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The LISA Pathfinder mission to space employs an optical metrology system(OMS) at its core to measure the distance and attitude between two freelyfloating test-masses to picometer and nanorad accuracy, respectively, withinthe measurement band of [1 mHz, 30 mHz]. The OMS is based upon an ultra-stableoptical bench with 4 heterodyne interferometers from which interference signalsare read-out and processed by a digital phase-meter. Laser frequency noise,power fluctuations and optical path-length variations are suppressed touncritical levels by dedicated control loops so that the measurementperformance approaches the sensor limit imposed by the phase-meter. The systemdesign is such that low frequency common mode noise which affects the read-outphase of all four interferometers is generally well suppressed by subtractionof a reference phase from the other interferometer signals. However, highfrequency noise directly affects measurement performance and its common moderejection depends strongly on the relative signal phases. We discuss how thedata from recent test campaigns point towards high frequency phase noise as alikely performance limiting factor which explains some important performancefeatures.

Fundamental Performance Determining Factors of the Ultrahigh-Precision Space-Borne Optical Metrology System for the LISA Pathfinder mission