How do they tell the number of times the laser bounces back and forth, in the 4km arms of the interferometer, before the laser light is picked out to the reading sensor?

See under “The Longer The Better” https://www.ligo.caltech.edu/page/ligos-ifo

How do they know the number of times the laser bounces back and forth, in the 4 km arms of the interferometer, before the laser light is picked out to the reading sensor?

See under “The Longer The Better” https://www.ligo.caltech.edu/page/ligos-ifo