GB2026716A

GB2026716A - A Glass Optical Fiber Coated with Organopolysiloxane Layers

Info

Publication number: GB2026716A
Application number: GB7918392A
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: 1978-05-25
Filing date: 1979-05-25
Publication date: 1980-02-06
Also published as: DE2921338A1; FR2426917A1; DE2921338C2; FR2426917B1; GB2026716B

Abstract

A reinforced glass fiber for use in optical transmission comprises an optical fiber 1 provided with a coating 5 of a first curable organopolysiloxane composition having a refractive index higher than that of the clad glass which forms the outermost layer of the optical fiber, said first curable organopolysiloxane composition being baked. A coating 6 of a second curable organopolysiloxane composition which can be the same as or different from the first curable organopolysiloxane composition is provided on the first curable organopolysiloxane and is baked. <IMAGE>

Description

SPECIFICATION A Glass Fiber for Use in Optical Transmission This invention relates to glass fibers for optical transmission (hereafter referred to as optical fibers).

With the recent advancement in fiber optics technology concerned with reducing light loss due to absorption by the fibers, many attempts have been made to use optical fibers as a medium for communication. While optical fibers housed in cables (hereinafter referred to as optical cables) can be used in a variety of communication modes, the electrically insulating properties of optical fibers has led to interest in combining optical cables with power cables in composite communication-power cables.

Therefore, various methods have been proposed for housing optical fibers within a power cable and using them to transmit control or communication signais. In addition, some proposals are being subjected to field testing.

In a power cable, especially a high-tension cable, it is known that the conductor and its nearby area are heated to about 900C by the Joule effect. Therefore, if an optical cable is combined with a power cable, it is necessary to ensure that exposure of the optical fiber to this level of temperature for a long period of time does not result in the fiber changing its transmission characteristics or mechanical characteristics.

Optical fibers are generally protected with a plastics coating to increase the mechanical strength and/or facilitate handling of the fibers, examples of conventional optical fibers being shown in Figures 1 to 3 of the accompanying drawings. According to a study by the present inventors, if a coated optical fiber (hereinafter referred to as the core of an optical fiber) is exposed to high temperatures, shrinkage due to the residual strain remaining after molding of the plastics coating or increased internal "clamping" due to a change in volume accompanying higher crystallinity will cause the effect generally referred to as "microbending", which in turn results in increased transmission loss.

However, since optical fibers should be 200 mm or less in diameter to retain their flexibility and are made of a fragile material, the mechanical strength of the fibers makes it almost impossible to use them as a transmission line without any protection.

In addition, it is well known that an inherent property of glass is a tendency to lose its strength with time due to the influence of moisture and other factors. Therefore, several prior art techniques have been proposed for covering an optical fiber with a protective coating of plastics or other suitable material so as to provide the optical fiber with a desired initial strength and a strength that withstands extended use.For example, a coated optical fiber obtained by the method disclosed in Japanese Patent Application (OPI) No. 125754/75 which comprises coating an optical fiber with a thermosetting resin composition (generally referred to as a primary coat) and baking the resin coating and further providing thereon a coating of a melt-extruded thermoplastic resin composition (a secondary coat), possesses satisfactory strength and weatherability sufficient to withstand extended use. Also, as disclosed in Japanese Patent Application (OPI) No. 100734/76, it is known that a spun optical fiber, prior to its contact with another solid object, can be coated with a resin composition which is then baked to provide the fiber with a strength not substantially lower than the virgin strength of the glass.

On the other hand, a stress absorbing layer of small Young's modulus has been provided between the primary coat of thermosetting resin and the secondary coat of thermoplastic resin to eliminate the increased transmission loss due to the so-called "microbending phenomenon" which occurs when an optical fiber is repetitiously bent in short cycles. Examples of the materials which have been proposed for the stress absorbing layer are silicone resin, urethane rubber, butadiene rubber, ethylene-propylene rubber and foamed plastics. Of these materials, the silicone resin has been used widely because of its high processability, good curability and weatherability. The term "silicone resin" as used herein refers to a two-part room temperature vulcanizing resin (RTV) which is generally referred to as a curable organopolysiloxane.

