August 1994

PT~CAL-

ELSEVIER Optical Materials 3 (1994) 187203

Tellurite glass: a new candidate for fiber devicesJ.S. Wang, E.M. Vogel 1, E. Snitzer

Rutgers University, Fiber Optics Materials Research Program, Piscataway, NJ 08855, USA

Abstract

The physicalpropertiesof R20-ZnO-TeO2 glasses have been studied for their feasibility for fiber drawing and rare earth dop-ing. A telluriteglass fiberwith less than 1 dB/m loss hasbeenmade by therod-in-tube method. The spectroscopicpropertiesofrareearthions (Pr

3~,Nd3~,Er3~,and Tm3~)in tellurite glass are discussed and compared with silica, fluoride andchalcogenideglasses.

1. Introduction given to optical fiber amplifiers which were firstdemonstrated in neodymium-doped glass fiber in

The first glass laser was obtained in a barium sili- 1965 on a pulse basis [2] and later with CW pump-cate glass [1]. This result initiated a new application ingby a laser diode [3]. However, the results did notfor glass. It is clearly evident that silicate glass has attract much attention until the mid-1980s whenbeen most extensively studiedas a laserhost because work with an erbium amplifier showed useful gainsofits excellent glass stability, chemical durability, and of40 dB or more when pumped with single mode lasergood fluorescence properties. However, with the diodes of 30 mW or less [4].widespread use of optical components for telecom- While erbium-doped fiber amplifiers operating atmunications, sensors and medical applications, new the 1.5 jim windows are commercially available now,glass hosts have emerged for special usage. They in- the majority of already-installed telecommunica-dude the less common oxides, halides and chalco- tions fibers are optimized for the 1.3 jim window. Be-genide glasses. Furthermore, with the success of fiber cause changing the fiber fort the 1.55 jim windowtechnology for optical communication, a variety of would be costly, 1.3 jim amplifiers have attracted soresearch efforts have been undertaken in glass and much attention.glass fibers. This diversity includes new applications, Nd3+ and Pr3 + are two active rare earth ions thatsuch as the use of ultraviolet-induced photo-refrac- have been investigated as potential candidates in ative gratings, and new glass families, particularly, variety of glass hosts since each has emission bandsthose that canbe fabricated into fiberwaveguides with near 1.3 jim. Recently, another rare earth ion, Dy3 ~,low loss. One of the very important advantages of in chalcogenideglass emitting at 1.3 jim with reason-glass is its ability to be fabricated into a fiber with able quantum efficiency based on lifetime measure-ultra low light attenuation.This also made fiberoptic ments also has been demonstrated [5]. A praseo-telecommunication possible and growing. dymium-doped fluoride fiber amplifier announced in

Recently there has been considerable attention 1991 [6] comes closest to a practical 1.3 jim fiber_______ amplifier todate. Apeak gain of 38.2 dB witharoundBelicore, Morristown, NJ07960, USA. 1.31 jim was obtained with a launched pump power0925-3467/94/$07.00 1994 Elsevier ScienceBy. All rights reservedSSD! 0925-3467(94)00043-P

188 .1.5. Wang et al. / Optical Materials 3 (1994) 18 7203

of 300 mW [7]. The high pump power required is amplifiers at long wavelength are possible in oxidedue to the low quantum efficiency (3%) of this pra- glasses, for example, Pr3 + at 1.3 jim. A detail discus-seodymium-doped fluoride glass. More recently, sion is given in section 3.3.2.1. It is also believed thatDejneka et al. [8] has found another fluoride glass tellurite glasses could have better glass stability andhost for Pr3 ~, based on Pb-In-Ga-Zn-F [9], to have chemical durability than halide glasses [12], betterlonger measured lifetime. The fluorescence rate of glass stability than chalcogenides, and better compat-Pr3~in La-Ga-S glass has also been reported [10], ibility with other oxide glasses than the nonoxidealthough there has been no report to date on their glasses. Another interesting fact is the degree offiberization. covalence among the four glass family, as given in

Although many fiber lasers and fiber amplifiers Table 1, decreases in the order: Chalcogen-have been studied for both silica-based and fluoride- ide> Tellurite> Silica> Fluoride glasses, which di-based glasses, the former suffer, in some cases, from rectly affects the wavelength of the transition peak.their inherent highphonon energy and the latter from Although tellurites havebeenknown for some timetheir poor glass stability and chemical durability. On as stable glass formers and selected properties of somethe other hand, chalcogenide glasses have not been rare earth ions in tellurite glasses havebeen reportedmuch studied. One of the major difficulties is their [13,14], their utility has not been extended to fiberlow solubilities for rare earths. However, Wei et al. optics where advantage could be taken of their non-[11] have recently demonstrated that the Ge-Ga-S linear propertiesand the desirable fluorescence prop-based glasses canbe madewith only a yellow tint and erties associated with their low energy phonon spec-with a high rare earth solubility. trum and high indices of refraction. Our goal is to

Tellurite glasses combine the attributes of (i) a fabricate active tellurite glass fiber devices based onreasonably wide transmission region (0.355 jim), the laser action of rareearth ions in tellurite glass and/versus only 0.23 jim for silicate glasses, (ii) good or on the nonlinear properties of tellurite glasses.glass stability and corrosion resistance, which pres- Some recent work wasdone to the searching the glassent difficulties in fluoride glasses, (iii) a relatively compositions [15] and investigating the spectro-low phonon energy among oxide glass formers, and scopic properties of various rare earth ions in tellur-(iv) high refractive index and high nonlinear refrac- ite glasses [16]. In this paper, the emphasis is placedtive index, which are generally low in both fluoride on the material properties of the TeO

2-based glassesand silicate glasses. A comparison of the selected op- and fibers, and the unique spectroscopic propertiestical properties among tellurite, silica, fluoride and in the near infrared for fiber laser oscillators andchalcogenide glasses is shown in Table 1. The high amplifiers.nonlinear refractive index and the low phonon en-ergy make the tellurite glass fibers uniquely suitablefor nonlinear and laser applications. For example, the 2. Experimental procedurelower phonon energy results in a lower nonradiativetransition rate between adjacent rare earth energy 2.1. Preparation ofbulk glasslevels, leading to fluorescence and laseremission fromadditional energy levels that are not possible for sili- High-purity commercial oxides (99.999% andcate glasses. The longest fluorescent wavelengthsthat 99.99% pure) were used as the starting materials.could be observed are about 2.8 jim, 2.2 jim, 4.4 jim, Powders of these materials, weighed to conform toand 7.4 jim for tellurite, silica, fluoride and chalco- the oxide molar percentages as indicated in the text,genide glasses, respectively. This is due to difference were mixed and than transferred to a gold crucible.in the highestphonon energy for different glass hosts, The crucible was heated to 8000 C in a resistance fur-as shown in Table 2. nace, and held in an air atmosphere for 2 hours. Melts

With lower phonon energy, Te02-based glasses in- were cast into preheated cylindrical brass molds andtrinsically could havehigher quantum efficiencies and allowed to cool to room temperature. Samples 0.51could provide more fluorescent emissions than sil- cm long were cut from these glasses, and the faces wereica-basedglasses. This implies that newfiber laser and polished to a 0.5 jim finish.

