LISA, the laser interferometer space antenna, requires the ultimate in lasers, clocks, and drag-free control

The existence of gravitational waves is the most prominent of Einstein's predictions that has not yet been directly verified. The space project LISA shares its goal and principle of operation with the ground-based interferometers currently being operated, the detection and measurement of gravitational waves by laser interferometry. Ground and space detection differ in their frequency ranges, and thus in the detectable sources. Toward low frequencies, ground-based detection is limited by seismic noise, and yet more fundamentally by "gravity-gradient noise," thus covering the range from a few Hz on upward to a few kHz. It is only in space that detection of signals below, say, 1 Hz is possible, opening a wide window to a different class of interesting sources of gravitational waves. The project LISA consists of three spacecraft in heliocentric orbits, forming a triangle of 5 million km sides. A technology demonstrator, the LISA Pathfinder, designed to test vital LISA technologies, is to be launched by ESA in 2009. LISA will face great challenges in reducing measurement noise, and thus, it will very strongly depend on the technologies of lasers, clocks, and drag-free control

Verification of polarising optics for the LISA optical bench

The Laser Interferometer Space Antenna (LISA) is a spacebased interferometric gravitational wave detector. In the current baseline design for the optical bench, the use of polarising optics is foreseen to separate optical beams. Therefore it is important to investigate the influence of polarising components on the interferometer sensitivity and validate that the required picometre stability in the low-frequency band (1 mHz - 1 Hz) is achievable. This paper discusses the design of the experiment and the implemented stabilisation loops. A displacement readout fulfilling the requirement in the whole frequency band is presented. Alternatively, we demonstrate improvement of the noise performance by implementing various algorithms in data post-processing, which leads to an additional robustness for the LISA mission

Phase noise contribution of EOMs and HF cables

Two key components of LISA's inter-spacecraft clock tone transfer chain are electro-optic modulators (EOMs) and high-frequency (HF) cable assemblies. At modulation frequencies of 2 GHz, we characterized the excess phase noise of these components in the LISA frequency range (0.1 mHz to 1 Hz). The upper phase noise limit was found to be almost an order of magnitude better than required. In addition, phase dependencies on temperature were determined. The measured coefficients are within a few milliradians per Kelvin and thereby negligible due to the specified on-board temperature stability

Improved spectrum estimation from digitized time series on a logarithmic frequency axis

We present a practical technique for spectrum and spectral density estimation from long time series by Fourier transforms. We apply Welch’s popular technique of “averaging over modified periodograms” which uses Fourier transforms of fixed length with time-domain windows and overlap. Our technique retains the basic properties of this method, but computes the optimal frequency resolution individually for each Fourier frequency on a logarithmic frequency axis, thus yielding results that are more useful than those of the standard techniques

Readout for intersatellite laser interferometry: Measuring low frequency phase fluctuations of HF signals with microradian precision

Precision phase readout of optical beat note signals is one of the core techniques required for intersatellite laser interferometry. Future space based gravitational wave detectors like eLISA require such a readout over a wide range of MHz frequencies, due to orbit induced Doppler shifts, with a precision in the order of $\mu \textrm{rad}/\sqrt{\textrm{Hz}}$ at frequencies between $0.1\,\textrm{mHz}$ and $1\,\textrm{Hz}$. In this paper, we present phase readout systems, so-called phasemeters, that are able to achieve such precisions and we discuss various means that have been employed to reduce noise in the analogue circuit domain and during digitisation. We also discuss the influence of some non-linear noise sources in the analogue domain of such phasemeters. And finally, we present the performance that was achieved during testing of the elegant breadboard model of the LISA phasemeter, that was developed in the scope of an ESA technology development activity.Comment: submitted to Review of Scientific Instruments on April 30th 201

Design and construction of an optical test bed for LISA imaging systems and tilt-to-length coupling

The laser interferometer space antenna (LISA) is a future space-based interferometric gravitational-wave detector consisting of three spacecraft in a triangular configuration. The interferometric measurements of path length changes between satellites will be performed on optical benches in the satellites. Angular misalignments of the interfering beams couple into the length measurement and represent a significant noise source. Imaging systems will be used to reduce this tilt-to-length coupling. We designed and constructed an optical test bed to experimentally investigate tilt-to-length coupling. It consists of two separate structures, a minimal optical bench and a telescope simulator. The minimal optical bench comprises the science interferometer where the local laser is interfered with light from a remote spacecraft. In our experiment, a simulated version of this received beam is generated on the telescope simulator. The telescope simulator provides a tilting beam, a reference interferometer and an additional static beam as a phase reference. The tilting beam can either be a flat-top beam or a Gaussian beam. We avoid tilt-to-length coupling in the reference interferometer by using a small photo diode placed at an image of the beam rotation point. We show that the test bed is operational with an initial measurement of tilt-to-length coupling without imaging systems. Furthermore, we show the design of two different imaging systems whose performance will be investigated in future experiments

Laser development for LISA

The two most promising configurations for the LISA laser are a stand-alone diode-pumped nonplanar ring oscillator (NPRO) or a fibre amplifier seeded by a low-power NPRO. The stand-alone laser was stabilized in frequency to a ULE cavity and in power to an electronic reference. For the first time the LISA requirement of relative power noise below 2 × 10-4/Hz1/2 was fulfilled in the whole frequency range from 0.1 mHz to 1 Hz. The LISA goal of frequency noise below 30 Hz/Hz1/2 was achieved for frequencies above 3 mHz. As a first step in the characterization of an oscillator-amplifier system, the excess frequency noise of an ytterbium-doped fibre amplifier was measured. For frequencies between 0.1 mHz and 1 Hz the excess noise was measured to be below 0.1 Hz/Hz1/2, which is significantly below the free-running frequency noise of NPROs

Lasers for LISA: Overview and phase characteristics

We have investigated two alternative laser systems for the Laser Interferometer Space Antenna (LISA). One consisted of the laser of LISA's technology precursor LISA Pathfinder and a fiber amplifier originally designed for a laser communication terminal onboard TerraSar-X. The other consisted of a commercial fiber distributed feedback (DFB) laser seeding a fiber amplifier. We have shown that the TerraSar-X amplifier can emit more than 1W without the onset of stimulated Brillouin scattering as required by LISA. We have measured power noise and frequency noise of the LISA Pathfinder laser (LPL) and the fiber laser. The fiber laser shows comparable or even lower power noise than the LPL. LISA will use electro-optical modulators (EOMs) between seed laser and amplifier for clock noise comparison between spacecraft. This scheme requires that the excess noise added by the amplifiers be negligible. We have investigated the phase characteristics of two fiber amplifiers emitting 1 W and found them to be compatible with the LISA requirement on amplifier differential phase noise.DLR/50 OQ 0501DLR/50 OQ 060

Fiber modulators and fiber amplifiers for LISA

We present the sideband phase characteristics of a fiber-coupled integrated electro-optical modulator (EOM) at a modulation frequency of 2 GHz for Fourier frequencies from 0.1 mHz to 1 Hz. The upper phase noise limit was almost an order of magnitude better than required for LISA. The EOM's phase dependencies on temperature and transmitted optical power were measured and found to be uncritical. Additionally we have investigated three optical amplifiers emitting 1 W. Their differential phase noise and optical pathlength noise as one contribution to differential phase noise were measured. The measured differential phase noise was within the requirement. The dependencies of differential phase noise on pump power were measured and requirements for the operation of the amplifier on the LISA satellite derived.DLR/50 OQ 0601DFG/EXC/QUES