  In current paper, distortions in digital demodulation schemes with harmonic
phase modulation for interferometric optical sensors are considered. In
particular, the influence of target signal variations on phase demodulation
errors is theoretically evaluated. An analytical expression, describing the
phase error magnitude dependence on first derivative and mean value of measured
signal and amplitude of the phase modulation in case of simple 4-point
demodulation algorithm is derived. After that, an approach for synthesizing
algorithms with suppressed sensitivity to target signal variations is
developed. Based on this approach, a novel 4+1 demodulation algorithm is
proposed. It is shown analytically that the demodulation error of new 4+1
algorithm is proportional to the second derivative of target signal, and
therefore, is typically several orders of magnitude smaller than in case of
4-point algorithm. The correspondence between analytical expressions and real
phase errors, induced by target signal variations is verified by means of
numeric simulation.
