Parametric instabilities and their control in advanced interferometer gravitational-wave detectors

Area of Science:

• Gravitational Wave Detection
• Optical Cavity Physics
• Materials Science

Background:

• Advanced LIGO utilizes optical cavities for high-sensitivity gravitational wave detection.
• Parametric instabilities can degrade the performance of these optical cavities.

Purpose of the Study:

• To simulate and analyze parametric instabilities in Advanced LIGO's fused silica and sapphire test mass optical cavities.
• To compare the susceptibility of fused silica versus sapphire test masses to parametric instabilities.
• To investigate methods for mitigating or eliminating these instabilities.

Main Methods:

• Detailed computational simulation of Advanced LIGO test mass optical cavities.
• Analysis of parametric gain (R) across various acoustic modes.
• Exploration of tuning strategies, including radii of curvature and thermal g-factor tuning.

Main Results:

• Fused silica test masses exhibit 7 excited acoustic modes with parametric gain R up to 7.
• Sapphire test masses show only 1 acoustic mode with R approximately 2.
• Tuning of test mass radii of curvature can increase parametric gain to R ~2000.
• Thermal g-factor tuning can completely eliminate instabilities in sapphire cavities.
• Instabilities can be minimized but not eliminated in fused silica cavities.

Conclusions:

• Sapphire test mass optical cavities offer a significant advantage in suppressing parametric instabilities compared to fused silica.
• Thermal g-factor tuning is a viable method for completely mitigating parametric instabilities in sapphire cavities.
• While instabilities can be reduced in fused silica cavities, complete elimination is not achievable with current methods.