CA1259211A

CA1259211A - Quartz glass optical fiber

Info

Publication number: CA1259211A
Application number: CA000476524A
Authority: CA
Inventors: Michihisa Kyoto; Shuzo Suzuki; Minoru Watanabe; Motohiro Nakahara
Original assignee: Nippon Telegraph and Telephone Corp; Sumitomo Electric Industries Ltd
Current assignee: Nippon Telegraph and Telephone Corp; Sumitomo Electric Industries Ltd
Priority date: 1984-04-12
Filing date: 1985-03-14
Publication date: 1989-09-12
Also published as: DE3566879D1; AU4087485A; DK158897B; KR850007236A; DK103185D0; DK155185D0; KR890001125B1; US4804247A; JPS647015B2; EP0160244B1; JPS60215550A; DK155185A; DK158897C; EP0160244A1; AU575387B2

Abstract

ABSTRACT OF THE DISCLOSURE

A quartz optical fiber comprising a core having a higher refractive index and made of pure quartz containing fluorine and phosphorus pentoxide and a cladding having a lower refractive index, a weight ratio of fluorine and phosphorus pentoxide in the core being larger than 1 (one), which is substantially free from unstability of the glass structure.

Description

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QUARTZ GLASS OPTICAI. FIBER

FIELD OF THE INVEMTION
The present invention relates to a quartz glass optical fiber. More particularly, it relates to a quartz glass optical fiber comprising a core made of pure quartz containing at least fluorine and phosphorus pentox~àe (P205 ) .
BACKGROUND OF T~E INVENTION
A glass preform for use in the fabrication of an optical fiber comprises a core and a cladding surrounding the core. l'he core mus~ have a higher refractive index than the cladding so as to allow easy propagation of light there-through.
In order to increase the refractive index of the ` core higher than that of silica, additives such as TiO2, GeO2 and Ai2O3 are usually added to the core material.
Among them, GeO2 is most commonly used (cf. Japanese Patent Kokai Publication (unexamined) Nos. 217744/1976 and 46742/1078). In a usual optical fiber, pure quartz glass is often used to form the cladding. In this case, pure quartz glass has a refractive index of 1.4585 and An = 0.

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- la -The background of the invention is described in further detail below and reference is made to some of the accompanying drawings. For the sake of convenience, therefore, all of the drawings are briefly explained as follows:
Figs. lA and lB are diagrams illustrating the refractive indices of optial fibers;
Fig. 2 is a similar diagram of a further optical fiber;
Fig. 3 is a graph showing the W spectra of various types of glass;
Figs. 4(a) and 4(b) are diagrams illustrating steps in the formation of optical fibers; and Fig. 5 is a graph showing the increase in attenuation of light transmission with respect to heating time for various optical fibers.
Referring to Figs. lA and lB~ there are shown diagrams illustrating distributions of the refractive index of two types of optical fibers. In ~hese figures, the regions A and B indicate the core and cladding, respec-tively. The difference in refractive index between the core and cladding is usually indicated in terms of a relative

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refractive index difference (in percent). That is, assuming that the refractive indices of the core and cladding are n and n2, respectively, the relative refractive index diffe-rence ~n ~ is represented by the following equation:

nl n2 n2 Fig. lA shows the general distribution of rerrac-tive index of a single mode optical fiber. In this case, ~n is usually 0.3 to 0.5 ~. Fig. lB shows the general distribution of refractive index of a multi-mode optical fiber. For an optical fiber for ordinary communication purposes, n is usually about 1 %, and for large aperture optical fibers used in computer ring communication appli-cations, n12 is usually about 2 to 4 %.
Oxide additives such as GeO2 added to increase refractive index of the core cause light scattering (Ray-leigh scattering) because of their inherent characteristics.
As the amount of the additive aaded is increased, the degree of light scattering (Rayleigh scattering) due to the addi-tive increases. This is not desirable for light trans mission.

If the additive is added in a large amount, bubbles and/or a crystal phase are formed in the glass preform. In the case of GeO2, for example, ~eO gas easily rorms, thereby producing bubbles. In the case of A1203, clusters of A1203 crystals easily forms. This is not desirable for light transmission characteristics and also for the strength of the final optical fiber. Furthermore, ~25~

