This entry summarizes the (too many) measurements performed last Thursday and Friday...
I started by checking the bulk reference sample as it was the last calibration measurement performed ( see entry 2510).
First, I had to change the HWP that controls the ir laser power from 118 degrees to 119 degrees in order to get the usual Pin~28 mW.
Then, I found out that the calibration was lower than expected at 0.5 cm/W.
This, together with the (small) laser power change seems to indicate that the beam parameters changed during a 2 weeks window.
My guess is that, the temperature/humidity increase of last week affected the laser beam parameters (effects of temperature inside TAMA central building can be found in entry 2493 where PR pitch motion seems to follow trends similar to temperature change troughout the day).
I replaced the bulk reference sample by the surface reference sample and started several measurements...
To summarize :
I checked the IU optimal position : 68 mm is still good.
I checked the sample Z position : after several tests, I found out that a good position for the surface reference sample is at Z = 39.5 mm
This corresponds indeed to have the surface of this sample at the ir beam waist and crossing with the red beam (details in entry 2446)
With the good sample Z position, I still could not achieve 'optimal' calibration factor.
I checked again the IU position and confirmed than IU at 68 mm is optimal.
I found out that having this sample Y center (ie Y = 121.2 mm) at the beam crossing was giving :
y = 121.2
AC_surfref = 0.4075;
DC_surfref = 3.966;
acdc = 0.1026;
P_in = 27.6e-3;
R_surf = 16.8972
while moving the Y position to 120.2 mm gives
AC_surfref = 0.411;
DC_surfref = 3.96;
acdc = 0.1038;
P_in = 27.6e-3;
R_surf = 17.0949
I will need to check if this sample has some dust in the center...
Finally, I moved on the spare viewport measurement :
I placed it on the TAMA holder using 2 spacers to fix it.
The good Z position to have the surface at the 2 beams crossing point is z = 38 + 60/2 = 68 mm
The good IU Z position is 68 - 3.4 = 64.6 mm (here I assumed 1 cm thickness of the viewport).
Unfortunately, at that point I moved by mistake the last lens on the IR path... So I performed again the surface calibration as :
AC_surfref = 0.4125;
DC_surfref = 3.974;
acdc = 0.1038;
P_in = 27.7e-3;
abs_surfref = 0.22;
R_surf = 17.0331 (screenshot saved at 20h35)
I re-installed the spare viewport and tried to measure its absorption.
28 mW did not show any signal.
428 mW (ie HWP at 89 degrees) did not show anything as well...
As this power is around the maximal that can be produced with the laser diode current of 1 A and because any current change requires ~1h to stabilize the laser power, I stopped the measurement at that point.
Anyway, this tells us that the absorption is quite low...