You have to take into account the real size and characteristics of the source, the wavelength, the aperture, aberrations, element positions, assembly tolerances, surface quality, clipping and, depending on the scale of the problem, diffraction and the wave properties of light.
For this reason, there is no single universal simulation tool for collimator design. Designing a collimating lens calls for different methods than analyzing a collimator with a real laser diode and housing, and different ones again than analyzing a microcollimator working with a waveguide. A practical workflow can use Lambda Research tools (OSLO and TracePro) together with Optiwave software, choosing the simulation method to match the specific physical problem.
What does light collimation actually mean?
An ideally collimated beam would have rays parallel to the optical axis and would not change diameter as it propagates. Real light sources do not behave ideally, however. An LED is a source with a finite emitting area and a wide angular distribution. A laser diode can have very different divergence in the fast and slow axes. A Gaussian beam, in turn, has a finite divergence that follows from the physics of propagation and depends, among other things, on the wavelength and the beam waist.
That is why, in practice, instead of asking "is the beam collimated?", it is better to define measurable requirements:
- what is the acceptable divergence?
- what should the beam diameter be?
- what uniformity is required?
- what fraction of the power should fall within the useful aperture?
- what is the acceptable deviation of the propagation direction?
- how does the beam profile change with distance?
- what is the tolerance for shift and tilt of the source or lens?
Only a problem defined this way can be simulated correctly.
The simplest collimator model
In the geometric approximation, light from a point source located at the focal point of a lens forms a parallel beam behind it. This is a very good starting point, but it is not enough to design a real device. If the source is shifted along the optical axis, convergence or divergence appears behind the lens. A transverse shift can change the direction of the output beam. Element tilt, surface manufacturing errors, or changes in refractive index also affect the result. Aberrations come on top of that. The first stage of a professional project is therefore usually the optical design of the collimating system itself.
OSLO: designing the collimator optics
OSLO (Optics Software for Layout and Optimization) from Lambda Research is intended for this type of task. OSLO uses ray tracing and tools for analyzing and optimizing optical systems. It can be used to design lenses and analyze system parameters, and the manufacturer's documentation also includes a design example of a laser diode collimator.
In a collimator model you can start by defining source → lens system → output beam and then analyze the influence of optical parameters on the result. In practice, the designer can vary radii of curvature, element thicknesses, spacings, materials, or surface positions, and then optimize the system against the chosen criteria.
A collimator is not just a lens
A correctly designed lens does not yet mean a correctly designed device. A real collimator can include a source, one or several lenses, a lens mount, an aperture, a housing, a protective window, positioning elements, and absorbing or reflecting surfaces. In such a model we are no longer interested only in the ideal optical path. Rays can be blocked by the mount. Part of the light can reflect off mechanical surfaces. The lens can be shifted or tilted. Light falling outside the aperture can be absorbed, reflected, or scattered. This is where a second class of simulation becomes useful.
TracePro: the collimator as a real opto-mechanical system
TracePro from Lambda Research uses non-sequential Monte Carlo ray tracing and lets you model sources, optics, and mechanical geometry in a single 3D model. Among other things, it supports surface sources, grid sources, and ray files, and lets you assign optical properties to surfaces and materials.
For a collimator, this means you can build a much more complete model:
real source → lens → aperture → mount → housing → measurement plane
and check what actually happens to the luminous flux.
The source model matters
One of the most important elements of a collimator simulation is a correct definition of the source. Assuming an ideal point source can lead to results that look good in simulation but do not correspond to the real device. This is particularly important for LEDs and laser diodes.
TracePro lets you define sources in several ways, including spatial and angular data and ray files. The manufacturer points out that for near-field analyses the starting position of the rays can be critical, so a simplified source model is not always sufficient. If the source manufacturer provides measured ray-file data, it can be a much better starting point than an arbitrarily assumed ideal model.
Laser diode collimation: fast axis and slow axis
A particularly interesting case is the edge-emitting laser diode. Its emission is usually strongly asymmetric. The characteristics in the fast axis differ from those in the slow axis. That is why separate elements may be used in such systems: an FAC (Fast Axis Collimator) and a SAC (Slow Axis Collimator). Such a source should therefore not automatically be described by a single divergence angle and a single symmetric lens.
For this problem, Lambda Research shows a TracePro workflow covering the diode model, FAC and SAC optics, apertures, mechanical elements, and detectors. The analysis can cover, among other things, divergence in both axes, beam width, pointing, clipping, transmitted flux, and irradiance distribution. It is a good example of why a collimator model should include the source and the mechanics, not just the lens.
