Inorganic Materials, Vol. 38, No. 10, 2002, pp. 1063–1068. Translated from Neorganicheskie Materialy, Vol. 38, No. 10, 2002, pp. 1260–1265. Original Russian Text Copyright © 2002 by Churbanov, Shiryaev, Gerasimenko, Pushkin, Skripachev, Snopatin, Plotnichenko.
Stability of the Optical and Mechanical Properties of Chalcogenide Fibers M. F. Churbanov*, V. S. Shiryaev*, V. V. Gerasimenko*, A. A. Pushkin*, I. V. Skripachev*, G. E. Snopatin*, and V. G. Plotnichenko** * Institute of Chemistry of High-Purity Substances, Russian Academy of Sciences, ul. Tropinina 49, Nizhni Novgorod, 603950 Russia ** Fiber Optics Research Center, General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 117756 Russia e-mail:
[email protected] Received March 25, 2002
Abstract—The variations in the optical losses and bending strength of high-purity As–S, As–Se, As–S–Se, and As–Se–Te glass fibers during storage in air were studied. The optical properties and strength of fibers with reflecting clads and well-protected surfaces were shown to be sufficiently stable for practical applications. The optical and mechanical properties of uncoated fibers degrade during storage because of adverse surface processes.
INTRODUCTION The interest in chalcogenide optical fibers is engendered by their low optical losses, broad transparency range in the mid-IR, and good mechanical and physicochemical properties. The minimum optical losses in core–clad As2S3 glass fibers are 23 dB/km at 2.4 µm, and those in As2S1.5Se1.5 glass fibers are 60 dB/km at 4.8 µm [1, 2]. The mean bending strength of these fibers is 1.2 and 0.8 GPa, respectively. The minimum optical losses in uncoated As2Se3 fibers are 76 dB/km at 4.3 µm [3]. Such fibers are suitable for a number of practical applications. In view of this, information about the stability of their optical losses and mechanical strength is of current interest. These properties of chalcogenide fibers have not yet been studied in sufficient detail. In this work, we focus on the stability of the optical and mechanical characteristics of optical fibers made of high-purity As–S, As–Se, As–S–Se, and As–Se–Te glasses. EXPERIMENTAL We studied three types of chalcogenide optical fibers. Fibers of the first type consisted of a glass core and glass clad protected with a polymeric or metallic coating. Fibers of the second type (glass–polymer) had a glass core and polymeric coating. Fibers of the third type had neither reflecting clad nor protective coating. The core and clad glasses were prepared by a standard technique: by melting mixtures of high-purity elements in evacuated silica tubes. The core glasses had the compositions As2S3, As2S1.5Se1.5, As2Se3 , and As2Se1.5Te1.5 .
Multimode core–clad optical fibers were produced by the double-crucible method [4]. The fibers were protected by a double coating consisting of tetrafluoroethylene/1,1-difluoroethylene copolymer (F-42) and polyvinyl chloride (PVC) layers. We also fabricated As2S3 glass fibers with an indium coating. Optical losses were measured using a standard twopoint technique with an accuracy of about ±4% at a loss level of 1000 dB/km and about ±8% at 100 dB/km. The mechanical strength of the fibers was determined in two-point bending between parallel plates [5] at 20°C in air. The fiber was bent into a loop and fixed between the plates, which were then driven closer together at a rate of 1 mm/s until fiber failure. The corresponding distance D between the plates was determined using an optoelectric sensor with an accuracy of 1%. The ultimate strain ε and fracture stress σ were determined by the formulas 2r ε = 1.198 ------------- , D–d
σ = Eε,
where E is Young’s modulus of the glass, r is the radius of the fiber without coating, and d is the total diameter of the fiber. The fiber radius r was measured on both fracture surfaces using an optical microscope. Each strength value was the average of at least 30 repeated measurements. The results were used to construct Weibull plots, representing the probability of fiber failure, F, as a function of applied stress σ. The accuracy in mechanical tests was ~3%. For some of the glasses, Young’s modulus was measured by an acoustic method [6]: E = 16.7 GPa for
0020-1685/02/3810-1063$27.00 © 2002 MAIK “Nauka /Interperiodica”
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β, dB/km
EXPERIMENTAL RESULTS
C–H C–H H2O
1200
Figure 1 illustrates how the total optical loss spectrum of uncoated As35Se65 fibers (initial loss level of 200 dB/km) varies during storage in air under laboratory conditions. It can be seen that storage for 1, 4, and 6 months increases the optical losses from 200 to 400, 600, and 640 dB/km, respectively.
