LIGO Document T1500542-v2

Cavity Alignment Using Fringe Scanning: LIGO SURF Final Report

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T - Technical notes
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23 Nov 2015, 21:46
LIGO employs two 4-km long FabryPérot arm cavities, which need to be aligned in order for an interferometer to be locked on a TEM00 mode. Once the cavity is locked, alignment signals can be derived from wave-front sensors which measure the TEM01 mode content. However, the alignment state is not always good enough for locking on TEM00. Even when this is the case, the alignment can be evaluated using a free swinging cavity, that shows ashes when higher-order modes become resonant. By moving test masses, small changes are made to the mirror orientation, and hence the TEM00 mode can be optimised iteratively. Currently, this is a manual procedure, and thus it is very time-consuming. Therefore, this project is aimed to study another possible way to lock the cavity on the TEM00 mode. Misalignment information can also be extracted from the power of the higher-order modes transmitted through the cavity. This final SURF written report presents an algorithm for this alternative and faster way to derive the alignment state of the arm cavities.
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