the coefficient of thermal expansion of glass increases, which makes the glass preform fragile. Therefore, also from the viewpoints of light propagation and glass streng h, it is preferred to reduce the amount of the additive added to the core.
For this reason, it is proposed to increase -the refraciive index difference between the core and cladding by lowering the refractive index of the cladding. For example, additives which lower the refractive index, such as B2O3, fluorine or a combination thereof, can be added to the cladding (cf. Japanese Patent Kokai Publication (unexamined) No. 111259/1982). B2O3, however, has disadvantages in that the coefficient of therma~ expansion of the resulting cladding greatly changes with the concentration of B2O3 and in that the refractive index changes upon heating. Further-more, with regards to light transmission characteristics, the cladding has an absorption loss due to B2O3 in a longer wavelength region. Thus, it is preferred to use fluorine as a refractive index-lowering agent.
It is known that addition of fluorine to ~uartz glass makes it possible to produce optical fibers with various refractive index distributions, and that, by the proper choice of structure, there can be obtained an optical fiber of low dispersion over a wide wavelength region.
The advantage that can be obtained by using fluorine as an additive is that, since the refractive index of the cladd1ng can be made lower than that of pure quartz, ~ r~

pure quartz or quartz glass with a small amount of additive added thereto can be used in the fabrication of the core.
In addition, GeO~ as an additive to increase the refractive index and fluorine as an additive to decrease the refractive index may be simultaneously added to the core and the cladding, respectively.
An optical fiber having a distribution of refrac-tive index shown in Fig. 2 was proposed (cf. A. D. Pearson, et al., Fabrication and Properties of Single Mode Optical Fiber Exhibiting Low Dispersion, Low Loss, and Tight Mode Confinement Simultaneously, The Bell System Technical L., Vol. 61, No. 2 (1982) 262). The new optical fiber comprises a core made of GeO2-SiO2 glass and a cladding made of SiO2-F-P2O5 and produced by MCVD (Modified chemical vapor-phase deposition) method.
However, structural unstability of the optical fiber comprising the core made of GeO2-SiO2 glass was revealed by UV absorption peak and coloring by radiation observed therein. Needless tc say, the deficiency of the glass structure adversely affects light transmission charac-teristics even in a near infrared region in which the optical fiber is used.
SUMMARY OF THE INVENTION
One object of the invention is to provide a quartz optical fiber substantially free from unstability of the glass structure.
Another object of the invention is to provide a quartz opt:Lcal fiber comprising a core containing fluorine and phosphorus pento~ide.

~ 5 ~ 125~

Accordingly, the present invention provides a quartz optical fiber comprising a core having a higher refractive index and made of pure quart~ containing fluorine and phosphorus pentoxide and a cladding having a lower refractive index, a weight ratio of rluorine and phosphorus pentoxide in the core being larger than 1 (one).
DETAILEV DESCRIPTION OF T~E INVENTION
Fig. 3 shows UV absorption spectra of SiO2-GeO2 glass tCurve A), SiO2-F-GeO2 glass (Curve B) and SiO2-P~O5 glass (Curve C). It is apparent from this figure that the fluorine-added glass shows less absorption, which means it includes less structural deficiency.
Tendency of the generation of the structural deficiency in the glass influences resistance of Ihe glass, which may be improved by the addition of fluorine.
Mechanism of the improvement of the structural stability by the addition of fluorine has not been thorough-ly proved. Probably, an oxide such as SiO~ and GeO2 tends to have a structure of MOX wherein x is less than 2 in the glass, and oxygen may be replaced with fluorine anion.
The deficiency of the glass can be reduced by the addition of fluorine, but never be vanished because it is necessary to heat the glass at a temperature o 1,600 to 1,800C for the fabrication of an optical fiber and the bondings in the glass, for example, Si-O~Si bondings are more easily broken at a higher temperature. This is due to the fact that the Si-O vibration becomes vigorous at a higher temperature so that this bonding is broken, which corresponds to transition from a solid phase to a liquid phase. Therefore, it is preferred to fabricate the optical fiber at a temperature as low as possible. For this end, phosphorus pentoxide is preferably added since it lowers a temperature at which the ~Jlass is made transparent and thereby reduces the structura:L deficiency.
The addition of phosphorus pentoxide is, howevér, not necessary ror the light transmission characteristics of the optical fiber, and not optically desirable since phos-phorus atom having a coordination number of 5 is unstable in the glass structure, that is, it tends to have a structure having a coordination number of 4, which results in the structural deficiency. This is affirmed by an éxperiment showing that the glass added with phosphorus pentoxide has more deficiency.
Namely, phosphorus pentoxide has, on the one hand, a property to lower the fabrication temperature of the optical fiber so as to reduce the structural deficiency, and on the other hand, a property to increase the structural deficiency of the glass having coordination number of ~ of quartz since the coordination number of phosphorus a~om is 5. Therefore, it is not possible to predict whether the addition of phosphorus pentoxide to the glass containing fluorine increase or decrease the structural deficiency of the glass.
It has now been found that the addition of phos phorus pentoxide to the glass containing fluorine suppresses generation of the structural deficiency, and further the ~,2S~32~