C=O
1000 4 800
4
600
5
The data on the variation of optical losses in different fibers during storage in air under laboratory conditions are summarized in Table 1.
3 2
400
1
200 2
3
4 5 6 Wavelength, µm
7
8
Fig. 1. Total optical loss spectra of uncoated As35Se65 fibers after storage in air under laboratory conditions for (1) 1, (2) 8, (3) 28, (4) 122, and (5) 347 days.
As40S60, 17.0 GPa for As40Se60, and 18 GPa for Ge5As38Se57. To study the aging behavior of the chalcogenide fibers, the total optical losses and bending strength were measured directly after drawing and then at different intervals during storage. The samples were stored in air under laboratory conditions. The storage time was varied from several days to 9 years, depending on the sample.
It follows from the data in Fig. 1 and Table 1 that the optical losses in uncoated fibers rise significantly during storage. The increase in optical losses in the glass– polymer fibers is smaller than that in uncoated fibers. For example, storage for 45 days increases the optical losses in the arsenic selenide fibers coated with F-42 by 100 dB/km (Table 1). The optical losses in sulfide fibers with a reflecting glass clad and two protective polymer coatings or a indium coating increase insignificantly during storage. For example, upon storage for 3 years, the optical losses in fibers with an initial level of 40 dB/km increased by ~3 dB/km. The evolution of the total optical loss spectrum of the core–clad As–S–Se glass fibers coated with F-42 and PVC is illustrated in Fig. 2. The increase in losses in these fibers is more significant than that in the analogous sulfide fibers and amounts to ~40 dB/km per month (Table 1, Fig. 2).
Table 1. Changes in optical losses (∆β) in chalcogenide fibers during storage under laboratory conditions Fiber type core/clad As35Se65 /–
Ge5As38Se57 /–
coating No
No
As30Se70 /–
F-42
As40S60/As35S65
F-42 + PVC, indium
As38S25Se37/As38S27Se35
F-42 + PVC
∆β
βmin , dB/km
Storage time
200
%
dB/km
1 month
100
200
4 months
200
400
6 months
220
440
40 days
40
240
4 months
80
480
2 years
100
600
240
45 days
42
100
40
3 years
5
2
300
1 month
13
40
2 months
30
90
100 days
50
150
160 days
80
240
10 months
80
240
600
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α, dB/km 3600 3200 2800 2400 2000 1600 1200
3
5
800 400 0
1 2
4
2
4 5 Wavelength, µm
3
6
7
Fig. 2. Total optical loss spectra of As38S25Se37/As38S28Se34 glass fibers coated with F-42 and PVC: (1) as-drawn, (2) 60 days of storage, (3) 100 days, (4) 160 days, (5) 10 months.
Figure 3 illustrates the effect of storage on the bending strength of uncoated As–S and As–Se–Te glass fibers. Data on the variation of bending strength with storage time are summarized in Table 2. The average fracture stress corresponds to a probability of 63.2%. Figure 4 shows the Weibull plots for uncoated and indium-coated sulfide fibers. The average mechanical strength of the as-drawn uncoated fibers is seen to be 600 MPa. Indium coating increases the strength to 750 MPa. Storage for 3 years reduces the strength of the uncoated fibers by 60%, while the strength of the indium-coated fibers remains unchanged to within the experimental error. The effect of the storage environment on the stability of the mechanical properties of fibers is illustrated in Table 3. It can be seen that water and, particularly, acetone accelerate the aging of fibers.
lnln(1/(1 – F)) 2 (a)
1 0 –1
2 –3 –4
0.2
1
–2
The observed changes in the optical and mechanical properties of fibers during storage are due to the nucleation and propagation of cracks on the fiber surface, impurity adsorption, and microstructural changes. Uncoated fiber surfaces are subject to a stronger influence of the storage environment. It is, therefore, reasonable to expect more significant changes in the optical and mechanical properties of uncoated fibers. The present experimental data lend support to this conclusion.
