As explained in the preceeding Section 4.3, high power laser light is needed to overcome limitations of a detector’s sensitivity due to photoelectron shot noise. The situation can be helped greatly if a multi-pass arrangement is used in the arms of the interferometer as this multiplies up the apparent movement by the number of bounces the light makes in the arms. The multiple beams can either be separate as in an optical delay line [107, 6], or may lie on top of each other as in a Fabry–Pérot resonant cavity  as shown in Figure 6.
Optimally, the light should be stored for a time comparable to the characteristic timescale of the signal. Thus if signals of characteristic timescale 1 msec are to be searched for, the number of bounces should be approximately 50 for an arm length of 3 km. With 50 bounces the required laser power is reduced to 2.4 × 103 W, still a formidable requirement.
Living Rev. Relativity 3, (2000), 3
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