Camilla, Sheila

We looked at some signals related to the ALS X beatnote, relating to the problems described in 78703 and 78691. 

This morning the PSL is down so there is no light coming out of the X end or Y end fiber.  For the Y arm, the beatnote PD gets about 0.2mW from the fiber; the X arm beatnote PD doesn't show a noticable step down when the PSL went offline, indicating that there is much less light coming from the fiber reaching the X beatnote PD than the Y.   This has been the case at least since last Novemeber, when the PLL seemed to be working fine. 

The total power on the two beatnote PDs are similar, both just over 2mW. When we aren't having these temperature dependent changes in the X beatnote, the two beatnotes have similar sizes according to H1:ALS-X_FIBR_A_DEMOD_RFMON.  As long as the amplification at the two end stations are the same, this means that the beatnote alignment and mode matching at end X must be better than at EY, to get the same beatnote strength with much less light from the PSL.

Without light from the PSL, the ALS X beatnote noise floor is -32dBm, ALS Y beatnote noise floor is -41dBm.  This means that when TJ saw an X beatnote of of around -30dBm, 78745 , there was effectively no beatnote.  Camilla and I toggled the noise eater for both lasers, shown in the third attachment here.  The X laser has a larger increase in noise than the Y laser, but both beatnotes returned to their previous values after we turned the noise eaters back on.

The last attachment shows a trend of the beatnote strengths over the last few weeks.  Although the problem had come and gone a few times in the past, it doens't seem to be resolving itself now.