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BIG fluoride glass optical fibers with improved NA
] O U R N A L OF
Journal of Non-Crystalline Solids 161 (1993) 161-164 North-Holland
BIG fluoride glass optical fibers with improved NA N. R i g o u t , J.L. A d a m a n d J. L u c a s Laboratoire des Verres et C~ramiques, UA CNRS 1496, Campus de Beaulieu, Universit~ de Rennes L 35042 Rennes c~dex, France
A low refractive index fluoride glass based on barium, indium and gallium has been synthesized. By addition of aluminum, sodium and potassium fluorides to the base glass (BIGaZYbTZr), an index as low as 1.482 is obtained. The glass physical parameters are given and are found to be fully compatible with those of the base glass. A preform suitable for fiber drawing and exhibiting a theoretical numerical aperture of 0.26 is obtained. Optical losses are measured for the best fiber.
1. Introduction The development of 1.3 txm amplifiers and other devices requires optical fibers with high numerical aperture (NA) [1]. Fluoride glasses based on barium, indium and gallium have been demonstrated to be good candidates for active optical fiber applications because of lower multiphonon relaxations and better quantum efficiencies in that host than in Z B L A N glass [2,3]. The purpose of this study is to improve the N A values obtained until now on non-doped fibers from the composition B I G a Z Y b T Z r . To the best of our knowledge, the N A value obtained in this work is the largest one reported to date for two compatible compositions of the B a - I n - G a - g l a s s series.
2. Experiment The glassy samples have been prepared following conventional methods [4]. They have been characterized by differential thermal analysis (DTA) and by thermal expansion coefficient measurements in the range 100-250°C. The stability has been investigated by means of the Hruby Correspondence to: Dr N. Rigout, Laboratoire des Verres et C~ramiques, UA CNRS 1496, Campus de Beaulieu, Universit~ de Rennes I, 35042 Rennes c~dex, France.Telefax: + 33 99 28 16 00.
factor defined by eq. (1): Hr=(Tc-
Tg)/(TI-
Tc),
(1)
where T~, Tc and T 1 are the glass transition, crystallization and liquidus temperatures. The refractive indices, n D, have been determined for the sodium D-line at room temperature.
3. Results Various substitutions have been carried out on the base glass whose composition is (in mol%): 30B a F 2 - 1 8 I n F 3 - 1 2 G a F 3- 2 0 Z n F 2 - 1 0 Y b F 36ThFn-4ZrF4 which has been chosen to form the core of the preform. It will be referred to as BIG glass in the text. In the aim of decreasing the refractive index, substitutions have been made with cations with smaller electronic polarizability such as AI 3÷, Na ÷ and K ÷ [5]. 3.1. A l u m i n u m substituted glasses
Three types of substitutions have been carried out: on barium, on indium and on both indium and gallium. The different characteristics of the new glasses are listed in table 1. Stable glasses containing up to 8% aluminum could by synthesized. They exhibit relatively high Hruby factors, comparable to that of the base
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
N. Rigout et al. / BIG fluoride glass optical fibers
162
Table 1 Physical constants of Al-substituted BIG fluoride glasses Glass
Tg (°C)
Te (°C)
Tm (°C)
T 1 (°C)
Hr
nD
ot (10 -7 K -1)
An (%)
BIG 4%Al/Ba 8%AI/Ba 4%Al/ln 8%Al/ln 8%A1/ln-Ga
332 338 341 337 351 342
460 480 480 470 468 470
576 580 596 587 576 588
609 617 629 612 655 640
0.86 1.04 0.93 0.94 0.63 0.75
1.505 1.503 1.494 1.502 1.499 1.502
171 167 158 178 174
0.73 0.40 0.20
glass. 12%-aluminum substitutions have been attempted as well. They lead to partially crystallized unstable glasses. It is noteworthy that the 8% A 1 / I n - G a substitution ( H r = 0.75) leads to better glasses than the 8% A1/In-alone substitution (H r = 0.63). This indicates that a constant 18 indium-12 gallium ratio is of great importance for obtaining stable BIG glasses. As expected, the refractive index variation, An, is smaller for the 8% A1/In-Ga-substituted glass than for the A1/In glass.
