Hello Raymond,

It’s difficult to comment in detail without knowing more about your design requirements. Especially since the fiber design in your graphics is not one with which I am familiar.

But I can make a few comments:

In addition to the RSRE operand, I also see TRCX and TRCY operands. I suspect these were generated by the wizard, and create a section of the merit function that is driving the RMS spot size to zero. This conflicts with the RSRE operand which is asking for a non-zero spot size. Optimization will compromise and satisfy neither requirement.

Also, the algorithms that determine spot size work by filling the entrance pupil with rays from a field and calculating spot size from the resulting ray strikes on the image plane. Different tools in OpticStudio can arrive at different results when the fill patterns are different.

In the attached file, I followed your design, as best I could understand it from the graphic. There is a single on-axis object height field. Aperture is float by stop size with Gaussian apodization. The merit function uses only an RSRE operand. There is only one constraint which is RMS spot size, so only one variable is needed. I fixed the lens radius and allowed the distance to the image to vary. (If you allow both to vary, you could control the working f number.) I also increased the sampling parameter on the RSRE operand.

The result achieves the required spot size, and the RSRE operand and spot diagram agree. If you have more questions, perhaps you could attach your design as a ZAR file in a ZIP folder.