We will calculate the far field distribution of a Gaussian beam propagating at 30 degrees to the z-axis with an azimuthal angle of 15 degrees from a near field FDTD simulation. This example is intended to help new users understand how to calculate and understand results from far field projections.
The simulation file is a simple simulation with a Gaussian source propagating at 30 degrees to the z-axis with an azimuthal angle of 15 degrees.
After running the simulation, right click on the monitor 'z2' and select Visualize -> farfield from the context menu. The following window will appear which allows you to select the frequency to project if more than 1 frequency point was recorded by the monitor.
You can click the "Far field settings" button from the bottom left corner of the frequency selection window to open the following window which allows you to set additional settings.
NOTE: Changing the frequency and far field settings
The window to select frequency and far field settings can always be opened again by right-clicking on the "farfield" result for the monitor under "Result View". This can be used to recalculate the far field projection without rerunning the FDTD simulation.
Far field settings - details
- PROJECTION DIRECTION: This can be set to auto, forward, or backward. Auto will project in the field of maximum power flow, forward will project towards the positive axis direction, and backward will project to the negative axis direction, similarly to the 1 and -1 projection directions respectively when using the farfield3d script command.
- MATERIAL INDEX: This is the refractive index of the medium to use for projection. For more details see Changing the far field refractive index.
- FAR FIELD FILTER: The far field filter alpha parameter. For more details see Far field projections - Spatial filtering.
- 2D: Number of points in the far field to project to for 2D simulations.
- 3D: Number of points in the far field to project to in each ux and uy for 3D simulations. For details on the coordinate system see Far field projections - Direction unit vector coordinates.
Assume structure is periodic
The settings in this section are discussed further in Far field projections - Periodic structures.
Near field sampling rate
- OVERRIDE NEAR FIELD MESH: If selected, enables spatial down sampling of the data in the near field that is used in the far field projection. This can be used to speed up far field projection calculations.
- NEAR FIELD SAMPLES PER WAVELENGTH: Target number of samples per wavelength to use for near field spatial down sampling of monitor data.
After clicking OK, the visualizer window will show up. Notice that you can choose to visualize the far field intensity E2 (that is |E|2), or the complex vector field components Es and Ep.
It is also possible to calculate the far field with a few script commands. The script file will perform a projection to the far field using the farfield3d function, which returns |E|2. The plot that is created is shown below. This plot represents the electric field intensity (|E|2) on a hemisphere of radius R = 1m for z > 0.
|Note: To obtain the far field vector components in spherical coordinates, use the farfieldpolar3d script function. Es corresponds to Ephi (Ez in 2D), while Ep corresponds to Etheta.
Note: The projection direction is automatically determined by the direction of net power flow through the monitor.
As we expect, the Gaussian beam is propagating at 30 degrees to the z axis with an azimuthal angle of approximately 15 degrees.