all investigated laser irradiances (i.e., 0.71–1.95 W cm−2), the numerical prediction demonstrated excellent agreement with experimental data.

Average intensity and spreading of a radially polarized multi-Gaussian Schell-model beam in anisotropic turbulence

Analytical expressions of average intensity and long-term beam width for a radially polarized multi-Gaussian Schell-model (MGSM) beam in anisotropic atmospheric turbulence are developed based on the extended Huygens-Fresnel principle. The influences of anisotropic non-Kolmogorov turbulence and source parameters on the average intensity, polarization and long-term beam spreading of radially polarized MGSM beam are examined in detail. The source correlations, free-space diffraction and turbulent effect jointly affect both the intensity and polarization profiles of the beam propagated in the atmosphere. As a radially polarized MGSM beam propagates in the atmosphere, increasing the associated beam order can lower the evolution rate from its flat-topped beam profile into a Gaussian beam profile because of the turbulent effect. Variations in the average intensity and long-term beam width are closely related to the propagation distance, power spectrum index, inner-scale, initial spatial coherence, and beam width. The atmospheric turbulence-induced beam spreading is generally suppressed with the enhanced anisotropy coefficient.

Focus shaping and optical manipulation using highly focused second-order full Poincaré beam.

Generation of vectorial optical fields with arbitrary polarization distribution is of great interest in plenty of applications. In this work, we propose and experimentally demonstrate the generation of a second-order full Poincaré (FP) beam and its application in two-dimensional (2D) flattop beam shaping with spatially variant polarization under a high numerical aperture focusing condition. In addition, the force mechanism of the focal field with 2D flattop beam profile is numerically studied, demonstrating the feasibility to trap a dielectric Rayleigh particle in three-dimensional space. The results show that the additional degree of freedom provided by the FP beam allows one to control the spatial structure of polarization, to engineer the focusing field, and to tailor the optical force exerted on a dielectric Rayleigh particle. The findings reported in the work may find useful applications in laser micromachining, optical trapping, and optical assembly.

Power loss mechanisms in small area monolithic-interconnected photovoltaic modules

Power loss mechanisms in small area monolithic-interconnected photovoltaic modules (MIM) are described and evaluated. Optical and electrical losses are quantified and individual loss components are derived for loss mechanisms of small area radial (radius = 1 mm) pie-shaped six-segment GaAs MIM laser power converter. At low monochromatic homogeneous illumination (Glow = 1.8 W/cm2, λ0 = 809 nm) conversion efficiency of the cell, designed for a low irradiance, is reduced by 3.7%abs. due to isolation trench optical losses and by 7.0%abs. due to electrical losses (mainly perimeter recombination). Electrical losses in a device designed for a high irradiance, result in 18%abs. decrease of output power under homogeneous monochromatic illumination (Ghigh = 83.1 W/cm2, λ0 = 809 nm), while 11.6%abs. losses are attributed to optical reasons. Regardless the irradiance level, optical losses further increase if the device is illuminated with a Gaussian instead of an ideal flattop beam profile. In this case, beam spillage losses occur and losses due to isolation trenches and reflections from metallization are elevated. On top of that, additional current mismatch losses occur, if individual MIM’s segments are not equally illuminated. For the studied device, a 29 μm off center misalignment of a Gaussian shaped beam (with 1% spillage) reduces the short circuit current Isc by 10%abs. due to the current mismatch between segments.

An approach to realisation of a Radial Phase mask using 3-D printing in transparent PLA

In the recent years, many different techniques and algorithms have been devised to design diffractive optical elements (DOE’s) for the purpose of beam shaping. This paper demonstrates an approach to realise a 3-D printed radial phase mask to be used in beam shaping to achieve a beam profile closer to the flattop. An iterative algorithm approach is employed to simulate the phase masks in greyscale and subsequently into STL format. These 3-D printed masks are used as an optical element and characterised using an experimental setup. The images of the light after the characterisation are examined and compared with the simulated results. Therefore, this method reduces the complexity as 3-D printing the masks eliminates the need for fabrication, processing time and number of components necessary to obtain a flattop beam profile.

