Liu and Poon's ring (gdsfactory's generic PDK), TE0 into the bus; materials at 1.545 um
the 500 x 220 nm strip's group index at 1.545 um (full-vector, 6.25 x 5 nm grid): 4.1792 dispersive, 4.0421 not (1.0339); the ring 75.747 um round
  free spectral range (nm), n_g and L     7.540394   paper 7.5        ± 1.5e-1   ok
all 1 within tolerance of the paper
