Enhancement of ultrabroadband Bi NIR emission via fluorination for all wavelength amplification of optical communication

Bismuth (Bi)-doped photonic glasses and fibers with broadband near-infrared (NIR) photoemission have potential applications in tunable lasers and broadband amplifiers. Yet, when it comes to all wavelength amplification of optical communication, it remains challenging to achieve efficient Bi NIR emission in the technically relevant C- and L- bands (1530-1625 nm). Here, we propose a scheme by fluorination triggered enhancement of ultra-broadband Bi NIR emission in nitrided germanate glasses. Besides, compared to previous research, a unique and efficient Bi-activated ultra-wideband NIR emission with new emission bands peaked at ~924 and ~1520 nm under excitation of 450 nm are obtained in nitrided germanate glasses after fluorination. Moreover, the fluorination can modulate the local chemical environment by forcing the conversion of aluminum species from AlO₄ to AlO₅ and AlO₆ and consequently increase the flexibility of the glass network structure, which finally induces the conversion of Bi species and then manipulates the relative emission intensity of different Bi NIR centers. Thus, a flat and tunable emission spectrum covering the entire optical communication band is obtained by optimizing the fluoride amount. We believe this work is helpful to design the Bi-doped tunable fiber lasers and ultra-broadband amplifiers for all wavelength amplification of optical communication.     

Manipulating Bi NIR emission by adjusting optical basicity,boron and aluminum coordination in borate laser glasses

Bismuth‐doped (Bi) glasses and fibers have raised considerable attention for broadband emission and tunable fiber lasers in the near infrared (NIR) region. However, they suffer from low efficiency and it remains challenging to enhance Bi NIR emission. Here, we propose a facile way to enhance and tune the Bi NIR emission by adjusting optical basicity and modulating the boron and aluminum coordination in borate glasses. We find that BO₄ and AlO₅ species favor Bi NIR emission, right followed by the analyses of static emission spectra, the Fourier transform infrared (FT‐IR), and nuclear magnetic resonance (NMR) spectroscopy. Furthermore, abnormal Bi NIR luminescence phenomenon and boron anomaly were observed, which are attributed to the synthetic effect of B and Al coordination transformation. Besides, we find that BO₄ tetrahedron plays a major role in enhancing Bi NIR emission at low Al content, while AlO₅ hexahedron group will dominate at high Al concentration. Our investigation may give an insight into the luminescent behaviors of Bi in borate glasses and contribute to improving the performance of Bi‐doped fiber and fiber lasers in future.     

New strategy to enhance the broadband near‐infrared emission of bismuth‐doped laser glasses

Bismuth‐doped glasses and fibers with broadband near‐infrared (NIR) emission have garnered much attention on account of their potential applications in new fiber lasers and broadband amplifiers. Yet the realization of high gain from Bi‐doped fibers and highly efficient NIR emission from Bi‐doped glasses are still a stubborn puzzle. The enhancement of Bi NIR emission is normally based on modifying the glass composition and topology, which will change the structure of the glass over a wide range and alter the thermal or mechanical properties of the glass simultaneously, making it more complicated for the designing and fabricating of Bi fibers with good performance. Here, we find that a trace addition of Si₃N₄ can efficiently enhance the Bi NIR emission without changing the glass structure significantly, right followed by spectral and structural analysis. ²⁷Al NMR measurement reveals that the short‐ to medium‐range order of this glass is unchanged. The EPMA measurement confirms the homogeneity of fabricated glass. The great enhancement and red‐shift under blue light excitation may originate from the conversion of Bi active centers to low valence. Our results indicate that the trace addition of nitride could be a facile and maneuverable way to control the valence of active ions in glasses, which may contribute to improving the performance of photonic glasses.     