3.2. Al, Na and K substituted glasses Substitutions of barium by alkali ions such as sodium and potassium have been realized. The most interesting results are presented in table 2. Two main pieces of information can be extracted. First, potassium and sodium fluorides are good stabilizers for the 8% A l / I n - G a composition mentioned previously. In particular, sodium fluoride leads to stability factors greater than 1. Second, zirconium fluoride has lost its stabilization function that was essential in the BIG base sys-
tem. The composition 3% Na-10% K shown in table 2, which contains no aluminum fluoride, is remarkable. It combines a relatively low refractive index of 1.490 with a high stability factor ( H r = 1.08). The glass transition, however, is lower than that for the base glass. Taking advantage of the good stability and low refractive index of the 3% Na-10% K composition and of the high-Tgs of Al-containing glasses, we have synthesized a (A1, Na, K)-substituted glass that exhibits physical parameters comparable to that of the BIG base glass and a low refractive index of 1.482. This represents an index difference of 1.53% and a NA of 0.26.
3.3. BIG(AI-Na-K) glass The infrared transmission spectra for both BIG and BIG(A1, Na, K) glasses are displayed in fig. 1. As expected, the infrared edge occurs at slightly shorter wavelengths for the (A1, Na, K)-substituted glass. The critical cooling rate, Re, of our glass has also been determined in order to compare it with the 6°C/min value of the base glass
Table 2 Physical constants of A1, Na, K-substituted BIG fluoride glasses Glass
Tg (°C)
Tc (°C)
Trn (°C)
T 1 (°C)
Hr
nD
a (10-7K -1)
An (%)
8% K / B a - 8 % AI (with Zr) 8% K / B a - 8 % AI (Zr free) 8% N a / B a - 8 % AI (with Zr) 8% N a / B a - 8 % A1 (Zr free) 3% N a - 1 0 % K / B a 3% N a - 1 0 % K - 6 % AI
337
469
569
637
0.79
1.486
-
347
475
590
635
0.80
1.491
175
0.93
322
459
572
593
1.02
1.490
180
-
326
467
565
598
1.08
1.494
172
0.73
327 335
471 480
576 571
604 632
1.08 0.95
1.490 1.482
174 178
1.00 1.53
-
N. Rigout et aL / BIG fluoride glass optical fibers
100
Thickness = 3mm 2,5
~ 80 z°ua ,~
163
60
0
I
4
5
6
"'~
n- 1.5
BIG(AI,Na,K)
3
"
2
BIG
2
~
7
8
L 9
\-
1
'
10
11
-
12
-
Rc = 20°C/rain -
2
4
0 0
WAVELENGTH (gm)
"--
BIG(AI,Na,K)
0.5 -'
Ill 6
Fig. 1. Infrared transmission spectra for BIG and BIG (A1, Na, K) glasses.
"
1 8
10 12 14 16 18 20 22
10 5 / & T c 2
Fig. 2. Critical cooling rate for BIG(A1, Na, K) glass.
BIG [6]. This parameter is obtained from eq. (2): In R = In R c - B / A T e
2,
(2)
where R (°C/min) is the cooling rate during the experiment, B is a constant and ATe the difference between the liquidus temperature, Tl, and the crystallization temperature, To, upon cooling. The plot of In R versus IO5/AT~ shown in fig. 2 gives a critical cooling rate of 20°C/min for the BIG(A1, Na, K) glass. We have therefore made preforms using BIG(AI, Na, K) glass as cladding and BIG base glass as core glass. Preforms have been mechanically and chemically polished before being drawn into fibers. Fibers with lengths up to 10 m were obtained with an average diameter of about 200 p~m. Attenuation measurements have been carried out by using the cut-back method. The most
interesting result exhibiting a total loss of 4 d B / m near 4 ~m is presented in fig. 3.
4. Discussion The theoretical NA of 0.26 obtained for our c o r e / c l a d structure ( B I G / B I G ( A I , Na, K)) even though moderate, is the highest value reported in this family of fluoride glasses. The challenge is to obtain a cladding glass having a low refractive index and being nevertheless thermally and mechanically compatible with the core glass. The glass transition temperatures of both glasses must remain close. We have succeeded here in combining the opposite actions of aluminum and sodium fluorides for Tg, these two elements having on the
35
J
Z
30 25 [.., ._. ~ . 20 ~-
10 5 0 2
/ Lk_
J
,,~./
2.5
3
3.5 4 4.5 W A V E L E N G T H (~m)
5
Fig. 3. Attenuation measurement for BIG/BIG(AI, Na, K).
