GB2029400A

GB2029400A - An Optical Fibre

Info

Publication number: GB2029400A
Application number: GB7926144A
Authority: GB
Original assignee: Nippon Telegraph and Telephone Corp; Sumitomo Electric Industries Ltd
Current assignee: Nippon Telegraph and Telephone Corp; Sumitomo Electric Industries Ltd
Priority date: 1978-07-31
Filing date: 1979-07-26
Publication date: 1980-03-19
Also published as: FR2433493A1; DE2930816C2; JPS5761699B2; FR2433493B1; DE2930816A1; GB2029400B; JPS5521059A; CA1126548A

Abstract

The mainly quartz glass of the core 1 contains GeO2 and at least one dopant selected from P2O5, B2O3, TiO2, Al2O3, SiF4 and Ga2O, the amount of GeO2 being less than about 15% by weight and the total amount of GeO2 and the other dopants being about 15% by weight or more. The glass cladding 2 is mainly quartz glass. <IMAGE>

Description

SPECIFICATION An Optical Fiber This invention relates to an optical fiber.

As shown in Figure 1, a typical optical fiber is constructed of an outer glass cladding 2 and a glass core 1 having a higher refractive index than the glass cladding.

The principle of optical transmission by an optical fiber is to confine the light in the core by the refractive index differentiai between the core and the glass cladding. The transmission loss of an optical fiber is the result of such factors as absorption inherent to the material used for the optical fiber, scattering emission loss increased by the fiber bending and loss due to microbending.

An object of the present invention is to provide an optical fiber having a wide transmission band and low transmission loss.

Accordingly, the invention resides in an optical fiber composed of a glass cladding comprising mainly quartz glass and a glass core comprising mainly quartz glass having a higher refractive index than the glass cladding, wherein the quartz glass of the core contains GeO2 and at least one dopant selected from P2O5, B203, TiO2, Al2O3, SiF4 and Ga2O3, the amount of GeO2 being less than that about 15% by weight and the total amount of GeO2 and the other dopants being about 15% by weight or more.

In the accompanying drawings; Figure 1 is a cross-sectional view of an optical fiber; Figure 2 is a graph of the refractive index distribution in the core of an optical fiber; Figure 3 is a schematic illustration of one stage during a method of producing an optical fiber; and Figure 4 is a schematic illustration of the refractive index distribution in an optical fiber.

Referring to the drawings, GeO2 is added to the core glass in order to increase the refractive index of the core and thereby establish a refractive index difference between the core and the cladding so as to provide an optical fiber having low transmission loss and wide transmission band. The refractive index increases in proportion to the amount of GeO2 in the core. While small amounts of GeO2 may be present in the cladding (as a result of accidental mixing of the GeO2 contained in the core) without defeating the purpose of the present invention generally GeO2 is not positively added to the cladding.

If the coefficient of thermal expansion of the core 1 is substantially different than that of the cladding 2, large stress is generated between the core 1 and the cladding 2, which can cause the composite member to split. In order to avoid this problem, the coefficient of thermal expansion is controlled by adding additives such as P205 and B203 to the core 1 which do not largely affect the refractive index of the core, as compared with that obtained when GeO2 is used alone, but reduce the softening temperature of the core. The addition of these additives is generally controlled such that the softening temperature of the core is about 500 to 2000C lower than the cladding and the coefficient of thermal expansion differential is 3x10-7/OC or less.The exact amounts added depend on several factors including the diameter of the preform and the preform and the type of optical fiber. Additives such as GeO2 and B203 are used in the present invention because they are accompanied by a minimum of light energy absorption and scattering.

Since scattering loss increases in proportion to the amount of the additives, the amount of additives such as GeO2, P205 and B203 in the core must be small in order to reduce scattering loss.

On the other hand, the refractive index of the core should be increased in order to reduce microbending loss and emission loss. For this purpose the amount of additives, in particular GeO2 should be increased.

