  The Advanced LIGO gravitational wave detectors are nearing their design
sensitivity and should begin taking meaningful astrophysical data in the fall
of 2015. These resonant optical interferometers will have unprecedented
sensitivity to the strains caused by passing gravitational waves. The input
optics play a significant part in allowing these devices to reach such
sensitivities. Residing between the pre-stabilized laser and the main
interferometer, the input optics is tasked with preparing the laser beam for
interferometry at the sub-attometer level while operating at continuous wave
input power levels ranging from 100 mW to 150 W. These extreme operating
conditions required every major component to be custom designed. These designs
draw heavily on the experience and understanding gained during the operation of
Initial LIGO and Enhanced LIGO. In this article we report on how the components
of the input optics were designed to meet their stringent requirements and
present measurements showing how well they have lived up to their design.
