Freeform optics are used to create complex application-adapted illuminance distributions due to their high number of adjustable degrees of freedom. As with conventional optics, this can result for some applications in large optics that exceed the available installation space or lead to high production costs. While the use of Fresnel lenses is common in such cases for conventional optics, the Fresnelization step in the design process of freeform optics is more complex due to the lack of rotational symmetry and only done for specific cases. Therefore, this paper presents a method to examine different segmentation strategies on freeform lenses and to optimize Fresnelization parameters. In this work, the method will be demonstrated using a head-up display (HUD) as example, in which the combination of a Fresnel lens and a display creates a moiré pattern in the illuminance distribution. In order to accelerate the simulation of complex Fresnel freeform optics considerably, a special ray tracing algorithm is developed, which takes advantage of the segmentation characteristics. The simulation approach of the illuminance distribution and the optimization of the Fresnelization are shown and discussed on a lens with about 300 segments visibly reducing the moiré pattern.
further shape or diffuse the light distribution (e.g. in non-imaging luminaires or automotive headlights). While the
geometry can be described in a parametric form by mapping the micro-optical features onto an underlying smooth
freeform surface, ray-tracing an optical system composed of NURBS or polynomial B-spline surfaces for each
optimization step can be costly.
Designing freeform optical surfaces with a large number of degrees of freedom has been a field of extensive research and development. Several design methods have been proposed. Starting point in the design process often is an idealized light source that has zero étendue (e.g. point source or collimated light). With this assumption the solution is unique and corresponds to the solution of an equation of Monge-Ampère type. We propose a method to solve the Monge-Ampère equation on convex bounded domains by using triangle meshes and by minimizing the difference between prescribed and actual target light distribution which is computed by tracing rays through the optical surface. The mathematical solution has to comply with two conditions: the boundary of the source domain has to be mapped onto the boundary of the target domain and the solution has to be convex. The boundary condition problem is solved using a signed distance function that is computed in advance by a fast marching algorithm. The actual light distribution is computed by tracing rays along the triangle nodes and computing the light irradiance on the target by dividing the light flux through a triangle by its mapped area on the target. Under certain conditions this is also an approximate solution to the Optimal Transportation Problem with quadratic cost.
Free-form optics have been proven to be a very powerful and efficient illumination strategy with applications ranging from automotive and architecture illumination to laser beam shaping. State of the art free-form optics design methods assume that the light has zero étendue, which is for example given if it is emitted from a point source or perfectly collimated. In some cases, this assumption is not valid and designing free-form optics with a zero-étendue method and using a non-zero étendue source will result in a blurring effect for sharp edges in the irradiance pattern. In previous work1, we derived an integral formulation for the irradiance distribution on a target screen for a non-zero étendue source. Furthermore, we showed for a 2D-application that it is possible to combine this irradiance calculation method with a surface optimization routine to obtain free-form optics that also take into account a non-zero étendue. As a continuation, we extend this approach to three dimensions. To this end, we show how the integral formulation can be approximated numerically in three dimensions and we present an optimization method for the free-form optics. We demonstrate the performance of the algorithm by using two different test cases. For the second test case, we additionally present how the achieved irradiance distribution varies with the étendue of the source.
We report on an extension of the previously published two-step freeform optics tailoring algorithm using a Monge-Kantorovich mass transportation framework. The algorithm's ability to design multiple freeform surfaces allows for the inclusion of multiple distinct light paths and hence the implementation of multiple lighting functions in a single optical element. We demonstrate the procedure in the context of automotive lighting, in which a fog lamp and a daytime running lamp are integrated in a single optical element illuminated by two distinct groups of LEDs.