Furthermore it is known that in an optical fiber the distribution of the refractive index of the core 1 varies across its diameter in the form on a parabola as shown in Figure 2, and the transmission band (width) of the optical fiber is inversely proportional to the second power of the difference (An) between the maximum value of the refractive index of the core and the refractive index of the cladding. In view of this, An should be small in order to broaden the transmission band and, thus, the amount of GeO2 should be maintained low. Thus, there is the anomally that high amounts of GeO2 are required in the core in order to increase its refractive index and reduce transmission loss but if too high an amount of GeO2 is present in the core An increases which leads to a narrower transmission band width.

As can be understood from the foregoing, the amount of the additives in an optical fiber having low transmission loss and a broad transmission band must be balanced and confined to certain ranges.

One example of a method of producing a preform for optical fibers by a flange hydrolyzing process is described below.

This manufacturing method is shown schematically in Fig. 3. From a first nozzle 3 and a second nozzle 4, a gaseous glass-forming material containing additives providing a higher refractive index than SiO2 (for example, a mixture of SiCI4, GeCI4, PoCI3, and BBr3) and a gaseous giass-forming material containing additives providing a lower refractive index than SiO2 (for example, a mixture of SiCI4 and BBr3) are blown together with a heat combustion gas (for example, oxgen and hydrogen, butane and propane). The glass-forming material gases are flame-hydrolyzed outside the multiple nozzles to form fine glass particles which accumulate on a starting rod 5.A fine glass particle body 6 then grows in the axial direction. the fine glass particle body gradually descends into a high temperature furnace having a temperature of 1400 to 1 6000C whereby the glass particle body is sintered and vitrified thereby producing a preform for an optical glass fiber.

The distribution of additives in a cross-section of a core glass is not uniform and the coefficient of thermal expansion also varies across the cross section. If the coefficient of thermal expansion varies greatly, stress occurs upon sintering the fine glass particle body 6 causing the fine glass particle body to crack. Furthermore, the preform tends to crack easily when the preform is removed from the high temperature furnace after the sintering, or at the time of preforming other heat-treaiments.

In forming SiO2 glass containing GeO2 by flame-hydrolyzed reaction, a noticeable phenomena is recognized. That is, a solid solution is formed between GeO2 and SiO2, and a part of GeO2 is precipitated into crystals in the form of a hexagonal system. The hexagonal system GeO2 has a melting point of about 1 0860C which is much lower than the melting point of SiO2 (1 6000C to 1 7000C). Therefore, bubbles of vaporized GeO2 are apt to be formed by thermal treatment during draw forming operations performed on such core materials to obtain glass fibers. If such bubbles are generated transmission loss becomes large. The probability of generation of bubbles is increased at high concentrations of GeO2.Various experiments have shown GeO2 amount should be less than 1 5 wt%, in order to ensure that the generation of bubbles is negligible and low transmission loss is obtained.

Furthermore, if the central portion of the fine glass particle body 6 has a higher melting point than that of the outer peripheral portion thereof, sintering of the glass particle body will begin vitrification at the surface of the body making it difficult to drive off bubbles in the central portion completely. Hence bubbles tend to remain in the central portion and cause scattering loss in the optical fibers. To prevent these bubbles from forming, glass particle body 6 is produced by varying the amount'of additives at the central portion and the peripheral portion within the above range, and sintering.

As will be understood from the above, the amounts of additives in the core portions of the preform for optical fibers are limited to certain ranges. When a fine glass particle body 6 containing 5 to 15% by weight of GeO2 at the core and containing 0 to 10% by weight P205 and B203 was sintered, a very good preform for optical fibers was obtained. Furthermore, a good preform for optical fibers was obtained from glass particle body 6 containing 15% by weight or more in total of GeO2, P205 and B203 at the core.

By adjusting the amount of GeO2 to ensure An (in this case to about 0.9 to 1.2%) (which is closely related to the transmission loss and the transmission band) is less than 1 5% by weight, a preform for optical fibers having high quality with a transmission loss of not more than 3.0 dB/km (at 0.85 micrometer) and a transmission band of at least 400 MHz. cm075 (0.75th power) was obtained.

