Construction techniques and commissioning of the Three-Backlink experiment for the LISA mission

Abstract Designed to detect gravitational waves in the lower-frequency band, the space mission Laser Interferometer Space Antenna (LISA) will open a new window to astronomy after its launch in the 2030s. Each LISA spacecraft houses two optical benches that require the exchange of a phase reference between them via an optical connection, called a Backlink. Here we present the construction and commissioning of an ultra-stable quasi-monolithic optical testbed to investigate different Backlink implementations: a direct fiber, a frequency-separated fiber, and a free-beam link, compared in the Three-Backlink experiment. Dedicated alignment techniques crucial for the construction of these optical benches are presented together with the development of a high-precision beam alignment and measurement tool—a Calibrated Quadrant Photodiode Singleton. In this paper we present the first part of a two-paper study, by showing an upper limit for the performance of all three investigated Backlink schemes. With initial experiments and commissioning, we reached a noise level of an 15 pm Hz − 1 -equivalent level within the LISA band, spanning 0.1 mHz to 1 Hz. The second part of the study deepens our understanding on the testbed noise, including technical and environmental noise and presents the successful noise suppression to validate the stability below 3 pm Hz − 1 . Our measurements were able to verify the successful construction and commissioning of this very complex interferometer as an interferometric laboratory testbed for LISA. We find no limitations due to the construction on the observed performance levels. Our results can support the construction of high-precision metrology testbeds for space-based laser interferometry for future gravitational wave or geodesy missions.