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Entry  Tue May 12 01:11:55 2009, Yoichi, Update, LSC, DARM response (DC Readout) SUS_Resp.pngDARM_Resp.png
    Reply  Tue May 12 15:22:09 2009, Yoichi, Update, LSC, Arm Finesse 
Message ID: 1577     Entry time: Tue May 12 15:22:09 2009     In reply to: 1575
Author: Yoichi 
Type: Update 
Category: LSC 
Subject: Arm Finesse 

Quote:

It looks as if the measured DARM response is skewed by an extra low pass filter at high frequencies. I don't know why is it so.


One large uncertainty in the above estimate is the cavity pole of X-arm because I simply assumed that the ITMX reflectivity to be the designed value.
I think we can directly measure the X-arm finesse from Alberto's absolute length measurements (i.e. from the width of the resonant peaks in his scans).
By looking at Alberto and Koji's posts (elog:1244 elog:838), it looks like the FWHM of the peaks are around 3kHz. With the FSR ~ 3.8MHz, it gives a finesse of about 1300, which is reasonable.
Alberto, can you check your data and measure the FWHM more precisely ?
Note that we want to measure the FWHM of the peak in the *power* of the beat signal. The beat amplitude is proportional to the electric field *amplitude* of the transmitted auxiliary laser. What we need to get a finesse is the FWHM of the transmitted laser *power*. Thus we need to take the power of the beat signal.
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