The origin of the heterogeneous distribution of bismuth in aluminosilicate laser glasses

As one kind of novel and burgeoning laser materials, bismuth‐doped silicate glasses have aroused increasing attention for the super broadband near‐infrared (NIR) emission. However, the large optical scattering loss, resulting from optical heterogeneity in glass color and refractive index, limits their further applications in telecommunication system. Thus, it is urgent to uncover the essence of heterogeneity in Bi‐doped silicate glasses and subsequently improve glass optical performance. It will give us some hint to homogenize the glass component and Bi active centers so as to boost the development of Bi‐based glass materials. Here, taking 1 typical Bi‐doped calcium aluminosilicate glass as an example, we revealed the origin of the optical heterogeneities in glass color and refractive index through the NIR emission spectra, electron probe microanalyzer (EPMA) of elements and X‐ray photoelectron spectroscopy (XPS) of Bi 4f_5/2, Bi 4f_7/2, and Al 2p. The inhomogeneous distribution of Bi and aluminum components is responsible for the heterogeneity in this glass system. In addition, we found that tetrahedral coordinated aluminum favors the existence of Bi NIR centers, consequently resulting in enhanced Bi NIR emissions. Furthermore, based on our results and the role of Al³⁺ in glass network, we demonstrate the homogenizing of glass component by finely tuning glass composition. This work will enrich the understanding of Bi‐doped laser glass and provide a guideline for the design of component‐derived Bi‐doped silicate glasses and fibers with efficient NIR emission and high optical quality.     

Experimental and theoretical studies on NIR luminescence of titanate-germanate glasses doped with Pr3+ and Tm3+ ions

Near-infrared (NIR) luminescence of Pr³⁺ and Tm³⁺ ions in titanate-germanate glasses has been studied for laser and fiber amplifier applications. The effect of the molar ratio GeO₂:TiO₂ (from 5:1 to 1:5) on spectroscopic properties of glass systems was studied by absorption, luminescence measurements, and theoretical calculations using the Judd–Ofelt theory. It was found that independent of the TiO₂ concentration, intense NIR emissions at 1.5 and 1.8 μm were observed for glasses doped with Pr³⁺ and Tm³⁺ ions, respectively. Moreover, several spectroscopic and NIR laser parameters for Pr³⁺ and Tm³⁺ ions, such as emission bandwidth, stimulated emission cross-section, quantum efficiency, gain bandwidth, and figure of merit, were determined. The results were discussed in detail and compared to the different laser glasses. Systematic investigations indicate that Pr³⁺-doped system with GeO₂:TiO₂ = 2:1 and Tm³⁺-doped glass with GeO₂:TiO₂ = 1:2 present profit laser parameters and could be successfully applied to NIR lasers and broadband optical amplifiers.     

Tunable luminescence from bismuth‐doped phosphate laser glass by engineering photonic glass structure

Because of ultra‐broadband near‐infrared (NIR) emission bismuth‐activated glasses and fibers offer a new promising platform for novel photonic devices such as new type of optical amplifiers and broadly tunable fiber lasers. Yet, challenge remains to manipulate the NIR emission behavior of bismuth (Bi) in photonic glasses for efficient Bismuth‐doped fiber and fiber lasers. Here, by engineering phosphorus and aluminum's topology, broadly tunable NIR emission has been realized in Bismuth‐doped phosphate laser glass. Structural and optical analyses on ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS NMR), ³¹P MAS NMR, fourier transform infrared (FTIR) and static emission spectra suggest that polymerization of glass network can be improved by proper addition of aluminum into the system, which can be evidenced by partial conversion of Q² to Q³ species of phosphorus and the shift of P–O–P asymmetric stretching vibration toward lower frequency, and this turns out beneficial to Bi NIR emission. Embedding aluminum tetrahedra into phosphorus glass network can reduce the local crystal field around bismuth and therefore lead to the blueshift of Bi emission. This work presents new insights into the luminescent behavior of Bi ions in phosphate glass and it helps the design and fabrication of Bismuth‐doped glasses and fibers in future.     

Photoemission from Bi-doped calcium aluminate glasses similar to sunlight

Light sources as the substitution for sunlight play an important role due to their practical applications in aircraft, medicine, and agriculture. Bi³⁺ ions show tunable emission from ultraviolet to deep red depending on local crystal field, presenting great potential in mimicking sunlight. However, these emissions are mainly limited to crystals. To stabilize Bi³⁺ ions inside photonic glasses with desirable emission and excellent physical properties remains challenging. Here, we managed to stabilize Bi³⁺ with ultrabroad and tunable emission inside glasses at air atmosphere. This emission spans the whole visible range of 350-780 nm and matches well with the sunlight spectra. For the first time, an external quantum efficiency of 34% was obtained from Bi-doped photonic glasses without optimizing glass composition. We find that the local oxidation atmosphere from super-oxide ions and proper crystal field formed by AlO₄ are responsible for this unique luminescence, which is revealed by the detailed comparison on the optical and structural properties of calcium aluminate glasses and Ca₁₂Al₁₄O₃₃ crystals with the same composition. Moreover, glasses with distortion structure further results in multiluminescent centers of Bi³⁺ so as to broaden the emission band. This work provides new insights into the luminescent behaviors of Bi ions in luminescent materials, and should contribute to designing new photonic glasses in future.     