Aperture and clipping: a larger lens does not always mean a simple answer
If the beam is too large relative to the clear aperture of an element, it is cut off, which is known as clipping or truncation. In a simple energy balance this means a loss of part of the power. In a real device, however, the consequences can be broader. Light hitting the edge of the aperture can be absorbed, reflected, scattered, directed onto a mechanical element, or turned into an additional source of stray light. For coherent beams, aperture limitation also has diffraction consequences.
Lambda Research draws attention to the need to analyze apodization and beam truncation in laser collimator projects as well. In this case, TracePro lets you track where the flux intercepted by the aperture and mechanical elements ends up.
When ray geometry stops being enough
OSLO and TracePro are very useful in designing classical collimators, but there are situations in which geometric optics alone does not describe all the phenomena of interest. This applies especially when diffraction, the amplitude and phase distribution of the field, interference, polarization, structures with dimensions comparable to the wavelength, propagation in waveguide structures, and coupling of light to a waveguide or photonic structure matter. This is exactly where Optiwave tools come in. It is important to make a distinction, however: OptiFDTD and OptiBPM are not replacements for classical software for designing macroscopic lens optics; they solve a different class of problems.
OptiFDTD: when the electromagnetic field must be analyzed
OptiFDTD uses the Finite-Difference Time-Domain method for numerical modeling of the electromagnetic field. The program supports, among other things, excitation with a plane wave, a point source, a waveguide mode, and a Gaussian beam. The Analyzer enables, among other things, analysis of power distribution, Poynting vectors, overlap integrals, and far-field transformation.
In the context of collimation it can be of interest primarily when the project concerns micro-optics or a photonic structure and wave effects are an essential part of the problem. You can then analyze not only ray trajectories but the distribution of the electromagnetic field. OptiFDTD also lets you define a Gaussian input field profile, with the program documentation specifying how the beam radius and size are defined.
OptiBPM: when collimation meets waveguide propagation
Yet another case occurs when the collimating system is tied to a waveguide or photonic integrated circuit. The question then may no longer be "what is the ray angle behind the lens?" but "how does the field leaving the waveguide propagate, and how efficiently can it be coupled to the next structure?".
OptiBPM uses the Beam Propagation Method and is intended for simulating light propagation in 2D and 3D waveguide structures. The initial field can be, among others, a waveguide mode, a Gaussian field, a rectangular field, or a user-defined field. This approach is particularly interesting when designing fiber → microlens → free space or free space → coupling element → waveguide systems. OptiBPM should therefore not be treated as a program for classical design of a collimating objective. Its role appears when the collimation problem is part of a field-propagation problem in a waveguide system.
How to choose software for collimator design
The most important thing is to determine what kind of engineering question we want to solve.
| Design problem | Suitable tool |
|---|---|
| Design of a collimator lens or lens assembly | OSLO |
| Optimization of a classical optical system | OSLO |
| Analysis of a real source and angular distribution | TracePro |
| LED, laser diode, ray file | TracePro |
| FAC/SAC for a laser diode | TracePro + optics design |
| Apertures and clipping | TracePro |
| Housing and mechanical geometry | TracePro |
| Reflections and stray light in the device | TracePro |
| Electromagnetic field analysis of a microstructure | OptiFDTD |
| Diffraction and full-wave effects in a small structure | OptiFDTD |
| Propagation in a waveguide structure | OptiBPM |
| Field leaving a waveguide / coupling | OptiBPM / OptiFDTD, depending on the problem |
The key point is that these tools do not have to compete with each other. They can be responsible for different levels of the same project.
A practical collimator design workflow
A professional design process is best started not by choosing a program, but by defining the source and the output parameters.
1. Define the source
Specify the wavelength or spectral range, the size of the emitting area, the angular distribution, the beam profile, the polarization (if it matters), the power or flux, and the fast/slow-axis parameters in the case of the relevant laser diodes. If measurement data is available, it is worth determining whether it can be used directly in the model.
2. Specify the collimation parameters
The requirement "collimated beam" alone is too imprecise. The specification can include, for example, the acceptable divergence, the beam diameter in a given plane, pointing, transmitted power, the irradiance distribution, or the tolerance for source shift.
3. Design the optical system
For a classical lens collimator, the natural tool is OSLO. At this stage you can select and optimize the optics, analyzing the system's behavior over a given range of parameters. OSLO also has tolerance analysis tools, which is important when moving from a nominal design to a device that can be manufactured and assembled.
