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A bi-directional optical propagation tool



FIMMPROP is a highly innovative tool for simulating propagation in optical waveguides in 2D and 3D, which is fully integrated as part of our optical mode solver FIMMWAVE and relies on the rigorous EigenMode Expansion (EME) method.

A powerful and versatile optical propagation tool

FIMMPROP is ideal for the modelling of optical propagation in structures with high refractive-index contrast, commonly found in silicon photonics and III-V integrated optics, for which approximate techniques such as the beam propagation method (BPM) would be inaccurate, and methods such as FDTD or FEM would be extremely slow.

This includes the simulation and optimisation of devices such as MMI couplers, optical gratings, co-directional couplers or polarisation converters. Thanks to its unique adaptive taper algorithm, it is also a very accurate and efficient method for the modelling of optical tapers (e.g. mode-size converters) and slowly z-varying structures such as ring resonators and Y-junctions.

For modelling optical gratings, FIMMPROP can use either EME or a form of RCMT (Rigorous Coupled Mode Theory) enhanced by Photon Design. The two methods are complementary, RCMT allowing you to model many grating geometries more efficiently and accurately than EME.

It can also model propagation in optical fibers, allowing you to simulate many types of fiber to chip couplers, tapered fibers and lensed fibers as well as fiber Bragg gratings.

FIMMPROP is an extremely versatile tool which can also model plasmonic waveguides, AR coatings for waveguide facets and photonic crystal fiber devices.

Silicon taper and MMI Coupler modelled in FIMMPROP

A silicon taper and an MMI Coupler simulated with FIMMPROP, showing its ability to
model wide-angle propagation and high delta-n structures

Explore the underlying physics of your designs

FIMMPROP is a great tool for the thinking optics designer since it provides not only a simulation result, but also a deep insight into what is going on inside the device thanks to its eigenmode approach, providing you with invaluable hints on how to improve your structures. Where methods such FDTD or BPM which would only provide you with the transmission, you will be able to study the evolution of mode power and mode properties along the structure, and find out exactly where and why your taper is radiating!

Benefits of EigenMode Expansion

FIMMPROP is based on the powerful EigenMode Expansion method (EME), with the following benefits: 

  • Rigorous: no approximation, unlike BPM. Fully vectorial 2D and 3D modelling.

  • Bi-directional: FIMMPROP is inherently bi-directional, and will take all internal reflections into account. It is therefore capable of modelling structures such as Bragg gratings or AR coatings, which are not solvable by other methods such as BPM. It can also model devices with with strong internal reflections, such as waveguides terminated by a tilted or straight facet.

  • Wide angle capability: FIMMPROP can model wide angle problems as shown in the above picture - just add more modes as the angle gets larger. Note that this is a true "wide angle" algorithm, compared to so called "wide angle BPM" which can only model light travelling at a wide angle if all the light is travelling close to that same angle!

  • Fast: EME enables the fields to be calculated using fast semi-analytical methods, making it much faster than FDTD or FEM for many problems. Once the modes of the waveguide are found then propagation along the length of the section is near-instantaneous. It also permit calculations to achieve high accuracy even for more complicated structures.

  • Highly efficient: Makes calculations much faster by taking advantage of all the symmetries in your structure, including symmetry planes, cylindrical symmetry, allowing you to reduce not only the computation time per mode, but also the number of modes that you need. Can also take advantage of repetition, mirroring and periodicity along the z-axis (useful for gratings, MZIs, ring couplers etc.).

  • Versatile: can model both continuously z-varying structures (e.g. tapers) and discrete structures (e.g. junction between a fiber and a planar waveguide). Structures with smoothly z-varying cross-sections such as tapers and Y-junctions are computed using sophisticated extensions of EME developed at Photon Design.

Find out more about EME

Comparison of the EME, BPM and FDTD methods

Component design your for PIC circuit model

Components modelled with FIMMPROP can be easily exported to PICWave where they can be integrated in the simulation of photonic integrated circuits (PICs).

Link to ray tracing tools

FIMMPROP can import beam profiles from FRED and Zemax, and can export optical profiles back into these ray tracing tools.


PC min spec: Win 8/10 x64, 4GB RAM, Intel Haswell, AMD Ryzen2 or later CPU.