**1. Introduction**

Holographic elements are broadly used in the laser technique for spectral narrowing, stabilizing, and tuning the laser emission, phase locking in resonators, angular narrowing and steering the laser beams, stretching, compressing, and shaping the laser pulses, and also in combining laser beams. These applications impose specific requirements in holographic media. One of important requirements is their stability to the effects of optical radiation and temperature. Another useful feature is the transparency in the infrared because IR lasers are

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

widely used in the modern techniques. In this paper, a holographic medium is described that satisfies both these requirements: it is extremely stable and allows for reading out holograms in the IR up to *~*10 μm. This medium is calcium fluoride crystals (CaF<sup>2</sup> , fluorite) with color centers. Color centers in fluorite are the combination of anion vacancies and electrons trapped by the latter; the centers exhibit the absorption bands in the visible region and, thus, color the crystal. To form the centers in the crystal bulk, heating the crystal in the reducing atmosphere of calcium vapor is used (so‐called "additive coloring" of the crystal). Color centers can also be created in the crystal volume under the impact of *γ*‐radiation or high‐energy electron beams; however, such coloration is less stable.

The photochromism of colored crystals that allows for their use as a holographic medium is due to the transformation of color centers under the illumination of the crystal in the absorption band of specific center at an elevated temperature.

In this paper, the technique of additive coloring of fluorite crystals is described. The types of color centers are discussed as well as their photochromic transformations. Special attention is paid to the mechanism of hologram recording because it is this mechanism that determines the most important features of holograms recorded in this medium. The phenomenon of self‐ organization of color centers under hologram recording is considered. In conclusion, possible applications of the medium are discussed.
