  We studied both theoretically and experimentally the intensity distribution
of a Gaussian laser beam when it was focussed by an objective lens with its
numerical-aperture (NA) up to 0.95. Approximate formulae for full widths at
half maximum (FWHM) of the intensity distribution at focus were derived for
very large and very small initial beam waists with respect to the entrance
pupil radius of the objective lens. In experiments the energy flux through a
0.5 micron pinhole was measured for various pinhole positions. We found that
the FWHM's at focus in the transverse and the longitudinal directions do not
increase much from the ultimate FWHM's until the input beam waist is reduced
below the half of the entrance pupil radius. In addition, we observed
significance of the spatial distribution of the input beam against a true
Gaussian beam profile in the case of small initial beam waist. For high NA with
resulting focal beam waists comparable to or smaller than the wavelength of the
laser, the interaction between the electric field and the conducting surface of
the pinhole caused the transverse FWHM to be measured slightly smaller than
FWHM of the unperturbed intensity distribution convoluted with the pinhole
opening.
