FDTD in OghmaNano

1. Overview

FDTD simulation of a ring resonator coupled to a bus waveguide showing field intensity distribution. — Field intensity in a ring resonator coupled to a straight waveguide.

Three-dimensional FDTD simulation visualising energy density build-up in a resonant photonic structure. — Three-dimensional FDTD snapshot showing energy density build-up in a resonant structure.

OghmaNano includes a full-wave Finite-Difference Time-Domain (FDTD) solver for simulating how electromagnetic waves propagate through complex optical structures. The solver is designed for problems where diffraction, interference, resonant build-up, scattering, mode conversion, or transient field evolution matter and cannot be described reliably by simpler optical models.

The implementation is full 3D and uses OpenCL so that the same fundamental model can run on either the CPU or the GPU. This gives a straightforward path between development-scale and accelerated runs while preserving the same physical setup, solver behaviour, and outputs.

If you want the underlying method, see What is FDTD? and FDTD derivations and mathematical background. This page is focused instead on what the solver in OghmaNano can actually do.

2. What the solver can do

The OghmaNano FDTD solver is built for practical wave-optics simulation. It can be used to model:

full-wave electromagnetic propagation in structured optical media,
full 3D photonic geometries,
OpenCL-accelerated execution on CPU or GPU,
waveguides, mode propagation, and reflections from discontinuities,
resonators including Fabry–Pérot cavities and ring structures,
photonic crystals and Bragg-type periodic structures,
diffraction, interference, and free-space propagation,
tapered structures, angled facets, and mode-expansion problems,
transient pulse propagation and time-domain detector response,
detector-based spectral extraction and field snapshot analysis.

In practical terms, this means the solver is suitable both for teaching-scale wave problems and for more realistic photonic-device structures where the full electromagnetic field must be resolved explicitly.

3. Typical use cases

OghmaNano’s FDTD module is particularly useful for integrated photonics, nanophotonics, resonant structures, and waveguide optics. It is a good choice when the geometry is strongly structured, when local field behaviour matters, or when the final answer depends on how the field evolves in time rather than on a simple steady-state approximation.

Typical examples include waveguide coupling, ring resonators, Mach–Zehnder-type devices, photonic crystal waveguides, interference problems, optical outcoupling from structured facets, and pulse propagation through complex dielectric layouts.

4. When to use FDTD

FDTD is most useful when full-wave behaviour is essential. That includes diffraction, sub-wavelength features, cavity build-up, interference, scattering from complex geometry, near-field coupling, and transient optical behaviour.

It is usually not the right first choice for large optical systems dominated by geometric propagation or for simple planar multilayer stacks. In those cases, ray tracing or transfer-matrix approaches are normally faster and more direct. OghmaNano includes those tools as well, so FDTD can be used where its additional generality is genuinely needed.

5. Theory and background

If you want a concise explanation of the method itself, start with the What is FDTD? page. That page explains what the method is doing and why time-domain field solving is useful.

If you want the full mathematical background, go to FDTD derivations and mathematical background. That page contains the more detailed derivation of the discrete update equations.

6. Core simulation components

In OghmaNano, an FDTD simulation is built from a small set of core components:

light sources for injecting time-domain optical excitation,
boundary conditions for controlling how the simulation region is terminated, and
detectors and spectral extraction for monitoring transmitted power, field response, and spectra.

Together, these make it possible to move from a simple demonstration to a realistic optical device using the same basic solver workflow.

7. Ready-to-run examples

OghmaNano includes a growing library of FDTD examples and tutorials that are effectively ready to run. These cover both fundamental wave physics and more structured device-like simulations.

Fundamental wave simulations include free-space propagation and double-slit interference.

Photonic-structure tutorials include photonic crystal waveguides, Bragg gratings, Fabry–Pérot cavities, tilted facet waveguides, and tapered waveguides.

Device-oriented examples include silicon Mach–Zehnder modulators, ring resonators, and anti-reflection coatings.

These examples are useful both as tutorials and as starting points for your own simulations.

8. Where to start

If you are new to the method, begin with What is FDTD? and then open one of the ready-to-run example pages. A good practical starting point is the photonic crystal waveguide tutorial or one of the simpler wave-propagation examples.

If you already know the basics and want the mathematics, go directly to FDTD derivations and mathematical background.