Tunable broadband photoluminescence from bismuth-doped calcium aluminum germanate glasses prepared in oxidizing atmosphere

Tunable photoluminescence (PL) from transparent inorganic glass matrices is of interest for applications demanding a semitransparent photoconverter that does not elastically scatter incoming light. For this purpose, bismuth (Bi)-doped optical materials exhibit unique spectral characteristics in terms of bandwidth and emission tunability. Here, we demonstrate a facile route for preparing such converters from Bi-doped calcium-aluminate and calcium-aluminogermanate glasses. These glasses offer tunable PL across the near violet and visible-to-near-infrared (NIR) spectral range, with an emission lifetime in the range of 300 μs. The addition of GeO₂ exerts a decrease in optical basicity, which in turn enables the stabilization of NIR-active low-valence Bi species for broadband NIR PL.     

Glass-forming region and enhanced Bi NIR emission in sodium tantalum silicate laser glass

Bismuth (Bi)-doped glasses and fibers are of current interest as promising active media for new fiber lasers and amplifiers due to their 800-1700 nm near-infrared (NIR) emission. However, the optically active Bi centers in silica are easily volatilized during high-temperature fiber drawing, which results in low Bi doping concentration and low gain NIR luminescence. Here, we explored the glass-forming region in a model glass system of sodium tantalum silicate (Na₂O-Ta₂O₅-SiO₂) glass and attained suitable glass host for enhancing Bi NIR emission, right followed by detailed analysis on optical and structural characterization. Glass-forming region roughly lies in where Ta₂O₅ ≤ 30 mol%, SiO₂ ≥ 40 mol%, and Na₂O ≤ 40 mol%. Not only is glass-forming ability improved but also Bi NIR emission is enhanced (~60 times) by the introduction of Ta into glass network. Dissociated Na cations are restricted beside Ta, the high-field-strength element, so that the negative impacts of Na cations on glass formation and Bi NIR emission are weakened, which is responsible for the highly enhanced Bi NIR emission. This work helps us understand the glass-forming of tantalum silicate glass systems and luminescent behaviors of Bi. Hopefully, it could contribute to designing the Bi-doped laser glasses and high gain fibers with stable luminescent properties in future.     

Controllable ultra-broadband visible and near-infrared photoemissions in Bi-doped germanium-borate glasses

The design of functional materials with tunable broadband luminescence performance is still of great interest in the fields of lighting, solar cells, tunable lasers, and optical amplifiers. Here, via a melt-quenching method, a series of bismuth (Bi)-doped germanium-borate glasses with composition of 40GeO₂–25B₂O₃–25Gd₂O₃–10La₂O₃–xBi₂O₃ have been prepared, in which multiple Bi active centers can be stabilized simultaneously. Dual-modulating modes of visible (380-750 nm) and near-infrared (NIR) (1000-1600 nm) broadband photoemissions were effectively controlled under flexible excitation scheme. Photoluminescence (PL) spectra at low temperature 10-298 K were appropriately employed to interpret such an unusual wide visible emission band. To further illustrate the origin of NIR component, transmission electron microscopy (TEM) measurement was carried out. It is demonstrated experimentally that the visible emission mainly originates from the collective contribution of the ³P₁/³P₀¹S₀ transitions of Bi³⁺, while the broadband NIR luminescence should be related to the formation of low valent Bi⁺ and (or) Bi⁰ centers. This work may help to enhance the knowledge of the complex luminescence mechanism for the Bi species and it also enables such transparent glass materials to be a promising candidate for the multifunctional tunable light source.     

