  Reduction of thermal noise in dielectric mirror coatings is a key issue for
the sensitivity improvement in second and third generation interferometric
gravitational wave detectors. Replacing an end mirror of the interferometer by
an anti-resonant cavity (a so-called Khalili cavity) has been proposed to
realize the reduction of the overall thermal noise level. In this article we
show that the use of a Khalili etalon, which requires less hardware than a
Khalili cavity, yields still a significant reduction of thermal noise. We
identify the optimum distribution of coating layers on the front and rear
surfaces of the etalon and compare the total noise budget with a conventional
mirror. In addition we briefly discuss advantages and disadvantages of the
Khalili etalon compared with the Khalili cavity in terms of technical aspects,
such as interferometric length control and thermal lensing.
