# Calculating 2d ring-resonator modes, from the Meep tutorial.
import meep as mp


def main():
    n = 3.4  # index of waveguide
    w = 1  # width of waveguide
    r = 1  # inner radius of ring
    pad = 4  # padding between waveguide and edge of PML
    dpml = 2  # thickness of PML
    sxy = 2 * (r + w + pad + dpml)  # cell size

    # Create a ring waveguide by two overlapping cylinders - later objects
    # take precedence over earlier objects, so we put the outer cylinder first.
    # and the inner (air) cylinder second.

    c1 = mp.Cylinder(radius=r + w, material=mp.Medium(index=n))
    c2 = mp.Cylinder(radius=r)

    # If we don't want to excite a specific mode symmetry, we can just
    # put a single point source at some arbitrary place, pointing in some
    # arbitrary direction.  We will only look for Ez-polarized modes.

    fcen = 0.15  # pulse center frequency
    df = 0.1  # pulse width (in frequency)

    src = mp.Source(mp.GaussianSource(fcen, fwidth=df), mp.Ez, mp.Vector3(r + 0.1))

    sim = mp.Simulation(
        cell_size=mp.Vector3(sxy, sxy),
        geometry=[c1, c2],
        sources=[src],
        resolution=10,
        symmetries=[mp.Mirror(mp.Y)],
        boundary_layers=[mp.PML(dpml)],
    )

    sim.run(
        mp.at_beginning(mp.output_epsilon),
        mp.after_sources(mp.Harminv(mp.Ez, mp.Vector3(r + 0.1), fcen, df)),
        until_after_sources=300,
    )

    # Output fields for one period at the end.  (If we output
    # at a single time, we might accidentally catch the Ez field when it is
    # almost zero and get a distorted view.)
    sim.run(mp.at_every(1 / fcen / 20, mp.output_efield_z), until=1 / fcen)


if __name__ == "__main__":
    main()