Yesterday we found a rather large contamination in the corner ALS COMM signal. There is highly non-stationary line around 24.5 MHz. Suspecting this maight be the green RF sidebands that are leaking through, we went to EY to look for their exact location. Measuring the transfer function from the FIBR servo to the PDH error signal, we were unable to identify the sidebands. Next, we took a close look at the notch feature which marks the first FSR. It is located at FSR = 37.4658 kHz with an uncertainty of 0.1Hz. The 3 dB points (down from the flat part) are at 37.426 kHz and 37.506 kHz, 80 Hz wide.

The cavity length then becomes FSR / 2 / c = 4000.88 m.

The RF frequency is f_M = 24.515730 MHz. Dividing by the FSR we get
f_M / FSR = 654.350, or in other words the sidebands ar expected at 0.35* FSR =  13.113 kHz +/- 0.065 kHz.

Looking again at the full sweep and zooming in near 13.1kHz we indeed see a tiny feature in the phase plot around 13.188 kHz. It moves to 12.868 kHz when the RF sidebanda frequency is lowered by 300 Hz.

The only other features in the sweep which are clearly identifyable are at 17.739 kHz and 19.731 kHz symmetrically around half the FSR. These are most likely the second order modes. Dividing 17.739 kHz by 2 we get a corresponding mode spacing of 8.87 kHz. For OAT the mode spacing was measured at 8.92 kHz with a different ITM at a different position.