Testing of a Prototype Optical Truss Interferometer for the LISA Telescope

The LISA telescopes are bidirectional components used to send and receive laser light between the three LISA spacecraft, each separated by 2.5 million km in a triangular constellation, to form long baseline interferometers between free flying test masses aboard each spacecraft. These telescopes lie directly in the optical path of the interferometers, and so their optical path length stability must exhibit a pm/√Hz noise level at mHz frequencies to allow for the detection of incoming gravitational waves. One method to monitor and verify the displacement noise along the optical path of the telescope is the implementation of an optical truss interferometer (OTI), which consists of three Fabry-Perot cavities mounted lengthwise around the telescope structure. We have developed prototype OTI cavities formed by compact input and return stages which integrate fiber injection, mode matching, and cavity mirrors into modular units housed in Zerodur and mounted to a test structure. The path length stability is monitored via Pound-Drever-Hall (PDH) frequency locking such that the laser frequency locked to each optical cavity will change in proportion to displacements in the test structure. The laser frequencies are measured as beat signals with respect to a reference frequency generated by a master laser locked to a stable reference cavity. We have tested our prototype OTI cavities to verify a pm/√Hz displacement sensitivity, and we will present on our experimental setup and initial results.