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778 J. Opt. Soc. Am. B/Vol. 26, No. 4 /April 2009 Ruan et al.

Spectral properties and broadband opticalamplification of Yb–Bi codopedMgO–Al2O3–ZnO–SiO2 glasses

Jian Ruan,1,2 E Wu,3 Botao Wu,3 Heping Zeng,3 Qiang Zhang,2,4 Guoping Dong,2,4 Yanbo Qiao,2,4

Danping Chen,2 and Jianrong Qiu5,*1State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics,

Chinese Academy of Sciences, Shanghai 201800, China2Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

3State Key Laboratory of Precision Spectroscopy, East China Normal University, Zhongshan North Road 3663,Shanghai 200062, China

4Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China5State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China

*Corresponding author: qjr@zju.edu.cn

Received October 27, 2008; revised February 3, 2009; accepted February 7, 2009;posted February 12, 2009 (Doc. ID 103164); published March 24, 2009

We reported on the spectral properties of Yb–Bi codoped MgO–Al2O3–ZnO–SiO2 glasses. The near-infraredluminescence intensity in Yb–Bi codoped glass was �31 times stronger than that of Bi-doped glass with980 nm laser diode excitation. No optical amplification was observed in Bi-doped glass, while apparent broad-band optical amplification between 1270 and 1340 nm was observed from Yb–Bi codoped glass with 980 nmlaser diode excitation. The highest optical gain and gain coefficient at 1270 nm of Yb–Bi codoped silicate glassreached to 1.57 dB and 7.84 dB·cm−1, respectively. Yb–Bi codoped silicate glass is a promising candidate forbroadband optical amplification. © 2009 Optical Society of America

OCIS codes: 160.2750, 250.4480.

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. INTRODUCTIONavelength division multiplexing (WDM) technology

ased on a rare-earth-doped fiber amplifier is significantor modern optical communication networks. It has highlyromoted the progress of the optical communication sys-em. However, the demand to increase the transmissionapacity of WDM systems is indispensable due to rapidevelopment of telecommunication and spread of the In-ernet. To meet these requirements, many investigationsave been focused on efficient broadband optical amplifi-rs operating at the telecommunication wavelength re-ion, especially at 1310 and 1550 nm. The Er-doped fibermplifier (EDFA), Pr-doped zirconium barium lanthanumluminum sodium fluoride (ZBLAN) fiber amplifierPDFA) [1], and Raman amplifier [2] were studied exten-ively. However, EDFA and PDFA cannot realize opticalmplification in the wavelength region beyond 80 nm,nd Raman amplifiers have a complex structure and re-uire high power consumption.Recent studies revealed that Bi-doped glass is a prom-

sing candidate for the broadband optical amplifier andunable laser, because it exhibits broadband near-infraredNIR) luminescence covering the 1.2–1.6 �m wavelengthegion [3–12]. From the viewpoint of practical application,ost efforts in the field were concentrated on Bi-doped

ilicate glasses. Broadband NIR luminescence and opticalmplification have been achieved in Bi-doped aluminosili-ate glass and fibers [4,10,11]. But high temperature1750 °C� and long melting duration �50 h� were required

0740-3224/09/040778-5/$15.00 © 2

or glass preparation [10]. Several studies have reportedchieving broader NIR luminescence and lower fabrica-ion temperature by adding alkali oxides or alkali-earthxides to the aluminosilicate glass. In particular, a full-idth at half maximum (FWHM) more than 500 nm haseen obtained in the photoluminescence (PL) spectra ofoth Bi-doped lithium aluminosilicate glasses with80 nm laser diode (LD) excitation [12]. Recently, opticalmplification and continuous-wave laser operation haveeen reported in Bi-doped silica fiber prepared by usinghe modified chemical vapor deposition method [13–15].owever, there are few reports on optical amplification ini-doped multicomponent glasses till now. According tour earlier research, this should be mainly ascribed to theow quantum yield of the NIR luminescence, since thetimulated emission cross section of Bi-doped silicatelasses is very small at those available wavelengths forbtaining NIR emission and optical amplification, such as00 and 980 nm LDs and Nd: YAG (yttrium aluminumarnet) laser at 1.064 �m.

