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Fritschel P.,Massachusetts Institute of Technology | Evans M.,Massachusetts Institute of Technology | Frolov V.,LIGO Laser Interferometer Gravitational-Wave Observatory
Optics Express

Balanced homodyne detection is typically used to measure quantum-noise-limited optical beams, including squeezed states of light, at audio-band frequencies. Current designs of advanced gravitational wave interferometers use some type of homodyne readout for signal detection, in part because of its compatibility with the use of squeezed light. The readout scheme used in Advanced LIGO, called DC readout, is however not a balanced detection scheme. Instead, the local oscillator field, generated from a dark fringe offset, co-propagates with the signal field at the anti-symmetric output of the beam splitter. This article examines the alternative of a true balanced homodyne detection for the readout of gravitational wave detectors such as Advanced LIGO. Several practical advantages of the balanced detection scheme are described. © 2014 Optical Society of America. Source

Harms J.,California Institute of Technology | O'Reilly B.,LIGO Laser Interferometer Gravitational-Wave Observatory
Bulletin of the Seismological Society of America

In April of 2009 a seismic survey utilizing explosive charges took place in Livingston parish, Louisiana. The area of the survey encompassed the location of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Livingston interferometer. In this paper, we present an analysis of seismic data recorded with three of the LIGO seismometers and a geophone array that was deployed during the time of the survey. In particular, the geophone-array data are used to study the propagation of surface waves, whereas first-arrival measurements with the LIGO seismometers provide estimates of the speed of compressional seismic waves as a function of depth. We find that fundamental Rayleigh waves have a speed close to 205 m/s consistent with results from previous borehole tests and that speed of compressional waves is 1650 m/s at 25 m depth, increasing to 2300 m.s at 1 km depth. Blast spectra are further investigated to determine the Q value of the ground medium experienced by Rayleigh waves (f > 1 Hz) and body waves deeper underground. The estimated Q value is approximately 50 for the surface waves and exceeds a value of 190 for body waves propagating at depths below 100 m. Source

Dwyer S.,LIGO Laser Interferometer Gravitational-Wave Observatory
Physics Today

You cant beat the Heisenberg uncertainty principle, but you can engineer systems so that most of the uncertainty is in the variable of your choice. Doing so can improve the precision of delicate measurements. Source

Meadors G.D.,University of Michigan | Kawabe K.,LIGO Laser Interferometer Gravitational-Wave Observatory | Riles K.,University of Michigan
Classical and Quantum Gravity

LIGO, the Laser Interferometer Gravitational-wave Observatory, has been designed and constructed to measure gravitational wave strain via differential arm length. The LIGO 4 km Michelson arms with Fabry-Perot cavities have auxiliary length control servos for suppressing Michelson motion of the beam-splitter and arm cavity input mirrors, which degrades interferometer sensitivity. We demonstrate how a post facto pipeline improves a data sample from LIGO Science Run 6 with feedforward subtraction. Dividing data into 1024 s windows, we numerically fit filter functions representing the frequency-domain transfer functions from Michelson length channels into the gravitational-wave strain data channel for each window, then subtract the filtered Michelson channel noise (witness) from the strain channel (target). In this paper we describe the algorithm, assess achievable improvements in sensitivity to astrophysical sources, and consider relevance to future interferometry. © 2014 IOP Publishing Ltd. Source

Amin R.S.,Louisiana State University | Giaime J.A.,LIGO Laser Interferometer Gravitational-Wave Observatory
Classical and Quantum Gravity

Thermal lensing has been one of the sensitivity-limiting factors for the LIGO detectors since their inception. Although estimates of such lensing were assumed when LIGO's core optics were specified, in practice a thermal lensing compensation system (TCS) was installed in 2004 in order to improve mode matching to the injected beam and ultimately detector sensitivity. This subsystem's primary purpose was to induce a corrective thermal lens in the input test mass mirrors or LIGO's 4 km Fabry-Perot arms. A few empirically motivated means of monitoring the focal parameters of the input couplers were employed for the 2005-2007 science run, 'S5'. We discuss results of a numerical model study, a set of signals, 'focal discriminants', that could have been used during S5 to set TCS compensation levels. Most of these signals would not have needed the installation of any new equipment or software. If investigated further, these 'focal discriminants' may find utility in pathfinder projects as the next-generation LIGO detectors are commissioned. © 2010 IOP Publishing Ltd. Source

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