LIGO Document T1700195-v3

Mirror Metrology using Mode Spectroscopy

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T - Technical notes
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The mirrors in the LIGO detectors are not ideal, and have imperfections due to various reasons such as manufacturing defects, thermal defects, etc. These imperfections introduce optical losses and hence cause an apparent rise in GW Shot Noise output. The total loss in aLIGO is estimated at ~100ppm out of which ~20 ppm is budgeted for the mirror figure error. It is the second highest contributor to the total intensity loss and hence it is significantly important to be able to characterize this loss. Mode Spectroscopy is the in-situ technique we aim at developing to do this. We do our experimentation on the 40m prototype, which is quite similar to the aLIGO. We scan the cavity for various ‘Higher Order Modes’ and compare our results with the ideally expected values. This data is to be further used to try and produce a mirror map telling us what are the defects in the mirror and how much is the total cavity loss due to the mirror figure error.
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