Of various organopolysiloxanes, dimethyl polysiloxane which is generally commercially available has a refractive index of about 1.40 which is lower than the refractive index of glass. Therefore, if dimethyl polysiloxane is directly coated on an optical fiber and then baked, the resulting glass fiber has the following disadvantages: a) When an optical fiber having a distribution of refractive index as illustrated in accompanying Figure 7 is coated with a layer of an organopolysiloxane composition having a refractive index of about 1.40, the resulting transmission system as shown in Figure 8 comprises the desired transmission system having I as the core and another transmission system having II as the core and the organopolysiloxane as the cladding.The second transmission system (or cladding transmission system) having the core of II is undesirable and makes the correct measurement of transmission loss difficult.

Since the transmission system having II as the core suffers an optically higher loss than the system having I as the core, light excited in the region II will be damped in a distance of about ten meters. As a result, estimation of optical transmission in terms of the ratio of the optical output at a point one to two meters away from the incident end (Pin) to the optical output at a point several hundred to thousand meters away from the incident end (Pout) cannot be made correctly because Pin includes the optical output from the transmission system having ll as the core and is therefore over estimated.

b) In addition, if the light loss in the region of ll is relatively low, light excited in the region ll will reach the receiving end. On the other hand, the core of an optical fiber is generally prepared by controlling the distribution of its refractive index to obtain a desired transmission band (or base-band frequency characteristic) which is one element of its transmission characteristic. Therefore, emergence of light excited in the region Il at the receiving end will seriously degrade the transmission band of the fiber.

It is to be understood that while Figures 7 to 9 illustrate examples of the distribution of refractive index of an optical fiber to which this invention is applicable, optical fibers having other distributions of refractive index are included within the scope of this invention.

A primary object of this invention is to provide a reinforced optical fiber which is free from clad transmission and has high mechanical strength as well as stable transmission characteristics that can withstand extended use.

Accordingly, the invention resides in a glass fiber for use in optical transmission comprising an optical fiber having provided thereon (1) a coating of a first curable organopolysiloxane composition having a refractive index higher than the glass which forms the outermost layer of the glass fiber, said first curable organopolysiloxane composition being baked, and (2) a coating of a second curable organopolysiloxane composition which can be the same as or different from the first curable organopolysiloxane composition, said second curable organopolysiloxane composition being provided on the first curable organopolysiloxane and being baked.

In order to produce the glass fiber of the invention, a spun optical fiber, before contacting other solid materials, is coated with a first curable organopolysiloxane composition having a refractive index larger than that of the clad glass which forms the outermost layer of the fiber and the first composition is then baked. It is also possible to increase the rate of production of the optical fibers by using a silicone resin which is cured by exposure to ultraviolet rays. The resulting fiber is further coated with a second curable organopolysiloxane composition which can be the same as or different from the first curable organopolysiloxane composition, and the second curable composition is also baked. The glass fiber thus obtained can optionally be coated with a thermoplastic resin composition on the baked second curable composition.The coatings of the first and second curable organopolysiloxane compositions of the thus-reinforced optical fiber function as what are generally referred to as a primary coating and a stress absorptive coating. The coatings thus provide the fiber with sufficient mechanical strength to withstand the stresses encountered during the assembling and sheathing steps for making a cable from the fibers and to withstand extended use in varying environments to be encountered after cable laying, as well as making the fiber retain stable transmission characteristics in environments which are likely to cause microbending.

As previously stated the first curable organopolysiloxane composition has a refractive index higher than that of the glass which forms the outermost layer of the optical fiber, and so it is able to absorb and inhibit undesirable transmission modes and achieve correct measurement of transmission loss without degrading the transmission band. A curable organopolysiloxane composition having a refractive index higher than that of glass may have a basic structure comprising the polysiloxane bond of Si--OO-Si and phenyl groups as side chain substituents. The phenyl content in this resin composition can vary within the range from about 1.40 to 1.52 to control its refractive index. A suitable phenyl polysiloxane composition basically comprises:- (i) a component a component a component

