Gaussian Beam Profile Research Articles

Design and validation of a ten nanosecond resolved resistive thermometer for Gaussian laser beam heating.

Pulsed laser processing plays a crucial role in additive manufacturing and nanomaterial processing. However, probing the transient temperature field during the pulsed laser interaction with the processed materials is challenging as it requires both high spatial and temporal resolution. Previous transient thermometry studies have measured neither sub-100 µm spatial resolution nor sub-10 ns temporal resolution. The temperature field induced by Gaussian laser beam profiles has also not been accounted for. Here, we demonstrate a 9 ns rise time, 50 µm sized Pt thin-film sensor for probing the temperature field generated by a nanosecond pulsed laser on a semiconductor thin film. The measurement error sources and associated improvements in the thin film fabrication, sensor patterning, and electrical circuitry are discussed. We carried out the first experimental and theoretical analysis of spatial resolution and accuracy for measuring a Gaussian pulse on the serpentine structure. Transparent silica and sapphire substrates, as well as 7-45 nm insulation layer thicknesses, are compared for sensing accuracy and temporal resolution. Finally, the measured absolute temperature magnitude is validated through the laser-induced melting of the 40 nm thick amorphous silicon film. Preliminary study shows its potential application for probing heat conduction among ultrathin films.

Ionization-assisted refocusing of femtosecond Gaussian beams

Ionization occurs ubiquitously in intense laser-matter interaction and often leads to rapid decrease in laser intensity via plasma defocusing, shortening the effective interaction length of desired high-field processes. Refocusing of pulses may compensate for this adverse effect. However, it typically relies on Kerr-induced self-focusing and requires sufficiently high power. Here, we present simulations showing the refocusing of intense pulses with an initial Gaussian beam profile in atmospheric pressure gases at relatively low power. We attribute this refocusing to the formation of ring-structure plasmas. We find that tighter focusing leads to stronger refocusing, and the initial chirp of the pulse greatly affects its dynamics due to spatiotemporal coupling of focused broadband pulses. Our results highlight a novel aspect of complex pulse dynamics and can be relevant to applications involving tightly focused ultrafast Gaussian beams.

Simulation and in vivo investigation of light-emitting diode, near infrared Gaussian beam profiles

Near infrared spectroscopy is an optical imaging technique which offers a non-invasive, portable, and low-cost method for continuously measuring the oxygenation of tissues. In particular, it can provide the brain activation through measuring the blood oxygenation and blood volume in the cortex. Understanding and then improving the spatial and depth sensitivity of near infrared spectroscopy measurements to brain tissue are essential for designing experiments as well as interpreting research findings. In this study, we investigate the effect of applying two common light beam profiles including Uniform and Gaussian on the penetration depth of an LED-based near infrared spectroscopy. In this regard, two Gaussian profiles were produced by adjusting plano-convex and bi-convex lenses and the Uniform profile was provided by applying a flat lens. Two experiments were conducted in this study. First, a simulation experiment was carried out based on scanning the intra space of a liquid phantom by using static and pulsating absorbers to compare the penetration depth of the configurations applied on the LED-based near infrared spectroscopy with that of a laser-based near infrared spectroscopy. Second, to show the feasibility of the best proposed configuration applied, an in vivo experiment of stress assessment has been performed and its results have been compared with that results obtained by laser one. The results showed that the LED-based near infrared spectroscopy equipped with bi-convex lens provides a penetration depth and hence quality measurements of near infrared spectroscopy and its extracted heart rate variability signals as well as laser-based near infrared spectroscopy especially in the application of stress assessment.

Free-Space Optical Communications With Detector Arrays: A Mathematical Analysis

Photon counting detector arrays are commonly used for free-space optical communications in deep space. Such detector arrays—by virtue of their size—help in the collection of the optical signal even when there is some misalignment between the transmitter and receiver systems. In this paper, we argue that, for the common Gaussian beam profile, a detector array receiver is more useful for minimizing the probability of error than a single detector receiver of the same dimensions. Furthermore, the improvement in the error probability is more pronounced for low signal-to-noise ratio conditions, and the probability of error decreases monotonically as a function of the number of detectors in the array. However, communication with detector arrays results in a larger computational and hardware complexity at the receiver.

