Diffractive phase elements for beam shaping: a new design method

Design and 3D printing of the Powell lens for sub-terahertz imaging

The aim behind this work is to design and manufacture a beam shaping lens for active terahertz imaging systems that boosts their performance in terms of sensitivity and image quality. The proposed beam shaper is based on an adaptation of the original optical Powell lens, where a collimated Gaussian beam is converted into a uniform flattop intensity beam. The design model for such a lens was introduced, and its parameters were optimized by a simulation study conducted using COMSOL Multiphysics software. The lens was then fabricated using a carefully chosen material [polylactic acid (PLA)] through a 3D printing process. The manufactured lens was implemented in an experimental setup to validate its performance using a continuous-wave sub-terahertz source around 100 GHz. Experimental results demonstrated a high-quality flattop beam maintained along the propagation path, which is highly recommended for terahertz and millimeter-wave active imaging systems to produce high-quality images.

A Hybrid Diffractive Optical Element Design Algorithm Combining Particle Swarm Optimization and a Simulated Annealing Algorithm

With the rapid development of computer hardware and the emergence of the parallel calculation of diffraction fields, a breakthrough has been made in terms of the limitation of the unacceptable amount of computational cost to design diffractive optical elements (DOEs), and more accurate global search algorithms can be introduced to the design of complex DOEs and holographic projections instead of traditional iterative algorithms. In this paper, a hybrid algorithm which combines particle swarm optimization (PSO) with a simulated annealing (SA) algorithm is proposed for the designing of DOEs and projecting holographic images with less noise. PSO is used to reduce the invalid disturbance in SA, and SA can jump out from local extreme points to find the global extreme points. Compared with the traditional Gerchberg–Saxton (GS) algorithm, the simulation and experimental results demonstrate that the proposed SA–PSO hybrid algorithm can improve uniformity by more than 10%.

Generating uniform irradiance in the Fresnel region by quasi-optical beam shaping of a millimeter-wave source

In this paper, we report on the design, simulation and measurement of a flat-top beam shaper (FBS) lens for a millimeter-wave (mm-wave) antenna coupled source. The beam shaper is designed to transform the pseudo-Gaussian beam of the mm-wave source into a flat-top beam of sharp roll-off at a specific distance in the Fresnel region. Firstly, relying on the geometrical optics principles, we propose an analytical formulation for the design of lens profiles. Next, a simple optimization method based on the full-wave simulation of the lens and the mm-wave antenna is suggested to tune the analytically extracted lens profile. The optimized lens profile is then studied to assess its performance sensitivity to various deviations. Finally, a prototype of the proposed FBS lens is fabricated and measured. As an example study, the radiated beam of a 100GHz conical horn antenna is shaped into a flat-top beam of radius 40 cm with a sharp skirt and low ripples at the distance of 3 meters. The radius of the generated flat-top beam is also shown to be easily tunable by displacing the FBS lens with respect to the horn antenna. The proposed procedure for the design of FBS lens results in a high-quality flat-top beam, which is promising for the applications where a uniform irradiance within the field-of-view with negligible spillover is of high importance. Terahertz (THz) and mm-wave imaging systems are potential candidates in this regard.

A Computational Design Framework for Efficient, Fabrication Error-Tolerant, Planar THz Diffractive Optical Elements

We demonstrate ultra-thin (1.5-3λ₀), fabrication-error tolerant efficient diffractive terahertz (THz) optical elements designed using a computer-aided optimization-based search algorithm. The basic operation of these components is modeled using scalar diffraction of electromagnetic waves through a pixelated multi-level 3D-printed polymer structure. Through the proposed design framework, we demonstrate the design of various ultrathin planar THz optical elements, namely (i) a high Numerical Aperture (N.A.), broadband aberration rectified spherical lens (0.1 THz-0.3 THz), (ii) a spectral splitter (0.3 THz-0.6 THz) and (iii) an on-axis broadband transmissive hologram (0.3 THz-0.5 THz). Such an all-dielectric computational design-based approach is advantageous against metallic or dielectric metasurfaces from the perspective that it incorporates all the inherent structural advantages associated with a scalar diffraction based approach, such as (i) ease of modeling, (ii) substrate-less facile manufacturing, (iii) planar geometry, (iv) high efficiency along with (v) broadband operation, (vi) area scalability and (vii) fabrication error-tolerance. With scalability and error tolerance being two major bottlenecks of previous design strategies. This work is therefore, a significant step towards the design of THz optical elements by bridging the gap between structural and computational design i.e. through a hybrid design-based approach enabling considerably less computational resources than the previous state of the art. Furthermore, the approach used herein can be expanded to a myriad of optical elements at any wavelength regime.

Composite method for precise freeform optical beam shaping

We present a composite freeform surface construction method for creating a high-accuracy irradiance distribution from a given incident beam under the influence of diffraction. The main idea is that we first determine a fully continuous freeform surface estimate by solving a standard Monge-Ampère equation and then refine it using an iterative Fourier-transform algorithm associated with over-compensation. Although this method can only be implemented in the paraxial approximation, it can significantly simplify the design and is applicable to many examples that fulfill this restriction. The resulting optical surface, unwrapped from the final phase, is an unusual discontinuous freeform surface that can produce very promising performances in terms of surface roughness and irradiance accuracy.

