  Mechanism of transient population inversion in graphene with multi-splitted
(interdigitated) top-gate and grounded back gate is suggested and examined for
the mid-infrared (mid-IR) spectral region. Efficient stimulated emission after
fast lateral spreading of carriers due to drift-diffusion processes is found
for the case of a slow electron-hole recombination in the passive region. We
show that with the large gate-to-graphene distance the drift process always
precedes the diffusion process, due to the ineffective screening of the inplane
electric field by the gates. Conditions for lasing with a gain above 100
cm$^{-1}$ are found for cases of single- and multi-layer graphene placed in the
waveguide formed by the top and back gates. Both the waveguide losses and
temperature effects are analyzed.
