  Coherent population trapping (CPT) is extensively studied for future vapor
cell clocks of high frequency stability. In the constructive polarization
modulation CPT scheme, a bichromatic laser field with polarization and phase
synchronously modulated is applied on an atomic medium. A high contrast CPT
signal is observed in this so-called double-modulation configuration, due to
the fact that the atomic population does not leak to the extreme Zeeman states,
and that the two CPT dark states, which are produced successively by the
alternate polarizations, add constructively. Here we experimentally investigate
CPT signal dynamics first in the usual configuration, a single circular
polarization. The double-modulation scheme is then addressed in both cases: one
pulse Rabi interaction and two pulses Ramsey interaction. The impact and the
optimization of the experimental parameters involved in the time sequence are
reviewed. We show that a simple sevenlevel model explains the experimental
observations. The double-modulation scheme yields a high contrast similar to
the one of other high contrast configurations like push-pull optical pumping or
crossed linear polarization scheme, with a setup allowing a higher compactness.
The constructive polarization modulation is attractive for atomic clock, atomic
magnetometer and high precision spectroscopy applications.
