Improvement of axial resolution in confocal microscopy using heterodyne illumination

A new technique for improving the axial resolution of confocal microscope is proposed. Based on the interference between two different frequency beams, which are separated axially, a frequency domain field confined focal spot is generated. The effective region made by the interference makes the point-spread function (PSF) of confocal microscope sharper. The three-dimensional imaging equations are derived. The intensity distribution of frequency domain field confined focal spot is proportional to the absolute value of the product of two fields. Three-dimensional intensity point-spread function (IPSF) is calculated numerically. The farther two beams are separated axially, the sharper IPSF is obtained. The numerical results show that the full width half maximum (FWHM) of the IPSF is improved by factor of 1.78 maintaining the strength of side lobe at 0.5 relative to main lobe. Also simulations for two-point resolution show the same improvement in the axial resolution.