Effect of TeO2 on Near‐Infrared Emission from Bismuth Centers in Borogermanate Glasses

Bi‐doped xTeO₂–(60?x)GeO₂–15B₂O₃–20MgO–5Al₂O₃ glasses were prepared by the conventional melt‐quenching method and their absorption and fluorescence spectra were characterized. Broadband near‐infrared (NIR) emission from Bi centers centered around 1240 nm with a full width at half maximum (FWHM) of 250 nm was observed, and the position of the emission peak strongly depends on the excitation wavelength. Increasing TeO₂ concentration results in the strong coloration of the glass, leading to the reduction and finally, complete quenching of the NIR emission. Based on Raman, X‐ray photoelectron spectroscopy and transmission microscopy observation, the coloration of the glass at high TeO₂ concentration can be ascribed to the precipitation of elemental Te nanoparticles of around 3–8 nm, which effectively suppresses the NIR emission by reabsorption. The precipitation of Te nanoparticles in an oxide glass may find novel applications in photonics and relevant fields.     

Predictable tendency of Bi NIR emission in Bi‐doped magnesium aluminosilicate laser glasses

Because of superbroad luminescence in the range of near infrared (NIR), Bi‐doped glasses and fibers have received more attentions recently for the applications in super broadband optical fiber amplifiers or new wavelength lasers. As the luminescence comes from the transitions between naked 6p orbitals of bismuth, it is very susceptible to slight changes of local field around Bi. Therefore, it is always very challenging to predict NIR emission of bismuth in advance. Here, we found bismuth NIR emission shows predictable tendency in ternary glass system of MgO–Al₂O₃–SiO₂. The emission peak shifts red along the content of magnesium upon the excitation of 484 nm, which follows a single exponential growth equation. In the meantime, the full width at half maximum (FWHM) is broadened while the lifetime keeps decreasing. Glass structure analysis on basis of FTIR, ²⁷Al NMR, ²⁹Si NMR spectra reveals that these changes correlate to integrity of glass network, the increased disorder of local field around bismuth and the enhanced interaction between bismuth and host, which are perhaps due to the linear increase of nonbridging oxygen, and the enhanced Si–O asymmetric stretching vibrations along with magnesium, respectively. Electron probe microanalysis shows good homogeneity of Si, Al, Mg, Bi, and O distribution within the samples, and yoyo experiments of heating and cooling between 30°C and 300°C reveal the good resistance of such doped glasses to thermal degradation. This makes the glasses promising in applications of fiber devices even under extreme condition such as at higher temperature. The finding in this work should be helpful for the design of Bi‐doped laser glasses in future.     

Effect of BaF2 Content on Luminescence of Rare‐Earth Ions in Borate and Germanate Glasses

Rare‐earth‐doped oxyfluoride germanate and borate glasses were synthesized and next studied using spectroscopic methods. Influence of fluoride modifier on luminescence properties of rare earths in different glass hosts was examined. The excitation and emission spectra of Pr³⁺ and Er³⁺ ions in the studied glasses were registered. The emission spectra of Pr³⁺ ions in germanate and borate glasses are quite different and depend strongly on the glass host. In samples doped with Er³⁺ ions emission bands located around 1530 nm corresponding to the main ⁴I_13/2→⁴I_15/2 laser transition were registered, independently of the glass host. Quite long‐lived near‐infrared luminescence of Er³⁺ ions was observed for germanate glasses with low BaF₂ content, while in borate glass systems influence of barium fluoride on luminescence lifetimes is not so evident. The Judd–Ofelt calculations were used in order to determine quantum efficiencies of excited states of rare‐earth ions in germanate and borate glasses.     

Efficient Enhancement of Bismuth NIR Luminescence by Aluminum and Its Mechanism in Bismuth‐Doped Germanate Laser Glass

As a new member of laser glass family, bismuth‐doped glasses have received rising interests due to the application of fiber amplifiers and laser sources in the new spectral range for the next‐generation optical communication system. For practical application of the glasses, it must be considered on how to improve the luminescence efficiency. Here, we demonstrate that addition of aluminum can enhance the bismuth near‐infrared luminescence by more than 10 000 times, which is right followed by the discussion on the mechanism on why this can happen. We believe this work can be helpful for designing bismuth‐doped multiple component laser glasses with high efficiency. In addition, because of high susceptibility of bismuth to local field change, it can be used as probe ion to envision glass structures. Using bismuth as a luminescent structural probe, we can see the modifier ions of Bi⁺ are not completely randomly distributed inside germanate glass and they prefer the residence around tetrahedral AlO₄ sites.     