wherein R is a substituted or unsubstituted univalent hydrocarbon group having no aliphatic unsaturation, (ii) an organohydrodiene polysiloxane component having in its molecule at least 3 hydrogen atoms directly bonded to a silicon atom contained in an amount sufficient to provide 0.7 to 5 such hydrogen atoms per vinyl group of the component and (iii) a catalytic amount of a platinum compound. Examples of suitable platinum compounds are those which are highly compatible with the above two components, such as an olefin complex or a chloroplatinic acid in which part of chlorine may or may not be substituted with an alcohol, aldehyde or ketone.For the purpose of increasing the mechanical strength of the cured product and fluidity of the composition, these three components may be combined with an organopolysiloxane composition comprising (CH2=CH)R2SiOo 5, R3SiOo and SiO2 (wherein R is a substituted or unsubstituted univalent hydrocarbon group having no aliphatic unsaturation) the molar ratio of the sum of (CH2=CH)R2SiO05 and R3SiO05 to SiO2 being in the range of from 0.5 to 2.0 and the content of vinyl group being in the range of from 0.5 to 3 wt.%. The phenyl content in this phenyl polysiloxane composition can be changed to control its refractive index within the range of from about 1.40 to about 1.52.In the formulae m and n are positive integers such that the phenyl polysiloxane composition has the desired refractive index and a viscosity at 250C of from 50 to 100,000 centistokes, preferably from 1,000 to 10,000 centistokes. The second curable organopolysiloxane composition may have a refractive index lower than that of glass and may basically comprise (i) a component

wherein R is a substituted or unsubstituted uni-valent hydrocarbon group having no aliphatic unsaturation and most preferably is a methyl group, (ii) an organohydrodiene polysiloxane component having in its molecule at least 3 hydrogen atoms directly bonded to a silicon atom contained in an amount sufficient to provide 0.7 to 5 such hydrogen atoms per vinyl group in the above defined component, and a catalytic amount of a platinum compound.Examples of suitable platinum compounds are those which are highly compatible with the above two components, such as an olefin complex or a chloroplatinic acid in which part of the chlorine may or may not be substituted with an alcohoi, aidehyde or ketone. This resin composition generally has a refractive index ranging from 1.40 to 1.41.For the purpose of increasing the mechanical strength of the cured product and fluidity of the composition, these three components may be combined with an organopolysiloxane composition comprising the units of (CH2=CH)R2SiOo s, R3 Si00,5, and SiO2 (wherein R is a substituted or unsubstituted univalent hydrocarbon group having no aliphatic unsaturation), the molar ratio of the sum of (CH2=CH)R2SiO05 and R3SiO0,5 to SiO2 being in the range of from 0.5 to 2.0 and the content of vinyl group being in the range of from 0.5 to 3 wt.%. Being a positive integer, n in the formula above of the curable polysiloxane composition is desirably such that said composition has a viscosity at 250C of from 50 to 1,000,000 centistokes, preferably from 500 to 10,000 centistokes.

Alternatively, the second curable organopolysiloxane composition may comprise a fluorinecontaining organopolysiloxane, preferably a trifluoroalkyl-containing organopolysiloxane, again conveniently having a refractive index lower than that of glass. As a further alternative the second curable organopolysiloxane may comprise a phenyl polysiloxane.

Both the first and second curable organopolysiloxane compositions should be capable of rapid curing and, to increase the rate of production of the optical fibers, at least one of the first and second organopolysiloxane compositions may be photosetting organopolysiloxane composition having incorporated at terminals of the main chain or in side chains a vinyl group, a mercapto group or acryl group. Examples of suitable photosetting organopolysiloxane compositions are those which comprise a component.

a component

a benzoin photosensitizer, or which comprise a component

a component

and a benzoin photosensitizer. The refractive index of the photosetting organopolysiloxane composition can be controlled by using either methyl groups for R or phenyl groups for R in the above formulae.

While the above examples refer to a benzoin sensitizer it will be apparent that other equally well known photosensitizers can be used.

The curable organopolysiloxane compositions used in this invention should afford, after curing, a Young's modulus which is sufficiently small to absorb any external stress and prevent the resultant optical fiber from bending in a small radius. According to the study of the present inventors, an organopolysiloxane composition which affords, after curing, a Young's modulus of 2.0 kg/mm2 or more is ineffective as a stress absorptive material and experiences an increase in transmission loss with changes in temperature and other factors. The cured organopolysiloxane composition preferably has a Young's modulus of 0.5 kg/mm2 or less.

