  The limits of applicability of the Lang-Kobayashi (LK) model for a
semiconductor laser with optical feedback are analyzed. The model equations,
equipped with realistic values of the parameters, are investigated below
solitary laser threshold where Low Frequency Fluctuations (LFF) are usually
observed. The numerical findings are compared with experimental data obtained
for the selected polarization mode from a Vertical Cavity Surface Laser (VCSEL)
subject to polarization selective external feedback. The comparison reveals the
bounds within which the dynamics of the LK can be considered as realistic. In
particular, it clearly demonstrates that the deterministic LK, for realistic
values of the linewidth enhancement factor $\alpha$, reproduces the LFF only as
a transient dynamics towards one of the stationary modes with maximal gain. A
reasonable reproduction of real data from VCSEL can be obtained only by
considering noisy LK or alternatively deterministic LK for extremely high
$\alpha$-values.
