Discussion Forum

Hello, I am currently working on 2D simulations studying the modes of slab waveguides with gain and loss materials in their core, and I am having issues with the field evolution in the waveguides.

I adapted the geometry, materials (added gain/loss) and polarization (TM for me) of the verification example “dielectric_slab_waveguide” from the Wave Optics module to obtain my simulations, and they run fine. The mode profiles at the ports and the propagation constants calculated using Boundary Mode Analysis seem good.

However, the frequency domain study step leads to strange result : when I plot the transverse magnetic field in the structure for a waveguide with gain, there seems to be a lot of reflection at the “out” port causing the field to explode not only towards the “out” port but also coming back towards the “in” port (at the start/left of the waveguide - see image). The reflectance R and transmittance T calculated by comsol are both >1, and R is extremely high. This reflection decreases drastically for a 1-waveguide simulation if I set the mesh size to an exceedingly fine size (using the custom settings), but the calculated R is still >1, though significantly smaller than with a bigger mesh. For another simulation with two parallel waveguides, one with gain and the other with loss, the reflection is still visible in the field plot and in the calculated R, and does not seem to decrease when I set the mesh with the custom extremely fine size.

Do you have any idea where this reflection might come from and/or how I could avoid it ?

I also want to mention that I am calculating the same mode at the “in” and “out” ports.

You can find both the 1-waveguide and 2-waveguide models in the attached files.

Thank you