Hi @Suguna,

This sounds like an interesting project! The file that you shared is only a ZDA file. For us to see your design, you need to share a ZMX, ZOS, or ZAR file. Without reviewing the design, I’ll just make some general comments/suggestions:

1.  You can use the OpticStudio optimization tools to create optimized surfaces that fit your specific design requirements, a compact system with a circular spot and specific working distance. For more information to get started, please see this Knowledgebase article: How to optimize non-sequential optical systems – Knowledgebase (zemax.com).
2. If you are having trouble designing the lens, you can consider first creating a design in Sequential Mode. Of course, you won’t be able to simulate all the complexity of the multi-modal fiber interacting with a light tube, but it could give you a place to start. In Sequential Mode, it will be very straightforward to study working distance and spot shape/size. Then you can port that design back to Non-Sequential and further modify as necessary.
3. It’s difficult to give you an exact lens recommendation, but using Sequential mode, there are many tools for optimizing the surfaces (or even creating a group of surfaces) that will get you closer to the intended design. Check out this Knowledgebase article: How to design a singlet lens, Part 3: Optimization – Knowledgebase (zemax.com)

I hope this helps.

Best,
Ethan