J.S. Wanget al. / Optical Materials 3(1994) 187203 189

Table 1A comparison ofselected properties for tellurite, silica, fluoride and chalcogenide glasses.

Property Tellurite Silica Fluoride Chalcogenide

Opticalproperties (typical values)Refractive index (n) 1.82.3 1.46 1.5 2.83Abbenumber(v) 1020 80 60110Nonlinearrefractive index (n2, m

2/W) 2.5x i0~ l020 l0_21 higherTransmission range (ism) 0.45.0 0.22.5 0.27.0 0.816Highest phonon energy (cm) 800 1000 500 300Longest fluorescent wavelength (i.tm) 2.8 2.2 4.4 7.4Bandgap(eV) ~3 ~l0 13Acousto-optical figure of merit, 24 119

p2n6/rv3 (10_li ~3/g)Physical properties (typical values)Glass transition (Tg, 0 C) 300 1000 300 300Thermalexpansion(107C) 120-170 5 150 140Density (g/cm3) 5.5 2.2 5.0 4.51Dielectric constant (~) 1335 4.0 Fiberloss 0.2 dB/km 1525 dB/km 0.4 dB/km

(1.5 ~m) (1.52.75 ~m) 6.5 ~tmBonding covalent-ionic ionic-covalent ionic covalentSolubilityinwater

190 J.S. Wang et al. / OpticalMaterials 3 (1994) 18 7203

loscope screen. Six hundred scans were routinely av- 3. Results and discussioneraged in making the measurements.

3.1. Material aspects oftellurite glass andfiber2.3. Preform preparation andfiberdrawing

To qualify tellurite glass as a practical fiberdevice,The melting procedure for making a core for a pre- the glass must posses gooddoping solubilitywith var-

form is similar to the preparation ofbulkglass. A sue- ious rare earth ions. It also must have enough glasstion method was employed to make the tube for the stability for drawing fibers. It shouldbe good enoughcladding. Then the rod-in-tube technique was em- for different core-cladding configuration redrawingsployed for fabricating fiber. In orderto achieve a sin- without sacrificing the glass stability. Additionally,gle mode fiber, a double rod-in-tube draw had to be glass fibers should have good chemical durability,performed because the ratio of core radius to clad- good mechanical strength, and reasonableoptical loss.dingwall thickness was not small enough. In fact, the need for low loss fiberalways imposes re-

quirements for ultra-highpurity starting materials and2.4. Thermal study very clean processes during fiber fabrication.

The selection of glass composition for fiber fabri-cation is a very important task and is often a compro-Thermal analysis was employed to determine themise among many factors. A briefevolution of glass

effect ofglass composition on glass stability. Samplesweighing 10 to 20 mg were sealed in aluminium pans composition development in the early stage of thisand heated at a rate of 10C/mm from 250 to 550C study is given in Table 3. While glasses with BaO orin a either Perkin-Elmer DSC7 or DSC4. Because of WO3 modifiers have good durability and rare earth

solubility, they appear yellow in color. With alkalithe temperature limitation of aluminium pans, DTAoxides present, the glasses have poor chemical dura-

was used to determine the liquids temperature (T1),glass transition temperature (Tg) and crystallization bility. ZnO-TeO2 glass has a short wavelength UV

edge and good durability but low rare earth solubil-onset temperature (Tx) for high temperature glasses.

ity. To solve this drawback, the addition of sodiumoxide in the ZnO-Te02 glasses significantly improves

2.5. Viscosity measurement the rare earth solubility (see Table 3) without affect-ing the other two properties. Later, it was observed

A Theta Instruments parallel plate viscometer was that Na2O could be replaced by other monovalentutilized toestablish the temperature needed to achieve ions, such as Li20, K2O, Rb20, Cs20, or Ag20, with-the appropriate viscosity for fiber drawing [18]. out deteriorating the UV edge, chemical durability,Samples 35 mm thick were prepared with flat and and rare earth solubility. Thus, for the R2O-ZnO-Te02parallel faces and heated at a rate of 1 C/minfrom system, R can be Li, Na, K, Rb, Cs, or Ag or combi-200C toa point where the samples had decreased in nations thereof. Also, the relative amounts of thesethickness tobelow 0.5 mm. constituents canbe varied overa wide range without

seriously affecting glass stability. Therefore, a wide2.6. UV- VIS absorption measurement range of compositions canbe achieved to match spe-

cific needs in the R20-ZnO-Te02 family.Absorption spectra were obtained using a Perkin

Elmer UV/VIS/NIR Lamda 9 double beam spectro- 3.1.1. Viscosityphotometer. The glass cylinder was usually scanned Since drawing fiber is a process involving a dy-from 3200 nm to 300 nm in air at ambient tempera- namic deformation at high temperature, it is neces-ture after running a background with air as a refer- sary to determine the relationship between viscosityence. Instrumental parameters were set as follows: and temperature in the glass. Fig. 1 shows the viscos-absorbance mode, slit width 0.8 nm, scan speed 60 ity data obtained through the use of the parallel platenm/mm and response time 0.5 s, which has good viscometer, which is suitable for measuring viscosi-enough resolution for glass samples. ties ranging from 1 0~to 1 0~poise. A stable glass is

J.S. Wang eta!. / OpticalMaterials 3 (1994) 187203 191

Table 3Properties ofselected tellurite glasses.

(l00x)%Te02+x%X UVedge (nm) C Durability (H20) RareearthX = (mol%) solubility

15%BaO 487 good good30%W03 458 good good1015% alkali oxides 370 poor good28%ZnO 373 good poor20%ZnO+5%Na20 370 good good20% ZnO+5% K20 365 good good

CAta5 cm

Temperature (C) Temperature (C)

~ 384 374 364 354 ~ 334 324 314 13000 8~7 677 577 ~ ~ .10000 ~ I _______W03-Te02 Na20-ZnO Te02~ 9.000 C.4 -o 10.000

8000 ~ Na2O.SI~,,,J Na20-zno ?2 ,,,IZBLAL4 , 7000

7.000 ____ ____ ____ ____ ____ 5 / ________ ________

6.000 ,..,...,,.......... 40001.5 1.54 1.58 1.62 1 66 1.7 08 1,2 16 2

1 000/T(K( 1 000/T)K)Fig. 1. The viscosity versus temperature ofWO3-TeO2 and Na20- Fig. 2. The viscosity-temperature behavior of tellurite (center),ZnO-Te02 glasses. silicate (left), and fluoride (right) glasses (ZBLAL=ZrF4-BaF2-

