clad compositions with high numerical aperture for single-mode fluoride fibres

JOURNALO¥

ELSEVIER

Journal of Non-CrystallineSolids 201 (1996) 237-245

Core/clad compositions with high numerical aperture for single-mode fluoride fibres M. Braglia, J. Kraus *, S. Mosso CSELT, Via Reiss Romoli 274, 1-10148 Turin, Italy

Received 21 June 1995; revised 15 September 1995

Abstract ZBLYALi core glasses modified by the substitution of LiF for BaF2 or the addition of PbF 2 and cladding glasses of ZBLYAN composition, into which HfF4 and NaF were substituted, are described and characterised by their thermal glass parameters and refractive index changes. Glasses studied, with Tg range from 533 to 573 K, yielded numerical apertures as high as 0.39, if ZBLYALiPb glasses were used as core. Since incorporation of PbF2 in ZBLYALi glass led to a strong decrease in glass stability, a closer look at the crystallisation behaviour of ZBLYALiPb was taken. Addition of PbF2 was found to suppress precipitation of LiBaZr2G j in favour of a (Ba, Pb)ZrF6 solid solution with orthorhombic structure.

1. Introduction The first prototype of a praseodymium-doped fluoride fibre amplifier (PDFFA) operating in the 1.3 Ixm window has been described [1]. The key for its realisation was the capability of manufacturing tens of meters of low-loss small core single-mode fluoride fibre with a numerical aperture (NA) as high as 0.4 [21. The fibre, the exact core-cladding compositions of which have not so far been disclosed, includes fluorides of Zr, Hf, Ba, La, A1, Y, Li, Na, and Pb. Based on those components, we present in this paper core glasses derived from ZBLYALi compositions and modified by PbF 2, and try to match them with cladding glasses of the Z B L Y A N type, the refractive

* Corresponding author. Tel.: +39-11 228 5764; fax: +39-11 228 5085.

index of which was lowered by substitution of Hf for Zr. The idea was to trace the scope and limitation of compositions thus modified if they are to be used in a PDFFA. Since the incorporation of refractive index modifiers in a given glass matrix is always accompanied by changes in glass thermal parameters, the suitability of promising c o r e / c l a d couples was evaluated not only in terms of attaining the highest NA but also in terms of a match in glass transition temperature, Tg, and sufficient thermal stability, AT. In addition, the crystallisation behaviour of ZBLYALi core glasses on addition of PbF 2 was investigated.

2. Experimental Glasses were prepared from optical grade BDH products. Almost all the core glasses were doped

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)00143-3

238

M. Braglia et al. / Journal of Non-Cr>'stalline Solids 201 (1996) 237-245

with 500 ppmw of PrF 3 of 99.9% purity purchased from Rare Earth. The starting fluoride mixture was melted at 1123 K in a platinum crucible under dry N 2 for 2 h; then, the temperature was decreased to 1023 K and the melt kept at that temperature for 2 h. The melt was then poured into a brass mould preheated near T~ of each glass and cooled slowly to room temperature, to give a glass slab of 100 mm X 15 mm × 5 mm in size. The four series of glasses prepared comprised: (i) a ZBLYAN cladding glass of nominal composition (mol%) 5 1 Z r - 1 8 . 5 B a - 4 L a - 2 Y - 4 . 5 A I - 2 0 N a with a modified HfF4 and NaF content; (ii) a ZBLYALi core glass of nominal composition 5 8 . 0 5 Z r - 2 1 B a - 4 . 6 L a - 2 . 2 5 Y - 4 A l - 1 0 . 1 Li into which BaF 2 was substituted for LiF; (iii) a ZBLYALi core glass of nominal composition 5 5 . 2 Z r - 2 1 . 9 B a - 4 . 4 L a - 2 . 1 5 Y - 3 . 3 5 A I - 13Li which was modified by the addition of PbF2; (iv) a ZBLYALiPb core glass of nominal composition 5 1 Z r - 2 0 B a - 4 L a - 2 Y - 3 A I - 1 2 L i - 8 P b which was modified in its LiF content. The refractive index, n, was measured with a Pulfrich refractometer at the wavelength of 633 nm on cut and polished samples. The accuracy of the measurement was estimated to be ___4 × 10 -4. The thermal analysis was performed by a Perkin Elmer DSC-7 differential scanning calorimeter under Ar atmosphere at a heating rate of 10 K / m i n on single glass chips of 10 to 20 mg sealed in aluminum pans. Thermal stability was evaluated as the difference AT between the extrapolated temperature for

