Flat-top Profile Research Articles

Sapphire Concave Microlens Arrays for High-Fluence Pulsed Laser Homogenization

Microlens array has been demonstrated to be an effective method for laser beam homogenization. However, the homogenization of high-fluence laser beam is still a great challenge due to the serious damage to the microlens-array-based homogenizer. Here, an ultra-smooth sapphire concave microlens-array-based homogenizer, which is fabricated by dry-etching-assisted femtosecond laser machining, is proposed for shaping high-fluence pulsed laser beam to a flat-top profile intensity distribution. By taking advantages of high transmittance (>70%) and low surface roughness (1.1 nm), high-fluence (5 J/cm2) and wide-band (266 nm, 532 nm and 808 nm) laser beams homogenization is successfully realized by the sapphire concave microlens arrays.

Assessment of light diffraction and focusing of chirped gratings by the beam propagation method

In this paper, we investigate the competence of the Fast Fourier Transform based beam propagation method (FFT-BPM) in the analysis of the diffraction of a Gaussian light beam by linearly binary chirped gratings. The visibility and the focusing of the diffraction orders are investigated thoroughly. The waist size and focal distance of the diffraction orders are extracted easily. Moreover, the key parameters that control the Gaussian beam deflection and focusing are examined for more improvement of the chirped grating lens operation and to design a beam splitter. The applicability and accuracy of the method is verified by comparing our results with those obtained by other analytical method. The comparison showed a good agreement. Furthermore, the illumination by flat-top beam profiles such as rectangular and super-gaussian are studied.

Simple intra-cavity beam shaping for generating a shape-invariant flat-top laser beam

For laser users, the usual Gaussian beam (GB) may not always be the best intensity profile. Often, a preferred option is a Flat-Top (FT) beam which is generally obtained by using diffractive optics in a reshaping operation. In this case, the resulting FT profile occurs only in the focal plane of the focusing lens, and its vicinity. We consider here the generation of a shape-invariant FT laser beam resulting from the incoherent weighted (50%-50% in power) mixing of LG00 (GB) and LG01 (doughnut) beams. For that we consider the insertion inside the laser cavity of a diaphragm and an absorbing ring, and optimising their sizes makes possible the simultaneous oscillation on LG00 and LG01 modes allowing the generation of a shape-invariant laser beam.

The 808 nm and 980 nm infrared laser irradiation affects spore germination and stored calcium homeostasis: A comparative study using delivery hand-pieces with standard (Gaussian) or flat-top profile

Photobiomodulation relies on the transfer of energy from incident photons to a cell photoacceptor. For many years the concept of photobiomodulation and its outcome has been based upon a belief that the sole receptor within the cell was the mitochondrion. Recently, it has become apparent that there are other photoacceptors operating in different regions of the electromagnetic spectrum. Alternative photoacceptors would appear to be water and mechanisms regulating calcium homeostasis, despite a direct effect of laser photonic energy on intracellular calcium concentration outwith mitochondrial activity or influence, have not been clearly demonstrated. Therefore, to increase the knowledge of intracellular‑calcium and laser photon interaction, as well as to demonstrate differences in irradiation profiles with modern hand-pieces, we tested and compared the photobiomodulatory effect of 808 nm and 980 nm diode laser light by low- and higher-energy (60s, 100 mW/cm2, 100 mW/cm2, 500 mW/cm2, 1000 mW/cm2, 1500 mW/cm2, 2000 mW/cm2) irradiated with a “standard” (Gaussian fluence distribution) hand-piece or with a “flat-top” (uniform fluence) hand-piece. For this purpose, we used the eukaryote unicellular-model Dictyostelium discoideum. The 808 nm and 980 nm infrared laser light, at the energy tested directly affect the stored Ca2+ homeostasis, independent of the mitochondrial respiratory chain activities. From an organism perspective, the effect on Ca2+-dependent signal transduction as the regulator of spore germination in Dictyostelium, demonstrates how a cell can respond quickly to the correct laser photonic stimulus through a different cellular pathway than the known light-chromophore(mitochondria) interaction. Additionally, both hand-piece designs tested were able to photobiomodulate the D. discoideum cell; however, the hand-piece with a flat-top profile, through uniform fluence levels allows more effective and reproducible effects.

