Introduction

In this chapter, the definitions of the spatial coherence functions described in the space-time domain and the space-frequency domain are reviewed. The correlation of optical wavefields is closely related to the interference effects. The visibility of the interference fringe is determined by the modulus of the complex degree of coherence. When we discuss the correlation in the wavefields with the wide spectral bandwidth, the concept of the spectral coherence plays an important role. The efficiency of the spectral interference is determined by the spectral degree of coherence in a similar way as the spatial interference fringes.

The phenomenon of the spectral change owing to the source correlation, that is so called the correlation-induced spectral changes, has been actively studied since the 1980's. Studies on the correlation-induced spectral changes from both the theoretical and experimental points of view were reported. The basic concept of the correlation-induced spectral changes and the condition for the spectral invariance that was derived by Wolf[3] is briefly reviewed in the followings.