Measured the power of the probes with and without the surface calibration sample to check the transmission of the sample at different wavelengths

633nm: 2.87 mW -> 1.89 mW  Transmission= 65%

1310nm: 24.8 mW -> 16.6 mW Transmission= 67%

----------------------------------------------------------------------------------------

To check linearity of the Si PD, I measured the HeNe power and changed it putting some filters while measuring the PD DC signal
 

2.87 mW -> 6.58V
2.44 mW -> 6.52V
1.45 mW  -> DC=4.28V
1.22mW (2 filters) -> 3.4V
328uW -> DC=0.88V

Result: the PD is saturated, see plot