Performance Testing of an Optical Truss Interferometer for the LISA Telescope

LISA is a space-based gravitational wave observatory aimed at detecting the gravitational waves emitted from astronomical sources in the frequency band of 0.1 mHz to 1 Hz. The observatory is comprised 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 optical system 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 lies directly in the path of the long-baseline interferometer, its structure must be dimensionally stable at the pm/√Hz level at mHz frequencies. The structural stability of the LISA telescope can be measured and verified with a compact optical truss interferometer (OTI), consisting of three Fabry-Perot cavities mounted along the telescope to monitor structural displacements over time. All three cavities are operated with a common laser source, which can be done by equipping each cavity with an acousto-optical modulator to shift the nominal laser frequency as well as an electro-optical modulator to modulate the laser phase for frequency locking. In such an optomechanical system, small displacements in each cavity’s length entail proportional 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, fabrication, and preliminary results in the testing of first-generation prototype cavities.