5.5
6
164
N. Rigout et aL / BIG fluoride glass optical fibers
other hand the same positive effect on n o. The Tg difference between core (332°C) and cladding (335°C) glasses is not significant. During fiber drawing, the temperature of the core glass, in the center of the heated preform, is usually lower than that of the cladding glass, close to the surface. The second critical point concerns the thermal expansion coefficients. Generally it should be lower for the cladding than for the core glass [7]. However, the difference obtained (7 x 1 0 - 7 K - 1), though not suitable for our purpose, is not significant. It does not exceed the error bars which have been found equal to 4%. The validity of our results can be checked by using the empirical formula given as eq. (3): aTg = N × 2 / z / 9 ( 1 - 2/z),
(3)
where /z is Poisson's ratio and N is a constant. Clare and Parker [8] have established that aTg should be about 1.11 × 10 -2 for fluoride glasses. For Z B L A N glass, the aTg product was found to be equal to 1.03 × 10 -2 with a 7% error. For BIG glasses, we have calculated a value of 1.03 _+ 0.06 × 10 -2 which corresponds to a 6% deviation from eq. (3). At last, the presence of aluminum fluoride in the cladding glass is responsible of the shift of the infrared edge to slightly shorter wavelengths. This is because of the A 1 - F vibration, which is at higher energies compared to other M - F bonds. However, this effect is weak as shown in fig. 1,
and no adverse effect on the optical properties of our waveguide structure is expected.
5. Conclusion In summary, we have obtained a substituted fluoride glass that exhibits a low refractive index and full compatibility with the b a r i u m - i n d i u m gallium base glass. We have been able to make a fiber with a waveguide structure and a theoretical numerical aperture of 0.26. Optical losses for this fiber are found to be equal to 4 d B / m at 4 Ixm. This work was supported by ' G r o u p e m e n t de R e c h e r c h e s sur les Verres d ' H a l o g 6 n u r e s ' (CNRS-Alcatel Alsthom Recherche).
References [1] Y. Miyajima, SPIE 1581 (1992) 304. [2] J.L. Adam, F. Smektala, E. Denoue and J. Lucas, SPIE 1513 (1991) 150. [3] J.L. Adam, N. Rigout, E. Denoue, F. Smektala and J. Lucas, SPIE 1581 (1992) 155. [4] J. Lucas and J.L. Adam, J. Alloys Compounds 180 (1992) 27. [5] S. Takahashi, J. Non-Cryst. Solids 95&96 (1987) 95. [6] J. Lucas, I. Chiaruttini, G. Fonteneau, P. Christensen and S. Mitachi, SPIE 1228 (1990) 56. [7] D.A. Krohn and A.R. Cooper, J. Am. Ceram. Soc. 52 (1969) 661. [8] A.G. Clare and J.M. Parker, SPIE 1048 (1989) 64.
Journal of Non-Crystalline Solids 161 (1993) 161-164 North-Holland
BIG fluoride glass optical fibers with improved NA N. R i g o u t , J.L. A d a m a n d J. L u c a s Laboratoire des Verres et C~ramiques, UA CNRS 1496, Campus de Beaulieu, Universit~ de Rennes L 35042 Rennes c~dex, France
A low refractive index fluoride glass based on barium, indium and gallium has been synthesized. By addition of aluminum, sodium and potassium fluorides to the base glass (BIGaZYbTZr), an index as low as 1.482 is obtained. The glass physical parameters are given and are found to be fully compatible with those of the base glass. A preform suitable for fiber drawing and exhibiting a theoretical numerical aperture of 0.26 is obtained. Optical losses are measured for the best fiber.
1. Introduction The development of 1.3 txm amplifiers and other devices requires optical fibers with high numerical aperture (NA) [1]. Fluoride glasses based on barium, indium and gallium have been demonstrated to be good candidates for active optical fiber applications because of lower multiphonon relaxations and better quantum efficiencies in that host than in Z B L A N glass [2,3]. The purpose of this study is to improve the N A values obtained until now on non-doped fibers from the composition B I G a Z Y b T Z r . To the best of our knowledge, the N A value obtained in this work is the largest one reported to date for two compatible compositions of the B a - I n - G a - g l a s s series.
2. Experiment The glassy samples have been prepared following conventional methods [4]. They have been characterized by differential thermal analysis (DTA) and by thermal expansion coefficient measurements in the range 100-250°C. The stability has been investigated by means of the Hruby Correspondence to: Dr N. Rigout, Laboratoire des Verres et C~ramiques, UA CNRS 1496, Campus de Beaulieu, Universit~ de Rennes I, 35042 Rennes c~dex, France.Telefax: + 33 99 28 16 00.
factor defined by eq. (1): Hr=(Tc-
Tg)/(TI-
Tc),
(1)
where T~, Tc and T 1 are the glass transition, crystallization and liquidus temperatures. The refractive indices, n D, have been determined for the sodium D-line at room temperature.
