Theoretical analysis and property study of optical focus detection based on differential confocal microscopy

The conventional differential confocal microscopy has been presented by Lee for surface profiling. This paper deals with a theoretical description of improvements in the signal to noise ratio and axial resolution of confocal microscopy, which is improved by detection of optical focus in differential confocal microscopy (DCM) using two differential detectors in the optical path, one located before and the other behind the focal plane. The optimum axial offset from the focal plane is obtained through the theoretical analysis of focus error signal (FES) of DCM, while the slope of the FES curve is maximum at the linear segment. Theoretical and experimental results show that, in comparison with conventional DCM, the common noise is suppressed and axial resolution is doubly improved.