A bit more analysis concerning the OMC scans, the first (blue) and last (black) measurement are with SR aligned. And with SR misaligned (green and red)

Figure 1. Shows highlighted the 56MHz sideband TEM00 and order 1 mode, both for lower and upper sideband. In addition the order 1 (~74MHz) and order 2 (~142MHz) of the carrier are shown. What is clear is that with SR misaligned the power of the 56MHz order 1 mode increase by a factor at least 5, which sounds reasonable. This explains the appearance of a peak around 20MHz in the FSR.

Figure 2 Shows the carrier modes from 1 to 12 from left to right. The peaks at ~45MHz that disappear with SR misaligned is the order 12 mode of the carrier. The HOM powers look reduced in power by at least a factor 2, and up to a factor 10 (for mode 4). The order 2 modes look all with similar power.

We can actually have 4 measurement of the order mode power in a single scan, two crossed when the OMC temperature is increasing and two when it is decreasing. This is not the case for the first (blue) measurement, as the interferometer unlocked during the down going scan.

Figure 3 shows all the measurents, there is lots of scatter with a single configuration having factor ~2 changes. So the order 2 mode is constant versus SR misalignement, but the error bars of that statement are that is constant within an error band of a factor 2.

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