Aoxiang Lin,1,3,* Xueming Liu,1 Pramod R. Watekar,2 Haitao Guo,1 Bo Peng,1
Wei Wei,1 Min Lu,1 Won-Taek Han,2 and Jean Toulouse3
1State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics,Chinese Academy of Sciences, Xi’an 710119, China
2Department of Information and Communications, School of Photon Science and Technology,Gwangju Institute of Science and Technology, Gwangju 500712, South Korea
([email protected])3Center for Optical Technologies and Physics Department, Lehigh University,
Upconversion lasers are among the most efficientsources of coherent visible and near-UV radiationand can provide practical solutions for deviceapplications, such as medical diagnosis and treat-ment, bioscience gene sorting, underwater surveil-lance and communications, red-green-blue all-soliddisplays, high-density data storage, high-resolutionprinting, and advanced scientific instrumentation[1,2]. Compared with frequency doubling schemes,upconversion lasers have a much broader pumpingwavelength tolerance, excellent output spatial-mode
quality, insensitivity to pump polarization, and highefficiency [2]. Although impressive progress in thedevelopment of GaN-based short-wavelength semi-conductor lasers has been made in the last few years,it is likely to take years before semiconductor laserscan produce diffraction-limited and continuous-waveoutput greater than 1W [3,4].
The strong sensitizing effect of codoped ytterbiumions on the emission of other rare-earth ions has beendemonstrated for several years [5,6]. Another advan-tage of codoped Yb3þ ions is that they can suppressthe concentration quenching effect by interferingwith the interaction between host ions [7]. Besidesthe already reported rare-earth ion systems forlasing applications, such as Er3þ, Ho3þ, Nd3þ,
Pr3þ, Pr3þ=Yb3þ, Tm3þ, Tm3þ=Yb3þ, and Tm3þ=Ho3þ[4,8–14], a group of newly upcoming Tb3þ=Yb3þcodoped systems, based on energy transfer upconver-sion (ETU) mechanisms, are motivating researchersto develop new green laser gain media. Although theTb3þ=Yb3þ codoped glasses and crystals have beendeveloped within the last decade [15–21], opticalglass fibers codoped with Tb3þ=Yb3þ may haveobvious advantages over the reported glasses or crys-tals because they are relatively inexpensive, compactand easy to install, length controllable, self-cooling atthe low-power level, and deliver a high-quality beamcompatible with current silica-based optical commu-nication systems.Initially and briefly investigated by Brown et al. in
1995 [22], Tb3þ=Yb3þ codoped silica-based fiberlasers have been rarely mentioned since, due tothe intrinsic high phonon energy of silicate materialsbelieved to decrease quantum efficiency for long-wavelength transitions [23]. However, as was demon-strated by Jackson [9] in Tm3þ=Ho3þ codoped silicafiber lasers, a high phonon energy does not necessa-rily set a practical limit to the laser quantum efficien-cy in codoped rare-earth ion systems if there exists anear-resonant ETU process. We have, therefore,worked to develop Tb3þ=Yb3þ codoped alumino-germano-silicate glass fibers as a practical laser gainmedium, overcoming the high phonon energy fromhost germano-silicate glass material. Experimentalresults of absorption, emission, and concentrationdependence are presented, and the mechanisms re-sponsible for the visible luminescence of these fibersare discussed in detail.
2. Experiments
Five Tb3þ=Yb3þ codoped alumino-germano-silicateglass fibers with different dopant concentration le-vels were fabricated in-house, using modified chemi-cal vapor deposition and solution doping techniques[24–30]. The concentration ratios in volume of Tb3þto Yb3þ were as follows: 1 : 4, 2 : 4, 4 : 4, 8 : 4, and12 : 4, as shown in Table 1. In order to preventpossible concentration quenching and improve theuniformity of the dopants, aluminum was also incor-porated in the fiber core. The absorption spectra ofthe fibers were measured with an optical spectrumanalyzer (OSA, Ando AQ6315B). A commerciallyavailable laser diode (LD) at 976nm (M-Tech. Co.Ltd., MSMF-10) with maximum power of 400mWwas used to measure the fluorescence of the fibers.
3. Results and Discussion
Figure 1 shows the absorption spectrum of one of theTb3þ=Yb3þ codoped fibers (Tb : Yb ¼ 2 : 4) fabri-cated, showing a typical broadband absorption inthe range of 910 − 990nm, corresponding to the2F7=2 →
2F5=2 transition of the Yb3þ ions [31,32].With different Tb3þ ion concentrations, but main-taining the same Yb3þ ion concentration, other fiberswith different concentration ratios (Tb : Yb ¼ 1 : 4,4 : 4, 8 : 4, and 12 : 4) also showed similar absorptionbands. It is important to note that the absorptionpeaks of Tb3þ appear in the deep and near-UV range,which is shorter than 400nm [33]. Therefore, nodirect absorption due to Tb3þ is observed in the 500 −
1600nm range in Fig. 1.To investigate the emission property of the fibers
upon pumping in the IR absorption band, the fiberswere pumped with a 976nm LD. Two emission bandsappeared, one in the visible from 480nm to 640nmand the other in the near-IR wavelength region from1000nm to 1200nm due to the Tb3þ ions and Yb3þions, respectively. However, no visible emission wasseen from the Tb3þ singly doped fibers when pumpedwith the 974nm LD and the 488nm argon-ion laser,which are beyond the typical UVabsorption bands ofTb3þ [22].
