  We present the analytical method enables one to compute the optical functions
i.e., reflectivity, transmission and absorption including the excitonic effects
for a semiconductor crystal exposed to a uniform electric field for energy
region above the gap, for external field suitable to appearance of
Franz-Keldysh (FK) oscillations. Our approach intrinsically takes into account
the coherence between the carriers and the electromagnetic field. We
quantitatively describe the amplitudes and periodicity of FK modulations and
the influence of Rydberg excitons on FK effect is also taking into account. Our
analytical findings are illustrated numerically for $P$ excitons in Cu$_2$O
crystal.
