Claude Leiner

Multiple interfacing between classical ray-tracing and wave-optical simulation approaches: a study on applicability and accuracy

In this study the applicability of an interface procedure for combined ray-tracing and finite difference time domain (FDTD) simulations of optical systems which contain two diffractive gratings is discussed. The simulation of suchlike systems requires multiple FDTD↔RT steps. In order to minimize the error due to the loss of the phase information in an FDTD→RT step, we derive an equation for a maximal coherence correlation function (MCCF) which describes the maximum degree of impact of phase effects between these two different diffraction gratings and which depends on: the spatial distance between the gratings, the degree of spatial coherence of the light source and the diffraction angle of the first grating for the wavelength of light used. This MCCF builds an envelope of the oscillations caused by the distance dependent coupling effects between the two diffractive optical elements. Furthermore, by comparing the far field projections of pure FDTD simulations with the results of an RT→FDTD→RT→FDTD→RT interface procedure simulation we show that this function strongly correlates with the error caused by the interface procedure.

Multi-scale simulation of an optical device using a novel approach for combining ray-tracing and FDTD

Optimizing the properties of optical and photonic devices calls for the need to control and manipulate light within structures of different length scales, ranging from sub-wavelength to macroscopic dimensions. Working at different length scales, however, requires different simulation approaches, which have to account properly for various effects such as polarization, interference, or diffraction: at dimensions much larger than the wavelength of light common ray-tracing techniques are conveniently employed, while in the (sub-)wavelength regime more sophisticated approaches, like the socalled finite-difference time-domain (FDTD) technique, are used. Describing light propagation both in the (sub-)wavelength regime as well as on macroscopic length scales can only be achieved by bridging between these two approaches. Unfortunately, there are no well-defined criteria for a switching from one method to the other, and the development of appropriate selection criteria is a major issue to avoid a summation of errors. Moreover, since the output parameters of one simulation method provide the input parameters for the other one, they have to be chosen carefully to ensure mathematical and physical consistency. In this contribution we present an approach to combine classical ray-tracing with FDTD simulations. This enables a joint simulation of both, the macro- and the microscale which refer either to the incoherent or the coherent effects, respectively. By means of an example containing one diffractive optical element (DOE) and macroscopic elements we will show the basic principles of this approach and the simulation criteria. In order to prove the physical correctness of our simulation approach, the simulation results will be compared with real measurements of the simulated device. In addition, we will discuss the creation of models in FDTD based on different analyze techniques to determine the dimensions of the DOE, as well as the impact of deviations between these different FDTD models on the simulation results.

A Simulation Procedure Interfacing Ray-Tracing and Finite-Difference Time-Domain Methods for a Combined Simulation of Diffractive and Refractive Optical Elements

A simulation procedure which enables integrated simulation of optical devices including both refractive and diffractive optical elements at different length scales is presented. The approach uses the Poynting vector to interface between a Ray-tracing and a finite-difference-time-domain tool for a step by step simulation of both the light propagation through a laser-written diffractive grating structure and its measurement by an appropriate setup. These simulated results are in great accordance with experimentally determined ones. Furthermore, the impact of parameter variations is analyzed and discussed in detail.

Optical design of freeform micro-optical elements and their fabrication combining maskless laser direct write lithography and replication by imprinting

Abstract. Today, freeform micro-optical structures are desired components in many photonic and optical applications, such as lighting and detection systems, due to their compactness, ease of system integration, and superior optical performance. The high complexity of a freeform structure’s arbitrary surface profile and the need for high throughput upon fabrication require sophisticated approaches for their integration into a manufacturing process. In this paper, we discuss a smart fabrication process of freeform micro-optical elements that ranges from their design by optical simulations to their cost-efficient fabrication by maskless laser direct write lithography (MALA) and replication from the as-fabricated master by imprinting. Aided by profilometry and optical microscopy, the fidelity of the fabricated freeform micro-optical elements to the design is characterized. Finally, the light intensity distribution on a target plane affected by the freeform micro-optical element illuminated with a light-emitting diode is determined and compared with the predicted one.

Manufacturing of freeform micro-optical elements by mask-less laser direct write lithography and replication by imprinting

Today, freeform micro-optical structures are desired components in many photonic and optical applications such as lighting and detection systems due to their compactness, ease of system integration and superior optical performance. The high complexity of a freeform structure’s arbitrary surface profile and the need for high throughput upon fabrication require novel approaches for their integration into a manufacturing process. For the fabrication of polymer freeform optics, in this contribution we discuss two principal technologies, mask-less laser direct write lithography (MALA) and replication from the as-fabricated master by imprinting. We show the high flexibility in design and rapid-prototyping of freeform optical microstructures that can be achieved by such an approach. First, the original structures known as masters are fabricated using MALA. Because of the specific requirements on shape and height (>50μm) of the microstructures, laser writing and photoresist processing have to be performed within a narrow range of fabrication parameters. Subsequently, UV-soft lithography based replication is used for serial production of the freeform micro-optical elements within a batch process. Aided by profilometry, optical microscopy and atomic force microscopy, the fidelity of the fabricated freeform microoptical elements to the design is characterised. Finally, the light intensity distribution on a target plane caused by the freeform micro-optical element illuminated with an LED is determined and compared with the predicted one.

