I order to reduce the size of the pump and keep the waist on the same position we should add a diverging lens before the converging one.
We designed a configuration on Jammt using a -100 lens and a +100 lens. See the attached image.
The new pump size will be 37.6um (radius).
A more precise design will be after a better characterization of the pump beam profile before the lenses
I made a measurement of the pump beam profile to make sure of the parameters without the last converging lens.
Result:
waist = 500um
waist position = -1m
the axis origin is the converging lens mount.
Then I updated the Jammt design using these parameters for the initial beam.
The current converging lens is f=150mm, so Jammt gives a resulting waist of 67um at 165mm from the lens. See figure.
I also made a measurement and fit after putting back the lens. the fit gives a waist of 70um at 171mm. Quite comparable with the jammt result.
then I updated the configuration to have a pump size of 70 - 75um (diameter).
See the diagram.
I need a diverging lens f=-125mm and a converging lens f=100mm