LaF3-AIF3-LiF).

characterized by rapid but smooth changes in viscos-ity in the entire region from 102 to 1014 poise. A vis 1~=1~oexp(E~/RT), (1)cous flow curve exhibiting a discontinuity or uns-mooth variation may be attributed to structure where i~is the viscosity and E~the activation energychange, perhaps devitrification, in the glass. for viscous flow. This Arrhenius-type relation rarely

The temperature dependence of the viscosity is holds for polymeric melts over a wide temperatureshown for Na20-ZnO-Te02, Na20-SiO2, and ZBLAN range, such as glass, because of depolymerization withglasses in Fig. 2. There are two important parameters increasing temperature. However, the linearity of thewe can derive from viscosity-temperature curve: (i) data in Fig. 2 with 1 / T indicates that the viscositythe temperature corresponding to fiber drawing vis- obeys the Arrhenius equation (1) at both the highcosity and (ii) the slope of te viscosity-temperature and low temperature regions, but not in the interme-curve in the low temperature region. Hu et al. [19] diate temperature region. The curve-fitting results forhave proposed that glasses with a flat viscosity-tern- Na20-ZnO-TeO2, ZBLAN,and Na2O-Si02 are givenperature curve are more resistant to crystallization in Table 4. A high activation energy for viscous flowthan glasses with steep one in the low temperature is indicative of a very steep viscosity-temperaturerange. Although the viscosity required for the fiber profile, which shortens the working-temperaturedrawing in the silicate glass is around 10~poise, it range. Additionally, the activation energy differencecould be slightly higher, around 1 ~ to 1065 poise, between the low and high temperature regions mdi-for the fluoride and tellurite glasses. cates the differences in the glass deformation struc-

The temperature dependence of the viscosity can ture Silicate glass exhibit~onlya small difference, tel-be roughly represented by the exponential equation lurite glass some difference, and ZBLAN a large

192 .1.5. Wang et al. / Optical Materials 3 (1994) 187203

Table 4Fitting parameters for Eq. (1) in variousglasses.

Glass High temperature region Low temperature regionii=,~oexp(E~/RT)

E,, (kcal/mole) lo E~(kcal/mole)

Na20-ZnO-Te02 1.6x109 33.6 i.4X 1021 7.9

W03-Te02 l.6x i0~ 41.4 l.3x 1021 1.9

ZBLAN 4.4x 10~2 37.6 8.9x i0~~ 103.6Na20-Si02 6.5X10

5 31.8 6.2x108 49

30.difference. This imposesgreatprocessingdifficulty for _-ZBLAN glasses. Attempting to reduce the activation 25. mote%Na

20

energy for viscous flow, or to keep it constant during 20temperature changes, was a major research effort influoride glasses [20]. 15. mule%Na20

The curve fitting results (derived from Fig. 1) for 10WO3-Te02 glasses are also listed in Table 4. They allshow a certain degree of glass structurevariation with ~, to mole%Nu20the temperature changes. 0

200 250 300 350 400 450 500 550 600 650 700

3.1.2. Thermal analysis Temperature (C)To investigate the glass stability, we used a differ- Fig. 3. DTA traces of ZnO-Te02 glasses with various concentra-

ential scanning calorimeter (DSC) and a differential tions of Na20.thermal analyzer (DTA). The purpose ofusing DTAinstead of DSC is to measure the liquidus tempera- one of our objectives, it is natural tostudy the effectsture of glasses, because the maximum temperature of rare earth oxide on glass composition. Althoughemployed in DSC is around 550Cif an aluminium P205-Te02 glass has a very large (Tx Tg), Er2O3 ad-pan was not replacedwith the goldsample holder. The dition decreases its stability from 147Cdown toglass transition temperature (T5), crystallization on- 101 C. A similar effect has beenobserved for 28ZnO-set temperature (Tx), and liquidus temperature (T1) 72TeO2 glass.are important information in the thermal analysis. For 5Na2O-2OZnO-75TeO2 glass, the (Tx 1s), i.e.The glass transition temperature, which corresponds glass stability, increases with Er203 doping. Table 6to a viscosity of 10i2_ 1013 P, was defined as the tern- shows the effect of rare earth doping (using Pr2O3,perature region inwhich the behaviorof the material Er2O3, Tm203, and Yb2O3) on the glass stability. Itchanges from solid-like to liquid-like [21]. The crys- is clear that all four rare earths improve the glass sta-tallization temperature indicates the region in which bility for the 5Na2O-2OZnO-75TeO2 family. As wethe glass viscosity is sufficiently low to permit rapid cansee, the increases in (T~ Tg) mainlyresult fromcrystal growth. It can be defined as the extrapolated the increase in the crystallization temperature (Tx).onset of the first crystallization exotherm. The quan- This suggests that the rare earth dopinginhibits crys-tity of T~ 7~has been frequently used as a rough tal formation. The dependence of (T~ Tg) withmeasure ofglass stability. To achieve a large working Er203 concentration has been investigated, as shownrange during operations such as preform preparation inTable 6. The experimental results indicate that thefor fiber drawing, it is desirable to have (T~ Tg) as glass stability increases (3.9 wt% Er2O3) and thenlarge as possible [22]. Typical DTA traces are illus- decreases with increasing Er2O3 concentration. How-trated in Fig. 3 for the Na2O-ZnO-TeO2 glasses. ever, even with 7.5 wt% Er2O3 doping, the glass sta-

Table 5 lists T5, T~and (T~ Is) for some tellurite bility does not decrease in comparison with the baseglasses studied. Since doping with rare earth ions is glass.

J.S. Wanget al. / OpticalMaterials 3(1994) 187203 193

Table 5The Tg, T~and (T~ Tg) of tellurite glasses.

Glass(Mole%) Tg(C) T~(C) TxTg

l0%P205-90%Te02 347 494 14710% P205-90% Te02-l wt% Er203 330 431 10128%ZnO-72% Te02 324 420 9628%ZnO-72% Te02-1 wt% Er203 330 425 955% Na20-20% ZnO-75% Te02 299 417 1185% Na20-20% ZnO-75% Te02- 1 wt% Er203 304 432 12815%W03-85%Te02 343 467 12415%WO3-85%Te02-0.5w1%Er203 357 505 1481l%BaO-89%Te02 325 468 14310%BaO-20%ZnO-70%Te02 339 495 1566%Na20-94%Te02 294 338 44

Table 6The Tg, T~and (T. Tg) of rare earthdoped tellurite glasses.