Radially polarized multi-Gaussian Schell-model beam and its tight focusing properties

In this paper, we introduce a new kind of vector multi-Gaussian Schell-model (MGSM) beam named radially polarized MGSM beam as an extension of recently introduced scalar MGSM beam [Opt. Lett.37, 2970 (2012)], and we obtain the realizability conditions of such beam. The tight focusing properties of a radially polarized MGSM beam passing through a high numerical aperture objective lens are investigated numerically based on the vectorial diffraction theory. It is interesting to find that the intensity distributions of both transverse and longitudinal fields near the focus display flat-top beam profiles due to the special correlation functions of the incident beam. Our results clearly show that engineering the structures of the correlation functions of a radially polarized partially coherent beam provides a novel way for shaping both transverse and longitudinal fields distributions, which have potential applications in particle acceleration and laser machining. Finally, we report experimental generation of a radially polarized MGSM beam with the help of a spatial light modulator and a radial polarization converter.

Numerical study of spatiotemporal distortions in noncollinear optical parametric chirped-pulse amplifiers

During amplification in a noncollinear optical parametric amplifier the spatial and temporal coordinates of the amplified field are inherently coupled. These couplings or distortions can limit the peak intensity, among other things. In this work, a numerical study of the spatiotemporal distortions in BBO-based noncollinear optical parametric chirped-pulse amplifiers (NOPCPAs) is presented for a wide range of parameters and for different amplification conditions. It is shown that for Gaussian pump beams, gain saturation introduces strong distortions and high conversion efficiency always comes at the price of strong spatiotemporal couplings which drastically reduce the peak intensity even when pulse fronts of the pump and the signal are matched. However, high conversion efficiencies with minimum spatiotemporal distortions can still be achieved with flat-top pump beam profiles.

Influence of the Laser Spot Size, Focal Beam Profile, and Tissue Type on the Lipid Signals Obtained by MALDI-MS Imaging in Oversampling Mode.

To improve the lateral resolution in matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) beyond the dimensions of the focal laser spot oversampling techniques are employed. However, few data are available on the effect of the laser spot size and its focal beam profile on the ion signals recorded in oversampling mode. To investigate these dependencies, we produced 2 times six spots with dimensions between ~30 and 200μm. By optional use of a fundamental beam shaper, square flat-top and Gaussian beam profiles were compared. MALDI-MSI data were collected using a fixed pixel size of 20μm and both pixel-by-pixel and continuous raster oversampling modes on a QSTAR mass spectrometer. Coronal mouse brain sections coated with 2,5-dihydroxybenzoic acid matrix were used as primary test systems. Sizably higher phospholipid ion signals were produced with laser spots exceeding a dimension of ~100μm, although the same amount of material was essentially ablated from the 20μm-wide oversampling pixel at all spot size settings. Only on white matter areas of the brain these effects were less apparent to absent. Scanning electron microscopy images showed that these findings can presumably be attributed to different matrix morphologies depending on tissue type. We propose that a transition in the material ejection mechanisms from a molecular desorption at large to ablation at smaller spot sizes and a concomitant reduction in ion yields may be responsible for the observed spot size effects. The combined results indicate a complex interplay between tissue type, matrix crystallization, and laser-derived desorption/ablation and finally analyte ionization. Graphical Abstract ᅟ.

Underwater annealing and texturing for enhancing electrical characteristics of n-aSi/p-cSi using Nd 3 + : YAG laser beam-overlap technique with a wavelength of 532 nm

The influence of the laser beam profile on simultaneous wide-area crystallization and texturing of amorphous silicon (a-Si) thin films in water ambience is investigated by using a pulsed laser-beam-overlap technique. A Q-switched Nd3+:YAG laser with the second harmonic wavelength of 532 nm and different beam profiles, namely Gaussian and flat-top, was used for the annealing of 1-μm thick a-Si films deposited on crystalline silicon (c-Si) substrates. High density and smaller-sized conical spikes with an increase in grain size of around 25% and improved photoconductivity characteristics (9% to 17%) were observed after laser treatment was carried out in water when compared with that in air. Further, crystalline characteristics were also improved with the flat-top beam profile as compared with that of the Gaussian beam profile. The necessary laser fluence range based on the thermal modeling in the underwater ambience is in good agreement with the experimentally measured values between 150 and 600 mJ/cm2. The improvement in morphological, crystalline, and electrical characteristics of a-Si films clearly show that underwater annealing and texturing with the Nd3+:YAG laser beam-overlap of 30% to 50% is suitable for photovoltaic applications.

