ABSTRACT
The propagation of a beam can be classified as a propagation in free space and a propagation in a guided structure. Free-space propagation can be described as propagation by diffraction of light, whereas propagation of light in a waveguide can be explained by the eigenmode expansion of the waveguide. This chapter discusses the propagation of light in a dielectric medium with a homogeneous, isotropic dielectic constant e, including vacuum. The state of the propagation of the Gaussian beam has been described. The chapter also discusses the propagation of an optical beam with an arbitrary complex amplitude distribution can be expressed by the Fresnel-Kirchhoff diffraction integral. In general, a light beam propagating in an optical system has an intensity distribution that is close to a Gaussian beam, but it is rare that they are exactly identical to the Gaussian beam. In the single-mode region, however, the spot size will not decrease even if the core thickness is reduced.
