Effects of impurities in cadmium halide glasses for 1.3 μm amplifier

J O U R N A L OF

ELSEVIER

Journal of Non-Crystalline Solids 184 (1995) 263-267

Effects of impurities in cadmium halide glasses for 1.3 Ixm amplifier Sophie Jordery a Mira Naftaly a Animesh Jha a,* Bryce N. Samson b a Department of Materials Technology, Brunel University, Kingston Lane, Uxbridge UB8 3PH, Middlesex, UK b Optoelectronics Research Centre, University of Southampton, Highfields, Southampton S095NH, Hampshire, UK

Abstract

Cadmium mixed halide glasses have been recently identified as a potential low-phonon energy host for a 1.3 ixm Pr 3+-doped optical fibre amplifier. The crystallization behaviour of the mixed halide glasses while reheated above their glass transition temperature is reported. The effect of impurities in the starting raw materials on the quality of the bulk glass is reported. The OH- absorption was monitored by Fourier transform infrared spectrometry, and related to the lifetime of the 1G4 excited state in pr3+-doped glass. The thermal stability of the glass was studied using heat treatment data obtained by a differential scanning calorimeter.

1. Introduction

Cadmium mixed-halide glasses have been shown to be a promising alternative to Z B L A N as a host glass for a praseodymium-doped fibre amplifier [ 1 3]. Cadmium-based glass has lower phonon energy than ZBLAN, leading to an increase in the non-radiative lifetime of the 1G4 excited state of praseodymium. The longer lifetime improves the quantum efficiency and gain of the 1.3 lxm transition. The lifetime of the 1G4 level of praseodymium in cadmium-based glass is ~-= 325 Ixs [1] compared with r = 100 Ixs in Z B L A N [4]. Although these glasses possess good thermal stability, confirmed by their large H R number ( H r = 1.3 compared with 0.9

* Corresponding author. Tel: +44-895 274 000, ext. 2402. Telefax: +44-895 812 636. E-mail: [email protected].

for ZBLAN), their use in the fabrication of optical devices is difficult due to their high sensitivity to moisture and low glass transition temperature (120°C). It has been shown [5-7] that a lower fluorine content in the glass reduces its chemical durability but enhances its thermal stability. Further, lower devitrification rates are observed in mixed fluorohalide glasses. The test composition used was ' C D ' (50CdF 2, 10BaC12, 40NaC1), which is a good compromise between the opposed requirements of stability and durability. In making cadmium-based glasses, it is important to maintain low impurity levels. This will ensure higher glass stability and produce lower loss fibres. Moreover, the presence of impurities such as O H - reduces the 1G4 lifetime in pr3+-doped glasses, which in turn leads to a decrease in the quantum efficiency of the 1.3 ~ m fibre amplifier [8]. The aim of this paper is to determine the effect of

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264

s. Jordery et aL /Journal of Non-Crystalline Solids 184 (1995) 263-267

impurities such as O H - , NO 3 and SO 2 - on the devitrification behaviour. The 1G4 lifetimes of pr3+-doped glasses were also correlated with the various levels of impurities of these glasses. In particular, the effect of O H - is reported.

0.8 ~e

v

0 t-

2. E x p e r i m e n t a l p r o c e d u r e

0.6

°m

E

The starting materials were supplied by Testbourne Ltd (CdF2), and Aldrich Ltd (BaC12 and NaC1). The glass samples were prepared and processed under a controlled atmosphere of dry nitogen in order to avoid hydrolysis, and then cast in a brass mould to form 0.5 cm thick slabs. A series of CD glasses, undoped and doped with water and nitrates, were prepared. There were indications that the melting schedule influences the final concentration of nitrates and hydroxyl ions dissolved in the glass. Since a rigid schedule coud not be adhered to for all glasses, the O H - , NO 3 and SO Econcentrations were estimated from the size of their principal peaks in the infrared aborption spectra. Fig. 1 shows two typical IR spectra: the dashed line spectrum is obtained from an undoped glass while the solid line spectrum is obtained from a glass doped with KNO 3. The thermal stability of glasses containing various levels of impurities was established using a differential scanning calorimeter (Perkin-Elmer DSC-7) set

tn t-

0.4

o

i--

0.2

0.0 4000

i

i

3000

2000

1000

Wovenumber (cm-1) Fig. 1. Absorption spectra of cadmium-based glass. - - - - - undoped; - - , doped with KNO3.

