Interferometry developments for spaceborne gravitational wave detectors

The existence of gravitational waves is predicted by Einstein's General Theory of Relativity. They can be considered as a wave-like distortion of four dimensional spacetime. Gravitational waves are produced by systems with time varying quadrupole mass moment. Spacetime is a stiff elastic medium, implying that waves traveling through it will have small amplitudes and this makes their detection very challenging. There are well established efforts towards the detection of gravitational waves using ground-based systems. These detectors are limited by a lower frequency limit of ~ 10 Hz set by the gravity gradient wall, which is a consequence of being in a gravitationally noisy environment. However, there are many predicted sources of gravitational radiation of relatively large amplitude at lower frequencies. Thus to complement the ground based network of detectors a spaceborne detector, the Laser Interferometer Space Antenna (LISA), is planned. Gravitational wave detection by interferometry on Earth involves displacement measurements of order 10[-18]m/√Hz on tens of millisecond timescales and over arm lengths of kilometers. In contrast, LISA requires the monitoring of 5 million kilometer baselines at a noise level of 10[-12]m/√Hz and over 1000 second timescales. So while the displacement sensitivities required of LISA may appear routine in the context of current ground-based detectors, the frequency regime and distances involved introduce new challenges. In order to try and reduce some of the technological risks of LISA, a precursor mission (called LISA Pathfinder) will be flown to demonstrate performance of technologies that cannot be adequately demonstrated on Earth. LISA Pathfinder contains an experiment called the LISA Technology Package (LTP). The work presented in this thesis deals mainly with investigations of the interferometry that will be used in the LTP and in LISA, with particular emphasis on the identification of sources of excess noise and of methods to minimise their effects. LTP will use interferometry to monitor the distance between two inertial proof masses. The goal is to demonstrate the performance of the intertial sensors to within an order of magnitude of that required for LISA. To do this the interferometer sensitivity is relaxed an order of magnitude from the LISA goal but is still technically very challenging. The approach adopted to demonstrate the interferometry for LTP was to build a stable optical bench using hydroxide-catalysis bonding of optical components to a low thermal expansion baseplate. This is the construction approach to be used in LTP and likely to be adopted for LISA. The stability of the optical bench was then tested using an LTP style heterodyne interferometer arrangement and demonstrated to be stable to 10pm/?Hz from 3 mHz to 30 mHz, with the exception of a minor spectral feature of temperature driven excess noise when operated in a laboratory environment. The experience gained by constructing and testing the optical bench strongly influenced the techniques used to construct the engineering model LTP bench and the techniques that will be used for building the flight model

Automated precision alignment of optical components for hydroxide catalysis bonding

We describe an interferometric system that can measure the alignment and separation of a polished face of a optical component and an adjacent polished surface. Accuracies achieved are ∼ 1μrad for the relative angles in two orthogonal directions and ∼ 30μm in separation. We describe the use of this readout system to automate the process of hydroxide catalysis bonding of a fused-silica component to a fused-silica baseplate. The complete alignment and bonding sequence was typically achieved in a timescale of a few minutes, followed by an initial cure of 10 minutes. A series of bonds were performed using two fluids - a simple sodium hydroxide solution and a sodium hydroxide solution with some sodium silicate solution added. In each case we achieved final bonded component angular alignment within 10 μrad and position in the critical direction within 4 μm of the planned targets. The small movements of the component during the initial bonding and curing phases were monitored. The bonds made using the sodium silicate mixture achieved their final bonded alignment over a period of ∼ 15 hours. Bonds using the simple sodium hydroxide solution achieved their final alignment in a much shorter time of a few minutes. The automated system promises to speed the manufacture of precision-aligned assemblies using hydroxide catalysis bonding by more than an order of magnitude over the more manual approach used to build the optical interferometer at the heart of the recent ESA LISA Pathfinder technology demonstrator mission. This novel approach will be key to the time-efficient and low-risk manufacture of the complex optical systems needed for the forthcoming ESA spaceborne gravitational waves observatory mission, provisionally named LISA

Construction of rugged, ultrastable optical assemblies with optical component alignment at the few microradian level

A method for constructing quasimonolithic, precision-aligned optical assemblies is presented. Hydroxide-catalysis bonding is used, adapted to allow optimization of component fine alignment prior to the bond setting. We demonstrate the technique by bonding a fused silica mirror substrate to a fused silica baseplate. In-plane component placement at the submicrometer level is achieved, resulting in angular control of a reflected laser beam at the sub-10-μrad level. Within the context of the LISA Pathfinder mission, the technique has been demonstrated as suitable for use in space-flight applications. It is expected that there will also be applications in a wide range of areas where accuracy, stability, and strength of optical assemblies are important

Mechanisation of Precision Placement and Catalysis Bonding of Optical Components

Precision-aligned, ultra-stable optical assemblies are needed for an increasing number of space applications, in areas such as science, metrology and geodesy

Sub-system mechanical design for an eLISA optical bench

We present the design and development status of the opto-mechanical sub-systems that will be used in an experimental demonstration of imaging systems for eLISA. An optical bench test bed design incorporates a Zerodur® baseplate with lenses, photodetectors, and other opto-mechanics that must be both adjustable - with an accuracy of a few micrometers - and stable over a 0 to 40°C temperature range. The alignment of a multi-lens imaging system and the characterisation of the system in multiple degrees of freedom is particularly challenging. We describe the mechanical design of the precision mechanisms, including thermally stable flexure-based optical mounts and complex multi-lens, multi-axis adjuster mechanisms, and update on the integration of the mechanisms on the optical bench

Optical Characterisation of Hydroxide Catalysed Bonds Applied to Phosphate Glass

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/cm2 (16 J/cm2) 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

An elegant Breadboard of the optical bench for eLISA/NGO

The Laser Interferometer Space Antenna, as well as its reformulated European-only evolution, the New Gravitational-Wave Observatory, both employ heterodyne laser interferometry on million kilometer scale arm lengths in a triangular spacecraft formation, to observe gravitational waves at frequencies between 3 × 10−5 Hz and 1 Hz. The Optical Bench as central payload element realizes both the inter-spacecraft as well as local laser metrology with respect to inertial proof masses, and provides further functions, such as point-ahead accommodation, acquisition sensing, transmit beam conditioning, optical power monitoring, and laser redundancy switching. These functions have been combined in a detailed design of an Optical Bench Elegant Breadboard, which is currently under assembly and integration. We present an overview of the realization and current performances of the Optical Bench subsystems, which employ ultraprecise piezo mechanism, ultrastable assembly techniques, and shot noise limited RF detection to achieve translation and tilt metrology at Picometer and Nanoradian noise levels

The status of GEO 600

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