www.elsevier.com/locate/jnoncrysol

Journal of Non-Crystalline Solids 345&346 (2004) 359–365

Rare earth–aluminum oxide glasses for optical applications

Richard Weber a,*, Jean A. Tangeman a, Paul C. Nordine a,Richard N. Scheunemann a, Kirsten J. Hiera a, Chandra S. Ray b

a Containerless Research Inc., 906 University Place, Evanston IL 60201, USAb University of Missouri-Rolla, Rolla, MO 65401, USA

Available online 23 September 2004

Abstract

Glasses based on rare earth oxides and aluminum oxide with 0–20mol% SiO2 provide a combination of optical properties,

mechanical and chemical stability, and process characteristics not available in other oxide materials. Properties of the glasses

include: refractive index 1.7–1.8, low dispersion (Abbe number �40), high solubility of optically active dopants, homogeneous

chemical composition, long fluorescence lifetimes at dopant concentrations up to 5mol%, broad fluorescence bandwidth, and infra-

red transmission to �5000nm. The glasses are hard, strong and resist chemical attack and they can be cast in sections 5–10mm

thick. This paper briefly describes glass processing and presents bulk glass properties including results of experiments to study infra-

red fluorescence at wavelengths �1550, �2900 and �1030nm from Er- and Yb-doped glasses that were optically pumped at 980nm.

� 2004 Elsevier B.V. All rights reserved.

PACS: 42.55.Rz; 78.20; 78.55; 81.05.Kf

1. Introduction

Glass materials have advantages over crystals in

many optical device applications. These advantages in-

clude: (i) a disordered ion environment that can broaden

fluorescence bandwidth, (ii) uniform (isotropic) optical

properties over a wide range of compositions, (iii) ease

of fabrication into complex shapes including fibers,and (iv) low fabrication cost, which lends itself to mass

production. In addition, many device applications re-

quire a high solubility of dopants to enable small, high

power density devices needed for local area network

and power laser applications. The quest for glasses that

combine desirable optical properties and environmental

stability, and that can be readily manufactured has led

0022-3093/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2004.08.044

* Corresponding author. Tel./fax: +1847 467 2678/2679.

E-mail address: weber@containerless.com (R. Weber).

to development of many oxide glass formulations, in

particular silicates, phosphates, and tellurites [1–6].

Silica-based glasses are widely used in long haul opti-

cal communications devices, notably EDFAs [7], but the

silica-rich glasses are limited to applications where there

is very low dopant concentration, hence relatively large

devices can be used [1]. Phosphate [3–5], tellurite [6] and

aluminosilicate glass materials [1,8] can dissolve rela-tively large concentrations of dopants compared to pure

silica glasses. At higher dopant concentrations,

>500ppm, �clustering� of dopant ions in silicate glasses

leads to energy transfer processes that decrease device

efficiency [2]. The high phonon energy of phosphates

and silicates limits their use to visible and near infrared

applications. Tellurite glasses have lower phonon ener-

gies and exhibit superior infrared transmission but sufferfrom poor environmental stability.

Recent research has established that pure rare earth

oxide–aluminum oxide materials can be formed into

bulk single phase glass [9,10] and glass fibers [11,12].

360 R. Weber et al. / Journal of Non-Crystalline Solids 345&346 (2004) 359–365

Glasses based on rare earth oxide aluminum oxide com-

positions with up to 20mol% silicon dioxide (REAlTM

Glass 1) can be formed into rods and plates. The glass

materials are strong, stable and can host large concen-

trations of dopant ions. In the present work, rare

earth–aluminum oxide glasses containing up to20mol% silica and up to 6mol% optically active rare

earth element oxides were synthesized and their physical

and optical properties were studied.

2. Experimental

Here we describe the synthesis of bulk glass, measure-ments of physical and environmental properties of the

glasses, and measurements of fluorescence behavior of

Er- and Yb-doped glasses. Glass formulations were

made from mixtures of 5N purity, –325 mesh metal

oxide powders that were blended in a ball mill. Repre-

sentative compositions and composition ranges that

were investigated are given in Table 1.

2.1. Production of bulk glass

Containerless processing techniques that avoid con-

tainer-derived nucleation of crystals in the undercooled

molten oxides were used to: (i) investigate an array of

compositions (see Table 1) and (ii) to determine the glass

forming limits for the rare earth–aluminum oxide mate-

rials [13]. Based on the results of these experiments,compositions were selected for processing in crucibles.

