Over the past two decades, fiber laser technologies have matured to such an extent that they have captured a large portion of the commercial laser marketplace. Yet, there still is a seemingly unquenchable thirst for ever greater optical power to levels where certain deleterious light-matter interactions that limit continued power scaling become significant. In the past decade or so, the industry has focused mainly on waveguide engineering to overcome many of these hurdles. However, there is an emerging body of work emphasizing the enabling role of the material. In an effort to underpin these developments, this paper reviews the relevance of the material in high power fiber laser technologies. As the durable material-of-choice for the application, the discussion will mainly be limited to silicate host glasses. The discussion presented herein follows an outward path, starting with the trivalent rare earth ions and their spectroscopic properties. The ion then is placed into a host, whose impact on the spectroscopy is reviewed. Finally, adverse interactions between the laser lightwave and the host are discussed, and novel composition glass fiber design and fabrication methodologies are presented. With deference to the symbiosis required between material and waveguide engineering in active fiber development, this review will emphasize the former. Specifically, where appropriate, materials-based paths to the enhancement of laser performance will be underscored.

Sustained progress in the development of optical fibers has led to the present state where further improvements in performance are limited by intrinsic optical nonlinearities. In order to circumvent such limitations, the user community has adopted two general approaches: (i) engineer the enabled systems accordingly; and/or (ii) microstructure the fiber to shift nonlinear thresholds to high optical power levels. In both cases, the nonlinearities are accepted as they are and performance is enhanced through added system or fiber design complexity. This paper, the first in a trilogy, along with two companion articles (in 3 parts) (Int J Appl Glass Sci.

The purpose of this paper, Part IIA in the trilogy (Int J Appl Glass Sci. 2018;9:263-277; Int J Appl Glass Sci. 2018 (in press); Int J Appl Glass Sci. 2018 (in press)), is to describe the continuum models employed to deduce the physical, acoustic, and optical properties of optical fibers that exhibit intrinsically low optical nonlinearities. The continuum models described herein are based on the additivity approaches of Winklemann and Schott (W-S). Initially developed over 120 years ago, W-S additivity works well for predicting the basic properties of bulk silicate glasses. While high-silica-content glasses are still the gold-standard for telecommunication and high energy laser fibers, the models have been systematically expanded to include deduction of the physical, thermophysical, and acoustic constants and coefficients that bear on parasitic nonlinearities. The stateof-the-art in W-S-based continuum materials models is reviewed here with specific examples provided based on canonical material systems suggested from the findings of Part I and treated in detail in Part III. K E Y W O R D Slasers, optical fibers, optical glasses, optical properties 1 | INTRODUCTION Without today's understanding of atomistic and quantum physics and chemistry and the benefits of modern high performance computing, our scientific ancestors developed continuum models for predicting the behavior of a range of materials. Amongst the original approaches was that of Adolph Winkelmann, who introduced an additivity approach for calculating the specific heat of (oxide) glasses of arbitrary composition based on the physical properties of the individual components. That said, one does today have the benefit of high performance computing and numerous atomistic and quantum chemical models and codes with which to simulate the performance of glasses.2-4 As a matter of fact, the simulation of glass properties and processing has become so effective, that glasses have been brought to commercial markets entirely designed and optimized based on modeling.5 Accordingly, the Readers are then within their rights to wonder why continuum-level additivity models as described herein are employed. The simple answer is simplicity . . . and speed and versatility. In the work described in Companion Paper III, 6 there are entire binary, ternary, and quaternary glass systems whose properties need to be estimated in order to predict their physical, acoustic, and optical behavior as relates to the nonlinearities described in Paper I. 7 A macroscale continuum approach offers a rapid yet sufficiently accurate method for screening large computational spaces.--

The purpose of this paper, Part IIB in the trilogy [Int J Appl Glass Sci. 2018 (not available); Int J Appl Glass Sci. 2018 (not available); Int J Appl Glass Sci. 2018(not available)], is to describe the continuum models employed to deduce the effects of differential thermal expansion in the clad optical fiber in addition to the nonlinear optical behaviors at high intensities of light. In this continuation of Part IIA, more of the state-of-the-art in W-S-based continuum material models are reviewed here with specific examples provided based on canonical material systems suggested from the findings of Part I and treated in detail in Part III. Part IIB pays particular attention to the optical fiber and its nonlinearities. K E Y W O R D Slasers, optical fibers, optical glasses, optical properties

In this paper, various types of high temperature fibre Bragg gratings (FBGs) are reviewed, including recent results and advancements in the field. The main motivation of this review is to highlight the potential of fabricating thermally stable refractive index contrasts using femtosecond (fs) nearinfrared radiation in fibres fabricated with non-conventional techniques, such as the molten core method. As a demonstration of this, an yttrium aluminosilicate (YAS) core and pure silica cladding glass optical fibre is fabricated and investigated after being irradiated by an fs laser within the Type II regime. The familiar formation of nanogratings inside both core and cladding regions are identified and studied using birefringence measurements and scanning electron microscopy. The thermal stability of the Type II modifications is then investigated through isochronal annealing experiments (up to T=1100°C; time steps, Δt=30 min). For the YAS core composition, the measured birefringence does not decrease when tested up to 1000°C, while for the SiO 2 cladding under the same conditions, its value decreased by ∼30%. These results suggest that inscription of such 'Type II fs-IR' modifications in YAS fibres could be employed to make FBGs with high thermal stability. This opens the door toward the fabrication of a new range of 'FBG host fibres' suitable for ultra-high temperature operation. ORCID iDsMaxime Cavillon https:/ /orcid.org/0000-0003-1341-6294 Peter Dragic https:/ /orcid.org/0000-0002-4413-9130 Figure 5. Normalized retardance (averaged over the 0.1 to 1.0 μJ pulse energy range) as a function of temperature for YAG-derived fibre at the core center (YAS) and in the cladding (SiO 2 ), for both // and ⊥ configurations. The measurements were performed at room temperature.