Plot attached showing that the Higher Order Modes around 10.4 to 10.6kHz are in a similar position during ER16 (blue) that they were at the end of O4a (yellow).
This is expected as we have not changed the TCS system (CO2 and RH settings the same). Unsure why in the last long (2024/03/28) they are larger, the noise floor changes with SQZ but wouldn't expect the peaks to change much.
Vicky, Camilla. Attached plot has comparisons of No SQZ times, the noise floor at 10kHz seems to have decreased since O4a. Using times in Jennie's 76516 show similar noise floors.
Late alog: After the Emergency vent and OFI crystal swap in July/August, in late August we saw that the two HOM peaks had moved into a single peak. Plot: Blue = pre vent in July, Brown and green = Post vent in August.
Dan Brown made the quick model attached showing that the IFO to OMC mismatch does have an impact on the height of those peaks. From Dan: The plot looks at mode mismatch change and how laser frequency to DCPD coupling scales with it. If changing the OFI has modified the mode-matching or more likely just reduced HOM scattering then seeing smaller peaks could be due to that.
