In this situation if the mirrors are of very low optical loss, nearly all of the light supplied to the interferometer is reflected back towards the laser. In other words the laser is not properly impedance matched to the interferometer. The impedance matching can be improved by placing another mirror of correctly chosen transmission – a power recycling mirror – between the laser and the interferometer so that a resonant cavity is formed between this mirror and the rest of the interferometer; in the case of perfect impedance matching no light is reflected back towards the laser [25, 87]. There is then a power build-up inside the interferometer. This can be high enough to create the required kilowatts of laser light at the beamsplitter, starting from an input laser light of only about 10 W.
To be more precise, if the main optical power losses are those associated with the test mass mirrors – taken to be A per reflection – the intensity inside the whole system considered as one large cavity is increased by a factor given by , where the number of bounces, or light storage time, is optimised for signals of timescale and the other symbols have their usual meaning. Then:
Living Rev. Relativity 3, (2000), 3
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