40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Thu Aug 9 00:14:02 2012, Eric, Summary, Locking, YARM Locking and Calibration cal_swept_sine3_tfmagcal_swept_sine3_tfphcal_swept_sine3_cohsensing_func_model.png
    Reply  Thu Aug 9 00:35:53 2012, Jenne, Summary, Locking, YARM Locking and Calibration 
       Reply  Thu Aug 9 10:09:32 2012, Eric, Summary, Locking, YARM Locking and Calibration cal_swept_sine3_magnitude.pdfcal_swept_sine3_phase.pdfcal_swept_sine3_coherence.pdf
Message ID: 7130     Entry time: Thu Aug 9 00:35:53 2012     In reply to: 7128     Reply to this: 7134
Author: Jenne 
Type: Summary 
Category: Locking 
Subject: YARM Locking and Calibration 

Quote:

 Once you've got C1:LSC-TRY_OUT as large as possible, you've locked the cavity.


 Both the transfer function and the coherence look good above roughly 30 Hz, but do not look correct at low frequencies. There's also a roll-off in the measured transfer function around 200 Hz, while in the model the magnitude of the transfer function drops only after the corner frequency of the cavity, around several kHz. I have attached a plot of the roughly analogous transfer function from the DARM control loop model (the gains are very large due to the large arm cavity gain and the ADC conversion factor of 2^16/(20 V) ). The measured and the modeled transfer functions are slightly different in that the model does not include the individual mirrors, while the excitation was imposed on ITMY for the measurement.

 

The next steps are to figure out what's happening in DTT with the transfer function and coherence at low frequencies, and to understand the differences between the model and the measurement.

 The cavity is actually "locked" as soon as the feedback loop is successfully closed.  One easy-to-spot symptom of this is that, as you mentioned elsewhere in your post, TRY is a ~constant non-zero, rather than spikey (or just zero).  Once you've maximized TRY, you've got the cavity locked, and the alignment optimized.

We didn't get to this part of "The Talk" about the birds, the bees, and the DTTs, but we'll probably need to look into increasing the amplitude of the excitation by a little bit at low frequency.  DTT has this capability, if you know where to look for it.

It would be great to see the model and your measurement overlayed on the same plot - they're easier to compare that way.  You can export the data from DTT to a text file pretty easily, then import it into Matlab and plot away.  Can you check and maybe repost your measured plots?  I think they might have gotten attached as text files rather than images.  At least I can't open them. 

ELOG V3.1.3-