  We investigate coherent lattice dynamics in optimally doped
YBa$_2$Cu$_3$O$_{7-\delta}$ driven by ultrashort ($\sim$ 12 fs) near infrared
(NIR) and near ultraviolet (NUV) pulses. Transient reflectivity experiments,
performed at room temperature and under moderate ($<$0.1 mJ/cm$^2$) excitation
fluence, reveal $A_g$-symmetry phonon modes related to the O(2,3) bending in
the CuO$_2$ planes and to the apical O(4) stretching at frequencies between 10
and 15 THz, in addition to the previously reported Ba and Cu(2) vibrations at
3.5 and 4.5 THz. The relative coherent phonon amplitudes are in stark contrast
to the relative phonon intensities in the spontaneous Raman scattering spectrum
excited at the same wavelength. This contrast indicates mode-dependent
contributions of the Raman and non-Raman mechanisms to the coherent phonon
generation. We show that the particularly intense coherent Cu(2) phonon,
together with its initial phase, supports its generation predominately via a
displacive mechanism, possibly involving the charge transfer within the CuO$_2$
planes. The small amplitude of the coherent out-of-phase O(2,3) bending mode at
10 THz also suggests the involvement of non-Raman generation mechanism. The
generation of the other coherent phonons can in principle be explained within
the framework of Raman mechanism. When the pump light has the polarization
component perpendicular to the CuO$_2$ plane, the coherent O(4) mode at 15 THz
is strongly enhanced compared to the in-plane excitation, corresponding to the
large polarizability component associated with the hopping between the apical
and the chain oxygens, O(4) and O(1).