Precise design of two-dimensional diffractive optical elements for beam shaping

Diffractive optical elements (DOEs) for beam shaping are widely used in many fields, and there are many kinds of optimization algorithms to design the DOEs for beam shaping. However, only the intensity distribution of the selected sampling points is controlled by these optimization algorithms. The intensity distribution of other points on the output plane is always far away from the ideal distribution. The reason is that the sampling interval on the output plane is not small enough. In this paper, a new modified GS algorithm is presented with a small enough sampling interval on the output plane. A two-dimensional DOE for beam shaping is designed, and the simulation results and the experimental results demonstrate the good performance of this algorithm.

Compact flattop laser beam shaper using vectorial vortex

In this paper, we demonstrate a compact flattop beam shaper to realize two-dimensional flattop focus through generating a second order full Poincaré beam. Liquid crystal material is used in the device as the voltage-dependent birefringent material to provide appropriate phase retardation modulation. The beam shaper is fabricated and tested. Experimental results show that high quality flattop profiles can be obtained with steep edge roll-off. The tolerance of different input beam sizes of the beam shaper is also verified in the experimental demonstration.

Replication of micro-optical elements with continuous relief by ultraviolet embossing with thiol-ene-based resist

We propose in this paper to use the delayed gel point character of thiol-ene-based resist to reduce the influence of polymerization shrinkage during the replication of micro-optical elements with continuous relief by UV embossing. Experiment results indicate that this resist can be used to bring down the fabrication error to less than 2% in the vertical direction at a proper thickness of the residual resist, which is far less than that of traditional acrylate-based resist. This resist can also be used to transfer continuous relief into a fused silica substrate through reactive ion etching because of its good etching resistance.

Flattop focusing with full Poincaré beams under low numerical aperture illumination

In this Letter, we propose and experimentally demonstrate the generation of second-order full Poincaré beams and its applications in two-dimensional flattop beam shaping with spatially variant polarization under low NA focusing condition. High-quality flattop profiles with steep edge roll-off can be obtained with this technique. The experiment results also demonstrate that flattop profile can be maintained for different input beam sizes by conveniently rotating a half-wave plate.

Experimental demonstration of Generalized Phase Contrast based Gaussian beam-shaper

We report the first experimental demonstration of Gaussian beam-shaping based on the Generalized Phase Contrast (GPC) approach. We show that, when using a dynamic spatial light modulator (SLM), this approach can rapidly generate arbitrarily shaped beams. Moreover, we demonstrate that low-cost binary-phase optics fabricated using photolithography and chemical etching techniques can replace the SLM in static and high power beam shaping applications. The design parameters for the binary-phase elements of the module are chosen according to the results of our previously conducted analysis and numerical demonstrations [Opt. Express 15, 11971 (2007)]. Beams with a variety of cross-sections such as circular, rectangular and square, with near flat-top intensity distributions are demonstrated. GPC-based beam shaping is inherently speckle-free and the shaped beams maintain a flat output phase. The non-absorbing components used in this beam-shaping approach have a high-damage-threshold and are thus ideally suited for high power applications.

Polar nephelometer based on a rotational confocal imaging setup

Rapid measurement of the angular distribution of light scattered by particles, the scattering phase function, is achieved by using a new type of polar nephelometer, a device for measuring the angular scattered-light intensity distribution, with a high angular precision and across many orders of magnitude of intensity. The design offers high-speed measurements and avoids many of the problems often associated with traditional goniometers when they are used for measurements of light scattering from small particles or biological cells in suspension. Our system relies on confocal imaging of the test space with off-axis parabolas, using a rotating mirror to scan the angular field of view at the second focus of a pair of conjugated parabolic mirrors, with the test space located at the first focus. The angular resolution of the system is limited mainly by the data-acquisition sampling frequency. In this proof-of-principle demonstration the system performs multiple scans of a 55 deg field of view in a very short time (<1 s). To significantly increase the signal-to-noise ratio, we averaged the successively acquired scans during this time. Polystyrene spheres dispersed in water at low concentrations were used to test the system. The scattering patterns obtained were found to be in good agreement with Mie theory calculations.

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.

Genetic local search algorithm for optimization design of diffractive optical elements

We propose a genetic local search algorithm (GLSA) for the optimization design of diffractive optical elements (DOE's). This hybrid algorithm incorporates advantages of both genetic algorithm (GA) and local search techniques. It appears better able to locate the global minimum compared with a canonical GA. Sample cases investigated here include the optimization design of binary-phase Dammann gratings, continuous surface-relief grating array generators, and a uniform top-hat focal plane intensity profile generator. Two GLSA's whose incorporated local search techniques are the hill-climbing method and the simulated annealing algorithm are investigated. Numerical experimental results demonstrate that the proposed algorithm is highly efficient and robust. DOE's that have high diffraction efficiency and excellent uniformity can be achieved by use of the algorithm we propose.

Design and optimization of an irradiance profile-shaping system with a genetic algorithm method

We develop a genetic algorithm (GA) optimization method and use it in the design of a refractive-beam profile-shaping system. In this application, we employ the GA to determine the shape of one surface of the primary beam profile-shaping element in our system. The GA is instructed to vary the shape of this surface such that the output intensity profile is flat on a spherical surface some distance away. The GA does this while insuring that only a specified area of the output surface is illuminated. The calculation of the intensity profile is based on geometrical optics and is accomplished exclusively through ray tracing, giving this method broad applicability.

Beam shaping and its solution with the use of an optimization method

We present an exact mathematical description of beam shaping and indicate that a rigorous solution does not exist: only an optimal solution can be found. An optimization method is proposed to search for the solution. The simulation results for an example are given in detail.