thomas.shaffer@LIGO.ORG - posted 16:39, Tuesday 21 August 2018 - last comment - 18:04, Wednesday 22 August 2018(43564)
HWS EY Realigned with 50um Core Fiber
Danny V., Georgia M., TJ S.
Today we continued to touch up the alignment of the EY HWS path with the 50um core fiber that was installed last week. We ended up translating the BS2, the polarizer, and L3 because we seemed to have a good alignment to the irises, but it was not centered on these optics. There was some bright fringes seen on the outside of the beam that we cut out using the iris on the fiber launcher. After some minor tweaks to the alignment, we were able to get rid of much of the clipping on the sides that we saw previously, and we now have an almost round beam profile as seen on the HWS camera. (Attached a screenshot of the stream with a the plate off.)
Tonight we will run a ring heater test and post results tomorrow.
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Some additional thoughts on HWS ETMY mode matching
Yesterday we also measured distances on the HWS path from the SLED to the polariser. They are roughly equal to that in Aidan’s mode matching mathematica notebook (T10001717).
According to that mode matching solution we should have a waist ~2.2 m after the fiber collimator (not technically a collimator but a lens after the fiber launcher, but I’ll call it a collimator anyway).
In reality (just looking at the beam size on the card) we see a waist roughly at the position of the diverging lens, 86 cm from the collimator. After the diverging lens the beam is diverging. We were not able to find a position of the 120mm lens where the beam continues to converge after the diverging lens.
L2, the distance from the fiber collimator to the diverging lens, is 4cm shorter on the table than in the mathematica notebook - 860 mm compared to 903 mm. I re-evaluated the waist size with the new distance, recreating plots on page 13 of the PDF, output 215 and 206, to see if this explained the difference in waist position. The notebook suggested the waist would move *further* from the fiber collimator, but not but very much. This is the opposite of what we see on the table.
Conclusion: if we need to be strict about mode matching we should measure take a beam scan to end-y to make sure we know the mode size coming out of the collimator, and look at this as a function of position of the collimator lens.