KAGRA MIR (Absorption)
ManuelMarchio - 23:29, Tuesday 02 October 2018 (1001)
1310nm probe alignment with the surface calibration sample

previous crossing point pinhole position

X 327.432
Y 121.255
Z 34.9000

now the 3 beams pass through the pinhole in this position
X 327.332
Y 120.823
Z 38.85
 
Since the pump is at 2deg (0.035rad) with the normal of the samples, I aligned the 1310nm probe to impinge the sample at 0.135rad.
After many attempts of alignment with the surface reference sample. This (attached screenshot) is the best signal I could get.
It is the same value I got last January (other screenshot): same AC, same DC, same phase.
But today it was much noisier.
The absorption of the reference is 0.2, the incident power 32mW and the AC/DC at the maximum of the scan is 0.028.
I put the sample on the peak of absorption and took 10min of data at 100ms of sampling period. See plot
The noise level in this case is 7000ppm.
no control loops or filtering were applied
 
Images attached to this report
1001_20181002142035_january2018.png 1001_20181002142039_october2018.png 1001_20181002162853_noisesurfref20181002.png
Comments related to this report
ManuelMarchio - 17:55, Wednesday 03 October 2018 (1003)

I tried to disconnect the cable (the signal from the PD) from the spectrum analyzer and the sr560.
The noise is still around 10'000ppm. So the noise doesn't come from some ground loops.

Then I changed the current of the laser diode and rotated the HWP to keep the DC at around 2V, but the noise didn't reduce a lot (still around 7000ppm)

Then I replaced the QWP with a PBS to clean the polarization and the noise reduced by a factor of 10

The alignment changed a bit after replacing the QWP. So the screenshot shows the new calibration scan. The AC signal went from 57mV to 45mV.

Now the noise is around 950ppm. See plot