The HOM/SB resonance of the current H1 OMC was recalculated with refined analysis.

Attachment 1 shows how the transverse mode spacing (TMS) depends on the PZT voltages. Note that the vertical axis is the ratio of TMS and FSR. PZT2 is the nominal OMC actuator for locking and PZT1 is currently used for the OMC shutter.

Attachment 2 shows the higher-order mode and sideband resonances that comes close to the carrier resonance. The calculation inludes up to 24th order HOMs. As per Sheila's suggestion, I have added the modes for CLF (+3.125123MHz and -3.125123MHz).

Over the entire range of the PZT (0~100V) , the 9th-order 9MHz upper sidebands is coincide with the carrier TEM00 resonance. The 23rd-order 9MHz lower sidebands ("-9MHz:23") and the 23rd order 3MHz lower sidebands ("-3MHz:23") are also resonant but probably it is not the primary issue. Otherwise, the CLF modes does not come into the TEM00 resonance.

So, in order to avoid the 9th-9MHz, we want to go to higher PZT2 voltage, but we limit the voltage to 100V to avoid possible "sudden death" of the PZT, which L1 OMC had.

Another possibility is to apply some voltage (<100V) to PZT1. This way, we can shift the structure to the left side. The results with PZT1=50V and 100V are shown in Attachments 3 and 4. This makes the 45MHz sidebans come into play but they are 13th order and 10th order. Considering the RIN of 45MHz sidebands is lower than the 9MHz one, this would help to improve the SB RIN coupling?

To allow this high but constant voltage application to PZT1, we need to modify the PZT driver circuit.