Since we're thinking about RAM (residual amplitude modulation), also known as RFAM (RF amplitude modulation), I did a quick Optickle simulation to see what effect RAM will have on some of our length degrees of freedom.

Here, all of the Optickle simulations plotted were run with 50W of PSL input power, and a +10pm DARM offset (+5pm for ETMX, -5pm for ETMY).  So far, I've just plotted the vertex length degrees of freedom, but I'll do a similar simulation for CARM and DARM soon.

The bottom panel of each of the attached PDFs shows power buildups that are relevant for that degree of freedom.  The other panels are the RF PDs that we use as error signals in full lock.  PRCL uses POP_9_I, MICH uses POP_45_Q, and SRCL uses a combination of POP_9_I and POP_45_I. 

One might note that even in the absence of RAM, the error signals do not have a zero crossing at 0m, and the power buildups for the carrier are not centered at zero.   This is a result of our finite DARM offset.  Removing the DARM offset centers things up closer to what your intuition would indicate. 

I've used a RAM modulation depth of 1e-3, which is roughly what we had measured at the 40m some time ago.  I haven't yet looked at Sheila's data from last night to update this value for us. 

Interestingly, the RAM always pushes the lock point offsets in the same direction, regardless of DARM offset.  But, the DARM offset changes which size of 0m each zero crossing is in the absence of RAM. So, with a positive DARM offset, having some RAM actually puts the zero crossing closer to 0m, while with a negative DARM offset the RAM pushes us farther away.