4. Add the real source and mechanics
The model can then be analyzed at the level of the entire device in TracePro. This is where source + optics + aperture + mount + housing + detector comes in. You can then check clipping, the flux on individual surfaces, the irradiance distribution, and the behavior of rays outside the ideal optical path.
5. Perform tolerance and assembly-error analysis
It is worth varying, among other things, the source position, the source-to-lens distance, lens decentration, tilt, aperture position, and detector position. Thanks to this, the nominal design turns into a design that can also be evaluated in terms of real assembly.
6. Add wave analysis where it is needed
If electromagnetic effects, diffraction, microstructure, or coupling to a waveguide are key, the relevant part of the problem can be analyzed with OptiFDTD or OptiBPM. This does not mean that the whole opto-mechanical model must be replaced by full-wave calculations. The most detailed model should be used where its physics really affects the result.
LED collimator versus laser collimator: not the same problem
For an LED, the main issue may be capturing as large a fraction of the flux as possible and achieving a specified angular distribution. A realistic model of the emitting surface and its angular characteristics is particularly important here. TracePro can use source data, including ray files, which is useful when modeling real emitters.
For a laser, the situation is different. Gaussian beam parameters, its divergence, asymmetry, apertures, and, depending on the application, wave effects can matter. That is why the phrase "we are designing a collimator" alone is not enough to choose a simulation method.
Tolerances: why an ideal collimator is not enough
The nominal design answers the question: how does an ideally made and aligned system work? Production requires an answer to a different question: how will a set of real devices behave? If a small shift of the lens causes a significant increase in divergence or pointing error, the problem is no longer just the optics: the mechanical design and the assembly process become part of it.
That is why tolerances should be analyzed before the prototype is built. OSLO enables design, optimization, and tolerance analysis of optical systems, while TracePro lets you place the optics in a real 3D model of the device. Combining these two perspectives is especially useful in designing a product, not just a nominal optical prescription.
The most common mistakes in collimator simulation
Idealizing the source
An ideal point or an ideal symmetric beam may not correspond to a real LED or laser.
Neglecting the mechanics
The mount and aperture are part of the optical system if they can intercept, reflect, or scatter light.
Analyzing only the nominal system
The collimator should also be checked for the shifts and tilts possible in real assembly.
Ignoring the fast and slow axes
For the relevant laser diodes, both axes may require separate modeling.
Using ray tracing for every problem
If the result depends on interference, diffraction, or the detailed field distribution in a microstructure, a wave method may be needed.
Using FDTD for an entire macroscopic device
A full-wave model is not automatically "better". The computational cost grows with the size of the domain and the required resolution. For a classical lens system, ray tracing is therefore usually a much more natural starting point.
One device, several models
Collimator design illustrates well why there is no single simulation method suitable for every problem in optical engineering. A possible workflow looks as follows:
- OSLO: design and optimization of the lens system.
- TracePro: real source, optics, mechanics, apertures, and flux analysis.
- OptiFDTD / OptiBPM: analysis of a selected fragment requiring a wave or waveguide description.
- Prototype and measurements.
- Correlation of the model with the real device.
This approach lets you use each method where it gives the designer the most useful information.
Summary
Collimator design does not end with calculating the focal length of a lens. In a real project you must take into account the source characteristics, aperture, aberrations, tolerances, mechanics, clipping, reflections, and, in the relevant applications, wave phenomena. That is why engineering tools should be chosen to match the level of the problem being analyzed.
OSLO is the natural tool for designing and optimizing a classical optical system. TracePro lets you extend the analysis to the real source, non-sequential propagation, apertures, mechanics, and the energy distribution in the device. OptiFDTD is used when full-wave electromagnetic analysis becomes important, while OptiBPM is intended primarily for propagation problems in waveguide structures.
In practice, the best collimator model does not have to be the most complicated one. It should, however, describe the phenomena that determine how the designed device works.
Manufacturer resources
Design example of a laser diode collimator in the OSLO environment: Lambda Research: OSLO
Technical information on modeling sources and geometry in TracePro: Lambda Research: TracePro Product Information
Fast- and slow-axis laser diode collimation workflow in TracePro: Lambda Research: Fast/Slow Axis Laser Diode Collimation with TracePro
Description of OptiFDTD capabilities, including Gaussian sources and far-field analysis: Optiwave: OptiFDTD Overview
Technical introduction to the beam propagation method: Optiwave: Introduction to OptiBPM