40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
 40m Log Not logged in
 Wed Mar 24 00:51:19 2021, gautam, Update, LSC, Schnupp asymmetry Wed Jul 28 17:10:24 2021, Anchal, Update, LSC, Schnupp asymmetry
Message ID: 16264     Entry time: Wed Jul 28 17:10:24 2021     In reply to: 15956
 Author: Anchal Type: Update Category: LSC Subject: Schnupp asymmetry

[Anchal, Paco]

I redid the measurement of Schnupp asymmetry today and found it to be 3.8 cm $\pm$ 0.9 cm.

### Method

• One of the arms is misalgined both at ITM and ETM.
• The other arm is locked and aligned using ASS.
• The SRCL oscillator's output is changed to the ETM of the chosen arm.
• The AS55_Q channel in demodulation of SRCL oscillator is configured (phase corrected) so that all signal comes in C1:CAL-SENSMAT_SRCL_AS55_Q_DEMOD_I_OUT.
• The rotation angle of AS55 RFPD is scanned and the C1:CAL-SENSMAT_SRCL_AS55_Q_DEMOD_I_OUT is averaged over 10s after waiting for 5s to let the transients pass.
• This data is used to find the zero crossing of AS55_Q signal when light is coming from one particular arm only.
• The same is repeated for the other arm.
• The difference in the zero crossing phase angles is twice the phase accumulated by a 55 MHz signal in travelling the length difference between the arm cavities i.e. the Schnupp Asymmetry.

I measured a phase difference of 5 $\pm$1 degrees between the two paths.

The uncertainty in this measurement is much more than gautam's 15956 measurement. I'm not sure yet why, but would look into it.

 Quote: I used the Valera technique to measure the Schnupp asymmetry to be $\approx 3.5 \, \mathrm{cm}$, see Attachment #1. The data points are points, and the zero crossing is estimated using a linear fit. I repeated the measurement 3 times for each arm to see if I get consistent results - seems like I do. Subtle effects like possible differential detuning of each arm cavity (since the measurement is done one arm at a time) are not included in the error analysis, but I think it's not controversial to say that our Schnupp asymmetry has not changed by a huge amount from past measurements. Jamie set a pretty high bar with his plot which I've tried to live up to.

 Attachment 1: Lsch.pdf  22 kB  Uploaded Wed Jul 28 18:21:46 2021
ELOG V3.1.3-