  As a light beam is produced by an amplification of modes of the zero point
field in its source, this field cannot be distinguished; consequently a
nonlinear optical effect is a function of the total field. However, we
generally prefer to use a conventional field which excludes the zero point
field; for a low conventional field, the total field may be developed to the
first order, so that the effect appears linear.
  This nearly trivial remark allows a correct computation of the signal of a
photocell used for photon counting and shows that the "impulsive stimulated
Raman scattering" (ISRS), a nonlinear, without threshold effect, which shifts
the frequencies, becomes linear at low light levels, so that the shifted
spectra are not distorted.
