
I scanned the X arm by changing an offset for the feedback to ETMX while the arm stayed locked by the green locking.
But the resultant plot is still far away from a beautiful one.
Changing the offset broke the lock frequently, so eventually I couldn't measure the stability of the IR-PDH signal at the resonance. 
The plot above is a result of the scanning. You can see there is a clear resonance at the center of the plot.
However the lock frequently became unstable when I was changing the offset.
It looked like this unstability came from the end PDH lock. I guess there are two possible reasons:
(1) feedback range for the laser PZT is not wide enough. Right now the range is limited by a SR560, which has been used for a summing amplifier.
(2) Length to Alignment coupling. Pushing ETMX causes a misalignment.
The issue (1) can be easily solved by engaging the temperature feedback, which helps actuating the laser frequency a lot at DC.
The issue (2) will be also solved by well align the IR beam, the arm cavity and the green beam. |