High optical power is essential for improving the sensitivity of advanced detectors. Thermal lensing, Sidles–Sigg instability and parametric instability are three dominant effects that limit the optical power required to achieve the target sensitivity. This chapter summarises these three instabilities and how they can be controlled. In particular, we emphasise parametric instability and its control.

Introduction

The first generation laser interferometer gravitational wave detectors (LIGO, Chapter 6 and Virgo, Chapter 7) have achieved their target design sensitivity, and yet this sensitivity is estimated to be only sufficient to detect large rare gravitational wave signals. To achieve detection of signals from predicted sources, a ten-fold improvement in sensitivity is required. For LIGO, this would increase the sensitive range from ~ 14 Mpc today to about ~ 200 Mpc, leading to event rates of many per year (Cutler and Thorne, 2002). To achieve this improvement the Advanced LIGO (Fritschel, 1994) and other second generation detectors require a ~ 100-fold increase in circulating laser power, in addition to improvement in the interferometer configuration, test mass thermal noise, and vibration isolation. Such high power levels create three significant challenges. The first is the control of thermal lensing in the core optics substrates and dielectric coating layers. The second is the control of parametric instabilities in the form of radiation pressure mediated opto-mechanical oscillation. The third is the control of Sidles–Sigg instabilities, which are the angular instabilities of the suspended test mass that result from radiation pressure induced optical torque.