ABSTRACT
Coherence means the degree of correlation of the optical field. If the field is fully deterministic, there is full correlation of the field, as in the case of a monochromatic point source that is realized by a single-mode laser. The stochastic nature of the field is caused by a superposition of many modes, each of which is varying in fluctuating phase relationships with each other. Coherence effects are observed through interferometers because the resultant fringe contrast is governed by the coherence. Coherence can be discussed in two categories, i.e., temporal coherence and spatial coherence. Temporal coherence, sometimes also called longitudinal coherence, is characterized by the temporal autocorrelation of light disturbance arising at a point with varying time delay and corresponds to the cross-correlation between the disturbances arising at two points separated in the direction of light propagation, while spatial coherence is described in terms of the cross-correlation between the disturbances arising at laterally separated points as a function of the separation and sometimes called lateral coherence. The former is related with spectrum, while the latter depends on the size of a light source. Temporal coherence is discussed for a point polychromatic source whereas spatial coherence is usually discussed for a quasi monochromatic extended source. The classical coherence theory can also be extended to analysis of laser speckle that is a spatial intensity fluctuation arising from random interference of coherent light scattered from rough surfaces. In this entry, basic relations involved with the classical coherence theory are presented.