For practical applications, it is necessary to increasehe stimulated emission cross section of Bi-doped multi-omponent silicate glasses with commercially availableDs. One probable approach is sensitizing Bi-related ac-

ive centers through an energy-transfer process betweenhem and sensitizers. Recent research has revealed thathe intensity and lifetime of Bi-related NIR luminescencean be largely increased through efficient energy transferssociated with Yb3+�2F – 2F � transition in Yb–Bi

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Ruan et al. Vol. 26, No. 4 /April 2009 /J. Opt. Soc. Am. B 779

odoped phosphate glasses [16]. Therefore, we also expecthat efficient optical gain in Bi-doped multicomponentilicate glasses can be achieved by this approach.

In this paper, we reported spectral properties and gainharacteristics of Yb–Bi codoped multicomponent silicatelass with a 980 nm LD as the pumping source. The spec-ral properties of Yb–Bi codoped with 808 nm LD excita-ion were also measured for comparison.

. EXPERIMENTALlasses with compositions of 3.9 MgO- 15.3 Al2O3-1.6 ZnO- 67.7 SiO2- 1.0 ZrO2- 1.5 Bi2O3- xYb2O3MAZS-x) (x=0, 2.0, 2.5, 3.0, 3.5, and 4.0 in mol%) and.9 MgO- 15.3 Al2O3- 11.6 ZnO- 67.7 SiO2- 1.0 ZrO2-Bi2O3- 3.5 Yb2O3 (MAZS-y) (y=0, 0.5, 1.0, 1.5, and 2.0 inol%) were prepared by the conventional melt-quenchingethod. Analytical reagents of SiO2, Al�OH�3,g�OH�2 ·4MgCO3·5H2O, ZrO2, ZnO, Bi2O3, and Yb2O3ere used as raw materials. Batches of �20 g wereelted in a corundum crucible in air at 1600 °C for 2 h

nd then cast into a stainless steel slab. The obtainedlass samples were cut into 2 mm thickness and polishedor optical measurements.

Absorption spectra were measured with a JASCO-570 spectrophotometer. The infrared luminescent spec-ra were measured by a ZOLIX SBP300 spectrofluorom-ter with an InGaAs detector excited with a 980 nm LD.he signals of fluorescent decay detected by an InGaAshotodetector in a TRIAX550 spectrofluorometer were re-orded using a storage digital oscilloscope (TektronixDS3052). All optical measurements were carried out atoom temperature. Characteristic temperatures wereeasured by differential scanning calorimetry (DSC) at a

eating rate of 10 °C/min in air.

. RESULTS AND DISCUSSIONigure 1 shows the transmission spectra of the MAZS-0nd MAZS-3.5 samples. The inset shows the correspond-ng absorption spectra between 800 to 1100 nm. Absorp-ion bands of Bi-related centers �Bin+� in silicate glassest 500, 700, and 800 nm are obvious in the absorption

ig. 1. (Color online) Transmission spectra of MAZS-0 and 3.5lasses. The inset shows the corresponding absorbance. Theample thickness L=2 mm.

pectrum [3]. Comparing with them, the absorption inten-ity at 980 nm is much weaker in single Bi-doped glassMAZS-0). The absorption intensity at 980 nm can beargely increased by Yb3+ codoping in the glass. Thishould be contributed to 2F7/2– 2F5/2 transition of Yb3+

ons [16]. Yb3+ ions exhibit not only a large absorptionross section but also a broad absorption band between00 and 1100 nm. There is an increment of �21 times inbsorption peak intensity at 980 nm compared with thatf single Bi-doped glass at most.

Figure 2 shows the NIR PL spectra of MAZS-0 andAZS-3.5 samples excited by a 980 nm LD at room tem-

erature. The dependence of emission intensity of Bin+

nd Yb3+ on Yb2O3 concentration is also presented in thenset of Fig. 2. Characteristic emission of 2F5/2– 2F7/2 tran-ition of Yb3+ ions at 1020 nm is observed in the spectrumf MAZS-3.5 glass. Both of MAZS-0 and MAZS-3.5 glassesave shown broadband NIR emission in the000–1600 nm wavelength range. The emission intensityf Bin+ at 1170 nm is highly enhanced in Yb–Bi codopedlass in comparison with Bi2O3 single-doped glass. Withhe increment of Yb3+ concentration, the intensity of bothin+ and Yb3+ emission increased at first and then de-reased. The decrease in intensity can be due to concen-ration quenching between Yb3+ ions when the Yb2O3 con-entration exceeds 4.0 mol%. There is an increment of31 times in the emission intensity of Bin+ as the Yb2O3

