This evening Lisa pointed out that the low carrier buildup can be explained by misalignment in the Y arm.
I spent an hour to carefully adjust the alignment. Indeed, after the alignment, I got a cavity flash of 45 (normalized for a single arm build up) which is the highest we ever obtained. Also, the funny REFL9I offset disappeared and I could see a PDH-looking signal there when CARM passed through a resonance.
What I did:
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Aligned ETMY to maximize the IR in Y. (CAREFUL: This is an opposite way of the proposed global alignment procedure)
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Then steered TMSY and added an offset in the TMSY QPDs to recover a high buildup for the green light.
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The resultant green buildup is now 10% smaller than the usual, but optimized for the infrared. Not terrible.
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ALS-Y_QPD_A_PIT now has an intentional offset of 0.8. This gave a high buildup for the green light.
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I did not steer ITMY or BS at all in this process. In other words, I relied on the PRMI local alignment.
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Then I did a mode scan of the Y arm to see how good the alignment is. The spatial HOMs are now small. Their power are roughly 10-20 times smaller than the carrier 00. Good.
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I tried the locking process again, starting from a CARM offset of 700 Hz with REFL45 for PRMI. This worked fine.
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The funny offset in REFL9I was much smaller this time. It was about 10 times smaller than yesterday. Good.
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I reduced the offset to let CARM resonate while holding PRMI with 135I and Q.
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I didn't have to re-adjust the demod phase at all this time. It was robust until the arm cavity started building up the carrier light.
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Apparently PRMI was not able to survive through the arm cavity resonances.
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However if the sweep through a resonance is fast enough, PRMI does not drop the lock since the carrier does not build up so high.
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I disconnected the REFL_DC_BIAS cable from the common mode board and connected the REFL9I signal in order to try self-lcoking.
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It seemed that a good control sign is positive at the input2 of the common mode board because this brought CARM closer to the resonance.
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I tried self-locking a couple of times with no success.
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I was not able to approach the resonance as close as 100 Hz because of the fragile PRMI before introducing REFL9I. So I tried to add REFL9I when CARM was as far as 150 Hz or so.
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Self-locking tends to push CARM to the resonance as expected, but IMC unlocked every time. The arm power went up to 3 or 4 before IMC unlocked.
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I tried different gains (almost randomly) in input2, but no luck. I guess that CARM is basically too far from the resonance and therefore self-locking can not squash such a big offset by its gain.
Also in the afternonn I did a simple, brief chek for the demod phase of all the REFL 1f PDs.
I swept the X arm through a resonance by using ALS comm and monitored the PDH signals. It seems that everyone is adjusted somewhat.
=== settings ===
REFL9 = 84.9 deg
REFL45 = 19.2 deg
REFLAIR9 = 90 deg
REFLAIR45 = 147.6 deg
The whitening gain for all the PDs was set to the maximum i.e. 45 dB in this measurement.
This is a summary of last week of locking work, for posterity.
https://dcc.ligo.org/LIGO-G1400576