SkewRaysGaussian: need DLL supporting general astigmatism

Hi Greg,

I have spoken with the author of the DLL. She has sent me the source code and a higher-resolution version of the documentation. I am attaching both here. In this version of the documentation, the image which shows how position is defined is a little clearer. Although I agree that it is not obvious that a positive position represents having the source placed +Z away from the waist. I will submit an update to the Help System files to modify that point. 

The provided code is in C++, which is now preferred over C. With the code, you can clearly see how each parameter is defined. You should feel free to update the DLL code with the parameters you require. Instructions for compiling the code are given in the Knowledgebase article 'How to compile a User-Defined DLL.' 

Best,

Allie

PS: I will ask about adding a Non-Sequential tag for use on the forums!

Hi, Allie and Mark (and Sandrine and David?).

Thank you, Allie, for providing the source code and additional documentation for the circular DLL.  It will help me understand how it is done, but I don't plan on learning C++ or implementing a modified DLL myself as soon as I would like...  I hope that this topic is interesting enough, easy enough, and strategic enough to elevate my request in Sandrine or David's priorities.

Mark, thank you for your endorsement and your musings about more tightly integrating such features into (especially Sequential) OpticStudio.  Right now the Sequential UDS DLL from Paul Colbourne is available through the KnowledgeBase article, but not through e.g. a menu.  That is not a problem for me.  And my own immediate need, as I said, was for a Non-Sequential Source DLL capable of generating rays for 'beams' having 'general astigmatism.'  If it comes through this Forum thread or in another KnowledgeBase article, that's fine!

I have never used the 'Skew Gaussian Beam' feature myself under the Laser and Fibers Group under the Analyze tab, which Mark referred to.  I think this is a bit different, tell me if I am wrong.  Note that the word 'skew' in this Skew Gaussian Beam tool refers to the Gaussian beam axis being skew... not to using 'skew rays' as implemented by Paul Colbourne to 'simulate' and optimize generally astigmatic Gaussian beams.  So we might want two, more different-sounding names for these two features in order to avoid confusion.  I'm not sure what to suggest, since the word 'skew' is correct for both and has already been used for both.

Finally, Mark's suggestion of a Rotation Angle Ø is very interesting.  It could apply to the NSC Source DLL that Sandrine wrote as well.  There is more than one way to define an ellipse.  For example, instead of WaistX and WaistY, a similar set of parameters (yet another 'Definition?') could be WaistMajor and WaistMinor (with, say, the major axis always being in the same starting orientation e.g. local Y) plus Rotation Angle (about the local 'beam' axis).  I was trying to use the Source DLL in global coordinates to point the beam along X, Y, or (best!) Z.  As a result, I always thought of defining the WaistX and WaistY in local coordinates for the Source DLL, using the Object Placement parameters in NSC (see manual p. 480) X, Y, and Z, and then Tilt X, Tilt Y, and Tilt Z to point and twist the Source.  But this could get confusing if the beam is not propagating e.g. in the purely Z (or other Cartesian) direction in global coordinates.

So having a Rotation Angle parameter that is in the local coordinates ('beam coordinates') of the Source DLL might be nice.  Think about it... is it easy enough to specify an arbitrary-oriented (and twisted) Source DLL using Object Placement parameters and Reference Objects, or would this Rotation Angle parameter defined within the Source DLL object be really handy?

Although I mentioned more possible variations above, I don't require ALL possible 'Definition?'s for parameter input.  The user can be expected to do SOME calculation!  But the less one has to do, the more intuitive and user-friendly the tool becomes.  This is a tradeoff between convenience for the programmer--what is easiest and fastest to implement, and convenience for the user.  I'd vote for the programmer...whatever makes it easiest/fastest to get a version of the generally astigmatic Source DLL done so I can start using it!

-- Greg

Thanks for your attention and the update, Sandrine.

I think the astigmatic version of the Non-Sequential DLL should behave fine if the rays are launched as prescribed by Colbourne.

The Skew Gaussian in Sequential appears to me to be a different beast,

i.e.,from that described in Colbourne's KA-01772 , but you might be able to get some ideas from his User Defined Surface (sequential DLL) us_gaussXY.c .  This sequential DLL takes in a set of diverging rays and offsets them in position in angle to generate the rays to be traced.

