LIGO Document G0900060-v3

Absolute Length Meaurement of the Caltech 40 Meter Interferometer's Optical Cavities

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In gravitational wave interferometers, a precise measurement of the absolute length and of the g-factor of each optical cavity would provide a very useful information for characterizing and modeling the system. Complex locking acquisition schemes such as Advanced LIGO's would benefit from knowing how actually to position the suspensions to match the optical design or the amount of phase shift induced on non-resonant sidebands. At the same time, analyzing the structures of higher order modes would provide a measure of the g-factor of the cavities and thus of the quality of the optics. We could then control effect like unwanted resonances of sidebands' higher order modes and monitor thermal lensing effects.

We present here the technique we developed at the Caltech 40m interferometer by which we measured such parameters with a precision as high as 1 in 10e-6.

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