www.elsevier.com/locate/jnoncrysol

Journal of Non-Crystalline Solids 345&346 (2004) 396–401

Quantifying Raman gain coefficients in tellurite glasses

Clara Rivero a,*, Kathleen Richardson a,1, Robert Stegeman a, George Stegeman a,Thierry Cardinal b, Evelyne Fargin b, Michel Couzi c, Vincent Rodriguez c

a School of Optics/CREOL, Florida Photonics Center of Excellence, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, USAb ICMCB (UPR 9048-CNRS) 87, Avenue du Docteur Schweitzer, 33608 Pessac cedex, France

c LPCM (UMR 5803-CNRS), University of Bordeaux, I 351 Cours de la Liberation, 33405 Talence cedex, France

Available online 23 September 2004

Abstract

This paper presents results obtained on bulk glasses that have been fabricated and characterized for their Raman gain properties.

We summarize relative values of the Raman gain coefficients of different tellurite-based glasses as compared to that of fused silica,

and compare data to values estimated from their Raman cross-section data. Spontaneous Raman spectra measurements demon-

strate a straight forward means to estimate Raman gain coefficients utilizing spontaneous Raman scattering cross-section data as

referenced to known standards. This technique shows, for an initial set of tellurite-based bulk glass samples, excellent agreement

with experimentally-obtained Raman gain coefficient data. Initial experimental measurement of Raman gain coefficients for several

tellurite glasses reveal that some compositions examined exhibit an absolute Raman gain coefficient up to 30 times higher than silica,

with an overall spectral bandwidth more than twice that of fused silica.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

The demands for high-speed optical communications

are increasing at a tremendous rate. In order to satisfythis need for information flow to short and long distance

networks, a more efficient use of available communica-

tion channels is required. This in turn, requires an

expansion of the available spectral bandwidth, which

in the existing communication sector is limited by the

transmission losses and water absorption peak of cur-

rent silica-based fibers. However, a recent breakthrough

in communication was achieved with the reduction ofthe water absorption peak at 1400nm, which has opened

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

doi:10.1016/j.jnoncrysol.2004.08.051

* Corresponding author. Tel.: +1 407 823 6869; fax: +1 407 823

6880.

E-mail address: crivero@mail.ucf.edu (C. Rivero).1 On leave at Schott Glass Technologies, Duryea, PA, USA.

up the available communication range to span from

1270 to 1650nm, corresponding to about 50THz band-

width [1]. This dramatic increase in bandwidth, and cor-

respondingly in the number of channels available, rulesout use of existing erbium-doped fiber amplifiers

(EDFA), which until recently have been considered the

primary means of amplification. This evolution leaves

Raman gain as the main mechanism for future amplifi-

cation needs.

Silica fibers and germanium doped silica fibers are

currently deployed in industry as Raman gain media.

In fact, fused silica has been, for the past century, thekey material used for long and short haul transmission

of optical signals because of its good optical properties,

and attractive figure of merit (i.e trade-off between Ra-

man gain and losses). However, one of the main disad-

vantages of fused silica is its limited usable bandwidth

for Raman amplification of about 5THz (�150cm�1).

With current telecom demands, the transmission of data

over larger bandwidths is becoming a crucial factor,

C. Rivero et al. / Journal of Non-Crystalline Solids 345&346 (2004) 396–401 397

consequently new materials which possess both large

Raman gain coefficients with broad bandwidth will be

required to satisfy these increasing demands.

Heavy metal oxide glasses have been previously pro-

posed for Raman gain applications due to their en-

hanced nonlinearity and transparency over the telecomwindow [2,3]. Tellurite fiber has been previously evalu-

ated for its Raman gain performance and exhibited sim-

ilar performance to that observed in our studies,

however no compositional information on this glass

was provided [4]. In this study we have examined a range

of tellurite glasses due to their high nonlinearity as com-

pared to other oxide glasses. Also, these glasses have

been selected over other oxide families due to their highspontaneous Raman intensities and large spectral band-

width as compared to fused silica.

