I tried to see the transmitted flash but I could not on Monday.
Indeed, the beam size was at its waist was much larger than the design value i.e., 100 um though 50 um is required.

I decided to change the mode matching lenses in order to achieve the designed spatial mode.
To be honest, the obtained minimun size was about 80 um radius.

I gonna try improving the mode matching until end of this week.

Note

The lenses are biconvex or biconcave in order to reduce the aberration of reflected beam.