RMS of X/Y arm length using POX/POY lock is <160 pm and <120 pm respectively. RMS of free swinging X/Y arm length is both 0.17 um.

I used ALS error signal for out-of-loop evaluation of IR lock. We can even use ALS error signal when arm is free swinging because phase tracking ALS error signal is linear to arm length.
ALS error signal might not be as good as POX/POY. So, this out-of-loop estimation might be not so good.

X arm lock using POX11:
- Openloop transfer function
   I adjusted filter (C1:LSC-XARM) gain and now, UGF ~150 Hz, phase margin ~20 deg.
  570 usec delay (number in the figure is wrong) - Edited by Yuta on July 9


- Arm length spectra
   Red is the free run spectrum. Measured using C1:ALS-BEATX_FINE_PHASE_OUT, calibration factor in frequency is 9.81 kHz/deg (see elog #6938), so calibration factor is 1.32 nm/deg.
   Green is the out-of-loop spectrum. Measured using C1:ALS-BEATX_FINE_PHASE_OUT.
   Blue is the in-loop spectrum. Measured using C1:LSC-POX11_I_ERR, calibration factor is 3.8e12 counts/m (see elog #6841).
   Black is the expected spectrum from openloop transfer function, such as (free run)/|1+G|.



  Out-of-loop estimation of RMS during POX lock is 160 pm. But since this looks too large, ALS error signal might not see actual arm length change when arm length is locked.
  Also, it is interesting that ALS error signal sees 24 Hz peak, but POX doesn't. Roll mode coupling to green?

Y arm lock using POY11:
- Openloop transfer function
   I adjusted filter (C1:LSC-YARM) gain and now, UGF ~150 Hz, phase margin ~20 deg.
  570 usec delay (number in the figure is wrong) - Edited by Yuta on July 9


- Arm length spectra
   Red is the free run spectrum. Measured using C1:ALS-BEATY_FINE_PHASE_OUT, calibration factor in frequency is 9.65 kHz/deg (see elog #6938), so calibration factor is 1.30 nm/deg.
   Green is the out-of-loop spectrum. Measured using C1:ALS-BEATY_FINE_PHASE_OUT.
   Blue is the in-loop spectrum. Measured using C1:LSC-POY11_I_ERR, calibration factor is 1.4e12 counts/m (see elog #6834).
   Black is the expected spectrum from openloop transferfunction, such as (free run)/|1+G|.



  Out-of-loop estimation of RMS during POY lock is 120 pm. But since this looks too large, ALS error signal might not see actual arm length change when arm length is locked.
  Also, it is interesting that ALS error signal sees 16.5 Hz peak, but POY doesn't. Bounce mode coupling to green?

Next:
  - Noise budgeting of phase tracking ALS
  - Is it possible to lock MI when RMS of arm length during POX/POY lock increased to ~100pm?