LIGO Document G1300524-v1

The Arm Length Stabilisation System for Advanced LIGO Lock Acquisition

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The Laser Interferometric Gravitational-Wave Observatory (LIGO) is currently being upgraded to a dual-recycled Fabry-Perot Michelson interferometer. This upgraded interferometer, referred to as Advanced LIGO, will be an order of magnitude more sensitive over the gravitational wave detection band than Initial LIGO. This increase in sensitivity comes at the cost of added complexity to the interferometer.

To bring the interferometer to the designed sensitivity, the five length degrees of freedom need to be held at their operating points by feedback control systems. However, initially the interferometer lengths are in a state of flux, and the act of bringing the interferometer into a controlled state is a non-trivial task due to a combination of coupled cavities, the narrow linear range of the length readouts, and weak actuators. This process of bringing the interferometer into a controlled state is called lock acquisition.

To reduce the complexity of the Advanced LIGO lock acquisition scheme, an Arm Length Stabilisation (ALS) system has been developed. This ALS system will employ auxiliary frequency doubled Nd-YAG lasers to sense the lengths of the fabry-perot cavities in the arms independent of the rest of the interferometer. This ALS readout will be used to suppress the residual arm motions to less than 1nm rms, and also detune the arm cavities off resonance with the primary science laser, so that the central degrees of freedom can be locked.

In this talk I will describe the Advanced LIGO ALS system, and present results from some prototype experiments.

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