In last measurements, we noticed a larger noise when measuring the Tama-size sapphire sample.

I did some measurement of the AC signal from the lock-in, in different conditions with two samples: small sapphire sample and Tama-size sapphire sample.

Acquisition time: 1h

Sampling rate: 100ms.

Pump OFF

Probe ON

In order to check if the blocks vibrations were the noise source, I placed the small sample on the blocks .

Plot1: Comparison of small sample sitted on the blocks and small sample attached at the translation stage. The noise is almost the same

To be sure the vibrations don't cause the noise, I removed the translation stage to make enough room, and I placed the Tama-size sample tightly fixed at the optical board. Picture1

Plot2: Comparison of Tama-size sample sitted on the blocks and Tama-size sample fixed at th eoptical board. The noise is almost the same.

Plot3: Comparison between small sample and Tama-size sample, both tightly fixed. The larger sample gives a larger noise.

Conclusion:

The only different thing is the thickness.

Let me make an hypotesis. Let's suppose the noise comes from the angle fluctuations of the probe, which cause a fluctuation of the spot position on the photodetector. The probe passing through a thick sample have a longer optical path. This is like if the detector was further. And a further detector sees more angular fluctuation, like in an optical lever. So, next check, I will see how the noise change when I change the detector distance.