Edward R. Freniere

Using Spline Surfaces in Optical Design Software

Splines are commonly used to describe smooth freeform surfaces in Computer Aided Design (CAD) and computer graphic rendering programs. Various spline surface implementations are also available in optical design programs including lens design software. These surface forms may be used to describe general aspheric surfaces, surfaces thermally perturbed and interpolated surfaces from data sets. Splines are often used to fit a surface to a set of data points either on the surface or acting as control points. Spline functions are piecewise cubic polynomials defined over several discrete intervals. Continuity conditions are assigned at the intersections as the function crosses intervals defining a smooth transition. Bi-Cubic splines provide C2 continuity, meaning that the first and second derivatives are equal at the crossover point. C2 continuity is useful outcome of this interpolation for optical surface representation. This analysis will provide a review of the various types of spline interpolation methods used and consider additional forms that may be useful. A summary of the data inputs necessary for two and three-dimensional splines will be included. An assessment will be made for the fitting accuracy of the various types of splines to optical surfaces. And a survey of applications of spline surfaces in optical systems analysis will be presented.

First-Order Design Of Optical Baffles

Optical systems which have stringent reouirements on stray light levels often need optical baffles. Some basic design principles and goals for baffles are Presented. Four generic optical designs and their advantages for stray light control are discussed: all reflective and all refractive reimaging and nonreimaging systems. The desirable shape and location of baffle vanes are shown, based on various design criteria. Presuming that the designer has the ability to control specular reflections from baffle surfaces, absorptive coatings which reflect specularly are preferred over those which reflect diffusely. Stray energy from edge diffraction can be controlled in a nonreimaging optical system by forcing light to undergo multiple diffractions.

Edge diffraction in Monte Carlo ray tracing

Monte Carlo ray tracing programs are now being used to solve many optical analysis problems in which the entire optomechanical system must be considered. In many analyses, it is desired to consider the effects of diffraction by mechanical edges. Smoothly melding the effects of diffraction, a wave phenomenon, into a ray-tracing program is a significant technical challenge. This paper discusses the suitability of several methods of calculating diffraction for use in ray tracing programs. A method based on the Heisenberg Uncertainty Principle was chosen for use in TracePro, a commercial Monte Carlo ray tracing program, and is discussed in detail.

Asymptotic approximation to the encircled energy function for arbitrary aperture shapes

Encircled energy can be easily estimated for imaging systems with oddly shaped apertures using a simple asymptotic approximation formula. A useful tool for the optical system designer, it linearly relates the encircled energy to the perimeter-to-area ratio of the aperture. Comparisons made with numerical calculations show that it is usefully accurate for circles in the focal plane as small in radius as λf #.

Polarization models for Monte Carlo ray tracing

In many optical systems, the polarization state of light is of critical importance and must be considered during their design. In other systems, the polarization state is used and manipulated as an integral part of the design. Three methods exist for modeling the polarization state of light: the Mueller calculus; the Jones calculus; and the polarization ray tracing calculus. The relative advances of these methods for simulating the polarization behavior of real devices using Monte Carlo ray tracing software is discussed, including simulation of the polarization properties of scattered light.

Modeling birefringence in optomechanical design and analysis software

Opto-mechanical engineers are taking advantage of the birefringence exhibited in uniaxial crystals to control light in a wide range of applications. Software tools are required which can handle light propagation through such crystals; but these tools must also offer an intuitive interface to the user. Rigorous physics calculations are required at the optical component level to deal with beam doubling and flux propagation. However, these components are immediately combined into sub-assemblies where opto-mechanical packaging concerns arise. An intuitive, CAD-like interface coupled with accurate ray propagation algorithms has been implemented in TracePro, a commercial optical analysis program. In this tool components, sub-assemblies, and assemblies can be readily positioned and oriented. The performance of the optical systems is evaluated via raytracing. In essence, the software presents a virtual laboratory or optical bench. The birefringence ray tracing capability in a three dimensional, computer aided design (CAD) environment will be described. This analysis provides the design engineer the capability to model a variety of optical components used in telecom applications such as polarization independent isolators, circulators, beam displacers and interleavers. Several examples illustrating the application of this analysis will be presented.