at the heating rate of 1 0 ° C / m i n . In these experiments, dry nitrogen was used as a purge gas through the head chamber in order to minimise the adsorption of O H - on the glass surface during heating. Isothermal experiments were carried out to estimate the time required for the crystallisation of the glass. From these results, time-temperature-transformation CITY) curves were constructed and the thermal stability of ' C D ' glasses was evaluated. The total

Table 1 Characteristic temperatures for cadmium mixed-halideglasses Glass

CD1 CD2 CD3 CD4 CD5 CD6 CD7 CD8 CD9 CD10 CD0

T~

Tx

Tp,

Tp2

Zp3

Trn

Z x - Tg

( + 2°C)

( + 1°C)

( + I°C)

( 4- l°C)

( + l°C)

( + I°C)

( + 2°C)

119 129 133 136 134 116 116 118 122 131 136

205 203 196 190 209 205 204 188 206 201 22Q

208 206 198 192 212 208 207 190 209 204 237

226 227 228 228 229 227 226 226 227 227 248

265 265 266 264 264 266 264 263 266 267 285

315 313 313 315 316 316 315 315 315 315 314

86 74 62 55 75 92 90 70 84 70 95

S a

2.5 4- 0.3 1.7 5:0.2 1.2 5:0.2 0.9 ___0.2 1.8 ___0.2 2.7 ~ 0.3 2.4 5:0.3 1.2 4- 0.2 2.2 4- 0.3 1.6 4- 0.2 5.6 4- 0.3

Tg, glass transition temperature; Tx, onset of crystallisationtemperature; Tp, peak crystallisationtemperature; Tin,melting temperature; S, stability parameter. a The errors on the characteristic temperatures are not independent.The errors in the stability parameters are related to the size of the measurement interval.

S. Jordery et al. /Journal of Non-Crystalline Solids 184 (1995) 263-267

integrated areas of the OH , NO 3 and SO 2 - absorption peaks were determined using a Nicolet FTIR spectrometer. Fluorescence lifetimes were measured using a Ti:sapphire laser as a pump and a solid state detector with a band-pass interference filter centered at 1.3 p~m.

265

centrations as in C P 5 , the characteristic temperatures are less affected, implying that the presence of Ca 2÷ cations adversely influences thermal stability to a greater extent than the anion NO 3 does. This was further confirmed by adding CaF 2 to the glass (CD8). Despite large differences in the size of the O H absorption peak (Table 2; CD1,6,7), the characteristic temperatures remain unchanged. By contrast, the incorporation of sulphates affects the glass stability even at very low concentrations. It is also worth noting that impurities generally influence the glass transition and the first crystallisation peak temperatures, but leave the other characteristic temperatures unchanged. Isothermal analysis was carried out on the glasses CD1, CD6, CD7, CD8. Fig. 2 presents their isothermal cur, es at the onset of crystallisatioq temperature. The CD1, CD6, CD7 isothermal curves have two well-defined devitrification peaks, corresponding to the first two primary crystalline phases, also observed in non-isoth rmal measurements, i.e., Tpl, Tp2 in Table 1. The third peak is not observed on an isothermal curve because its crystallisation energy is 20 times lower than that of the first two peaks, and the crystallisation occurs at higher temperatures. The isothermal curve for CD8 glass shows one peak only, which is explained by the fact that the first two peaks of crystallisation in CaF2-doped glass are much farther apart. The OH concentration has a smaller effect on the q-TT curves, as shown in Fig. 3 and Table 3, compared with the dopant CaF 2. Nevertheless, it is evident from Fig. 3 that the glass stability

