LIGO Document T1100220-v1

Reduction of degenerate modes excitation in a imperfect FP cavity for LG33 beam

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T - Technical notes
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The excitation of degenerate modes in a FP cavity with imperfect mirrors can contaminate the signal when LG33 beam is used and the contrast defect degradation affects the SN ratio. In this note, artificial irregularity is placed on the mirror surface at the node radius of the LG33 field, and the reduction of the degenerate mode excitation is studied.
It is shown that these methods do suppress the degenerate mode excitation, but it is accomplished at the expense of the signal loss.
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