756 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
Bragg Spectroscopy of ultracold atoms loaded in an optical lattice
We study Bragg spectroscopy of ultra-cold atoms in one-dimensional optical
lattices as a method for probing the excitation spectrum in the Mott insulator
phase, in particular the one particle-hole excitation band. Within the
framework of perturbation theory we obtain an analytical expression for the
dynamic structure factor and use it to calculate the imparted
energy which has shown to be a relevant observable in recent experiments. We
test the accuracy of our approximations by comparing them with numerically
exact solutions of the Bose-Hubbard model in restricted cases and establish the
limits of validity of our linear response analysis. Finally we show that when
the system is deep in the Mott insulator regime, its response to the Bragg
perturbation is temperature dependent. We suggest that this dependence might be
used as a tool to probe temperatures of order of the Mott gap.Comment: 4 pages, 3 figure
First demonstration of electrostatic damping of parametric instability at advanced LIGO
Interferometric gravitational wave detectors operate with high optical power in their arms in order to achieve high shot-noise limited strain sensitivity. A significant limitation to increasing the optical power is the phenomenon of three-mode parametric instabilities, in which the laser field in the arm cavities is scattered into higher-order optical modes by acoustic modes of the cavity mirrors. The optical modes can further drive the acoustic modes via radiation pressure, potentially producing an exponential buildup. One proposed technique to stabilize parametric instability is active damping of acoustic modes. We report here the first demonstration of damping a parametrically unstable mode using active feedback forces on the cavity mirror. A 15 538 Hz mode that grew exponentially with a time constant of 182 sec was damped using electrostatic actuation, with a resulting decay time constant of 23 sec. An average control force of 0.03 nN was required to maintain the acoustic mode at its minimum amplitude
Angular instability in high optical power suspended cavities
Advanced gravitational wave detectors use suspended test masses to form
optical resonant cavities for enhancing the detector sensitivity. These
cavities store hundreds of kilowatts of coherent light and even higher optical
power for future detectors. With such high optical power, the radiation
pressure effect inside the cavity creates sufficiently strong coupling between
test masses whose dynamics are significantly altered. The dynamics of two
independent nearly free masses become a coupled mechanical resonator system.
The transfer function of the local control system used for controlling the test
masses is modified by the radiation pressure effect. The changes in the
transfer function of the local control systems can result in a new type of
angular instability which occurs at only 1.3 \% of the Sidles-Sigg instability
threshold power. We report experimental results on a 74~m suspended cavity with
a few kilowatts of circulating power, for which the power to mass ratio is
comparable to the current Advanced LIGO. The radiation pressure effect on the
test masses behaves like an additional optical feedback with respect to the
local angular control, potentially making the mirror control system unstable.
When the local angular control system is optimized for maximum stability
margin, the instability threshold power increases from 4~kW to 29~kW. The
system behavior is consistent with our simulation and the power dependent
evolution of both the cavity soft and hard mode is observed. We show that this
phenomenon is likely to significantly affect proposed gravitational wave
detectors that require very high optical power.Comment: 7 pages, 7 figures, accepted for publication in Review of Scientific
Instrument
The role of cardiorespiratory fitness on the risk of sudden cardiac death at the population level: A systematic review and meta-analysis of the available evidence
Cardiorespiratory fitness (CRF) has been widely studied as a powerful and independent predictor of all-cause and disease-specific mortality. Sudden cardiac death (SCD) is recognized as a significant cause of mortality among the general population, including the general population without previous symptoms of any coronary heart disease (CHD). Consequently, SCD is an important public health problem, which constitutes a clinical challenge. Thus, prevention of SCD by detecting early risk factors could be a useful tool, contributing to the American Heart Association‘s goal of decreasing the incidence of SCD at the population level. The identification of these risk factors for CVD would facilitate the large-scale screening of those participants at higher risk of SCD. This systematic review collects information about the role of CRF on the risk of SCD at the available evidence, and analyzes the long-term influence of CRF as a risk factor and independent predictor of SCD
Analytical model for ring heater thermal compensation in the Advanced Laser Interferometer Gravitational-wave Observatory
Advanced laser interferometer gravitational-wave detectors use high laser power to achieve design sensitivity. A small part of this power is absorbed in the interferometer cavity mirrors where it creates thermal lenses, causing aberrations in the main laser beam that must be minimized by the actuation of “ring heaters,” which are additional heater elements that are aimed to reduce the temperature gradients in the mirrors. In this article we derive the first, to the best of our knowledge, analytical model of the temperature field generated by an ideal ring heater. We express the resulting optical aberration contribution to the main laser beam in this axisymmetric case. Used in conjunction with wavefront measurements, our model provides a more complete understanding of the thermal state of the cavity mirrors and will allow a more efficient use of the ring heaters in the Advanced Laser Interferometer Gravitational-wave Observatory
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