3. Results

3.1. Impurity effect on the crystallisation behaviour Two different parameters can be used to estimate glass stability against devitrification. The difference between the onset of crystallisation and the glass transition temperatures (T x - Tg) is an indication of the nucleation rate, large values corresponding to low rates and increased stability. The parameter S = (T x - T g ) ( T p - Tx)//Tg [9] (Tp being the peak crystallisation temperature) emphasises the crystallisation behaviour upon reheating the glass. Given the assumption that the crystalline fraction is constant when the exothermic peak reaches its maximum value, a wider crystallisation peak implies a lower rate of devitrification at a given heating rate. The parameter S thus combines two types of glass stability evaluation. When C a ( N O 3 ) 2 is incorporated into the glass structure ( C D 1 - C D 4 ) , the two parameters decrease sharply indicating reduced stability of these glasses (see Tables 1 and 2). However, if K N O 3 replaces Ca(NO3-) 2 in the glass structure at comparable con-

Table 2 Integrated absorption peaks and impurity concentrations in cadmium halide glasses Glass

CD1 CD2 CD3 CD4 CD5 CD6 CD7 CD8 CD9 CD10 CD0

Integrated absorption peak (+ 10%)

Concentration (ppm) (+ 5%)

OH

Ca(NO3 )2,4H 20

19 31 47 310 570 130 160 55 210 250

NO3

SO4z -

6 . 11 42 22 38 40 35 95 90 8 33 8 . 4 15 14 1000 8 2200 excessively large

. 460 1970 3170 . . . .

KNO ~-

.

.

CaF2

. . . . 4350 -

.

. . .

. . . -

. . .

2080 . .

.

BaSO4

xI xz -

250 520

. . .

. . .

H 20

.

.

.

266

S. Jordery et al. /Journal of Non-Crystalline Solids 184 (1995) 263-267

Table 3 Time of the peak of crystallisationfor an isothermal transformation at the onset crystallisationtemperature Glass: Time (min) (+5%):

CDI 7.1

CD6 6.8

CD7 6.5

CD8 1.6

240

0 0

230

CD0 9.1 o

~

0 0

220

0 L

against devitrification is marginally improved when OH concentration decreases. The CD0 glass was prepared using CdF 2 supplied by Alfa Product and BaCI2, 2 H 2 0 supplied by BDH (Merck). The characteristic temperatures, as shown in Table 1, indicate that the glass is more stable, with larger values of T× - Tg and S, and its devitrification rate is slower (see Fig. 3), despite the fact that this glass was prepared using chemicals of purity lower than the other ' C D ' glasses, and therefore contained high concentrations of O H - , nitrates and also sulphates. The glass appeared translucent due to the presence of microscopic crystals. This suggests that

2o

21o

E

200

L.

[] 0 V z~ 0

z~ A

i.-

190

A Zk

180 0

i

i

i

5

10

15

Time

CD1 CD6 CD7 CD8 COO 25

20

(min)

Fig. 3. Time-temperature-transformation curves for the CD1, CD6, CD7, CD8 and CD0 glasses.

the presence of large concentrations of impurities induces extensive bulk crystallisation during glass preparation. The composition of the remaining matrix may therefore be expected to change. It is possible that such compositional change could improve the thermal stability of the glass, explaining the delay of crystallization in the CD0 glass.

60

5o E -,-, 40 O

3.2. Impurity effect on the P r 3 + 1G 4 lifetime

.,6 3o

7

i 20

The main obstacle in achieving an efficient 1.3 txm praseodymium-doped amplifier is the short lifetime of the 1G4 metastable level [1-3]. pr3+-doped cadmium-based glass, having a phonon energy lower than silica and ZBLAN glasses [4], gives rise to 1.3 Ixm fluorescence with a lifetime of 325 Ixs [1]. However, impurities in glass can reduce the lifetime via two different mechanisms: they can act as activators in energy transfer in which the energy migrates

V

lO 0

2

4

6 Time

8

10

14

12

(min)

Fig. 2. Isothermalcurves at the onset of crystallization.