3. Results Various substitutions have been carried out on the base glass whose composition is (in mol%): 30B a F 2 - 1 8 I n F 3 - 1 2 G a F 3- 2 0 Z n F 2 - 1 0 Y b F 36ThFn-4ZrF4 which has been chosen to form the core of the preform. It will be referred to as BIG glass in the text. In the aim of decreasing the refractive index, substitutions have been made with cations with smaller electronic polarizability such as AI 3÷, Na ÷ and K ÷ [5]. 3.1. A l u m i n u m substituted glasses
Three types of substitutions have been carried out: on barium, on indium and on both indium and gallium. The different characteristics of the new glasses are listed in table 1. Stable glasses containing up to 8% aluminum could by synthesized. They exhibit relatively high Hruby factors, comparable to that of the base
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
N. Rigout et al. / BIG fluoride glass optical fibers
162
Table 1 Physical constants of Al-substituted BIG fluoride glasses Glass
Tg (°C)
Te (°C)
Tm (°C)
T 1 (°C)
Hr
nD
ot (10 -7 K -1)
An (%)
BIG 4%Al/Ba 8%AI/Ba 4%Al/ln 8%Al/ln 8%A1/ln-Ga
332 338 341 337 351 342
460 480 480 470 468 470
576 580 596 587 576 588
609 617 629 612 655 640
0.86 1.04 0.93 0.94 0.63 0.75
1.505 1.503 1.494 1.502 1.499 1.502
171 167 158 178 174
0.73 0.40 0.20
glass. 12%-aluminum substitutions have been attempted as well. They lead to partially crystallized unstable glasses. It is noteworthy that the 8% A 1 / I n - G a substitution ( H r = 0.75) leads to better glasses than the 8% A1/In-alone substitution (H r = 0.63). This indicates that a constant 18 indium-12 gallium ratio is of great importance for obtaining stable BIG glasses. As expected, the refractive index variation, An, is smaller for the 8% A1/In-Ga-substituted glass than for the A1/In glass.
3.2. Al, Na and K substituted glasses Substitutions of barium by alkali ions such as sodium and potassium have been realized. The most interesting results are presented in table 2. Two main pieces of information can be extracted. First, potassium and sodium fluorides are good stabilizers for the 8% A l / I n - G a composition mentioned previously. In particular, sodium fluoride leads to stability factors greater than 1. Second, zirconium fluoride has lost its stabilization function that was essential in the BIG base sys-
tem. The composition 3% Na-10% K shown in table 2, which contains no aluminum fluoride, is remarkable. It combines a relatively low refractive index of 1.490 with a high stability factor ( H r = 1.08). The glass transition, however, is lower than that for the base glass. Taking advantage of the good stability and low refractive index of the 3% Na-10% K composition and of the high-Tgs of Al-containing glasses, we have synthesized a (A1, Na, K)-substituted glass that exhibits physical parameters comparable to that of the BIG base glass and a low refractive index of 1.482. This represents an index difference of 1.53% and a NA of 0.26.
3.3. BIG(AI-Na-K) glass The infrared transmission spectra for both BIG and BIG(A1, Na, K) glasses are displayed in fig. 1. As expected, the infrared edge occurs at slightly shorter wavelengths for the (A1, Na, K)-substituted glass. The critical cooling rate, Re, of our glass has also been determined in order to compare it with the 6°C/min value of the base glass
Table 2 Physical constants of A1, Na, K-substituted BIG fluoride glasses Glass
Tg (°C)
Tc (°C)
Trn (°C)
T 1 (°C)
Hr
nD
a (10-7K -1)
An (%)
8% K / B a - 8 % AI (with Zr) 8% K / B a - 8 % AI (Zr free) 8% N a / B a - 8 % AI (with Zr) 8% N a / B a - 8 % A1 (Zr free) 3% N a - 1 0 % K / B a 3% N a - 1 0 % K - 6 % AI
337
469
569
637
0.79
1.486
-
347
475
590
635
0.80
1.491
175
0.93
322
459
572
593
1.02
1.490
180
-
326
467
565
598
1.08
1.494
172
0.73
327 335
471 480
576 571
604 632
1.08 0.95
1.490 1.482
174 178
1.00 1.53
-
N. Rigout et aL / BIG fluoride glass optical fibers
100
Thickness = 3mm 2,5
~ 80 z°ua ,~
163
60
0
I
4
5
6
"'~
n- 1.5
BIG(AI,Na,K)
3
"
2
BIG
2
~
7
8
L 9
\-
1
'
10
11
-
12
-
Rc = 20°C/rain -
2
4
0 0
WAVELENGTH (gm)
"--
BIG(AI,Na,K)
0.5 -'
Ill 6
Fig. 1. Infrared transmission spectra for BIG and BIG (A1, Na, K) glasses.
