  GRB 090902B, detected by Fermi Large Array Telescope (Fermi/LAT), shows
extend high-energy emission (>100 MeV) up to 10^3 s after the burst, which
decays with time in a power-law as t^{-1.5}. It has been also observed by
several follow-up low-energy instruments, including an early optical detection
around 5000 s after the burst. The optical emission at early time decays faster
than t^{-1.6}, which has been suspected to originate from the reverse shock. We
here explore the models that can possibly explain the the broadband afterglow
emission of GRB 090902B. We find that the reverse shock model for the early
optical emission would overpredict the radio afterglow flux that is
inconsistent with observations. A partially radiative blast wave model, which
though is able to produce a sufficiently steep decay slope, can not explain the
broadband data of GRB 090902B. The two-component jet model, which consists of a
narrow and bright jet component in the core and a surrounding wider and less
energetic jet component, is shown to be able to explain the broadband afterglow
data, including the LAT high-energy data after ~50 s and low-energy (radio,
optical and X-ray) afterglow data. The early-time high-energy emission detected
by LAT before ~50 s is likely due to internal origin as that of the sub-MeV
emission. The highest energy (33 GeV) photon of GRB090902B detected at 80 s can
be marginally accommodated within the forward shock emission under the
optimistic condition that electrons are accelerated by the Bohm diffusive
shock.
