Gaussian Irradiance Profile Research Articles

Simulation of laser-induced retinal thermal injuries for non-uniform irradiance profiles and the applicability of the laser safety standard

Significance: A validated biophysical computer model simulating retinal thermal damage thresholds is used to investigate elongated retinal images. The International Commission on Non-Ionizing Radiation Protection Guideline and the laser safety standard IEC 60825-1:2014 include a method for averaging non-uniform extended sources, however, there are no studies that have examined the applicability in detail. Our study represents a method that can also support future research in the field of eye safety. Aim: As there is currently no experimental data available for non-uniform irradiance profiles, the calculation procedure given in the laser safety standard is derived from symmetric retinal images. We aim to verify this calculation procedure for such profiles on the retina in the thermal hazard regime. Approach: A three-dimensional computer model, which solves the heat transfer equation and the Arrhenius equation describing the denaturation of the proteins in the retina, is used to simulate the threshold values for the retinal thermal injury. Three different non-uniform irradiance profiles, elliptical Gaussian, elliptical top-hat, and rectangular top-hat distributions, are investigated for a wavelength of 530 nm. The profiles are varied in their sizes and simulated for different single-pulse durations. By applying the laser safety standard, the maximum allowed energies are calculated and divided by the corresponding threshold values to obtain the reduction factor (RF) which is a crucial parameter. Results: Due to the thermal behavior in the retinal tissues, the Gaussian irradiance profiles yield larger threshold values than both top-hat profiles. Furthermore, the ratio between the threshold values and the maximum allowed energies are found to be the lowest for the Gaussian profiles. Conclusion: The simulated retinal thermal injury thresholds for the three investigated non-uniform irradiance profiles show larger RFs than the minimum RF found for symmetric profiles. This supports the applicability of the evaluation scheme of the laser safety standard for non-uniform retinal images.

High concentration thin profile solar concentrator utilizing toroidal confocal relay

A thin profile solar concentrator is designed based on the novel off-axis toroidal confocal relay design principle. The solar beam is collected, converged, re-collimated, and relayed to the solar cell simultaneously by the radially cascaded dielectric concentrator modules. This concentrator has properties of high concentrating ratio, thin aspect ratio, zero back focal distance, Gaussian irradiance profile, and single molding manufacture capability. In the Monte Carlo ray tracing simulation shown, one of the proposed concentrator designs has aspect ratio of 1/8 and achieves over 43100X peak concentration with the theoretical thermodynamic concentration limit, within the dielectric, of 71290X.

Comprehensive numerical design approach for refractive laser beam shapers to generate annular irradiance profiles

A refractive laser beam shaper typically consists of either two plano-aspheric lenses or one thick lens with two aspherical surfaces. Ray mapping is a general optical design technique for irradiance reshaping based on geometric optics. Although ray mapping, in principle, allows generating any rotational-symmetric irradiance profile, in the literature this technique is mainly used to transform a Gaussian irradiance profile to a uniform rotational-symmetric profile. For more complex profiles, especially with low intensity in the inner region (such as annular profiles), a high sampling rate is required to ensure an accurate calculation of the surfaces. In practice, the high sampling rate increases the numerical effort to calculate the aspherical surfaces and the simulation time to verify the design considerably. In this work, we evaluate different sampling approaches and surface construction methods. This allows us to propose and demonstrate a comprehensive numerical approach to efficiently design refractive laser beam shapers to generate rotational-symmetric collimated beams with annu- lar irradiance profiles. Ray tracing analysis for several annular irradiance profiles demonstrates the excellent performance of the designed lenses and the versatility of our design procedure. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of

Temporal widening of a short polarized pulse focused with a high numerical aperture aplanatic lens

We present a theoretical analysis of the field distribution in the focal plane of a dispersionless, high numerical aperture (NA) aplanatic lens for an x-polarized short pulse. We compare the focused pulse spatial distribution with that of a focused continuous wave (CW) field and its temporal distribution with the profile of the incident pulse. Regardless of the aberration free nature of the focusing aplanatic lens, the temporal width of the focused pulse widens considerably for incident pulses with durations on the order of a few cycles due to the frequency-dependent nature of diffraction phenomena, which imposes a temporal diffraction limit for focused short pulses. The spatial distribution of the focused pulse is also affected by this dependence and is altered with respect to the diffraction limited distribution of the CW incident field. We have analyzed pulses with flat top and Gaussian spatial irradiance profiles and found that the focused pulse temporal widening is less for the Gaussian spatial irradiance pulse, whereas the spatial distribution variation is similar in both cases. We present results of the focused pulsewidth as a function of the NA for the two spatial irradiance distributions, which show that the Gaussian irradiance pulse outperforms the flat top pulse at preserving the incident pulse duration.

