FIMMPROP

A 1x8 Planar MMI Coupler

3D simulation with FIMMPROP software

In this example we used FIMMPROP to model a 1x8 planar MMI coupler based on a AlGaAs/GaAs rib waveguide geometry.

Benefits of using FIMMPROP to model MMI couplers

EME (EigenMode Expansion) is the ideal method to model MMI couplers. The physics of MMIs results from constructive and destructive interferences between a small number of guided modes; it is these interferences that give MMIs their ability to generate multiple images of the input beam. EME is a mode-based approach, and it therefore offers an extremely efficient way of modelling such devices.

The implementation of EME in FIMMPROP offers an optimal framework for this application, with a potential for highly efficient optimisation and fully vectorial solutions:

• FIMMPROP allows you to simulate the structure with a minimal amount of calculation: you only need to calculate three sets of modes and their overlaps at two joints.
• FIMMPROP will model the structure in a fully vectorial way, allowing you to model MMIs designed in SOI (silicon-on-insulator) or other high index contrast technologies.
• FIMMPROP allows you to optimise an MMI coupler with great efficiency: when a parameter is scanned, FIMMPROP will only recalculate what is needed thanks to its automatic update scheme. The length of the central section can for instance be scanned almost instantaneously!
• The fully bidirectional algorithm will account for all internal reflections and will allow you to simulate the structure simultaneously in both directions.

Modelling a 1x8 MMI coupler

FIMMPROP was used to design and simulate in rigorous fully vectorial 3D a 1x8 MMI coupler based on a AlGaAs/GaAs rib waveguide geometry, working at a wavelength of 1.103um.

The single-mode input waveguide was located in the centre of the device. The length of the central section and the lateral positions of the output waveguides were optimised using scanners.

A power transmission of 92.46% was obtained. The power was transmitted to each output waveguide in a fairly equal way, with a standard deviation of only 0.11% for each waveguide.

Schematic view of the MMI Coupler in FIMMPROP with the cross-section of the output waveguide

Intensity profile of the MMI Coupler when the fundamental TE-like mode is launched at the input

Ex-field profile at the output of the MMI Coupler

Scan the length of the MMI instantly!

The device was optimised by scanning the length of the multimode section and the lateral positions of the output waveguides.

FIMMPROP relying on EME, the length of a section of the device can be changed at a minimal cost in calculations and time.

Simulating the MMI coupler with FIMMPROP took 5 minutes 30 seconds with an Intel Pentium 4 CPU running at 3GHz, with 80 modes included in the multimode section. Once the MMI coupler simulated, obtaining the evolution of transmission when scanning the length of the multimode section over 200 values only took 7 seconds!

The results are plotted in the graph below.

Scan of power transmission v. length of multimode section: 200 points in 7s!

The graph below shows the evolution of transmission when the lateral positions of the output waveguides are varied. Varying the position of the waveguides implied that the modes of the last section had to be recalculated, as well as the overlap integrals in the joint between the multimode section and the output section. Nevertheless, performing this scanner only took 13 seconds per point!

Scan of power transmission v. lateral position of the output waveguides,
characterised by the offset of the first waveguide from the centre (um)

It is also possible to optimise both parameters simultaneously thanks to our integrated optimiser Kallistos. You can see an example of 1x4 MMI optimisation here.