Continuing our exploration of the optical techniques that allow LIGO to reach its unprecedented sensitivity to gravitational waves, it’s time to discuss signal recycling, one of the most confusing of them all!
When a gravitational wave changes the length of the two LIGO arms differentially – meaning that one gets longer and the other shorter – the interference of the two laser beams coming back to the beam splitter is not perfectly destructive anymore, and some light passes through. We call this light that is generated by the passing gravitational wave the signal sidebands.
In LIGO we added a Signal Recycling Mirror between the beam splitter and the photodetectors. This mirror reflects back the light produced by a gravitational-wave signal, the signal sidebands, and makes it recirculate into the main interferometer.
By tuning the microscopic position of the signal recycling mirror, we can change the resonance condition of the signal light in the Signal Recycling Cavity that is formed between the mirror and the rest of the interferometer. In this way we can obtain some additional amplification of the instrument response, especially at high frequency, thus increasing the instrument bandwidth from a few tens of Hz to several hundreds of Hz.
This is how we manage to have a detector that has simultaneously very high sensitivity but also large bandwidth.
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