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Q&A: Confusion using apertures with Gaussian beam apodization

Question
“I am attempting to acquaint myself with the Gaussian Beam propagation options in OSLO (ABCD Analysis, Astigmatic Gaussian Beam and Fiber Coupling). I have read and followed the “Gaussian Beam and Fiber Coupling” tutorial offered on your website. The problem is that when I change the system aperture or physical apertures on individual surfaces, the calculations do not respond the way I expect. Can you explain this?”

Synopsis
Confusion using apertures with Gaussian beam apodization

Solution
“Note that this note does not apply to ‘ABCD’ or ‘Astigmatic (Skew) Gaussian beam’ analyses – see the aforementioned tutorial for a good explanation of the differences in these analyses.

When using real ray tracing to simulate Gaussian beam propagation in OSLO, people sometimes become confused with how the different apertures interact. There are three different aspects of apertures that you should be aware of when trying to use real ray tracing to calculate Gaussian beams in OSLO.

1. SYSTEM APERTURE: Think of this as the parameter that limits the size of the bundle of rays that enter your system. If, for example, your lens apertures are larger than your System Aperture, then rays will only be traced through a small subsection of your lens aperture. If your lens apertures are much smaller than the system aperture, then perhaps only a small fraction of the rays from the starting ray bundle will make it through your lens apertures.The system aperture can be defined in a number of ways: “Entrance Beam Radius”, “Object NA”, …etc., by setting different values in the Paraxial Setup Editor dialog. It makes no difference which of these parameters you use to define the system aperture, it always results in defining the cross-sectional size of the ray bundle that is used in OSLO calculations. For calculation purposes, a certain number of rays are always defined or sampled across the aperture. The user can change the number of rays across the aperture that are used in calculations:

  • In Spot Diagram calculations, the number of rays across the aperture used for calculations can be changed in the Paraxial Setup Editor dialog. If left unchanged, the default is 17.03 rays across the aperture.
  • In Fiber Coupling calculations, the number of rays across the aperture is defined when the calculation is performed. Look in the dialog that results from the “Source>>Fiber Coupling” menu option and you will note that the default number of rays across the aperture is 32.
  • In Point Spread Function (PSF) calculations, the number of rays across the aperture is also defined when the calculation is performed. Look in the dialog that results from the “Source>>Truncated Gaussian Beam” menu option and you will note that the default number of rays across the aperture is 32.

2. GAUSSIAN APODIZATION FACTORS can be thought of as transmission multipliers (always 1) that reduce the energy of the beam entering your system at different radial locations in the beam.The Gaussian apodization factors (SDGX and SDGY) dictate the size of the Gaussian apodization that is applied to the system, by defining the size (semi-diameter) of the 1/e^2 point. These factors work as if you placed a physical Gaussian apodizing filter on your system. These factors are also applied in the Paraxial Setup Editor.
3. THE PHYSICAL APERTURES on individual surfaces act as light blocks.The physical apertures on each optical surface can potentially limit the amount of light passing through the system. The light rays will be blocked (vignetted) and the ray calculation will be affected if the surface apertures are BOTH (a) smaller than the size of the limiting beam on that surface, AND (b) they are “checked” by setting the “Checked(K)” flag in the “Aperture Radius” column of the surface data spreadsheet. The description of “checking” rays can be found on page 93 of the hardcopy version of the “Optics Reference Manual” (page 102 of the electronic version).
Some examples of what can cause odd interaction between these aperture settings:

  • If the system aperture or individual surface apertures that are checked are not ~2x larger than the 1/e^2 Gaussian apodization size, then the rays at the edges of the Gaussian apodized bundle will be clipped too close to the 1/e^2 value, and the far field diffraction pattern of the Gaussian bundle will display artefacts.
  • If the system aperture is too large, then the Gaussian apodization size will only extend over a small portion of the system aperture and only a few sampled rays will be used in the calculation – the resulting analysis will be unreliable. You can correct this problem by making the system aperture size smaller (but not too small – see the previous item), or increasing the number of sampled rays across the aperture.”

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