Periodic thin-film interference filters as one-dimensional photonic   crystals
  We review photonic band gap related properties of a simple periodic system of thin dielectric layers. Properties associated with forbidden and allowed bands of such one-dimensional photonic crystals are presented. A revision of forbidden bands properties leads to an omnidirectional Bragg mirror design. The anisotropy of allowed bands suggests the formation of photon-focusing caustics in one-dimensional photonic crystals. 
Dmitry N. Chigrin and Clivia M. Sotomayor Torres