weight ratio of fluorine to phosphorus pentoxide plays an important role to suppress the generation of the structural deficiency. The amour.t of fluorine should be larger than that of phosphorus pentoxide since when the amount of phosphorus pentoxide is larger than that of rluorine, the generation of the structural deficiency dominates the stabilization of the glass structure by the addition of fluorine.
Prererably, the core of the optical fiber of the invention contains not larg~r than 3 % by weigh-t of fluorine and less than 3 % by weight, more preferably less than 1 %
by weight of phosphorus pentoxide.
The core of the optical fiber according to the present invention may contain GeO2 in an amount of not larger than 17 % by weight.
Production of Soot Preform In producing a soot preform consisting of a quartz glass fine particle mass by flame hydrolysis, as indicated in Fig. 4A, oxygen 2, hydrogen 3, and a starting material gas 5, namely SiC14, POC13 or a gaseous mixture of SiC14, POC13, GeC14, AlC13, SF6, and the like, are introduced into an oxyhydrogen flame with Ar or He gas as a carrier gas by means of a coaxial multi-tube burner 1 made or quartz. In Fig. 4A, numeral 4 lndicates Ar gas which is introduced as a barrier gas so that the starting material gas reacts in a space several millimeters apart from the top of the burner 1. If it is intended to produce a fine glass particle rod, the fine glass particle mass is deposited in the axial Zl:~L

direction from the top of rotating seed member 6. If it is intended to produce a pipe-like fine glass particle mass, as shown in Fig. 4B, a fine glass particle mass is deposited around a rotating quartz bar or carbon bar 7 while horizon-tally travelling a burner 8 and, thereafter, the bar 7 is removed. The bar 7 may be a glass preform for the core. In this case, the bar need not be removed. A plurality of burners 8 may be used. The conditions for depositing the fine quartz particles on the seed member are substantially the same as in the conventional method.
The same soot preform as produced by the method of Figs. 4A and 4B can be produced by hydrolysis of alcoholate.
This method is referred to as a "sol-gel method".
Sinterin~ of Soot Preform The above produced soot preform is placed in a muffle tube made of pure quartz. It is heated to a tempera-ture from 1,200 to 1,600C, particularly to 1,400C at a temperature-raising rate of 2 to 10C/min in an inert gas atmosphere.
When fluorine is added to the glass preform, a gaseous fluorine~containing compound (eg. SF6, CF4, C2F6, C3F8, CC12F2, COF2, etc.) is added to the inert gas.
Fluorine liberated from the fluorine-containing compound is added to the glass according to, for example, the following equation:
SiO2 (s) + 1/2F2 (g) ~ SiOl 5F (s) + 1/42 (s) ~1) wherein (s) and (g) indicate solid and gas states, respec-tivel~.

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For dehydration, the inert gas atmosphere may contain a chlorine-contalning compound (eg. C12, SOC12, COC12, CC14, etc.).
The glass preform may be prepared by other conven-tional methods, for example the MCVD method.
The thus produced transparent glass preform is drawn to fabricate an optical fiber in a muffie tube made of quartz by a per se conventional method.
The degree of the structural deficiency of the glass fiber is expressed in terms of the increased content of hydroxyl groups after heatlng the optical fiber in an atmosphere containing hydrogen at 200C for 24 hours. The increase of the content of the hydroxyl groups is apparently due to the increase of the structural deficiency (cfo J. E.
Shelby, et al, "Radiation-included Isotope Exchange in Vitreous Silica" J. Appl. Phys~, 50 (8) l1979) 5533).
The present invention is described in greater detail with reference to the following ~xamples.
EXAM~LE 1 A soot preform of SiO2 containing GeO2 and phos-phorus pentoxide and having a diameter of 60 mm and a length of 300 mm was produced by the method as shown in Fig. lA and heated in a stream of helium at a rate of 10 liters/mln.
including chlorine at a rate of 50 ml/min and SF6 at a rate of 100 ml/min. at 1,300C to obtain a transparent glass preform containing 17 % by weight of GeO2, 0.5 % by weight of phosphorus pentoxide and 2 ~ by weight of fluorine. The thus produced glass preform was drawn to form a rod having a ~L2~

diameter of 10 mm, which is jacketed with quartz having an outer diameter of 26 mm and a thickness of 6 mm and further drawn to fabrlcate an optical fiber having a diameter of 125 micrometers.
The content of hydroxyl groups in the glass preform was 0.02 ppm, namely, attenuation of l dB/km at a wavelength of 1.38 micron.

A soot preform prepared in the same manner as in EXAMPLE 1 was heated in a stream of helium at a rate of 10 liters/min. including chlorine at a rate of 50 ml/min at a temperature higher than l,600C to obtain a transparent glass preform containing 17 % by weight of GeO2 and 0.5 % by weight of phosphorus pentoxide. The thus produced glass preform was drawn, jacketed with quartz and again drawn in the same manner as in E~AMPLE 1 to fabricate an optical fiber having a diameter of 125 micrometers.
The content of hydroxyl groups in the glass preform was 0.02 ppm, namely, attenuation of 1 dB/km at a wavelength of 1.38 micron.

