Coherent microscopy and optical coherence tomography for biomedical applications

Abstract

In recent years many new methods of 3D optical imaging have been introduced that are applicable to the study of micro- and nano-scale flows. Coherent microscopy and optical coherence tomography join more established methods such as coherence scanning interferometry and confocal microscopy. These methods are very closely related and, using linear systems theory, can be compared in terms of their point spread and transfer functions. This paper introduces linear theory, demonstrates the main differences between the methods and discusses their use in micro- and nano-scale flow measurement. It is shown that coherent microscopy is currently the only method capable of single shot recording and consequently simultaneous whole-field flow measurement. Its use is limited to sparsely seeded flows however, such that individual particles can be identified. The other techniques provide increased 3D discrimination. Using a large numerical aperture, confocal microscopy and coherence scanning interferometry provide the most detailed 3D images making use the additional information available when the object is illuminated with plane waves propagating at different angles. In contrast optical coherence tomography uses the information that is available when the object is illuminated with different wavelengths. It is shown that the fundamental difference between these approaches is that the lateral and axial resolutions are decoupled in OCT making the technique easily scalable. This and the development of modern tunable laser sources, make OCT the method of choice for many biomedical applications.