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

A method for the experimental measurement of bulk and shear loss angles in amorphous thin films

Brownian thermal noise is a limiting factor for the sensitivity of many high precision metrology applications, among other gravitational-wave detectors. The origin of Brownian noise can be traced down to internal friction in the amorphous materials that are used for the high reflection coatings. To properly characterize the internal friction in an amorphous material, one needs to consider separately the bulk and shear losses. In most of previous works the two loss angles were considered equal, although without any first principle motivation. In this work we present a method that can be used to extract the material bulk and shear loss angles, based on current state-of-the-art coating ring-down measurement systems. We also show that for titania-doped tantala, a material commonly used in gravitational-wave detector coatings, the experimental data strongly favor a model with two different and distinct loss angles, over the simpler case of one single loss angle

Characterization of Core Optics in Gravitational-Wave Detectors: Case Study of KAGRA Sapphire Mirrors

We report the characterization of superlow-loss optics used in the second-generation gravitational-wave detectors currently in operation. The sapphire test-mass mirrors in the KAGRA detector are introduced as an example, but the techniques here are common to all detectors. In this work, we discuss mainly the surface topography obtained by interferometric techniques and the optical properties obtained with special setups

Sapphire mirror for the KAGRA gravitational wave detector

KAGRA, the Japanese interferometric gravitational wave detector currently under construction, will employ sapphire test masses for its cryogenic operation. Sapphire has an advantage in its higher thermal conductivity near the operating temperature 20 K compared to fused silica used in other gravitational wave detectors, but there are some uncertain properties for the application such as hardness, optical absorption, and birefringence. We introduce an optical design of the test masses and our recent R&D results to address the above properties. Test polish of sapphire substrate has especially proven that specifications on the surface are sufficiently met. Recent measurements of absorption and inhomogeneity of the refractive index of the sapphire substrate indicate that the other properties are also acceptable to use sapphire crystal as test masses

Structural Evolution that Affects the Room-Temperature Internal Friction of Binary Oxide Nanolaminates: Implications for Ultrastable Optical Cavities

Internal friction in oxide thin films imposes a critical limitation to the sensitivity and stability of the ultrahigh finesse optical cavities for gravitational wave detectors. Strategies like doping or creating nanolaminates (NL) are sought to introduce structural modifications that reduce internal friction. This work describes an investigation of the morphological changes SiO₂/Ta₂O₅ and TiO₂/Ta₂O₅ nanolaminates undergo with annealing and their impact on room-temperature internal friction. It is demonstrated that thermal treatment results in a reduction of internal friction in both nanolaminates but through different pathways. In the SiO₂/Ta₂O₅ nanolaminate, the layers of which remain intact after annealing, the total reduction in internal friction follows the reduction in the composing SiO₂ and Ta₂O₅ layers. In contrast, interdiffusion initiated by annealing at the interface in the TiO₂/Ta₂O₅ nanolaminate leads to the formation of a mixed phase. It is the interfacial reaction upon annealing that dictates the more significant reduction in internal friction to ∼2.6 × 10⁻⁴, a value lower than any other Ta₂O₅ mixture coating with similar cation concentration

A method for the experimental measurement of bulk and shear loss angles in amorphous thin films

Brownian thermal noise is a limiting factor for the sensitivity of many high precision metrology applications, among other gravitational-wave detectors. The origin of Brownian noise can be traced down to internal friction in the amorphous materials that are used for the high reflection coatings. To properly characterize the internal friction in an amorphous material, one needs to consider separately the bulk and shear losses. In most of previous works the two loss angles were considered equal, although without any first principle motivation. In this work we present a method that can be used to extract the material bulk and shear loss angles, based on current state-of-the-art coating ring-down measurement systems. We also show that for titania-doped tantala, a material commonly used in gravitational-wave detector coatings, the experimental data strongly favor a model with two different and distinct loss angles, over the simpler case of one single loss angle

Zirconia-titania-doped tantala optical coatings for low mechanical loss Bragg mirrors

The noise caused by internal mechanical dissipation in the high refractive index amorphous thin films in dielectric mirrors is an important limitation for gravitational wave detection. The objective of this study is to decrease this noise spectral density, which is linearly dependent on such dissipation and characterized by the loss angle of the Young’s modulus, by adding zirconia to titania-doped tantala, from which the current mirrors for gravitational wave detection are made. The purpose of adding zirconia is to raise the crystallization temperature, which allows the material to be more relaxed by raising the practical annealing temperature. The Ta, Ti and Zr oxides are deposited by reactive magnetron sputtering in an Ar:O2 atmosphere using radio-frequency and high power impulse plasma excitation. We show that thanks to zirconia, the crystallization temperature rises by more than 150◦C, which allows one to obtain a loss angle of 2.5 × 10−4 , that is, a decrease by a factor of 1.5 compared to the current mirror high-index layers. However, due to a difference in the coefficient of thermal expansion between the thin film and the silica substrate, cracks appear at high annealing temperature. In response, a silica capping layer is applied to increase the temperature of crack formation by 100◦C

Apparatus to Measure Optical Scatter of Coatings Versus Annealing Temperature

Light scattered by amorphous thin-film optical coatings limits the sensitivity of interferometric gravitational-wave detectors. We describe an imaging scatterometer to assess the role that crystal growth during annealing plays in this scatter