We apply the Hydroxide Catalysis Bonding (HCB) technique to phosphate glass and measure the reflectivity and Light Induced Damage Threshold (LITD) of the newly formed interface. HCB is a room temperature, high performing process which was designed for astronomical research glass assemblies and played a key role in the detection of gravitational waves, a breakthrough in contemporary science. The bonds have numerous assets including mechanical strength, stability, no outgassing and resistance to contamination which are of high interest in the precision optics industry. However only little research has been done on their optical properties and mostly on silica based materials. In this paper, we use HCB to bond phosphate glass at room temperature with the goal of designing composite components for solid state laser gain media. We change the solution parameters to identify how they influence the final properties of the bonds: the LIDT at 1535 nm in long pulse regime and the reflectivity at 532 nm are investigated. The measurement of the incidence dependent reflectance allows estimating the thickness and refractive index of the bond in a non destructive process. The best performing set of parameters yields a LIDT of 1.6 GW/cm<sup>2</sup> (16 J/cm<sup>2</sup>) and a reflectivity below 0.03 % which makes it suitable for use in high power lasers. The bond thickness is derived both from Scanning Electron Microscopy and the reflectivity measurements and is in the range of 50-150 nm depending on the parameters. Finally, the bonds survive cutting and polishing which is promising for manufacturing purpose.
The Laser Induced Damage Threshold (LIDT) and material properties of various multi-layer amorphous dielectric optical coatings, including Nb<sub>2</sub>O<sub>5</sub>, Ta<sub>2</sub>O<sub>5</sub>, SiO<sub>2</sub>, TiO<sub>2</sub>, ZrO<sub>2</sub>, AlN, SiN, LiF and ZnSe, have been studied. The coatings were produced by ion assisted electron beam and thermal evaporation; and RF and DC magnetron sputtering at Helia Photonics Ltd, Livingston, UK. The coatings were characterized by optical absorption measurements at 1064 nm by Photothermal Common-path Interferometry (PCI). Surface roughness and damage pits were analyzed using atomic force microscopy. LIDT measurements were carried out at 1064 nm, with a pulse duration of 9.6 ns and repetition rate of 100 Hz, in both 1000-on-1 and 1-on-1 regimes. The relationship between optical absorption, LIDT and post-deposition heat-treatment is discussed, along with analysis of the surface morphology of the LIDT damage sites showing both coating and substrate failure.
We have been developing a series of novel technological solutions to address the challenges posed by the adaptive optic
requirements for extremely large telescopes. Our deformable mirror surface material, a compliant from of silicon
carbide, offers a Young's Modulus comparable to glass but with greater, non-catastrophic, resistance to fracture. In
combination with the extraordinary new material we have been working on a new low power actuator with a deflection
capability of tens of microns. We have considered the systems requirements for our deformable mirror and developed
both a coating technology and a unique use of hydroxide catalysis bonding.
Adaptive optic requirements for instrumentation such as EAGLE for the European extremely large telescope present an
enormous challenge to deformable mirror technology. We have developed a unique approach using fabricated arrays of
multilayer actuator technology to address the requirements of actuator density and deflection. Our programme of work
has uncovered a novel approach which has led to a built in test capability. We will present the outcomes of our work
which we believe will lead to a compact deformable mirror.
Einstein's General Theory of Relativity predicts waves in spacetime caused by oscillating masses. Such waves, known as gravitational waves, are predicted to be created by binary black hole or neutron star inspirals, super-nova, or other catastrophic astronomical events. Even with such large masses moving so repidly, the expected size of the waves is extremely small, typically of order 10<sup>-21</sup> in unitless strain as seen on Earth. LIGO, the Laser Interferometer Gravitational Wave Observatory, is a basic physics experiments designed to detect and study these waves. The next generation interferometers, known as Advanced LIGO, are currently being designed. Thermal noise from mechanical loss in the optical coatings of the mirrors is expected to be an important limiting noise source. Reducing this noise by developing lower mechanical loss coatings, while preserving optical and thermal properties needed in the interferometer, is an area of active research.
The GEO 600 laser interferometer with 600m armlength is part of a worldwide network of gravitational wave detectors. GEO 600 is unique in having advanced multiple pendulum suspensions with a monolithic last stage and in employing a signal recycled optical design. This paper describes the recent commissioning of the interferometer and its operation in signal recycled mode.
Gravitational waves are a prediction of Einstein's General Theory of Relativity. Astrophysical events like supernova and binary neutron star inspirals are predicted to create potentially detectable waves. The Laser Interferometer Gravitational-wave Observatory (LIGO) is an experiment to detect these waves using Michelson interferometers with 4 km long arms. The effect of gravitational waves, even on an interferometer with such a long baseline, is extremely, with mirror displacements around 10<sup>-18</sup>m. Reducing noise is thus a primary design criterion. For the next generation interferometers now being designed, thermal noise from the optical coatings of the interferometer mirrors could prove a problematic limiting noise source. Reducing the mechanical loss of these coatings to improve thermal noise, while preserving the sub-ppm optical absorption, low scatter, and high reflectivity needed in the interferometer is an important area of research.
To meet the overall isolation and alignment requirements for the optics in Advanced LIGO, the planned upgrade to LIGO, the US laser interferometric gravitational wave observatory, we are developing three sub-systems: a hydraulic external pre-isolator for low frequency alignment and control, a two-stage active isolation platform designed to give a factor of ~1000 attenuation at 10 Hz, and a multiple pendulum suspension system that provides passive isolation above a few hertz. The hydraulic stage uses laminar-flow quiet hydraulic actuators with millimeter range, and provides isolation and alignment for the optics payload below 10 Hz, including correction for measured Earth tides and the microseism. This stage supports the in-vacuum two-stage active isolation platform, which reduces vibration using force feedback from inertial sensor signals in six degrees of freedom. The platform provides a quiet, controlled structure to mount the suspension system. This latter system has been developed from the triple pendulum suspension used in GEO 600, the German/UK gravitational wave detector. To meet the more stringent noise levels required in Advanced LIGO, the baseline design for the most sensitive optics calls for a quadruple pendulum, whose final stage consists of a 40 kg sapphire mirror suspended on fused silica ribbons to reduce suspension thermal noise.
The GEO600 laser interferometric gravitational wave detector is approaching the end of its commissioning phase which started in 1995.
During a test run in January 2002 the detector was operated for 15 days in a power-recycled michelson configuration. The detector and environmental data which were acquired during this test run were used to test the data analysis code. This paper describes the subsystems of GEO600, the status of the detector by August 2002 and the plans towards the first science run.
To obtain improved sensitivities in future generations of interferometric graviational wave detectors, beyond those proposed as upgrades of current detectors, will require different approaches in different portions of the gravitational wave frequency band. However the use of silicon as an interferometer test mass substrate, along with all-reflective interferometer topologies, could prove to be a design enabling sensitivity improvements at both high and low frequencies. In this paper the thermo-mechanical properties of silicon are discussed and the potenial benefits from using silicon as a mirror substrate material in future gravitational wave detectors are outlined.
The LIGO project has completed the installation of large fused silica optical components in the vacuum systems of its observatories. Commissioning work on the Hanford 2 km interferometer has determined an upper limit to the optics losses, allowing comparison with design and pre-installation testing. Planning and development of sapphire optics for the next generation, advanced LIGO detector is now underway, including polishability, optical homogeneity, absorption, and birefringence. The advanced optics development also includes research aimed at lowering coating loss.