Idea:
Try REFL_DC as the error signal for CARM rather than PO_DC.
Reasoning:
The PO signal is dominated by sideband light when the arms are detuned so that any misalignment in the recycling cavity introduces spurious signals. Also, the transfer function from coupled cavity excitation to REFL signal is not so steep and hence REFL should give a little more phase. Finally, the slope of the REFL signal should make it easier to hand over to RF CARM.
Conclusion:
The REFL signal showed no clear improvement over PO signals. We've gone back to PO.
During the night we also discovered that the LO for the MC loop is low. |