  In this paper we propose and discuss coherent terahertz sources based on
charge density wave (plasmon) amplification in two dimensional graphene. The
coupling of the plasmons to interband electron-hole transitions in population
inverted graphene layers can lead to plasmon amplification through stimulated
emission. Plasmon gain values in graphene can be very large due to the small
group velocity of the plasmons and the strong confinement of the plasmon field
in the vicinity of the graphene layer. We present a transmission line model for
plasmon propagation in graphene that includes plasmon dissipation and plasmon
interband gain due to stimulated emission. Using this model, we discuss design
for terahertz plasmon oscillators and derive the threshold condition for
oscillation taking into account internal losses and also losses due to external
coupling. The large gain values available at terahertz frequencies in graphene
can lead to integrated oscillators that have dimensions in the 1-10 micron
range.