"
1 8
10 12 14 16 18 20 22
10 5 / & T c 2
Fig. 2. Critical cooling rate for BIG(A1, Na, K) glass.
BIG [6]. This parameter is obtained from eq. (2): In R = In R c - B / A T e
2,
(2)
where R (°C/min) is the cooling rate during the experiment, B is a constant and ATe the difference between the liquidus temperature, Tl, and the crystallization temperature, To, upon cooling. The plot of In R versus IO5/AT~ shown in fig. 2 gives a critical cooling rate of 20°C/min for the BIG(A1, Na, K) glass. We have therefore made preforms using BIG(AI, Na, K) glass as cladding and BIG base glass as core glass. Preforms have been mechanically and chemically polished before being drawn into fibers. Fibers with lengths up to 10 m were obtained with an average diameter of about 200 p~m. Attenuation measurements have been carried out by using the cut-back method. The most
interesting result exhibiting a total loss of 4 d B / m near 4 ~m is presented in fig. 3.
4. Discussion The theoretical NA of 0.26 obtained for our c o r e / c l a d structure ( B I G / B I G ( A I , Na, K)) even though moderate, is the highest value reported in this family of fluoride glasses. The challenge is to obtain a cladding glass having a low refractive index and being nevertheless thermally and mechanically compatible with the core glass. The glass transition temperatures of both glasses must remain close. We have succeeded here in combining the opposite actions of aluminum and sodium fluorides for Tg, these two elements having on the
35
J
Z
30 25 [.., ._. ~ . 20 ~-
10 5 0 2
/ Lk_
J
,,~./
2.5
3
3.5 4 4.5 W A V E L E N G T H (~m)
5
Fig. 3. Attenuation measurement for BIG/BIG(AI, Na, K).
5.5
6
164
N. Rigout et aL / BIG fluoride glass optical fibers
other hand the same positive effect on n o. The Tg difference between core (332°C) and cladding (335°C) glasses is not significant. During fiber drawing, the temperature of the core glass, in the center of the heated preform, is usually lower than that of the cladding glass, close to the surface. The second critical point concerns the thermal expansion coefficients. Generally it should be lower for the cladding than for the core glass [7]. However, the difference obtained (7 x 1 0 - 7 K - 1), though not suitable for our purpose, is not significant. It does not exceed the error bars which have been found equal to 4%. The validity of our results can be checked by using the empirical formula given as eq. (3): aTg = N × 2 / z / 9 ( 1 - 2/z),
(3)
where /z is Poisson's ratio and N is a constant. Clare and Parker [8] have established that aTg should be about 1.11 × 10 -2 for fluoride glasses. For Z B L A N glass, the aTg product was found to be equal to 1.03 × 10 -2 with a 7% error. For BIG glasses, we have calculated a value of 1.03 _+ 0.06 × 10 -2 which corresponds to a 6% deviation from eq. (3). At last, the presence of aluminum fluoride in the cladding glass is responsible of the shift of the infrared edge to slightly shorter wavelengths. This is because of the A 1 - F vibration, which is at higher energies compared to other M - F bonds. However, this effect is weak as shown in fig. 1,
and no adverse effect on the optical properties of our waveguide structure is expected.
5. Conclusion In summary, we have obtained a substituted fluoride glass that exhibits a low refractive index and full compatibility with the b a r i u m - i n d i u m gallium base glass. We have been able to make a fiber with a waveguide structure and a theoretical numerical aperture of 0.26. Optical losses for this fiber are found to be equal to 4 d B / m at 4 Ixm. This work was supported by ' G r o u p e m e n t de R e c h e r c h e s sur les Verres d ' H a l o g 6 n u r e s ' (CNRS-Alcatel Alsthom Recherche).
References [1] Y. Miyajima, SPIE 1581 (1992) 304. [2] J.L. Adam, F. Smektala, E. Denoue and J. Lucas, SPIE 1513 (1991) 150. [3] J.L. Adam, N. Rigout, E. Denoue, F. Smektala and J. Lucas, SPIE 1581 (1992) 155. [4] J. Lucas and J.L. Adam, J. Alloys Compounds 180 (1992) 27. [5] S. Takahashi, J. Non-Cryst. Solids 95&96 (1987) 95. [6] J. Lucas, I. Chiaruttini, G. Fonteneau, P. Christensen and S. Mitachi, SPIE 1228 (1990) 56. [7] D.A. Krohn and A.R. Cooper, J. Am. Ceram. Soc. 52 (1969) 661. [8] A.G. Clare and J.M. Parker, SPIE 1048 (1989) 64.