Experimental generation of a flat-top beam profile in a stable ring cavity.

In this Letter, we present a spatially homogeneous field inside of a ring cavity that was created by combining two transverse modes generated by a single laser through modulation. The interference term between the two modes averages out because of the frequency difference that exists between them, eliminating the need for interferometric control of their relative phase. The use of a ring cavity allows for a large waist for the flat-top profile, big enough to cover the atoms in an atomic trap. The cavity is mechanically and thermally isolated, and the laser light is locked to the cavity using the Pound-Drever-Hall technique. The flat-top profile technique reported here fulfills the vanishing curvature criterion at the center of the profile.

Multiple Fano interferences due to waveguide-mediated phase coupling between atoms

We examine quantum interference effects due to absorption and emission from multiple atoms coupled to a waveguide and highlight the modifications they entail in regards to single-photon transport properties. A prominent upshot of these interference phenomena is the resonant suppression of the reflection amplitude, which leads to the observation of multiple Fano minima in the reflection spectrum. Such minima determine the points at which transparency is induced in the system. By taking recourse to the real-space Hamiltonian framework, we calculate analytically the reflectivity and transmissivity for a one-dimensional waveguide that evanescently couples to a chain of equally spaced quantum emitters. The inter-emitter spacing relative to the wavelength of the propagating photon, leading to a waveguide-mediated "phase-coupling" between the atoms, is found to fundamentally affect the existence of Fano minima. For a chain of $N$ atoms, the number of minima can be at most $N-1$. However, suitable choices of the phase can suppress the discernibility of the full range of roots in the reflection spectrum. A principal observation for the case of multiple emitters is the emergence of super-Gaussian characteristics close to zero-detuning and consequently, a plateau-shaped broadband spectrum in the region of high reflectivity. For a large chain size, the plateau gets transformed into a flat-topped quasi-rectangular profile in the frequency domain.

Model of a rectangular multi-Gaussian Schell-model vortex beam and its properties

Considering the theory of coherence, the rectangular multi-Gaussian Schell-model vortex beam (RMGSMV) beam generated by multi-Gaussian Schell-model source has been introduced, and the propagation properties of RMGSMV beam propagating in free space has been analyzed by using the derived equations. The effects of beam parameters on the properties of RMGSMV beam propagating in free space have been discussed. The results show that the RMGSMV beam propagating in free space can remain the dark hollow center at the short propagation distance, and the beam will gradually evolve into a beam with similar rectangular flat-topped profile as the propagation distance increases.

Hyperbolic phase function used in a spatial light modulator for flat top focus generation.

We report on a novel transversal beam shaping approach to convert a Gaussian beam to a flat top focus profile by employing a hyperbolic phase function in a liquid crystal on silicon (LCOS) spatial light modulator (SLM). The well-known Powell lens approach is compared to ours. The analytically obtained phase function for the flat top focus conversion problem is compared to our approach theoretically and experimentally. A strategy for the generation of squared and circular top hat foci is derived.

Partially Coherent Flat-Topped Beam Generated by an Axicon

The intensity distribution of a partially coherent beam with a nonconventional correlation function, named the multi-Gaussian Schell-model (MGSM) beam, focused by an axicon was investigated in detail. Our numerical results showed that an optical needle with a flat-topped spatial profile and long focal depth was formed and that we can modulate the focal shift and focal depth of the optical needle by varying the width of the degree of coherence (DOC) and the parameters of the correlation function. The adjustable optical needle can be applied for electron acceleration, particle trapping, fiber coupling and percussion drilling.

Light-emitting-diode-pumped active Q-switched Nd:YLF laser.

An active Q-switching light-emitting diode (LED)-pumped laser is demonstrated by Nd:YLF crystal with acousto-optic modulation for the first time. The spectrum-band pump characteristic is grasped to describe the essential difference between an LED pump and single-absorption-peak matching of laser-diode pump or no matching of lamp pump. An effective absorption spectrum concept is proposed to characterize the absorption features of the gain material with LED-band pumping. According to this new theory, a flat-top beam profile is designed for pumping Nd:YLF crystal with only a 14 W/cm2 peak power, resulting in 165μJ output energy at 1047nm. More importantly, by using the acousto-optic Q-switching technique, this LED-pumped Nd:YLF laser has successfully realized a TEM00 mode output with a pulse energy of 10.6μJ and a pulse width of 452ns.

