I checked lock scheme of FNC and found some control signals saturated.
And also, guardian says FNC continues to lock even if feedback signals saturate.
FNC-X
Fig1 shows control signals for FNC-X around locking (~9s).
- Just after the lock, the feedback signal to the tweeter which is also the error signal of woofer saturated on ADC.
- The feedback signal to the woofer saturated by software limit a few seconds after locking.
FNC-Y
Fig2 shows for FNC-Y around locking (~8.5s).
- Just after the lock, the feedback signal to the tweeter which is also the error signal of woofer saturated on ADC.
According to the ALS user manual (JGW-T2113031), fringe signals has nominal amplitude.
(+/-1000ct for X and +/-900ct for Y. Is X slightly small...?)
I' not sure if it was originally saturated or if recent changes (e.g. obs mode in PSL room, some works around POP table etc.)
have made it so, but we had better to investigate.
Anyway, before touching the optics alignment as described in the "manual", it might be better to check the power got back from the POP table on the PSL table; do we have such a monitor port???
I can find only two green PDs on POP table.
One of them is DCPD which monitors fiber output coming from PSL. The other is RFPD for the arm lock.
As for the saturation issue, I also checked last month data.
Woofer output was already saturated in last month.
Fig. 1 shows the 1month-long data of control signals and guardian flags for X (left) and Y (right).
There is no long-time lock according to the guardian flags (brown line).
It was locked for a relatively long time around June 24, and at this time we can see the saturation of the woofer output.
Fig. 2 is an enlarged view of around 6/24, and the woofer output is represented as green line.
This means that saturation was occurring even before the recent works around POP table.
Difference from last time klog17464 is a fringe amplitude. (~2000ct p2p => ~4000ct p2p)
On the other hand, FNC-Y feedback saturated just after requesting lock.
A fringe amplitude of FNC-Y is smaller than one in last time. (~2000ct p2p => 1500ct p2p)
I also checked the fringe amplitude recent 1 week (See Fig2)
Fringe amplitude changed in time around nominal value in the manual.
And also, somehow X and Y tend not to get better at the same time.
Is it coming from instability of fiber coupling?
I want to know laser power also at Prometheus output.
But there is no such a monitor...