the onset of crystallisation, Tx, and the glass transition temperature, Tg. Within one batch, the estimated error of the thermal parameters was ___2 K for Tg and _ 3 K for Tx, respectively. The identification of the crystalline phases formed on heating was carried out on crushed and powdered samples with Siemens Crystalloflex 80 X-ray diffractometer using the CuKo~ radiation. If the lattice parameters of solid solutions were to be determined, LaF 3 served as an internal standard and a SIMPLEX program was used for their calculation taking the reflections from 28 ° to 80 ° 2 O.

3. Results 3. l. Substitutions o f HfF4 and N a F into a ZBLYAN cladding glass The continuous substitution of HfF4 for ZrF4 in ZBLAN glass is known to decrease the refractive index and increase Tg [3,4]. However, little work has been done on the way the thermal stability is affected [5]. Taking as a starting point a ZBLYAN composition with high AT (denoted AI in Table 1), previously described by Poignant et al. [6], Hf was substituted for Zr. In a second step, the Na concentration of the Z B L Y A N was decreased from 20 to 10 tool%, while the molar ratio of other components was kept constant. This series was then repeated with Hf in place of Zr. Table 1 gives compositions, thermal parameters and refractive indices, respectively.

Table 1 Batch compositionand physical parameters of a ZBLYAN cladding glass, and after substitutionswith HfF4 and NaF Glass

ZrF4

HfF4

BaF2

LaF3

AIF3

YF3

NaF

Tg (K)

Tx (K)

AT (K)

n

AI A2 A3 A4 A5 A6 A7 A8 A9

51 50 49 41 54.2 57.4 -

1 2 l0 51 54.2 57.4

18.5 18.5 18.5 18.5 18.5 19.6 20.8 19.6 20.8

4 4 4 4 4 4.3 4.5 4.3 4.5

4.5 4.5 4.5 4.5 4.5 4.8 5.1 4.8 5.1

2 2 2 2 2 2.1 2.2 2.1 2.2

20 20 20 20 20 15 10 15 10

542+3 a 542 543 543 552 553 564+2 ~ 567 + 2 c 575

655+9 a 648 652 641 658 681 674+4 b 693 + 10 c 712

113+ 12 a 106 109 98 106 128 110+3 b 126 + 12 c 137

1.4945 1.4943 1.4941 1.4920 1.4845 1.4972 1.5037 1.4878 1.4980

Average value from 18 glass batches. b Average value from two glass batches. c Average value from eight glass batches.

M. Braglia et a l . / Journal of Non-Crystalline Solids 201 (1996) 237-245

239

1.4950

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0

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20

30

40

L 50 525

HIF4 concentration ( mol % ) Fig. I. Variations in refractive index with HfF4 concentration in a ZBLYAN cladding glass. The line is drawn as a guide for the eye.

As can be seen, slight modifications in composition on a level of up to 2 mol% of Hf (A2 and A3, respectively) did not significantly affect either thermal stability or refractive index. When we replaced 10 mol% of Zr by Hf (A4), the AT value decreased, as was the case previously when half of the Zr content was replaced by Hf [5]. Total substitution of Hf for Zr (A5) resulted in a AT value comparable to those obtained after small additions of Hf. In Fig. 1, the decrease in refractive index with increasing concentration of Hf is illustrated. By completely substituting Hf for Zr, a maximum difference in refractive index of 1 × 10 -2 could be achieved. Reducing the Na concentration to 15 mol% produced an increase in glass stability of at least 13 K in both cases (A6, A8). When the Na content was decreased to 10 mol%, the AT value of the HBLYAN remained high (A9) whereas that of the ZBLYAN fell well short with an average AT of 110 K (A7). These results are qualitatively in agreement with the findings of Seddon et al. [7] who attained the highest thermal stability at around 15 mol% of Na by substituting NaF in a ZBLAN composition which was quite similar to our ZBLYAN as far as its content of Zr and Ba was concerned. As an illustration of the slow crystallisation kinetics of the Z B L Y A N / H B L Y A N compositions which translate into large crystallisation peaks and low

