LIGO Document P1000103-v1

[Thesis] Calibrating and Improving the Sensitivity of the LIGO Detectors

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The Laser Interferometer Gravitational wave Observatory (LIGO) is network of three, power recycled Fabry-Perot Michelson interferometers built to detect gravitational waves from astrophysical sources at frequencies between 40 and 6000 Hz. For their fifth science run, from 2005 to 2007, the detectors observed at designed sensitivity, achieving equivalent strain amplitude noise of 3x10^-23 strain/rtHz at 100 Hz. To date, the observatory has not detected gravitational waves. However, even at such sensitivity, the expected detection rate for known astrophysical sources of gravitational waves is likely 0.02 yr^-1.

The fundamental noise source of these ground-based detectors limiting the sensitivity below 40 Hz is seismic motion. They use multi-stage passive isolation platforms from which their test masses are suspended from piano wire as single pendula providing isolation from ground motion. The residual test mass motion is controlled by electromagnetic actuators on the suspension system in response to the output of the interferometers, keeping them at their operating point. In the first portion of this thesis, I discuss the absolute calibration of the first generation of LIGO interferometer's gravitational wave readout during their fifth science run, the uncertainty of which is limited by the precision to which we can measure the control system above residual seismic noise.

A second generation of detectors, called Advanced LIGO, is currently under construction which will completely replace the first generation. Scheduled to become operational in 2014, they are predicted to improve the sensitivity by ten-fold or more, and will likely improve the detection rate to as much as 40 yr^-1. To achieve this sensitivity at the lower limit of the band, the test masses will be suspended from from multiple cascading pendula. In addition, the multi-stage passive isolation platforms will be replaced with single- and double-stage suspended platforms with built-in active feedback control systems. Prototypes of single-stage active control systems have been in use for two years for a non-invasive upgrade of the LIGO interferometers. In the second portion of this thesis, I present results from these prototypes and demonstrate that their performance can meet the stringent requirement of the second generation of interferometers.

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