Since we are interested in the emission at visiblewavelengths for laser applications, the visible emis-sion bands from 480nm to 640nm are discussed ingreater detail. Four visible emission bands from480nm to 640nm are shown in Fig. 2(a), correspond-ing to 5D4 →
7FJ (J ¼ 6, 5, 4, and 3) transitions of theTb3þ ions. The four emission peaks appear at 490,546, 589, and 623nm in Fig. 2(a) [16–22], whichare due to 5D4 →
7FJ (J ¼ 6, 5, 4, and 3) transitionsof Tb3þ ions, resulting from possible cooperative ETUprocesses indicated in Fig. 2(b). Population inversionof Tb3þ ions in the 5D4 excited-state level wasthought to be produced through these cooperativeETU processes among a pair of Yb3þ donor ions
Table 1. Concentration of Dopants in Alumino-Germano-SilicateFibers
As shown in Fig. 2(b), Tb3þ ions radiatively relaxfrom the 5D4 level to the 7FJ lower lying levels, gen-erating the recorded visible emission. The intensityof the visible upconversion emission was investi-gated as a function of laser power and was foundto exhibit an approximately quadratic behavior,characteristic of the two-photon upconversion pro-cesses [23,36]. Among the emission peaks, the peakat 546nm (green emission) showed the highest inten-
sity for all fibers, indicating that the Tb3þ=Yb3þ co-doped alumino-germano-silicate glass fibers madeare suitable for green fiber laser application. Asshown in Fig. 3, the intense green fluorescence fromthe fibers is easily seen by the naked eye.
The emission intensity of the fibers was found toincrease with pump power, regardless of the dopants,but the Tb3þ=Yb3þ ratio played a key role. In order todetermine the optimum ratio, the emission intensityfrom fibers with different dopant concentrations wasmeasured for different pump powers, from 5mW to160mW. The results are shown in Fig. 4. Of allthe fibers fabricated, those made with a concentra-tion ratio of Tb : Yb ¼ 8 : 4 clearly resulted in thehighest emission intensity at 546nm. From the com-parison in Fig. 4, we conclude that an appropriate in-crease of the relative concentration of Tb3þ increasesthe probability of ETU from Yb3þ to Tb3þ under ex-citation with a 976nm LD, while, inversely, an excessof Tb3þ ions raises the probability of concentrationquenching and that of phonon-assistant backward
Fig. 2. (Color online) (a) Visible emission spectrum of theTb3þ=Yb3þ codoped alumino-germano-silicate fiber (Tb : Yb ¼2 : 4) and the energy level transitions responsible for the corre-sponding fluorescence peaks. (b) Energy level diagrams of Yb3þ
and Tb3þ ions to explain the two-photon assistant ETU processes.
Fig. 3. (Color online) Green light illumination along theTb3þ=Yb3þ codoped fiber tested during pumping experiments:from top to bottom: “in daylight,” “against dark background,”and “against dark background (enlarged).”
Fig. 4. (Color online) Effect of Tb3þ=Yb3þ concentration ratio onthe green photoluminescence intensity at 546nm.
energy transfer from Tb3þ to Yb3þ and, therefore,decreases the intensity of the observed green emis-sion at 546nm. The concentration ratio of Tb3þ toYb3þ, therefore, plays a key role in enhancing thegreen emission at 546nm and the optimum nominalconcentration ratio between Tb3þ and Yb3þ wasfound to be 8 : 4 or 2 : 1 in the case of alumino-germa-no-silicate optical glass fibers.
4. Conclusions
By overcoming the intrinsic high phonon energy fromhost germano-silicate glass materials, Tb3þ=Yb3þ co-doped alumino-germano-silicate optical fibers weresuccessfully fabricated with the goal of developingfiber lasers in the visible range of the spectrum,especially green 546nm fiber lasers. The macrolumi-nescent phenomenon was explained by a microco-operative ETU mechanism between Tb3þ and Yb3þ.For the chosen host alumino-germano-silicate glassmaterials, a 2 : 1 concentration ratio of Tb3þ to Yb3þwas found to optimize the green photoluminescence.The new Tb3þ=Yb3þ codoped glass system studied of-fers a new solution for the development of green fiberlaser applications.
This work was partially funded by grants60907039, 60807034, 10874239, and 10604066 fromthe National Natural Science Foundation of China(NSFC) and by the “Hundreds of Talents Programs”from the Chinese Academy of Sciences (CAS). The re-search work of Aoxiang Lin and Jean Toulouse isbeing funded by a grant from the National ScienceFoundation (NSF) by grant DMR-0701526. Fiberfabrication and characterization were financiallysupported by the BK-21 Information Technology Pro-ject, Ministry of Education and Human ResourcesDevelopment, by the National Core Research CenterProgram from the Ministry of Science and Technol-ogy (MEST)/Korea Science and Engineering Founda-tion (KOSEF) (R15-2006-022-02001-0), and by theGIST Top Brand Project (Photonics 2020) of MEST.
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