The Michelson interferometer output is determined by the laser wavelength , the reflectivity and
transmittance of the beam splitter and the end mirrors, and the length of the interferometer arms. In many
cases the end mirrors are highly reflective and the beam splitter ideally a 50:50 beam splitter. In that case,
we can compute the output for a monochromatic field as shown in Section 2.4. Using Equation (20
) we can
write the field in the South port as
The above seems to indicate that the macroscopic arm-length difference plays no role in the Michelson output signal. However, this is only correct for a monochromatic laser beam with infinite coherence length. In real interferometers care must be taken that the arm-length difference is well below the coherence length of the light source. In gravitational-wave detectors the macroscopic arm-length difference is an important design feature; it is kept very small in order to reduce coupling of laser noise into the output but needs to retain a finite size to allow the transfer of phase modulation sidebands from the input to the output port; this is illustrated in the Finesse example below and will be covered in detail in Section 6.4.
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