Glass(Mole%) Tg(C) T~(C) T~T~5%Na20-20%ZnO-75%Te02 299 417 118

+lwt%Pr203 302 452 150+lwt%Er203 304 432 128+lwtkTm2O3 298 424 126+lwt%Yb203 303 430 127

5% Na~O-20%ZnO-75% Te02 299 417 1185% Na~O-20%ZnO-75% Te02-1 wt% Er203 304 432 1285% Na20-20% ZnO-75% Te02-3.9 wt% Er203 311 447 1365% Na20-20% ZnO-75% TeO2-7.5 wt% Er203 324 443 119

To illustrate the effects ofNa2O additionon T, T~, ~ I 6 7w1% Nd 0T1 and glass stability in ZnO-Te02 glasses, DTA traces 25are given in Fig. 3. Comparing the three traces (0

20 2 7w1% Nd203mole%, 5 mole% and 10 mol% Na20), one can con-dude the following: 15 1 4wt% Nd203 /\

The crystallization temperature (Tx) increasesvery slightly with increasing Na2Oconcentration. O5~%Nd 0 /\

The glass transition (Tg) temperature decreases. 5Therefore, with unchanged T~and decreased 7s, themagnitude of (Tx Tg) definitely increases so that 200 ~ ~ , 5 . . , . .glass stability increases. Temperature )~C)

The liquidus temperature (T1) decreases and also Fig. 4. DTA traces of 5Na2O-2OZnO-75TeO2 glasses with variousbecomes broad. concentrations ofNd203.

DTA curves of x wt% Nd2O3 in 5Na2O-2OZnO-75TeO2 glasses (x=0.05 wt%, 1.4%, 2.7%, and 6.7 the intensity of the crystallization exotherm iswt%) are illustrated in Fig. 4. The effects of Nd203 reduced;concentration on Tg, T~,and T1 are clear. With in- the glass transition temperature only slightly in-creasing Nd203 concentration, creases; and

the crystallization temperature increases; the liquidus temperature increases slightly.

194 J.S. Wang eta!. / OpticalMaterials 3(1994) 18 7203

According to Hrubys criterion: to be 3.02 jim, which are both comparable to theHR(T T )/(T T) ~2 fluorideglass. Butthelossisaboutoneorderofmag-

x g I C nitude better than the chalcogenide glass. The V curvewhere HR (Hrubys Ratio) is a measure of glass sta- for the tellurite glass is given in Fig. 5. With OHbility. The closer T1 is to T~(the maximum ofexoth- absorption present at 3.33 and 2.86 jim, the pre-erm), the better is the glass stability. Nd2O3 addition dicted minimum loss would be shifted to 1.8 x 1 0_2not only increases (Ti Tg) but also decreases dB/km at 1.9 jim. However, the measured minimum(T1 T~)for 5Na2O-2OZnO-7 5TeO2 glasses, both of loss for fiber is about 0.9 dB/m at 1.35 jim. The highwhich are favorable to improving glass stability. Note loss are probably due to the starting powder impuri-the different roles of Nd203 and Na2O play in in- ties, crucible contamination, and melting in air. Thecreasing glass stability. The former may participate fundamental lattice vibration of tellurite glass, ob-in prohibiting crystalline formation whereas the lat- served in thin film samples, is 690 cm~,as shown inter act mainly as network breaking agents to decrease Fig. 5.the liquidus and glass transition temperatures. Presently, we are using the (i) rod and tube method

and (ii) suction technique for preform fabrication.3.2. Tellurite glassfiber The result is a fiber geometry with core diameter 5

jim (core composition Nd203-l.5Bi2O3-6Na2O-3.2.1. Fiber loss 1 5.5ZnO-77TeO2) and overall fiberdiameter 125 jim

To estimate the intrinsic minimum loss of tellurite (cladding composition 5Na2O-2OZnO-75TeO2).glass fiber, the V curve is generated by the following Tellurite glass fiberswith losses less than 1 dB/m haveequations [23]: been achieved. The loss spectra are shown in Fig. 6.

4 Tellurite glass fibers with 1.4% fracture strain, mea-a0=A0(l/..t )+B0exp(B1/).)+C0exp(C1/A),(3 ,~ sured by the two point bending test, have been ob-

/ tamed, as listed in Table 8. However, many tech-where A0, B0, B1, C0 and C1 are constants. The first niques have also been developed to introduce rareterm indicates a loss that is due to light scatteringfrom earth ions in the core of the fiber, i.e., solution dop-microscopic density and composition fluctuations in ing [24], chelate vapor deposition [25], aerosolthe material. These effects decrease rapidly with in- doping [26], the sol-gel method [27], and the pow-creasing wavelength. The second and third terms de- der-in-tube method [28].scribe, respectively, losses due to ultraviolet absorp-tion from the electronic band edge (Urbach tail) and 3.3. Spectroscopic properties of rare earth in telluriteinfrared edge losses arising from multiphonon ab- glasssorption. With an assumption ofa single componentof tellurite glass, the first term could be calculated by In contrast to silicate, borate, fluorophosphate andthe followingequation, as described in Ref. [23]. fluoride glasses, which havebeen investigated exten-

~ 3 8 2Q1 p 8 31 2 ~2~I ,, sively as laser glasses, tellurite glasses have not re-03~ n p ,,~ -~~r~,n 1, ~ , ~ .cerred much attention. Among the many lasing

where n is the refractive index of glass, p is the aver- wavelength of rare earth ions, only one rare earthage photoelastic constant, /3 is the isothermal corn- transition (Nd

3~at 1.06 jim) has been reported in apressibility, Tg is the glass transition temperature and bulk tellurite glass [29].k is Boltzmanns constant.

The second and third terms of Eq. (3) are used to 3.3.1. Nonradiative relaxationfit the experimental data on the UV and infrared Since the role of multiphonon electron-lattice re-spectra of fibers, bulksand film. The fitting parame- laxation is so critical in determining the efficiency ofters for the tellurite, fluoride and chalcogenide glasses fluorescence, studies of the multiphonon nonradia-are listed in Table 7. The projected minimum loss of tive rate in various hosts have been reportedthe tellurite glass fiber is calculated to be around [9,30,31]. They have shown that when the phonon3.6 x 1 0~dB/km and the minimum loss wavelength occupation number is smaller than 1 and coupling

J.S. Wang eta!. /Optical Materials 3 (1994) 18 7203 195

Table 7Fitting parameters for Eq. (3) in various glasses.Glass [Ref.] Rayleigh scattering Ultraviolet absorption Infra-red absorption Projected minimum

loss (dB/km) loss (dB/km) loss (dB/km) loss (dB/km)(atA [jim])

Te02 [this work] 0.29/A4 6.47x l06 5.75x 1015 3.5x l0~

exp(9.84/A) exp( 126.67/A) (3.02 jim)5i0

2 [50] 0.70/A4 3.4x 1011 l.2xl0~

exp(31.4/A) (1.55 jim)BaF

2-GdF3-ZrF4[23] 0.112/A4 2.56x10 2.82xl06 l.1x103

exp(6.76/A) exp(163/A) (3.44 jim)GeS

3 [23] 3.97/A4 5.26x l0~ 5.63X 1012 1.1 X 102

exp(15.2/A) exp(l64/A) (4.54 jim)

