Microstructure optical fiber with uniform intensity distribution of the fundamental mode is proposed. The design guide line and characteristics of this kind fiber are demonstrated. The relationship between refractive index profile and structure parameters is investigated. The mechanism of forming uniform fundamental mode in these fibers is analyzed.
Compounds of Sr3Al2O6 ： Eu, SrgAl14O25 ： Eu, and BaZnSiO4 ： Eu were synthesized by high-temperature solid state reactions. The doping Eu^3 ＋ ions were partially reduced to Eu^2＋ in Sr4Al14O25：Eu and BaZnSiOg：Eu prepared in an oxidizing atmosphere, N^2 ＋ O2. However, such an abnormal reduction process could not be performed in Sr3Al2O6：Eu, which was also prepared in an atmosphere of N^2 ＋ O2. Moreover, even though Sr3A1EO6：Eu was synthesized in a reducing condition CO, only part of the Eu^3 ＋ ions was reduced to Eu^2 ＋ . The existence of trivalent and divalent europium ions was confirmed by photoluminescent spectra. The different valence-change behaviors of europium ions in the hosts were attributed to the difference in host crystal structures. The higher the crystal structure stiffness, the easier the reduction process from Eu^3 ＋ to Eu^2 ＋ .
Near-infrared to visible upconversion luminescence was observed in a multicomponent silicate （BK7） glass containing Ce^3 ＋ ions under focused infrared femtosecond laser irradiation. The emission spectra show that the upconversion luminescence comes from the 4f-5d transition of the Ce^3 ＋ ions. The relationship between the intensity of the Ce^3 ＋ emission and the pump power reveals that a three-photon absorption predominates in the conversion process from the near-infrared into the blue luminescence. The analysis of the upconversion mechanism suggests that the upconversion luminescence may come from a three-photon simultaneous absorption that leads to a population of the 5d level in which the characteristic luminescence occurs.
A new method was used to prepare erbium-doped high silica （SiO2 % 〉 96 % ） glasses by sintering nanoporous glasses. The concentration of erbium ions in high silica glasses can be considerably more than that in silica glasses prepared by using conventional methods. The fluorescence of 1532 nm has an FWHM （Full Wave at Half Maximum） of 50 nm, wider than 35 nm of EDSFA （erbium-doped silica fiber amplifer）, and hence the glass possesses potential application in broadband fiber amplifiers. The Judd-Ofelt theoretical analysis reflects that the quantum efficiency of this erbium-doped glass is about 0.78, although the erbium concentration in this glass （6 × 10^3） is about twenty times higher than that in silica glass. These excellent characteristics of Er-doped high silica glass will be conducive to its usage in optical amplifiers and microchip lasers.
A new kind of Nd^3＋-doped high silica glass （SiO2 〉 96% （mass fraction）） was obtained by sintering porous glass impregnated with Nd^3 ＋ ions. The absorption and luminescence properties of high silica glass doped with different Nd^3 ＋ concentrations were studied. The intensity parameters Ωt （t = 2, 4, 6）, spontaneous emission probability, fluorescence lifetime, radiative quantum efficiency, fluorescence branching ratio, and stimulated emission cross section were calculated using the Judd-Ofelt theory. The optimal Nd^3＋ concentration in high silica glass was 0.27% （mole fraction） because of its high quantum efficiency and emission intensity. By comparing the spectroscopic parameters with other Nd^3 ＋- doped oxide glasses and commercial silicate glasses, the Nd^3 ＋ -doped high silica glasses are likely to be a promising material used for high power and high repetition rate lasers.
