Chapter 18: The input optics system

The light which is injected into the main interferometer in all advanced ground-based laser interferometric gravitational-wave detectors comes from high-power lasers operating at near infrared wavelengths, with powers exceeding 100 W. Their frequency stability in the detection frequency band with respect to their relevant references, the arm cavities, is unrivaled by any other experimental method. Similarly, the relative intensity noise (RIN) of the laser beams, especially at lower frequencies of the detection band, defines the state of the art in laser power stabilization. Their spatial eigenmode is very close to an ideal Gaussian mode. This chapter describes the input optics of the advanced detectors that prepare and inject these near ideal laser beams into the main interferometers.