Containerless experiments were performed using 2 to

3.5mm diameter samples that were levitated in a conical

nozzle levitator [13,14]. The levitated samples were

heated and melted using a partially focused continu-

ous-wave CO2 laser beam. After complete melting, the

samples were cooled by blocking the heating laser beam

to achieve cooling rates of 50–300K/s. Smaller samplescooled faster due to their larger specific surface area.

Compositions that melted at temperatures up to

1650 �C were processed in platinum crucibles. Powdered

crystalline precursor materials were homogenized,

placed in a crucible and heated to 1650 �C using a box

furnace. Melts were removed from the furnace after 2–

6h and cast into rods or plates.

2.2. Property measurements

Property measurements were performed on selected

glasses. Properties measured were density, hardness,

glass transition temperature, crystallization tempera-

ture, corrosion resistance, index of refraction, infrared

1 REAlTM Glass is a trademark used by Containerless Research,

Inc.

transmission, ground-state absorption cross section [8]

and fluorescence behavior of optically active ions.

Properties of the bulk glass materials were measured

using standard techniques. Density was measured by

displacement using a 2ml pycnometer, a microbalance

and deionized water as the immersion fluid. Hardnesswas measured using a microhardness indenter. Glass

transition and crystallization temperature ranges were

measured by a Perkin-Elmer DT7 differential thermal

analyzer (DTA). DTA measurements were performed

at a heating rate of 10 �C/min in a nitrogen atmosphere

on both bulk and crushed glass samples (approximate

particle size approximately 45–75lm) to obtain a quali-

tative measure of the contribution of surface effects tothe onset of crystallization. The dissolution rate of the

glass was investigated by immersing samples in agitated

deionized water at 90 �C and measuring the specific mass

change at intervals of 2 days over a period of 16 days.

Index of refraction was measured at wavelengths of

486, 589 and 659nm (F, D and C Fraunhofer lines)

using the Becke line method with index-matched oils.

Abbe numbers were calculated from the measuredrefractive indices.

The ground-state absorption cross section and fluo-

rescence behavior were measured using polished glass

disks. These were prepared by mounting glass in

25mm diameter, 3mm thick epoxy resin mounts and

plane polishing them on two surfaces to form nearly-

parallel disks 1–2.5mm thick [8]. The surfaces were

polished to a 0.05lm finish using water-based aluminaslurry. Spectral transmission and ground-state absorp-

tion cross sections were measured using a dual beam

spectrophotometer (Cary model 500 for 300–3300nm

range and Perkin–Elmer model 1725X FTIR for 2000–

6000nm range). Transmitted intensity versus wave-

length data were acquired by computer and the raw data

were corrected for the measured background. For doped

glass compositions, the data were normalized to thespectrum of an undoped glass. The spectral absorption

cross section, rk, was obtained from the spectral absorp-

tion coefficient, kk, per unit thickness of sample, and the

average number of dopant ions per cm3, N, using the

relationships:

kk ¼ � ln sk

lrk ¼ kk

N; ð1Þ

where sk is the spectral transmission through the opticalpath length, l, after correcting for reflection at the sam-

ple surfaces. The value of N is 3.1 · 1020 RE3+ ions/cm3

per mol% of trivalent RE oxide. For purposes of the

analysis, a uniform distribution of dopant ions was

assumed.

Fluorescence output from doped glass disks was

investigated by optically pumping samples at a wave-

length of 980nm with the output from a 1W laser diode.The apparatus (illustrated in Fig. 1) comprised a pump

Fig. 1. Apparatus for investigation of fluorescence behavior in doped glass materials. The components were mounted on an optical table using

adjustable mounts. Data were acquired with a laptop computer. Measurements of fluorescence bandwidth were made by substituting a

monochromator for the filter.

Table 1

Compositions investigated values are in mol%

Al2O3 (A) Y2O3 (Y) La2O3 (L) Gd2O3 SiO2 (S) Dopant

62.5 30.5 7.0 0 0 1.0–5.0 Er2O3 or Yb2O3 sub. for Y

62.5 27.5 10.0 0 0 None

62.5 0 37.5 0 0 0.5–5.0 Er2O3 sub. for La

71.5 21.5 7.0 0 0 0.5–5.0 Er2O3 sub. for Y

50.0 10.0 20.0 0 20.0 0.5–5.0 Er2O3 sub. for Y

55.0 7.5 15.0 2.5 20.0 1.0–6.0 Er2O3 sub. for Y

55.0 28.0 0 0 15.0 2.0 Yb2O3

60.0 20.0 15.0 0 5.0 None

50.0 8.0 20.0 0 20.0 2.0 Yb2O3

50.0 3.0 25.0 0 20.0 2.0 Yb2O3

55.0 8.0 12.5 2.5 20.0 2.0 Yb2O3

50.0 10.0 20.0 0 20.0 None

Note: As needed, samples are identified in the text as e.g., 62.5A27.5Y10.0L.