Enhancement of optical penetration depth of LED-based NIRS systems by comparing different beam profiles

Near-Infrared Spectroscopy (NIRS) is a non-invasive brain imaging technique involving the quantification of oxy and deoxy-hemoglobin concentrations resolved from the measurement of Near-Infrared (NIR) light attenuation within the tissue. Previous studies have shown that NIR light is more influenced by the optical properties of the superficial layers than those of the deeper target layers such as cortex. NIR light produced by the Laser source penetrates deeper regions of the tissue rather than the LED source although Laser needs more expensive instrumentation. In this study, we investigate the effect of Uniform and Gaussian beam profiles on the enhancement of LED light penetration depth. The latter beam profiles were generated and compared using Flat and Aspherical lenses applied to the LED sources. In order to increase the signal to noise ratio, the lenses were also applied to the light detector. For performance analysis, two experiments were carried out by scanning the intra space of a liquid phantom by static and dynamic (pulsating) absorbers. Monte Carlo simulations were also carried out to be compared with the experiment. The results showed that Gaussian beam profile and in particular, Bi-Convex lenses applied to both source and detector leads to a greater light penetration depth in the liquid phantom close to that of a Laser source.

Third-order optical nonlinearity of tungsten disulfide atomic layer with resonant excitation

The third-order optical nonlinearity of tungsten disulfide atomic layer was characterized with open and closed Z-scan techniques with a resonant excitation near an exciton absorption peak using a pulsed laser which has a temporal pulse width of ~6 ns and a Gaussian spatial beam profile. The normalized transmittance traces with open and closed Z-scan for the WS2 atomic layers displayed positive nonlinear absorption and negative nonlinear refraction. The nonlinear absorption and nonlinear refraction coefficients of WS2 with the resonant excitation were estimated to be ~6.0 × 10−8 m/W and ~-3.8 x 10−15 m2/W.

π/2 mode converters and vortex generators for electrons

In optics, mode conversion is an elegant way to switch between Hermite Gaussian and Laguerre Gaussian beam profiles and thereby impart orbital angular momentum onto the beam and to create vortices. In optics such vortex beams can be produced in a setup consisting of two identical cylinder lenses. In electron optics, quadrupole lenses can be used for the same purpose. Here we investigate generalized asymmetric designs of a quadrupole mode converter that may be realized within the constraints of existing electron microscopes and can steer the development of dedicated vortex generators for high brilliance electron vortex probes of atomic scale.

Customized exposure strategies for manufacturing hybrid parts by combining laser beam melting and sheet metal forming

Subjects of this work are the mechanical, geometrical, and microstructural properties of hybrid parts from Ti6Al4V sheet metal bending and laser beam melting (LBM) in dependence of the extended LBM process parameters. In this context, the effects of different laser exposure strategies for the interface area to increase the reproducibility of the joint strength are investigated. Also, the effect of varying scan patterns and two different laser beam sources (Gaussian beam profile, spot size: 120 μm, maximum laser power: 400 W and irregular beam profile, spot size: 680 μm, maximum laser power: 1000 W) on the LBM process and the resulting distortion of the parts are examined. Furthermore, three heat-treatment temperatures at 450, 850, and 1050 °C were applied to the hybrid samples, resulting in variable microstructures and different mechanical properties for the sheet metal body and the LBM structure.

Simulation of thermal lensing in an end-pumped Nd:YAG laser rod with Gaussian and super-Gaussian pump beam profile

A numerical simulation is made on the thermal lensing effect in an laser diode end-pumped Nd:YAG laser rod. Based on finite element method (FEM), the laser rod temperature distribution is calculated and the focal length is deduced for a Gaussian and super-Gaussian pump beam profiles.
 At the pump power of 20W, the highest temperature located at the center of end-pumped face was 345K, and the thermal lens focal length was 81.4mm along the x-z axis. 
 The results indicate that the thermal lensing effect sensitively depend on the pump power, waist radius of the pump beam and the pump distribution in a laser rod geometry.

