kiwamu.izumi@LIGO.ORG - posted 05:49, Thursday 11 February 2016 - last comment - 14:03, Thursday 11 February 2016(25501)
Left the interferometer in observing
Evan and I left the interferometer in the observing mode at around 20:00 pm local last night. The ISS 2nd loop now keeps correcting the diffraction power all the time via a feedack loop and maintains it at 8%.
Comments related to this report
kiwamu.izumi@LIGO.ORG - 14:03, Thursday 11 February 2016 (25506)
By the way, we had a difficulty in engaging the ASC soft loops yesterday. Those loops kept dragging the optics to a point where the recycling gain is low enough to unlock the interferometer.
This issue is not resolved yet although we were able to manually engage the loops through a very careful and slow process.
I suspect some part of the interferometer has misaligned in a very subtle way.
[The symptoms]
The recycling gain could reach as high as roughly 38 which is more than enough to stably achieve full lock. However, as soon as the guardian came across the ASC_PART3 state, the interferometer unlocked. This happened multiple times yesterday. The error signals for all four soft loops had signals were as big as 0.2 before engaging the loops. Since the error signals are derived from the TRX and Y QPDs, this roughly means that the beam spots on TRX and TRY were off from the optimal by 20% of the QPD range. With these big offsets, once we closed the loops, the interferometer started drifting slowly away from the optimal alignment on a time scale of probably a few minutes which was then followed by rapid decrease in the sideband power in the recycling cavity and the carrier power every where on a time scale of about 10 seconds and then unlocked.
[ I don't think TMSs are culprit]
Then I looked for suspicious drift or jump in the angles of the TMSs in trend because this is very similar to the case we had during O1 (alog 22575). I did not find obvious misalignment with the TMSs in the past few days. In addition, the digital offsets in the TR QPDs seem to have unchanged at least in the past week.
For the record, in the past we had a trouble when the input optics were not well aligned (alog 23538). Looking at trends of those optics, I did not find obvious misalignment in the past few days either. The IM4 TRANS and POP B QPDs seem to have almost the same values as they should be. Performing a full initial alignment sequence did not help this issue either.
[Manual engagement]
Later, we tried engaging the loop manually and succeeded in it. This is just a test and not a permanent solution. We added an intentional digital offset to each loop so as to cancel the large signals at the error points. Then we engaged all the loops with the nominal gains. Since the error signals are already close to zero by the digital offsets, the servo did not really do on the soft degrees of freedom as expected. We then gradually reduced the digital offsets to zero very slowly so that the soft loops come back to the intended operating points. Since we did not want to lose the lock we went very slowly and took about 20 minutes or so to complete the process. Lame.
A good thing is that as we reduced the offsets, TRX, TRY and POP signal kept increasing. This indicates that the QPD signals from TRX and TRY are still good reference point. I conclude that the interferometer is the one which has been misaligned rather than the TMSs. Since the recycling gain can be still as high as 38, the misalignment we are chasing here may be something subtle.
By the way, we had a difficulty in engaging the ASC soft loops yesterday. Those loops kept dragging the optics to a point where the recycling gain is low enough to unlock the interferometer.
This issue is not resolved yet although we were able to manually engage the loops through a very careful and slow process.
I suspect some part of the interferometer has misaligned in a very subtle way.
[The symptoms]
The recycling gain could reach as high as roughly 38 which is more than enough to stably achieve full lock. However, as soon as the guardian came across the ASC_PART3 state, the interferometer unlocked. This happened multiple times yesterday. The error signals for all four soft loops had signals were as big as 0.2 before engaging the loops. Since the error signals are derived from the TRX and Y QPDs, this roughly means that the beam spots on TRX and TRY were off from the optimal by 20% of the QPD range. With these big offsets, once we closed the loops, the interferometer started drifting slowly away from the optimal alignment on a time scale of probably a few minutes which was then followed by rapid decrease in the sideband power in the recycling cavity and the carrier power every where on a time scale of about 10 seconds and then unlocked.
[ I don't think TMSs are culprit]
Then I looked for suspicious drift or jump in the angles of the TMSs in trend because this is very similar to the case we had during O1 (alog 22575). I did not find obvious misalignment with the TMSs in the past few days. In addition, the digital offsets in the TR QPDs seem to have unchanged at least in the past week.
For the record, in the past we had a trouble when the input optics were not well aligned (alog 23538). Looking at trends of those optics, I did not find obvious misalignment in the past few days either. The IM4 TRANS and POP B QPDs seem to have almost the same values as they should be. Performing a full initial alignment sequence did not help this issue either.
[Manual engagement]
Later, we tried engaging the loop manually and succeeded in it. This is just a test and not a permanent solution. We added an intentional digital offset to each loop so as to cancel the large signals at the error points. Then we engaged all the loops with the nominal gains. Since the error signals are already close to zero by the digital offsets, the servo did not really do on the soft degrees of freedom as expected. We then gradually reduced the digital offsets to zero very slowly so that the soft loops come back to the intended operating points. Since we did not want to lose the lock we went very slowly and took about 20 minutes or so to complete the process. Lame.
A good thing is that as we reduced the offsets, TRX, TRY and POP signal kept increasing. This indicates that the QPD signals from TRX and TRY are still good reference point. I conclude that the interferometer is the one which has been misaligned rather than the TMSs. Since the recycling gain can be still as high as 38, the misalignment we are chasing here may be something subtle.