M. Todd, C. Compton

Camilla and I re-profiled the QCL 0918 unit again today to see where things were left and what the astigmatism in the beam looks like. The whole idea that's been planned was to rotate one of the lenses in the path to induce the right amount of astigmatism the other way (trying to correct the astigmatism present in the laser output) in order to bring the beamsizes on the ITM closer together. Sophie put together a nice analysis for each QCL unit to calculate the right angle and L2 translation to give the right beamsize and removal of astigmatism, alog 89719.

As she points out in her table, the L2 lens needs to be translated back by some millimeters to get the optimal beam size on the ITM. As it was, the L2 lens position was sitting forward so that the fitted beam should be smaller than this nominal value. What some of my modeling has shown is that with the rotation in L1 lens, any translation away from L2's optimal position will re-introduce astigmatism. In a sense this means that to get the adaptive telescope working without astigmatism would require the ability to change the L1 rotation angle actively as well, which we are not able to do. So we profiled the beam after M4 with L2 in its nominal position to see if there was astigmatism. We fit the profile and propagated the beam to the distance the ITM will sit at, and indeed there is a little over 2mm of astigmatism and (as expected) the beam size in both directions is smaller than the nominal by about 2mm (which is as expected). The beamsizes as propagated to the ITM were: tan=49.4mm sag=51.7mm.

I want to take the profile with L2 translated to the -12mm in Sophie's table, and see if the astigmatism goes away as expected before making any conclusions.

However, this issue has motivated me to try reworking the layout a bit, mainly featuring a longer "dog-leg" which pushes L1 further away from the laser. This is an attempt as putting the lenses in the right place so that we minimize the astigmatism while also maintaining acttive control of the beam size on the ITM. The new design was for the QCL 0920 unit, whose table is at a much lower readiness level. This design shows a much lower astigmatism as a function of the L2 translation, however the ITM beam size is much more sensitive to the L2 translation as the design favors a waist after the last steering mirror into vacuum.