Specific Beam Shapes Research Articles

Micro-diffractive optical element arrays for beam shaping

We describe the design of Fourier type array generators and beam shapers as periodic configurations of refractive-diffractive optical elements in microscale to provide specific beam shaping and imaging functionaliies. We investigate how the addition of micro-nanostructures to regular microstructure arrays enables new degrees of freedom for the design of micro-optical systems, in combination with adapted fabrication techniques yields a better optical performance and leads to enhancement of the array concept, uniformity and efficiency.

Efficient molecule discrimination in electron microscopy through an optimized orbital angular momentum sorter

We consider the problem of discriminating macromolecular structures in an electron microscope, through a specific beam shaping technique. Our approach is based on maximizing the which-molecule information extracted from the state of each electron. To this aim, the optimal observables are derived within the framework of quantum state discrimination, which allows one to fully account from the quantum character of the probe. We simulate the implementation of such optimal observable on a generalized orbital angular momentum (OAM) sorter and benchmark its performance against the best known real space approach.

Effect of an input beam’s shape and curvature on the nonlinear effects in graded-index fibers

We present a general framework capable of describing the nonlinear propagation of pulsed optical beams of arbitrary shapes and phase fronts inside a graded-index (GRIN) fiber. The main assumption made is that the spatial self-imaging features of the beam are not affected by the temporal evolution of optical pulses. A propagation kernel known from the work done in the 1970s is used to obtain a distance-dependent nonlinear coefficient that captures all spatial effects within an effective nonlinear Schrödinger equation. We consider three specific beam shapes (Gaussian, circular, and square) to study the impact of the shape, position, and curvature of optical beams on the complex spatiotemporal dynamics specific to GRIN fibers. In particular, we focus on the impact of an input beam’s shape on the modulation-instability sidebands and the generation of multiple dispersive waves when higher-order solitons form inside a GRIN fiber. The results of our numerical analysis indicate that for beam widths chosen to yield the same value of the effective mode area at the input end of the fiber, the nonlinear effects are pronounced considerably when a Gaussian beam is launched into the fiber. We also found that even though the self-imaging period is doubled when an off-centered Gaussian beam is launched into a GRIN fiber, it does not affect the nonlinear evolution because the effective beam area still maintains the same periodicity, as long as the shift in the beam’s center is not so large that it does not remain confined to the fiber’s core.

Ray mappings and the weighted least action principle

Two basic problems in optics are presented. The solutions to both problems are formulated in terms of the associated ray mappings. An alternative formulation based on a weighted sum of the actions along the rays is derived. Existence of solutions is established via the Weighted Least Action Principle. Numerical methods for computing the ray mappings are discussed. Finally, we demonstrate the theoretical considerations by presenting complete solutions to a phase retrieval problem and to a specific beam shaping lens design.

Optimisation of arbitrary light beam generation with spatial light modulators

Phase only spatial light modulators (SLMs) have become the tool of choice for shaped light generation, allowing the creation of arbitrary amplitude and phase patterns. These patterns are generated using digital holograms and are useful for a wide range of applications as well as for fundamental research. There have been many proposed methods for optimal generation of the digital holograms, all of which perform well under ideal conditions. Here we test a range of these methods under specific experimental constraints, by varying grating period, filter size, hologram resolution, number of phase levels, phase throw and phase nonlinearity. We model beam generation accuracy and efficiency and show that our results are not limited to the specific beam shapes, but should hold for general beam shaping. Our aim is to demonstrate how to optimise and improve the performance of phase-only SLMs for experimentally relevant implementations.

Numerical sensitivity analysis of single pulse laser welding with a C-shaped beam

Even though Gaussian and top-hat beam profiles are suitable for most laser welding applications, for certain cases other beam distributions can be favored in terms of weld quality or performance. One promising method to generate a tailored beam shape is diffractive optical elements. A numerical model on the temperature field generated by specific beam shapes is therefore under development to iteratively identify desired beam shapes for specific applications. The present study is based on two thin steel sheets that are conduction welded in a lap joint mode by a C-shaped single laser pulse. The main aim is to ensure a specified weld width along the C-weld shape at the overlap interface between the two sheets in a robust manner. The sensitivity of main criteria like the interface weld width and phase changes at the workpiece top and bottom is studied and discussed in a systematic manner by applying a numerical heat transfer model for various parameters and conditions.

SU-E-T-71: A Time Dependent Model of a Passive Scattering Proton Therapy Nozzle Using TOPAS

Purpose: To model and validate a commercial passive scattering proton therapy nozzle using the time dependent features of the TOPAS Monte Carlo toolkit. Methods: The geometry and material composition of a commercial passive scattering nozzle were modeled in TOPAS based on specifications found in the literature (Smith, et al. 2009, Med. Phys. 36:4068; Arjomandy et al. 2009, Med. Phys. 36:2269). Energy specific beam shaping devices including range modulator wheels (RMWs) and second scatterers were implemented in TOPAS. Simulations were performed using the time features of TOPAS, which allow geometry update during a single Monte Carlo simulation. The time features allowed us to simulate the rotation of the RMWs to obtain a full spread‐out Bragg peak (SOBP) in a single run. Initial parameters of the beam such as initial energy spread, beam spot size and angular spread have large uncertainties and therefore they were considered free parameters of our model. By varying these parameters, simulated dose distributions were matched as closely as possible to measured ones. We compared simulation and measurement results of normalized dose as a function of depth for 4 and 8 cm SOBPs and 140 and 250 MeV beam energies. Results: In the buildup and SOBP regions, simulations and measurements agreed within 3.6%. Ranges agreed within 1.3 and 0.5 mm for 140 and 250 MeV, respectively. Conclusions: TOPAS has the potential to accurately model passively scattered therapeuticproton beams. Adjustments of the source parameters and physics list are needed to further improve the agreement between our simulations and measurements. Once validated, our TOPAS model may be used to more accurately perform retrospective studies of the impact of physicalparameters, including linear energy transfer, on patient outcome. The model may also be used to investigate the response of detectors to therapeuticproton beams. Natural Sciences and Engineering Research Council of Canada and Ontario Graduate Scholarship Program, Ontario Ministry of Training, Colleges and Universities

Chaotic and integrable reflective tube shapes for anamorphic, adiabatic transformation of diffuse light

Reflective light-guiding tubes having chaotic and integrable cross-sectional shapes are investigated. The applicability of chaotic billiard shapes as cross-sections versus integrable ones for specific beam shaping purposes is demonstrated. Three important cases are considered: homogenizing, anamorphic collimation and concentration of diffuse light.