2. Experimental procedures

2.1. Glass fabrication

Glasses in the system TeO2–PbO–P2O5–Sb2O3 were

prepared from high purity raw materials: TeO2 (Cerac

99.99%), (NH4)2 HPO4 (Merck minimum 98%), PbO

(Cerac 99.99%), Sb2O3 (Cerac 99.999%). Before melting,

a pre-heat treatment was conducted at 200 �C and 400 �Cto eliminate the water and ammonia respectively, pre-

sent in the primary starting materials. The batch mixture

was melted in alumina crucibles at a temperature rangeof 900 �C to 1000 �C depending on the composition, for

30min. Following melting, the glasses were quenched on

to a pre-heated carbon plate, and annealed at a temper-

ature of 40 �C below their glass transition temperature

(Tg). Approximately 1mol% of alumina attributable to

the crucible material was detected in the glass using ele-

mental dispersive spectroscopy (EDS). Finally, the

glasses were cut and optically polished. The composi-tions examined in the study are listed in Table 1.

2.2. Measurements

The volumetric weight of the different glass samples

was measured by the Archimedes method in diethyl-

Table 1

Physical properties of different tellurite-based glasses and reference material

Glass composition (mol%) Sample code q (

76.5TeO2–9PbO–9P2O5–5.5Sb2O3 sam 1 5.3

56TeO2–20PbO–20P2O5–4Sb2O3 sam 2 5.1

48TeO2–17PbO–17P2O5–18Sb2O3 sam 3 5.3

85TeO2–15WO3 sam 4 5.8

85TeO2–10Nb2O5–5MgO sam 5 5.2

SiO2 sam 6 2.2

SF6 sam 7 5.1

phtalate at room temperature (24�C). The accuracy ofthe measurement is within 0.02g/cm3.

The linear absorption spectra of the different compo-

sitions were obtained using a Cary 5E (VARIAN) spec-

trophotometer. However, no absolute absorption

measurement has been conducted in the IR region ofthe spectra, where the application of these glasses is ex-

pected. To ascertain these data for our glasses at key

wavelengths, we are currently setting up a photothermal

deflection apparatus (PDS) to accurately measure the

absorption coefficients of these glasses at 1.3lm. Resultsof these measurements will not be discussed here.

The linear refractive index of the glasses was meas-

ured using the Brewster�s angle method at a wavelengthof 532nm and 1064nm respectively. The recorded signal

was fitted using Fresnel equations. The experimental er-

ror was found to be ±0.05.

The IR transmittance spectra of 2mm thick bulk

glasses were recorded using a NicoletTM 740 (BRUKER)

FTIR spectrometer, in the range of 2000–5000cm�1,

with a spectral resolution of 4cm�1.

In order to estimate the spontaneous Raman cross-section of the different tellurite-based glasses, the polar-

ized (VV and VH) spontaneous Raman spectra of the

vitreous materials were measured using a micro-Raman

setup with an excitation wavelength of 514nm. A 100X

microscope objective, with a spatial resolution of about

2lm, was used to focus the light on the front polishedsurface of the sample. A polarizer and quarter-wave

plate (k/4) combination were used to select the polariza-tion direction (vertical, V or horizontal, H) of the back-

scattered light. The scattered light was collected and

spectrally analyzed with a CCD detector mounted on

the exit port of a single grating spectrograph, with a typ-

ical resolution of about 6cm�1. The Rayleigh line was

suppressed with a holographic notch filter.

3. Results

Table 1 summarizes the values of volumetric weight

(q) and linear refractive index (n) measurements for

the three families of compositions studied in this paper.

Note that all samples evaluated represented glasses in

s

g/cm3) ±0.02g/cm3 n ± 0.05

532nm 1064nm

8 2.05 1.99

3 1.99 1.94

4 1.98 1.97

9 2.20 2.08

6 2.15 2.13

05 1.462 (514.5nm)

8 1.82 (514.5nm)

398 C. Rivero et al. / Journal of Non-Crystalline Solids 345&346 (2004) 396–401

the tellurite family, but varied significantly in their net-

work constituents between phospho-tellurites (sam 1–

3), tungsten tellurites (sam 4) and niobium tellurites

(sam 5). It can be seen that the density and refractive in-

dex decreases with decreasing tellurite content for sam-

ples 1 and 2; however, sample 3 shows an increase inthe density and refractive index value at 1064nm. The

results obtained for sample sam 4 and sam 5 have been

previously discussed [5], but their q, and n values are

also summarized for completeness in the discussion. A

fused silica (Suprasil) sample from Heraeus and

SCHOTT SF6 glass, (provided by Schott Glass Tech-

nologies Inc.), have also been included in Table 1 for

comparison purposes.The visible and IR absorption spectra of bulk sam-

ples 1–3 are shown in Fig. 1(a) and (b) respectively. No-

tice the blue shift in the absorption edge with decreasing

tellurite concentration for compositions 1 and 2, and the

apparent red shift of the optical band gap for sample 3.

