I used OD filters to reduce the probe power.
I avoided using polarizing optics to limit the polarization fluctuations effects on the intensity.
I made the beam pass through the pinhole in 2 positions.

I used the surface reference to maximize the signal and made a scan. See screenshot 1.

The beam was not exactly passing through the center of the lens, so I think it made it astigmatic on the PD. 

I started over the alignment of the imaging unit. I moved the sphere with respect to the lens to find the sharp image of the blade.

In order to not saturate the PD, I changed the laser current from 300mA to  200mA, and the OD 2 to OD3. 

I found a larger signal and I maximized it moving the whole imaging unit, I made a scan for 3 positions of the imaging unit 20,25,30mm, and the maximum is at 30mm. See last 3 screenshots.