  A clear physical meaning of the Carruthers-Nieto symmetric quantum phase
fluctuation parameter (U) has been provided in Susskind Glogower and Barnett
Pegg formalism of quantum phase and it is shown that the reduction of phase
fluctuation parameter U with respect to its coherent state value corresponds to
an antibunched state. Thus nonclassicality of a state may be manifested through
the phase fluctuation parameters. As examples, quantum phase fluctuations in
different optical processes, such as four wave mixing, six wave mixing and
second harmonic generation have been studied by using Carruthers-Nieto quantum
phase fluctuation parameters. The operators required for the calculation of
quantum phase fluctuations are expressed in closed analytical forms (up to
second order in coupling constant). It is also found that the reduction of
phase fluctuations compared to their initial values are possible in all three
cases which means nonclassical (antibunched) state exists in all these cases.
