Interferometric measurement of refractive index inhomogeneity on polished sapphire substrates: application to LIGO-II

In order to improve the detection sensitivity of the Laser Interferometer Gravitational-wave Observatory (LIGO) the use of 40-kg sapphire test masses is being considered for the next instrument upgrade. Currently, sapphire material of adequate size is only available with the optical axis aligned with the m axis of the crystal. To determine the material's suitability it is necessary to characterize the refractive index inhomogeneity of the sapphire substrates for two orthogonal directions of polarisation, to a fraction of a part per million (ppm). We report on a method used to measure the refractive index inhomogeneity which requires three separate measurements of the polished sapphire blank in a Fizeau interferometer. These measurements are of the surface shapes or figures of the two polished sides of the blank and that of the wavefront entering side one propagating through the blank, reflected off side two and exiting through side one. The phase maps corresponding to these three measurements are combined to obtain the refractive index inhomogeneity map distribution. Measurements were carried out on two sapphire substrates (m axis) produced by the heat exchange method. The inhomogeneity maps show features which depend on polarisation direction. The physical origin of the inhomogeneities is discussed as well as the probable impact on the detection of a gravitational wave signal

Observation of Parametric Instability in Advanced LIGO

Parametric instabilities have long been studied as a potentially limiting effect in high-power interferometric gravitational wave detectors. Until now, however, these instabilities have never been observed in a kilometer-scale interferometer. In this Letter, we describe the first observation of parametric instability in a gravitational wave detector, and the means by which it has been removed as a barrier to progress

Observation of Parametric Instability in Advanced LIGO

Parametric instabilities have long been studied as a potentially limiting effect in high-power interferometric gravitational wave detectors. Until now, however, these instabilities have never been observed in a kilometer-scale interferometer. In this work we describe the first observation of parametric instability in an Advanced LIGO detector, and the means by which it has been removed as a barrier to progress

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead