Optical Design of the LISA Interferometric Metrology System

Within the context of the LISA Mission Formulation Study, we have developed a detailed concept for the optical layout of the LISA payload, which consists of two movable assemblies per spacecraft, each pointing to its respective remote spacecraft to form a constellation triangle of 5 million kilometer arm length. The movable assemblies comprise a Cassegrain telescope, an optical bench, and the gravity reference sensor with its free floating proof mass, which delimits the respective arm. Differential changes in the distances between the two proof masses of each arm, caused by the passage of a gravitational wave, are detected by a combination of heterodyne interferometry and differential wavefront sensing. The optical architecture is further characterized by a “strap‐down” approach with proof mass optical readout, as well as a “frequency swap” between transmitted and local reference beams. The nominal performance and sensitivity of the complete system is verified by extensive optical modeling.