Although the first curable organopolysiloxane composition must have a refractive index higher than that of glass, there is no limit on the refractive index of the second curable organopolysiloxane composition. Since a phenyl polysiloxane composition is more expensive than a dimethyl polysiioxane composition it is desirable to use a dimethyl polysiloxane composition rather than a phenyl polysiloxane composition as the second curable organopolysiloxane composition of the present invention. Moreover, in addition to its low price, a dimethyl polysiloxane composition of low refractive index has the advantage that its molecular structure permits faster curing of the composition than a phenyl polysiloxane composition of high refractive index.

It is to be understood that at least one of the first and second curable organopolysiloxane compositions may contain one or more fillers selected from the group consisting of fume silica, precipitated silica, aluminum silicate, quartz powder, fused quartz powder, diatomaceous earth, calcium carbonate, titanium dioxide and carbon black. Such fillers provide an improved mechanical strength.

It is to be noted that the second curable organopolysiloxane composition need not be formed as a single layer; instead, it may have a multilayer structure composed of the same or different materials.

The second curable organopolysiloxane layer may be coated with a thermoplastic resin composition to produce a thermoplastic layer which functions as an additional stress absorbing layer. The thermoplastic resin composition may be used in the thermoplastic layer independently or as a mixture with one or more of an additive resin, an inorganic filler, an organic filler, a cross-linking agent, pigment or dye. The resin composition should be such that it can be melt-extruded for coating on a glass fiber, preferred examples being polyamide, polyester, polycarbonate, polyurethane, polyethylene, polypropylene, ionomer resin, polyvinyl chloride, and an ethylene-vinyl acetate copolymer.

The production speed of optical fibers may be increased by replacing the curable organopolysiloxane composition with a non-photosetting resin composition containing a vinyl group, a mercapto group or an acrylate group.

In the accompanying drawings; Figures 1 to 3 are each a cross-sectional view of the core of a conventional optical fiber, Figure 4 is a cross-sectional view of the core of an optical fiber according to one example of the present invention, Figure 5 is a cross-sectional view of a composite cable comprising optical cables produced from optical fibers as shown in Figure 4, and a power cable, Figure 6 is an enlarged view of the optical cable shown in Figure 5, Figures 7 and 8 illustrate the refractive index distributions of conventional plastics-jacketed optical fibers, Figure 9 illustrates the distribution of refractive index of a coated optical fiber according to the invention, and Figure 10 illustrates apparatus for use in the production of an optical fiber according to the invention.

Referring to Figure 4, in the optical fiber of said one example of this invention, 1 is a glass fiber, 5 is a first coating composed of a silicon resin mainly comprising a phenyl group-containing polysiloxane, and 6 is a second coating composed of (1) a silicone resin similar to the first coating, or (2) a silicone resin mainly comprising polysiloxane which contains fluorine in the form of a trifluoro-alkyl group and which has a refractive index below 1.40.

The silicone resin which mainly consists of a phenyl group-containing polysiloxane or a polysiloxane containing a trifluoroalkyl group used for each of the first and second coatings is highly resistant to heat and has excellent mechanical strength and will suffer small change in its physical properties if it is exposed to a temperature higher than 1000C. Therefore, coating an optical fiber with said silicone resin will provide a core suffering no substantial ligh loss due to the aforementioned effect of microbending.On the other hand, since the silicone resin which mainly consists of a trifluoroalkyl group-containing polysiloxane has a refractive index lower than 1.40, a glass fiber directly coated with this resin will involve an unwanted clad mode transmission. to avoid such disadvantage, the layer of the silicone resin mainly comprising trifluoroalkyl-containing polysiloxane is separated from the glass fiber by the layer of a silicone resin having a refractive index higher than that of glass and which mainly consists of phenyl-containing polysiloxane.