Non-linear optics study of the samples which strongly diffuse the Gaussian beam

The Z-scan experiment is the fastest method to determine non-linear optical (NLO) coefficients of materials. Classic Z-scan technique described by Sheik-Bahae et al. (1989) and Wang et al. (1994) uses pulsed or CW lasers with the Gaussian beam profile. As long as the Gaussian profile is not diffused by the examined materials, this method can be used. However, by extending the classic Z-scan technique using a CCD beam profiler and analyzing the beam deviation from the Z axis, the NLO coefficients can be estimated even for a highly diffused Gaussian beam. The method proposed in this paper tracks the centroid position of the diffused beam and on this basis the two photon absorption coefficient β and the nonlinear refractive index n2 are obtained. The accuracy depends on the samples geometry and the CCD detector used. The new approach presented in this paper can be used in substitution to the classic technique for the materials which strongly diffuse the Gaussian beam.

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.

Influence of laser wavelength and beam profile on the coagulation depth in a soft tissue phantom model

In laser tissue soldering (LTS), a protein solution is thermally denatured and cross-linked to obtain a strong bond between tissues or tissue and a wound dressing. However, if the extension of the heat-affected zone is too large, wound healing is impaired by thermal tissue injuries. Therefore, heat input and coagulation depth have to be limited. We investigate the influence of wavelength and beam profile on coagulation depth using a soft tissue phantom in case of weakly (980 nm) and strong (1540 nm) absorbed laser radiation. The soft tissue phantom is doped with polystyrene (PS) beads to obtain similar scattering properties as natural tissue. The propagation of the laser radiation in the phantom is simulated by Monte–Carlo method and the optical penetration (OPD) depth calculated from isophotes. The simulation results are compared with the experimental determination of the coagulation volume. The results reveal that scattering effect of tissues on laser radiation increases the losses of a Gaussian beam profile laterally leading to a half-sphere coagulation volume. The depth profile of the coagulation follows approximately the intensity distribution of the laser beams as long as scattering effects are weak. As scattering effects become significant, as for 980-nm radiation, the intensity distribution of the laser beam in the tissue deviates from the original one, leading to different profile of the coagulation depth.

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.

Evaluation of heat conduction in a laser irradiated tooth with the three-phase-lag bio-heat transfer model

In this study, a dental short pulse laser with a Gaussian beam profile was applied normally to the top surface of a mineral organ i.e. the human tooth for a root canal therapy. A numerical method of finite difference is adopted to solve the time-dependent heat transfer equation. The real boundary conditions of thermal insulation on the sharp segment of the root canal and periodic heat flux on the top boundary of the tooth were applied. The comparison of a three-phase-lag (TPL) bio-heat transfer model with other heat transfer studies has shown that this new bio-heat model (TPL) could accurately predict the thermal behaviour of a non-homogeneous structure such as the human tooth. It was observed that decreasing of the pulse time, increasing of the pulse number and laser spot radius are the logical approaches for dental pulp ablation in comparison with increasing of laser intensity or increment of continuous irradiation time. Based on the results of this investigation, an optimum dental laser with heat flux of 5 W/cm2 that could make adequate temperature increments in the range of 5 °C to 8 °C inside the dental pulp after 30 s is suggested. The time of each pulse was 0.1 s, and eight times were iterated in the first ten seconds of treatment time while the laser radius was 3 mm.

Radiation-driven rotational motion of nanoparticles.

Focused synchrotron beams can influence a studied sample via heating, or radiation pressure effects due to intensity gradients. The high angular sensitivity of rotational X-ray tracking of crystalline particles via their Bragg reflections can detect extremely small forces such as those caused by field gradients. By tracking the rotational motion of single-crystal nanoparticles embedded in a viscous or viscoelastic medium, the effects of heating in a uniform gradient beam and radiation pressure in a Gaussian profile beam were observed. Changes in viscosity due to X-ray heating were measured for 42 µm crystals in glycerol, and angular velocities of 10-6 rad s-1 due to torques of 10-24 N m were measured for 340 nm crystals in a colloidal gel matrix. These results show the ability to quantify small forces using rotation motion of tracer particles.

