Saturated absorption linewidth: Influence of the source frequency noise

The effect of the source FM noise on the lineshape in sub-Doppler saturation spectroscopy is examined both theoretically and experimentally. We show that the line profile is simply given by the convolution of the homogeneous profile with an effective spectrum, depending on the relative values of the sample relaxation time and of the source frequency correlation time but not departing significantly from the source field power spectrum in the usual cases. This result is well supported by a millimeter wave experiment involving a nearly perfect source, frequency modulated by a Gaussian noise.