We present a model and results of simulations and experiments investigating the L-I characteristics of electrically pumped (EP-) VECSELs in the single- and multi-mode regime. In our model we use a mode expansion ansatz to treat the electromagnetic field inside the VECSEL cavity. The eigenmodes of the passive cavity are computed using the bidirectional beam propagation method (BDBPM) to solve the Helmholtz equation. The BDBPM allows us to account for the complex refractive index distribution within the semiconductor heterostucture, composed of approximately thousand interfaces along the optical axis in addition to lateral refractive index variations in oxide-confined devices as well as the macroscopic external cavity. We simulate the time evolution of the modal powers of several transverse modes and the spatial distribution of the inversion carriers in the quantum well plane. Therefore we solve an differential equation system composed of multimode rate equations and the carrier diffusion equation. With this ansatz we are able to identify cavity geometries suitable for single-mode operation assuming typical current profiles that are taken from photoluminescence measurements of the devices under investigation. Furthermore, we identify effects limiting the single-mode efficiency, such as poor gain and mode matching, reabsorption in unpumped regions of the quantum wells or enhanced carrier losses due to strong spatial hole burning. Critical parameters of the equations, such as optical losses, injection effciency, carrier recombination constants and gain parameters are obtained from experiments, microscopic models and literature. The simulation results are compared to experimental results from EP-VECSELs from Philips Technologie GmbH U-L-M Photonics.
The advent and rapid development of efficient high power LED sources with their unique emission characteristics
enables the development of illumination systems that meet very strict requirements concerning light distribution
and efficiency. Most of the algorithms used to design the necessary optical freeform surfaces rely on the point
source assumption. As long as the distance between LED and those surfaces is sufficiently large, this is a good
approximation. One further important design goal is to make the optical components as small as possible, which
makes the point source assumption less accurate. The existing design algorithms thus have to be accompanied
by methods to treat the finite-sized LED sources. We examine the limits that are set by the finite size of the light
sources and present algorithms to optimize optical freeform surfaces up to these limits. Point source results are
iteratively improved to get the desired illumination pattern employing finite sized LEDs. At each iteration step
the illumination pattern used in the point source computations is adapted so that the real illumination pattern
of an LED approximates the originally desired pattern.
Based on the Monge-Kantorovich theory of optimal mass transport, the computation of a ray mapping between source and target irradiances is used to design two-sided freeform lenses fulfilling the constraints of an automotive application: compactness and sharp bright-dark cutoff. A generic segmentation technic resulting in Fresnel-type optics is presented and the whole procedure is illustrated with the design of a fog light lens. Finally Monte Carlo simulation of the virtual model and measurements of a polycarbonate prototype are presented.
We report on the development and experimental analysis of an LED lighting module for use in a high-end food lighting
environment which puts high demands on color homogeneity and color rendering. The system is built from highly
reflecting and partly scattering PVD coated metal reflector sheet that has limited deformability and uses RGBW LEDs.
We develop an optical design that is adapted to allow for color mixing and to take into account manufacturing
constraints and include this into a prototypical module. Results of measurements and field tests are in good agreement
with simulations.
LEDs are a promising alternative to existing illuminants for a wide range of lighting applications. Besides efficiency
and high durability, the small light source dimensions compared to conventional light sources open up new possibilities in
optical design. In many lighting setups, it is desired to realize a prescribed intensity distribution, for example homogeneous
irradiance on a given area on a wall or floor. This can be realized using LEDs in combination with specially designed
freeform lenses and/or mirrors. For high efficiency, it is necessary to collect at least 70 - 80 degrees half-angle (measured
against the z axis) of the light that the LED emits into a 90 degree half-angle. This results in a lens that resembles a
hemisphere. The numerical design problem thus requires a mathematical description that can handle such strongly curved
surfaces with strongly varying surface slopes. Surface parametrizations with a rectangular topography, like e.g. Cartesian
tensor product B-splines, have severe drawbacks when handling such surfaces. We report on the use of an alternative
surface approximation scheme that uses a triangular mesh. We describe an algorithm that optimizes the two surfaces of a
lens for a wall washer that generates homogeneous irradiance on a wall area of 2.8 × 2.8 m2 while mounted to the ceiling. The homogeneity is better than 80% and the optical efficiency including Fresnel losses is about 85%.