Enhancement in Photoconductivity of a-Si thin Films by Annealing and Texturing Technique with the Third Harmonic Output from a Pulsed Nd3+:YAG Laser

Influence of the third harmonic pulsed Nd3+:YAG laser on the formation of a polycrystalline-silicon (poly-Si) on a-Si thin film with thickness of 1000nm and 400nm in water and air ambience was investigated. In order to induce texturing of surface along with annealing, laser beam overlap technique with different percentages of spot overlap was used. Crystalline characteristics and electrical characteristics were studied to confirm the extent of crystallization. The crystalline characteristics of the film obtained with the Gaussian and the flat-top beam profiles were comparable for higher percentage of overlapping. Based on the theoretical modeling, the laser treatment without the ablation with the third output from the Nd3+:YAG laser was limited to the a-Si film thickness up to 800 nm. This was in qualitative agreement with the experimental observations.

New Ways to Generate Flat‐Top Profiles

AbstractPreviously, flat‐top beam profiles were usually generated in laser technology through homogenizers. These beam shaping optics are, however, both expensive and diminish the laser output. Optical fibers with a rectangular core geometry offer an alternative, by being able to homogenize the intensity distribution of a laser beam without reducing its performance.

Influence of pulsed Nd3+ : YAG laser beam profile and wavelength on microscribing of copper and aluminum thin films

Evenly spaced conductive grids of copper and aluminum thin films on polyimide substrate are used for parabolic reflector antennas aboard telecommunications satellites. Laser microscribing of thin films using a flat-top and Gaussian laser beam profile is analyzed with 95% overlapping of the diameter of the laser spot. Laser scribing is performed using the Q-switched Nd3+: YAG (355, 532 nm) laser. The influence of laser irradiation and beam shape on the scribed microchannel width, depth, and surface characteristics is experimentally analyzed using a noncontact optical profilometer and scanning electron microscope (SEM). Laser scribing using a flat-top profile produced near rectangular microchannels in copper thin films. Using the Gaussian profile, the probability of melting is greater than vaporization as observed using SEM images; this melt pool plays a prominent role in resolidification at the edges. The depth of the scribe channel is observed to be 20% higher for the 532-nm wavelength compared to the 355-nm wavelength. The effect of different environments such as air, water, and vacuum on the channel depth and quality is reported. The response of aluminum and copper thin films for high fluences is also studied. Thermal modeling of the laser-material interaction has been attempted by assuming the plasma electron temperature as the laser ablation temperature for modeling the recession rate and depth for a single laser pulse. Model results agree with experimental data showing greater depth for 532 nm compared to 355 nm.

Effect of laser path overlap on surface roughness and hardness of layer in laser cladding

A Ni–Cr–Si–B alloy coating was produced on a type 304 stainless steel plate by laser cladding. A diode laser with a flat-top beam profile was used in this experiment. The effects of focused laser spot overlap on surface roughness (Ra) and Vickers hardness were investigated. Ra and the Vickers hardness of the cladding layers decreased as the overlap ratio increased. Cladding layers that maintained a hardness comparable to that of the source powder material were produced at overlap ratios of < 70%.

Generalized multi-Gaussian correlated Schell-model beam: from theory to experiment.

A new kind of partially coherent beam with non-conventional correlation function named generalized multi-Gaussian correlated Schell-model (GMGCSM) beam is proposed. The GMGCSM beam of the first or second kind is capable of producing dark hollow or flat-topped beam profile in the focal plane (or in the far field). Furthermore, we carry out experimental generation of a GMGCSM beam of the first or second kind, and measure its focused intensity. Our experimental results verify theoretical predictions. The GMGCSM beam will be useful for free-space optical communications, material thermal processing, particle or atom trapping.