Morphologies of femtosecond laser ablation of ITO thin films using gaussian or quasi-flat top beams for OLED repair

This study compares the ablation morphologies obtained with a femtosecond laser of both Gaussian and quasi-flat top beam profiles when applied to indium tin oxide (ITO) thin films for the purpose of OLED repair. A femtosecond laser system with a wavelength of 1030 nm and pulse duration of 190 fs is used to pattern an ITO thin film. The laser fluence is optimized for patterning at 1.38 J/cm2. The patterned ITO thin film is then evaluated through both optical microscope and atomic force microscope. Ablations with a square quasi-flat top beam are demonstrated using slits with varying x–y axes. With the Gaussian beam, the pattern width of the ablated area is shown to range from 9.17 to 9.99 μm when the number of irradiation pulse increases from one to six. In contrast, when slit control is used to obtain a quasi-flat top beam, the ablated pattern width remains constant at 10 μm, despite the increase in the number of pulse. The improved surface roughness is correlated with the quasi-flat top beam through measured Ra values. Furthermore, when using the Gaussian beam, the minimum resolution of the controllable ablation depth on the ITO thin film is found to be 60 nm. In contrast, when the quasi-flat top beam is used, the minimum ablation depth decreases to 40 nm.

Acousto-Optic Control of the Energy Profile of Laser Radiation

An acousto-optic shaper is developed for transformation of the Gaussian beam profile into almost rectangular profile. As distinct from lens devices, an acousto-optic device allows modification of the shape of the output beam in a fixed optical configuration using changes in radio control with relatively short response times of about 10 μs. The method is based on multibeam high-efficiency Bragg diffraction when the diffraction orders are overlapped with respect to angle. It is shown that the beam profile can be transformed into rectangular and more complicated profiles with an efficiency of no less than 80%. The experiments are performed using the TeO2 crystal.

High-power, high repetition rate, tunable, ultrafast vortex beam in the near-infrared

We report on experimental demonstration of high power, ultrafast, high repetition rate (RR) vortex beam source tunable in the near-IR wavelength range. Based on single-pass optical parametric generation of Yb-fiber laser of vortex order lp = 1 in a 50 mm long MgO doped periodically poled LiNbO3 crystal, the source produces signal beam in vortex profile of order ls = 1 across 1433–1553 nm. Additionally, the source produces broadband idler radiation tunable across 3379–4132 nm in the Gaussian beam profile. We observed that the vortex profile of the pump beam is always transferred to the signal beam due to the highest overlapping integral among the interacting beams and the idler maintains a Gaussian spatial profile owing to the conservation of orbital angular momentum in optical parametric processes. For a pump power of 4.72 W, the signal and idler beams have a maximum power of 1.7 W at 1509 nm and 0.48 W at 3625 nm respectively. The signal vortex beam has output pulses of width ∼637 fs at a RR of 78 MHz. The signal (idler) has a spectral width of 4.3 nm (129.5 nm) and a passive peak-to-peak power fluctuation better than 3% (1.1%) over 30 min, respectively.

Three dimensional characterization of laser ablation craters using high resolution X-ray computed tomography

Laboratory X-ray computed tomography is an emerging technology for the 3D characterization and dimensional analysis of many types of materials. In this work we demonstrate the usefulness of this characterization method for the full three dimensional analysis of laser ablation craters, in the context of a laser induced breakdown spectroscopy setup. Laser induced breakdown spectroscopy relies on laser ablation for sampling the material of interest. We demonstrate here qualitatively (in images) and quantitatively (in terms of crater cone angles, depths, diameters and volume) laser ablation crater analysis in 3D for metal (aluminum) and rock (false gold ore). We show the effect of a Gaussian beam profile on the resulting crater geometry, as well as the first visual evidence of undercutting in the rock sample, most likely due to ejection of relatively large grains. The method holds promise for optimization of laser ablation setups especially for laser induced breakdown spectroscopy.

Self-mode-locked AlGaInP-VECSEL

We report the mode-locked operation of an AlGaInP-based semiconductor disk laser without a saturable absorber. The active region containing 20 GaInP quantum wells is used in a linear cavity with a curved outcoupling mirror. The gain chip is optically pumped by a 532 nm laser, and mode-locking is achieved by carefully adjusting the pump spot size. For a pump power of 6.8 W, an average output power of up to 30 mW is reached at a laser wavelength of 666 nm. The pulsed emission is characterized using a fast oscilloscope and a spectrum analyzer, demonstrating stable single-pulse operation at a repetition rate of 3.5 GHz. Intensity autocorrelation measurements reveal a FWHM pulse duration of 22 ps with an additional coherence peak on top, indicating noise-like pulses. The frequency spectrum, as well as the Gaussian beam profile and the measured beam propagation factor below 1.1, shows no influence of higher order transverse modes contributing to the mode-locked operation.