Measurement of submicron laser beam radii

Three methods for direct measurement of the intensity distribution in laser beams focused by microscope optics to waists of submicron width are described and compared. They use scans of the beam waist with (1) a knife-edge, (2) a submicroscopic point fluorescent source, and (3) convolution scans generated by the photobleached pattern of the focused beam. An indirect photographic technique is also evaluated. The laser beam is found to propagate ideally down to a minimum size usually limited by the aberrations of the optics.