| The fast increase of telecommunication traffic demands raising the transmission capacity of Dense Wavelength Division Multiplexing (DWDM). To achieve the purpose, one of the efficient ways is to broadening the gain band width of optical amplifiers. Thus, explorating the new materials for the gain medium which can covers the whole low loss window becomes the most urgent task. In recent years, more researches center on the rare-earth ions doped materials, but the gain bandwidth of rare-earth ions doped materials are hard to surpass 100 nm due to the nature of the inner shell transition of rare earth ions, which limits the transition capacity of optical fiber communications. Recently, Fujimoto et al reported the near infrared broadband emission centered at about 1300 nm from bismuth-doped (simplifized Bi-doped) silicate glass excited by 800 nm. Since this, many researchers commit themselves to the research of near infrared (reffered to NIR) fluorescence of Bi-doped materials. Broadband NIR emission was observed in various Bi-doped materials including crystals, silica, phosphate, germanate glasses and so on. The bandwidth of NIR fluorescence from Bi-doped materials can reach to 200 nm and the fluorescence lifetime of near infrared emission is enough long. If the Bi-doped materials are used to be gain media of optical amplifiers in future, the purpose of using a single fiber and a single pumping source to bring the NIR fluorescence covering the whole low loss optical communication window is achieved. Nevertheless, the mechanism of NIR fluorescence remains unknown. In this paper, the dependences of NIR properties on the optical basicity and glass composition under different pump sources are investigated. The mechanism of NIR fluorescence is also studied.It is considered that the valence of the multivalence metal ions depended upon the optical basicity of glass matrix, and the characteristics of the NIR fluorescence lie on the valence of doped active ions. Therefore, the effect of optical basicity on the NIR properties under different pumping sources was investigated. In the case of 690 nm excitation, the NIR luminescence located at about 1100 nm with FWHM of about 220 nm intensity decreases with increasing of alkaline-earth metal radius. However, under 808 nm excitation, the NIR luminescence at about 1270 nm with FWHM of 280 nm intensity increases with the increase in radius. It is suggested that the NIRs may be from two kinds of bismuth ions with different valence state. What is more, the NIR spectra of Bi-doped aluminosilicophosphate glass under 532 nm excitation has two peaks at around 1110 nm and 1270 nm by Gaussian peak separation. The fluorescence centered at about 1110 nm is similar to that excited by 690 nm, whereas the peak at about 1270 nm is like that pumped by 808 nm. In Bi-doped aluminophosphate glass, the NIR spectra excited by 532 nm has similar Gaussian peak separation results with that of the spectra from Bi-doped aluminosilicophosphate glass under 532 nm excitation, which further suggest the NIRs at 1100 and 1260 nm are from two luminescence centers. According to the differences in change trends of NIR properties with optical basicity, the mechanism of NIR fluorescence is also discussed.Since the NIR properties are not only depended on the optical basicity, but also related with the glass microstructure. Subsequently, the effects of glass former and intermediate on the NIR properties are investigated. First, the relationship between SiO2 concent and NIR property is investigated. With the increase in SiO2 concent, both the peak position and FWHM change, and the NIRs excited by 690 and 808 nm intensity increase. Subsequently, the effect of B2O3 concent on the NIR properties is discussed. With the increase in B2O3 concent, the NIR intensity upon excitation of 690 nm increase, while the NIR intensity under 808 nm excitation decreases. The phenomenon and the mechanism of near infrared fluorescence are discussed from the angle of microstructure. |
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