In alog 52927 I posted up a video of the phase camera data taken for each sideband and carrier from DRMI to just after nominal low noise. Attached are some initial fits of a model to this data to see how each sideband is behaving during the process of locking.

The model

The model I have used for this analysis consists of two gaussian beams, one being the reference field and the other being the IFO field. The phase camera images are these two multiplied together. To get a good idea of the reference beam I took stretch of data looking at just the reference field without any IFO beam and measured the intensity at the camera. Fitting the intensity of a gaussian beam to this data we can get a good idea of the size of the reference field. The intensity of the reference field stays constant enough for each sideband over a lock stretch. For the curvature I assume the reference field is flat, which should be a good assumption considering the beam is coming out of a single mode fiber and collimated into a large beam about 70cm before the camera. "Knowing" the reference beam we can now fit the combined beam model to the actual amplitude and phase images coming from the phase camera. This fits beam size, radii of curvature, offset from the reference, angle from the reference, and amplitude of the IFO beam.

Results

This isn't an exhaustive or quantative analysis, more to come no doubt, but just what was noticed so far. On most of the plots I have plotted the average of both the upper and lower, as they are very similar.

• The sidebands look well balanced in amplitude and shape.
• There's a change in all beam size and RoC just before we power up. I need to double check when I get onto the site, but I believe this is during the MOVE_SPOTS state.
• To optimise the dynamic range of the phase camera to image all the fields you need to adjust both the camera exposure and the amount of reference light being used. Given limited time and a lack of data as we aren't connected to the CDS network, we don't do this. We just optimise to get the best SNR at full power, which also provides just enough SNR at DRMI to do something with the images
  • From carrier resonance to before powering up the SNR of the sideband RoC isn't great or the beam 
• During powerup we can see that the sidebands have different curvature and also trend differently to the carrier field
• Beam size of the sidebands and carrier also trend differently, X direction being moreso than the Y. Y direction is showing the sidebands having different shape.
• Amplitudes:
  • the 45 sidebands experience a dip in power but then recover around 1800s, not sure why
  • The 9 have a sqrt(2) drop in amplitude at ~2200s, from reducing the modulation depth