Optical Configuration and Control of Ultra-sensitive Gravitational Wave Detectors

In order to constantly detect gravitational waves from astronomical sources, the required sensitivity for a detector is better than 10−23 in strain. To achieve such a high sensitivity, variants of Michelson interferometer with additional mirrors are used as the second generation interferometric gravitational wave detectors. The extra-mirrors form a total of four Fabry-Perot cavities in an interferometer to enhance the optical power and increase the observation bandwidth. While this complex optical configuration enables us to tailor the quantum noise shape to maximize the astronomical output and provides ways to cope with technical constraints, a sophisticated control scheme is required to keep the interferometer at its optimal operating point. We will give a brief review of the evolution of the interferometer configuration from a simple Michelson interferometer as well as the signal extraction techniques to control the advanced interferometers.