daniel.sigg@LIGO.ORG - posted 19:05, Friday 31 August 2012 - last comment - 19:42, Friday 31 August 2012(4062)
ALS WFS path change
(Keita K., Daniel S.) We changed the reflection path towards the wavefront sensors on the ALS table. Previously, they were picked up through the rejected return beam out of the Faraday isolator. Unfortunately, this Faraday isolator uses a wedged calcite as a polarizer which generates numerous ghost beams which were impossible to separate. Instead, we placed a 10% beamsplitter just before the first PZT mirror to pick-up a fraction of the return beam. We then duplicated the lens in the return path and steered the beam towards the wavefront sensor setup. The beam size on the two wavefront sensors was still too large, so we moved them as close to the last steering mirror as we could. Hopefully, this will yield a more reproducible sensing matrix.
This means that the injected beam power was reduced by 90%.
Everything in the Hartman sensor path including the REFL_B_PWR diode is receiving 90% smaller light.
Offset of the REFL_B_PWR was adjusted so that it becomes zero when the cavity was unlocked.
Faraday-rejected light see the 90% splitter twice, and PDH diode is the only thing that receives 0.9^2 = 81% smaller light.