|Session: Advanced Gravitational Wave Detectors and Advanced LIGO Technology (February 24, 2010)|
Title: Arm Length Stabilisation for Advanced LIGO
|Speaker: Bram Slagmolen|
The quadruple suspension system (quad) designed for the test masses, provide isolation of the ground motion to the test mass at frequencies above 10 Hz. Displacements at frequencies below and near the quadruple suspension resonance frequencies will not be well attenuated. Complex feedback will damp the suspension resonance modes. Even with the BSC ISI platforms, this results in the test mass displacement of ~10−7 m/√Hz below 0.5 Hz. This displacement is too much for the actuators mounted on the quad to acquire lock of the arm cavities in a deterministic manner. To make the lock acquisition more deterministic, the arm cavities are locked independently from the central recycling cavities. The independent arm locking scheme will enable to introduce a small offset of the cavity resonance with the main laser. The lock acquisition of the recycling cavities will then not be hindered by the arm cavities going through resonance and breaking the acquisition process. Once the recycling cavities have been locked, the offset on the arm cavities is reduced to bring them into resonance with the main laser. To achieve the deterministic locking procedure, the residual arm cavity length fluctuations need to be reduced to within the line-width of the cavity, the line-width equivalent displacement is 1.3 nm. This document describes the conceptual design for the Arm Length Stabilisation system.
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