Fundamental Research On Bi-doped NIR-emitting Aluminosilicate Laser Glass

In the era of today's information and intelligent development,upgrading the data transmission capacity in the field of optical fiber communications,and pursuing ultra-broadband optical fiber amplifiers have become the key to the further development and improvement of the optical fiber communications industry.In this case,researchers have been looking for novel fiber gain media with ultra-broadband near-infrared?NIR?emission that can effectively cover the low-loss window of quartz fiber.Bi-doped aluminosilicate glass,as one kind of new fiber-core host material,have become the research focus because of its advantages of ultra-broadband NIR emission,low melting point,good mechanical,chemical and thermal stabilities,and easy preparation process.However,there are still a series of problems in the Bi-doped aluminosilicate glass,such as low NIR emission efficiency,inhomogeneity of glass composition,unclarity of the relationship between Bi NIR emission and glass network,unstability of Bi NIR centers and serious thermal degradation.This paper mainly aims at solving these problems.A systematic study has been conducted to reveal the intrinsic relationship between Bi NIR emission and glass network structure through a series of NIR emission spectral and glass structural characterizations.The research results obtained in this dissertation are as follows:?1?The NIR emission efficiency of bismuth has been improved in aluminosilicate glass.In this dissertation,enhanced Bi NIR emission could be realized through precipitating a series of Sr₂Ln F₇?Ln=La,Gd,Y,Yb,Lu?fluoride nanocrystals in aluminosilicate glass matrix.Specifically,first,we precipitated Sr ₂Yb F₇ nanocrystals in Bi-doped aluminosilicate glass system.This precipitated fluoride crystal have a low phonon energy,which effectively reduces the nonradiative transition probability of Bi and further promotes Bi NIR emission by more than 40 times.In addition,we systematically studied the NIR fluorescence characteristics and the crysta llization kinetics of a series of Bi-doped Sr₂Ln F₇?Ln=La,Gd,Y,Yb,Lu?glass-ceramics.It was found that with the increase of rare earth ion radius,the crystallization active energy of Sr₂Ln F₇ crystal gradually decreased,and the NIR emission intensi ty of Bi were gradually weakened.In brief,this part of the dissertation provides an effective method to enhance the Bi NIR emission,and lays the foundation for the realization of efficient and controllable broadband NIR emission of Bi centers in the fut ure;at the same time,the study of the crystallization kinetics process also has a guiding significance for controlling the crystallization process in glass.?2?The homogeneity of Bi active centers has been improved in aluminosilicate glass host.Taking Bi-doped heterogeneous calcium aluminosilicate glass for example,we explored the origination of the inhomogeneity of Bi distribution.It was found that the heterogeneous distribution of Bi centers was caused by the inhomogeneity of Al with different coordinations in glass network.Based on this,we finally achieved homogeneous distribution of Bi in the calcium aluminosilicate glass by adjusting the coordination of aluminum in the glass network structure.This work lays the foundation for obtaining Bi laser glass and optical fiber with good optical homogeneity in future.In addition,it has been revealed that f our-coordinated Al O₄facilitates the formation of Bi NIR centers in aluminosilicate glasses,which in turn promotes the NIR emission;moreover,it is easy to cause the expansion of the silicon network structure when Al O₄ is incorporated into the network structure of silicon since the length of Al-O bond is greater than that of Si-O bond,which will weaken the crystal field strength around Bi NIR centers,resulting in the blue-shifted NIR emissions of Bi.In brief,improved homogeneity of glass and tunable Bi NIR emissions can be obtained by modulating Al coordination and distribution in Bi-doped aluminosilicate glass.?3?The relationship between the Bi NIR emission behavior and glass network structure has been clarified.The NIR emission characteristics of Bi-doped calcium and magnesium aluminosilicate glass systems were systematically studied.The NIR emission of Bi is not only affected by the coordi nation of Al,but also by the polymerization degree of the silicon network structure?or the amount of non-bridge oxygen?NBOs??.When Al acts as the glass network former,the increase of Al O ₄ is conducive to the enhancement of Bi NIR emission.When the ne twork structure of silicon is depolymerized,the increase of NBOs in glass network will hamper the existence of Bi NIR centers,which in turn leads to the weakened Bi NIR emission.In a word,in Bi-doped calcium and magnesium aluminosilicate glass,it is t he combined effects of both Al coordination and polymerization degree of silicon network that determine the final NIR emission behavior of Bi.In addition,we also found that there is an exponential function relationship between the NIR emission peak posit ion of Bi and the content of alkaline-earth metal oxides in these two glass systems,while the FWHM is linearly related to the content of alkaline-earth metal oxides.Therefore,to some extent,we can modulate and even predict the NIR emission behaviors of Bi by designing glass components and structures in Bi-doped calcium and magnesium aluminosilicate glass,which makes is possible to achieve efficient,tunable ultra-broadband NIR emission of Bi,and promotes the promising applications of our Bi-doped multi-component glass systems in the broadband optical fiber amplifiers.?4?Bi NIR centers have been stabilized in aluminoslicate glass.A local excess charge model was proposed.Taking Bi-doped lanthanum and yttrium aluminosilicate glass systems for examples,according to the results of a series of spectroscopic and structural analyses,we found that Bi⁰ and Bi⁺NIR centers coexisted in these two glass systems.Moreover,Bi⁰ centers are mainly located in the multimember rings of silicon,while Bi⁺are situated at the interstices of silicon network.More inspiring,based on our local excess charge model,Bi⁰ and Bi⁺centers can be stabilized in their respective sites by managing glass network microstructure,during the process of which the integrality of silicon network needs to be maintained.Our results indicate that it is an effective and feasible method to stabilize and modulate Bi NIR emission to manage glass network structure,which will not only help understand the origination and distribution of Bi N IR centers,but also provide a guidance for designing Bi-doped aluminosilicate laser glasses with efficient super-broadband NIR emission and permanent NIR emission peak position.?5?The method of improving the anti-thermal degradation property of Bi-doped aluminosilicate glass has been found.Taking Bi-doped heterogeneous magnesium aluminosilicate glass for example,we explored the effects of alkaline earth metal oxides contents on the thermal degradation properties of Bi-doped aluminosilicate glass.It was found that most glass samples fabricated in this dissertation have a good resistance to thermal degradation,especially 60Si O₂-20Al₂O₃-20Mg O glass sample,after two yoyo experiments of heating and cooling between 30°C and 300°C,still maintain 97.7%NIR emission intensity compared to the one before yoyo experiments of heating and cooling.More meaningful,the thermal degradation become more and more serious with increasing content of alkaline earth metal oxides,which is closely related to the depolymerization of glass network and the amount of NBOs,since it will be beneficial to the diffusion of oxygen from ambient inside glass network and speed up the oxidizing reaction of partial Bi NIR emission centers at elevated temperatures,and Bi NIR emission consequently are declined.This work will help us obtain the glass with good resistance to thermal degradation by decreasing the amount of NBOs and the content of alkaline earth metal oxides in glass composition.