Stefan Hild January 2009 Pick-off beams in the central Interferometer of Advanced Virgo

S. HildJanuary 2009 Slide 2 Creation points of Pick-off beam  All Anti-reflex coatings potentially cause secondary beams.  Pick-off beams have about 10 to 20mW (6ppm x 2.7kW)  Pick-off beams are of similar size as the main IFO beam (LARGE!)  These 15 (18) beams should either be detected or properly dumped 3 beams from PRC (+3 beams from SRC) 2 beams 4 beams 2 beams 4 beams

S. HildJanuary 2009 Slide 3 Why more pick-off beams than in Virgo? Pick-off portInitial VirgoAdvanced Virgo Input mirror AREtalon2 beams each Compensation platesNo CPs4 beams each BSAR PRC1x used (B5) 2x unused (not dumped) 3 beams BSAR SRC3x unused (not dumped) (3 beams ?)

S. HildJanuary 2009 Slide 4 Input mirror AR coating  Can an Etalon (A) be control- led well enough with the CP so close by?  C should be avoided, because of scattered light.  D: one beam is strongly divergent.

S. HildJanuary 2009 Slide 5 Pick-offs from the CPs  CP is probably primary pick-off for ISC.  For ASC it might be important to have no overlapping beams (distributing the secondary beams in 3D).  Divergence of CP pick-offs depends on IM backsurface (lens or not).  If we get the CP pick-offs to work for ISC, we can probably dump the B5 beam.

S. HildJanuary 2009 Slide 6 Secondary reflections from BS  3 beams from within PRC needs to be dealt with for AdV.  What to do about the 3 beams from within the SRC (in contrast they are not differential)?  Do we need B5 or can it be dumped? Currently not dumped beam