Design, fabrication, and testing of an optical truss interferometer for the LISA telescope

Abstract

LISA is a space-based gravitational wave observatory aimed at detecting gravitational waves in the frequency range of 0.1 mHz to 0.1 Hz. The observatory is composed of three spacecraft, each separated by 2.5 million km in an equilateral triangle formation, trailing the Earth in a heliocentric orbit. One of the many crucial components to the mission is the LISA telescope, a bidirectional component used to expand an outgoing laser beam to the far spacecraft as well as compress a large incoming beam to a diameter of a few mm at the optical bench. Since the telescope is in the path of the long-baseline interferometer, its structure must be dimensionally stable at the pm/√Hz level at mHz frequencies. A way to measure the stability of the LISA telescope is with a compact optical truss interferometer (OTI), consisting of three Fabry-Perot cavities mounted along the telescope to monitor structural distortions over time. All three cavities are operated with a common laser source, and each cavity is equipped with an acousto-optic modulator to shift the nominal laser frequency as well as an electro-optical modulator to modulate the laser phase for Pound-Drever-Hall locking. Variations in each cavity’s length create variations in their corresponding laser frequency, which can be measured against a reference frequency that is locked to an external ultra-stable cavity. We will present the design and preliminary results in the fabrication and testing of first-generation OTI prototypes. © 2021 SPIE.