  We investigate the characteristics of temporal phase locking states observed
in the route to phase synchronization. It is found that before phase
synchronization there is a periodic phase synchronization state characterized
by periodic appearance of temporal phase locking state and that the state leads
to local negativeness in one of the vanishing Lyapunov exponents. By taking a
statistical measure, we present the evidences of the phenomenon in
unidirectionally and mutually coupled chaotic oscillators, respectively. And it
is qualitatively discussed that the phenomenon is described by a nonuniform
oscillator model in the presence of noise.