500 1000 1500 2000 2500 30000

5

10

15

20

350 400 450 5000

5

10

15

20

Abs

orpt

ion

Coe

ffici

ent α

(cm

-1)

Wavelength λ (nm)

sam 1 sam 2 sam 3

(a)

2000 2500 3000 3500 40000.0

2.5

5.0

7.5

10.0

Abs

orpt

ion

Coe

ffici

ent α

(cm

-1)

Wavenumber (cm-1)

sam 1 sam 2 sam 3

(b)

Fig. 1. (a) Absorption spectra and (b) IR spectra.

Fig. 2(a) and (b) illustrate the VV and VH polarized

Raman spectra of the various tellurite glasses, as well as

that for fused silica and SF6. These data were used to

calculate absolute Raman cross-section as described in

[3]. The VH spectra were multiplied by a factor of 5

for all the compositions due to lower intensity, to allowfor comparison. The absolute intensities were compared

to that of SiO2, and the Raman scattering cross-section

for each glass was calibrated to the absolute values of

SiO2 and SF6, as reported in [3]. Fig. 3 shows the rela-

tive differential Raman cross-section values obtained

for the various bulk glasses after correction for Fresnel

reflection and internal solid angle as given in [6,7]. Data

has been normalized to that of SiO2, for Raman dataobtained for the TeO4 Raman vibration at 665cm�1.

The cross-section results have been plotted against the

number density (N) of tellurium atoms in the glass

system.

As predicted by Hellwarth in 1963, there is a relation

between spontaneous and stimulated Raman scattering

processes. The results of this derivation are presented

in [8], where the Raman gain coefficient (g [cm/W]), alsoknown as the gain factor, is given by:

g ½cm=W� ¼ 4p3Nc2

�hx2SxPn2S

o2r

oxoX

� �; ð1Þ

where N is the number density of molecules, xS and xP

are the Stokes (signal) and pump (laser) frequencies

respectively, nS is the refractive index at the Stokes

wavelength, and o2roxoX is the differential Raman cross-

section.

The differential spontaneous Raman cross-sectioncan be obtained from the absolute measurement of the

Raman scattered intensity I(xS) of a given Raman active

mode, yielding the relationship described in [6]:

IðxSÞdX ¼ NV EðxLÞoroX

� �dX; ð2Þ

where NV represents the number density of molecules in

the scattered volume V, and E(xL) is the irradiance of

the excitation laser beam. The relative values of the Ra-

man cross-section as compared to SiO2 illustrated inFig. 3, were used to calculate the relative Raman gain

coefficient, at the 665cm�1 Stokes Raman peak, for

the different tellurite compositions (sam 1–5). The re-

sults of this calculation are also shown in Fig. 3. The

experimentally determined Raman gain spectra of com-

positions 4 and 5 have been reported previously in Ref.

[9]. The estimated results presented in Fig. 3 for these

two glasses are in very good agreement with the experi-mental values obtained in [9], which are also illustrated

in Fig. 3 for comparison purposes. The experimental er-

ror of the Raman cross-section measurements are within

±15%.

200 400 600 800 1000 1200 1400

Wavenumber (cm-1)

sam 3

sam 1

Ram

an In

tens

ity (

a.u)

sam 2

VV VH ( x5)

200 400 600 800 1000 1200 1400

SiO2 (VV)

SiO2 (VH)

SF6 (VV) SF6 (VH)

Wavenumber (cm-1)

sam 4

sam 5

Ram

an In

tens

ity (

a.u)

VV VH ( x5)

(a) (b)

Fig. 2. Parallel (VV) and perpendicular (VH) Raman spectra.

