Hello there friends from the Zemax community,

I would like to ask for your help with a situation that I have. I am trying to optimize for a lens which is supposed to couple light from a collimated beam into a single mode fiber.

This is the procedure that I always try to use:

1. I use the FICL operand in order to estimate the fiber in-coupling efficiency, i.e, the overlap between the incident field and the defined fiber mode. For the fiber mode I always use the NA of my fiber.
2. Additionally I use a standard spot size operand in order to obtain a non-aberratated spot at the fiber plane.
3. I optimize for the spot and adjust the length of my collimating beam “free space” region in order to match the size of the spot needed for coupling into the fiber. In general, the wider the input collimated beam, the longer my free space region needs to be in order to have the right spot size at the fiber plane.
4. I use the FICL operator just as a monitor to have an estimate on the coupling efficiency

Instead of using this approach, I would like to directly use the FICL operand to optimize for the spot size. However, I have notice that if I just use the FICL operand, usually my spot becomes aberrated. Specifically, the spherical aberration contributions increase and I can observe from the ray diagrams that I do not have a nice a clean spot.

Is there a way to efficiently use the FICL operator to optimize for the spot size? Or Is there any other operand which I could use in this case? 

Any comment or feedback will be highly appreciated.