  Evolution of modulus and phase of complex order parameter associated with
field induced phase transition (FIPT) of the vacuum state interacting with time
dependent Sauter pulse is studied to analyse different evolution stages of the
order parameter e.g., quasi electron positron plasma (QEPP), transient, and
residual electron-positron plasma (REPP) stages. By revisiting FIPT in presence
of single-sheeted and multi-sheeted pulses, we attribute the transient stage to
the nonlinear coupling in the differential equations governing the dynamics of
the phase and the modulus of the order parameter. The appearance of the rapid
oscillations in the modulus is shown to be associated with the abrupt change in
the phase of the order parameter in the transient stage. FIPT is also studied
for multi-sheeted Sauter pulse with linear and quadratic frequency chirp. QEEP
stage is found to show complex dynamical behaviour with fast and irregular
oscillations due to the frequency chirp. The formation of pre-transient region
due to the quadratic frequency chirping is observed in the accelerating part of
the QEPP stage before the electric field attains the maximum value. As the
quadratic chirp is increased the pre-transient and transient stages move closer
to the electric field maximum which leads to a decrease in temporal separation
between the two stages. The early appearance of the transient stage and hence
of the following REPP stage results in the enhancement of pair production rate.
