5.4 Readout schemes

There are various schemes that can be applied to readout the gravitational-wave signal from an interferometer. A good discussion of some of these can be found in [177Jump To The Next Citation Point]. If the interferometer laser has a frequency of f l then a passing gravitational wave, with frequency f gw, will introduce sidebands onto the laser with a frequency of fs = fl ± fgw. A readout scheme must be able to decouple the gravitational-wave component, with frequencies of order ∼ 100 Hz, from the far higher laser frequency at hundreds of tera-Hz. To do this it needs to be able to compare the sideband frequency with a known stable optical local oscillator. Ideally this oscillator would be the laser light itself (a homodyne scheme), but the initial generation of gravitational-wave detectors are operated at a dark fringe (i.e., the interferometer is held, so as the light from the arms completely destructively interferes at the beam splitter), so no light at the laser frequency exits (a gravitational wave will alter the arms lengths and constructively interfere, causing light to exit, but only at the sideband frequency).

The standard scheme used by the initial interferometers is a radio frequency (RF) heterodyne readout. In this case the laser light is modulated at an RF (called Schnupp modulation [281]) prior to entering the interferometer arms, giving rise to sidebands offset from the laser frequency at the RF. The interferometer is set up to allow these RF sidebands to exit at the output port. This can be used as a local optical oscillator with which to demodulate the gravitational wave sidebands. However, the demodulation will introduce a beat between the RF and the gravitational wave frequency, which must be removed by a second (hence heterodyne) demodulation at the RF.

The preferred method for future detectors is a DC scheme (see [147, 309, 177Jump To The Next Citation Point] for motivations and advantages of using such a scheme). In this no extra modulation has to be applied to the light. Instead the interferometer is held just off the dark fringe, so some light at the laser frequency reaches the output to serve as the local oscillator.

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