Yuhang and Michael

Unfortunately in 2398 the measurement was performced incorrectly. Random noise from the spectrum analyser was injected into the "Perturb IN" port of the IRMC servo controller rather than the IN port of the IRMC HVD. This mistake was corrected, and we measured the following

• Power spectrum of dark noise, LO jitter and phase shifter excitation noise from the X, Y and T (Total) channels of the 500 Ohm position sensitive detector (PSD) - figures 1, 2 and 3 respectively
• Transfer functions and coherence of source noise to the X, Y and T channels of the PSD - figure 4
• Transfer functions of source noise to the IRMC error signal (at EPS1), correction signal (at Servo Out) and IRMC reflection - figure 5

The high frequency numbers for pitch are now much more in line with 1904. Notably, the mid frequency discrepancy between 2398 and 1904 still remains, where the mid frequency noise magnitude in this rearranged configuration is much higher than for 1904 despite having similar levels at the extremes of the measurement window. The PSD of yaw noise from the excited phase shifter is now signifcantly reduced compared to 1904, although the LO jitter is slightly above the PSD dark noise this time. We also measured the T-channel of the PSD to obtain the amplitude noise of the LO beam.

There is quite a lot of coherence from the source noise to both T-channel noise and pitch noise, especially amplitude noise above 30 Hz. The T-channel noise transfer function and coherence is fairly similar to the IR reflection, which is to be expected since they essentially just amplitude noise transmitted and reflected from the IRMC.

Source noise coupling to the error signal increases above 100 Hz, while the correction signal transfer function does not follow said increase. Howver, they have similar coherence across all frequencies.

Just as a note, when I checked and realigned the IRMC at the start of the measurements for the day, it seemed to have drifted out of alignment with a pitch HOM prominent in the reflection spectrum.