I found the big green beat note for the Y arm. The alignment of the green optics on the PSL table was crappy.
What I did:
1. By adjusting PSL laser temperature, I found tiny beat note when
PSL laser temperature on display: 31.35 deg C (PSL HEPA 100%)
C1:PSL-FSS_SLOWDC = 1.75
PSL laser temperature on display: 33.21 deg C (PSL HEPA 100%)
C1:PSL-FSS_SLOWDC = -6.82
Y end laser temperature settings are fixed as follows during the measurement.
Y end laser "T+": 34.049 deg C
Y end laser "ADJ": 0
Y end laser measured temperature: 34.13 deg C (*)
C1:GCY-SLOW_SERVO2_OFFSET = 29845
Bryan's formula (swapped one; see elog #6746), suggests the paring
(Yend laser temp, PSL laser temp) = (34.13 deg C, 31.09 deg C).
2. Checked that beat PD is working by swapping the beat PDs for Y arm and X arm.
3. Checked that the mode-matching of the two beams, one from Y arm and the other from PSL, is OK by moving mode-matching lens and measuring the beam spot size at near/far field are the same.
4. When checking the beam spot size at far field(~ 1 m from the BS), I noticed the relative beam tilt by ~ 1 mrad. We aligned them few days ago, but I think the green beam from the Y arm has shifted. Of course we align IR to the Y arm first, but we difinitely need dither servo or A2L for the arm, too.
5. As soon as aligning the PSL green optics near the BS, I found a large beat note. The measured amplitude was ~ -26 dBm, without any amplifiers after the PD.
Currently the measured green beam power onto the beat PD from Y end is 75 uW and from PSL is 92 uW. So the calculated beat amplitude will be ~ -10 dBm (see calculation in elog #6746). So there is about 84% loss. Anyway, I will go on to the mode scan.