Coherent methods in confocal microscopy

The authors review the use of 2 kinds of coherent imaging methods in confocal microscopy: interferometry and the use of optical fibres. In both cases, the emphasis is concerned with detecting both the confocal field amplitude and its phase. It is usual in confocal systems to detect only the image intensity and so to loose all phase information. In order to detect the phase with an incoherent photodetector, it is necessary to use some form of interferometry. In the classical interferometer, the authors show that by detecting the phase and amplitude in a conventional interference microscope employing a large area detector it is possible to obtain confocal imaging. Indeed, the interference term images in conventional and confocal interference microscopes are found to be identical. An optical fibre-based confocal interference microscope is shown to have some practical advantages over a conventional, noncommon path implementation. The question of resolution in phase imaging is discussed and it is shown that due to the inherent nonlinearity of the process, great care should be taken in interpreting these images. The introduction of optical fibres instead of pinholes is shown to lead to simplification in system design. However, the introduction of 2-mode fibres makes the system more versatile. In essence, the fundamental mode detects the image amplitude and the first order mode detects the differential of the image amplitude. These modes then propagate along the fibre with different speeds and, hence, emerge from the fibre with a relative phase difference determined by the length of the fibre. These 2 modes are finally allowed to interfere. In this way, it is possible to obtain both confocal differential amplitude contrast, and differential phase contrast images simultaneously, without any other alteration to the optical system.