Fig. 1 shows use of a coherent (laser) source. Narrowband spectral light from a discharge or even white light can also be used, however to obtain significant interference contrast it is required that the differential pathlength is reduced below the coherence length of the light source. That can be only micrometers for white light.
  LIGO uses two 4-km Michelson interferometers to detect the waves. (We will discuss why it needs two interferometerts in the next paragraph.) 

Gires–Tournois etalon

  It is a transparent plate with two reflecting surfaces, one of which has very high reflectivity.
  By the use of partially reflecting mirrors(the surfaces), Fabry–Pérot cavities are created in both arms that increase the effective path length of laser light in the arm. The power of the light field in the cavity is 100 kW.\cite{Abbott_2017}