Validation of optical pathlength stability in a LISA test-bench demonstrator

The Laser Interferometer Space Antenna (LISA) observatory is a future L3 mission of the European Space Agency (ESA) to detect gravitational waves, set to launch in 2035. The detector constellation will conduct interferometry to picometer stability over an unprecedented arm length of 2.5 million km. In this paper, we present the development and testing results for the Zerodur interferometer (ZIFO), an optical demonstrator built to validate critical technology for the test setup of the interferometric core of LISA. Optical path length stability measurements on the ZIFO demonstrate successful reduction of bench noise to maintain the 10 pm/$\sqrt{\text{Hz}}$ specification across the 1 mHz to 1 Hz frequency band. We also identify and characterize dominant noise sources from phasemeters and correlations of beam tilt into the path length that were observed during the test campaign.