  Graphene is an interesting debated topic between scientists because of its
unique properties such as tunable conductivity. Graphene conductivity can be
varied by either electrostatic or magnetostatic gating or via chemical doping,
which leads to the design of various photonic and electronic devices. Among
various graphene-based structures, plasmonic graphene couplers have attracted
the attention of many researchers because of their fascinating applications in
the THz frequencies. There are four main types of graphene couplers proposed in
the literature, which are: 1- directional, 2- non-reciprocal, 3- dielectric,
and 4- nano-ribbon couplers. This paper aims to study and investigate the
various types of graphene-based couplers published in the literature.
