  Using far-infrared spectroscopy we have studied the magnetic field and
temperature dependence of the spin gap modes in the chains of
Sr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_1$ and T$_2$ were found in the
center of the Brillouin zone at $\Delta_1=9.65$ meV and $\Delta_2=10.86$ meV in
zero magnetic field. The T$_1$ mode was excited when the electric field vector
${\bf E}$ of the light was polarized along the b axis (perpendicular to the
planes of chains and ladders) and T$_2$ was excited for ${\bf E}\parallel {\bf
a}$ (perpendicular to the chains and along the rungs). Up to the maximum
magnetic field of 18 T, applied along the chains, the electron $g$ factors of
these two modes were similar, $g_{1c}=2.049$ and $g_{2c}=2.044$. Full linewidth
at half maximum for both modes was 1 cm$^{-1}$ (0.12 meV) at 4K and increased
with $T$. The temperature dependence of mode energies and line intensities was
in agreement with the inelastic neutron scattering results from two groups
[Matsuda {\it et al.}, Phys. Rev. B {\bf 59}, 1060 (1999) and Regnault {\it et
al.}, Phys. Rev. B {\bf 59}, 1055 (1999)]. The T$_1$ mode has not been observed
by inelastic neutron scattering in the points of the $k$-space equivalent to
the center of the Brillouin zone. Our study indicates that the zone structure
model of magnetic excitations of Sr$_{14}$Cu$_{24}$O$_{41}$ must be modified to
include a triplet mode at 9.65 meV in the center of the magnetic Brillouin
zone.
