Dear community

I’m working on a design which use the OAP to focus collimated beam into a SM fiber. In sequential mode, I want to check the focusing Gaussian beam size on the fiber tip, by using GBPS operand.  (the collimated beam is generated by fiber + collimator combination, where the collimator is a vendor blackbox.  So GBSS and POPD are not working in my real design).

The output quite confused me. It seems the GBPS reported the focusing spot by calculating from Parent Focal Length instead of Reflection Focal Length.

I duplicated this in a standard vendor zemax file. Below is a Thorlabs OAP with 90deg tilt angle, PFL=1.5”, RFL=3”. I set wavelength at 550nm, collimated input beam waist 1mm. The reported GBPS is 6.67um, and GBSS is 13.34um, POPD equal to GBSS.

It’s possible that I find a way to use POPD later, but really want to understand is there a way to get the GBPS reported data close enough. Thanks.