Sending Collimated Light onto OAP

  • 25 January 2023
  • 3 replies

Hey all,

First question here and new user of the software (~2 months). Let me know if I have violated any rules of posting.

Question: I have downloaded the following reflective fiber collimator from ThorLabs and am attempting to send collimated light ‘into’  the OAP. The downloaded lens files has collimated light entering the image plane, which I do not want.

My goal is to analyze the spot size/fiber coupling efficiency of this OAP if I were send collimated light onto the OAP, with a single mode fiber at the focus. How can I go about using this lens file? How can I have an infinite-finite conjugate relationship instead of a finite-infinite?


~ Luke


backup link for reflective collimator:


Best answer by Jeff.Wilde 25 January 2023, 22:29

View original

3 replies

Userlevel 7
Badge +3

Hi Luke,

Welcome to Zemax!  You’ve actually picked a rather tricky problem to cut your teeth on.  Modeling OAPs in sequential mode is definitely a test of using offset apertures and coordinate breaks (decenters/tilts). 

Unfortunately, there is no “easy” way that I’m aware of to reverse the OAP fiber collimator model in order to make it a fiber coupler model.  You have to roll up your shelves and make several changes manually, including changing the aperture as well as the orientation of the OAP relative to the starting surface.  Here is an article on modeling OAPs that may be helpful:

How to model an off-axis parabolic mirror

A search of the user’s forum will likely produce other helpful posts.

On top of modifying the OAP layout, to add fiber coupling analysis introduces another layer of complexity, as there are various ways to approach the analysis.  For example, using rays to find the coupling efficiency (via a Fourier transform of the associated exit-pupil wavefront error, followed by a mode overlap integral) is computationally very efficient, but requires the aperture to be Gaussian apodized without clipping the wings of the Gaussian (assuming you are inputting a collimated Gaussian beam).  

I have quite a bit of experience with the software, so to save you some effort I went ahead and made the necessary changes, including incorporation of the coupling analysis using both ray-based and POP-based approaches.  However, I would encourage you to try and make the changes yourself, using my model to help if you get stuck.





Wow Jeff! Too kind… I truly appreciate it.

My initial approach when attempting to alter the OAP myself was to somehow rotate all surfaces of the OAP about the optical axis. Then, I thought I would be able to switch the aperture from object space NA to entrance pupil size, with light entering from infinity. Its good to know I was not too far off. I have much to learn.

I’m going to spend some time reviewing the changes you made so that I may be able to replicate them. I will be posting my progress when I get to it. Coordinate breaks stack in a confusing manner!


Thank you for help!

Userlevel 7
Badge +3

Hi Luke,

You’re most welcome.  One tip: when working with OAPs it may be helpful to remove the offset aperture, make changes to the position/orientation of the parabolic surface,

and then re-apply the offset aperture as a last step.