16Fundamental Lattice Vibratiorr

Table 812 Comparison of fiber strengths of various glasses in two-point

OH Absorption b d

8 1k _______________ Film Fracture strain Tellurite glass Silica Fluoride glass

\ ~~Ik (%) fiber fiber fiber1.4% 67% 0.42%

.4 2, 3 with the phonon is weak, the internal multiphonon0.2 Log(Wavelength, 5m( 10 30 relaxation transition probability, Wm~,can be given

byFig. 5. The projected loss spectrum and measured minimum lossfor Na

20-ZnO-Te02 glass fiber; Eq. (3) was employed to fit the Wmi.t = /3[ n (w, T) + 1 ] exp ( azS.E) , (5)experimental data on UV and infrared spectra of fibers, bulks,and film. Curve 1: electronic edge; Curve 2: infrared edge with where /3 is a constant characteristic of the host mate-OH presence; Curve 3: infrared edge without OH presence; rial, LiE is the energy gap between two successive 1ev-and Curve 4: Rayleigh scattering. els, and a is expressed by

a=(hw)_i(ln p l\. g(n+l)3500 __--- = (hw)~ln(e) , (6)

.3000E N where g is the electronphonon coupling constant (~

~25OO ~z~-~7;;~ is the coupling constant), hw is the energy of the~ 2000 ~ phonon which contributes predominantly to the re-

1500 .~ laxation process,pis the numberofphonons emittediooo ~ma= _~ intheprocess,namelyp=AE/hw,andnisthephonon

500 ~__ occupation number defined through the Bose-Em-0 stein equation as1000 1100 1200 1300 1400 1500 1600

Wavelength )nm) n = [exp(hw/kT) 1] , (7)Fig. 6. The absorption spectraof tellurite glass fibers. Core: Nd2O3-1 .5Bi2O3-6Na2O-l 5.5ZnO-77TeO2. Cladding: 5Na2O-2OZnO- a, /3 and e are dependent on the hostbut independent75TeO2. (a) Core diameter 45 jim, fiber OD 85 jim. (b) Core of the specific electronic level of the rare earth fromdiameter 5 jim, fiber OD 155 jim. which the decay occurs.

196 J.S. Wang et al. / OpticalMaterials 3 (1994)187203

From the equations above, it is clear that for a given pulling possible. Their high index is also desirable,glass the probability of non-radiative relaxation be- since it increases the local field correction at the raretween two electronic levels decreases with increases earth site, leading to large radiative transition prob-in the energy gap AE and in the number of phonons abilities [32]. The fluorescence spectrum of a PrSh~required to bridge the gap, provided that there is no doped Te02 glass, pumped at 1.02 jim, is shown insignificant change in the value of e. Table 9 presents Fig. 7. The peak fluorescence appears at 1.33 jim andtypical values for various glasses [48,49]. Therefore, the bandwidth is around 90 nm. Table ] 0 summa-the nonradiative transition rate decreases in the rizes the Judd-Ofelt parameters as well as calculatedsequence: Phosphate> Silicate> Germanate> Tel- and measured lifetime values. The fluorescent life-lurite> Fluoride> Chalcogenide. Tellurite glasses timecalculated from Judd-Ofelt parameters is 460 jishave the lowest nonradiative transition rate among and the measured lifetime is 22 jis. Therefore, the ra-oxide glasses. In the case of 5Na2O-2OZnO-75TeO2, diative quantum efficiency is expected to be aroundthe highest phonon energy is about 690 cmi, based 4%. The e-folding times obtained from fluorescenceon infrared measurements of glass film. It is interest- measurements suggest that the quantum efficiencyofing to note that while chalcogenides have the lowest tellurite glasses is comparable to that of the first re-nonradiative rate, their coupling constants are one ported Pr

3~in ZBLAN fluoride glass [6].order of magnitude higher than heavy metal oxide Anotherway of increasing the performance oftheseglasses. glasses is by codoping with another rare earth ion in

order to improve absorption of the pump. The flu-3.3.2. Spectroscopic properties ofPr~,Nd3t Ho3~, orescent spectrum of a Yb3~-codopedPr3~telluriteEr3~,and Tm3~in tellurite glasses glass is also shown in Fig. 7. The spectrum demon-

In this section, the spectroscopic propertiesof Pr3 ~, strates reasonably efficient energy transfer from theNd3~,Ho3~,Er3~,and Tm3~in tellurite glasses will 2F

312 level ofYb3~to the G

4level ofPr3~sothat the

be discussed. At particular, a comparison of the se- weak absorption of the Pr3 level does not limit thelectedproperties for different glasses are listed, pump efficiency. In ZBLAN it was shown by Ohishi

et a!. [33] that direct pumping of Pr34 alone is more3.3.2.1. Spectroscopic properties ofPr3 4 efficient than pumping by energy transfer from Yb3 4.

One approach to achievinggain at 1.3 jim is to use To utilize this advantage, long lengths ofPr34~in a host with lower phonon energy than silicate Pr34 : ZBLAN fiber are required. Given the highglasses. Glass hosts with lower characteristic phonon background optical loss for the tellurites, Yb34-Pr34energies are expected to provide an environment with codoping may be favored because a given inversionconsiderably lower non-radiative decay rates, which (and gain) can be obtained in a shorter length fiber.is more favorable for 1.3 jim emission in Pr34-doped Fig. 8 presents the comparison of fluorescence spec-glass. tra of tellurite, fluoride and chalcogenide glass hosts

Tellurite glasses are being investigated because they doped with Pr3~.The emission bandwidth in tellur-combine the desired lower phonon energies with ite glass is the smallest, around 88 nm. However, thethermal and mechanical properties which makefiber peak emission has been shifted from 1.32 jim in

Table 9Typical nonradiative parameters for different glass hosts [49]

Glass fl(s~) a(cm) hw(cm~) 6Phosphate 5.4x 1012 4.7X l0~ 1200 0.0037Silicate 1.4x1012 4.7x103 1100 0.0057Germinate 3.4x101 4.9xl03 900 0.013Tellurite 6.3x lOb 47x l0~ 700 0.037Fluoride l.88x10 5.77X 10~ 500 0.056Chalcogenide lxlO6 2.9x103 350 0.36

J.S. Wangeta!. / OpticalMaterials 3 (1994) 18 7203 19712 ..---

Ibi trend: silicate (32 nm) > tellurite (27nm) > phos-sult implies that tellurite glass has less multiple-sites

6-broadening for Nd34 and/or less crystal field split-10. ~ ~ ~ phate (21 nm) > fluoroberyllate (19 nm). This re-

