Coated optical fiber

Abstract: A coated optical fiber (11) and an optical fiber unit (20) including a plurality of coated optical fibers (11) as well as a method of forming the optical fiber unit. The coated optical fiber (11) is able to minimize transmission losses of the optical fiber unit by utilizing a pigment in a resin covering layer (3) that covers an optical fiber strand in a specific range, the range being from 0.01 to 1.40 percent by weight.

Description: Fig. 1 is a cross sectional view showing a structure of a typical coated optical fiber Generally, a coated optical fiber 11 is made up of a secondary layer 3 and an optical fiber strand 10 including an optical fiber 1 made of SiO₂ as a main component and a primary layer 2 made of a material such as a silicone resin or an ultraviolet curable resin. The primary layer 2 coats the optical fiber 1, and the secondary layer 3, made of a thermoplastic resin, e.g., nylon, coats the optical fiber strand 10. The resin of the covering layers 2 and 3 is originally transparent, however where a plurality of coated optical fibers are gathered, as will be described below, the coated optical fiber 11 is frequently colored by mixing pigment into the secondary layer 3, in order to distinguish the coated optical fibers from one another. Generally, the coated fiber thus constructed is dimensionally designed such that the diameter of the optical fiber 1 is approximately 125µm, the diameter of a strand as formed by covering the fiber with a primary layer 2 is approximately 400µm and the diameter of a coated optical fiber formed by covering the strand with a secondary layer 3 is approximately 600 to 900µm. The coated optical fibers are usually used in such a way that a plurality of coated optical fibers are collectively arranged into an optical fiber unit or an optical fiber cable. Fig. 2(a) is a cross sectional view showing a typical structure of an optical fiber unit using coated optical fibers. As shown, in an optical fiber unit 20, a tensile strength member 21 is disposed at the center. A plurality of coated optical fibers 11 are disposed around the member 21. An ultraviolet curable resin 22 is applied to an arrangement of the tensile strength member 21 and the coated optical fibers 11, and is cured into a single solid structure. In manufacturing the optical fiber unit 20, coated optical fibers 11 are twisted and disposed around the tensile strength member 21. The ultraviolet curable resin 22 is then applied to an arrangement of the fibers and the member 21, forming a single solid structure. Finally, the ultraviolet curable resin layer 22 is cured by irradiating it with ultraviolet rays. In the above described optical fiber unit, a coated optical fiber is covered with a plurality of covering layers, to form an optical fiber. A plurality of coated optical fibers are gathered, twisted, disposed around a tensile strength member, and covered again with an ultraviolet curable resin. Even with such a solid structure, when the fiber unit is bent, the transmission loss thereof is often remarkably increased. Such a tendency of the fiber unit limits the practical bendable range of the fiber unit. This is one of the problems urgently needed to be solved in this technical field. Chemical Abstracts, vol. 109, no. 10, September 1988, abstract no. 78494c, Columbus, Ohio, US, discloses a coated optical fiber comprising a core and a silicon resin containing 0.1% TiO₂. WO-A-8 403 085 relates to a process for coating an optical fiber with a coating being hard on its outside but soft on its inside, i.e. the fiber-covering side, which comprises applying to the fiber an opaque layer containing pigment and subsequently radiation crosslinking the thus coated fiber. Patent Abstracts of Japan, vol. 12, no. 175 (p-707) [3022], 25 May 1988, describes a body wherein no lateral pressure is applied to the optical fibers contained which comprises a central tension resisting body surrounded by twisted optical fibers that are fixed into said body with a thermosetting or ultraviolet ray hardened resin and a heat resistent coating on the outside of the body. It is the object of the present invention to provide an optical fiber unit with a broadened bendable range and a method for producing same. According to the present invention this object is attained with an optical fiber unit comprising an opaque tensile strength member, a plurality of coated optical fibers disposed around the tensile strength member, and an ultraviolet curable resin layer covering said tensile strength member and said coated optical fibers to form a single solid structure, wherein each of said coated optical fibers comprises an optical fiber strand and a resin covering layer containing pigment for covering said optical fiber strand, wherein said resin covering layer contains said pigment in the range of 0.01 to 1.40 percent by weight. The method of forming an optical fiber unit according to the present invention comprises the steps of: mixing 0.01 to 1.40 percent by weight of pigment into a resin covering layer for a coated optical fiber; applying said resin covering layer to a plurality of coated optical fibers; arranging said plurality of coated optical fibers around an inner tensile member; applying an ultraviolet curable resin around the arrangement of the plurality of coated optical fibers and said tensile member; and curing said ultraviolet curable resin by irradiating it with ultraviolet light. This invention will be described with reference to the drawings in which: Figure 1 is a cross sectional view showing a struture of a typical known coated optical fiber to which the present invention is applicable; FIGURE 2(a) is a cross sectional view showing a typical known structure of an optical fiber unit using the coated optical fibers shown in FIGURE 1; and FIGURE 2(b) shows an enlarged cross sectional view of a part of the cross section of the optical fiber unit shown in FIGURE 