  We investigated in detail the optical properties of several
Cu(1-delta)Mg(delta)GeO3 (with delta=0,0.01), and CuGe(1-x)B(x)O3 with B=Si
(x=0,0.007,0.05,0.1), and Al (x=0,0.01) single crystals, in the frequency range
20-32000 cm^-1. We report temperature dependent reflectivity and transmission
measurements, performed with polarized light in order to probe the anisotropy
of the crystals along the b and c axes, and optical conductivity spectra
obtained by Kramers-Kronig transformation or direct inversion of the Fresnel
formula. Special emphasis is given to the far-infrared phonon spectra. The
temperature dependence of the phonon parameters is presented and discussed in
relation to the soft mode issue in CuGeO3. For T<Tsp we could detect zone
boundary folded modes activated by the spin-Peierls phase transition. Following
the temperature dependence of these modes, which shows the second order
character of the phase transition, we were able to study the effect of doping
on Tsp. Moreover, in transmission experiments we detected a direct
singlet-triplet excitation at 44 cm^-1, across the magnetic gap, which is not
understandable on the basis of the magnetic excitation spectrum so far assumed
for CuGeO3. The optical activity of this excitation and its polarization
dependence confirm the existence of a second (optical) magnetic branch,
recently suggested on the basis of inelastic neutron scattering data. The
anisotropy in the magnetic exchange constants along the b axis, necessary for
the optical triplet mode to gain a finite intensity, and the strong effect of
Si substitution on the phonon spectra are discussed in relation to the
alternative space group P2(1)2(1)2(1), recently proposed for CuGeO3 in the high
temperature uniform phase.
