There is not propagation inside the waveguide

konstalekkkonstalekk Member Posts: 3

I have a silicon waveguide 5um x 220nm which, the BOX and the cladding are SiO2. I use a mode source and I select the fundamental mode to be propagated. I had used the mode source and that waveguide dimensions in another application and it seemed to work well, but here it doesn't work. I would like to plot the E field in the waveguide by using a frequency domain field and power monitor.

The result of the E field is

From the visualizer, we can see that there is not propagating in the waveguide. I was expecting the wave to travel inside the waveguide. Why that happens and how can I fix it?


  • kjohnsonkjohnson Posts: 172Ansys Employee

    Hello @konstalekk ,

    It looks like the mode source is too small for the waveguide, so it is not properly calculating the mode. The span of the mode source needs to be large enough such that the mode field decays to around 10^-3 or 10^-4 at the edges of the mode source. Visualizing the mode profile is helpful for this. Please see the "source span" section of this post for more information:

  • konstalekkkonstalekk Posts: 5Member

    Hello, thank you for the reply

    I read about the the key FDTD simulation settings and I increased the source span. Also, I checked the other settings to be OK. I run my simulation and again there is not propagation in the waveguide. Also, the simulation stops of around 14%. The simulation reaches the auto shut off min (which is 1e-05) and then is finished. Moreover, I increased the simulation time but again the simulation is finished at 14%.

    Here is a image of the source fundamental mode. I select the option "fundamental mode" and then visualize data.

    The plot of the E field from "monito_1" is the following. Again no propagation in the waveguide

    As you can see, propagation in the waveguide does not happen. It is like there is not waveguide there. Could you make me any suggestion about how to fix that?

  • konstalekkkonstalekk Posts: 5Member

    Hello @kjohnson

    In the previous simulations, I used a waveguide and I had given specific values at the poles so as to create a straight waveguide.

    As I have already shown in the previous comment, no propagation happened. But when I replaced the waveguide with a rectangle and I used exactly the same dimensions, I noticed that there is propagation.

    Why when I use the waveguide, no propagation happens and when I use the rectangle, the propagation goes very well?

    Is the waveguide only to create S bends and not straight waveguides?

  • kjohnsonkjohnson Posts: 172Ansys Employee

    It looks like the top oxide and the waveguide geometry objects are overlapping, what are their mesh orders? The waveguide object should have a lower mesh order to it's index is used. If they have the same mesh order, the object that is lower in the Objects Tree will be used. I suspect that this is what is happening in your simulation, when you replaced the waveguide object with the rectangle the rectangle was placed lower in the tree:

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