R. Weber et al. / Journal of Non-Crystalline Solids 345&346 (2004) 359–365 361

diode with collimating optics, gradient neutral density

filter, focusing lens, sample, calcium fluoride infrared

transmitting collecting lens, spectral filter, aperture

and detector. Samples were placed in a precision mountand aligned with the pump beam. Pump light transmit-

ted by the sample was blocked using a silicon filter lo-

cated in front of the detector.

Temporally resolved measurements of fluorescence

output were made using a pump source that was modu-

lated at 5–10Hz with a square wave control signal.

Fluorescence output in the wavelength ranges 1000–

2000nm and 2000–4000nm was measured using an InGaAs photodiode and a cooled HgCdTe photodiode,

respectively. The amplified voltage output from the

detectors was acquired by computer. Spectrally resolved

measurements of fluorescence output were made in

the wavelength range 1000–2000nm. These measure-

ments were made using a continuous-wave pump source

and the fluorescence output was analyzed by substitut-

ing a calibrated monochromator for the spectral filter(see Fig. 1). The transmission of the monochromator

and the fluorescence intensity were not sufficient to per-

mit spectral measurements in the 2000–4000nm

waveband.

3. Results

3.1. Glass processing

The use of containerless techniques enabled glasses to

be made over a wide range of compositions including

materials that contained no silica. Some of the glasses

made using containerless melting exhibited a two phase

character, making them unsuitable for optical applica-

tions [9]. Selected compositions were formed in glass sec-

tions approximately 1cm thick by casting the melt from

crucibles. Crystalline regions of a few millimetres in ex-tent occasionally formed on the free surface of as-cast

samples. A photograph of a cast glass rod containing

3mol% erbium oxide and �20mol% silica is shown in

Fig. 2.

3.2. Glass properties

Typical ranges of values for the measured propertiesare given in Table 2. Trends with composition are briefly

discussed in Section 4. Measured values of the glass

transition and crystallization temperatures for several

glasses are given in Table 3. Values of Tg were in the

Fig. 2. Photograph of an erbium doped glass made by casting liquid

from a platinum crucible.

362 R. Weber et al. / Journal of Non-Crystalline Solids 345&346 (2004) 359–365

range 850–880 over the composition range shown in

Table 3. Differences in Tg values between bulk and

crushed samples were within the measurement error.

The temperature at which crystallization occurred

increased with increasing silica content. The crystalliza-tion onset temperatures were higher for bulk than for

crushed samples by as much as 63 �C.Measured dissolution rates for the 62.5A27.5Y10L

and 50A20L10Y20S glasses in water at 90 �C were

nearly the same, being 1.0 and 1.2 · 10�8gcm�2min�1,

respectively. The pH of the test solution increased

Table 2

Properties of REAlTM Glasses

Property

Density

Hardness

Chemical stability (in water at 90�C)Refractive index (nD, k = 589nm) a

Abbe number (nD � 1)/(nF � nC)

Infrared transmission

a Wavelengths: nF = 486, nD = 589, and nC = 656nm.

Table 3

Summary of DTA results for bulk and powdered samples

Glass Tg (�C) Crystallizatio

Onset

55A28Y15S2Yb (Powder) 882 ± 3 967 ± 2

55A28Y15S-2Yb (Bulk) 878 ± 3 972 ± 2

50A8Y20L20S2Yb (Powder) 855 ± 3 955 ± 2

50A8Y20L20S2Yb (Bulk) 857 ± 3 1018 ± 2

50A3Y25L20S2Yb (Powder) 853 ± 3 961 ± 2

50A3Y25L20S2Yb (Bulk) 848 ± 3 997 ± 2

50A10Y20L20S (Powder) 851 ± 3 974 ± 2

50A10Y20L20S (Bulk) 862 ± 3 1004 ± 2

62.5A37.5Y (Powder) 874 ± 3 923 ± 2

* Based on onset of crystallization.

slightly, from 5.5 to 5.8 during the course of a 16 day

test. The 62.5A27.5Y10L-composition glass formed a

surface residue as a result of reaction with the water.

