  A digital signal acquisition system for an Apertureless SNOM (ASNOM) based on
a digital signal processing (DSP) card is presented. An electromagnetic wave
scattered by an AFM-like tip is initially detected by an optical homodyning in
a Michelson interferometer, with a homodyne phase modulation caused by a
periodic travel of the reference arm mirror. Utilizing a non-linear dependency
of the tip scattering amplitude on a tip-sample separation distance, the
non-fundamental components of a tip oscillation frequency are recovered in
optical signal within each tip oscillation period. A detected value,
corresponding mostly to the near-field modes of a tip-sample electromagnetic
interaction, is then averaged over all reference phases. A modular structure of
the data acquisition and signal recovery program allows modify easily the modes
of operation and the recovery algorithms.