A dispersion engineered silica-based photonic crystal fiber for supercontinuum generation in near-infrared wavelength region

We report computational modeling of photonic crystal fibers in silica glass for near-infrared supercontinuum generation. Optical parameters like chromatic dispersion, nonlinearity and the effective area of the fundamental mode have been computed and tailored by using a full vectorial finite element method. At pump wavelength of 1300 nm, a flat-top dispersion profile has been obtained with very low dispersion value + 0.6402 ps/nm/km in the anomalous region. The reported structure offers high nonlinear coefficient 26.27W−1. km−1with effective mode area 4.78 μm2 at the pump wavelength. An ultra-broadband supercontinuum spanning, from 0.67 μm up to 2.4 μm can be easily achieved by the proposed fiber of 37 cm length using a 63 fs secant laser pulse source with 8 kW peak power in the near-infrared wavelength region. Such type of fiber can be advantageous in the field of near-infrared spectroscopy, optical communication, gas sensing, medical, frequency metrology, optical coherence tomography, and food quality control.

Flat-top TIRF illumination boosts DNA-PAINT imaging and quantification

Super-resolution (SR) techniques have extended the optical resolution down to a few nanometers. However, quantitative treatment of SR data remains challenging due to its complex dependence on a manifold of experimental parameters. Among the different SR variants, DNA-PAINT is relatively straightforward to implement, since it achieves the necessary ‘blinking’ without the use of rather complex optical or chemical activation schemes. However, it still suffers from image and quantification artifacts caused by inhomogeneous optical excitation. Here we demonstrate that several experimental challenges can be alleviated by introducing a segment-wise analysis approach and ultimately overcome by implementing a flat-top illumination profile for TIRF microscopy using a commercially-available beam-shaping device. The improvements with regards to homogeneous spatial resolution and precise kinetic information over the whole field-of-view were quantitatively assayed using DNA origami and cell samples. Our findings open the door to high-throughput DNA-PAINT studies with thus far unprecedented accuracy for quantitative data interpretation.

Multi-Gaussian Schell-model source with a complex coherence state

The effects of non-trivial phase of the source degree of coherence on the far-field spectral density for the class of one-dimensional, multi-Gaussian Schell-model beams are explored. While in the absence of the phase, the flat-top spectral density profile with a Gaussian edge drop is symmetrically formed in the far field about the optical axis, various phase functions introduce asymmetric modifications to such distribution. Since the phase of the source degree of coherence must be an odd function, three such examples are considered in detail: linear phase leading to shift, signum phase producing either tilt or split, and cubic phase forming Airy-like radially accelerating profiles. We have established that unless the phase of the source degree of coherence is sufficiently small at all separations, the resulting far-field spectral density profiles are qualitatively very different from those with the trivial phase.

A Comparative Study Between the Effectiveness of 980 nm Photobiomodulation Delivered by Hand-Piece With Gaussian vs. Flat-Top Profiles on Osteoblasts Maturation.

Photobiomodulation (PBM) is a clinically accepted tool in regenerative medicine and dentistry to improve tissue healing and repair and to restore the functional disability. The current in vitro study aimed to investigate the photobiomodulatory effects of 980 nm wavelength (the real energy at the target: ~0.9 W, ~0.9 W/cm2, 60 s, ~55 J/cm2 and a single energy ~55 J in CW) on MC3T3-E1 pre-osteoblast, delivered with flattop profile in comparison to the standard profile. The laser groupings and their associated energies were: Group 1 - once per week (total energy 110 J); Group 2 - three times per week (alternate day) (total energy 330 J); Group 3 - five times per week (total energy 550 J). The metabolic activity and the osteoblasts maturation were analyzed by alkaline phosphatase assay, alizarin red S histological staining, immunoblot and/or double immunolabeling analysis for Bcl2, Bax, Runx-2, Osx, Dlx5, osteocalcin, and collagen Type 1. Our data, for the first time, prove that laser irradiation of 980 nm wavelength with flat-top beam profile delivery system, compared to standard-Gaussian profile, has improved photobiomodulatory efficacy on pre-osteoblastic cells differentiation. Mechanistically, the irradiation enhances the pre-osteoblast differentiation through activation of Wnt signaling and activation of Smads 2/3-βcatenin pathway.

