  The next generation of e+/e- colliders will require a very intense flux of
gamma rays to allow high current polarized positrons to be produced. This can
be achieved by converting polarized high energy photons in polarized pairs into
a target. In that context, an optical system consisting of a laser and a
four-mirror passive Fabry-Perot cavity has recently been installed at the
Accelerator Test Facility (ATF) at KEK to produce a high flux of polarized
gamma rays by inverse Compton scattering. In this contribution, we describe the
experimental system and present preliminary results. An ultra-stable
four-mirror non planar geometry has been implemented to ensure the polarization
of the gamma rays produced. A fiber amplifier is used to inject about 10W in
the high finesse cavity with a gain of 1000. A digital feedback system is used
to keep the cavity at the length required for the optimal power enhancement.
Preliminary measurements show that a flux of about $4\times10^6 \gamma$/s with
an average energy of about 24 MeV was generated. Several upgrades currently in
progress are also described.
