Xu Tiefeng

On the enhancement of superconductivity due to Van Hove singularity

We have rigorously solved the standard BCS gap equation with the density of states of Van Hove type. The result shows that the enhancement ofTc is not as effective as previously estimated. It seems unlikely that Van Hove singularity inN(E) can be theTc-enhancement mechanism for the high-Tc Cu oxides.

Tm3+-doped tellurite glass with Yb3+ energy sensitized for broadband amplifier at 1400–1700 nm bands

A kind of novel experiment was disclosed as it possessed two bands of fluorescence emission at 1.4 and 1.6 μm, which were perfectly complimentary to the current C band of optic communication. The fluorescence was based on energy transfer and up-conversion processes between Tm3+ and Yb3+ under direct pumping of 975 nm LD. The spectra and lifetimes of Tm3+ fluorescence in the tellurite glass were described. The corresponding fluorescence characteristics and energy migration process were analyzed by the method of lifetime and intensity comparison. The mechanism of the up-conversion based IR fluorescence was presented upon analyzing the multi-photon pumping process. The potential advantages of Tm3+/Yb3+ co-doped tellurite glass as amplifier material were concluded.

Investigation of 2.9 μm luminescence properties and energy transfer in Tm3+/Dy3+ co-doped chalcohalide glasses

Abstract A series of chalcohalide glasses based on the composition of 0.9(Ge 30 Ga 5 Se 65 )-0.1CsI with different Tm 3+ /Dy 3+ -codoped ions concentrations were synthesized by melt-quenching technique. The absorption spectra and 2.9μm mid-infrared fluorescence spectra of glass samples under 800 nm laser excitation were measured. The results showed that Tm 3+ was an efficient sensitizer, which could enhance the Dy 3+ :2.9 μm fluorescence intensity significantly. The effective energy transfer between the two rare-earth ions were mainly attributed to the resonance energy transfer from Tm 3+ : 3 F 4 to Dy 3+ : 6 H 11/2 level. Emission cross section of 2.9 μm mid-infrared luminescence was also investigated according to Judd-Ofelt theory, σ e =2.51×10 −20 cm 2 .

Influence of B2O3 on Spectroscopic Properties of Er3+/Yb3+ Co-Doped Tungsten-Tellurite Glasses

Abstract A series of novel Er 3+ /Yb 3+ co-doped (85 – x)TeO 2 -15WO 3 - xB 2 O 3 (TWB; x = 2%, 5%, 8% (mole fraction)) glasses were prepared. Influence of B 2 O 3 on the spectroscopic properties of Er 3+ /Yb 3+ co-doped tungsten-tellurite glasses were investigated. It is found that the intensity of 1.5 μm fluorescence, lifetime of the 4 I 13/2 level and upconversion fluorescence all decrease with the increase of B 2 O 3 content. The product of full width at half maximum (FWHM) and stimulated emission cross-section (σ) of Er 3+ : 4 I 13/2 → 4 I 15/2 transition has an optimum when B 2 O 3 is 5% (mole fraction). The emission spectra of Er 3+ : 4 I 13/2 → 4 I 15/2 transition was analyzed using peak-fit routine, and an equivalent four-level system was proposed to estimate the stark splitting for the 4 I 15/2 and 4 I 13/2 levels of Er 3+ ions in TWB glasses at room temperature.

Investigation on energy transfer from Er3+ to Nd3+ in tellurite glass

Abstract A study of energy transfer of Er 3+ /Nd 3+ codoped tellurite glasses was presented. By Nd 3+ co-doping, both the Er 3+ green emission corresponding to the Er 3+ : ( 4 S 3 / 2 , 2 H 11 / 2 ) → 4 I 15 / 2 transitions and the red emission corresponding to the Er 3+ : 4 F 9 / 2 → 4 I 15 / 2 transitions were quenched. The energy transfer mechanism between Er 3+ and Nd 3+ was discussed based on their energy level characteristics. The interaction parameters, C D-A , for the energy transfer processes from Er 3+ to Nd 3+ in tellurites glass were calculated. Finally, the resonant transfer Er 3+ : 4 I 9/2 → Nd 3+ : ( 4 F 5/2 , 2 H 9/2 ) was proposed to be the most probable microscopic process to occur in contrast with the other processes.

FPGA implementation of Augmented Hammerstein predistorters for RF power amplifier linearization

In this paper, FPGA implementation of Augmented Hammerstein predistorters for linearizing wideband RF power amplifiers with memory effects is presented. The Augmented Hammerstein nonlinear model based predistorter is briefly introduced. The implementation method of the Augmented Hammerstein predistortion on FPGA is described in details. The experimental platform based on Agilent vector signal generators for baseband digital predistortion is discussed. The validation results show that the predistortion system exhibits good performance in mitigating the signal deterioration caused by memory effects with two-carrier WCDMA testing signals.

Improved Fluorescence from Tm3+/Er3+/Ce3+ Triply Doped Bismuth-Silicate Glasses for S+C-bands Amplifiers

Fluorescence of Tm3+/Er3+ codoped bismuth-silica (BS) glasses and the sensitization of Ce3+ are investigated. It shows that Ce3+ codoping with Tm3+/Er3+ in BS glasses results in a quenching of Tm3+ ion emission from 3F4 to the 3H6 level. Consequently, the 1.47 μm emission occurs after the population inversion between the 3H4 and 3F4 levels. Furthermore, the codoped glasses show the broad emission spectra over the whole S and C bands with full-width at half-maximum (FWHM) up to about 119 nm, as it combines 1.55 μm emission band of Er3+ with 1.47 μm emission band of Tm3+ under 800 nm excitation.

