Sheila, Camilla. Saved to camilla.compton/Documents/sqz/templates/dtt/Fringe_wrapping.xml

ZM2,5 Fridge Wrapping with No SQZ light

As Sheila suggested in 86836, the amount of backscatter we are seeing from SQZ should be due to light from the IFO being backscattered rather than any excess SQZ light. To confirm this I took SQZ DOWN, mis- aligned FC, blocked all three SQZ beams on SQZT0 and re-opened the beam-diverter so that the IFO light could get into HAM7 but there was no SQZ generated light. The ZM2 and ZM5 scatter shelves were the same in both this and the nominal SQZ configurations, confirming all backscattered light is coming from the IFO. Plot attached. This plot does show our ZM5 scatter has increased since 2024, ZM2 is the same.

I did the same with the 30Hz injection too, saved to Fringe_wrapping_nosqz.xml, refs 0-8 are no sqz beams, refs 9-17 are nominal FDS. Plot attached, if anything, the shoulders of the 30Hz peak are larger with no sqz beams, so again, there is no difference. 

ZM2,5 Fridge Wrapping with Different Alignments on OFI_PD_A

In FRS # 35457 and 87071, we theorized that the backscatter could be off the OFI_PD_A which we shows out beam is clipping, when we change the alignment to this PD using ZM4/5, we change the level of light on OFI_PD_A, showing we are clipping and not centered on the PD, but this also changes the level of SQZ so it's hard to compare. Unsure if there's a real difference, plot attached and ndscope. It's possible that with more light on PD_A (less clipping) there is a more scatter and a secondary ZM5 scatter shelf ~25-40Hz. Would want to repeat or misalign more to confirm. 

Other ideas that haven't been done yet: change the temperature of SFI2 and repeat backscatter measurements, repeat above with increased alignment changes the OFI_PD_A changes.