i

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575

600

625

650

675

700

temperature, T (K) Fig. 2. Normalized DSC traces of ZBLYAN (HBLYAN) cladding glasses A1, A4, A5, and A8.

enthalpies of crystallisation, the DSC scans of the glasses A1, A4, A5 and A8 are displayed in Fig. 2 in the temperature range from Tg to the crystallisation peak temperature. The effect of reduction of NaF content on refractive index of both ZBLYAN and HBLYAN series is outlined in Fig. 3. While the difference in refractive index between the two series remained virtually con15050

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Fig. 3. Variations in refractive index with NaF concentration in ZBLYAN and HBLYAN cladding glasses, respectively. Lines are drawn as a guide for the eye.

240

M. Braglia et al. // Journal of Non-Crystalline Solids 201 (1996) 237-245 572

1.5160

570

1.5155

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1.5150

566

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LiF concentration

(mol%)

5 10

Fig. 4. Variationsin glass transitiontemperature with LiF concentration in a ZBLYALi core glass. The line is a best fit to the measured data.

stant in the case of concentrations ranging from 20 to 15 mol% of NaF, the situation changed drastically when the concentration was lowered to 10 mol%. The index difference was found to be nearly halved because of a steeper rise in the refractive index of the HBLYAN glass. 3.2. Substitution o f B a F 2 f o r L i F into a Z B L Y A L i core glass

To find matches for cladding glasses with high Tg values, that is compositions A5 to A9, a ZBLYALi composition was chosen (denoted B1 in Table 2) with a Tg at 553 K. Basically, it derived from the Z B L Y A N with 10 mol% of Na (A7) in which LiF was totally substituted for NaF to produce an increase in refractive index. Because of solubility problems, the AIF3 content had to be decreased to 4

F

;

!

~

6

7

8

9

J 10

LiF concentration (tool %)

Fig. 5. Variations in refractive index with LiF concentrationin a ZBLYALi core glass. The line is drawn as a guide for the eye.

mol%. The ratio of the other components was adjusted to match that in the Z B L Y A N glass. Subsequently, a series of glasses was made in which BaF 2 was substituted for LiF. Compositional changes, thermal parameters and refractive indices are summarised in Table 2. While both Tg and refractive index increase in a linear fashion with decreasing concentration of LiF (Figs. 4 and 5), no clear trend is found for thermal stability. 3.3. Addition o f P b F 2 to a Z B L Y A L i core glass

PbF: is known to be efficient in raising the refractive index when incorporated in Z B L A N type glasses with simultaneous Tg reduction. It was observed that, the more the amount of lead added, the less stable the glass becomes against devitrification. Thus, the maximum concentration of PbF: in

Table 2 Batch compositionand physicalparameters of a ZBLYALi core glass, and after substitutionof BaF2 for LiF Glass

ZrF4

BaF2

LaF3

YF 3

AIF3

LiF

Tg (K)

Tx (K)

