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Studies on crystallisation behaviour of erbium doped phosphate glasses.
Abstract
Phosphate glasses can dissolve high concentrations of rare earths and
have excellent spectroscopic properties making them useful solid-state
laser materials. Solid-state lasers doped with different rare-earth ions
find applications in a wide range of LIDAR (Light Detection and Ranging)
and sensing applications; phosphate glasses are useful host materials
for many applications in the visible and near-infrared spectral regions.
For example, trivalent erbium (Er3+) doped phosphate glasses
operate at the eye-safe wavelength of 1.54 μm and are used for range
finding and sensing applications. Tm3+ doped solid-state
lasers operating at ~ 2 μm can be used for wind-shear and turbulence
monitoring. Similarly, Nd-doped metaphosphate glasses are the preferred
gain medium for high-peak-power lasers used for fusion energy research
because they can store optical energy at greater densities than other
glass-types and can be fabricated in large sizes with high rare-earth
ion concentration. This paper discusses issues affecting glass quality,
with particular focus on defect formation, especially crystallisation.
Avoiding crystallisation during processing is essential to form high
quality laser cavities. The work presented explores some of the factors
controlling these defects including contamination during melting. The
crystallisation behaviour of the glass was investigated for several
different phosphate glass compositions and different melting conditions,
including melting duration, temperature and crucible material.
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Continuous melting of phosphate laser glass is being used for the first time to prepare meter-scale amplifier optics for megajoule lasers; a description of the melting process is given. Two key factors in the successful melting of laser glasses are the elimination of damage-causing Pt-inclusions and dehydroxylation of the glass to concentrations less than ∼100 ppmw OH. Oxidizing conditions using 100% O2 or O2 + Cl2 mixtures (at one atmosphere) can be used to dissolve Pt inclusions and the effects of different gases on the dissolution of Pt-inclusions show the trend O2+Cl2>O2≫N2. The removal of hydroxyl groups is achieved by reactive (O2 + Cl2) or non-reactive (O2) gas bubbling; model calculations are used to simulate this process.
Properties of N31 phosphate laser glass by continuous melting were studied, and key technologies were explored during continuous-melting phosphate laser glass. Crux of the matter such as dehydroxylating, Pt-inclusions and so on were solved through continuous-melting experiments. 80×50×5 cm3 big plates were made and the glass properties were measured. Compared with pot melting, the glass by continuous-melting in full compliance with the spectrum and laser requirements, furthermore continuous-melting has more advantages.
The mechanisms of dehydroxylation of some polyphosphate glasses have been studied between ambient temperature and 1000°C while the residual pressure of water vapor above the sample was controlled. The results indicate that devitrification of the glass affects the water release. Only one mechanism takes place both under atmospheric pressure with an inert flowing gas and under a residual water pressure of 5 mbar. Until the fixed residual water vapor pressure is larger than 10−3 mbar, only one mechanism is present. But when the residual pressure of water vapor is decreased to a value of 10−3 mbar, several mechanisms occur for both a Graham's salt and a sodiumcalcium polyphosphate glass. We observe water release governed by a mechanism of nucleation-growth of nuclei. We have determined the activation energy of this reaction and proposed mechanisms for the dehydroxylation of polyphosphate glasses.
To secure the special technological properties of phosphate laser glasses, a series of preparation techniques, including a hermetic platinum melting system with a special stirrer, removal of water from the glass melt, and the forming process have been developed to produce high quality glass blanks of large dimensions for building high-power laser systems. The results are given.
Recent developments of phosphate glasses for a variety of technological applications, from rare-earth ion hosts for solid state lasers to low temperature sealing glasses, have led to renewed interest in understanding the structures of these unusual materials. In this review, spectroscopic and diffraction studies of simple phosphate glasses, including v-P2O5 and binary phosphate compositions, are described. Special attention is given to the structures of anhydrous ultraphosphate glasses, which have received close attention from the glass community only in the past six years.
Advances in laser glass compositions and manufacturing have enabled a new class of high-energy/high-power (HEHP), petawatt (PW), and high average power (HAP) laser systems that are being used for fusion energy ignition demonstration, fundamental physics research, and materials processing, respectively. The requirements for these three laser systems are different, necessitating different glasses or groups of glasses. The manufacturing technology is now mature for melting, annealing, fabricating, and finishing of laser glasses for all three applications. The laser glass properties of major importance for HEHP, PW, and HAP applications are briefly reviewed and the compositions and properties of the most widely used commercial laser glasses are summarized. Proposed advances in these three laser systems will require new glasses and new melting methods, which are briefly discussed. The challenges presented by these laser systems will likely dominate the field of laser glass development over the next several decades.