Step 3b: Non-circular? We can fix that...
A quick Beamscan sweep of the beam after the Faraday:
Position A1_13.5%_width A2_13.5%_width
(bench) (um mean) (um mean)
25.8 503.9 478.8
25 477.5 489.0
24 447.1 512.4
21 441.6 604.5
20 476.3 645.4
19 545.4 704.1
18 620.3 762.8
OK. It looks not too bad - doesn't look too different from what we had. Note that the x axis is in local table units - I found this useful for working out where things were relative to other things (like lenses and the FI) - but it means the beam propagates from right to left in the plot. in other words, the horizontal waist occurs first and is larger than the vertical waist. Also - they're not fitted curves - they're by-eye, best guesses and there's no solution for the vertical that doesn't involve offsets... discussion in a later part of the thread.
Anyway! The wonderful thing about this plot is that the horizontal and vertical widths cross and the horizontal focussing at this crossing point is shallower than the vertical. This means that we can put a lens in at the crossing point and rotate it such that the lens is stronger in the horizontal plane. The lens can be rotated until the effective horizontal focal length is right to fix the astigmatism.
I used a 200mm lens I had handy - a rough check sweeping the Beamscan quickly indicated should be about right though. Adjusting the angle until the beam size at a distant point is approx circular - I then move the profiler and adjust again. Repeat as required. Now… taking some data. with just that lens in:
24 371.7 366.1
21 360.3 342.7
20 447.8 427.8
19 552.4 519.0
18 656.4 599.2
17 780.1 709.9
16 885.9 831.1
Well now. That looks quite OK. Fit's a bit rubbish on vertical but looks like a slight offset on the measurement again.
The angle of the lens looks awful, but if it's stupid and it works then it isn't stupid. If necessary, the lens can be tweaked a bit more, but there's always more tweaking possible further down the line and most of the astigmatic behaviour has been removed. It's now just a case of finding a lens that works to give us a 50 um beam at the oven position...