M. Notcutt

Ultrahigh Q pendulum suspensions for gravitational wave detectors

Pendulum suspensions for laser interferometer gravitational wave detectors need to have an extremely high Q factor to minimize Brownian motion noise. In this paper we analyze the limits to the Q factor of the compound pendulum. We show that the observed acoustic loss of niobium can allow pendulum Q factors of 1010 to be achieved. This should enable a 3 km terrestrial laser interferometer detector to achieve strain sensitivity of 10−22/√Hz at frequencies as low as 10 Hz. At cryogenic temperatures Q factors up to 1012 should be achievable.

Sapphire beamsplitters and test masses for advanced laser interferometer gravitational wave detectors

We present a feasibility study of using sapphire beamsplitters and test masses in laser interferometer gravitational wave detectors. The internal thermal noise, optical losses and birefringence effects are analysed. Suspension losses are investigated. Experimental data on birefringence is presented. The conclusions are generally positive.

Cryogenic, all‐sapphire, Fabry–Perot optical frequency reference

This paper presents a design for a Fabry–Perot optical frequency reference resonator which utilizes the high dimensional stability of cryogenically cooled sapphire. We show that cryogenic sapphire cavities can achieve substantial improvements in frequency stability compared with room‐temperature cavities. The design of a laser stabilization system based on such a resonator is discussed. Estimates of fundamental and practical limitations on the frequency stability of such a system suggest that a fractional frequency stability of 10−16 over integration times from 10 to 104 s is possible. The fundamental limits to stability from quantum shot noise, radiation pressure fluctuations, and thermal noise are overwhelmed by practical limits which arise due to mechanical, thermal, and optical effects.

Temperature compensation for cryogenic cavity stabilized lasers

We describe a cryogenic Fabry-Perot cavity in which a frequency-temperature turning point at 43 K is created by differential thermal contraction between the mirrors and the spacer. The cavity uses a sapphire spacer and fused silica mirrors. The expansion coefficient of the spacer is balanced by a negative term arising from the outward flexing of the mirrors caused by differential radial expansion of the mirror substrate and spacer. A simple model indicates that is possible to construct cavities with shallow turning points of frequency-temperature at temperatures between 10 and 65 K.

POSITION CONTROL SYSTEM FOR SUSPENDED MASSES IN LASER INTERFEROMETER GRAVITATIONAL WAVE DETECTORS

We review and present an analysis of the major issues in the design of test mass control systems for laser interferometer gravitational wave detectors. Based on this analysis, we present a design for a computer controlled modular analog servo control system which is well suited to the control of a large number of degrees of freedom in long baseline instruments. The system has been tested on an interferometer using multistage cantilever spring isolators. The system enables simple monitoring, testing, and display of many channels simultaneously, while retaining the advantages of analog proportional‐integral‐derivative control electronics.

Development of low-loss sapphire mirrors

We report on the successful development of low-loss sapphire mirrors for use at a 1-mum wavelength. Methods for polishing and coating are described. The analysis of each process shows a roughness of better than 0.1 nm, a coating scattering of 1 ppm, and a surface scattering of 13 ppm. The mirrors have been characterized in a Fabry-Perot cavity, having a finesse of 100, 000. Mode doublets result from the birefringence of the coatings.

Cryogenic system for a sapphire Fabry-Perot optical frequency standard

This paper describes the design and performance of a cryogenic system for a sapphire Fabry-Perot optical frequency reference cavity. The cryogenic system utilises a single vacuum system, a liquid helium coupled heat exchanger, low mechanical vibration mounting and an axial, bottom-entering optical path. The cryogenic and thermal performances are also described.

X-ray induced absorption of high-purity sapphire and investigation of the origin of the residual absorption at 1064 nm

We report a set of optical absorption measurements on x-irradiated high-purity sapphire. The results of the photothermal deflection absorption measurements at 1064 nm show an increase of the absorption at the x-irradiated areas. The x-ray damage recovery is achieved through annealing, where the absorption level at 1064 nm is brought from an average level of ~80 ppm cm-1 to ~24 ppm cm-1 . UV-Vis spectroscopy results suggest that the residual absorption at 1064 nm is due to complex clusters of Ti and Fe ions and oxygen vacancies. We suggest that a further reduction of Ti and Fe in sapphire (<0.1 ppm) as well as oxygen vacancies (through post growth oxidizing annealing) would further reduce the absorption. Moreover, the absorption level of 20 ppm cm-1 remains within the requirements of the second generation laser interferometric gravitational-wave detectors.

Birefringence measurements of sapphire test masses for laser interferometer gravitational wave detector

Abstract We report measurements of the birefringence of a high-purity polished sapphire sample to determine the fringe contrast attainable in a laser interferometer gravitational detector. The results show that birefringence-induced losses in the interferometer would be comparable whether sapphire or silica is used.

Rayleigh scattering in sapphire test mass for laser interferometric gravitational-wave detectors: I. Measurement of scattering attenuation coefficient

Abstract We give a theoretical estimate of the lower limit of Rayleigh scattering in sapphire crystals. We report measurements of scattering attenuation coefficient in sapphire at wavelengths of 633 and 1064 nm using an imaging measurement technique. The measured scattering loss in sapphire is ∼13 ppm/cm. Using sapphire as a mirror and beamsplitter substrate in laser interferometric gravitational-wave detectors, Rayleigh scattering losses are similar in magnitude to estimates of mirror losses. However, the scattering level still remains higher than the theoretical limit because of the presence of impurities–vacancies in the sapphire crystal.