I don't think this is the case but assuming that you're using PML boundaries, make sure they are far enough from the mode fields so that mode does not overlap with the PMLs as it propagates. If the calculated phase velocity is not changing, then your mesh is probably fine. For meshing, try different auto-mesh settings. 0.99 is a good value, try not to go below that unless you have to. To get the most accurate phase velocity, dt should be as close to 1 as possible, but smaller. For the mode solver this is not an issue because the mode solver calculates in frequency domain and there is no need to fit the refractive index whereas FDTD is in time domain, hence the need for the material fit. In case the problem is how you set up the simulation, take a look at these:Īre your materials dispersive? If so, you should make sure Lumerical FDTD fits the material properly to account for the wavelength dependence of the refractive index. Am I missing something fundamental about FDTD? I've tried being very careful with the meshing, dt stability factor, source pulse width and monitor sampling. My "pulses" are essentially monochromatic over the time scale of the simulation. I start the simulation and then note the first time at which each monitor reaches an extrema in field strength (=the pulse has reached the monitor). Technique: to measure the speed of light, I'm placing two monitors a fixed distance apart (several wavelengths). This tells me it's something to do with how I've set up the simulation, rather than something to do with my waveguide. I find that the speed of light has the same relative error - 1-5% off no matter what I do. To troubleshoot this, I deleted the waveguide entirely and tested the speed of light in my simulation. However, the phase velocity is quite off, usually by 1-5%. I've used a mode solver and I know what the mode cross section should look like, and when I take the model over to FDTD, it gets the mode shape right. I'm trying to simulate a single-mode slab waveguide. I'm a bit new to FDTD and so I apologize if this is a noob question but:
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