AT (K)

n

B1 B2 B3 B4

58.05 58.05 58.05 58.05

21 23 24 26.1

4.6 4.6 4.6 4.6

2.25 2.25 2.25 2.25

4 4 4 4

10.1 8.1 7.1 5

553 560 563 570

654 673 670 668

101 113 107 98

1.5115 1.5136 1.5139 1.5154

M. Braglia et al./Journal of Non-Crystalline Solids 201 (1996) 237-245

single-mode fibres with high NA achieved so far varies from 8 to 12 mol% depending on the glass composition [3,6]. ZBLALi glasses, for example, have been shown to accept higher amounts of lead than ZBLAN [6]. In order to provide appropriate core glasses for the claddings with low Tg values, we chose another ZBLYALi composition (C1) which had a Tg of 542 _ 2 K and a AT value of 108 _ 8 K. In contrast to previous work in which PbF 2 was substituted for BaF 2 [6,8-10], lead fluoride was added continuously to 10 mol% while the ratio of the other components remained unchanged. The results are shown in Table 3. With increasing concentration of PbF 2, the Tg decreased continuously but not in a linear way from 542 K to 533 K. Across the series, the stability of the glass went from 108 K to 83 K, while the refractive index increased by 2.7 × 10 -2 (Fig. 6). In an attempt to shift Tg upwards without sacrificing the benefits of a high lead concentration as far as the refractive index was concerned, the ZBLYALi composition with 8 mol% of PbF 2 (C4) was modified by continuously reducing its Li content from 12 to 7 mol%. Again, the ratio of the other compounds was kept constant (Table 4).

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1.5250 .__. 13 .

1.5200

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1.5150

1.5100

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2

4

6

8

10

PbF 2 ~ n c e n t m t i o n ( mol % ) Fig. 6. Variations in refractive index with PbF 2 concentration in a ZBLYALi core glass. The line is drawn as a guide for the eye.

As can be seen from Table 4, decreasing the Li content to 7 mol% resulted in an increase of both Tg and refractive index without compromising thermal stability. Since the Tg increased by more than 10 degrees, it should be possible to match the composition C7 with the HBLYAN cladding A5 which had the lowest refractive index of all the claddings studied, which would result in a NA of 0.39.

Table 3 Batch composition and physical parameters of a ZBLYALi core glass, and after addition of PbF 2 Glass

ZrF4

BaF 2

LaF 3

YF3

AIF3

LiF

PbF 2

Tg (K)

Tx (K)

AT (K)

n

CI C2 C3 C4 C5

55.2 53 52 51 49.7

21.9 21 20.6 20 19.7

4.4 4.2 4.1 4 4

2.15 2.1 2 2 1.9

3.35 3.2 3.1 3 3

13 12.5 12.2 12 11.7

4 6 8 10

542+2 a 536+ 1 b 535 533+ 1 c 533

650-4-6 a 6394-4 b 634 6194-6 c 616

1085: 8 ~ 1035:4 b 99 864-7 ¢ 83

1.5111 1.5206 1.5270 1.5323 1.5378

a Average value from 10 glass batches. b Average value from two glass batches. c Average value from seven glass batches.

Table 4 Batch composition and physical parameters of a ZBLYALiPb core glass, and after reduction of the LiF content Glass

ZrF4

BaF 2

LaF 3

YF3

AIF3

LiF

PbF2

Tg (K)

Tx (K)

AT (K)

n

C4 C6 C7

51 52.2 54

20 20.4 21.1

4 4.1 4.2

2 2.0 2.1

3 3.1 3.2

12 10 7

8 8.2 8.4

533+ 1 a 536 544

619+6 a 631 634

865:7 ~ 95 90

1.5323 1.5335 1.5345

a Average value from seven glass batches.

242

M. Braglia et al. / Journal of Non-Crystalline Solids 201 (1996) 237-245

Table 5 Crystalline phases found in ZBLYALi and ZBLYALiPb by interrupting DSC runs at the end of the first and the second crystallisation peak. The main phase of each sample is indicated by italic letters. The orthorhombic modification of BaZrF6 is denoted as [3-BaZrF6 (o)

CI (0% PbF2)

C2 (4% PbF2)

5

C4 (8% PbF2)

~'-

C5 (10%PbF2)

~ ' 1

% PbFz

1st peak

2nd peak

0

LiBaZr2 FII

LiBaZr 2 Fjl

[3-BaZr2Flo (traces)

[3-BaZr2Flo I3-BaZrF6 (traces)

4

8 ,

1

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525

550

575

600

625

650

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700

l0

LiBaZr 2 F H

fl-BaZrF6 (o)

[3-BaZrF6 (0) 13-BaZr2Fio (traces)

LiBaZrEFj i

fl-BaZrF6 (o)

fl-BaZrF6 (o)

LiBaZr2FI I (traces)

LiBaZr2Fi i

fl-BaZrF 6 (o)

fl-BaZrF6 (o)

LiBaZr2Flt

temperature, T (K) Fig. 7. Normalised DSC traces of ZBLYALiPb glasses with different PbF2 content.