According to the experiments conducted by the present inventors, the core of the optical fiber having the cladding shown in Figure 4 suffers no substantial increase in transmission loss if it is exposed to 1 600C for a period of 30 days or longer. Estimation of the measurements by the Arrhenius formula indicates that the characteristics of the fiber will remain stable at 900C for a period of 10 years or longer.

Figure 5 illustrates a composite cable comprising three optical cables produced from optical fibers shown in Figure 4 and a power cable. In Figure 5, the reference numeral 7 represents the power cable, 8 is an optical cable, 9 is an outer coat, 10 is an inner coat, 11 is an external conductor, 12 is an insulating layer, and 13 is an internal conductor. It is to be appreciated that more than three optical cables may be housed in the composite cable shown in Figure 5.

Figure 6 is an enlarged view of the optical cable of Figure 5 and in Figure 6, the reference numeral 14 represents a jacket, 1 5 is an interstitial quad, 1 6 is the core of an optical fiber, and 1 7 is a tension member. While four cores are housed in the cable of Figure 6, any desired number of cores can be used. In addition, the characteristics of the cable can be more effectively stabilized by a V- or U-shaped interstitial spacer or heat insulating material.

Figures 7 and 8 illustrate examples of the distribution of refractive index of conventional plastic jacketed optical fibers, whereas Figure 9 shows the distribution of refractive index of an optical fiber coated according to this invention. In Figures 7, 8 and 9, the reference number 1 represents a core, 2 is a cladding A, 3 is a cladding B, and 4 and 5 are each a coat made of an organopolysiloxane composition.

Figure 10 illustrates one embodiment of apparatus for coating and baking the organopolysiloxane compositions used in producing the optical fiber according to the invention. In Figure 10, 1 9 is a spinning furnace, 20 is a fiber rod, 21 is a coating die, 22 is a curing furnace, and 23 is a take-up bobbin.

This invention will now be described in greater detail by reference to the following Examples which are given for illustrative purposes only and are by no means meant to limit the scope of thisinvention.

Example 1 A fiber rod composed mainly of quartz and having an outside diameter of about 15 mm was heated in a resistance-inductance furnace (designated by the number 19 in Figure 10) and spun into a fiber having an outside diameter of 125 ,um. Before the fiber contacted another solid object, it was passed through a coating die (21-A in Figure 10) where it was coated with a phenyl polysiloxane composition (OF 103, a product of Shinetsu Chemical Industries Co,, Ltd.) transferred to an electric heating type curing furnace (22-A in Figure 10) for curing, then passed through a coating die (21-B in Figure 10) where it was coated with a dimethyl polysiloxane composition (KE 103 RTV, a product of Shinetsu Chemical Industries Co., Ltd.) transferred to an electric heating type curing furnace (22-B ini Figure 10) for curing, and finally accumulated by a take-up bobbin (23 in Figure 10). Cured OF 103 and KE 103 each had a Young's modulus of about 0.05 kg/mm2. A screw type extruding machine was used to coat the fiber with Nylon-12 (Diamide N-1940, a product of Daicel Ltd., Japan) by melt extrusion.

The thicknesses of the phenyl polysiloxane composition, dimethyl polysiloxane composition and Nylon12 coated were 200 yam,350 ym and 0.9 cm, respectively. The coating speed for each layer was 30 m/min.

Example 2 The procedure of Example 1 was repeated to produce a reinforced optical fiber except that the phenyl polysiloxane composition was CY-52-151 (a product of Toray Silicone), the dimethyl polysiloxane composition was CY-52-01 6 (a product of Toray Silicone) and the thermoplastic resin was HDPE (Hizer 5300, a product of Mitsui Petrochemical Industries, Japan). Cured CY-52-1 51 and CY-52-016 each had a Young's modulus of 0.05 kg/mm2. The thickness of each coating and its coating speed were the same as in Example 1.

Example 3 The procedure of Example 1 was repeated to produce a reinforced optical fiber except that the phenyl polysiloxane composition was photosetting X-32-296 (a product of Shinetsu Chemical Industries Co., Ltd.) the dimethyl polysiloxane composition was photosetting X-62-719 (a product of Shinietsu Chemical Ind. Co., Ltd.) both of which were cured with a 20 cm long mercury lamp (rated power: 2 kw), and that the thermosetting resin was polybutylene terephthalate (PBT 1401, a product of Toray). Cured X-32-296 and X-62-719 each had a Young's modulus of 0.05 kg/mm2. The coating and baking sped of phenyl and dimethyl polysiloxane compositions were 100 m/mm, and the extrusion/coating speed of polybutylene terephthalate was 30 m/min. The thickness of each coating was the same as in Example 1.