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0.4 2
(b)
0
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2
1
–4
3
4
–1
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–2
0.1
0.2
0.3 0.4 Stress, GPa
0.6
0.8
Fig. 3. Weibull plots for the bending strength of uncoated fibers. (a) As32S68, d = 556 µm: (1) as-drawn, (2) after storage for 9 years. (b) As40Se30Te30, d = 200 µm: (1) asdrawn, (2) 1 day of storage, (3) 4 days, (4) 2 weeks.
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lnln(1/(1 – F))
1 0
3
1
–1 –2
2
4 –3 –4 0.1
0.2
0.4
0.6 0.8 1.0
2.0
Stress, GPa Fig. 4. Weibull plots for the bending strength of arsenic sulfide fibers: (1) as-drawn uncoated fibers, (2) as-drawn indium-coated fibers, (3) uncoated fibers after 3 years of storage, (4) indium-coated fibers after 3 years of storage.
The increase in the optical losses in unclad, uncoated fibers is twice as large as that in the F-42coated fibers after storage for the same time.
In the course of storage, the average mechanical strength of uncoated fibers decreases significantly, by about 50–60% compared to as-drawn fibers. The strength of fibers decreases most rapidly during the first 2–4 days and then varies more gradually. For example, after storage for 3 years in air, the strength of uncoated arsenic sulfide fibers is comparable to that after 2 days of storage. The mechanical strength and optical losses of As2S3 fibers protected by a metallic or bilayer polymeric coating remain unchanged, to within the experimental error, after 3 years of storage under ordinary conditions. This implies that no significant changes in the bulk properties of As2S3 glass occur. It is of interest to note that the mechanical strength of As–S–Se glass fibers with well-protected surfaces decreases little during storage, while their optical losses rise for as long as 6 months, almost uniformly over the entire spectral region of interest, and are probably due to bulk scattering. As–S–Se glasses are believed to be more microinhomogeneous than As–S and As–Se glasses. Some of their properties are commonly interpreted under the assumption that there are zones formed by AsS3/2 or AsSe3/2 structural units and AsS2/2Se1/2 and AsS1/2Se2/2 mixed units. It was shown by Treacy et al. [7] that
Table 2. Variation in the bending strength of chalcogenide fibers during storage in air Composition Coating –
–
core
clad
As40Se30Te30
–
As40S60
As39S61
400
Storage time
∆σav , %
1 day
14
4 days
28
2 weeks
47
1 day
48
2 days
55
3 years
60
350
9 years
25 12
800
F-42
As32S68
F-42 + PVC
As40S60
As39S61
800
1.5 years
As38S25Se37
As38S27Se35
600
1 day
8
2 days
10
4 days
12
45 days
12
Indium
As40S60
–
σav , MPa
As39S61
750
60 days
12
100 days
12
160 days
12
8 months
12
3 years
5
Note: σav is the average strength of as-prepared fibers, and ∆σÒ is the strength loss over the storage time. INORGANIC MATERIALS
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Table 3. Variation in the bending strength of chalcogenide fibers during storage in different environments Composition Environment
core Water
Acetone
clad
F-42
As35S65
F-42 + PVC
As38S25Se37
F-42 + PVC –
Argon, 85 MPa
σav , MPa
Coating
F-42 + PVC
Storage time
∆σ %
MPa
–
300
4 days
11
33
As38S27Se35
500
40 days
20
100
70 days
25
125
As40S60
As39S61
850
1 months
40
200
As40S60
As39S61
850
1 day
53
450
4 days
63
310
1h
30
210
As40S60
As39S61
700
Note: ∆σ is the strength loss over the storage time.
