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Future GRACE-like geodesy missions could benefit from adopting accelerometer technology akin to that of the LISA Pathfinder, which employed laser interferometric readout at the sub-picometer level in addition to the conventional capacitive sensing, which is at best at the level of 100 pm. Improving accelerometer performance holds great potential to enhance the scientific output of forthcoming missions, carrying invaluable implications for research in climate, water resource management, and disaster risk reduction. To reach sub-picometer displacement sensing precision in the millihertz range, laser interferometers rely on suppression of laser-frequency noise by several orders of magnitude. Many optical frequency stabilization methods are available with varying levels of complexity, size, and performance. In this paper, we describe the performance of a Mach–Zehnder interferometer based on a compact monolithic optic. The setup consists of a commercial fiber injector, a custom-designed pentaprism used to split and recombine the laser beam, and two photoreceivers placed at the complementary output ports of the interferometer. The structural stability of the prism is transferred to the laser frequency via amplification, integration, and feedback of the balanced-detection signal, achieving a fractional frequency instability better than 6 parts in 10 13 , corresponding to an interferometer pathlength stability better than 1 pm / Hz . The prism was designed to host a second interferometer to interrogate the position of a test mass. This optical scheme has been dubbed "single-element dual-interferometer" or SEDI. [ABSTRACT FROM AUTHOR]

The precise tracking of distance variations between two satellites in low Earth orbit can provide key data for the understanding of the Earth's system, specifically on seasonal and sub-seasonal water cycles and their impact on water levels. Measured distance variations, caused by local variations in gravitational field, serve as inputs to complex gravity models with which the movement of water on the globe can be identified. Satellite missions GOCE (2009–2013) and GRACE (2002–2017) delivered a significant improvement to our understanding of spatial and temporal gravity variations. Since 2018, GRACE Follow-On has been providing data continuity and features for the first time through the use of a laser interferometer as the technology demonstrator, in addition to a microwave ranging system as the main instrument. The laser interferometer provides an orders-of-magnitude lower measurement noise, and thereby could enable a significant improvement in the measurement of geoids if combined with suitable improvements in auxiliary instrumentation and Earth system modelling. In order to exploit the improved ranging performance, the ESA is investigating the design of a 'Next Generation Gravity Mission', consisting of two pairs of satellites with laser interferometers, improved accelerometers and improved platform performance. In this paper, we present the current design of the laser interferometer developed by us, the development status of the individual instrument units and the options available. [ABSTRACT FROM AUTHOR]

Intersatellite laser interferometers feature quadrant photoreceivers to produce electrical signals from the interfered optical beams. In the particular case of Laser Interferometer Space Antenna, the expected optical ac beat note has an amplitude of the order of nanowatts. This requires photoreceivers with an input current noise density of a few ${\mathrm {pA}} \cdot {\mathrm {Hz}}^{-1/2}$ in each channel up to 25 MHz. In addition, 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 ${\mathrm {pA}} \cdot {\mathrm {Hz}}^{-1/2}$ at 25 MHz, a 3-dB bandwidth of 37 MHz, and a total power consumption of 178 mW. [ABSTRACT FROM AUTHOR]

We report on the progress in the realization of an electronic / optical simulator for space based, long arm interferometry and its application to the eLISA mission. The goal of this work is to generate realistic optics and electronics signals, especially simulating realistic propagation delays. The first measurements to characterize the simulator are also presented. With the present configuration, noise reduction factors of 5×10 for optical beat notes and 10 for RF beat notes have been achieved using the Time Delay Interferometry algorithm. The principle of the experiment has been validated and further work is ongoing to identify the residual noise sources and optimize the apparatus. [ABSTRACT FROM AUTHOR]

We report on the design, implementation, and characterization of fully digital control loops for laser frequency stabilization, differential phase-locking, and performance optimization of the optical metrology system on-board the LISA Pathfinder space mission. The optical metrology system consists of a laser with modulator, four Mach–Zehnder interferometers, a phase-meter and a digital processing unit for data analysis. The digital loop design has the advantage of easy and flexible controller implementation and loop calibration, automated and flexible locking and resetting, and improved performance over analog circuitry. Using the practical ability of our system to modulate the laser frequency allows us to accurately determine the open-loop transfer function and other system properties. Various noise sources and their impact on system performance are investigated in detail. [ABSTRACT FROM PUBLISHER]

Tracking moving masses in several degrees of freedom with high precision and large dynamic range is a central aspect in many current and future gravitational physics experiments. Laser interferometers have been established as one of the tools of choice for such measurement schemes. Using sinusoidal phase modulation homodyne interferometry allows a drastic reduction of the complexity of the optical setup, a key limitation of multi-channel interferometry. By shifting the complexity of the setup to the signal processing stage, these methods enable devices with a size and weight not feasible using conventional techniques. In this paper we present the design of a novel sensor topology based on deep frequency modulation interferometry: the self-referenced single-element dual-interferometer (SEDI) inertial sensor, which takes simplification one step further by accommodating two interferometers in one optic. Using a combination of computer models and analytical methods we show that an inertial sensor with sub-picometer precision for frequencies above 10 mHz, in a package of a few cubic inches, seems feasible with our approach. Moreover we show that by combining two of these devices it is possible to reach sub-picometer precision down to 2 mHz. In combination with the given compactness, this makes the SEDI sensor a promising approach for applications in high precision inertial sensing for both next-generation space-based gravity missions employing drag-free control, and ground-based experiments employing inertial isolation systems with optical readout. [ABSTRACT FROM AUTHOR]