Using skew rays to model Gaussian beams - webinar – Knowledgebase (zemax.com)

From his journal article on the method:

Generally astigmatic Gaussian beam representation and optimization

using skew rays

Paul D. Colbourne

SPIE-OSA/ Vol. 9293 92931S-8

'This was accomplished in Zemax

(optical design software available from Zemax, LLC) by creating a user-defined surface which offsets rays according to

their angle and the beam waist radii ω0x and ω0y which are entered as parameters of the user-defined surface:

. (28)

The system aperture type is set to “Object Space NA” with aperture value corresponding to θ0x so that rays originate

diverging from a point. It can be verified that the user-defined surface placed at the input converts each launched ray

(with arbitrary pupil value) to a skew ray meeting the conditions of Eq. (8), or Eq. (12) with γ=δ=0, and appropriate

values of p and α or β. The absolute value of θ0x is used in Eq. (28) so that by specifying positive or negative values of

ω0x and ω0y, right skew or left skew rays may be generated. The use of Eq. (28) requires that the input beam is elliptical

and aligned with the x-y axes, with both x and y waists located at the input surface. If the desired input beam does not fit

this condition, for example if it is astigmatic with x and y waists located at different planes, paraxial lenses or coordinate

breaks may be inserted so that an input beam meeting the condition is transformed into the desired input beam.

...'

The method is currently completely available in Sequential if one follows Colbourne's instructions, so I am interested in the Non-Sequential Source DLL above all.

Mark's suggestions to integrate Colbourne's method more tightly into Sequential OpticStudio would be icing on the cake!

-- Greg

Hi Greg!

Yes I was thinking. We also added another source DLL called GaussianSource.dll that was written by a user. It defines the spatial and angular distribution of the rays using the beam waist, the beam position and the M^2 factor. Would that DLL work for you?

Sandrine

I see that Sandrine replied while I was typing my latest message.  Thank you!

Sure, a circular Gaussian with specified M^2 is useful for our multimode VCSEL modeling.  I just found the GaussianSource.dll in the list of currently-available NSC Source DLLs.  Didn't know it was there!  Of course, it only does a circular, non-astigmatic Gaussian.

But please comment on Source Diode, too, and whether its independent control of divergence (both X and Y) accomplishes the same thing as a M^2 factor, and whether the astigmatism implementation is general.

We're going to need some new names to tell these apart, there are too many features that all sound like Source Gaussian and GaussianSource!!

-- Greg

Thank you, Allie, for chiming in again. This is helpful.

1. Your explanation of theSource Diode Astigmatism parameter does help to clarify what it does.

2. I'm afraid I didn't understand your discussion of divergence and M^2.

An elliptical Gaussian beam has two waists w0x and w0y, not just one w0. Divergence --and M^2-- can be different in x and y, too; in fact, Siegman's tutorials discuss the possibility of an Mx^2 and an My^2.  So a Source DLL for an elliptical beam should probably accept two parameters for Mx^2 and My^2 (which can be the same)--much like Source Diode has separately-defined divergences for x and y.  In addition, of course to parameter(s) for the positions (or separation) between the waists along the propagation axis.

There are also more complicated beams having various kinds of 'twist' as they propagate (according to Siegman), but I'm happy with just a general elliptical Gaussian with astigmatism.  And some way to rotate the x and y axes conveniently (if only using the local Tilt About Z for the NSC Source Object I am requesting; but as Mark mentioned, more flexibility is also needed in the Sequential versions than exist now).

-- Greg

Hi Greg,

Yes, OpticStudio provides a GaussianSource DLL that can be used to model the spatial and angular distribution of a Gaussian beam using the beam waist, the beam position and the M^2 factor. It scales beam waist size and far field divergence angle based on the M2 factor provided, as shown below. However, as you've noticed, this is for a circular beam with no astigmatism. And we cannot provide the source code for sharing and modificuation at this point due to proprietory reason. 

We also provide two built-in source models, Source Gaussian and Source Diode, to model Guassian beam, however they don't include aberration or effect of M2 factors. Source Gaussian models an ideal Gaussian distribution emitting from a point, while Source Diode allows an elliptical beam with different X and Y divergence angles as well as different X and Y waist sizes. But again, they don't take in M2 factors. 

It seems there isn't any model, built-in or source DLL, that's currently available in OpticStudio that can consider both astigmatism (different divergence angles in X and Y) and different M2 values along X and Y. If that's your goal, you'll probably need to compile your own source DLL to accomplish the task. And i agree, if we could introduce more flexibility in source modelling, it'll be very helpful for our customers. 

Hope this helps clear things up a bit. Please feel free to let us know if you have any other questions. 

Best regards,

Hui