6 ting than silicate glass. TheJudd-Ofelt parameters andcalculated lifetimes for the 4F

312 level are listed inTa-0 ble 11. Again, the more covalent the glass, the greater

spectroscopic and laser properties among variousis its radiative transition rate. The comparisons of

2~

1200 1250 1300 13521400 1450 1500 glasses are given in Table 12. The laserwavelengthsWavelength nm) varies from 1.306 jim in fluorides to 1.34 jim in sili-

Fig. 7. G4.3H

5 fluorescence spectra of Pr34 in (a) 5Na

2O- dates, and in tellurite and chalcogenide glasses to 1.372OZnO-75TeO2 glass, (b) Yb

34-codoped 5Na2O-2OZnO-75TeO2 jim. Generally speaking, tellurite and chalcogenide

glass.glasses have larger absorption and emission cross sec-tions in comparison with other oxide and halide

fluoride glass to 1.33 jim in tellurite and chalcogen- glasses.ide glasses due to the nephalauxetic effect. As Neodymium-doped tellurite glass appears to be aJrgensen et al. [34] described, the Q2 parameter is less promising material for 1.3 jim applications be-strongly affected by covalent chemical bonding and cause the excited state absorption eliminates gain inthe Q4 and Q6 parameters are related to the rigidity much of the target wavelength region, and there is aof the medium inwhich the ions are situated. There-

red shift of this emission. The 1.3 jim transition forfore, a large Q2 indicates a greater degree of cova- Nd

3+ in Te02 glasses has notbeenstudiedvery much.

lency and longer emission wavelength, as indicated It is conceivable that tellurite glasses lower the ESAin Table 11 and Table 12.

and/or shift it toward shorterwavelengths.3.3.2. 2. Spectroscopic properties ofNd

34 3.3.2.3. Spectroscopicproperties ofEr3 ~Although our goal is to optimize 1.3 jim amplifi- Erbium doped glasses were first investigated for

cation, we began with the 1.06 jim characterization eye-safe range finders, but more recently have re-ofNd34-doped tellurite glass, since numerous results ceived a great deal of attention as fiber amplifiers foron both materials and devices with Nd3+ operating the 1.55 jim telecommunications windowat 1.06 jim are widely available. A comparison of [35,36,37,38].4F

312e41

1112 emission among several typical glasses, The Judd-Ofelt parameters [39,40] and radiativeincluding Na2O-Te02 glass, is shown in Fig. 9. Their lifetimes for the transitions 41i3,2_e2Iis,2 andemission peak wavelengths are quite different: 1046 Iii/2_6115/2 for R20-Te02 glass with about 1 wt%nm for fluoroberyllate, 1052 nmfor phosphate, 1062 Er2O3 dopant (where R20 = Li20, Na20, K20, Rb2O,nm for silicate, and 1065 nmfor tellurite glasses. Ad- Cs20) and for Na2O-ZnO-Te02 glass with 1 wt%ditionally, their emission full widths at half maxi- Er203 have been reported [16]. The fluorescencemum height are also different with the following spectra of the ~Ii3,2e~Ii5,2transition for Na2O-ZnO-

Table 10TheJudd-Ofelt parameters, calculated and measuredlifetimes, emission cross sections, and branching ratios ofPr~for G4e

3Hg transition.

Glass Q2 24 Q6 T i,,,~ Branching ~x 1021 Ref.

(pm2) (pm2) (pm2) (jis) (jis) ratio (cm2)

ZnNaTe 8.7 9.1 8.7 460 22 63% 8.7 [this work]ZBLAN 1.4 4.2 4.9 3240 110 64% 3.48 [6]LaGaS 9.8 2.8 5.5 500 300 10.5 [10]

198 J.S. Wang eta!. / OpticalMaterials 3 (1994) 18 7203- the lifetime of the I13/2e~Iis,2transition did not

Fluoride ~P Chalcoge ide o

0.8 .~ ~ show any decrease but then dropped by SOlo at 2.87Tellurite .. .

t 0.6 as sensitive to Er2O3 concentration as it is in theNa2O-Si02 glasses.~ ~ / mole% [16]. However, the decrease in lifetime is not

0,4 The absorption and emission cross sections, andtheir difference are shown in Fig. 12 and Fig. 13 for

0,2 silicate and tellurite glasses, respectively [16]. For the0 ~ tellurite glass, the difference spectrum has larger val-1200 12501300135014001450 1500 ues (0.3 to 0.2x l0_20 cm

2) at the short wavelengthWavelength (nrc) side ofthe peak and a wider width for the fluorescent

Fig. 8. The G4e

3H5 fluorescent spectra ofPr~in 5Na2O-20ZnO- spectrum on the long wavelength side (1.531.62

75TeO2, chalcogenide and fluoride glasses. jim), which could makewider bandwidth lasers pos-sible. One problem with Er

34-doped active fiberTable 11 pumping at 0.8 jim is excitedstate absorption (ESA)Typical valuesof Judd-Ofelt parameters and calculated lifetimes [40]. The Judd-Ofelt calculated ratios of GSAofNd04 forfluorescence from the 4F

312 level in fluoride, silicate, ~ 4 4 2tellurite and chalcogenide glasses. ( Ii 5/2~ 9/2) to ESA ( L 3/2~ H1 I/2) in silicate glass

and tellurite glass are 0.34 and 1.23, respectively. TheGlass Q2 24 Q6 Tmnan line shapes of these absorption spectra also have to

(pm2) (pm2) (pm2) (ms) be considered, but based on the total transition prob-

Fluoride 2.0 4.0 4.5 0.5 abilities there appears to be a three-fold improve-Silicate 4.5 3.0 3.0 0.4 ment for tellurite glasses against the detrimental ESABaO-TeO

2 5.8 4.9 5.8 0.14 at 0.8 jim pumping. The ratios of (~Ii3,2~II5,2)flu-GLS 6.65 4.39 4.67 0.1 orescence to (41i3/2_2I9/2) ESA for silicate and tellur-

ite glasses are 4.6 and 3.5, respectively, which arecomparable.

TeO2 glasses with various amounts of Er2O3 dopant The three e-folding lifetime dependencies on glass(mole% Er203=0.055, 1.085 and 7.513) are shown composition are given in Table 13. The three e-fold-in Fig. 10. They are similar for high and low concen- ing lifetime is very dependent on the glass modifiers.tration. As shown in Fig. 11, up to 1.46 mol% Er2O3, It varied from 2.6 ms to 7.8 ms. Forcomparison, Fig.

Table 12Spectroscopic and laser properties of Nd

3 4 in tellurite glasses ascompared to other glasses.