Near‐Infrared Broadband Photoluminescence of Bismuth‐Doped Zeolite‐Derived Silica Glass Prepared by SPS

Through the order–disorder transition process of zeolites, bismuth‐doped zeolite‐derived silica glasses with broadband near‐infrared (NIR) photoluminescence have been successfully prepared by spark plasma sintering (SPS). The samples were characterized by X‐ray diffraction, UV‐vis, photoluminescence, and fluorescence lifetime. The results showed that as‐prepared samples possessed favorable broadband NIR luminescence. The NIR emission (peaked at ~1140 nm) intensity decreased with increasing the bismuth doping concentration when excited by 500 and 700 nm. The tendency was different from the emissions (peaked at ~1240 nm) excited by 800 nm. In addition, the NIR fluorescence peaks of the fixed Bi concentration sample can be observed almost around 1140 or 1240 nm when excited by different wavelengths from 500 to 950 nm. These phenomena implied that the NIR emission peaked at different wavelengths may originate from different bismuth species. Three kinds of Bi active centers Bi⁺, Bi⁰, and (Bi₂)^2? were proposed to contribute to the NIR emission peaks at ~1140, 1240, and 1440 nm, respectively. Interestingly, a broadband NIR emission peaked at 1207 nm with a full‐width at half maximum of 273 nm was observed when excited by 600 nm, whose intensity was stronger than that excited by 800 nm. This property might be useful for broadband fiber amplifiers and tunable lasers.     

Dielectric Properties of Glasses in the Systems Bi2O3-CdO-SiO2, Bi2O3-CdO-B2O3, and Bi2O3-CdO-GeO2 and Their Relation to the Structure of Glass

Areas of glass formation and regions of “neo-ceramic” glasses in the systems Biz03-CdO-Si02, Bi203-Cd0-B203, and Bi203-Cd0-GeO2 are demarcated. Properties of glasses in the neoceramic regions were determined before and after nucleation and heat treatment. There are no maxima or minima in the dielectric properties versus composition curves. The dielectric constants increase after nucleation and heat treatment. The dissipation factors show a remarkable change toward higher values. Transparent glasses with Bi₂O₃. CdO as the predominant constituents have unusual dielectric constants ranging from 30 to 42 and dissipation factors ranging from 3 to 50 × 10⁻⁴., Since these glasses contain only from 0.5 to 2.0 wt% SiO₂ or B₂O₃ or 5 wt% GeO₂, the conventional concept of glass structure composed of a random spacial network of SiO₄ tetrahedra, whose interstices are filled with network modifiers, cannot be sustained in this case. It is suggested that the dominant cations Bi^{3 +} and Cd²⁺ form the network and that the interstices are occupied by Si4 + cations. The function of the Si⁴⁺ ion apparently is to distort the highly polarizable Bi3 + ion enough to form a random network. It appears to be possible that Bi³⁺ and Cd²⁺ can form the network in sixfold coordination. The dielectric losses observed are explained in terms of the relative instability of the structure.     

Quantitative prediction of the glass-forming region and luminescence properties in Tm3+-doped germanate laser glasses

Germanate laser glasses have received much attention as a promising host materials for mid-infrared fiber lasers in recent years because of the outstanding infrared transparency, low phonon energy, and high rare earth solubility of such glasses. However, the development of high-performance germanate laser glasses is usually based on intuition and a trial-and-error method, which can involve long experimental periods and high costs, and thus, this approach is highly inefficient. Recently, with proposals for materials genome engineering, the concept of the “glass genome” has grown of interest to us. Herein, the structures of Tm³⁺-doped germanate laser glasses (BaO–GeO₂ and BaO–La₂O₃–GeO₂) were investigated by Fourier transform infrared spectra (FTIR) and Raman spectra analyses, which revealed that the resulting glass contains similar structural groups to the neighboring congruently melted glassy compounds (NCMGCs) in the composition diagram. What is more, the structure and properties of the resulting laser glasses largely depend on NCMGCs. Then, the glass-forming region, physical properties, and luminescence properties were calculated via the use of NCMGCs in Tm³⁺-doped BaO–GeO₂ binary and BaO–La₂O₃–GeO₂ ternary laser glass systems. The calculated results were in good agreement with the experimental results, thus demonstrating that our approach is practical for predicting the glass-forming region, physical properties, and luminescence properties in Tm³⁺-doped BaO–GeO₂ binary and BaO–La₂O₃–GeO₂ ternary laser glass systems. This work may provide an effective method to develop Tm³⁺-doped germanate laser glasses rapidly and at low cost.     

