[Koji, Jenne, EricQ, Manasa]
We had a short discussion this evening about what our game plan should be for transitioning from using the ALS system to IR-generated error signals.
The most fundamental piece is that we want to, instead of having a completely separate ALS locking system, integrate the ALS into the LSC. Some time ago, Koji did most of the structural changes to the LSC model (elog 9430), and exposed those changes on the LSC screen (elog 9449). Tonight, I have thrown together a new ALS screen, which should eventually replace our current ALS screen. My goal is to retain all the functionality of the old screen, but instead use the LSC-version of the error signals, so that it's smoother for our transition to IR. Here is a screenshot of my new screen:
You will notice that there are several white blocks in the center of the screen. From our discussion this evening, it sounds like we may want to add 4 more locking servo paths to the LSC (ALS for each individual arm, and then ALS for CARM and DARM signals). The reason these should be separate is that the ALS and the "regular" PDH signals have different noise characteristics, so we will want different servo shapes. I am proposing to add these 4 new servo blocks to the c1lsc model. If I don't hear an objection, I'll do this on Monday during the day, unless someone else beats me to it. The names for these filter modules should be C1:LSC-ALS_XARM, C1:LSC-ALS_YARM, C1:LSC-ALS_DARM and C1:LSC_ALS_CARM. This will add new rows to the input matrix, and new columns to the output matrix, so the LSC screen will need to be modified to reflect all of these changes. The new ALS screen should automatically work, although the icons for the input and output matrices will need to be updated.
The other major difference between this new paradigm and the old, is the place of the offset in the path. Formerly, we had auxiliary filter banks, and the summation was done by entering multiple values in the ALS input matrix. Now, since there is a filter bank in the c1lsc model for each of the ALS signals precisely where we want to add our offsets, and I don't expect us to need to put any filters into those filter modules, I have used the offset and TRAMP of those filter banks for the offsets. Also, you can access the offset value, and the ramp time, as well as the "clear history" button for the phase tracker, all from the main screen, which should help reduce the number of different screens we need to have open at once when locking with ALS. Anyhow, the actual point where the offset is added has not changed, just the way it happens has.
When we make the move to using the ALS in the LSC, we'll also need to make sure our "watch arm" and "scan arm" scripts are updated appropriately.
As an intermediary locking step, we want to try to use the ALS system to actuate in a CARM and DARM way, not XARM and YARM. We will transition from using each ALS signal to feed back to its own ETM, to having DARM feed back to the ETMs, and CARM feed back to MC2. We may want to break this into smaller steps, first lock the arms to the beatnotes, then find the IR resonance points. Transition to CARM and DARM feedback, but only using the ETMs. After we've done that, then we can switch to actuating on MC2. If we do this, then we'll be using the MC to reduce the CARM offset.
Once we can do this, and are able to reduce the CARM offset, we want to switch CARM over to a combination of the 1/sqrt(transmission) signals. The CARM loop has a tighter noise requirement, so we can do this, but leave DARM locked to the beatnotes for a while.
After continuing to reduce the CARM offset, we will switch CARM over to one of the RF PDs, for its final low-noise state.
We'll then do a quick swap of the DARM error signal to the AS port (maybe around the same time as CARM goes over to a PDH signal, before the CARM offset is zero?).
During all of this, we hope that the vertex has stayed locked. If our 3f sensing matrix elements are totally degenerate when the arms are out of resonance, then we may need to acquire lock using REFL 1f signals, and as we approach the delicate point in the CARM offset reduction, move to 3f signals, and then move back to 1f signals after the arm reflection has done its phase flip. Either way, we'll have to move from 3f to 1f for the final state.