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.

Spectral broadening effects on metal photoemission by femtosecond laser pulses

Emitted electronic densities from an Al surface have been computed for various temporal shapes of near-infrared femtosecond laser pulses (less than or equal to 20 fs) over a wide range of low and intermediate laser intensities. At low intensities photoemission originates from the high-energy tail of the pulse spectrum leading to linear absorption in all cases, even if several photons are necessary to eject an electron at higher intensities. We propose an analytical method to calculate the transition intensity in which the behavior of emitted densities evolves from the single-photon ${I}_{m}$ slope to the multiphoton ${I}_{m}^{n}$ one (with ${I}_{m}$ the maximum intensity and $n$ the number of absorbed photons). We show that the use of a flat-top profile or at least of a super-Gaussian envelope enables strong electron emission at very low intensities for realistic pulse durations and should allow experimental observation of the transition between these two regimes of photoemission.

Highly efficient thulium-doped high-power laser fibers fabricated by MCVD.

We report a hybrid process by combining both vapor-phase and solution-doping techniques of rare-earth doped preform fabrication in conjunction with the MCVD technique, in order to fabricate highly efficient Tm-doped laser fibers. The proposed fabrication route takes advantage of co-doping silica with high alumina content through the vapor-phase doping process, which is otherwise difficult to achieve using conventional solution doping technique. In addition, by employing the solution doping method, high-purity thulium halide precursors that have low vapor pressures up to several hundred degree Celsius. These high-purity thulium halide precursors can be used to dope the fiber core region with a high thulium concentration that is optimized for an efficient two-for-one cross-relaxation process for 79xnm diode pumped thulium-doped fiber laser. Fibers fabricated using the hybrid approach show more homogeneous and flat-top dopant profiles, compared with the conventional approach, where both aluminum and thulium are incorporated in the core through solution doping. This will ensure that more doped region will take part in the cross-relaxation process. Superior laser performance with a slope efficiency of >70% in the two-micron band has been demonstrated when diode pumped at ~790nm.

Novel method to design laser beam shaping lenses using PSO techniques

To design beam shaping lenses converting the light from Gaussian intensity distribution into flat-top profile, a new method—particle swarm optimization (PSO) algorithm—was employed. The theoretical values of ray intersections at the output plane are precisely calculated by the energy conservation when a Gaussian beam is converted to a flat-top one, and these values are then used as target values in the optimization process. Real values of ray intersections are obtained from the ray tracing. The absolute values of the differences between real and theoretical values are used to construct the merit function of a shaping lens system, which are also used as the fitness function values for the PSO. By the manipulation of minimizing the fitness function, refractive two-element beam shapers and a refractive single-element beam shaper have been designed. The analyses for the design results demonstrated the feasibility and validity of the PSO method in the design of laser beam shaping lenses.

Focusing properties of spirally polarized sinh Gaussian beam

The tight focusing properties of spirally polarized sinh-Gaussian beam is investigated numerically by vector diffraction theory. Results show that the optical intensity in focal region of spirally polarized sinh Gaussian beam can be altered considerably by the beam order, relative waist width and the spiral parameter that indicates the polarization spiral degree of the spirally polarized sinh Gaussian beam. Many novel focal patterns including flattop profile, focal hole axially separated focal spots and focal spot with long focal depth are evolved considerably for the suitable beam parameters. We expect such a tunable focal patterns are useful for optical manipulation of micro particles.

Mean irradiance profile of a Gaussian beam under random jitter.

The mean irradiance profile has been widely applied in performance analysis and atmospheric channel research for free space optical communication; however, the modeling method and a closed-form expression of the mean irradiance profile under random jitter have rarely been discussed. To the best of our knowledge, this work presents a modeling method and a closed-form expression for a mean irradiance profile under random jitter through the ensemble average principle for the first time. We find that, with an increase in random jitter variance, the mean irradiance profile varied from an approximately Gaussian profile to a flat-topped profile to a hollow profile. These findings were verified using multilayer random phase screen simulations. The model of the mean irradiance profile under random jitter developed in this study will improve the accuracy of performance analysis and atmospheric channel research.