RMS Pulse Width of the Ultrashort Lorentz Pulses

The pulse width(full width at half maximum,FWHM) doesn't give an accurate description of the ultrashort Lorentz pulse when the FWHM pulse width is shorter than one optical oscillation period.The relation between the FWHM pulse width and the root-mean-square(RMS) pulse width is studied in theory.The results show that the FWHM pulse width is the same as the RMS pulse width when the FWHM pulse width is longer than one optical oscillation period;when the FWHM pulse width is shorter than one optical oscillation period,the RMS pulse width is different from the FWHM pulse width,and the absolute phase seriously affects the RMS pulse width of Lorentz pulse.