8 10 12 14 16 180

10

20

30

40

50

60

70

80

90

100

0

5

10

15

20

25

30

35

Rel

ativ

e D

iffer

entia

l Ram

an C

ross

-Sec

tion

Number Density of Te Atoms

Raman Cross-Section

Relative R

aman G

ain Coefficient

sam 1

sam 2sam 3

sam 4

sam 5

Estimated Raman gain coeff Experimental Raman gain coeff

atυ =665cm-1

Fig. 3. Differential Raman cross-section and Raman gain coefficient at

665cm�1, relative to SiO2.

C. Rivero et al. / Journal of Non-Crystalline Solids 345&346 (2004) 396–401 399

4. Discussion

The three bulk glass samples within the family TeO2–

PbO–P2O5–Sb2O3 were engineered such that, in the case

of samples 1 and 2, the Te–Sb ion ratio is almost identi-

cal, while the Te–Pb and Te–P ratios decrease by almost

three times. On the other hand, compositions 2 and 3

possess almost the same Te–Pb and Te–P ratio, and dif-ferent Te–Sb ion ratio. These compositional modifica-

tions were chosen to evaluate the impact of glass

former/modifier types on resulting structure and corre-

sponding spontaneous Raman bandwidth. Recall, that

the goal of our study is to both maximize Raman inten-

sity and spectral bandwidth within a given system.

From the density results shown in Table 1 we can see

that, for compositions 1 and 2, as the tellurite concentra-

tion decreases, the density also decreases. However,sample 3 shows an increase in density, which is corre-

lated to the concurrent molar increase of antimony,

resulting in a corresponding increase in the total amount

of heavy atoms inside the glass matrix, as compared to

sample 2. Notice that the index of refraction values at

1064nm exhibit the same behavior as the density meas-

urements since there is a well-established correlation be-

tween the index of refraction and the density, (ornumber density of oscillators) given by the simple har-

monic oscillator model [10]. Using this argument, there

is a slight discrepancy in the index measurements at

532nm, however the results are within the experimental

error of the measurement.

Fig. 1(a) illustrates the absorption spectra of compo-

sitions 1–3 respectively. As shown in the figure, the

transparency window of these glasses spans the rangefrom approximately 400nm to 2500nm, making them

suitable for telecom applications. In fact, the key

trade-off in Raman amplification applications is given

by the ratio between the gain and material�s loss spectra.Absolute absorption measurements at 1.3lm will be

conducted in the near future using a photothermal

deflection setup (PDS). It can be seen, in the inserted

graph in Fig. 1(a), that there is an evident blue shift ofthe absorption edge for sample 2, resulting from the de-

crease in tellurite concentration. However, as we in-

crease the antimony content (sam 3) with a slight

decrease in Te content, there is a relative red shift of

the absorption edge. This absorption edge shift we be-

lieve is due to a charge transfer from the oxygen to the

metal, although it is not clear for the moment which me-

tal, Te, Pb, or Sb, is responsible for this shift in absorp-tion. Once again, there is a correlation between the

behavior of the absorption and index of refraction meas-

urements given by Kramers–Kronig relation [10].

400 C. Rivero et al. / Journal of Non-Crystalline Solids 345&346 (2004) 396–401

Fig. 1(b) illustrates the IR absorption spectra of the

same three compositions previously described. As

shown in the figure, sample 2 exhibits a higher absorp-

tion than sample 1 due to the increase in phosphate con-

centration, as illustrated by the peak around 3000cm�1.

This vibration has been assigned to the (R)–OH groupsparticipating in oxygen bonding inside the glass net-

work, such as P–O bonds. Moreover, the vibration at

about 3500cm�1 has been attributed to the stretching

of (R)–OH free groups [11]. Furthermore, it can be seen

that the addition of antimony decreases the IR absorp-

tion of the glass. This effect could be related to the for-

mation of different phosphate groups, with formation of

P–O–Sb bonds, for large antimony content. Thisassumption remains to be confirmed using reflectance

IR spectroscopy.

