To confirm that the concept of interferometry with free floating test masses works, ESA launched the LISA Pathfinder in 2015. This single spacecraft hosted two test masses at a distance of 40 cm to test LISA technology. Won’t there be too much noise from the continuously moving test masses? Will the noise of LISA’s detectors be low enough to pick up the laser light after its millions of kilometres long journey? After two years of testing through LISA Pathfinder, the concept of LISA technology appears to be more than ready to meet the mission’s requirements.

The quadrant photo receivers (QPRs), detect the infrared laser beams between the three LISA spacecraft on which the passing gravitational waves are imprinted. One could say that they are the eyes of LISA. The laser beams are emitted with a power of 1 Watt, but over their course of 2.5 million kilometres, they diverge substantially and by the time they arrive at the opposite spacecraft, a mere 250 picoWatt is all that remains within the reach of the receiving LISA telescope. When mixed with the local laser, the relative amplitude of the interference signal is less than 1%. Furthermore, the interference is heterodyne, meaning that it changes with a frequency in the order of a few MHz. One of the challenges in developing the QPRs is to beat down the noise to a sufficiently low level that this weak and rapidly changing interference signal can be measured with enough accuracy.

24 QPRs are needed in each LISA spacecraft, not only to measure the distance to the other two spacecraft with an accuracy of 1 picometer, but also to measure other distances within the spacecraft to correct the long-distance measurement and attain that picometer accuracy. Also, every QPR is redundant to have a backup in the unlikely event that a QPR is lost. This is a common practice in space instrumentation, which cannot be repaired after launch. Each QPR consists of a millimetre-large quadrant photo diode, nearby readout electronics and a firm housing for the former two to keep the photodiode aligned with the interfering laser beams at sub-micron accuracy. The development of the photodiode is carried out by NWO institutes Nikhef and SRON, and the Dutch companies BRIGHT Photonics and SMART Photonics. SRON and Nikhef have been granted funding in 2019 from the National Science Agenda and in 2021 from an NWO-M investment. The housing, including a stage that allows finetuning of the position of the photodiode with sub-micrometre accuracy, is being designed by SRON and Nikhef. The sensitive readout electronics, crucial for the required low noise level, is being designed in Belgium, with support from the Albert Einstein Institute in Hannover (Germany).

TNO is developing the point-ahead angle mechanism (PAAM). This mechanism adjusts the pointing of the laser beam on each optical bench to where the receiving spacecraft will be 8 seconds later, when the laser beam will have traversed the 2.5 million kilometres distance. This inter-spacecraft distance varies slightly within a ten thousand kilometres margin. Also, the angle between two spacecraft, as seen from the third spacecraft, varies continuously. Therefore, the laser beam pointing often needs to be adjusted slightly but accurately. The PAAM is designed to be able to adjust the pointing with an accuracy of milliarcseconds, equivalent to a dime on the Eiffel tower as seen from The Netherlands. TNO is developing the PAAM since more than 10 years and has already built several prototypes.

A central element in LISA is the optical bench, of which there are six – two on each spacecraft. This disk-shaped bench, 30 cm in diameter and made out of Zerodur glass, carries tens of optical components to steer and measure the laser beams including the QPRs. Some of those components, called ‘mechanisms’, will be adjustable, either to repoint the laser beam, change the beam width or switch between different optical fibres. Electronics are needed to read out positions of the mechanisms and change them. These are called ‘control electronics’. For each of the three mechanisms, dedicated control electronics will be developed on separate boards. These will all be brought together in the so-called mechanisms control unit (MCU). The three mechanisms are envisaged to be provided by Belgium, Czechia and the Netherlands. One mechanism is the point-ahead angle mechanism (PAAM) being developed by TNO in the Netherlands, as described above. The control electronics for the PAAM are in development by SRON. SRON is also candidate to assume responsibility for the MCU. This entails the definition of the requirements for the three control electronics boards so that they fit in the MCU and the interface to the rest of the LISA system, integration of all boards in one housing, and development of an electronics board for input/output of signals to the rest of the LISA system and the distribution of electrical voltages to the various boards.