I. Introduction
Coherence in optics is an important parameter that quantifies the quality of interference [1], [2]. The most commonly used concepts are temporal and spatial coherence, which have been extensively studied in the past [3]– [6]. The concept of field correlations in the space-time domain has been expressed clearly in [1]. Spatial coherence describes the correlation between signals at different points in space. Temporal coherence describes the correlation between signals observed at different moments. In a typical interferometer, a lightwave is split into two beams, and the two beams are recombined together with different delay times. The two beams are perfectly coherent when the lengths of the two paths are identical. For a certain delay difference, the degree of coherence depends on the linewidth and wavelength stability of the light beam. There are other concepts on coherence in accordance with different physical parameters, such as polarization coherence, quantum coherence, and spectral coherence. Spectral correlation, which is not so widely used as temporal and spatial coherence, describes the correlation that exists between the spectral components at a given frequency in the light oscillations at two points in a stationary optical field [1].