2(a). In an optical fiber unit 20 in which a plurality of coated optical fibers 11 are assembled into a solid structure by an ultraviolet curable resin, the gaps between a tensile strength member 21 and the coated optical fibers 11 are filled with the ultraviolet curable resin. Consider the case where the fiber unit 20 is irradiated with ultraviolet rays. The member 21 is generally opaque and pigment is contained in the coated fibers 11. Accordingly, the irradiated ultraviolet rays are absorbed or scattered by those components. The quantity of the irradiated rays in an area A within the fiber unit is remarkably smaller than that of the irradiated rays in the remaining area. In the area A, the resin is insufficiently cured, resulting in an imcomplete fixing of the coated fibers 11 within the fiber unit. When such a fiber unit is bent, the coated fibers 11 are displaced within the fiber unit 20, so that the optical fiber 1 is locally bent. This increases transmission losses in the coated fiber or fiber unit. The above fact was found and confirmed by the inventor in the present patent application. Constructing the coated fibers so that they are colorless or transparent should be avoided. Consider the case where colorless optical coated fibers are used for an optical fiber unit. The optical fiber unit is extremely long and both ends of the fiber unit are separated by an extremely long distance. Therefore, it is very difficult to identify a specific coated optical fiber at both ends of the fiber unit, because the coated fibers are all transparent and hence cannot be distinguished from one another. To identify each coated optical fiber, it is necessary to enter light into the coated fiber at one end and receive it at the other end. This work must repeatedly be done for all coated fibers. However, it is almost impossible to apply such a coated fiber identifying technique for a fiber unit containing a great number of coated optical fibers. In this respect, the use of pigment for coated optical fibers is essential in practical use. One possible way to accelerate the curing of the ultraviolet curable resin in the optical fiber unit is to increase the energy of the irradiating ultraviolet rays. The ultraviolet rays' energy (expressed usually in mJ) is the product of the intensity of the ultraviolet rays (usually in mW) and the irradiation time (usually in s). It is clear that to accelerate the curing of the resin, it is necessary to increase the irradiation time or the intensity of the ultraviolet rays. Actually, a light source for emitting ultraviolet rays also emits infrared rays for heating the ultraviolet curable resin. According, if the quantity of ultraviolet rays is increased, the resin deteriorates due to heat. Ideally, the curing of the entire resin is saturated, but unfortunately, such is not the case. When the quantity of the energy of ultraviolet rays is merely increased, the portion of the optical fiber unit which is not shaded with the coated fibers is fully saturated and undergoes deterioration already, while in a portion shaded with the coated fibers, the curing of the resin is not yet saturated and the resin is insufficiently cured. The inventor has found that a strictly optimized density of pigment mixed into the covering layer of a coated optical fiber can provide both excellent visual identification of a coated optical fibers and perfect using of an ultraviolet curable resin. According to the present invention, the concentration of pigment mixed into the resin covering layer, or a secondary layer, of the coated optical fiber for coloring the layer is set to 0.01 to 1.40 percent by weight in a standard coated optical fiber in which the diameter of the optical fiber strand is approximately 400µm, and the thickness of the resin layer covering the fiber strand is 100 to 250µm. When the concentration of the pigment exceeds the above range, the ultraviolet curable resin used for the optical fiber unit cures irregularly. On the other hand, when it is below the above noted range, it is difficult to visually perceive the coloration of the coated fiber. Accordingly, it becomes meaningless to use the pigment. When the thickness of the covering layer made of the resin mixed with the pigment exceeds the above range, the concentration of the pigment must be properly decreased, in order to secure an optimum quantity of transmitted ultraviolet rays. Generally, where the particle diameter of the pigment exceeds the wavelength of transmitted light, the transmissivity is abruptly decreased. Therefore, it is preferable to use pigment of the smallest possible particle diameter. A preferable particle diameter of the pigment is 50µm or less, although it depends on the wavelenght of the ultraviolet rays used. White pigment, such as TiO₂, may be used as pigment providing white color, but usually it is used together with another pigment, in order to make the color clear. In the case where a plurality of pigments are used, the total concentration of the pigments must be selected so as to allow a smooth transmissivity of the ultraviolet rays. In case where pigment of TiO₂ is solely used, the concentration of the pigment in the covering layer is preferably within the range of 0.01 to 0.80 percent by weight. In case where a combination of the pigment TiO₂ and another pigment is used, the concentration of the pigment TiO₂ is preferably within the range of 0.01 to 0.80 percent by weight, the concentration of the remaining pigment is preferably within the range of 0.03 to 1.00 % by weight. In this case, the total concentration of those pigments must be within the above range. To manufacture an optical fiber unit having such a structure that the coated fibers are disposed in an alignment fashion, it is preferable that the concentration of pigment contained in each coated fiber is properly decreased. Some examples of the present invention will has be described. Example 1 Coated optical