I have looked at your file. I think one challenge of your simulation is that it has very strong resonance, meaning light is trapped in the cavity and it will take a long time for light to leave the simulation region. For this reason, you will need a very long simulation time to reduce the ripples.
Other than the simulation time, you can also reduce the Auto Shutoff (ASO) level from 1e-5 (default) to say 1e-6. This means that the solver will determinate the simulation at a lower ASO level, meaning more time to simulate the light trapping behavior.
After all, this is more like testing the convergence. The concept of results convergence is that you will be changing simulation parameters until the results become stable. This can be done to reduce simulation errors. Common parameters for convergence testing includes, mesh size, PML layers, PML proximity, etc. You will need to perform iterative tests to assess the effects of difference parameters on your simulations. Therefore, convergence testing can be time consuming. You can refer to this page for more information and example.
I downloaded your file and try to plug in some parameters to see if it reduces the ripples, basically I have used,
- longer simulation time
- lower ASO level
- smaller mesh
- larger PML proximity
- more PML layers
compared to your original file. However, you will need to perform convergence tests to confirm the optimized values for the parameters.
For the plot below, you can see that the ripples are reduced noticeably. For the amplitude of the dip, you can refer to @bkhanaliloo’s comments to increase the number of frequency point for a higher resolution plot to better resolve the peak.
The plot was created using this file 192) P=1180 - n=1.33 - Dgold=250 - scripted - fs=10000.fsp (249.5 KB)