Table 4 The 1G4 lifetime in pr3+-doped 'CD' glass as a function of the integrated intensity of the OH absorptionpeak Glass: OH- peak area (+ 10%): NO3 peak area (+ 10%): SO4 peak area ( + 10%): Lifetime ( + 5 Its):

CD1 19 6 328

CD8 55 4 5 325

CDll 24 12 16 320

CD6 130 8 33 304

CD7 160 8 300

CD12 250 8 291

CD13 295 13 287

CD5 570 95 90 240

CD0

CD9

CD10

210

1000 205 14 310

2200 250 8 312

S. Jordery et al. /Journal of Non-Crystalline Solids 184 (1995) 263-267

to the rare-earth ions. It is also worth noting that the CD0 glass, having the highest content of all three impurities, i.e., O H - , nitrates and sulphates, also demonstrated the shortest lifetime (210 Ixs). The role of NO 3 ion on fluorescence lifetime is not very clear at this stage, and further analysis is required to establish its effect on t h e 1G4 lifetime.

340

I+

.~ 330 E

k~ q)

320

._

310

++

(9 c(D

(9 300 (0

0

~

290

L

280 0

I

I

50

100

I

I

150 200

OH a b s o r p t i o n

267

++ I

I

250

300

peak area X 10 - 3

Fig. 4. The I G 4 lifetime as a function of the integrated OHabsorption peak.

from a Pr 3+ ion to an impurity ion and is lost non-radiatively, or, if they are incorporated in the structure, as origins of high-energy phonons which facilitate non-radiative transitions. In halide glasses, the main impurities are the hydoxyl group, nitrates and sulphates. The effect of the OH dopant concentration on the lifetime of the l G4 excited state of praseodymium in ' C D ' glasses is shown in Table 4. It is clear that the strongest effect on the lifetime is due to the presence of OH in the glass. Fig. 4 shows the relationship between the m e a s u r e d I G 4 lifetime and the integrated intensity of the O H - absorption peak. The graph indicates that a further reduction in impurity levels may lead to even longer lifetimes. It is of interest that, although the two glass samples doped with BaSO 4 have large O H - and SO4:- absorption areas, their lifetime is not reduced as much as could be expected from the previous data. A possible reason may be that a large fraction of O H - ions are sited near SO~ ions which do not couple efficiently

4. Conclusions

It has been shown that the OH , NO 3, SO 2- and dopants adversely influence the kinetics of crystallisation in cadmium-based glasses, but not to equal extent. Moreover, O H - ions are highly detrimental to the I G 4 excited state lifetime in the praseodymium-doped cadmium halide glass. C a 2+

This work is supported by E E C / R A C E gramme (Project No. 2038).

Pro-

References [1] B.N. Samson, M. Naftaly, D. Hewak, S. Jordery, J.A. Medeiros Neto, H.J. Tate, R.I. Laming, A. Jha, R.S. Deol and D.N. Payne, in: Proc. Conf. OAA'93 (Optical Amplifiers and their Applications), July 1993, Yokohama, Japan. [2] D.W. Hewak, R.S. Deol, J. Wang, G. Wylangowski, J.A. Medeiros Neto, B.N. Samson, R.Y. Laming, W.S. Brocklesby, D.N. Payne, A. Jha, M. Poulain, S. Otero, S. Surinach and M.D. Baro, Electron. Lett. 29 (1993) 237. [3] M.A. Newhouse, R.F. Bartholomew, B.G. Aitken, A.L. Sadd and N.F. Borelli, in: Proc. Conf. OAA '92 (Optical Amplifiers and their Applications), Santa Fe, NM, USA, PD16, p. 70. [4] Y. Ohishi, T. Kanamori, T. Kitagawa, S. Takahashi, E. Snitzer and G.H. Sigel, Opt. Lett. 16 (1991) 1747. [5] J.L. Mouric, M. Matecki, M. Poulain and M. Poulain, Mater. Sci. Forum, 5 (1985) 135. [6] M. Matecki, M. Poulain and M. Poulain, Mater. Sci. Forum 19&20 (1987) 47. [7] M. Matecki and M. Poulain, J. Non-Cryst. Solids 140 (1992) 82. [8] E.G. Bondarenko, E.I. Galant, S.G. Lunter, A.K. Przhevuskii and M.N. Tolstoi, Sov. J. Opt. Technol. 42 (6) (1975) 333. [9] M. Saad and M. Poulain, Mater. Sci. Forum 19&20 (1987) 11.