3.3. Optical properties

The infrared transmission of rare earth aluminate

glasses containing 5 and 20mol% silica is shown in

Fig. 3. Data for pure silica and single crystal sapphire

are shown for comparison [15,16]. The �normalized�transmission in which the maximum value is set equal

to unity, nominally correcting for reflection at the sam-

ple surfaces is plotted in the figure. The ground state

absorption cross sections for Er and Yb in the wave-length range around 1000nm are presented in Fig. 4.

For a 62.5A27.5Y10.0L-composition base glass, the flu-

orescence lifetime of 5F7/2 energy level in Yb3+ decreased

from approximately 800 to 600ls when the concentra-

tion of Yb2O3 was increased from 1 to 5mol%. Substitu-

tion of 20mol% silica in a base glass containing 2mol%

Yb2O3 resulted in a decrease in Yb3+ fluorescence life-

time from approximately 800 to 600ls. The fluorescenceemission spectrum from the 4I13/2 energy level

(�1550nm) in Er3+ is shown in Fig. 4 for glasses con-

taining 1 and 6mol% erbium oxide. The 1/e fluorescence

lifetime of the 4I13/2 state in Er3+ is shown as a function

of Er2O3 concentration in Fig. 5. Erbium-doped glasses

emitted light at �2900nm by the 4I11/2 ! 4I13/2 transi-

tion. The 4I11/2 state exhibited a complex fluorescence

decay behavior indicative of the radiation, quenching,

Range of values

3.5–4.6g/cm3

800–1000 Vickers

Dissolution rate �1 · 10�8g/cm2/min

1.7 to >1.8

Typically �40, values up to 60 for some compositions

To 5000nm

n temperature (Tx, �C) Tx � Tg*(�C)

Peak End

984 ± 1 995 ± 2 85 ± 5

977 ± 1 983 ± 2 94 ± 5

991 ± 1 1033 ± 2 100 ± 5

1026 ± 1 1031 ± 2 161 ± 5

998 ± 1 1041 ± 2 108 ± 5

1053 ± 1 1080 ± 2 149 ± 5

1009 ± 1 1046 ± 2 123 ± 5

1054 ± 1 1062 ± 2 142 ± 5

931 ± 1 933 ± 2 49 ± 5

0.5

0.6

0.7

0.8

0.9

1.0

2.5 3.5 4.5 5.5Wavelength (Microns)

Norm

aliz

ed T

rans

mitt

ance

100 % silica

20 % silica

5% silica

Sapphire

Fig. 3. Infrared transmission of glasses as a function of silica content.

Data for pure silica and single crystal sapphire are shown for

comparison.

R. Weber et al. / Journal of Non-Crystalline Solids 345&346 (2004) 359–365 363

and upconversion processes that govern its concentra-

tion. Emission from this state was considerably more in-

tense than from Er-doped crystalline YAG.

4. Discussion

4.1. Glass processing and properties

Bulk glass was cast in bubble- and crystal-free sec-

tions 5–10mm thick from compositions that were melted

0

1

2

820 860 900 940 980 1020 1060

Wavelength (nm)

GSA

Cro

ss S

ectio

n (1

0-20 cm

2 )

-20

2

(a) (b

Fig. 4. Ground-state absorption cross section of Yb3+ (left) and Er3+ (rig

concentration and given as absorption cross sections as discussed in the text

0

200

400

600

800

1000

1200

1400 1450 1500 1550 1600 1650 1700

Wavelength (nm)

Inte

nsity

(arb

. uni

ts)

((a)

Fig. 5. Fluorescence cross section line shape for Er-doped glas

in platinum crucibles. Crucible melting was achieved for

compositions with 10–20mol% silica. Based on results of

the containerless experiments, it is expected that some

silica-free compositions can also be vitrified by casting

from the melt. At the limits of glass formation, contai-

nerless techniques enabled vitrification of pure binaryalumina–yttria compositions with critical cooling rates

up to �300 �C/s in �2mm sections.

The density of the glasses increased with increasing

average formula weight of the oxide mixtures. The bin-

ary alumina–yttria glasses had the highest hardness,

approximately 1000 Vickers. Substitution of the larger

La ion for Y decreased hardness by �10%. The meas-ured corrosion rate of the glasses is comparable to win-dow glass. Addition of 20mol% silica had no

measurable effect o the dissolution rate in water. The sil-

ica-free glass compositions resisted attack by concen-

trated aqueous hydrofluoric acid [8]; this acid fogged

the surface of the silica-containing glass after a few

hours exposure.

