# The LIGO Detectors Controls

Daniel Sigg^\#^, P.O. Box 159, Richland, WA 99352.\
(for the LIGO Scientific Collaboration)

Abstract

![](media/image1.png){width="6.45in" height="3.8604166666666666in"}All
three LIGO detectors have reached their design sensitivities. A
sky-averaged detection range (SNR \> 8) of more than 15 Mpc for inspiral
binary neutron stars with masses of 1.4 Msol has been achieved with the
two 4 km instruments. The fifth LIGO science started in November 2005
and ended September 2007. About 365 days of coincidence data have been
collected. The feedback controls system is a major component to make
LIGO work and its performance has been crucial to achieve the present
sensitivity.

## Introduction

Interferometric gravitational wave antennas are based on Michelson
interferometers whose sensitivity to small differential length changes
has been enhanced by adding multiple coupled optical resonators. The use
of optical cavities is essential for reaching the required sensitivity,
but sets challenges for the control system which must maintain the
cavities near resonance. The goal for the strain sensitivity of the
Laser Interferometer Gravitational-wave Observatory is 10^-21^ rms,
integrated over a 100 Hz bandwidth centered at 150 Hz \[1,2\].

LIGO implements a power-recycled Michelson
![](media/image2.png){width="6.7in"
height="4.583333333333333in"}interferometer with Fabry-Perot arm
cavities (see Fig. 1). Using optical cavities is essential in reaching
the ultimate sensitivity goal but it requires an active electronic
feedback system to keep them 'on resonance'. The control system must
keep the round-trip length of a cavity near an integer multiple of the
laser wavelength so that light newly introduced into the cavity
interferes constructively with light from previous round-trips. Under
these conditions the light inside the cavity builds up and the cavity is
said to be on resonance \[3\]. Attaining high power buildup in the arm
cavities also requires that minimal light is allowed to leave the system
through the antisymmetric port, so that all the light is sent back in
the direction of the laser where it is reflected back into the system by
the power recycling mirror. Hence, an additional feedback loop is needed
to control the Michelson phase so that the antisymmetric port is set on
a dark fringe.

## Sensitivity

The current displacement sensitivities of the three LIGO detectors are
shown in Fig. 2. The sensitivity above approximately 200 Hz is limited
by shot noise of the laser light. Below about 40 Hz the sensitivity is
limited by anthropogenic and seismic activities. The noise in the
intermediate frequency band is not completely understood. Thermal noise,
electronics noise, back scattering of stray beams and upconversion of
low frequency excitations may all contribute.

## The 5^th^ Science Run

The LIGO observatories consist of three detectors at two different
sites. At the Hanford Observatory in Washington a 4 km and a 2 km long
interferometer share the same vacuum envelop. A single 4 km long
interferometer is implemented at Livingston site in Louisiana.

The 5^th^ LIGO science run started on November 4, 2005 and ended on
September 30, 2007. Just over 365 days of triple coincidence data have
been acquired. A little more than 400 days of two site coincident data
have been archived. The duty factor for the 4 km interferometer at
Hanford was around 78%, it was about 79% for the 2 km interferometer at
Hanford and it was about 66% for the Livingston interferometer. The
triple coincidence duty factor was 53%, whereas the two site coincidence
duty factor was 60%.

## Results and Conclusions

The data of the 5^th^ science run are still being analyzed. Results from
earlier runs are available in Refs. 4-26.

Initial LIGO has achieved its major goals. Plans for the next generation
advanced LIGO detectors are well underway.

## Acknowledgement

The authors gratefully acknowledge the support of the United States
National Science Foundation for the construction and operation of the
LIGO Laboratory and the Particle Physics and Astronomy Research Council
of the United Kingdom, the Max-Planck-Society and the State of
Niedersachsen/Germany for support of the construction and operation of
the GEO600 detector. The authors also gratefully acknowledge the support
of the research by these agencies and by the Australian Research
Council, the Natural Sciences and Engineering Research Council of
Canada, the Council of Scientific and Industrial Research of India, the
Department of Science and Technology of India, the Spanish Ministerio de
Educacion y Ciencia, The National Aeronautics and Space Administration,
the John Simon Guggenheim Foundation, the Alexander von Humboldt
Foundation, the Leverhulme Trust, the David and Lucile Packard
Foundation, the Research Corporation, and the Alfred P. Sloan
Foundation.

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