Benjamin Sheard

Intersatellite laser ranging instrument for the GRACE follow-on mission

The Gravity Recovery and Climate Experiment (GRACE) has demonstrated that low–low satellite-to-satellite tracking enables monitoring the time variations of the Earth’s gravity field on a global scale, in particular those caused by mass-transport within the hydrosphere. Due to the importance of long-term continued monitoring of the variations of the Earth’s gravitational field and the limited lifetime of GRACE, a follow-on mission is currently planned to be launched in 2017. In order to minimise risk and the time to launch, the follow-on mission will be basically a rebuild of GRACE with microwave ranging as the primary instrument for measuring changes of the intersatellite distance. Laser interferometry has been proposed as a method to achieve improved ranging precision for future GRACE-like missions and is therefore foreseen to be included as demonstrator experiment in the follow-on mission now under development. This paper presents the top-level architecture of an interferometric laser ranging system designed to demonstrate the technology which can also operate in parallel with the microwave ranging system of the GRACE follow-on mission.

Phasemeter core for intersatellite laser heterodyne interferometry: modelling, simulations and experiments

Intersatellite laser interferometry is a central component of future space-borne gravity instruments like Laser Interferometer Space Antenna (LISA), evolved LISA, NGO and future geodesy missions. The inherently small laser wavelength allows us to measure distance variations with extremely high precision by interfering a reference beam with a measurement beam. The readout of such interferometers is often based on tracking phasemeters, which are able to measure the phase of an incoming beatnote with high precision over a wide range of frequencies. The implementation of such phasemeters is based on all digital phase-locked loops (ADPLL), hosted in FPGAs. Here, we present a precise model of an ADPLL that allows us to design such a readout algorithm and we support our analysis by numerical performance measurements and experiments with analogue signals.

Laser interferometer for spaceborne mapping of the Earth's gravity field

The Gravity Recovery and Climate Experiment (GRACE) is one of the present missions to map the Earths gravity field. The aim of a GRACE follow-on mission is to map the gravitational field of the Earth with higher resolution over at least 6 years. This should lead to a deeper insight into geophysical processes of the Earths system. One suggested detector for this purpose consists of two identical spacecraft carrying drag-free test masses in a low Earth orbit at an altitude of the order of 300 km, following each other with a distance on the order of 50 to 100 km. Changes in the Earths gravity field will induce distance fluctuations between two test masses on separate spacecraft. These variations in the frequency range 1 to 100 mHz are to be monitored by a laser interferometer with nanometer precision. We present preliminary results of a heterodyne interferometer configuration using polarising optics, demonstrating the required phase sensitivity.

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.

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 resonators 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.

ACIGA's high optical power test facility

Advanced laser interferometer detectors utilizing more than 100 W of laser power and with ~106 W circulating laser power present many technological problems. The Australian Consortium for Interferometric Gravitational Astronomy (ACIGA) is developing a high power research facility in Gingin, north of Perth, Western Australia, which will test techniques for the next generation interferometers. In particular it will test thermal lensing compensation and control strategies for optical cavities in which optical spring effects and parametric instabilities may present major difficulties.

The design and construction of a prototype lateral-transfer retro-reflector for inter-satellite laser ranging

The Gravity Recovery and Climate Experiment (GRACE) mission, launched in 2002, is nearing an end, and a continuation mission (GRACE Followon) is on a fast-tracked development. GRACE Follow-on will include a laser ranging interferometer technology demonstrator, which will perform the first laser interferometric ranging measurement between separate spacecraft. This necessitates the development of lightweight precision optics that can operate in this demanding environment. In particular, this beam routing system, called the triple mirror assembly, for the GRACE Follow-on mission presents a significant manufacturing challenge. Here we report on the design and construction of a prototype triple mirror assembly for the GRACE Follow-on mission. Our constructed prototype has a co-alignment error between the incoming and

Status of the Australian Consortium for Interferometric Gravitational Astronomy

We report the status of research and development being undertaken by the members of the Australian Consortium for Interferometric Gravitational Astronomy.

Laser link acquisition demonstration for the GRACE Follow-On mission

We experimentally demonstrate an inter-satellite laser link acquisition scheme for GRACE Follow-On. In this strategy, dedicated acquisition sensors are not required-instead we use the photodetectors and signal processing hardware already required for science operation. To establish the laser link, a search over five degrees of freedom must be conducted (± 3 mrad in pitch/yaw for each laser beam, and ± 1 GHz for the frequency difference between the two lasers). This search is combined with a FFT-based peak detection algorithm run on each satellite to find the heterodyne beat note resulting when the two beams are interfered. We experimentally demonstrate the two stages of our acquisition strategy: a ± 3 mrad commissioning scan and a ± 300 μrad reacquisition scan. The commissioning scan enables each beam to be pointed at the other satellite to within 142 μrad of its best alignment point with a frequency difference between lasers of less than 20 MHz. Scanning over the 4 alignment degrees of freedom in our commissioning scan takes 214 seconds, and when combined with sweeping the laser frequency difference at a rate of 88 kHz/s, the entire commissioning sequence completes within 6.3 hours. The reacquisition sequence takes 7 seconds to complete, and optimizes the alignment between beams to allow a smooth transition to differential wavefront sensing-based auto-alignment.

Laser frequency noise suppression by arm-locking in LISA:progress towards a bench-top demonstration

Overcoming laser frequency noise is a significant technical challenge for achieving the design sensitivity of the Laser Interferometer Space Antenna (LISA) gravitational wave detector. Arm-locking is a recently proposed technique for suppressing frequency noise in LISA and can be used in addition to the established techniques of pre-stabilization and time-delay interferometry. Incorporation of arm-locking into LISA could provide many benefits, however experimental verification and testing is needed. We present the progress of an experimental test of arm-locking which uses 10 km of optical fibre to generate a large propagation time delay, analogous to the propagation delay in LISA.