Sheila, Jenne, Evan

We have suspected for some time that the POPAIR B diode (which we use to extract POPAIR18 and POPAIR90 signals) is saturated in full lock, because these signals do not scale appropriately when increasing the PSL power. Now that we are seriously trying to use POPAIR90 to diagnose DRMI angular behavior during power up, we would instead like to use a signal that we can trust.

We tried a few different things today:

• Take the rf monitor for POP45 (in-vac) and feed it into the POPAIR90 demodulator. This seems to give a signal that scales appropriately with the PSL power (good), but the SNR is worse (bad).
• Take the rf monitor for POPAIR45 and feed it into the POPAIR90 demodulator. Same performance as above.
• Insert a 12 dB coupler directly into the POPAIR45 line. Feed the coupled output into the POPAIR90 demodulator. This gives a signal that scales appropriately with the PSL power and has comparable SNR to POPAIR90.

The last of these is the configuration that we have now. Whitening and dark offsets were set to give appropriate signal levels.

Careful examination will actually show that there appears to be a bit of saturation with this POPAIR45 signal as the power is increased. According to the POPAIR_A_LF channel, there is 83 mW (!) of dc power on this PD at 25 W. Probably we should be closing the POP beam diverter at this power, or else we need to swap some optics on the table.

Ideas for how to improve:

• Put the coupler on POP45 instead, since the POP PD has four times less light than POPAIR A. Requires rechecking the phasing of POP45 for MICH and SRCL in full lock. Has the advantage of giving us POP90 information even when the POP beam diverter is closed.
• Investigate whether POPAIR B can be made to give sensible signals. Perhaps we should modify it, just as we did in order to make REFLAIR B give sensible signals.