Fig. 2(a) illustrates the polarized VV and VH Raman

spectra of samples 1–3 respectively. The Raman vibra-

tion at 460cm�1 is attributed to the Te–O–Te chain unit

symmetric stretching mode, while the spectral features

from 610cm�1 to 670cm�1 and 750cm�1 correspond

to the TeO4 bi-pyramidal arrangement and the TeO3+1(or distorted TeO4), and TeO3 trigonal pyramids struc-

tures respectively [12]. It can be clearly observed that

the evolution of the TeO3+1 and TeO3 units occurs, as

the TeO2 concentration decreases. Also, the VH Raman

spectra show that these last mentioned entities are polar-

ization sensitive as compared to TeO4 bi-pyramidal

structures. The spontaneous Raman spectra of composi-

tions 4, 5, SiO2, and SF6 are also shown in Fig. 2(b) forcomparison purposes. Note that, as discussed previ-

ously, the TeO4 Raman vibrations increase with increas-

ing TeO2 content. In the case of sample 4, the Raman

band located at 920cm�1 is attributed to the isolated

W–O short bond [13]. Finally, the VV and VH Raman

spectra of SiO2 and SF6 glass are shown for comparison

purposes and completeness of our analysis. The experi-

mentally recorded values of the relative Raman intensi-ties (counts/s) of these two last compositions are

illustrated in the figure. The main SiO2 Raman vibration

has been assigned to the O–Si–O symmetric stretching,

while the main Raman bands of SF6 glass, at the low

frequency range and at 1000cm�1, are attributed to

the lead introduction in the glass network [3].

The differential Raman cross-section, relative to SiO2,

was obtained from the relative intensity values obtainedin the experiment, following the use of various correc-

tions as described in Ref. [6]. Substituting these values

into Eq. (1), the relative Raman gain coefficients of the

five different tellurite compositions, at the 665cm�1 Ra-

man peak, were estimated. The results are illustrated in

Fig. 3. As predicted in the literature [14] and also illus-

trated in Fig. 2(a) and (b), the higher the TeO2 concen-

tration, the more TeO4 units, and thus the higher thedifferential Raman cross-section and Raman gain coeffi-

cient respectively. These results are consistent for the

data from this study depicted in Fig. 3. The exponential

behavior of the spontaneous Raman cross-section data,

and consequently the Raman gain coefficient, observed

in the figure can be correlated to the fact that as the

TeO2 concentration increases, the relative amount of

TeO3+1 and TeO3 units inside the network structure de-creases, and consequently, the number of TeO4 units has

to increase to maintain a constant TeO2 concentration.

This result also agrees with the fact that the TeO4bi-pyramidal arrangement possesses the highest polariz-

ability value as compared to TeO3+1 and TeO3 struc-

tures [14] and leads to the observable index variation

in the glasses.

Lastly, we compared the estimated Raman gain val-ues for samples 4 and 5 with the experimental values re-

ported by our group previously in Ref. [9]. The

estimated results presented in Fig. 3 for these two glasses

are in very good agreement (within ±10%) with the

experimental values obtained in [9], within the errors

of the theoretical and experimental predictions. The next

step will be to experimentally acquire the Raman gain

spectra of samples 1–3, using the experimental setup de-scribed in [9], to observe if they too match with the Ra-

man gain values predicted here.

5. Conclusions

Based on an evaluation of a range of oxide glasses

examined to date, tellurite based glasses appear to bepromising candidates for Raman amplification applica-

tions. The proposed method to predict the Raman gain

coefficient from spontaneous Raman cross-section esti-

mations provides a means to estimate to the right order

of magnitude, for expected Raman gain coefficients. The

theoretical values obtained in the present study are in

very good agreement with the experimental data ob-

tained in Ref. [9]. The results obtained reveal that someof these compositions exhibit an absolute Raman gain

coefficient of up to 30 times higher than silica, with an

overall spectral bandwidth of twice of that of fused

silica.

Acknowledgments

This work was carried out with the support of a num-

ber of research, equipment, and educational grants,

including ECS-0123484, ECS-0225930, INT-0129235,

NSF IGERT grant # DGE-0114418, and NSF-CNRS

# 13050. The authors acknowledge the assistance and

support of all the staff at ICMCB/LPCM, and particu-

larly to Laeticia Petit and Frederic Adamietz. Finally,

special thanks to Schott Glass Technologies Inc. forproviding us with the SF6 glass sample.

C. Rivero et al. / Journal of Non-Crystalline Solids 345&346 (2004) 396–401 401

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