G. M. Harry

Thermal noise from optical coatings in gravitational wave detectors.

Gravitational waves are a prediction of Einsteins general theory of relativity. These waves are created by massive objects, like neutron stars or black holes, oscillating at speeds appreciable to the speed of light. The detectable effect on the Earth of these waves is extremely small, however, creating strains of the order of 10(-21). There are a number of basic physics experiments around the world designed to detect these waves by using interferometers with very long arms, up to 4 km in length. The next-generation interferometers are currently being designed, and the thermal noise in the mirrors will set the sensitivity over much of the usable bandwidth. Thermal noise arising from mechanical loss in the optical coatings put on the mirrors will be a significant source of noise. Achieving higher sensitivity through lower mechanical loss coatings, while preserving the crucial optical and thermal properties, is an area of active research right now.

Dissipation of mechanical energy in fused silica fibers

To determine the dissipation induced by the surface of fused silica fibers, we measured the quality factor of fibers having various diameters. We measured a maximum quality factor of 21 million and extrapolated to obtain an intrinsic quality factor for fused silica of 30 million. Dissipation in the surface dominated at diameters less than about 1 mm. We developed a method for characterizing surface-induced dissipation that is independent of sample geometry or mode shape.

High quality factor measured in fused silica

We have measured the mechanical dissipation in a sample of fused silica drawn into a rod. The sample was hung from a multiple-bob suspension, which isolated it from rubbing against its support, from recoil in the support structure, and from seismic noise. The quality factor, Q, was measured for several modes with a high value of 57.1±0.1 million found for the mode number 2 at 726 Hz. This result is about a factor of 2 higher than previous room temperature measurements. The measured Q was strongly dependent on handling, with a pristine flame-polished surface yielding a Q three to four times higher than a surface which had been knocked several times against a copper tube.

Pendulum mode thermal noise in advanced interferometers: A comparison of fused silica fibers and ribbons in the presence of surface loss

The use of fused-silica ribbons as suspensions in gravitational wave interferometers can result in significant improvements in pendulum mode thermal noise. Surface loss sets a lower bound to the level of noise achievable, at what level depends on the dissipation depth and other physical parameters. For LIGO II, the high breaking strength of pristine fused silica filaments, the correct choice of ribbon aspect ratio (to minimize thermoelastic damping), and low dissipation depth combined with the other achievable parameters can reduce the pendulum mode thermal noise in a ribbon suspension well below the radiation pressure noise. Despite producing higher levels of pendulum mode thermal noise, cylindrical fiber suspensions provide an acceptable alternative for LIGO II, should unforeseen problems with ribbon suspensions arise.

Comparison of advanced gravitational wave detectors

We compare two advanced designs for gravitational-wave antennas in terms of their ability to detect two possible gravitational wave sources. Spherical, resonant mass antennas and interferometers incorporating resonant sideband extraction (RSE) were modeled using experimentally measurable parameters. The signal-to-noise ratio of each detector for a binary neutron star system and a rapidly rotating stellar core were calculated. For a range of plausible parameters we found that the advanced LIGO interferometer incorporating RSE gave higher signal-to-noise ratios than a spherical detector resonant at the same frequency for both sources. Spheres were found to be sensitive to these sources at distances beyond our galaxy. Interferometers were sensitive to these sources at far enough distances that several events per year would be expected.

Effect of optical coating and surface treatments on mechanical loss in fused silica

We report on the mechanical loss in fused silica samples with various surface treatments and compare them with samples having an optical coating. Mild surface treatments such as washing in detergent or acetone were not found to affect the mechanical loss of flame-drawn fused silica fibers stored in air. However, mechanical contact (with steel calipers) significantly increased the loss. The application of a high-reflective optical coating of the type used for the LIGO test masses was found to greatly increase the mechanical loss of commercially polished fused silica microscope slides. We discuss the implications for the noise budget of interferometers.

Optical Coatings and Thermal Noise in Precision Measurement: List of contributors

1. Theory of thermal noise in optical mirrors Y. Levin 2. Coating technology S. Chao 3. Compendium of thermal noises in optical mirrors V. B. Braginsky, M. L. Gorodetsky and S. P. Vyatchanin 4. Coating thermal noise I. Martin and S. Reid 5. Direct measurements of coating thermal noise K. Numata 6. Methods of improving thermal noise S. Ballmer and K. Somiya 7. Substrate thermal noise S. Rowan and I. Martin 8. Cryogenics K. Numata and K. Yamamoto 9. Thermo-optic noise M. Evans and G. Ogin 10. Absorption and thermal issues P. Willems, D. Ottaway and P. Beyersdorf 11. Optical scatter J. R. Smith and M. E. Zucker 12. Reflectivity and thickness optimisation I. M. Pinto, M. Principe and R. DeSalvo 13. Beam shaping A. Freise 14. Gravitational wave detection D. Ottaway and S. D. Penn 15. High-precision laser stabilisation via optical cavities M. J. Martin and J. Ye 16. Quantum optomechanics G. D. Cole and M. Aspelmeyer 17. Cavity quantum electrodynamics T. E. Northup.

Optical Coatings and Thermal Noise in Precision Measurement: Frontmatter

1. Theory of thermal noise in optical mirrors Y. Levin 2. Coating technology S. Chao 3. Compendium of thermal noises in optical mirrors V. B. Braginsky, M. L. Gorodetsky and S. P. Vyatchanin 4. Coating thermal noise I. Martin and S. Reid 5. Direct measurements of coating thermal noise K. Numata 6. Methods of improving thermal noise S. Ballmer and K. Somiya 7. Substrate thermal noise S. Rowan and I. Martin 8. Cryogenics K. Numata and K. Yamamoto 9. Thermo-optic noise M. Evans and G. Ogin 10. Absorption and thermal issues P. Willems, D. Ottaway and P. Beyersdorf 11. Optical scatter J. R. Smith and M. E. Zucker 12. Reflectivity and thickness optimisation I. M. Pinto, M. Principe and R. DeSalvo 13. Beam shaping A. Freise 14. Gravitational wave detection D. Ottaway and S. D. Penn 15. High-precision laser stabilisation via optical cavities M. J. Martin and J. Ye 16. Quantum optomechanics G. D. Cole and M. Aspelmeyer 17. Cavity quantum electrodynamics T. E. Northup.