Lee, S. and Gweon, D.
Improvement of the axial resolution in confocal microscopy by the use of heterodyne interference. Measurement Science and Technology 19: 105502-9 (2008).

A technique for improving the axial resolution of confocal microscopy is proposed and demonstrated. Based on the interference between two diffraction-limited spots separated along the optical axis, a field domain confined by the heterodyne interference is generated. The effective region of illumination made by the interference makes the point-spread function (PSF) of the confocal microscope sharper. The three-dimensional imaging equations for both coherent and incoherent systems are derived. Numerical simulation and experiment show that the full-width-at-half-maximum (FWHM) of the PSF can be improved by a factor of 1.78 and 1.41, respectively.