Simulating micro-optical systems is therefore one of those areas of photonics where choosing the right numerical method matters more than in classical optics — the wrong tool can produce results that look plausible while not reflecting how the system actually behaves.
What are micro-optical systems?
Micro-optics covers elements with characteristic dimensions ranging from a few micrometers to fractions of a millimeter: microlenses and their arrays, diffractive elements, micro-prisms, micro-mirrors, and coupling structures used in miniature imaging systems, sensors, and integrated photonics. In many cases, micro-optics is part of a larger system — a camera, a sensor, an AR/VR device — where space constraints force the optics to be miniaturized.
Why scale changes the rules of the game
Classical geometric ray tracing assumes that light can be treated as rays travelling in straight lines between surfaces, with refraction and reflection described by Snell's and Fresnel's laws. This approximation works well when structures are much larger than the wavelength of light. As an element's dimensions approach the wavelength, the geometric approximation stops being sufficient — diffraction, interference, and mode coupling start to significantly shape the field distribution, and these phenomena require a wave-based description that ray tracing alone does not provide.
Diffraction and wave effects in micro-optics
A diffractive element, by definition, works through the interference of waves bent by a periodic or quasi-periodic structure — its function cannot be properly described in purely geometric terms. Likewise, microlens edges, apertures comparable to the wavelength, or densely packed micro-element arrays can generate optical crosstalk between neighboring elements that ray tracing will not predict.
When ray tracing is enough, and when you need to switch to wave optics
The practical rule is: if a structure is many times larger than the wavelength and has no strongly diffractive features, classical ray tracing in OSLO gives a reliable result at a much lower computational cost than a full electromagnetic simulation. But when structure dimensions are on the order of the wavelength, or the design relies on diffractive effects, you need an analysis in OptiFDTD, which solves Maxwell's equations directly and accounts for the wave effects that ray tracing omits.
Manufacturing tolerances at the micro scale
In micro-optics, the same absolute manufacturing error carries relatively more weight than in classical optics — a deviation of a single micrometer on an element tens of micrometers in diameter represents a far larger percentage of its characteristic dimension than the same error would on a lens several centimeters across. That is why tolerance and sensitivity analysis matters especially early in micro-optics design, not only when validating a finished element.
OSLO: classical design of micro-lens systems
Wherever a micro-element behaves according to geometric optics — for example, designing a single imaging microlens with dimensions much larger than the wavelength — OSLO lets you design, analyse aberrations, and optimize the system using the same tools applied in classical imaging optics, just at a smaller scale.
OptiFDTD: full electromagnetic analysis of microstructures
When a micro-element requires a wave-based description — a diffractive element, a periodic structure, a microlens with dimensions comparable to the wavelength — OptiFDTD solves Maxwell's equations directly using the finite-difference time-domain method, making it possible to model diffraction, interference, and scattering across the full geometry of the structure.
Combining both approaches in one project
A practical system that includes micro-optics often combines both levels: the classical optics surrounding the micro-element — for example, a coupling or collimating path — is designed in OSLO or TracePro, while the micro-element itself, with dimensions on the order of the wavelength, is analysed with a full-wave method in OptiFDTD. This hierarchical approach avoids a computationally expensive electromagnetic simulation of the entire system, while still preserving accuracy where it is actually needed.
Typical applications
Micro-optics simulation is used in designing microlens arrays for cameras and imaging sensors, diffractive elements that shape the beam in projectors and scanners, micro-couplers in integrated photonics, and miniature sensor components where the space constraints of the whole device force the design into the micro scale.
Summary
Simulating micro-optical systems requires a deliberate choice of numerical method — not because micro-optics is inherently harder, but because the scale of the structure determines whether the geometric approximation is still valid. Wherever element dimensions remain much larger than the wavelength, ray tracing in OSLO delivers a fast and accurate result. Wherever a structure approaches the wavelength scale or relies on diffraction, a full electromagnetic analysis in OptiFDTD is needed — and in many projects, both approaches work together within a single hierarchical design process.