oncentration increases from 0 to 3.5 mol%. This can bescribed to efficient energy transfer from Yb3+ to Bin+.The NIR PL spectra of MAZS-0 and MAZS-3.5 samples

xcited by 808 nm LD were also measured for compari-on. As shown in Fig. 3, a strong NIR emission at275 nm along with a much weaker shoulder at 1020 nmre observed in MAZS-3.5 glass. They should be assignedo transition of Bi-related centers and 2F5/2– 2F7/2 transi-ion of Yb3+ ions, respectively. In comparison with singlei-doped glass (MAZS-0), the emission intensity of theAZS-3.5 sample is almost 2.2 times higher. This should

e also ascribed to the energy transfer between Yb3+ andin+ ions. The inset of Fig. 3 presents the dependence ofin+ emission intensity on Yb2O3 concentration. It is simi-

ig. 2. (Color online) NIR emission spectra of MAZS-0 and −3.5lasses pumped by 980 nm LD. The inset shows the dependencef Yb3+ and Bin+ emission intensity on Yb O content.

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780 J. Opt. Soc. Am. B/Vol. 26, No. 4 /April 2009 Ruan et al.

ar to those observed with 980 nm LD excitation. And theeak position of the NIR emission shows a red shift of25 nm with Yb2O3 additive. This indicates that the local

nvironment of Bin+ ions has been changed by additionalb3+ ions. However, because no obvious differences were

ound in the micro-Raman spectra of those glass samples,he changes should be mainly described as the dispersionffect of the Bin+ ions.

In order to estimate the potential applications of aboveamples in broadband fiber amplifiers and tunable lasers,he single-pass optical amplification is also measured.he traditional configuration adopted for internal gaineasurement and the LD used as a pumping source are

he same as described in the previous paper [16]. The ex-itation beam was manually chopped to make a probetate without excitation, I0, and with excitation, I. The in-ident intensity of the probe beam is defined as I0��I0 /T�, where T is transmittance. The internal net gainnd net gain coefficient are defined as 10 log�I /I0�� and0/L log�I /I0��, respectively, where L is the thickness ofamples �0.2 cm�. Figure 4 shows the wavelength-

ig. 3. (Color online) NIR emission spectra of MAZS-0 and −3.5lasses pumped by 808 nm LD. The inset shows the dependencef Bin+ emission intensity on Yb2O3 content.

ig. 4. (Color online) Wavelength-dependent internal net gainoefficient of MAZS-0 and −3.5 between 1270 and 1340 nm. Thexcitation power is 1.00 W.

ependent internal gain coefficient of MAZS-0 and 3.5ith 980 nm excitation. The points represented in Fig. 4re the measured optical gain from 1270 to 1340 nm. Noptical amplification phenomenon was observed in Bi-oped glass, while apparent optical amplification was de-ected in Yb–Bi codoped glass. There is a resemblance be-ween the measured spectral dependence of the opticalain coefficient and the fluorescence spectrum of Yb–Biodoped glass. The highest net gain and net gain coeffi-ient of MAZS-3.5 glass are 1.57 dB and 7.84 dB·cm−1 at270 nm, respectively. The values are larger than the re-orted results in Bi-doped aluminosilicate glass and fiber11,12]. The efficient gain from 1270 to 1340 nm demon-trates that the MAZS-3.5 glass can be used as a broad-and optical amplifier especially at the second telecom-unications window. Figure 5 depicts the results of

nternal gain at 1300 nm as a function of pumping power.he inset of Fig. 5 shows the signals with and withoutumping. It is observed that the gain coefficient increasesinearly with excitation power up to 1.00 W for MAZS-3.5lass. It is expected that the optical gain can be improvedy increasing the pumping power.Obvious enhancement in optical gain was acquired in

b–Bi codoped glass in comparison with single Bi-dopedlass. It resembles those that occurred in rare earth (RE)on-doped materials containing Yb3+ as a sensitizer. Opti-al gain from RE ions has been observed in glasses andrystals due to energy transfer between Yb3+ and RE ionenters. In our work, MAZS-3.5 glass exhibits �31 timestronger NIR emission. It could be ascribed to the effec-ive energy transfer between Yb3+ and Bi-related centers.