KEYWORDS: Light emitting diodes, Lenses, Ray tracing, Light sources and illumination, Polymers, Lens design, Monte Carlo methods, LED lighting, Freeform optics, LED displays
We report on the development and experimental analysis of novel freeform polymer lenses for the uniform illumination
of a target area using high-brightness LED. In a first step, the LED module was centered over the illuminated area and
the two faces of a freeform lens were designed for homogeneous and efficient light distribution. In a second step, the
system was extended for an off-axis target, the overall size was reduced and structures for automated alignment with
respect to the LED were included. Photometric analyses were used for each system and were found to be in good
agreement with simulations.
An Innoslab based Nd:YV04 MOPA system with pulse energy of 7.25 mJ at 40 kHz repetition rate and pulse
duration of 11.4 ns has been used for third harmonics generation in Lithium Triborate (LBO) crystals. We report
UV pulses of 8.9 ns duration at pulse energy of 1.65 mJ, which means an average power of 66 W. We have been
able to show UV beam qualities (M2) of 1.7/2.4 (stable/instable direction with 90/10 knife edge method), while
IR beam quality is 1.8/5.2. A sinc2-shape transversal distribution of beam intensity has been used in instable
direction of the Innoslab MOPA system for conversion. Due to high average power and short pulse length at
355 nm the laser meets the demands for high-throughput micro material processing as stereolithography or edge
isolation of solar cells. The thermal dependence of the conversion efficiency (due to heating power of the beam)
has been investigated theoretically, using a time resolved numerical simulation model for the nonlinear process in
both LBO crystals. Scaling effects of the absorption coefficients of LBO and the pulse power on the conversion
efficiency are presented in this article.
A Master-Oscillator-Power-Amplifier (MOPA) design combining rod and slab laser technology for high pulse energy, high average power and near diffraction limited beam quality for industrial use has been developed. To achieve the good beam quality at high average and high pulse power, an advanced birefringence compensation scheme, which ensures a high mode overlap while simultaneously minimizing the power densities on optical surfaces, has been developed and applied. The prototypes deliver an average power of up to 860 W with M2 < 2 or 1.3 kW with M2 < 12 at 10 kHz repetition rate and 5-16 ns pulse duration. At 1 kHz up to 420 mJ pulse energy can be achieved. The prototypes are fully computer controlled and can be operated from 0 to 100 % output power and from single shot to 10 kHz. They are currently operated for plasma generation in a laboratory surrounding and have run for more than one thousand hours without failure up to now. An analytical solution of the thermally induced refractive index profile in dependency of a radially symmetric pump light distribution including the effect of thermally induced birefringence, temperature dependency of the thermal conductivity and the second derivative of the refractive index with the temperature (d2n/dT2) has been derived. This allows a fast calculation of thermally induced aberrations without the use of FEA. Experimental results are compared to predictions from analytical and FEA modelling. Based on experimental and theoretical results, scaling limits of rod based MOPAs are predicted.
High power diode pumped solid state lasers are auspicious sources for various applications in material processing. The solid state laser we report on is a Nd:YAG slab laser, that is partially end pumped from two ends by the line focus of two diode laser stacks. The pumped volume has a rectangular cross section. The resonator is configured, so that it is stable in the plane of small dimension and off axis unstable in the plane of large dimension of the gain cross section. Unlike conventional slab laser design, in which the laser beam takes zick zack path inside the slab crystal, in the present design the beam goes straight through the crystal with perpendicular end faces.
KEYWORDS: Nd:YAG lasers, Semiconductor lasers, Diode pumped solid state lasers, Lamps, Diodes, Solid state lasers, Resonators, Rod lasers, High power lasers, Beam propagation method
High power diode pumped solid state lasers are attractive sources for various applications in material processing. One of the advantages of diode pumping of solid state lasers is the higher brilliance (maximum output power/poorest beam parameter product using equal plan-plane resonators) compared to arc lamp pumped solid state lasers.
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