This topic explains how the Mode source works when injecting into waveguides at non-normal incidence and how the multifrequency mode profile calculation can benefit angled injection in broadband simulations.
Features
The Mode source can be configured to inject at an angle, just like the plane wave and gaussian beam sources. The additional complication with the Mode source is that there are two planes of interest:Source Configuration
1) Before the main FDTD simulation is run, the mode source must use it's built-in mode eigensolver to calculate the supported modes of the waveguide. This calculation requires a cross section of the waveguide normal to the waveguide propagation direction. This plane is drawn in the CAD with a white outline.
2) During the actual FDTD simulation, the source must inject along one of the global simulation axes (x,y, or z). This plane is shown in the CAD as a shaded plane with a while outline.
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Mode profile calculation and injection planes |
Rotations and multifrequency calculation settings |
Angled Injection and Broadband Simulations
It is matter of fact that the injected fields have constant in-plane wavevector at all frequencies. The in plane wave vector is chosen such that the actual injection angle at the center frequency of the simulation matches the nominal injection angle. Since the magnitude of the wavevector is proportional to frequency, the actual injection angle will change as a function of frequency. Higher frequencies will be injected at smaller angles, while lower frequencies will be injected at larger angles as explained here.
To avoid this problem, multifrequency mode calculation can be enabled in order to update the properties of the injected fields at specified frequency points and keep the injection angle constant across the simulation bandwidth. To demonstrate this, we have modified the simulation file above to inject wavelengths from 800nm to 1200nm. If the default single frequency calculation is used, the injection angle will be calculated for the center frequency of 1000nm. Figure 3 shows that the longer wavelengths are injected at steeper angle and the light bounces from the waveguide walls. Figure 4 then shows the same wavelength with the multifrequency calculation on. It is apparent that the injection angle error is eliminated.
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Injected fields at 1200nm for a simulation with center wavelength at 1000nm and single frequency mode profile calculation |
Injected fields at 1200nm for a simulation with center wavelength at 1000nm and multifrequency mode profile calculation |
