Precision absolute positional measurement of laser beams

We describe an instrument which, coupled with a suitable coordinate measuring machine, facilitates the absolute measurement within the machine frame of the propagation direction of a millimeter-scale laser beam to an accuracy of around ±4  μm in position and ±20  μrad in angle

Construction and testing of the optical bench for LISA pathfinder

eLISA is a space mission designed to measure gravitational radiation over a frequency range of 0.1–100 mHz (European Space Agency LISA Assessment Study Report 2011). It uses laser interferometry to measure changes of order $10\,{\rm pm /\sqrt{Hz}}$ in the separation of inertial test masses housed in spacecraft separated by 1 million km. LISA Pathfinder (LPF) is a technology demonstrator mission that will test the key eLISA technologies of inertial test masses monitored by laser interferometry in a drag-free spacecraft. The optical bench that provides the interferometry for LPF must meet a number of stringent requirements: the optical path must be stable at the few ${\rm pm /\sqrt{Hz}}$ level; it must direct the optical beams onto the inertial masses with an accuracy of better than ±25 μm, and it must be robust enough not only to survive launch vibrations but to achieve full performance after launch. In this paper we describe the construction and testing of the flight optical bench for LISA Pathfinder that meets all the design requirements

Interferometric characterization of the optical window for LISA Pathfinder and LISA

In LISA Pathfinder and LISA the position fluctuations of drag free test masses will be determined interferometrically to picometer precision. To this end, laser light is brought to interference on an ultra stable optical bench after being reflected on the test mass, which needs to be in an ultra-high vacuum. The present baseline for both missions includes a separate vacuum enclosure for each test mass, so that the sensing laser beam has to pass through an optical window. This window is therefore a transmissive element in the interferometer and its associated pathlength fluctuations are potentially significant. We have selected an athermal glass that should minimize the thermally induced pathlength changes.Several prototype windows, both mounted and unmounted, have been produced and characterized. The pathlength sensitivity to both temperature fluctuations and temperature gradients has been measured with a dedicated interferometer prototype. We have also compared the long-term stability of the LISA Technology Package interferometer when an optical window is present in the optical path to the situation without window. Finally, glass samples have been radiated and the absorption in the glass after the radiation tests has been measured to be negligible at the wavelength of interest (1064 nm). We present here the results of our measurements, which indicate that using a window does not influence the interferometer performance

Construction of the LISA back-side fibre link interferometer prototype

The Laser Interferometer Space Antenna (LISA) is a joint ESA NASA mission to be launched in 2018. It is an interferometric gravitational wave detector with a measurement band going from 0.1 mHz to 1 Hz. The conceptual interferometer design is unique and includes many challenging aspects that must be analysed in terms of their stability in advance to the mission. One of these new features is the so-called back-side fibre link, which connects the two optical benches on-board each spacecraft. In its optical fibre, two frequency shifted laser beams are counter-propagating. LISA will only reach its design sensitivity, if these two beams inside this fibre experience the same pathlength changes down to a level of approximately 1 pm/\sqrt{\rm Hz} in the mHz range. In this paper, we present the construction of a quasi-monolithic interferometer that represents a cutout of the LISA interferometry concerning the back-side fibre link. In order to ensure a high thermal and mechanical stability of the interferometer, the hydroxide-catalysis bonding technique was applied. For the construction of the interferometer, a number of new alignment techniques and solutions were developed that are suitable for LISA prototype experiments

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

Design and construction of a telescope simulator for LISA optical bench testing

LISA (Laser Interferometer Space Antenna) is a proposed space-based instrument for astrophysical observations via the measurement of gravitational waves at mHz frequencies. The triangular constellation of the three LISA satellites will allow interferometric measurement of the changes in distance along the arms. On board each LISA satellite there will be two optical benches, one for each testmass, that measure the distance to the local test mass and to the remote optical bench on the distant satellite. For technology development, an Optical Bench Elegant Bread Board (OB EBB) is currently under construction. To verify the performance of the EBB, another optical bench - the so-called telescope simulator bench - will be constructed to simulate the beam coming from the far spacecraft. The optical beam from the telescope simulator will be superimposed with the light on the LISA OB, in order to simulate the link between two LISA satellites. Similarly in reverse, the optical beam from the LISA OB will be picked up and measured on the telescope simulator bench. Furthermore, the telescope simulator houses a test mass simulator. A gold coated mirror which can be manipulated by an actuator simulates the test mass movements. This paper presents the layout and design of the bench for the telescope simulator and test mass simulator

Optical bench development for LISA

For observation of gravitational waves at frequencies between 30 μHz and 1 Hz, the LISA mission will be implemented in a triangular constellation of three identical spacecraft, which are mutually linked by laser interferometry in an active transponder scheme over a 5 million kilometer arm length. On the end point of each laser link, remote and local beam metrology with respect to inertial proof masses inside the spacecraft is realized by the LISA Optical Bench. It implements further- more various ancillary functions such as point-ahead correction, acquisition sensing, transmit beam conditioning, and laser redundancy switching. A comprehensive design of the Optical Bench has been developed, which includes all of the above mentioned functions and at the same time ensures manufacturability on the basis of hydroxide catalysis bonding, an ultrastable integration technology already perfected in the context of LISA's technology demonstrator mission LISA Pathfinder. Essential elements of this design have been validated by dedicated pre-investigations. These include the demonstration of polarizing heterodyne interferometry at the required Picometer and Nanoradian performance levels, the investigation of potential non-reciprocal noise sources in the so-called backlink fiber, as well as the development of a laser redundancy switch breadboard

State space modelling and data analysis exercises in LISA Pathfinder

LISA Pathfinder is a mission planned by the European Space Agency to test the key technologies that will allow the detection of gravitational waves in space. The instrument on-board, the LISA Technology package, will undergo an exhaustive campaign of calibrations and noise characterisation campaigns in order to fully describe the noise model. Data analysis plays an important role in the mission and for that reason the data analysis team has been developing a toolbox which contains all the functionalities required during operations. In this contribution we give an overview of recent activities, focusing on the improvements in the modelling of the instrument and in the data analysis campaigns performed both with real and simulated data.Comment: Plenary talk presented at the 9th International LISA Symposium, 21-25 May 2012, Pari