OmniSim

A vertical fibre to waveguide grating coupler

Simulated with OmniSim software

New! OmniSim now includes a Surface Grating Coupler Design Utility to automatically design and simulate surface grating couplers in 2D and 3D.

OmniSim's FDTD and FETD engines were used to model a vertical grating structure used to couple light from a planar silicon waveguide to an optical fibre. The design that was chosen is a popular fiber-to-chip coupler for a silicon waveguide (silicon on insulator, SOI) with a grating optimised for vertical fibre coupling, taken from a popular publication from Ghent University [1]. The structure is modelled here in 2D but it can also be modelled in 3D with OmniSim.

With OmniSim you can choose between the Finite-Difference Time-Domain and Finite-Element Time-Domain methods, and run the same simulation using two independent tools. This is very useful to check the accuracy of your calculations. The FDTD Engine allows you to run initial calculations quickly and with reasonable accuracy, and you can then use the FETD Engine once you want to obtain more precise results.

This structure can also be modelled in 2D with our EingenMode Expansion tool FIMMPROP; you can find more details here.

Description of the structure
Benefits of FIMMPROP for this simulation
Modelling of the structure in FIMMPROP

Description of the structure

A 220nm-thick Si layer is grown on top of a 2um-thick SiO2 layer. An additional epitaxial silicon layer of thickness 150nm is grown locally on top in order to increase the directionality of the grating. A grating is created by etching equally-spaced slits in the epitaxial silicon layer, with an etch width of 160nm and an etch depth of 220nm. On the side of the excitation, an additional slit of same width and depth is etched on the 220nm silicon layer to minimise the reflection of the input beam. The distance "d" between the edge of the epitaxial silicon layer and the edge of the additional slit is varied.

The coupler is designed to work optimally for a wavelength of 1550nm.

The structure is represented below. A single mode fiber can be placed on top of the structure to couple light into the grating or to collect it; the simulation allows you to vary the properties of the fibre and its angle of inclination in the plane.

Principle of operation of the planar waveguide to fibre grating coupler.

Benefits of OmniSim for this simulation

OmniSim presents many benefits allowing it to model this structure extremely quickly and accurately compared to other methods.

• The calculation of the 2D structure can be performed quickly. Note that FIMMPROP is faster in this case, as it is able to perform a single-frequency calculation in less than 1.5 second!
• With OmniSim you can use three different methods to model this structure: FDTD, FETD and even FEFD. This allows you to check the benchmark your results. You can use FDTD for quick initial calculations; once you are close to an optimal design you can move to FETD to obtain higher accuracy.
• The calculations are very simple, and everything is set up from the GUI. There is no need to worry about setting up the PMLs or choosing the number of modes, and no need for data post-processing.
• The FETD and FDTD engines allow you to calculate the broadband response of the structure with a single calculation, by injecting a pulse in the time-domain.
• The built-in scanners allow you to study the effect of the fiber beam width, the inclination of the fiber, the number of periods etc. They can also be used to automate convergence tests.

Modelling of the structure in OmniSim

This structure was designed and simulated in OmniSim's 2D layout. The structure is shown below, as well as a screenshot of the real-time fields plotted during the FDTD and FETD calculations with different orientations of the Gaussian excitor.

Grating to fiber coupler designed in OmniSim with tilted Gaussian excitor on top

Real-time visualisation of the fields during the FDTD simulation for a tilt angle of -6 degrees

Real-time visualisation of the fields during the FETD simulation for a different tilt angle of +5 degrees;
you can see the adaptive triangular mesh of the FETD Engine superimposed over the fields.

For this simulation we can inject light either:

• with a mode excitor placed across the waveguide to inject the fundamental mode,
• or with a Gaussian excitor in the air region to model light injected from the fiber, as shown above.

We used the TE polarisation for the excitation (field polarised in the plane of the slab, i.e. orthogonal to the 2D layout). PMLs were introduced at all boundaries in order to absorb outgoing light.

Calculating coupling efficiency

After sending a pulse of light through the excitor we can calculate the coupling efficiency. You can see in the plot below the coupling efficiency spectra to the left-hand side and right-hand side waveguides, obtained with the FDTD Engine (grid of 10nm) and the FETD Engine (resolution of 0.5um, 4th order elements) for a 5um wide Gaussian beam launched vertically (no tilt). The FDTD data is shown in solid lines, the FETD data in markers only. As you can see the two curves are very well matched.

Coupling efficiency spectra to the left-hand side and right-hand side waveguides,
obtained with FDTD (solid lines) and FETD (markers) for a 5um-wide Gaussian beam launched vertically

Effect of tilt and beam width

We used the FDTD Scanner to study the effect of beam width and tilt angle on coupling efficiency. You can see below the results of coupling efficiency from the fiber to the waveguide mode at the left-hand side of the Device when varying the tilt angle.

FDTD Scanner showing coupling efficiency versus tilt angle as the orientation of the Gaussian beam is varied;
the excitor was parameterised so that the centre of the beam would always be aligned with the centre of the grating.

In the plot below you can see a scan showing the effect of beam width on coupling efficiency for the optimised tilt angle of -6 degrees; you can see that the optimal width is 5um.

FDTD Scanner showing coupling efficiency versus beam size for the optimal tilt angle.

Reference

[1] G. Roelkens, D. Van Thourhout, R. Baets, "SOI grating structure for perfectly vertical fiber coupling", ECIO Proceedings 2007 (PDF)