In Non-Sequential Mode a Gaussian point source can be generated using the object type “Source Gaussian”. A Source Gaussian is a point source that emits rays with a Gaussian probability distribution. When selected, the object type has two additional parameters that are different from a typical source:

• Beam Size (parameter 6)
• Position (parameter 7)

You can define the beam radius in the Beam Size parameter space. This should be the radius of your beam at the 1/e2 intensity of the beam. (Note: if you’re working with the FWHM measurement, see this Knowledgebase article on how to convert).

The Position parameter is the distance (along the local Z axis) between the location of your source waist and the apparent divergence location of the rays. This is used to set the divergence angle of the source. A special case is when set to zero, the rays are collimated. If you know the divergence angle of your beam, then you will need to use a little bit of trigonometry to determine what number to enter into parameter 7:

A Source Diode is another object type that can simulate a Gaussian point source and is useful if your system has any additional characteristics like Astigmatism, divergence, etc.

Thanks for the equation to calculate the Position parameter, it would be useful to have that in the documentation!

I just wanted to check the tan(...)^(-1) inverse power - is it 1/tan(...) or arctan(...) ? I.e. could one re-write your equation as:

Position = Beam size / Tan(angle/2)

Or

Position = Arctan(angle/2) * (beam size)

Also, are you using divergence angle as defined in Wikipedia’s diagram below?

Thanks!

Joe

Hi Joe,

Your diagram helps to answer your question. The right triangle is defined by W as the beam size and theta/2 as the half angle, with the Z coordinate as position. Then tan(theta/2) = W/Z, or Z = W/tan(theta/2). So it is the reciprocal of the tangent here, not the inverse tangent.

Re: converting FWHM into 1(e^2) radius...

Allie’s link to the old Knowledgebase article location no longer works.

Here is the new location:

https://support.zemax.com/hc/en-us/articles/1500005488161-How-to-convert-FWHM-measurements-to-1-e-2-halfwidths

The Knowledgebase Article number is at the bottom of the article:

KA-01545

Hope this helps.

-- Greg

BTW, I have found that if you are searching for a Knowledgebase Article by number, such as KA-01545 as referred to above, the Search function WON’T find it by typing “1545” into the search field…

you have to type the leading zero, e.g. “01545”

and then the Knowledgebase search will find it and place it at the top of the search results--which contain several other entries, in no discernable order, but all having “KA-015...” But if you type “015” it won’t find any of these at all! Best to type the whole KA number with initial zero.

The list of results is different from the Knowledgebase search field than from the Community search field.

I’d call this a bug, or at least a shortcoming in the Search functions.

-- Greg

This handy post from Allie of Zemax from 3 years ago really deserves to be in the User Manual: how to set the Position parameter to get the desired divergence using the Source Gaussian built-in NSC source:

But remember, the Source Gaussian is not quite physical, since the rays generated by this source, while weighted with a Gaussian probability distribution, all appear to emanate from the same point.  So this is OK in the far field (outside the Rayleigh range zR), but not good within zR of the source point.

I recommend using the AstigmaticGaussian.dll as a general NSC Source DLL for Gaussian beams these days, since it was posted in Code Exchange a few months ago:

https://community.zemax.com/code%2Dexchange%2D10/dll%2Dsource%2Dnon%2Dsequential%2Dastigmatic%2Dgaussian%2D1695

-- Greg