Design and Fabrication of an Optical Truss Interferometer for the LISA Telescope

The LISA telescope is a bidirectional component that is used to expand a laser beam that is transmitted to the far spacecraft and compress a large incoming beam that is received to a diameter of a few mm at the optical bench. Since the telescope is directly in the path of the LISA long-arm interferometer, it must have a very high structural stability in the pm/$\surd $Hz level at mHz frequencies. One way to measure the stability of the LISA telescope structure is by using compact optical truss interferometers (OTI). The OTI setup consists of three Fabry-Perot cavities mounted around the mirrors of the telescope to monitor structural distortions over time, which are operated with a common laser source. Each cavity is to be equipped with acousto-optic and electro-optical modulators to shift the nominal laser frequency for each cavity as well as to modulate the laser phase for Pound-Drever-Hall locking. Changes in each cavity's length will translate to changes in their corresponding laser frequency, which can be measured against a reference laser that is locked to an external ultra-stable cavity. We have created a design and initiated the fabrication of prototypes of the OTI and will present on our design process and prototype testing.