Simulation tool and geometry



Dear Lumerical community,

I would like to simulate a ridge waveguide followed by a slabwaveguide for broadband wavelengths, as shown below. My orignal structure is hundreds of µm to mm long and also includes tilted waveguide. My structure is symmetrical so I can scale down the length to tens of µm, but I need to sweep it with respect to slab waveguide length so I would still need lengths of tens of µm. My questions are
1 - Which solver is better in my case for accurate results and time efficiency?
2- How can I simulate my stucture for single TE mode but broadband wavelengths. Which solver can accomplish this?
3 - Can i tilt the waveguide by tilting the sources and monitors are same angle. Which solver for this purpose?
I have also attached an FDTD version of my device which can give you more idea about my design.tilted_facet_2.lsf (8.3 KB)


Hi @nouman.zia,

I’m not too sure if a bit of the script is missing, but I got an error when running it (the variable Lp is not defined).
Regardless of the actual structure, here’s a few elements of response.

Typically, for long propagation distances, you can consider the EME and varFDTD solvers of MODE Solutions. Note the varFDTD solver can be used for planar waveguiding structures where there’s little coupling between the vertical slab modes.
If the waveguides are tilted in the propagation plane, then varFDTD could be possible, provided the waveguides have the same thickness.
I’m not too sure the EME solver can easily be used here, first because of the slab (the modes are not confined laterally, so it can b tricky) and also because of the tilt (that said, it could be worth trying!). The EME advantage is you can easily do sweeps over the length.
In fine, a full 3D FDTD simulation is recommended to validate the results from other solvers.

EME is a frequecy domain method, so you would have to run a sweep over the wavelength to get the full results.
On the other hand, varFDTD and FDTD are time domain method, meaning you can get the full spectrum in 1 simulation.

The mode source in FDTD and varFDTD can be tilted. On the other hand, monitors can only record data from the mesh, so they will be along the axis. The mode expansion monitor can be tilted and provide the coupling efficiency to a selected mode.
Regarding EME, I believe the ports can be tilted, although I never used them that way.

It would be great if you could provide the missing part of the script, and also let us know what result you would like to extract from the simulation. That will allow to provide a better recommendation to which solver to use.

Modal reflection (back reflection) of tilted ridge waveguide

Hi @gbaethge,
Thank you for your comments. I have attached my script file. I would like to calculate the modal reflection in to ridge waveguide (RWG) when the length of slabwaveguide (SWG) is changed.
I have placed a mode source in SWG and mode expansion monitors in RWG. Is it a right configuration to study light coupling (reflection) from SWG to RWG. I see an unexpected behavior (i.e. reflection in RWG increases when slab length is increased) when mode source is placed in RWG. My other question is that can I adjust the span and position of mode source in z-axis independently, as I want to launch my mode in ‘barrier’ or ‘QW’ layer those have high index.tilted_facet_3.lsf (8.6 KB)


Hi @nouman.zia,

Thank for the additional information and the script. I’d have a couple of comments regarding the simulation:

  • I’m not sure if this is intended, the structure stops before the FDTD boundaries, creating some additional interfaces and, therefore, adding reflections from both ends (reflection at the air/RWG and SWG/air interfaces).
  • The simulation region, mode source and mode expansion monitor are probably too small. The boundaries should be far enough so it doesn’t affect the mode calculation and propagation. This will be important in the vertical direction (for both RWG and SWG) and lateral direction (for RWG). The idea is to get the fields small enough near the boundaries.

Also, I’m not completely sure to understand the goal. In the simulation, there is no tilt at the interface between the 2 waveguides. As the second WG is a slab, there is nothing, laterally, that causes any angle, so the main effect will be the angle at the end of the waveguide.

Finally, as I mention, the source (and simulation) should be large enough. This means you cannot adjust the span and position to select a particular mode in a layer. The mode solver takes into account the full index profile to calculate the possible modes. You can change which mode is selected (fundamental or higher order).
Another difficulty is the thickness of some layers. For instance, QW is only 10nm thick, while the mesh size is quite larger.


Thank you for comments @gbaethge!

  • Yes, air gap is intended.

  • I have reduced the span for initial simulations. I will increase it for more accurate calculations. Is there any example or explanation related to this in lumerical KB that could help me to understant this?

My goal is to reduce the light reflection in to RWG. For that I want to study the effect of slab length on light reflection in RWG.
There is no angle between RWG/SWG (its 90 degree by default) interface as I have shown in schematic diagram of my first post. In the real case the whole geometry, including the RWG and SWG, is tilted with respect to facet, but in simulation I have tilted the facet with respect to the rest of geometry.
What I understood that if my source span in z direction is large enough then mode will be launched in to high index layer irrespective to the central position of mode source?
Can I reduce the mesh size locally into QW layer?
Thank you!