**1. Introduction**

Photo-thermo-refractive (PTR) glasses are a new class of photosensitive materials intended for recording three-dimensional (3D) phase holograms. This class was developed based on photo-sensitive sodium zinc aluminosilicate glasses that were first put into practice by

© 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. © 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.

Corning, Inc., in 1977 and were referred to as polychromatic (PC) glasses [1–3]. In Russia, similar photosensitive glasses [4, 5] were denoted by multichromatic (MC) ones. PC/MC glasses are known to contain, in addition to Na2O, Al2O3, ZnO, and SiO2, some other ingredients such as (i) photosensitive dopants playing the roles of electron donors (Ce3+) and acceptors (Ag+ , Sb5+, and Sn4+) and also (ii) halogen ions (F− and Br− ) that participate in the formation of the crystalline phases. The main specific feature of PC/MC glasses is the selective absorption in the visible. Namely, PC glasses can acquire, under the effects of the UV exposure and subsequent heat treatment, a wide variety of colors. In brief, the final stages of photochemical and diffusion processes responsible for this coloration were assumed to be as follows. The colloidal silver particles formed under the above effects play the role of nucleation centers. Around such centers, the growth of NaF and (Ag, Na)Br nano- or microcrystals occurs. Under particular growth conditions, the microcrystals acquire complicated anisotropic shapes such as the elongated pyramid-like structures stretched along an axis [1, 3, 6]. Additional multistage UV irradiation and heat treatment lead to the photolytic precipitation of silver layer on the surfaces of these anisotropic structures (so-called "decoration of the latter with silver"). The anisotropy of metallic silver shells thus formed results in a certain shift of the corresponding absorption band into the visible. So the substantial anisotropy of metallic silver particles was considered to be the principal condition for the occurrence of "PC/MC coloration" in PC/MC glasses.

In the late 1980s–early 1990s [7–10], it was proposed first in Vavilov State Optical Institute to apply PC/MC glasses for recording the 3D phase holograms. Unlike the case of PC/MC coloration, only a single stage of photo-thermo-induced crystallization was used, and this stage included the UV irradiation and subsequent heat treatment. When developing the corresponding procedures, the principal attention was paid to a difference obtainable in the refractive indices of vitreous and crystalline phases rather than the anisotropic shapes of microcrystals. As a result, a new class of materials was developed in Vavilov State Optical Institute, this class was denoted [10] by a specific term such as "photo-thermo-refractive (PTR) glasses" (i.e., glasses whose refractive index varies due to the UV irradiation and subsequent heat treatment). Later [11, 12], this term started to be used widely in other countries as well. Now, there is an increased interest in PTR glasses because the volume Bragg gratings recorded on these glasses reveal a unique combination of working characteristics such as the high angle and spectral selectivity, high diffraction efficiency, high mechanical and optical strength, and also high thermal and chemical durability. Based on PTR glasses, a broad variety of optical devices are developed including extra narrow-band filters, wavelength division multiplexing (WDM) devices, combiners of high-intensity light beams, chirped gratings for compressing the light impulses, filters for increasing the spectral brightness of laser diodes, etc.

The given paper is a survey of recent achievements of ITMO University (St. Petersburg, Russia) in developing new holographic media such as fluoride, chloride, and bromide silicate photothermo-refractive (PTR) glasses as well as the holographic diffractive optical elements that are the volume Bragg gratings recorded in the glasses for improving dramatically the parameters of laser systems of different types.
