Alexa, Sheila

We were able to lock the x arm again today.

First we lowered the bandwidth of the ALS COMM loop.  Alexa and I had looked at her model a few days ago and saw that this will give us an improvement at high frequencies.  This looks like factor of 2 decrease in the RMS when we plot the noise with the cavity pole removed.  (The plot with the cavity pole still in is more relevant for the CARM handoff, but since this was an easy improvement we tried it anyway).  The first plot attached shows the noise with the new and old nominal settings.  The low frequency noise is not stationary, the new settings haven't really given us an improvement there although it looks that way from the plot.  The improvements from 700Hz up are real and repeatable.  After making this plot, we saw that the crossover was unstable, or we were ringing up one of the modes of MC2 possibly.  We changed the gain settings to -4dB on In1 and 3dB fast gain to move the crossover back down and keep the ugf the same. 

We also replaced the beamsplitter that splits the X arm transmission for DIFF and COMM with a high reflector to get more light on the COMM BBPD.  While it seems like this might have improved the 1/f noise, we aren't sure that this will be repeatable since this noise is nonstationary. 

In the last plot, we added an ND filter, to bring the light level back to what it would have been with the BS in place.  The high frequency noise stayed low, so this isn't related to the power, (we don't know why this changed in the first place).  The noise from 60-8Hz is back to the level with the BS, suggesting again that this is related to power. 

We have locked the arm on a 01 mode for IR at times, and we see that the high frequency noise is larger when we do that.