Example 4 The procedure of Example 1 was repeated to produce a reinforced optical fiber except that the phenyl polysiloxane compound was OF 103 (a product of Shinetsu Chemical Industries Co., Ltd.) and the dimethyl polysiloxane composition was OF 10 (a product of Shinetsu Chemical Ind. Co., Ltd.) mixed with silicon dioxide powder. The thicknesses of the first and second coatings were 300,um and 600 ,um, respectively.

The reinforced optical fibers produced in Examples 1 to 4 had the advantages that (1) they were free from undesirable transmission modes, (2) they had an average strength of 3.5 GN/m2, (3) they could be used at a temperature in the range of from -600C to + 1 700C without experiencing an increase in transmission loss, and that (4) no variation in transmission loss would occur during cable making and laying procedures.

In addition, none of these reinforced optical fibers suffered an increase in transmission loss even if they were exposed at a temperature of 1 6O0C for a period of 30 days or more.

Claims

1. A glass fiber for use in optical transussion comprising an optical fiber having provided thereon (1) a coating of a first curable organopolysiloxane composition having a refractive index higher than the glass which forms the outermost layer of the glass fiber, said first curable organopolysiloxane composition being baked, and (2) a coating of a second curable organopolysiloxane composition which can be the same as or different from the first curable organopolysiloxane composition, said second curable organopolysiloxane composition being provided on the first curable organopolysiloxane and being baked.

2. The glass fiber of Claim 1, wherein at least the first curable organopolysiloxane composition is a phenyl polysiloxane composition.

3. The glass fiber of Claim 1, wherein at least one of the first and second curable organopolysiloxane compositions is a photosetting curable organopolysiloxane composition.

4. The glass fiber of Claim 1, wherein the first curable organopolysiloxane composition is composed of a silicone resin mainly consisting of a phenyl group-containing curable organopolysiloxane.

5. The glass fiber of any one of Claims 1 to 4, wherein at least one of the first and second curable organopolysiloxane compositions has stress absorbing effect and a Young's modulus lower than 2.0 kg/mm2 at room temperature.

6. The glass fiber of Claim 1, wherein the second curable organopolysiloxane composition is a dimethyl polysiloxane composition.

7. The glass fiber of Claim 1, wherein said second curable organopolysiloxane composition is composed of a fluorine-containing organopolysiloxane.

8. The glass fiber of Claim 7, wherein said second curable organopolysiloxane composition is composed of a silicone resin mainly consisting of a trifluoroalkyl group-containing curable organopolysiloxane.

9. The glass fiber of any one of Claims 1 to 8, wherein at least one of the first and second curable organpolysiloxane compositions contains one or more fillers selected from the group consisting of fume silica, precipitated silicate, aluminum silicate, quartz powder, fused quartz powder, diatomaceous earth, calcium carbonate, titanium dioxide and carbon black.

10. The glass fiber of any one of Claims 1 to 9, wherein the second curable organopolysiloxane composition is coated with a thermoplastic resin composition.

11. The glass fiber of any one of Claims 1 to 10, wherein the second curable organopolysiloxane composition has a multilayer structure composed of the same or different materials.

12. A glass fiber as claimed in Claim 1 substantially as hereinbefore described with reference to, and as shown in, Figure 4 of the accompanying drawings.

13. An optical cable including a glass fiber as claimed in any one of the preceding claims.

14. An optical cable as claimed in Claim 13, substantially as hereinbefore described with reference to, and as shown in, Figure 6 of the accompanying drawings.

15. A composite cable including an optical cable as claimed in Claim 13 or Claim 14 and an electrical power cable.

16. A composite cable as claimed in Claim 15, substantially as hereinbefore described with reference to, and as shown in, Figure 5 of the accompanying drawings.