Random source generating far field with elliptical flat-topped beam profile

Circular and rectangular multi-Gaussian Schell-model (MGSM) sources which generate far fields with circular and rectangular flat-topped beam profiles were introduced just recently (Sahin and Korotkova 2012 Opt. Lett. 37 2970; Korotkova 2014 Opt. Lett. 39 64). In this paper, a random source named an elliptical MGSM source is introduced. An analytical expression for the propagation factor of an elliptical MGSM beam is derived. Furthermore, an analytical propagation formula for an elliptical MGSM beam passing through a stigmatic ABCD optical system is derived, and its propagation properties in free space are studied. It is interesting to find that an elliptical MGSM source generates a far field with an elliptical flat-topped beam profile, being qualitatively different from that of circular and rectangular MGSM sources. The ellipticity and the flatness of the elliptical flat-topped beam profile in the far field are determined by the initial coherence widths and the beam index, respectively.

Detection of buried layers in silicon devices using LIBS during hole drilling with femtosecond laser pulses

Femtosecond laser micromachining together with Laser Induced Breakdown Spectroscopy (LIBS) allows us to drill precise hole in materials to internal buried layers as well as characterize the materials while drilling. We report detection of a metal layer buried deep inside silicon by creating an access hole through the semiconductor. We used 800 nm femtosecond laser pulses to carry out the drilling while monitoring the plasma emission with a spectrometer system. Higher drilling rates of 1 μm per shot were achieved using a Gaussian laser beam profile with peak fluences of 42 J/cm2. Lower drilling rates of 30 nm per pulse with better accuracy could be achieved using lower intensity flat top beam profiles at fluences of 1.4 J/cm2.

Peak Intensity Stabilization of Fundamental Laser through Second-Harmonic Generation Process

In this paper, we report on numerical and experimental results for the peak intensity stabilization of a fundamental laser through the second-harmonic generation process by introducing a phase-mismatching factor. The peak intensity fluctuation of a nanosecond-order fundamental laser at 1064 nm is reduced from 7.0 to 1.1% with a type-I noncritical phase-mismatching lithium triborate crystal heated at a phase-mismatching temperature of 146.7 °C. The method is applicable to the power stabilization of the laser with a flat-top beam profile.

Peak Intensity Stabilization of Fundamental Laser through Second-Harmonic Generation Process

In this paper, we report on numerical and experimental results for the peak intensity stabilization of a fundamental laser through the second-harmonic generation process by introducing a phase-mismatching factor. The peak intensity fluctuation of a nanosecond-order fundamental laser at 1064 nm is reduced from 7.0 to 1.1% with a type-I noncritical phase-mismatching lithium triborate crystal heated at a phase-mismatching temperature of 146.7 °C. The method is applicable to the power stabilization of the laser with a flat-top beam profile.

Homogeneous one-dimensional optical lattice generation using a digital micromirror device-based high-precision beam shaper

A homogeneous one-dimensional optical lattice is demonstrated by using a high-precision beam shaper based on a digital micromirror device (DMD) with an imaging system containing a pinhole low-pass filter (LPF). This system is capable of producing a high-quality flattop beam profile to form a standing-wave optical lattice with a 50×50 μm2 flattop region. The periodic potential generated by the optical lattice confines ultracold atoms in Bose-Einstein condensate experiments. We conducted beam shaping tests at several wavelengths by implementing various coherent and incoherent light sources in the visible and infrared wavelength ranges. Experiments produced flattop and other well-controlled beam profiles with 0.2% to 0.26% root-mean-square (RMS) error after applying a digital LPF and nearly flat phase. Several concerns for the system design are presented. First, the energy requirement was determined by power conversion analysis and DMD diffraction efficiency simulation. In addition, a LabVIEW program was written to accelerate the speed of the iterative process for beam profile refinement. Finally, various camera calibrations improved the measurement accuracy. We achieved a 1.25% RMS error flattop beam with diameter of 70.4 μm at the atoms' plane. Other beam profile measurements in different diagnostic planes demonstrated a good intensity uniformity of the optical lattice.