J. Oberling, R. Short

The FSS RefCav TPD had been trending down again, and Ryan did not get enough improvement from a remote alignment tweak last week.  So today we went into the enclosure to do an on-table alignment of the FSS RefCav beam path.  As usual, we began with a power budget:

• FSS In: 282.9 mW
• AOM In: 283.2 mW
• AOM Out, SP: 174.4 mW
  • SP Diffraction Efficiency: 61.6%
• AOM Out, DP: 122.8 mW
  • DP Diffraction Efficiency: 70.4%
• EOM Out: 121.4 mW

No, we're not getting magic amplification between the FSS In and AOM In power measurements; that kind of small discrepancy is normal with the small stick power meter head we use for this alignment (they are also very AOI dependent, so it's entirely possible we had the head better aligned to the beam for the AOM In measurement).  Immediately we see the single-pass diffraction efficiency is lower than it normally is (generally hangs out in the lower 70% range these days), so we began by adjusting the AOM alignment to improve it.  This also means we have to then adjust M21 to improve the double-pass diffraction efficiency.  After the adjustments we had:

• AOM Out, SP: 199.4 mW
  • SP Diffraction Efficiency: 70.4%
• AOM Out, DP: 143.3 mW
  • DP Diffraction Efficiency: 71.9%

This is the lowest we've seen the diffraction efficiencies of this AOM, and suggests maybe it's time to swap the AOM for a new one (pretty sure this is the same AOM installed with the PSL in 2012, would have to go back through the alog to see if it's been swapped since install).  As usual, we had to do some small tweaks to the FSS EOM (provides the 21.5 MHz PDH sidebands for RefCav locking), and had to tweak the beam alignment into the RefCav using the input iris.  Once done, the RefCav locked without issue and our picomotor mounts were used to maximize the RefCav TPD; we began with a TPD of 0.57 V and ended with a TPD of 0.82 V.  We've had it higher in the past, but we have less power out of the PMC than usual (due to our slowly increasing PMC Refl that we have yet to figure out the cause of), and less power out of the PMC means less power available for the FSS RefCav which means lower maximum RefCav TPD.  To end we realigned the beam onto the RefCav RFPD and took a visibility measurement:

• Unlocked: 1.162 V
• Locked: 0.221 V
• Visibility: 81.0%

This finished the FSS RefCav tune-up, so we left the enclosure.  We left the ISS OFF while the enclosure returns to thermal equilibrium; it will be turned back ON once that equilibrium is reached, and Ryan will do a rotation stage calibration.  This closes LHO WP 12057.