First Experimental Demonstration of Noise Cancellation for LISA Interferometry

We report on the first experimental demonstration of post-processed, time-delay interferometry with USO noise cancellation for the Laser Interferometer Space Antenna. The demonstration used simplified optical benches constructed from ultra-low expansion glass and the LISA phasemeter prototype. Each bench was incorporated into an end station, consisting of two NPRO lasers, and three phase measurements (two inter-bench measurements and one local measurement). The interferometer has been designed to provide signals representative of LISA interferometry. The laser frequency noise-free combination, alpha, was formed by combining the six phase measurements from the two end stations, each of which contained a clock which was used as the master reference for that bench's three phase measurements. The phase measurements were interpolated in post-processing to correct for errors in sampling times caused by relative drift of the end stations' clocks. After interpolation, the laser frequency noise in alpha was reduced by almost 4 orders of magnitude (relative to the noise level in the individual phase measurements) down to the performance limit of the interferometer. As expected, the independent clocks' phase fluctuations also caused a large increase in the measured noise. This clock phase noise was removed by phase modulating the clock frequencies onto laser beams exchanged between spacecraft in the manner planned for LISA. The clock noise was suppressed by almost 5 orders of magnitude. Notably, both the laser frequency noise and clock noise were suppressed down to our interferometer displacement noise level of 20 pm/√Hz at 0.1 Hz. Although this noise meets the LISA minimum mission requirement, work is ongoing to reduce the pathlength noise low enough to demonstrate laser frequency noise and clock noise suppression to below the error allocation of approximately 13 pm/√Hz. The current results validate both the expected performance of TDI as well as the performance of the LISA phasemeter prototype.