  We study both experimentally and theoretically modulation of light in a
planar aligned deformed-helix ferroelectric liquid crystal (DHFLC) cell with
subwavelength helix pitch, which is also known as a short-pitch DHFLC. In our
experiments, azimuthal angle of the in-plane optical axis and electrically
controlled parts of the principal in-plane refractive indices were measured as
a function of voltage applied across the cell. Theoretical results giving the
effective optical tensor of a short-pitch DHFLC expressed in terms of the
smectic tilt angle and the refractive indices of FLC are used to fit the
experimental data. Optical anisotropy of the FLC material is found to be weakly
biaxial. For both the transmissive and reflective modes, the results of fitting
are applied to model phase and amplitude modulation of light in the DHFLC cell.
We demonstrate that, if the thickness of the DHFLC layer is about 50 $\mu$m,
the detrimental effect of field-induced rotation of the in-plane optical axes
on the characteristics of an axicon designed using the DHFLC spatial light
modulator in the reflective mode is negligible.