3.4. C r y s t a U i s a t i o n o f a Z B L Y A L i with PbF 2

core glass doped

F r o m Table 3 it is evident that the continuous addition of lead fluoride was very effective in increasing the refractive index but led to a decrease in thermal stability with increasing Pb 2+ concentration. The D S C traces of some o f the glasses from Table 3 are collected in Fig. 7. Invariably, two crystallisation peaks were observed. At low P b F 2 content, that is from 0 to 4 mol%, the m a i n crystallisation event is represented by a large first peak, while the second one is m u c h smaller and is partially overlapped by the first one. At a lead concentration of 8 mol%, the situation is reversed. The second peak evolves most of the crystallisation heat. O n further addition of P b F 2 to 10 mol%, the two crystallisation events are well separated. In order to identify the crystalline phases formed, two samples of each glass were heated in the D S C at a rate of 10 K / m i n and the run interrupted at the end o f the first and the second crystallisation peak. Crushed and powdered samples were subjected to X R D analysis and phases found are summarised in Table 5. As is k n o w n from a previous study on Z B L A L i glass [11,12], the first crystal phase to appear on heating above Tg is the ternary phase LiBaZr2FI1.

m

| 0 X

20

25

30

35

40

45

50

55

60

2 Theta

Fig. 8. XRD diffractograms of PbZrF6 (trace a); (Bao.sPb0.5)ZrF6 solid solution (trace b, S = standard LaF3); ZBLYALiPb (C5) interrupted after the I st crystallisation peak (trace c), and after the 2nd crystallisation peak (trace d, + = LiBaZr2Fl~), respectively. The shift of corresponding reflections (same hkl index) on incorporation of Pb2÷ is partially indicated by dotted lines. For the sake of clarity, the internal standard LaF3 is only shown in trace b.

M. Braglia et al. / Journal of Non-Crystalline Solids 201 (1996) 237-245

This could be confirmed for our basic ZBLYALi glass where LiBaZr2Fll was the predominant phase formed. The addition of PbF 2, however, suppressed the formation of this phase and gave instead orthorhombic 13-BaZrF6 (Table 5). At a lead fluoride concentration of l0 mol%, only orthorhombic 13BaZrF6 precipitated during the first crystallisation event (Fig. 8, trace c), while LiBaZr2F~I began to crystallise as a minor phase in the subsequent exotherm (Fig. 8, trace d). When the XRD pattern of the 13-BaZrF6 was examined in crystallised ZBLYALiPb glass, its reflections were found to have shifted towards higher 219 angles whereas those of LiBaZr 2F~ never changed position. This suggests an incorporation of Pb 2+ in the 13-BaZrF6 lattice. As is known, I3-BaZrF6 and PbZrF6 are isomorphous and crystallise in an orthorhombic structure. Of the two phases, PbZrF6 has the smaller lattice parameters [13,14]. The first hint of incorporation of Pb 2+ into 13BaZrF6 was given previously by Carter et al. [15] in a study on the crystallisation of a ZBLAN core with low PbF 2 content. As ZBLAN glass follows a different devitrification pathway than ZBLALi, Pb 2÷ was found to be incorporated in disordered 13-BaZrF6 which, together with NaZrF5, precipitates first when the glass is heated above Tg. The cell of disordered I3-BaZrF6 could be described by a tetragonal system [161. In order to ascertain the amount of Pb 2+ incorporated, we used a calibration curve based on XRD data from various orthorhombic (Ba, Pb)ZrF6 solid solutions prepared as standards, in which the volume of the unit cell was plotted as a function of the Pb 2+ content in the solid solution (Fig. 9). To obtain the calibration curve, pure PbZrF6 was synthesised by melting stoichiometric amounts of PbF 2 and ZrF4 at 1123 K under N 2 for 0.5 h and then subjecting them to rapid cooling. The lattice parameters were found to be a = 7.552 A, b = 11.127 A, c = 5.312 ,~ and match available literature [13,14]. The X-ray diffractogram of PbZrF6 is shown in Fig. 8, trace a. By melting together BaF 2 and ZrF4 as described above, monoclinic et-BaZrF6 was obtained. However, all the mixtures of et-BaZrF6 and P b Z r F 6 studied, i.e., from 50% of et-BaZrF6 up to 90%, were transformed into solid solutions with orthorhombic o

