I’m running the attached simulation in order to extract S-parameters using FDTD for the shown structure. The simulation is using a combination of input source (pointing down), field monitors, and mode expansion monitors to simulate the S-parameters. The results obtained from mode expansion for frequency monitor at port1 shows that Port1.a and Port1.b only contain 0+0i, and hence S11(=Port1.a/Port1.b), S21(=Port2.b/Port1.b), S31(=Port3.a/Port1.b), and S41 have Indefinite values because a division by 0 occurs. I have checked all the design parameters to make sure there is nothing wrong. Everything looks fine. This problem only happens at port1. What do you think? What can cause this problem?
Through power at center wavelength (Input1): -1.#IND%.
Cross power at center wavelength (Input1): -1.#IND%.
Loss (Input1): -1.#IND%.
Through power at center wavelength (Input2): 72.1603%.
Cross power at center wavelength (Input2): 7.6635%.
Loss (Input2): 20.1762%.