Glass Transition - Laser Abs. coef. AA o~

.1.5. Wanget al. lOptical Materials 3(1994)187203 199

1,20.8-

I : N 08 Absorption Emission_____ j:~

1020 1040 1060 1080 1100 1120011evWavelength (nm(

Fig. 9. The 4F5,2-.41, 1/2 fluorescence spectrum ofNd3 in differ- 1500 1600 - 1700

ent glasses at 295K;fluoroberyllate (A), phosphate (x); sili- Wavelength (crc)cate () [30]; and tellurite glasses. Fig. 12. Absorption cross section, emission cross section andtheir

difference for Er203 in Al203-Si02 glass.

1.2-

12

1430 1480 1530 1580 1630 1680 04Wavelength (nm) - 1400 1500 1600 1700

Wavelength (nnliFig. 10. The fluorescence spectra of Er

3 at 0.055 mole% (1),1.085 mole% (2) and 7.513 mole% (3) Er

203. Fig. 13. Absorption cross section, emission cross section and theirdifference for Er203 and 5Na2O-2OZnO-75TeO2 glass.

________ ________ ________ emission spectra as a function of composition [41],~Silicate\~ ~ Tellarite~ almost all spectra in the tellurite glasses are similar in_______ _______ _______ -_______ emission widths, number of main maxima, and

I structure details. They are slightly different in the in-5. ________ ~ tensity ratio. In fact, the possibility of rare earth ions

entering in the Te02 network [40] was proposed toii 6 explain the smaller dependence on the glass modi-

- Er 0, vrvi hr fiers. Note that the emission width in most tellurite4 4 . . glasses is wider than in silicate based glasses.Fig. 11. Lifetimes for the Ii3/2~ 115/2 transition at 1.54 jim as a

function of the Er203 concentration in Na20-2SiO2 [41] and5Na2O-2OZnO-75TeO2 glasses. 3.3.2.4. Spectroscopicproperties of Tm~4

In addition to Er34, Nd34 and Pr34, thulium (III)

14 presents the fluorescence spectra ofEr3~in tellur- ion is another rare earth ion, which has been inten-ite, silicate and fluoride glasses. It indicates the sen- sively studied for use in optical amplifiers [41,421.sitivity of fluorescence spectra on hosts, not only in Both 2.3 jim (3H

4~3H

5)and 1.88 jim (3F

4e3H

6)the fluorescence peak wavelength but also the peak emissions in Tm

3 are attractive for chemical sen-width. More than 50 compositions have been pre- sing, medical and atmosphere transmission applica-pared and their emission spectra were investigated. tions, and the 1.47 jim emission is near the third te-A few typical samples are shown inFig. 15. Unlike in lecommunication window ofsilica fiber. Additionally,the silicate glasses, which show a great variety of Tm34 has a strong absorption band around 0.79 jim

200 J.S. Wang et al. / Optical Materials 3 (1994) 18 7203

Table 13Lifetime dependence of ~I,3,2~I,5f2transition of Er3 on glass composition.

Composition (mole%); Er203 (wl%) e, (ms) e2 (ms) e3 (ms)

(l00x)%Te02+x%;X=

PbO-B203-W03-La203 0,99 2.64 2.59 2.64l5%B203 1.29 3.08 3,13 3.225%Nb205 1.15 3.91 4.15 4.42lO%P2O5 0.1 4,03 4.49 5.05l0%Ge02 1.22 4.10 4.22 4.4225%W03 1,19 5.29 6.14 6.5115%TiO2 1.65 5.74 5.88 6.087%La2O3 1.03 6.25 6.66 7.03l0%Na20-lO%TiO2 1.29 6.45 6.45 6.4918%Bi2O3 1.42 6.98 7,13 7.3710%Bi203-l0%Ti02 1.04 7.15 8,06 9.0330% BaO 1.16 7.88 7.37 7.62

12-

rozirconate fiber lasers [42] have been reported, as1 ~~ we described previously, the former suffer from in-

0.8. ,,,,,,,,,,, herent high phonon energy and the latter from theirpoor glass stability and chemical durability. The low-

06 est phonon energy of Te02 glass is around 600 cm ~ 54. ______ ______ ______ ______ ______ which is less than Si02 glass (1100 cm ) but larger

than fluoride glass (500 cm i)~ Since it is necessary02- to have 5 and 6 phononsto bridge the

3H4

3H5 (4500

0- ~ .. ~-,.. cm) and3F

4~3H

6(5300 cm~)transitions, re-1420 1470 1~0 I lb 1620 1670 spectively, we expect Te02 glass will provide muchhigher quantum efficiencies than Si02 glass at 1.47

Fig. 14. The fluorescence spectra of Er3 in tellurite (x ), silicate jim and 1.88 jim.

(A) and fluoride glasses (0). For 1.47 jim lasing action to occur, it is necessary

1 4 ______ _______ ______ to quench the relatively long-lived lower laser level toeliminate the self-terminating behavior of -thulium.

1.2 Otherwise, the advantage of four level laser system intrivalent thulium will be lost. Codopingwith terbium

21 08 [44] and holmium [45] for thulium in the fluoride______ / I ______ ______ ______ glasses has beendemonstrated. The results indicated

06 4)( 7,I ______ ~-~ ~ that holmium codoping decreased the lifetime of the04 ~ / -.,~ ~ lower laser level by nearly two orders of magnitude0.2 N~~ with much less effect on the upper laser level. There-0 ~ ~ fore, Ho3 was selected for quenching the lower laser1430 1480 1530 1580 1630 1680 level ofTm3 emitting at 1.47 jim.

Wavelength (om) In order to obtain an efficient system, it is alwaysFig. 15. The fluorescence spectra of Er3~in R

20-ZnO-Te02 necessary to keep the doping concentration below aglasses, R=Rb (l),Li (2), Cs (3),K(4), and Na (5). certain value, which depends on the host and the

quenching mechanisms. This phenomena, so-called(3H

6e3F

4) which is a convenient pumping wave- concentration quenching, is caused by the stronglength for A1GaAs semiconductor diode lasers. Al- coupling among active ions themselves. The concen-though Tm

3-doped silica fiber lasers [43] and fluo- tration effects on the lifetimes of 3F4 and

3H4 levels

J.S. Wang et aL / OpticalMaterials 3 (1994) 18 7203 201

of Tm3~and Tm3-Ho34-codoped barium tellurite 10,9glasses havebeen reported recently [46]. It has shown 0,8~O,7

that for 1.47 jim emission in the 1 lBaO-89TeO2 glass, .06

Ho2O3 is a very efficient co-dopantto eliminate pop- -~ 0.50 0.4

ulation in the long-lived lower laser level (3F

4) with-0,2

out decreasing the upper laser level population. Forexample, up to 0.8 wt% Ho203 can be added to re- 01300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300duce the

3F4 lifetime 83% with only a 5% reduction Wavelength nm)

in the3H

4 lifetime, and the upper limit ofthe thulium Fig. 17. Emission spectra of Tm3 (0.2 wt% Tm

203) codopingconcentration is around 0.4 wt% in order to avoid with various concentrations of Ho203 in BaO-Te02 glasses (1=0,

2=0.2,3=0.4,4=0.8,5=1.6 wt% Ho203) [46].concentration quenching via the two-for-one process(see Figs. 2a and 2b in Ref. [46]). Moreover, since __________ __________ __________tellurite glasses have lower phonon energies in com- Silicate ~ _______ ~Tellu6teparison with silicate glasses, tellurite glass as a hostcould improve the efficiency for lasing at ].88 jim 09 A Emission(3F

43H

6, Tm3).

