987 research outputs found
Parametric Instability in Long Optical Cavities and Suppression by Dynamic Transverse Mode Frequency Modulation
Three mode parametric instability has been predicted in Advanced
gravitational wave detectors. Here we present the first observation of this
phenomenon in a large scale suspended optical cavity designed to be comparable
to those of advanced gravitational wave detectors. Our results show that
previous modelling assumptions that transverse optical modes are stable in
frequency except for frequency drifts on a thermal deformation time scale is
unlikely to be valid for suspended mass optical cavities. We demonstrate that
mirror figure errors cause a dependence of transverse mode offset frequency on
spot position. Combined with low frequency residual motion of suspended
mirrors, this leads to transverse mode frequency modulation which suppresses
the effective parametric gain. We show that this gain suppression mechanism can
be enhanced by laser spot dithering or fast thermal modulation. Using Advanced
LIGO test mass data and thermal modelling we show that gain suppression factors
of 10-20 could be achieved for individual modes, sufficient to greatly
ameliorate the parametric instability problem
Analysis of a four-mirror cavity enhanced Michelson interferometer
We investigate the shot noise limited sensitivity of a four-mirror cavity
enhanced Michelson interferometer. The intention of this interferometer
topology is the reduction of thermal lensing and the impact of the
interferometers contrast although transmissive optics are used with high
circulating powers. The analytical expressions describing the light fields and
the frequency response are derived. Although the parameter space has 11
dimensions, a detailed analysis of the resonance feature gives boundary
conditions allowing systematic parameter studies
Mechanical loss in state-of-the-art amorphous optical coatings
We present the results of mechanical characterizations of many different
high-quality optical coatings made of ion-beam-sputtered titania-doped tantala
and silica, developed originally for interferometric gravitational-wave
detectors. Our data show that in multi-layer stacks (like high-reflection Bragg
mirrors, for example) the measured coating dissipation is systematically higher
than the expectation and is correlated with the stress condition in the sample.
This has a particular relevance for the noise budget of current advanced
gravitational-wave interferometers, and, more generally, for any experiment
involving thermal-noise limited optical cavities.Comment: 31 pages, 14 figure
Feedback control of thermal lensing in a high optical power cavity
This paper reports automatic compensation of strong thermal lensing in a suspended 80 m optical cavity with sapphire test mass mirrors. Variation of the transmitted beam spot size is used to obtain an error signal to control the heating power applied to the cylindrical surface of an intracavity compensation plate. The negative thermal lens created in the compensation plate compensates the positive thermal lens in the sapphire test mass, which was caused by the absorption of the high intracavity optical power. The results show that feedback control is feasible to compensate the strong thermal lensing expected to occur in advanced laser interferometric gravitational wave detectors. Compensation allows the cavity resonance to be maintained at the fundamental mode, but the long thermal time constant for thermal lensing control in fused silica could cause difficulties with the control of parametric instabilities.This research was supported by the Australian
Research Council and the Department of Education,
Science and Training and by the U.S. National Science Foundation,
through LIGO participation in the HOPF
Measurements of mechanical thermal noise and energy dissipation in optical dielectric coatings
In recent years an increasing number of devices and experiments are shown to
be limited by mechanical thermal noise. In particular sub-Hertz laser frequency
stabilization and gravitational wave detectors, that are able to measure
fluctuations of 1E-18 m/rtHz or less, are being limited by thermal noise in the
dielectric coatings deposited on mirrors. In this paper we present a new
measurement of thermal noise in low absorption dielectric coatings deposited on
micro-cantilevers and we compare it with the results obtained from the
mechanical loss measurements. The coating thermal noise is measured on the
widest range of frequencies with the highest signal to noise ratio ever
achieved. In addition we present a novel technique to deduce the coating
mechanical losses from the measurement of the mechanical quality factor which
does not rely on the knowledge of the coating and substrate Young moduli. The
dielectric coatings are deposited by ion beam sputtering. The results presented
here give a frequency independent loss angle of (4.70 0.2)x1E-4 with a
Young's modulus of 118 GPa for annealed tantala from 10 Hz to 20 kHz. For
as-deposited silica, a weak frequency dependence (~ f^{-0.025}) is observed in
this frequency range, with a Young's modulus of 70 GPa and an internal damping
of (6.0 0.3)x1E-4 at 16 kHz, but this value decreases by one order of
magnitude after annealing and the frequency dependence disappears.Comment: Accepted for publication in Phys. Rev.
The upgrade of GEO600
The German / British gravitational wave detector GEO 600 is in the process of
being upgraded. The upgrading process of GEO 600, called GEO-HF, will
concentrate on the improvement of the sensitivity for high frequency signals
and the demonstration of advanced technologies. In the years 2009 to 2011 the
detector will undergo a series of upgrade steps, which are described in this
paper.Comment: 9 pages, Amaldi 8 conference contributio
Parametric instabilities and their control in advanced interferometer GW detectors
A detailed simulation of Advanced LIGO test mass optical cavities shows that
parametric instabilities will excite acoustic modes in the test masses in the
frequency range 28-35 kHz and 64-72 kHz. Using nominal Advanced LIGO optical
cavity parameters with fused silica test masses, parametric instability excites
7 acoustic modes in each test mass, with parametric gain R up to 7. For the
alternative sapphire test masses only 1 acoustic mode is excited in each test
mass with R ~ 2. Fine tuning of the test mass radii of curvature cause the
instabilities to sweep through various modes with R as high as ~2000. Sapphire
test mass cavities can be tuned to completely eliminate instabilities using
thermal g-factor tuning with negligible degradation of the noise performance.
In the case of fused silica test mass, instabilities can be minimized but not
eliminated.Comment: 5 pages, 4 figure
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