As indicated by the data given in Table 3, addition

of silica expands the temperature window between Tg

and the onset temperature for crystallization of the

supercooled liquid. Results of DTA experiments on

bulk and powdered samples show that the powdered

material crystallizes more easily suggesting that surface

GSA

Cro

ss S

ectio

n (1

0cm

)

0

0.2

0.3

820 860 900 940 980 1020 1060

Wavelength (nm)

0

0.1

0.2

0.3

)

ht) in a rare earth aluminate glass host. Values are normalized for

.

0

200

400

600

800

1000

1200

1400 1450 1500 1550 1600 1650 1700

Wavelength (nm)

Inte

nsity

(arb

. uni

ts)

b)

ses containing 1 (left) and 6 (right) mol% erbium oxide.

364 R. Weber et al. / Journal of Non-Crystalline Solids 345&346 (2004) 359–365

nucleation of crystals would be the limiting factor in

glass working operations. Some crystals were occasion-

ally observed to form on the free surface of the as-cast

glasses, which consistent with the conclusions drawn

from the DTA data [17].

4.2. Optical applications

The glasses provide infrared transmission to wave-

lengths considerably beyond that obtained with conven-

tional silicate glasses. Transmission of 75% at 4000nm is

achieved in 5mm sections for compositions that can be

cast near net shape. The combination of IR transmission

and environmental stability are of potential interest indeveloping low cost infrared windows. The IR transmis-

sion is further extended by reducing the silica content of

the glass. The index of refraction is from 1.7 to >1.8, the

maximum value that could be measured with the avail-

able test oils. Abbe numbers of 40–60 exceed the values

of other glasses [18] designed to achieve low chromatic

dispersion.

The value of ground state absorption cross sectionfor ytterbium ions is 2.0 · 10�20cm2 at a wavelength

of 978nm. The strong absorption provides a convenient

means to pump the materials using 980nm laser diodes.

The ability to pump lasers at 980nm rather than the

940nm scheme employed in Yb:YAG lasers [19], can re-

duce the optical conversion losses by approximately 50%

and reduce the heat burden in the gain medium.

The full width half maximum (�3dB) bandwidths ofthe emission spectrum from the 4I13/2 energy level

(�1550nm) in Er3+ is 44 and 76nm for glasses contain-

ing 1 and 6mol% Er2O3, respectively. From the form of

the spectral distribution, it is evident that self absorption

at higher Er concentrations flattens the fluorescence

spectrum and increases the spectral bandwidth. The 1/

e fluorescence lifetime of the 4I13/2 state is approximately

6ms and independent of dopant concentration up to

2

4

6

8

0 2 4 6 8Mole % Er2O3

1/e

Life

time

(ms)

Fig. 6. Fluorescence lifetime of the 4I13/2 state in Er-doped rare earth

aluminate glass as a function of erbium concentration. Error bars are

0.05ms based on a data acquisition rate of 20kHz.

approximately 5mol% Er2O3 in silica-free REAlTM

glass. The lifetime decreases with added silica, but re-

mains quite large in glasses with upto 20mol% silica,

e.g., 3.6ms in a 50A20L7Y20S3Er. The erbium-doped

glass exhibits a combination of long fluorescence life-

time and large bandwidth at high dopant concentra-tions, properties that make these materials potentially

useful in small and high power density c-band optical

communications devices (Fig. 6).

The high output of the Er-doped glasses compared to

crystalline YAG, suggests applications for the glass in

�3000nm light sources. Further research is required to

investigate this opportunity, since the 4I11/2 ! 4I13/2transition exhibits rather complex behavior.

5. Conclusions

1. Glasses based on rare earth oxides and aluminum

oxide and containing up to 20mol% silica can be cast

in plate and rod forms in sections of approximately

1cm thickness to form hard, strong, thermally andchemically stable products.

2. The materials can dissolve large amounts of rare

earth oxides to form homogeneous glasses.

3. Heavily doped glasses provide a large bandwidth for

optical transitions.

Acknowledgments

This research was partially supported by: Air Force

Office of Scientific Research, Contract No. F49620-02-

C-0028, NASA Physical Sciences Division, Contract

No. NAS8-98092, and the National Science Foundation

Contract No. DMI-0216324.

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