sentences:  The first of these is the approximations associated with the Weak Field Limit, where the square modulus of the Rabi frequency of the field that is coupled to two-level atom is much smaller than the rate of spontaneous emission of the atom.This means that the difference in the population between the excited state of the atom and the ground state of the atom is approximately independent of time.If you also take the limit in which the time period is much larger than the time for spontaneous decay, the coherences of the light can be modeled as 
  formula_27, where formula_28 is the rabi frequency of the driving field and formula_29 is the spontaneous decay rate of the atom.