A soot preform prepared in the same manner as in EXAMPLE 1 but not adding phosphorus pentoxide was heated to obtain a transparent glass preform containing 17 % by weight of GeO2 and 2 ~ by weight of fluorine. An optical fiber fabricated from the glass fiber contained the hydroxyl groups of 0.02 ppm, namely, attenuation of 1 dB/km at a wavelength of 1.38 micron.

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In the same manner as in COMPA~ATIVE EXAMPLE 1, a soot pre'orm was produced. The produced soot preform was heated and fabricated in the same manner as in EXAMPLE 1 to fabricate an optical fiber containing 17 ~ by weight of GeO2. The content of hydroxyl groups iIl the optical fiber was 0.02 ppm.

A soot preform produced in the same manner as in EXAMPL~ 1 was heated in a stream of helium at a rate of 10 liters/min. including SF6 at a rate of 20 ml/min. at 1,400C
to obtain transparent glass preform containing 17 % by weight of GeO2, 0.5 % by weight of phosphorus pentoxide and 0.5 % by weight of fluorine. The glass preform was drawn in the same manner as in EXAMPLE 1 to fabricate an optical fiber.

In the same manner as in EXAMPLE 1, a transparent glass pre~orm containing 17 % by weight of GeO2, 1 % by weight of phosphorus pentoxide and 0.5 % by weight of fluorine was produced and drawn to fabricate an optical fiber.
EXPERIMENT
In order to flnd structural deficiency in the optical fibers produced in EXAMPLE 1 and COMPAXATIVE EXAM-PLES 1, 2 and 3, rollowing experilnent was carried out.
Each optical fiber was covered with a first covering of silicone resin and a second covering of Nylon.

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The 500 m long optical fiber in a bundle form was heated in a furnace at 200C for a predetermined period of time. Then attenuation of light transmission at a wavelength of 1.38 micron was measured, The amount of fluorine added to the core was 0.05 % in terms of an. The jacket tube was made of natural quartz for all the optical fiber.
Table Curve Phosphorus Fluorine in Fig. 5 pentoxide Comp. Ex. 2 ¦ A ¦ Yes No Comp. Ex. 3 ¦ B ¦ No No Comp. Ex. 1 ¦ C No Yes Example 1 D Yes Yes Increase in the attenuation of liyht transmission due to the hydroxyl groups was shown in thé graph of Fig. 5, in which the ordinate and the abscissa correspond to the increase of the attenuation of light transmission at a wavelength of 1. 38 micrometer and the heating time, respec-tively.
From these results, it is understood thal the attenuation of light transmission can be reduced from about 10 dB/km to about 5 dBtkm by the addition of fluorine or removal of phosphorus pentoxide. Co-addition of fluorine and phosphorus pentoxide further decrease the attenuation of light transmission from about 5 dB/km to about 3 dB/km.
The reason why the addition of fluorine to the core reduces the attenuation of the light transmission may ~5~Z~

be that the fluorlne is bonded to the structural deficiency due to the "generation of hydroxy groups" in the glass.
l'he reason why the co-addition of fluorine and phosphorus pentoxide further reduces the attenuation of light transmission may be that the lowering of the sintering temperature by the addition of the phosphorus pentoxide suppresses the thermal generation of the structural defi-ciency, which overwhelms the increase of the structural deficiency caused by ihe addition of phosphorus pentoxide.
The optical fiber produced in EXAMPLE 2 had substantially the same result as in EXAMPLE 1, that is, the increase of the attenuation of light transmlssion had tendency like the curve D in Fig. 5.
The optical fiber produced in E'~AMPLE 3 had substantially the same result as in COMPARATIVE EXA~PLE 2, that is, the increase of the attenuation of iight trans-mission had tendency like the curve A in Fig. 5.
This means that the amount of phosphorus pentoxide should be smaller than that of fluorine.

Claims

Claims:

1. A quartz optical fiber comprising a core having a higher refractive index and made of pure quartz containing fluorine and phosphorus pentoxide and a cladding having a lower refractive index, a weight ratio of fluorine and phosphorus pentoxide in the core being larger than 1 (one).

2. A quartz optical fiber according to claim 1, wherein the amount of fluorine is not larger than 3 % by weight.

3. A quartz optical fiber according to claim 1, wherein the amount of phosphorus pentoxide is less than 3 %
by weight.

4. A quartz optical fiber according to claim 3, wherein the amount or phosphorus pentoxide is less than 1.0 % by weight.

5. A quartz optical fiber according to claim 1 wherein the core contains GeO2.

6. A quartz optical fiber according to claim 5, wherein the amount of the GeO2 is not larger than 17 % by weight.