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Message ID: 9219     Entry time: Tue Oct 8 00:21:01 2013
Author: manasa 
Type: HowTo 
Category: Green Locking 
Subject: ALS arm stabilization 

Step by step procedure for stabilizing arms using ALS servo:

The procedure is the same for both the arms. 

0. Check that the ALS arm servos are turned OFF and not sending any signals to the ETM suspensions. 

1. Find the beat note by varying the laser temperature (moving the slider for SLOW_SERVO2_OFFSET).
Tip: It is easier to have the arms locked using IR PDH while searching for the beat note. Also check the stability of the MC. Unstable MC will cause the PSL temperature to drift and thereby affect the beat frequency.

2. Once you have the beat note, check if the beat amplitude is ~ -15 to -20 dBm. If the amplitude is small, then the alignment needs to be fixed (either the green input pointing at the end tables or the PSL green alignment). This is important because the UGF of the phase tracking loop (should be above 1KHz) changes with the amplitude of the beat note.
Also the beat frequency should be < 100 MHz; preferably below 80 MHz.

3. Disable IR PDH locking if you had used it while searching for the beat note. 

4. Press CLEAR HISTORY button for the phase tracker servo. Check if the phase tracking loop is stable (phase tracker servo output counts should not be ramping up). If the phase tracker is not stable, check the servo gain and phase margin of the loop.

5. Turn OFF all filters in the ALS arm servo filter module except for FM5 (phase compensation filter). With ALS arm servo gain set to zero, enable the arm servo and allow ALS control signals to be sent to the ETM suspensions.

5. Open dtt and look at the power spectrum of the ALS error signal (C1:ALS-BEAT?_FINE_PHASE_OUT_HZ). 

6. Set ALS arm servo gain +/- 0.1 to check the sign of the servo gain. Wrong sign of gain will make the loop unstable (beat note moving all over the frequency range on the spectrum analyzer). If this happens, set the gain to zero immediately and clear history of phase tracker servo. If you have set the correct sign for gain, the servo will stabilize the beat note frequency right away. 

7. Once you know the correct sign of the servo gain, increase the gain in steps while simultaneously looking at the power spectrum of error signal on dtt (it is convenient to set dtt measurements to low bandwidth and exponential measurement settings). Increase the gain until you can see a slight bump close to the UGF of the ALS servo (>100Hz). 
There have been times when this servo gain was in a few hundreds; but right now it varies from +/- 10-20 for both the arms. So we are stepping up gain in steps of +/- 2.

8. Enable filters (FM2, FM3, FM6, FM7, FM8). Wait to see the rms noise of the error signal go down (a few seconds).

9. Enable boost filter (FM10). There also exists a weaker boost filter (FM4) which we don't use any more. 


1. Beat frequency changes affect both the servo gain and sign of gain. So if you lose stability of ALS servo at any point, you should go through all the steps again.

2. At any point if the ALS arm servo becomes unstable (which can happen if the MC loses lock or if the beat frequency was too high ), change the servo gain to zero immediately. Turn OFF all the filters except for FM5 (if they were enabled) and reset phase tracker servo (CLEAR HISTORY button in the phase tracker filter module). Masayuki has written the down script that does all this. The script will detect arm servo loop instability by continuously looking at the feedback signal. Details about the script can be found here

Here is a cheat sheet that can give you an idea of the SLOW SERVO2 offset range to scan in order to find the beat note:

PSL temperature  X offset   Y offset
31.58                     5278       -10890
31.52                     5140       (not recorded)
31.45                     4810       (not recorded)
31.33                     4640       -10440
31.28                     4500       -10340

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