Fig. 16 illustrates the emission spectra of Tm3 + 0.6

It is clear that the dependencies on the concentration ______ -from 3H4 and

3F4 levels as the concentration was in-creased in the order 0.2, 0.4, 0.8 and 1.6 wt% [46]. 83

of Tm34 of the 3H

4 and3F

4 emission intensities are __________ __________ __________in opposite directions, that is, the relative

3H4 emis- 1300 1600 1900 2200Wavelength (cm)

sion intensity decreases with increasing Tm2O3 con- Fig. 18 The absorption and emission spectra ofthe3F

43H

6 tran-centration, but the

3F4 emission intensity increases. sition ofTm

3 in tellurite and silicate glasses.The reason is due to the existence of a cross-relaxa-tion process occurring between 3H

4-+3F

4 and tion, but 1.47 jim emission slightly decreased. This is3H6.

3F4. This process results in two ions remaining due to energy transfer from Tm

3 (3F4) to Ho

3in the excited 3F

4 state at the expense of the popula- (5J~),This led to an apparent increaseofHo3 emis-

tion of the 3H4 state and is usually referred to as the sion at 2.05 jim. Fig. 18 shows the overlap of the ab-

two-for-one conversion, as illustrated in Fig. 16. Fig. sorption and emission spectra of Tm3 + at 3F

43H

61 7 shows that the Ho

3 concentration increases in transitions. Its red shift is large, about 117 nm, andthe order 0, 0.2, 0.4, 0.8 and 1.6 wt% [46], while the the emission linewidth is wide, around 234 nm, bothTm3 concentration is held at 0.2 wt%. All peaks are ofwhich are a little bit less than that of silicate glassesnormalized with respect to the 1.47 jim peak. It is with a red shift of 136 nm and an emission width ofclear that 1.8 jim Tm3 emission decreased re- 260 nm [47], as shown in Fig. 18. This result makesmarkedlydue to the increaseofthe Ho

2O3 concentra- Tm3 a good candidate for a tunable fiber lasersource

around 1.9 jim. Additionally, the emission peak__________________________________ wavelength has been shifted to a longer wavelength

in tellurite glass which is favorable for air transmis-sion applications. Moreover, the strong two-for-oneprocess with a concentration optimization could fur-ther improve the 1.9 jim emission. Although this isas three level system (3F

43H

6 transitions), it will not________________________________ be a serious problem with a strong pumping source

1300 1400 1500 1600 1700 1800 1900 2000 2100 available, e.g. AlGaAs. Note that in a three level sysWavelength nm)

tern the branching ratio is always one (as shown inFig. 16. Emission spectra of Tm

3 at various concentrations inBaO-Te0

2 glasses (1=0.2, 2=0.4, 3=0.8, 4= 1.6 wt% Tm203) Table 14) while in four level system the branching[46]. ratio is usually considerably less than one.

202 J.S, Wang eta!. / OpticalMaterials 3 (1994) 18 7203

Table 14Judd-Ofelt parameters, Q2= 5.7 0.4, Q4= 1.40.3, Q6= 1.6 0.1, and selected transitions of Tm3 in BaO-Te02 glass.

)~(emission), nm Transition Branching ratiotr~d~abivn,J.tS

1800 3F4

3H6 1.00 1879

23003H

43H

5 0.015 2901470

3H,,3F4 0.086

8003H

43H

6 0.899

4. Conclusions [41W.J. Miniscalco, BA. Thompson, E. Eichen and T. Wei,Proc. OFC90, Paper FA2, (1990).

[5] K. Wei, D.P. Machewirth, J. Wenzel, E. Snitzer and G,H.Tellurite glasses possess (i) a reasonably wide SigelJr., Optics Lett. (to be published).transmission region (0.355 jim), (ii) good glass [6]Y. Ohishi, T. Kanamori, T. Kitagawa, E. Snitzer and G.H.stability and corrosionresistance, (iii) a relatively low Sigel Jr., OpticsLett. 16 (1991) 1747.phonon energy spectrum among oxide glass formers, [7] Y. Miyajima, T. Sugawa and Y. Fukasaku, Electron. Lett.and (iv) high linear and nonlinear refractive indices. 27 (1991) 1706.[81M. Dejneka, RE. Riman and E. Snitzer, Fiber OpticWe have studied the glass formation and properties Materials Research Program, Fall Meeting, Piscatawayof rare earth ion doped tellurite glasses. Glass corn- (1993).positions based on Na20-ZnO-TeO2 showed excel- [9] T. Miyajawaand DL. Dexter, Phys. Rev. B 1 (1970) 2961.lent properties in terms of UV edge, chemical dura- [10] P.C. Becker, MM. Broer, V.G. Lambrecht, A.J. Bruce andbility, rare earth solubility, stability, viscosity and G. Nykolak, Topical Meeting on Optical Amplifiers andfiberization. Tellurite glass fiber with a loss less than Their Applications, Santa Fe, NM, PDP5, (1992),[11] K. Wei, D.P. Machewirth, J. Wenzel, E. Snitzer and G.H.1 dB/m has been achieved by using the rod-in-tube Sigel Jr., J. Non-Cryst. Solids (to be published).fabrication method. As compared with silicates, rare [12] J.E. Stanworth, J. Soc. Glass Technol. 36 (1962) 217.earth elements in tellurite glass generally have lower [13] N. Spector, R. Reisfeld and L. Boehm, Chem. Phys. Lett.nonradiative decay rates, larger values of the radia- 49 (1977) 49.tive cross section (or transition strength), shorter [l4]R. Reisfeld and L. Boehm, Chem. Phys. Lett. 49 (1977)251.fluorescent lifetimes, and a red shift of the radiative 115] J.S. Wang, EM. Vogel, E. Snitzer and J.G.H. Sigel, Americantransitions. The emission spectra seem less sensitive Ceramic Society Annual Meeting, Cincinnati, OH (1993).to glass modifiers in tellurite glass than in silicate [161 J.S. Wang, E. Snitzer and G.H.J. Sigel, MRS Symp. Proc.,glass, i.e. Nd

3, Er3. BostonMA (1991).[l7]S.E. Stokowski, R.A. Saroyan and M.J. Weber, (ed), in:

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