Abstract
Advanced LIGO and other ground-based interferometric gravitational-wave detectors use high laser power to minimize shot noise and suspended optics to reduce seismic noise coupling. This can result in an opto-mechanical coupling which can become unstable and saturate the interferometer control systems. The severity of these parametric instabilities scales with circulating laser power and first hindered LIGO operations in 2014. Static thermal tuning and active electrostatic damping have previously been used to control parametric instabilities at lower powers but are insufficient as power is increased. Here we report the first demonstration of dynamic thermal compensation to avoid parametric instability in an Advanced LIGO detector. Annular ring heaters that compensate central heating are used to tune the optical mode away from multiple problematic mirror resonance frequencies. We develop a single-cavity approximation model to simulate the optical beat note frequency during the central heating and ring heating transient. An experiment of dynamic ring heater tuning at the LIGO Livingston detector was carried out at 170 kW circulating power and, in agreement with our model, the third order optical beat note is controlled to avoid instability of the 15 and 15.5 kHz mechanical modes. We project that dynamic thermal compensation with ring heater input conditioning can be used in parallel with acoustic mode dampers to control the optical mode transient and avoid parametric instability of these modes up to Advanced LIGO’s design circulating power of 750 kW. The experiment also demonstrates the use of three mode interaction monitoring as a sensor of the cavity geometry, used to maintain the g-factor product to g 1 g 2 = 0.829 ± 0.004.
Highlights
The Advanced LIGO detectors [1] are modified Michelson interferometers that use 4 km long orthogonal arms to measure changes in strain induced by passing gravitational waves
We present the first demonstration of parametric instability (PI) avoidance using dynamic thermal compensation (DTC), which was implemented at LIGO Livingston to suppress the transverse mode spacing transient, allowing stable operation at 170 kW intra-cavity power
We demonstrate a system which expands on static thermal compensation by using the existing ring heaters to dynamically control the radius of curvature (RoC) of the end test masses and maintain the cavity optical mode spacing, offering a fundamental approach to PI control
Summary
The Advanced LIGO detectors [1] are modified Michelson interferometers that use 4 km long orthogonal arms to measure changes in strain induced by passing gravitational waves. Opto-mechanical parametric instability (PI) was first observed in the LIGO detectors in November 2014 when the circulating arm power exceeded 25 kW [2] These instabilities involve three modes that become coupled in the interferometer: the fundamental optical mode, a higher-order transverse optical mode, and a mechanical resonance of an optic [3], in this case a LIGO test mass. During the first two observing runs (O1: 9/15–1/16, O2: 9/16–8/17), PIs were controlled using a combination of static thermal compensation and electrostatic damping [2, 4], allowing the power circulating in the arm cavities to be increased up to 120 kW. We demonstrate a system which expands on static thermal compensation by using the existing ring heaters to dynamically control the RoC of the end test masses and maintain the cavity optical mode spacing, offering a fundamental approach to PI control. We show how measurement of the parametric gain was used to tune DTC, an implementation of three mode interaction monitoring [8]
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