FRED performs diffraction and interference calculations using a technique called coherent beam superposition. The coherent beam superposition technique works by modeling arbitrary optical fields with the coherent summation of smaller fundamental beams. In FRED, these smaller fundamental beams are generally astigmatic Gaussian beamlets. It was demonstrated by J. Arnaud that Gaussian beams could be represented and propagated with real rays. Those real rays can be traced through an optical system while maintaining the Gaussian beam representation. The near and far field diffraction patterns can be calculated coherently summing the Gaussian beams, which are represented by real rays traced through the system.
If the Source is set as coherent on the Coherence page, then each of the ray positions defined in the source becomes a Gaussian beamlet that is represented by two, four, or eight secondary real rays. In addition to choosing the number of secondary rays, the overlap of the adjacent beam overlap factor, and the secondary beam scaling can be set on the Coherence page.
This feature can be accessed by selecting the Coherence tab from a Detailed Source dialog box.
The Number of Sample Points for Coherent Source Power Scaling controls on the Coherence tab of a detailed source address specific scenarios in which the algorithm used to set the total power of a coherent source fails to converge on the requested value. When a coherent source is created, the following steps are taken:
An irradiance distribution requires an area over which the calculation is performed and a grid resolution (number of pixels) over that area. The "Field sample points in X and Y" controls allow the user to specify the grid resolution of the internal irradiance calculation and these controls are sufficient for being able to achieve the correct coherent source power scaling in most cases. The area over which the irradiance calculation is performed is controlled by the "Spatial sample plane size scaling in X and Y" settings. The default size of the internal irradiance grid used in step (2) above is determined by the maximum extend of the secondary rays in the initial rayset. This default size can be determined by performing a gaussian ray size spot diagram at the source location and looking at the extents of the resulting plot window.
The scenario in which the default size of the internal irradiance grid is insufficient occurs when the extent of the secondary rays is significantly larger than the spatial extent of the irradiance distribution itself. In such a case, the default size of the internal irradiance grid will potentially have insufficient spatial resolution over the area of the power distribution to recover an accurate scale factor for the ray fluxes. The most common case that may encounter this scenario is a point source.
How do you know when you need to use these scale factor settings? With coherent sources, it is always good practice to perform an irradiance calculation on the source alone (prior to propagation, at the source position), since it will allow you to confirm that the spatial distribution at the source plane matches the requested specifications supplied to the source dialog and that the total integrated power in the distribution matches the requested source power. When the total integrated power returned by the irradiance calculation does not match the requested power, consider adjusting the spatial scale factor parameters if you are defining a point source.
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