Enhanced Efficiencies for High-Concentration, Multijunction PV Systems by Optimizing Grid Spacing under Nonuniform Illumination

The design of a triple junction solar cell’s front contact grid can significantly affect cell conversion efficiency under high concentration. We consider one aspect of grid design, choosing a linear grid within a distributed resistance cell model to optimize finger spacings at concentrations between 500 and 2500 suns under uniform and nonuniform illumination. Optimization for maximum efficiency under Gaussian irradiance profiles is done by SPICE analysis. Relative to the optimized uniform illumination designs, we find enhancements of 0.5% to 2% in absolute efficiencies for uniform spacing. Efficiency enhancement with nonuniform spacing under nonuniform illumination is also evaluated. Our model suggests that, at lower concentrations (<1000 suns), the penalty for using uniformly spaced fingers instead of nonuniformly spaced fingers is <0.1%. However, at a concentration of 2500 suns the penalty increases to 0.3%. Thus, relative to a uniform irradiance optimization, an absolute efficiency increase of 2.3% can be attained for an optimized nonuniform spacing given the Gaussian irradiance profile under consideration.

Maximum-likelihood estimation of a laser system pointing parameters by use of return photon counts

A maximum-likelihood estimator used to determine boresight and jitter performance of a laser pointing system has been derived. The estimator is based on a Gaussian jitter model and uses a Gaussian far-field irradiance profile. The estimates are obtained using a set of return shots from the intended target. An experimental setup with a He-Ne laser and steering mirrors is used to study the performance of the proposed method. Both Monte Carlo simulations and experimental results demonstrate excellent performance of the estimator. Our study shows that boresight estimation is more challenging than jitter estimation when both quantities are estimated. Furthermore, their estimation performance improves with an increase in the number of shots. The experimental results are found to agree well with the simulation results.

Investigation of Performance Degradation of Shack Hartmann Wavefront Sensing Due to Pupil Irradiance Profile

Wavefront sensing using a Shack-Hartmann sensor has been widely used for estimating wavefront errors or distortions. The sensor combines the local slopes, which are estimated from the centroids of each lenslet image, to give the overall wavefront reconstruction. It was previously shown that the pupil-plane irradiance profile effects the centroid estimation. Furthermore, a previous study reported that the reconstructed wavefront from a planar wavefront with a Gaussian pupil irradiance profile contains large focus and spherical aberration terms when there is a focus error. However, it has not been reported yet how seriously the pupil irradiance profiles, which can occur in practical applications, effect the sensing errors. This paper considered two cases when the irradiance profiles are not uniform: 1) when the light source is Gaussian and 2) when there is a partial interference due to a double reflection by a beam splitting element. The images formed by a Shack-Hartmann sensor were simulated through fast Fourier transform and were then supposed to be detected by a noiseless CCD camera. The simulations found that sensing errors, due to the Gaussian irradiance profile and the partial interference, were found to be smaller than RMS <TEX>${\lambda}/50$</TEX> when <TEX>${\lambda}$</TEX> is <TEX>$0.6328\;{\mu}m$</TEX>, which can be ignored in most practical cases where the reference and test beams have the same irradiance profiles.

Laser Beam Shaping

Laser beam shaping is the process of redistributing the irradiance and phase of a beam of optical radiation. This process of controlling optical beams is an enabling technology, which is used in a number of sectors of scientific, engineering, and industrial R&D. The shape of a laser beam generally refers to its irradiance profile, while the phase of a beam generally affects its propagation characteristics. Earlier work of Frieden and Kreuzer during the 1960s articulated well the goals of some contemporary laser beam shaping applications. Namely, geometrical optics was used to determine the configuration of a twoelement optical system that would transform an input plane wave with a Gaussian irradiance profile into an output plane wave with uniform irradiance. For some contemporary applications, physical optics must be used for the optical design of the laser beam shapers. A general presentation of the theory and techniques of laser beam shaping is given in a book edited by Dickey and Holswade. Laser beam shaping has become an important component of many laser-based applications, such as materials processing, medical applications, lithography, optical data storage, laser printing, isotope separation, optical data processing, and laboratory research. The first conference on laser beam shaping was held in 2000 at the Annual Meeting of the SPIE. This Laser Beam Shaping conference included a discussion of the fundamental limits of any beam shaping technique and the foundation for successful beam shaping design, including geometric techniques and optimization-based techniques. Applications included high-power laser fiber injection, UV and deep-UV homogenizers, micromachining in the electronics industry, and beam shaping techniques for laser printing. The Laser Beam Shaping II conference continued to bring together both workers in the field and potential users of the technology. Theory and design was again addressed, but the papers were primarily in the area of de-

