However, the main difference is that the measurement is made differentially by comparing two lengths. This allows one to separate a larger number of possible noise contributions, for example noise in the laser light source, such as amplitude or frequency noise. This is why the main instrument for gravitational-wave measurements is a Michelson interferometer. However, the resonant enhancement of light power can be added to the Michelson, for example, by using Fabry–Pérot cavities within the Michelson. This construction of new topologies by combining Michelson and Fabry–Pérot interferometers will be described in detail in a future version of this review.
The Michelson interferometer has two longitudinal degrees of freedom. These can be represented by the positions (along the optical axes) of the end mirrors. However, it is more efficient to use proper linear combinations of these and describe the Michelson interferometer length or position information by the common and differential arm length, as introduced in Equation (97):
The Michelson interferometer is intrinsically insensitive to the common arm length .
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