  We propose a superradiant laser based on two-photon Raman transition of
caesium-133 atoms which collectively emit photons on an ultra narrow transition
into the mode of a low Q resonator known as optical bad-cavity regime. The
spin-spin correlation which characterizes the collective effect is
demonstrated. We theoretically predict that the optical radiation has an
extremely narrow linewidth in the 98 (1) *10-2 mHz range, smaller than the
transition itself due to collective effects, and a power level of 7 (1)*10-10 W
is possible, which can provide a possible new way to realize an optical clock
with a millihertz linewidth.
