Position of temperature attribute in a thermally tuned waveguide and mesh

    • xh19363

      Hi everyone

      I try to simulate a thermally tuned waveguide and I have already read the example that is presented in Lumerical website. I use the same example with Lumerical but with slightly different dimensions and different material. I have set a Silicon Nitride waveguide. I used HEAT and then FDE in MODE. My final purpose is to find the phase shift using a script. I did everything as Lumerical example describes.

      In Lumerical example, the position of the temperature attribute is set in the middle ymin of the waveguide and after the run of the script to calculate the phase shift, I see the blue box and blue lines. This is exactly from Lumerical example


      Image 1


      In my simulation, I set the position of the temperature attribute in the middle of the ymin of the SiN waveguide (see the image below)

      Image 2


      After I run the script to find the phase shift, a blue box is appeared with the blue lines.

      Q U E S T I O N S  A B O U T   T H E  B L U E  B O X  A N D  L I N E S 

      1) What is that blue box and the lines?

      2) Why in Lumerical example (first image) the blue box covers not only the Si waveguide but also part of the SiO2?

      3) Why in my simulation (second image) the blue box covers only half of the SiN waveguide and not the whole waveguide? 

      4) Is this an error that only half of the SiN waveguide is covered by the blue box?

      5) I set the position of the temperature attribute at the center of the SiN waveguide (see the image below). In that case, the blue box covers the whole waveguide but the phase shift that is calculated from the script, is different. Which way is the correct position, second image of third image?

      Image 3


      Q U E S T I O N S  A B O U T   M E S H

      I set mesh to be the same as lumerical example.

      Image 4


      1) Why we need that extra mesh in the waveguide?

      2) What is the meaning of "override x mesh" and "override y mesh" and why dx and dy are set as 0.025 and 0.01 in Lumerical example?

      3) Is it correct to use that values for dx and dy in my simulation with the SiN waveguide? 


      Best Regards

    • Guilin Sun
      Ansys Employee

      The blue box should be the temperature attribute, and the orange box is the override mesh region. Ideally they should coincide.but the mesh can cover some SiO2 where may have relatively strong field.

      That said, you can modify the override mesh region. The temerature attribute contains temperature data. So it will affect the result if you place it at different locations.

      When the result changes with mesh size, it means it is sensitive to mesh size. you can do a convergence testing.

      The override mesh is to resolve the small change of refractive index due to heat.

      "override x mesh" and "override y mesh" are to override the background mesh size by the specified size. I think "dx and dy are set as 0.025 and 0.01 in Lumerical example " is arbitrary after a certain test. You can change them and see if the result changes.





      • xh19363

        I would like to know the correct position of the temperature attribute. In my example, where should I place the temperature attribute, in the center of the waveguide, in the lower side or somewhere else? Every time I change the position I get different neff. In Ansys website, I have not found any information about that. 

    • Guilin Sun
      Ansys Employee

      I guess you know where the temperature is, right? it should match the HEAT simulation. In HEAT, the monitor is to record the temperature. After importing in FDE, the temperature will be converted to index change.

      You should not arbitrarily change the position.  It is understandable why the result changes. In addition, the temperature monitor may need to be larger.

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