Two-position IR zoom lens with low f-number and large format

We describe the optical, mechanical and servo designs for a motorized, two-FOV (field of view) IR objective lens for use in the 8 - 12 micrometers spectral band. The FOV is changed by moving lenses axially instead of the more traditional approach which is to add and remove lenses. The advantages of this approach include: simple mechanics, since a single mechanism can be used for both adjusting focus and changing FOV; only one lens group need be moved; no stow space is needed for removed lenses; and fewer total lenses are needed (four elements). The lens is used with a low-cost, uncooled focal plane array. This dictates relatively fast F- number, large image format (F/1.1, 7.8 degree(s) narrow FOV, 155-mm narrow-field focal length), and low cost. This combination of wide field and large collecting aperture pose a difficult optical design challenge. The lens meets a range of military environmental requirements including immersion in one meter of water. We describe how the requirements were met. We have fabricated and tested five lenses and we describe the assembly and testing process and present a summary of test results.

Accurate source simulation in modern optical modeling and analysis software

Modern optical modeling and analysis programs allow users to create and analyze accurate optical and opto-mechanical systems in the software environment prior to building actual hardware based systems. The resultant accuracy of these models depends on the accuracy of the components that make up the model including the light source characteristics, surface and material properties, and the model geometry. In this paper we will consider factors that lead to improved modeling of the light source such as spectral and angular properties, the spatial distribution of light within the source, and the interaction of the light with the structure of the source. These factors are extremely important for near field modeling, especially for fiber and light pipe coupling. Several options will be discussed including simple source models such as point sources, ray files, surface properties that define optical parameters such as spectral and angular distribution, and detailed 3D solid models of the source. Simulated results for spectral, angular, and spatial distributions will be compared to actual measurements. Discussion will also include the appropriateness of each modeling approach with respect to different applications.

Design for manufacturablility (DFM) in the life sciences : Fluorescence spectroscopy product platform realized with TracePro® suite of opto-mechanical design software tools

This paper describes the design-for-manufacture (DFM) process for a multi-channel fluorometer product platform. The multi-disciplinary team eliminated the cost of quality by design, using a formal design method, facilitated by Lambda Research Corporations suite of TraceProTM suite of optical design software. Development of this platform presented rigorous design challenges - from identifying feasible design alternatives to minimizing the exponential cumulative effect of component quality and quantity to optimizing tolerances to thoroughly documenting the design. The design was highly constrained in terms of cost and the ability of the platform to accommodate a breadth of fluorescence-tagged media. Furthermore, the inherently interdisciplinary nature of developing medical devices required a high level of collaboration between scientists and engineers across the areas of optics, mechanics, materials, biology and clinical chemistry. While fluorescence tag technologies enable very sensitive detection of molecules, the anisotropic nature of fluorescence in both intensity and polarization severely complicate system design. TracePros fluorescence modeling capability enabled adherence to a methodical design process of (1) testing system design alternatives, (2) evaluating off-the- shelf and custom optical component and fluorophore feasibility, and (3) tolerancing for robustness without the cost and time associated with iterative hardware prototyping.

Numerical experiments in modeling diffraction phenomena with Monte Carlo ray tracing

Monte Carlo ray tracing programs routinely simulate the phenomena of reflection, refraction, and scattering by redirecting rays when they intersect a surface. We desire to simulate the diffraction of light by apertures, a wave phenomenon, by a method that melds easily into ray tracing algorithms. The proposed method redirects rays in random directions according to a Gaussian probability distribution. The width of this distribution varies inversely with distance of the ray intersection point from the edge of the aperture, and is derived from the Heisenberg uncertainty relation. Previous results have shown good agreement of incoherent summing of rays traced as compared with results obtained by integral methods. Here we present the coherent summation of rays, showing results that substantially agree with those obtained by integral methods including the fringes that result from interference of coherent light.