As2SxSe3 – x glass contains AsS1/2Se2/2 and AsS2/2Se1/2 heteropyramids, as well as AsS3/2 and AsSe3/2 homopyramids. It was also pointed out by Zhukov et al. [8] that As40S60 – xSex glass contains Sen chains. The structure of As–Se and Ge–Se–S glass fibers was reported to partially relax during room temperature storage for several months [9, 10]. At low temperatures, so-called quasicrystallization occurs: the glass structure transforms into a more ordered, quasi-crystalline structure. The changes in the glass structure and relative amounts of structural units in the course of relaxation reflect on the scattering of radiation passing through the fiber. Our results demonstrate that the strength of fibers depends on glass composition, the presence of coating, its material, and storage environment. Water is known to initiate crack propagation on the fiber surface. Protective polymer coatings prevent, though incompletely, water molecules from penetrating into the surface layer of the glass. The strength of fibers with water-tight coatings (metals, carbon) varies insignificantly with storage time. In the case of uncoated chalcogenide fibers, aging is additionally accelerated by photoinduced effects: photopolymerization, photocrystallization, and others [11]. Under irradiation, the metastable glass structure relaxes owing to polymerization processes. Photoinduced effects may raise the refractive index of chalcogenide glasses by 2–7%, depending on glass composition, because of the reduction in the concentration of homobonds [12], which has a significant effect on the optical characteristics of fibers. CONCLUSION The optical and mechanical characteristics of fibers with reflecting clads and well-protected surfaces are sufficiently stable for practical applications. The optical INORGANIC MATERIALS
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and mechanical properties of uncoated fibers degrade during storage because of adverse surface processes. Aging is additionally accelerated by structural changes and the associated changes in the physicochemical and optical characteristics of the glass. ACKNOWLEDGMENTS This work was sponsored through the Support to Leading Scientific Schools Program, grant no. 00-15-99008. REFERENCES 1. Churbanov, M.F., Scripachev, I.V., Snopatin, G.E., et al., High-Purity Glasses Based on Arsenic Chalcogenides, J. Optoelectron. Adv. Mater., 2001, vol. 3, no. 2, pp. 341–350. 2. Scripachev, I.V., Churbanov, M.F., Gerasimenko, V.V., et al., Optical and Mechanical Characteristics of Fibers Made of Arsenic Chalcogenides, J. Optoelectron. Adv. Mater., 2001, vol. 3, no. 2, pp. 351–360. 3. Dianov, E.M., Plotnichenko, V.G., Devyatykh, G.G., et al., Middle-Infrared Chalcogenide Glass Fibers with Losses Lower Than 100 Db/km, Infrared Phys., 1989, vol. 29, pp. 303–307. 4. Skripachev, I.V., Plotnichenko, V.G., Snopatin, G.E., et al., Fabrication of Core–Clad Optical Fibers Based on High-Purity As–S, As–Se, and Ge–As–Se Glasses, Vysokochist. Veshchestva, 1994, no. 4, p. 34. 5. Dianov, E.M., Kr”steva, V.M., and Plotnichenko, V., Mechanical Strength of Chalcogenide Glass Optical Fibers Produced by the Crucible Method, Vysokochist. Veshchestva, 1990, no. 4, pp. 208–214. 6. Bogatyrev, V.A., Dianov, E.M., Skripachev, I.V., et al., Mechanical Strength of High-Purity Chalcogenide Glass Optical Fibers, Vysokochist. Veshchestva, 1987, no. 2, pp. 202–205.
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7. Treacy, D.J., Greenbaum, S.G., Strom, U., and Taylor, P.C., Structure and Bonding in the Mixed Chalcogenide System As2SxSe3, J. Non-Cryst. Solids, 1983, vol. 59/60, pp. 847–850. 8. Zhukov, E.G., Dzhaparidze, O.I., Dembovskii, S.A., and Popova, N.P., System As2S3–As2Se3, Izv. Akad. Nauk SSSR, Neorg. Mater., 1974, vol. 10, no. 10, pp. 1886– 1887. 9. Hari, P., Taylor, P.C., King, W.A., and LaCourse, W.C., Metastable, Drawing-Induced Crystallization in As2Se3 Fibers, J. Non-Cryst. Solids, 1998, vols. 227–230, pp. 789–793.
10. Griffiths, J.E., Espinosa, G.P., Remeika, J.P., and Phillips, J.C., Reversible Quasicrystallization in GeSe2 Glass, Phys. Rev. B: Condens. Matter, 1982, vol. 25, p. 1272. 11. Fritzsche, H., On the Understanding of Photoinduced Changes in Chalcogenide Glasses, Fiz. Tekh. Poluprovodn. (S.-Peterburg), 1998, vol. 32, no. 8, pp. 952–956. 12. Frumar, M., Jedelsky, J., Ernosek, Z., et al., Optically Induced Phenomena in Amorphous Chalcogenides and Their Application, Proc. XII Int. Symp. on Non-Oxide Glasses and Advanced Materials, Florianopolis, 2000, pp. 331–334.
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