I have increased the CO2 Y laser from 0 to 110 mW as a part of the tuning of the differential lensing. No obvious imporvements or degradation were observed so far.

I am leaving CO2 Y as it is. If this setting turns out to make some noise performance worse, one can feel free to decrease the power.

[Background]

We have three tests/adjustments to do in this week and perhaps the coming as follows:

• Tuning of the CO2 differential heating
• Tuning of the CO2 common heating
• Implementation of the pre thermal loading

This alog reports an activity related to the first topic. The work has not been concluded yet. So we need to return to this point at some point.

[RF9 3rd HOM?]

There was a theory that the differential heating may be efficiently sensed by monitoring a 3rd order HOM of the 9 MHz rf sidebands. We obtainted this idea from a measurement that Dan did a while ago (alog 22175). However, according to today's test with PSL at 2W, this theory does not look promising. We should repeat this at 20 W.

[The test]

I have increased CO2Y from 0 to 110 mW. According to Aidan's simulator (alog 25587), this introduces a defocus of about 6 uD. Also, from the experience on this Monday, 400 mW of CO2 Y makes some part of the interferometer unstable and eventually unlocks. So this time, I chose 100 mW.

The interferometer stayed locked with AS RF45 Q with a nominal DARM offset (a few 10 pm) throughout the test. The PSL power was at 2 W. The OMC alignment was first optimized by engaging the length and dither alignment loops then I updated the QPD loops' operating points. I unlocked the OMC and changed the PZT voltage until I found the 3rd order HOM of RF9. The alignment was servoed bt the QPD loops. The 3rd order HOM was as high as 0.1 mA while the carrier was about 20 mA in OMC DCPD SUM. I wanted to lock on the HOM but unfortunately this was not practical. As the alignment chnages the peak height of this particular mode changes and also there was a 2nd order HOM of RF45 which is more stable and high. So for these reasons, the loop was not stable. I instead decided not to lock the OMC length and continuously adjust the pzt voltage by hand every few minutes or so. By the way, this mode was able to increase its power by a factor of 3 by changing the OMC alignment. Not great.

The attached trend shows some relavant channels over 1 hour. I do not see any obvious change in the carrier power (i.e. TRX, TRY and REFL_LF). The RF9 3rd HOM stayed constant throughout the test which is visible in OMC DCPD SUM since the OMC length was set such that it resonates with this mode. There was a slight change in POP RF18 which slightly increased by 6% or so with some amount of rf phase rotation. The interferometer stayed locked throughout the test. I have done a OMC scan after and before the test. I will process the data and post the results later.

Ideally, I should have monitored the laser noise couplings at the same time, but I did not do that this time. So, perhhaps next time.