Midinfrared Emission Properties of Ho 3+ -doped Ge-Ga-S-CsI Glasses: Midinfrared Emission Properties of Ho 3+ -doped Ge-Ga-S-CsI Glasses

A serial of chalcogenide glasses based on Ge-Ga-S-Csl system doped with the different Ho3+ ions were synthesized by melt-quenching technique. The properties of glasses including refractive indexes, absorption spectra, mid-infrared emission spectra and lifetimes of I-5(6) level of Ho3+ ions were measured. The Judd-Ofelt intensity parameters Omega(i)(i=2,4,6), oscillator strength f(cal), spontaneous transition probabilities A(rad) for Ho3+ ion were calculated by Judd-Ofelt theory. Multiphonon relaxation rates of the Ho3+ : I-5(5)-> I-5(6) and I-5(6)-> I-5(7) in Ge-Ga-S-Csl glasses were evaluated. Effect of Ho3+ ion concentration on the fluorescence spectra was investigated. The results indicate that the fluorescence under 900 nm excitation with peak wavelength at 2. 81 mu m and 3. 86 mu m are due to the Ho3+ : I-5(6)-> I-5(6) and I-5(6)-> I-5(7) transition, respectively. The intensity of the mid-infrared fluorescence are enhanced with the Ho3+ ion concentration increasing from 0. 5wt% to 1. 0wt%. Multiphonon relaxation rates are 29s(-1) and 34s(-1) for the Ho3+ : I-5(5)-> I-5(6) and I-5(6)-> I-5(7) transition, respectively.采用熔融冷却法制备了系列不同Ho 3+ 离子掺杂浓度的Ge-Ga-S-CsI玻璃样品，测试了样品的折射率、吸收光谱以及中红外荧光光谱和Ho 3+ 离子5I6能级荧光寿命.应用Judd-Ofelt理论计算了Ho 3+ 离子在该基质玻璃中强度参数 Ω i （ i =2,4,6）、能级跃迁振子强度fcal、自发跃迁几率Arad等光谱参数.计算了Ho 3+ ∶ 5 I 5 → 5 I 6 (3.86μm)和 5 I 6 → 5 I 7 (2.81μm)跃迁的多声子驰豫速率.讨论了中红外荧光特性与Ho 3+ 离子掺杂浓度之间的关系.结果表明，在900nm 激光泵浦下观察到了2.81和3.86μm两处中红外荧光，分别对应于Ho 3+ ∶ 5 I 6 → 5 I 7 和 5 I 5 → 5 I 6 跃迁，当Ho 3+ 离子掺杂浓度从0.5wt%增加到1.0wt%时，两处中红外荧光强度都随相应增加，计算的Ho 3+ ∶ 5 I 5 → 5 I 6 和 5 I 6 → 5 I 7 跃迁多声子驰豫速率分别为29s -1 和34s -1 .

Fabrication and gain performance of Er3+/Yb3+-codoped tellurite glass fiber

Abstract Er3+/Yb3+-codoped TeO2 -ZnO-BaO-La2O3 tellurite glass fiber was fabricated by rotation and rod-in-tube technologies. The thermal stability and optical refractive index of the core and cladding glasses were determined by DTA and optical coupler, respectively. The average background loss of tellurite glass fiber was 1.8 dB/m at 1310 nm. Optical microscopy and field emission scanning electron microscope (FESEM) were used to study structural characteristics of preforms and optical fibers. The main loss of tellurite glass fiber could be attributed to scatter centre due to core-cladding interface defects. The amplifier performance of tellurite glass fiber was investigated by pumping with 980 nm laser diode (LD). The gain coefficient and maximum signal gain were 0.21 dB/mW and 10 dB, respectively, for a pumping power of 120 mW. Gains exceeding 5 dB were obtained over 30 nm bandwidth from 1535 to 1565 nm. The minimum noise figure was 4.8 dB at 1557 nm.

High Efficiency Infrared-to-Visible Upconversion Emission in Er3+,Yb3+, and Ho3+ Codoped Tellurite Glasses

A novel method of codoping the Er3+, Yb3+, and Ho3+ ions in tellurite glasses is demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Three intense emission bands observed in Er3+, Yb3+, and Ho3+ codoped tellurite glasses centred at 525, 547, and 657 nm correspond to Er3+: 2H11/2→4I15/2, Er3+: 4S3/2→4I15/2+Ho3+: 5S2(5F4)→5I8, and Er3+: 4S3/2→4I15/2+Ho3+: 5F5→5I8 transitions, respectively. No visible upconversion quenching phenomenon is observed when three rare-earth ions are codoped together in tellurite glasses. In contrast, the upconversion intensity of red and green emissions in Er3+, Yb3+, and Ho3+ codoped glasses is enhanced largely when compared with Er3+/Yb3+-codoped glasses. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms are evaluated. The three emissions are based on two-photon absorption processes.