5 Basic Interferometers

The large interferometric gravitational-wave detectors currently in operation are based on two fundamental interferometer topologies: the Fabry–Pérot and the Michelson interferometer. The main instrument is very similar to the original interferometer concept used in the famous experiment by Michelson and Morley, published in 1887 [42]. The main difference is that modern instruments use laser light to illuminate the interferometer to achieve much higher accuracy. Already the first prototype by Forward and Weiss has thus achieved a sensitivity a million times better than Michelson’s original instrument [18]. In addition, in current gravitational-wave detectors, the Michelson interferometer has been enhanced by resonant cavities, which in turn have been derived from the original idea for a spectroscopy standard published by Fabry and Pérot in 1899 [16]. The following section will describe the fundamental properties of the Fabry–Pérot interferometer and the Michelson interferometer. A thorough understanding of these basic instruments is essential for the study of the high-precision interferometers used for gravitational-wave detection.

 5.1 The two-mirror cavity: a Fabry–Pérot interferometer
 5.2 Michelson interferometer
 5.3 Finesse examples
  5.3.1 Michelson power
  5.3.2 Michelson modulation

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