In the above experiments, GeO2,P2O5 and B203 were used as additives for the glass cladding. The amounts of the additives in the glass cladding should be adjusted in accordance with the composition of the core portion. These additives are not restricted to GeO2, P205 and B203 and it will be recognised from the foregoing that other additives which permit low loss and low scattering but affect the required control of the refractive index, the coefficient of thermal expansion and the melting point, such as Rio,, Al203, SiF4 and Ga203, can also be used.

Furthermore, the cladding may be pure quartz glass consisting of SiO2.

While the above description has been directed to graded index optical fibers in which the refractive index of the core varies in the form of a parabola as shown in Figure 2, the above experimental facts are also applicable to other optical fibers.

The following Examples illustrate the present invention in more detail. Unless otherwise indicated all percentages are by weight.

Comparative Example 1 Amount of base material SiO2 86.70% Amount of additives for the core glass: GeO2 12% P205 0.3% B203 1% Total 13.3% Amount of base material SiO2 95% Amount of additive for the glass cladding: B203 5% A fine glass particle body having the above composition was produced and sintered. Cracks occurred in the glass cladding.

Comparative Example 2 Amount of base material SiO2 88.89% Amount of additives at the core glass: GeO2 8.2% P205 0.51% B203 2.4% Total 11.11% Amount of base material SiO2 92% Amount of additive for the glass cladding: B203 8% A fine glass particle body having the above composition was produced and sintered in a high temperature furnace at 1 55O0C. A porous portion remained in the core portion and a completely solid preform for optical fibers could not be obtained.

Example 1 Amount of base material SiO2 79.27% Amount of additives for the core glass: GeO2 13.8% P205 0.53% B203 6.4% Total 20.73% Amount of base material SiO2 93.42% Amount of additives for the glass cladding: GeO2 3.5% P205 0.28% 8203 2.8% Total 6.58% A fine glass particle body having the above composition was produced and sintered in a furnace at 1 50O0C. A good preform for optical fibers was obtained. The preform was spun into a fiber having an outer diameter of 1 50 microns.

The transmission loss and the transmission band of the fiber per kilometer were 4.5 dB/km (loss at 0.83 micrometer) and 225 MHz . km, respectively.

Example 2 Amount of base material SiO2 81.72% Amount of additives for the core glass: GeO2 12.5% P205 0.48% B203 5.3% Total 18.28% Amount of base material SiO2 90.02% Amount of additives for the glass cladding: GeO2 3.2% P205 .0.38% B203 6.4% Total 9.98% A fine glass particle body having the above composition was produced and sintered in a furnace at 1 5500C. A good preform for optical fibers was obtained. The preform was stretched to a diameter of 10 mm, and inserted into a quartz pipe having an inner diameter of 11 mm and an outer diameter of 20 mm. The two were fused together and spun into a fiber having an outer diameter of 1 50 micrometers. The transmission loss and the transmission band of this fiber were 2.64 dB/km (loss at 0.83 micrometer) and 405 MHz. km '0.75 power: coefficient for distance conversion), respectively.

Claims

1. An optical fiber comprising a glass cladding comprised mainly of quartz glass and a glass core comprising mainly quartz glass having a higher refractive index than the glass cladding, wherein the quartz glass of the core contains GeO2 and at least one dopant selected from P205, B203, TiO2, Al2O3, SiF4 and Ga203, the amount of GeO2 in the glass core being less than about 15% by weight and the total amount of GeO2 and the other dopants being about 15% by weight or more.

2. The optical fiber of Claim 1, wherein said quartz glass of the core contains GeO2 and at least one of P205 and 8203.

3. The optical fiber of Claim 1 or Claim 2 wherein said glass cladding consists of SiO2.

4. The optical fiber of Claim 1 or Claim 2 wherein said glass cladding is a mixture of SiO2 and at least one of GeO2, P205 and 8203.

5. An optical fiber substantially as hereinbefore described with reference to either of the examples.