243

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,,

455O0 ~

[

" ~

452.50 450.00 447.5O i 445.00 ~ 0

10

~ 20

,

---

30

,

40

--w

50

60

= 70

-,

80

90

100

Pb =* concentration (%)

Fig. 9. Volume of the unit cell of an orthorhombic (Bal_x,Pbx)ZrF 6 solid solution versus its Pb 2+ concentration. The best fit straight line is shown.

structure on melting and subsequent cooling. In Fig. 8, trace b, the diffractogram of the (Ba0.5,Pb0.5)ZrF 6 solid solution is depicted. The lattice parameters of I3-BaZrF6 were determined from crystals found in partially devitrified ZBLAN glass. In the two crystallised samples of the glass with 10 mol% of PbF 2 (C5), cell volumes of 465.9 A and 466.2,4, were found, which translate to lead concentrations of 34.1% and 33.2%, respectively. This correlates satisfactorily with the theoretical value of 33.7% if it is assumed that the lead is randomly incorporated in the (Ba, Pb)ZrF6 solid solution that forms first. The situation is more complicated for glasses with minor PbF 2 content where the precipitation of 13BaZrF6 and LiBaZr2Fll is not properly separated. For example, the Pb 2+ content in the (Ba,Pb)ZrF6 solid solution of two glasses of C4 composition (8 mol% of PbF2), whose samples had been interrupted after the second crystallisation peak, amounted to 29. 1% and 35.0%. The deviation of the latter from the batch value of 28.6% may be accounted for the slightly different crystallisation kinetics of that glass. While in the DSC thermogram of the former glass the first crystallisation peak is clearly discernible from the second one though not completely separated (Fig. 7, C4), in

244

M. Braglia et al. / Journal of Non-Crystalline Solids 201 (1996) 237-245

the DSC trace of the latter it merely appeared as a shoulder. This suggests that, depending on the formation of the Li phase which deprives the glass matrix of barium, the 13-BaZrF6 tends to incorporate more lead. Further support is provided by glass C2 (4 mol% of PbF2), in which Pb 2÷ concentrations of 25.6% and 23.2% were found in the precipitated 13-BaZrF6, which exceed the theoretical value of 16% by roughly 50%.

2.5

0% I~F 2 _ ~ _

f

~

2.0

O ®

t--

1.5

4. Discussion 1.0

4.1. Glass stability In contrast to other studies in which compositional changes were caused by the substitution of one component for another, a different approach was adopted for most of the glass series presented here. Glass compositions with a large AT were chosen in which only one component was altered or added, while the ratio of all the other constituents was kept constant. This worked well in the case of the ZBLYAN and HBLYAN claddings where a decrease in the NaF content produced a marked increase in glass stability over a wide compositional range. However, a comparison of Tables 1-4 clearly shows that it is the core glass modified by PbF 2 for which glass stability constitutes an issue of major concern. If there is to be a sharp rise in the refractive index, a certain degree of thermal stability must be sacrificed. Across our ZBLYALiPb series, AT dropped to 83 K at 10 mol% of PbF 2. Since data on thermal stability of core compositions are scarce in the literature, we reproduced three glasses from a ZBLALi series of nominal composition 54Zr-(30 - x)Ba-2.5A1-3.5La-10Li-xPb, in which Pb had been substituted for Ba [6]. The glasses with x = 0, 4, 6 mol% of lead fluoride, whose DSC thermograms are shown in Fig. 10, exhibited TgS of 545, 539 and 540 K, and AT values of 81, 86 and 81 K, respectively. Since their TgS and devitrification kinetics were similar to those of the corresponding ZBLYALiPb compositions in Table 3, a direct comparison of the AT values could be made. There clearly emerged an enhanced stability of the ZBLYALiPb compositions. For instance, at a concentration of 6 mol% of PbF 2, the difference in

