peter.king@LIGO.ORG - posted 12:57, Tuesday 07 August 2018 - last comment - 19:00, Tuesday 07 August 2018(43291)
PMC work related to the ISS
Rick reminded me that the current pre-modecleaner was meant to be operated with a UGF of 1 kHz, as per
T1700543. There are also loop shape changes that are not
implemented with this servo card. However in order to reduce the UGF to 1 kHz, both the optical and electronic
gains had to be reduced. The unlocked light level was previously ~1.35 V, is now 0.28 V. This has been
reflected in changes to the locking thresholds on the MEDM screen.
SCRN0006.jpg shows the pre-modecleaner open loop transfer function after the above changes were made.
SCRN0007.jpg shows the power noise measured with an out of loop photodiode. Gone are the sharp ~22 kHz
and harmonics peaks.
Images attached to this report
Comments related to this report
Back here (alog 30682) and here (alog 30701) we saw some noise increase at higher UGFs. So we should keep the capability of reducing the UGF to ~few hundred Hz for future noise hunting.
Right now, the reflected light on the RFPD is turned down using the transmitted beam through a rotatable (thin film?) polarizer and it is at about the minimum level. Thus, the light on the RFPD is mostly the unwanted polarization. I suspect this is not what we want. If we replace the folding mirror directly upstream with a less polarization-selective beamsplitter (or two) we should be able to improve the ratio of wanted to unwanted polarization and get more attenuation for reducing the servo UGF, when desired.