Test mass optical levers preliminary design

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Abstract:: The Advanced LIGO optical levers will comprise Super-luminescent LEDs coupled with monomode angle-cut fibers to telescope launchers mounted on Opto-Sigma lockable goniometer pitch-yaw stages bolted to pyramidal, folded-sheet, welded stainless steel pylons that are bolted to base-plates that are grouted to the LVEA floor.
The Test mass optical lever launcher telescopes will be 50 mm diameter telescopes, to generate 1 to 2 mm light spot on 1 cm2 quadrant photo diodes up to 78 m away. The optical levers main beam impinge almost perpendicularly and hit the center of each test mass. The quadrant photo-detector receivers are mounted on remote-controlled X-Y micrometric translation stages that are also mounted on similarly designed pyramidal pylons. The X-Y translation stages are used to null the quadrant photo-detector signal where maximum sensitivity to displacement occurs and several causes of noise cancel out.
The beam launcher telescopes for the test masses are designed to include a beam splitter to generate auxiliary, paraxial beams that are sensed by two or three auxiliary CCD sensors mounted next to the quadrant photo sensor, to monitor the test Radius of Curvature changes (thermal lensing). This option will be implemented on a risk reduction basis only. The End test mass receiver piers are shared with the receiver boxes for the Photon Calibrator subsystem.

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LIGO Document T1000219-v2