Beyond LIGO

Advanced Optics Research

Adaptive Mode Matching for Advanced LIGO

Mode matching.During the last decade the technology for generating squeezed vacuum states of light has reached maturity. Tests showed a remarkable interferometer sensitivity improvement, promising an increase of more than a factor of three in the observable event rate. This sensitivity gain is also available as significant risk mitigation in case excessive optical absorption or thermal lensing prevent full power operation. It does however require minimal optical losses on the squeezed light path. For a gravitational wave interferometer, a significant chunk of the losses are due the mode matching into the output mode cleaner. Those losses are particularly worrisome for two reasons. First, the initial static mode matching is a challenge because the relevant mode, namely the mode resonant in the arm cavities, is not necessarily identical with the total light visible at the interferometer’s dark port. Second, the arm optical mode will be affected by thermal lensing inside the interferometer. Such lensing is expected to occur at the full input power level of the Advanced LIGO design, and can also affect the interferometer optical gain. To accommodate this lensing, the design included thermal lens correction actuators in the recycling cavities, but the project does not currently include adaptive correction of the mode matching into the output mode cleaner. A squeezer upgrade for Advanced LIGO will thus put adaptive mode matching into the output mode cleaner on the critical path. The Syracuse University Gravitational Wave Group is constructing two kinds of telescope to significantly reduce losses due to static and thermal mismatch. The first type will be based on prototype adaptive optics, which use applied heat to dynamically change the optics’ radius of curvature. The second type is based on a set of lenses, which can be remotely moved to apply corrections to the mode matching.