fibers were manufactured according to the present invention, and the characteristics were measured. An optical fiber of 125µm in diameter was covered with a primary layer made of an ultraviolet curable resin, thereby forming an optical fiber strand of 400µm in diameter. The strand thus formed was further covered with a secondary layer 100µm thick, thereby forming a coated optical fiber as shown in Fig. 1. The secondary layer was made of nylon containing pigment and TiO₂ at the concentrations shown in Table 1. The diameter of each coated fiber was 0.6mm. An optical fiber unit structured as shown in Fig. 2(a) was manufactured by using six coated optical fibers. The six coated optical fibers were twisted and disposed around a tensile strength member, a steel wire of 0.6mm in diameter. Then, an ultraviolet curable resin was applied to the arrangement of the tensile strength member and the twisted coated fibers, thereby forming an optical fiber unit of about 2.7mm in diameter. The layer was cured by irradiating it with ultraviolet rays of 100mJ. The transmission loss of the optical fiber unit thus formed was measured. Then, it was wound around a drum of 100mm in diameter by three turns, and the increase of the transmission loss was measured. The measurement result is shown in Table 2. As seen from Table 2, in the optical fiber unit using the coated optical fibers 11 according to the present invention, the increase of the transmission loss due to bending was negligible. The color of each coated fiber was satisfactorily perceived by naked eyes. Example 2 To form a colored covering layer material, pigments (of white, blue, yellow, red, and black) were mixed into 12 nylon as the mixing ratios shown in Table 3. In Table 3, the concentration of each pigment contained in the colored covering material at each mixing ratio is expressed in weight percent. In the table, the colors of each pigment in the column of "Mixed pigment" are shown in the left column. For white, its pigment in the column of "Mixed pigments" is blue pigment. In Table 3, materials whose pigment's concentrations are outside the concentration range as defined by the present invention were used for preparing comparative coated fibers. By using the colored resin thus prepared, coated optical fibers were manufactured as in Example 1. An optical fiber of 125µm in diameter was covered with a primary layer made of an ultraviolet curable resin, thereby forming an optical fiber strand of 400µm in diameter. The strand thus formed was further covered with a secondary layer, thereby forming a coated optical fiber of 600µm in outer diameter. An optical fiber unit was manufactured for each type of the coated fibers in the same ways as that for Example 1. The specifications of the fiber unit were the same as those of Example 1. As in Example 1, the increase of the transmission loss of each of the 26 optical fiber units was measured. The results of the measurements are shown in Table 4. As evident from Table 4, when an optical fiber unit using the coated optical fibers as the comparative coated optical fibers in which pigment of more than 1.4 percent by weight is contained in the resin, is bent, the transmission loss is remarkably increased. When an optical fiber unit using coated optical fibers as manufactured according to the present invention is bent, no increase of the transmission loss was measured. Also in the case of an optical fiber unit using coated optical fibers in which pigment of less than 0.01 percent by weight is contained in the resin, no increase of the transmission loss was measured. However, it was difficult to perceive the color of the coated fiber. In the examples and the comparative examples as mentioned above, 12 nylon was used for the covering material of the coated fiber. Alternatively, the covering material may be any of vinyl chloride, polyethylene, 6 nylon, and nylon or nylon copolymer as a mixture of various types of nylon. As described above, the present invention succeeded in providing an optical fiber unit in which each coated fiber can be visually identified, and when the fiber unit is bent, no increase of transmission loss occurs in the fiber unit or coated fibers. When an optical fiber unit is manufactured using such coated fibers, the coated fibers allow ultraviolet rays to transmit therethrough with little loss. Accordingly, the ultraviolet curable resin can be cured uniformly when it is irradiated with ultraviolet rays, and a reliable support of the coated fibers is ensured. Consequently, an optical fiber unit manufactured using the coated fibers suffers from little transmission loss when it is bent.

Claims: An optical fiber unit comprising: an opaque tensile strength member; a plurality of coated optical fibers disposed around said tensile strength member; and an ultraviolet curable resin layer covering said tensile strength member and said coated optical fibers to form a single solid structure; wherein each of said coated optical fibers comprises an optical fiber strand and a resin covering layer containing pigment for covering said optical fiber strand, wherein said resin covering layer contains said pigment in the range of 0.01 to 1.40 percent by weight. A method of forming an optical fiber unit, said method comprising the steps of: mixing 0.01 to 1.40 percent by weight of pigment into a resin covering layer for a coated optical fiber; applying said resin covering layer to a plurality of coated optical fibers; arranging said plurality of coated optical fibers around an inner tensile member; applying an ultraviolet curable resin around the arrangement of the plurality of coated optical fibers and said tensile member; and curing said ultraviolet curable resin by irradiating it with ultraviolet light.

Inventors: MIZUTANI AKIHIKO C O YOKOHAMA, MIZUTANI, AKIHIKO, MIZUTANI, AKIHIKO, C/O YOKOHAMA WORKS,
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C03C25
IPC codes: G02B6, C03C25