K. A. Strain

Invited article: CO2 laser production of fused silica fibers for use in interferometric gravitational wave detector mirror suspensions.

In 2000 the first mirror suspensions to use a quasi-monolithic final stage were installed at the GEO600 detector site outside Hannover, pioneering the use of fused silica suspension fibers in long baseline interferometric detectors to reduce suspension thermal noise. Since that time, development of the production methods of fused silica fibers has continued. We present here a review of a novel CO(2) laser-based fiber pulling machine developed for the production of fused silica suspensions for the next generation of interferometric gravitational wave detectors and for use in experiments requiring low thermal noise suspensions. We discuss tolerances, strengths, and thermal noise performance requirements for the next generation of gravitational wave detectors. Measurements made on fibers produced using this machine show a 0.8% variation in vertical stiffness and 0.05% tolerance on length, with average strengths exceeding 4 GPa, and mechanical dissipation which meets the requirements for Advanced LIGO thermal noise performance.

The Glasgow 10 m prototype laser interferometric gravitational wave detector

We present a description of the prototype interferometric gravitational wave detector at Glasgow. The detector, which has been under development for a number of years, consists of two perpendicular 10‐m‐long high finesse Fabry–Perot cavities formed between test masses hung as pendulums and is illuminated with a cw argon ion laser. The differential displacement sensitivity of the detector is ∼7×10−19u2009m/√Hz from 500u2009Hz → 3u2009kHz and is close to being limited by photoelectron shot noise in the detected photocurrent.

Aspects of the suspension system for GEO 600

The GEO 600 gravitational wave detector is currently under construction in Germany. To ensure that the detector sensitivity is not limited by seismic noise above 50 Hz a significant degree of seismic isolation has to be provided for each test mass. To achieve this level of isolation each test mass, which will be made from fused silica (mass ∼14 kg), will be suspended as the final stage of a triple pendulum from an isolation stack consisting of layers of stainless steel masses and graphite loaded silicone rubber. Extending from the stack will be three cantilever springs supporting an upper mass from which a double pendulum stage will be suspended. This double pendulum will incorporate cantilever springs in the upper stage in order to enhance the vertical isolation, and will use fused silica fibers in the lower pendulum stage in order to minimize thermal noise from the pendulum modes. The expected performance from this arrangement of stacks, vertical spring stages, and double pendulums should achieve our go...

Test of an 18‐m‐long suspended modecleaner cavity

Fluctuations in laser beam geometry such as lateral beam movement and beam width variation can produce perturbations in the output signal of a laser interferometer through various coupling mechanisms. In order to avoid this type of laser noise from degrading the sensitivity of interferometers designed to detect gravitational radiation, currently planned long‐base‐line detectors will employ resonant optical filters called modecleaners to suppress beam geometry noise. Here we describe a prototype modecleaner cavity constructed at Glasgow having an optical path length of 18 m and containing four suspended mirrors. We present results detailing various aspects of its performance.

Measurements of losses in high reflectance mirrors coated for λ = 514.5 nm

Abstract We have measured the losses of high reflectance mirrors coated for λ = 514.5 nm using the decay time of light in a Fabry Perot cavity. Currently the lowest losses we have measured in air, for substrates polished by General Optics and coated by British Aerospace using the ion beam sputtering technique, are 80±20 ppm. As expected, both the quality of the substrate and the method of deposition of the coating are important for achieving the best results.

Experimental demonstration of the use of a Fabry–Perot cavity as a mirror of variable reflectivity

The use of a spherical mirror Fabry–Perot cavity as a mirror of variable reflectivity for near monochromatic light is demonstrated herein. The results of an experimental test of two control systems for the variable mirror are presented. It is shown that the reflectivity of our mirror can be controllably varied over a wide range. This technique has application in laser interferometric gravitational‐wave detection.

Measurement of the absorptance of fused silica at λ = 514.5 nm

Optical absorption in fused silica components can limit the performance of high power optical systems. Measurements of the absorption coefficients for several grades of fused silica show widely differing absorption values at the test wavelength (514.5 nm), allowing selection of the type best suited to a particular application. The method employed can readily be adapted to other wavelengths.