Members: Akihiro T., Matteo L. and Yuhang W.

Some remarks about the filter cavity locking

On 07 December, we tried to lock the filter cavity at first, and then we found some problems.

When we switched on the loop, the yaw of IM mirror went to auto-oscillation.

Trying to figure out where the problem came from, we measured the meachanical transfer function and the open loop transfer function of each mirror (pictures to be posted later, sorry).

As a result, the MTF and OLTF measured had similar form. We consulted Eleonora for this problem and she advised us that it was possible that the loop filters were not applied on the LABVIEW program which we usually use. This was because the PC was wrongly shutted down the other day. When the PC is shutted down, a program which offers us the loop filters is also shutted down. In this case, one need to boot all programs again correctly. Finally, after applying the loop filters, the whole system (the filter cavity) seemed decent. 

In the case of re-booting the PC, please boot the LABVIEW programs used to manipulate yaw and pitch of each mirror,and also a program named "my_Filter_Bank.vi" and load a file "filtrotot.txt" in it.

AOM miss-alignment

We also found that the first order diffraction of the green beam, which is inserted to the filter cavity, had less power. It is posssible that AOM is not alingned properly. We did rough measurement of beam power at some different points (shown below).

Right after SHG cavity: 57mW

Right after EOM: 49mW

After beam spriter before AOM: 9mW

Before iris in front of the chamber: 8mW

After the iris (the 1st order diffraction): 0.25mW