J.-Y. Vinet

Thermal noise in mirrors of interferometric gravitational wave antennas

Abstract We present a direct and very fast calculation of the low frequency tail of the spectral density of thermal noise due to the thermal motion of matter in the cylindrical mirrors generally used in gravitational wave interferometric detectors. This calculation is based on the method recently proposed by Levin, that we extend to the case of finite mirrors, allowing one to study the aspect ratio dependence of the noise. Results are found in good agreement with those found in heavier preceding approaches, and provide a useful cross-check.

The Virgo Project

The Virgo project is a Italian-French collaboration aiming at the construction of a long baseline interferometric antenna for the detection of gravitational radiation signals of cosmic origin. We describe the principles of the system, and high-light the technical challenges we need to overcome for reaching a sensitiity as low as 10−23Hz−1/2.The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime; (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in hierarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. Using this model and some simple extensions, it is possible to understand the transfer of power from large to small scales and the behaviour of higher order correlation functions. The ideas presented here will also serve as a powerful analytical tool to investigate nonlinear clustering in different models.

An overview of the second round of the Mock LISA Data Challenges

The Mock Data Challenges (MLDCs) have the dual purpose of fostering the development of LISA data-analysis tools and capabilities and of demonstrating the technical readiness already achieved by the gravitational-wave community in distilling a rich science payoff from the LISA data. The first round of MLDCs has just been completed and the second-round data sets are being released shortly after this workshop. The second-round data sets contain radiation from an entire Galactic population of stellar-mass binary systems, from massive-black-hole binaries, and from extreme-mass-ratio inspirals. These data sets are designed to capture much of the complexity that is expected in the actual LISA data, and should provide a fairly realistic setting to test advanced data-analysis techniques, and in particular the global aspect of the analysis. Here we describe the second round of MLDCs and provide details about its implementation.

On Special Optical Modes and Thermal Issues in Advanced Gravitational Wave Interferometric Detectors

The sensitivity of present ground-based gravitational wave antennas is too low to detect many events per year. It has, therefore, been planned for years to build advanced detectors allowing actual astrophysical observations and investigations. In such advanced detectors, one major issue is to increase the laser power in order to reduce shot noise. However, this is useless if the thermal noise remains at the current level in the 100 Hz spectral region, where mirrors are the main contributors. Moreover, increasing the laser power gives rise to various spurious thermal effects in the same mirrors. The main goal of the present study is to discuss these issues versus the transverse structure of the readout beam, in order to allow comparison. A number of theoretical studies and experiments have been carried out, regarding thermal noise and thermal effects. We do not discuss experimental problems, but rather focus on some theoretical results in this context about arbitrary order Laguerre-Gauss beams, and other “exotic” beams.

Evanescent-field coupling between a monomode fiber and a high-index medium of limited thickness

By applying a coupled-mode theory to the angle degenerate modes of a planar waveguide and to the single mode of a neighboring monomode fiber, we derive an integrodifferential system describing the transfer of light intensity from the fiber to the planar waveguide. By numerically solving this system, the transmission loss of the fiber is computed as a function of various parameters such as the planar-guide thickness, the planar index, and the remaining fiber-cladding thickness.

Continuous laser amplification in a monomode fiber longitudinally pumped by evanescent field coupling

Amplification of continuous laser radiation propagating in a monomode optical fiber was obtained by evanescent field coupling within a pumped dye solution deposited in the neighborhood of the fiber core. A factor-of-25 signal gain was recorded for 632.8-nm He-Ne radiation and DCM dye solution.

Report on the second Mock LISA data challenge

The Mock LISA data challenges are a program to demonstrate LISA data-analysis capabilities and to encourage their development. Each round of challenges consists of several data sets containing simulated instrument noise and gravitational waves from sources of undisclosed parameters. Participants are asked to analyze the data sets and report the maximum information about the source parameters. The challenges are being released in rounds of increasing complexity and realism: here we present the results of Challenge 2, issued in Jan 2007, which successfully demonstrated the recovery of signals from nonspinning supermassive-black-hole binaries with optimal SNRs between ~10 and 2000, from ~20 000 overlapping galactic white-dwarf binaries (among a realistically distributed population of 26 million), and from the extreme-mass-ratio inspirals of compact objects into central galactic black holes with optimal SNRs ~100.

RADIATION PRESSURE AND STABILITY OF INTERFEROMETRIC GRAVITATIONAL-WAVE DETECTORS

The effect of radiation pressure on the stability of Fabry-Perot cavities with hanging mirrors is investigated. Such cavities will form an integral part of the laser interferometric gravitational-wave detectors that are being constructed around the globe. The mirrors are hung by means of a pendulum suspension and are locked by servo controls. We assume a realistic servo-control transfer function that satisfies the standard stability criteria. We find that for positive offsets from the resonance of the cavity the system is stable. However, we show that for negative offsets instabilities can occur, although the servo system has the effect of increasing the instability threshold, compared with the nonservoed case. Conditions for stability are finally given, involving the finesse of the cavity, the input power, the mass of the mirrors, the servo gain, and the phase detuning from perfect resonance. Gravitational-wave detectors with arm cavities having a finesse as low as approximately 200 could exhibit instabilities. Some implications for the locking of these detectors are finally given.

Kerr effect in all-fiber cavities of optical gyros

The transmission spectrum of an all-fiber cavity is theoretically analyzed, in order to evaluate the perturbation induced in the eigenfrequency measurement by the Kerr effect. Existence of a threshold separating the bistable operation region from the monostable is shown. In the weakly nonlinear operation, the asymmetry of the line is evaluated and the resulting error in the frequency measurement is analytically derived.

Report on the first round of the Mock LISA Data Challenges

The Mock LISA Data Challenges (MLDCs) have the dual purpose of fostering the development of LISA data analysis tools and capabilities, and demonstrating the technical readiness already achieved by the gravitational-wave community in distilling a rich science payoff from the LISA data output. The first round of MLDCs has just been completed: nine challenges consisting of data sets containing simulated gravitational-wave signals produced either by galactic binaries or massive black hole binaries embedded in simulated LISA instrumental noise were released in June 2006 with deadline for submission of results at the beginning of December 2006. Ten groups have participated in this first round of challenges. All of the challenges had at least one entry which successfully characterized the signal to better than 95% when assessed via a correlation with phasing ambiguities accounted for. Here, we describe the challenges, summarize the results and provide a first critical assessment of the entries.