  High precision metrology systems based on heterodyne interferometry can
measure position and attitude of objects to accuracies of picometer and
nanorad, respectively. A frequently found feature of the general system design
is the subtraction of a reference phase from the phase of the position
interferometer, which suppresses low frequency common mode amplitude and phase
fluctuations occurring in volatile optical path sections shared by both, the
position and reference interferometer. Spectral components of the noise at
frequencies around or higher than the heterodyne frequency, however, are
generally transmitted into the measurement band and may limit the measurement
accuracy. Detailed analytical calculations complemented with Monte Carlo
simulations show that high frequency noise components may also be entirely
suppressed, depending on the relative difference of measurement and reference
phase, which may be exploited by corresponding design provisions. Whilst these
results are applicable to any heterodyne interferometer with certain design
characteristics, specific calculations and related discussions are given for
the example of the optical metrology system of the LISA Pathfinder mission to
space.
