I assembled the filters on the PDs, now each PD has a FEL1250 and a FELH1250, for a total OD=11. On the spectrum analyzer the pump peak disappeared.

I moved the imaging unit closer to the sample to increase responsivity. I looked for the sharp image of the beam and maximized the signal with the surface reference sample.
I maximized the signal changing the imaging unit position.
I attach a screenshot of the best calibration scan I got. R = 7.7 W^-1, similar to the expected half of the HeNe one. (R = 18 W^-1)
I also plot all the scans in the same 3d plot. I chose to set the IU at 27mm.

I measured the LMA sample that absorbs about 6ppm using 10W of pump power. I made some scans and a map.

I measured the LMA sample that absorbs 0.65ppm, with open loop and with closed loop.

I plot all the scans together