EOM sideband phase characteristics for the spaceborne gravitational wave detector LISA

The Laser Interferometer Space Antenna (LISA) is a joint ESA/NASA mission proposed to observe gravitational waves. One important noise source in the LISA phase measurement will be on-board reference oscillators. An inter-spacecraft clock tone transfer chain will be necessary to remove this non-negligible phase noise in post processing. One of the primary components of this chain are electro-optic modulators (EOMs). At modulation frequencies of 2 GHz, we characterise the excess phase noise of a fibre-coupled integrated EOM in the LISA measurement band (0.1 mHz to 1 Hz). The upper phase noise limit was found to be almost an order of magnitude better than required by the LISA mission. In addition, the EOM's phase dependence on temperature and optical power was determined. The measured coefficients are within a few milliradians per kelvin and per watt respectively and thereby negligible with the expected on-board temperature and laser power stability.DLR/50 OQ 0601DFG/EXC/QUES

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

In-Orbit Performance of the GRACE Follow-on Laser Ranging Interferometer

The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degrees of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wave front sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of 1 nm/Hz at Fourier frequencies above 100 mHz. © 2019 authors. Published by the American Physical Society

AVONET: morphological, ecological and geographical data for all birds

Functional traits offer a rich quantitative framework for developing and testing theories in evolutionary biology, ecology and ecosystem science. However, the potential of functional traits to drive theoretical advances and refine models of global change can only be fully realised when species‐level information is complete. Here we present the AVONET dataset containing comprehensive functional trait data for all birds, including six ecological variables, 11 continuous morphological traits, and information on range size and location. Raw morphological measurements are presented from 90,020 individuals of 11,009 extant bird species sampled from 181 countries. These data are also summarised as species averages in three taxonomic formats, allowing integration with a global phylogeny, geographical range maps, IUCN Red List data and the eBird citizen science database. The AVONET dataset provides the most detailed picture of continuous trait variation for any major radiation of organisms, offering a global template for testing hypotheses and exploring the evolutionary origins, structure and functioning of biodiversity

High-bandwidth laser frequency stabilization to a fiber-optic delay line

Stabilization of laser frequency to interferometers with a large time delay in one arm is of significant interest to space-based gravitational wave detectors such as the Laser Interferometer Space Antenna. A recently proposed technique allows a control bandwidth larger than the inverse delay time to be achieved. We present experimental results demonstrating laser frequency stabilization to an optical fiber delay line. A control bandwidth approximately 50 times the inverse delay time is demonstrated.This work was supported by the Australian Research Council

A low-power, low-noise 37-MHz photoreceiver for intersatellite laser interferometers using discrete heterojunction bipolar transistors

Intersatellite laser interferometers feature quadrant photoreceivers to produce electrical signals from the interfered optical beams. In the particular case of LISA, the expected optical AC beat note has an amplitude of the order of nW. This requires photoreceivers with an input current noise density of a few pA Hz–1/2 in each channel up to 25MHz. Additionally, the significant number of photoreceivers in a single spacecraft imposes tight constraints on the power consumption per device. We present the experimental characterization of a quadrant photoreceiver based on discrete heterojunction bipolar transistors and an off-the-shelf 0.5 mm diameter InGaAs quadrant photodiode, showing an input current noise density of 1:9 pA Hz–1/2 at 25MHz, a 3 dB bandwidth of 37MHz and a total power consumption of 178mW. CCB

Photothermal effects in passive fiber Bragg grating resonators

Photothermal effects in passive Fabry–Perot resonators are caused by the conversion of circulating optical energy into heat as a result of absorption. This results in thermal change in the resonator’s optical path length, the round-trip phase, and hence the resonance condition. We describe a simplified dynamic numerical model for photothermal effects in passive fiber Bragg grating resonators and present results of their experimental observation.This research was supported by the Australian Research Council under the auspices of the Australian Consortium for Interferometric Gravitational Astronomy, with partial assistance from the Centre for Ultrahigh Bandwidth Devices for Optical Systems

Highspeed multiplexed heterodyne interferometry

Digitally enhanced heterodyne interferometry is a metrology technique that uses pseudo-random noise codes for modulating the phase of the laser light. Multiple interferometric signals from the same beam path can thereby be isolated based on their propagation delay, allowing one to use advantageous optical layouts in comparison to classic laser interferometers. We present here a high speed version of this technique for measuring multiple targets spatially separated by only a few centimetres. This allows measurements of multiplexed signals using free beams, making the technique attractive for several applications requiring compact optical set-ups like for example space-based interferometers. In an experiment using a modulation and sampling rate of 1.25 GHz we are able to demonstrate multiplexing between targets only separated by 36 cm and we achieve a displacement measurement noise floor of < 3 pm/ √ Hz at 10 Hz between them. We identify a limiting excess noise at low frequencies which is unique to this technique and is probably caused by the finite bandwidth in our measurement set-up. Utilising an active clock jitter correction scheme we are also able to reduce this noise in a null measurement configuration by one order of magnitude

Retroreflector for GRACE follow-on: Vertex vs point of minimal coupling

The GRACE Follow-On mission will monitor fluctuations in Earth’s geoid using, for the first time, a Laser Ranging Interferometer to measure intersatellite distance changes. We have investigated the coupling between spacecraft rotation and the intersatellite range measurement that is incurred due to manufacturing and assembly tolerances of the Triple Mirror Assembly (TMA), a precision retroreflector to ensure alignment between in- and outgoing laser beams. The three TMA mirror planes intersect in a virtual vertex to which satellite displacements are referenced. TMA manufacturing tolerances degrade this ideal vertex, however, a Point of Minimal Coupling (PMC) between spacecraft rotation and displacement exists. This paper presents the experimental location of the PMC under pitch and yaw rotations for a prototype TMA. Rotations are performed using a hexapod, while displacements are monitored with heterodyne laser interferometry to verify the PMC position. Additionally, the vertex of the three TMA mirror planes is measured using a Coordinate Measuring Machine and compared to the PMC position. In the pitch and yaw axes, the biggest deviation between TMA vertex and PMC was 50 ± 64 μm. Thus, within the measurement uncertainties, no difference between TMA vertex and PMC could be observed. This is a key piece of information for integration of the TMA into the spacecraft: It is sufficient to use the readily-available TMA vertex location to ensure minimal rotation-to-displacement coupling during the mission.This work was partly funded by the “Bundesministerium fur Bildung und Forschung” (BMBF, ¨ project number: 03F0654B), by the “Deutsche Forschungsgemeinschaft” (DFG) within the Cluster of Excellence QUEST (Centre for Quantum Engineering and Space-Time Research), and by the Australian Government’s Australian Space Research Program (ASRP)