Effect of introduction of TiO2 and GeO2 oxides on thermal stability and 2 μm luminescence properties of tellurite glasses

In this paper, Tm³⁺ doped pure tellurite glass, TiO₂ modified tellurite glass and TiO₂/GeO₂ co-modified tellurite glasses were prepared. The effects of the introduction of TiO₂ and GeO₂ oxides on the thermal, structural, optical properties of tellurite glasses were compared and analyzed intensively. Besides, the Judd-Ofelt intensity parameters and absorption and emission cross sections were calculated. DTA curves indicated the thermal stability of pure tellurite glass enhance obviously by GeO₂ modification. The introduction of TiO₂ and GeO₂ oxides in tellurite glasses arise a series of variation on the absorbance, luminescence peak position and fluorescence intensity which have been explained and analyzed in detail. Based on the analysis, the addition of TiO₂ in tellurite glass contributes to the improvement of 2 μm fluorescence performance, and the introduction of GeO₂ has great advantages in enhancing the thermal stability of glass.     

Newly developed BGO glasses: Synthesis,optical and nuclear radiation shielding properties

In this research paper, we studied the optical and nuclear shielding efficiency of newly developed BGO glasses with the following compositions (in wt%): 32Bi₂O₃–68GeO₂, 42Bi₂O₃–58GeO₂, 47Bi₂O₃–53GeO₂, 52Bi₂O–48GeO₂, 62Bi₂O₃–38GeO₂. BGO glasses were prepared by traditional melt quenching method. To obtain the band gap values of fabricated BGO glasses, optical absorption spectra were used for evaluation of optical properties. The mass attenuation coefficients (μ/ρ) were achieved for prepared glasses at 0.015–15 MeV photon energies employing MCNPX Monte Carlo code and WinXcom program. Moreover, broad-range of nuclear shielding parameters for gamma ray, neutrons and charged particles such as mass attenuation coefficient, half value layer, effective atomic number, buildup factors, mass stopping powers, projected ranges, fast neutron removal cross sections and damage factors were calculated. The refractive index is calculated from E_opt, As Bi₂O₃ concentration is enhanced, E_opt is also increased as well as the optical electronegativity and consequently the refractive index. In addition, the results showed that BIGE5 glass sample with highest Bi₂O₃ contribution has excellent nuclear radiation shielding ability among the other fabricated glass samples.     

Mechanism for broadening and enhancing Nd3+ emission in zinc aluminophosphate laser glass by addition of Bi2O3

Nd³⁺-doped phosphate laser glasses have been attracting much attention and widespread investigation due to their high solubility of rare earth (RE) ions, excellent spectroscopic properties, and large damage threshold. However, the narrow NIR emission bandwidth (less than 30 nm) of these Nd³⁺-doped phosphate glasses limits their further application toward ultrahigh power field and efficient fiber laser in new region. Here, we demonstrate the broadening and enhancing of Nd³⁺ NIR emission in laser glass of zinc aluminophosphate through tuning the glass structure and covalency of Nd-O bond without limiting the radiative properties of Nd³⁺. The maximum bandwidth of 1.05 μm emission is broadened to 50 nm, which is comparable to that of Nd³⁺-doped aluminate laser glasses. Simultaneously, the lifetime of ⁴F_3/2 level is elongated nearly by two times. Structural and optical properties of prepared glasses were discussed systematically to reveal the mechanism. Detailed analysis on optical spectra and glass structure indicates that the bandwidth is affected by not only the covalency of Nd-O but also the compactness of glass structure. Our results could enrich our understanding about the relationship between local glass structure and luminescence behaviors of active centers, and may be helpful in designing new RE-doped laser glass systems.