Introduction

Pick-off beams from the AR-coatings of BS and Input mirrors might be necessary for length and alignment sensing and control. The scope of this page is NOT to find out whether or which of these auxiliary are needed for Advanced Virgo, but rather HOW A FEASIBLE SCHEME COULD LOOK LIKE to make these beams available for the interferometer control. Or in other words: at the bottom of this page we should know for each auxiliary beam:

• Where it really passes through the vacuum system and at which tank it is accessible for detection (either inside the vacuum at a suspended bench or outside the vacuum at a viewport)?
• How many suspended beam directors of which size are necessary to get the beam to the suspended bench or the viewport?
• How do the baffles inside the vacuum system need to be modified?
• How large do the corresponding wedges of BS and input mirrors need to be?
• Rough estimate of costs.
  

Assumptions / Boundaries

• We only consider short, classical recycling cavities (i.e. NOT non degenerate recycling cavities).
• We consider to have NO compensation plates.
• The pick-off beams from the BS-AR (north arm pick-off and SR pick-off)  need to somehow "go around" the Signal-Recycling mirror.
• The pick-off beams from the input test masses need to somehow "go around" the beam splitter.
• We have to consider proper geometries of BS and SRM (What diameter have these optics, what diameter have their reference masses? - I will check with the payload people).
• The rim of any beam director needs to be separated from any main beam by 3 beam radii. For a start we can use beam directors with a radius of two beam radii for the input mirror pick-offs. For the pick-off from the beam splitter, which is likely to be used for the power stabilization we will probably have to use a larger beam director with a radius of three beam radii. - I will check this with the detection people.
  
• The beam splitter should for noise reasons be detected at the suspended detection bench. The pick offs from the input mirrors can be detected outside the vacuum. - I will check this with the detection people.
  
• Otherwise we should take all freedom to not consider any other constraints. ( For example I believe at the moment we don't care at which tank or view port the auxiliary beam leaves the vacuum system, as long as it does...)
  

Project Progress