In order to estimate the efficiency of Yb–Bi energyransfer process, the Bi2O3 concentration dependent life-ime of Yb3+: 2F5/2 level in MAZS glasses containing.5 mol% Yb2O3 was investigated. The results were alsohown in Fig. 5. The energy transfer efficiency ��ET� wasalculated by following formula [17]:

�ET = 1 − �f�x�/��0�,

here �f�x� and ��0� were the 2F5/2 lifetimes of Yb3+ ionsith and without Bi2O3 additive, respectively. As shown

ig. 5. (Color online) Gain coefficient of MAZS-3.5 glass at300 nm as the function of excitation power. The line is a guideor eyes. The inset shows an oscilloscope image of the amplifica-ion phenomenon.

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Ruan et al. Vol. 26, No. 4 /April 2009 /J. Opt. Soc. Am. B 781

n Fig. 6, the highest �ET in the Yb–Bi codoped silicatelass is nearly 60%, which is �10% higher than that ofb–Bi codoped phosphate glass (the highest is �50%).The stimulated emission cross sections �em can be esti-ated from the Füchtbauer–Ladenburg equation by as-

uming a Gaussian-shaped emission band [7]:

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2

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here �0 is the peak wavelength, n is the refractive indexf the host material, � is the emission lifetime, and �� ishe FWHM of the emission. We can obtain �em=1.5210−20 cm2, with �0=1170 nm, n=1.57, �=520 �s, and

�=1400 cm−1 for the Yb–Bi codoped silicate glass, andem=0.9110−20 cm2, with �0=1200 nm, n=1.55, �900 �s, and ��=1330 cm−1 for the Yb–Bi codoped phos-hate glass in [16]. The product of the stimulated emis-ion cross section and the lifetime, �em�, is an importantarameter for characterizing laser materials, because theaser threshold is proportional to ��em��−1. The �em� prod-cts of the Bi-related centers in the Yb–Bi codoped sili-ate and phosphate glasses are 7.910−24 cm2 s and 8.110−24 cm2 s, respectively. The �em� product of the

lasses is about 4.6 times larger than that of Ti:Al2O3�em�=1.410−24 cm2 s�. It indicates that both Yb–Bi

ig. 6. Bi2O3 concentration dependent lifetimes of Yb3+: 2F5/2evel and the calculated energy transfer efficiency ��ET� in Yb–Biodoped silicate glasses.

ig. 7. Probable energy transfer mechanism in Yb–Bi codopedlass. NIR: near-infrared emission.

odoped silicate and phosphate glasses are promising ma-erials for optical amplification.

The mechanism of infrared luminescence from Bi-oped glasses is still in controversy. We ascribe the originf the luminescence to “active bismuth,” e.g., Bi2+ or Bi+

18,19]. The energy transfer mechanism in Yb–Bi codopedlass was given in Fig. 7. Addition of Yb3+ ions into Bi-oped glass results in effective absorption at 980 nm dueo Yb3+ ions. When the glass was excited with 980 nm LD,b3+ ions were excited to 2F5/2 level, then they transfer

he energy to “active bismuth” centers, and return toround state 2F7/2.

Figure 8 shows the DSC profile of the host glass. Theransition temperature �Tg�, the crystallization onset tem-erature �Tx�, and the crystallization temperature �Tc�re 712, 921, and 976°C, respectively. Tx–Tg is 209°C,hich is the index of glass thermal stability against crys-

allization. The Tx and Tx–Tg values of ZBLAN glassesre not more than 270°C and 102.5°C, respectively. Thetudied glass shows better thermal stability compared toBLAN glass, which indicates that it could be drawn intober without crystallization.

. CONCLUSIONSn summary, spectral properties of Yb–Bi codoped silicatelasses were investigated. Enhanced broadband NIR lu-inescence and efficient optical gain have been observed

n Yb–Bi codoped silicate glass with a commercial 980 nmD. The NIR luminescence excited at 980 nm increasesreatly with the additional Yb2O3, owing to the sensitiza-ion of Yb3+ for the active bismuth centers. Yb–Bi codopedilicate glass is a promising candidate material for broad-and fiber amplifiers and tunable lasers.

CKNOWLEDGMENTShis work was financially supported by the Nationalatural Science Foundation of China (NSFC) (Grant Nos.0872123 and 50802083), the National Basic Researchrogram of China (2006CB806000b), and the Program forhangjiang Scholars and Innovative Research Team inniversity (IRT0651).

Fig. 8. DSC curve of MAZS glass.

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