![fdtd example lumerical fdtd example lumerical](https://i.ytimg.com/vi/wmXSnptIctc/maxresdefault.jpg)
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: