5.2 Signal recycling

To enhance further the sensitivity of an interferometric detector and to allow some narrowing of the detection bandwidth, which may be valuable in searches for continuous wave sources of gravitational radiation, another technique known as signal recycling can be implemented [231, 292, 169]. This relies on the fact that sidebands created on the light by gravitational-wave signals interacting with the arms do not interfere destructively and so do appear at the output of the interferometer. If a mirror of suitably-chosen reflectivity is put at the output of the system as shown in Figure 11View Image, then the sidebands can be recycled back into the interferometer, where they resonate, and hence the signal size over a given bandwidth (set by the mirror reflectivity) is enhanced.
View Image

Figure 11: The implementation of signal recycling on a Michelson interferometer with Fabry–Pérot cavities.

The centre of this frequency band is set by the precise length of the cavity formed by the signal recycling mirror and the cavities in the interferometer arms. Thus, control of the precise position of the signal recycling mirror allows tuning of the frequency at which the performance is peaked.

Often signal recycling will be used to provide a narrow bandwidth to search for continuous wave sources as mentioned above, however it may also be used with a relatively broad bandwidth, centred away from zero frequency, and this application is useful for matching the frequency response of the detector to expected spectral densities of certain broadband or “chirping” signals.

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