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 ) 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, 177] 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.
Living Rev. Relativity 14, (2011), 5
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