Photodetection in intersatellite laser interferometers

Abstract

This thesis describes the design and characterization of photoreceivers for present and future intersatellite laser interferometers. The photoreceiver performs the optoelectronic conversion of the interfered signals and, as the first stage of the readout chain, is critical for the interferometer sensitivity. The research focused on photoreceivers for the Laser Interferometer Space Antenna (LISA), a future space-based gravitational wave detector; and the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, whose aim is to measure time variations of the Earth’s gravity field. Intersatellite interferometry requires a heterodyne scheme to cope with Doppler-induced frequency changes due to relative spacecraft motion. Therefore, the photoreceivers need to operate at frequencies up to 25 MHz. Additionally, beam divergence and a large separation between spacecraft lead to low power received signals. This requires photoreceivers with an input current noise of the order of pA/sqrt(Hz) in the heterodyne frequency band to measure the weak optical signals. The thermally induced noise of the photoreceiver must also comply with the design sensitivity of the interferometer. Another aspect to consider in LISA is the significant number of photoreceivers in a single spacecraft, which imposes tight constraints on the power consumption per device. The thesis covers the basics of intersatellite laser interferometry and the derivation of photoreceiver requirements. It also contains an in-depth study of photoreceiver solutions. Quadrant PIN photodiodes based on indium gallium arsenide (InGaAs) and bipolar amplifiers were identified as optimal for intersatellite interferometry. The thesis also describes photoreceiver characterization techniques and provides performance results of several photoreceiver implementations. A new functionality upgrade for the IfoCAD simulation software was developed to use experimentally measured spatial responses of photodiodes in simulated environments. The characterization of the GRACE-FO photoreceiver flight models, developed at the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Berlin in collaboration with the Albert Einstein Institute (AEI), showed that the requirements for noise, bandwidth, thermal stability and radiation hardness were fulfilled, allowing the use of the units in the spacecraft. A new photoreceiver topology featuring heterojunction bipolar transistors and an off-the-shelf 0.5 mm diameter quadrant photodiode was implemented. The experimental characterization of a prototype was found to have a 3 dB bandwidth of 37 MHz and an input current noise of 1.9 pA/sqrt(Hz) at 25 MHz, 0.2 pA/sqrt(Hz) less than in previous photoreceiver designs. The reduction of photoreceiver electronic noise translates to a 0.13 pm/sqrt(Hz) decrease of the photoreceiver contribution to the LISA readout displacement noise. The measured power consumption of the full photoreceiver is 178 mW, thanks to the low power, transistor-based input stage and the OpAmps used. This constitutes an improvement compared to standard OpAmp-only photoreceiver designs, not optimized for power consumption. Due to the described performance, this new type of photoreceiver is a competitive candidate for future intersatellite interferometry missions.