500

i

i

I

i

~

i

i

i

525

550

575

600

625

650

675

700

temperature, T (K) Fig. t0. Normalised DSC traces of ZBLALiPb glasses with different PbF2 content.

thermal stability was about 18 degrees. And even after incorporation of 10 mol% of PbF 2 into ZBLYALiPb, its thermal stability still equals that of the ZBLALiPb with only 6 mol% of PbF 2.

4.2. Refractive index Fig. 11 displays the refractive indices of all the glasses prepared versus their glass transition temperature. In addition, and as a guide for the eye, highest and lowest refractive indices, respectively, were connected by dotted lines across the whole Tg range. If we assume that, in view of fibre pulling, it is beneficial for the cladding glass to have a Tg which is a few degrees higher than that of the core, four core/clad pairs can be identified, i.e., C5/A4, C7/A5, B 3 / A 8 and B 4 / A 9 , each of them representing the highest NA possible in a special Tg range. The NAs were calculated to be 0.37, 0.39, 0.28, and 0.23, respectively. It becomes evident that for the realisation of a single-mode fibre suitable for amplification at 1.3 Ixm, which requires the highest NA, only core compositions containing PbF 2 at a level of at least 8 mol% will be of interest, hence, appropriate couples will necessarily lie in the low Tg range between 533 and 553 K. Although even 4 mol% of PbF 2 in ZBLYALiPb

M. Braglia et al. / Journal of Non-Crystalline Solids 201 (1996) 237-245 1.5.~0 • 0 • El

C5

0 O0 i-

C7

ZBLYALi ZBLYALiPb

ZBLYAN/ZHBLYAN HBLYAN

0

1.5300

245

expected to decrease further due to the easy formation of a (Ba, Pb)ZrF6 solid solution, whose predominance over LiBaZr2F,~ was found to depend on the Pb concentration in the glass.

g •1o e- 1.5200

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Acknowledgements

¸

1.5100

1.5000

o

IIi 1.4900

The authors wish to thank Dr F. Taiariol for conducting the XRD measurements.

A4

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References

i

i

i

~

i

i

530

540

550

560

570

580

glass tmnsit~n temperature, Tg (K) Fig. 11. Refractive indices, n, of all the ZBLYALi, ZBLYALiPb, ZBLYAN/ZHBLYAN and HBLYAN glasses, respectively, as a function of their glass transition temperature, Tg. The core/clad couples C5/A4, C7/A5, B3/A8, B4/A9 represent the highest NA possible in a given Tg range.

suffice to lower the Tg considerably, an off-set could be reached by decreasing the Li content (Table 4). Thus, it was possible to raise the Tg to such an extent that the full potential of HfF4 as index decreaser in the cladding could be exploited, which, in the case of couple C 7 / A 5 , resulted in a NA of 0.39. On the other hand, ZBLYALi alone led only to a NA of 0.28, even if matched with a HBLYAN (couple

B3/A8). 5. Conclusion Core/clad couples with very high NAs, i.e., 0.37 to 0.39, have been achieved by combining ZBLYALiPb core glasses with claddings of the ZBLYAN or HBLYAN type. The amount of PbF 2 added varied from 8.4 mol% in the latter case to 10 mol% in the former, but both figures should not be considered to be the upper limit for the incorporation of Pb in the ZBLYALi glass matrix. A further raise in NA is therefore still possible. Glass stability is

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