Laser beam shaping with polarization-selective diffractive phase elements

A new scheme for converting a Gaussian irradiance profile beam on the input plane into a uniform irradiance profile beam on the output plane is presented based on polarization-selective diffractive phase elements. The relevant elements were designed by use of the simulated annealing method. The simulation design shows that the shaping quality is substantially improved and is much better than that obtained with traditional diffractive phase elements.

A Study on the Vision Sensor Using Scanning Beam for Welding Process Automation

The vision sensor which is based on the optical triangulation theory with the laser as an auxiliary light source can detect not only the seam position but the shape of seam. In this study, a vision sensor using the scanning laser beam was investigated. To design the vision sensor which considers the reflectivity of the sensing object and satisfies the desired resolution and measuring range, the equation of the focused laser beam which has a Gaussian irradiance profile was firstly formulated, Secondly, the image formaing sequence, and thirdly the relation between the displacement in the measuring surface and the displacement in the camera plane was formulated. Therefore, the focused beam diameter in the measuring range could be determined and the influence of the relative location between the laser and camera plane could be estimated. The measuring range and the resolution of the vision sensor which was based on the Scheimpflug's condition could also be calculated. From the results mentioned above a vision sensor was developed, and an adequate calibration technique was proposed. The image processing algorithm which and recognize the center of joint and its shape informaitons was investigated. Using the developed vision sensor and image processing algorithm, the shape informations was investigated. Using the developed vision sensor and image processing algorithm, the shape informations of the vee-, butt- and lap joint were extracted.

Analysis of probe volume effects associated with laser-saturated fluorescence measurements

Time-dependent and steady-state lateral fluorescence intensities are obtained analytically as a function of the saturation parameter along the laser axis for Gaussian and quasi-Lorentzian irradiance profiles. The sensitivity of these fluorescence intensities to quenching is compared with the sensitivity of the center-line fluorescence emission. Experimental saturation curves are analyzed similarly and compared to the steady-state predictions. The results show that the desired insensitivity of laser-saturated fluorescence (LSF) measurements to quenching is adversely affected by the fractional laser power in the wings of the transverse irradiance distribution. Ambiguities in the analysis of LSF measurements arising from these effects can be essentially eliminated by conducting the analysis with respect to the center-line fluorescence emission as opposed to the fluorescence intensity.

Effect of gaussian irradiance profile on the switching characteristics and the bit size of the fe/pc memory device

In the ferroelectric/photoconductor (FE/PC device, the effect of a gaussian irradiance profile is to give rise to a variable series resistance, in series with the ferroelectric. Low and high frequency hysteresis loops and the FE-switching transients are obtained as a function of series resistance for pulses of fixed amplitude and variable pulse width. Using a four-quadrant plot, the spatial distribution of the charge switched is estimated. Bit size is defined as the distance between the points where the switched polarization drops to half the amount of polarization switched at the center. The bit sizes are calculated for different photoconductor sensitivities and for different drive pulses. The sublinear lux-ampere characteristic of the photoconductor combined with the gaussian irradiance profile causes the recorded bit size to be larger than the light beam diameter.

Characteristic Parameters of a Beam with a Gaussian Irradiance Distribution

This describes a simple method to find two important characteristic parameters of a beam with a Gaussian irradiance profile—the position of the plane of the beam waist and the value of ω0 in that plane. It is necessary to observe the positions of maximum or minimum irradiances along the beam axis in the field diffracted by a circular aperture. These positions occur at places where the direct geometric-optical wave is in phase or out of phase with the boundary-diffraction wave.

Mixing Partially Coherent Fields with Gaussian Irradiance Profiles; Optimization Criteria

A detailed analysis of the average signal-to-noise ratio (SNR) resulting from the mixing or heterodyning of partially coherent, collinear beams with gaussian irradiance profiles is undertaken. It is shown that SNR can be expressed as a function of the beam cross sections and coherence areas and the area of the photodetector. The dependence of SNR on each of these parameters is examined. The previously observed saturation phenomenon, as well as coherence suppression and degradation effects, are observed and explained theoretically. Optimum values of the local oscillator beam size and coherence area are derived which generally produce the best SNR for a given set of signal beam parameters.