R. Short, J. Oberling

Today we tuned up the RefCav alignment for the PSL FSS, since the TPD had been doing its usual wander and also had a drop after last week's power outage.  As usual, we began with a power budget:

• FSS In: 266.3 mW
• AOM In: 267.2 mW
• AOM Out, Single Pass: 170.1 mW
  • Single Pass Diffraction Efficiency: 63.7%
• AOM Out, Double Pass: 119.4 mW
  • Double Pass Diffraction Efficiency: 70.2%
• EOM Out: 117.1 mW

The AOM signle pass diffraction is lower than usual, so we adjusted the AOM alignment to improve it.  While we were doing this we kept seeing small drops in the AOM output that we couldn't recover with alignment, which was making us think that we were walking the alignment off.  Turns out the PMC transmission was slightly dropping.  We remeasured the power incident on the AOM and sure enough, it had dropped a little, to 265.1 mW.  The diffraction efficiency looked good, so we stopped moving the AOM and moved on with adjusting M21 to improve the double pass diffraction (always have to adjust this mirror if the AOM is moved).  The results:

• AOM Out, Single Pass: 189.6 mW
  • Single Pass Diffraction Efficiency: 71.5%
• AOM Out, Double Pass: 133.6 mW
  • Double Pass Diffraction Efficiency: 70.5%
• EOM Out: 131.9 mW

There was no clipping through the EOM, but we adjusted it to better center the beam through the input and output apertures.

In recovering the RefCav, we decided to check beam alignment on the transmission PD, so I set up a LEMO splitter so we could see the DC voltage on a voltmeter.  The RefCav locked without much issue, and Ryan tweaked the beam alignment with the picomotor mirrrors.  Interestingly, the TPD voltage in MEDM did not match the voltage read by the voltmeter, so a recalibration there was necessary.  The TPD readings were not close to each other, at all, so we trended back to see the last time the TPD was calibrated.  Turns out this was at original PSL install in 2012, it hasn't been updated since.  Well then.  Alignment tweaking results:

• Initial TPD
  • MEDM: 0.508 V
  • Voltmeter: -0.320 V
• Final TPD
  • MEDM: 0.850 V
  • Voltmeter: -0.532 V

The negative sign is normal on these PDs from AEI, as they were designed to output a negative voltage; the sign gets flipped in the MEDM filter module.  I tweaked the beam alignment on the TPD, which increased the voltmeter voltage to -0.535 V, so it was decently aligned already.  We recalibrated the MEDM reading to match the voltmeter, and updated the offset to cancel out the small dark offset observed when the TPD was blocked.  Results:

• Gain
  • Old: -0.000336
  • New: -0.0002063
• Offset
  • Old: 130
  • New: 74.02

We then aligned the beam onto the RFPD and measured the RefCav visibility:

• Unlocked: 1.161 V
• Locked: 0.181 V
• Visibility: 84.4%

We were done in the enclosure at this point, so we exited and returned the enclosure to Science Mode.  Outside, we set up a network analyzer to measure the FSS TF.  With a common gain of 14 the FSS UGF was ~380 kHz.  We increased the common gain to 15 and the UGF was 445.6 kHz.  This is closer to where we generally like it, so we left the common gain at 15 (this can be reverted should there be an issue).  Ryan has a picture of the TF that he'll post as a comment to this alog.  With the lower, but now correct, TPD in MEDM, a couple of guardian thresholds required updating.  The TPD light on the Ops Overview screen now comes on if the TPD is less than 0.4 V and the "FSS Transmission Low" warning in DIAG_MAIN now comes on when the TPD is less than 0.3 V.  We'll monitor this over the coming days/weeks and adjust as needed.  This closes WP 12796.