The beat note out of the pre-cavity beat note detector was too small to be workable and the effort going into tweaking alignment was costing more time that it was worth. The problem was poor MM between north and south beams whose waists were offset by above 20 cm.
The setup of the BN detector is shown in the attached schematic. The lens in the south path was placed to give a waist of approximately 300 µm. The beam was profiled and parameters were:
Fitted horz beam waist: 275.2 +/- 9.564 um
Fitted horz beam position: -68.86 +/- 5.584 mm
Fitted vert. beam waist: 367.8 +/- 3.561 um
Fitted vert. beam position: 48.46 +/- 11.23 mm
Mean waist size is 321.5 um at position -10.2 mm.
data attached as 20170809_profileSouthPreCavBND.csv
To match the north path, I first profiled it. Parameters, referenced to the first steering mirror after the north PBS were:
Fitted horz beam waist: 186.2 +/- 1.054 um
Fitted horz beam position: -2.287 +/- 1.417 mm
Fitted vert. beam waist: 188.6 +/- 1.211 um
Fitted vert. beam position: 46.74 +/- 1.5 mm
data attached as: 20170809_profileNPreCavBNDFromPBS.csv
To fit the beams I passed a set of lenses available through a la mode (taking the average of the waists and positions as a la mode is only one axis) and it came up with a 0.986 overlap for a f=200 mm lens placed at 215 mm from the first steering mirror after the north PBS. I then re profiled the beam, getting beam parameters:
Fitted horz beam waist: 397.8 +/- 2.791 um
Fitted horz beam position: -47.48 +/- 13.3 mm
Fitted vert. beam waist: 346.5 +/- 2.717 um
Fitted vert. beam position: -171.2 +/- 3.256 mm
Mean waist size is 372.1 um at position -109.3 mm.
A la mode predicted 389 µm at -61 mm, so the lens positioning error is a bit off.