Smart freeform optics solution for an extremely thin direct-lit application

Light-emitting diodes (LEDs) based lighting solutions offer many advantages like the huge potential for energy saving, long lifetime, high reliability and the compact size compared to incandescent light bulbs. Especially the last-mentioned aspect favors new concepts for the design and integration of light points and luminaires. In case of a direct-lit system for general lighting applications the LEDs are separated by a certain distance, arranged in a regular array and illuminate an out-coupling surface which is placed in a certain height above the LED array. One approach to fulfill the demand of a uniform luminance of the out-coupling surface is to replace the surface by a diffuser sheet. In order to allow for a flat luminaire design the distance of the out-coupling surface has to be modified synchronously with a variation of the distance between the LEDs. This means, in order to maintain the uniformity of the luminance the distance to height ratio (DHR) of the optical arrangement has to be kept constant. A (from the viewpoint of costs desirable) reduction of the number of the LEDs and as a consequence thereof an increase of the DHR value can be achieved by using additional optical elements like, e.g., freeform optics. However, this can cause additional design problems due to the size of such optical elements. In this contribution we discuss a smart design of an extremely flat direct-lit luminaire for general lighting applications. The main advantage of this concept is the increased DHR ratio compared to diffuser sheet only-approaches and a smaller thickness of the whole set-up compared to common freeform approaches. For this demand we have designed very thin freeform optical elements with a maximal height of 75 μm that allow to maintain a uniform illumination in direct-lit applications using an LED-array with a comparably large distance between the individual LEDs. The presented design concept in addition emphasizes the use of cost-effective manufacturing methods like grey scale laser lithography for mastering and roll-to-roll processing for large area manufacturing of these optical elements.

Thin direct-lit application for general lighting realized by freeform micro-optical elements

ABSTRACT Common LED based direct-lit luminaires for general lighting applications are using LEDs as light sources, which are placed with a certain distance in a regularly arranged array. In order to achieve a homogenous light distribution a diffuser sheet has to be placed on the out-coupling side in a certain height above the LED array. The required height is determined by the distance between the LE Ds. For this so-called DHR (distance (of the LEDs) to height (of the diffuser sheet placement) ratio) values of 1 are hardly achievable. To overcome this limitation additional optical elements like freeform lenses are necessary. In this contribution we discuss a smart design concept for an extremely flat direct-lit lighting system. It is characterized by an improved distance (LEDs) to height (diffuser sheet) ratio compared to diffuser sheet only-approaches and a smaller thickness compared to common freeform approaches. For this demand we designed very thin freeform lenses with a maximal height of 75 µm that allow to maintain a uniform illumination in a flat direct-lit backlight using an LED-array with a comparably large distance be tween the individual LEDs. The concept emphasizes the use of mask-less laser direct write lithography for the cost-effective fabr ication of the thin freeform micro-lens array. Keywords: Free-form optics, Optical simulation, Solid State Lighting

Improving the effectiveness of photovoltaic devices by light guiding optical foils

A photovoltaic device comprising of areas which are partly covered by solar cells and a light guiding film is investigated. In particular results on the feasibility of combined daylighting and photovoltaic energy generation are presented. Optical simulations have been conducted for a device-design optimized to redirect most of perpendicular impinging light rays onto photovoltaic areas. Two application cases are investigated for integrating the photovoltaic device into windows and/or glazings in middle (northern) latitudes. The first application case deals with an overhead glazing and the second deals with a window integrated in a roof tilted by 30° towards south. For the latter case encouraging results have been derived. In particular it is calculated that during summer time more than 70% of the direct sunlight is absorbed by photovoltaic areas and less than 10% is transmitted. Consequently, effective shading in summer against direct sunlight can be achieved and most of the shaded solar irradiation can be used for photovoltaic energy conversion. In contrast, in winter time about 40% of the direct sunlight is transmitted through the device and enables decent daylighting.

Multiscale optical simulation settings: challenging applications handled with an iterative ray-tracing FDTD interface method

We show that with an appropriate combination of two optical simulation techniques-classical ray-tracing and the finite difference time domain method-an optical device containing multiple diffractive and refractive optical elements can be accurately simulated in an iterative simulation approach. We compare the simulation results with experimental measurements of the device to discuss the applicability and accuracy of our iterative simulation procedure.

Laser-assisted manufacturing of micro-optical volume elements for enhancing the amount of light absorbed by solar cells in photovoltaic modules

The laser-generation of micro-optical volume elements is a promising approach to decrease the optical shadowing of front side metal contacts of solar cells. Focusing a femtosecond laser beam into the volume of the encapsulation material causes a local modification its optical constants. Suchlike fabricated micro-optical elements can be used to decrease the optical shadowing of the front side metallization of c-Si solar cells. Test samples comprising of a sandwich structure of a glass sheet with metallic grid-lines, an Ethylene-vinyl acetate (EVA) encapsulant and another glass sheet were manufactured in order to investigate the optical performance of the volume optics. Transmission measurements show that the shadowing of the metalling grid-lines is substantially decreased by the micro-optical volume elements created in the EVA bulk right above the grid-fingers. A detailed investigation of the optical properties of these volume elements was performed: (i) experimentally on the basis of goniometric measurements, as well as (ii) theoretically by applying optical modelling and optimization procedures. This resulted in a better understanding of the effectiveness of the optical volume elements in decreasing the optical shadowing of metal grid lines on the active cell surfaces. Moreover, results of photovoltaic mini-modules with incorporated micro-optical volume elements are presented. Results of optical simulation and Laser Beam Induced Current (LBIC) experiments show that the losses due to the grid fingers can be reduced by about 50%, when using this fs-laser structuring approach for the fabrication of micro-optical volume elements in the EVA material.