Hi Jildy

I think I would change the definition of your input. Since you want to focus the fiber, I would define the System Explorer...Aperture using an object space NA and then the core can be modelled as different field points (for example 2 or 3 to start with). You can maybe tick Telecentric Object Space. It will make each field ray parallel to the optical axis.

The software will then be able to optimize to image the fiber. 

Let us know if that helps.

Dear Sandrine,
thank you for your reply.
I don't understand... I tried to use Aperture Type: Object Space NA, but nothing changes.
Why telecentric object space? I want the entrance pupil to be the optical fiber, that is my source (fiber coupled laser diode).
Is there a list of operands to be included in the merit function with their meaning? I don't know how to tell it to optimize in order to have the smallest possible image.

Regards

Hello Jildy

I have attached an example of a starting point.
For your optimization, I have set up a merit function for the smallest spot for each field but maybe you are looking at the smallest overall spot. Have a look at our free tutorials if you are quite new to the software as it shows how to set up the optimization: https://www.zemax.com/blogs/free-tutorials/imaging-system-fundamentals
How to design a singlet lens, Part 3: Optimization 

Let us know if that helps.