Heterodyne confocal microscopy using symmetrical shifted-focus phase filters.

We propose a heterodyne confocal microscopy technique that uses the symmetry properties of conjugate phase-pupil filters to shift the axial intensity-response curves of two confocal systems symmetrically with respect to the geometric focal plane. A monotonic position-measuring curve is obtained by subtracting the output signals of the two confocal systems collected synchronously, while eliminating the influence of environmental noise. The system's three-dimensional tomographic ability can be improved by optimizing the pupil filter. Theoretical analyses and experiments indicate that the proposed method improves the axial and transverse resolutions of differential confocal microscopy and simplifies system installation and integration.