Optical waveguide tube

Abstract: In an optical waveguide tube comprising a tubular cladding (2) filled with a solid core (3) having a higher refractive index than the cladding, a light transmissive window member (4) is disposed in close contact with at least the end face of the core on a light incident side for protecting the core end face from thermal influence and contamination. Means disposed around the cladding and the window member for securing the window member to the cladding may be a tape wrapped around them or a heat shrinkable sleeve or a ferrule. Alternatively, a ferrule surrounding the outer periphery of the cladding at each end is fastened for securing the cladding to the core at each end for preventing the core from projecting out of the cladding. The ferrule may have a collar disposed in abutment with the core end face.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an optical waveguide tube comprising a tubular cladding filled with a solid core having a higher refractive index than the cladding. Prior Art Optical waveguide tubes comprising a transparent core and a tubular cladding around the core having a lower refractive index than the core have been used in a variety of optical transmission applications. It is a routine to increase the diameter of the core to 3 mm or greater in order to effectively transmit a larger amount of light. Both solid and liquid cores are known in the art. The solid cores, especially plastic single cores are advantageous because of ease of handling, for example, ease of cutting to a desired length. Optical waveguide tubes of this type receive light from a light source at one end face of the solid core, transmit light therethrough, and emit light from an opposite end face of the core. Since the one end face of the core is given by cutting, it contains minute irregularities so that entrance efficiency is low. Even if the one end face is worked to a smooth surface by polishing, dust and debris deposit thereon from the outside. When light from a light source directly reaches the one end face, part of light is absorbed by such contaminants and converted into heat. This means that all the light incident on the one end face of the core is not transmitted by the core so that the efficiency of light transmission may be reduced. Additionally, since the incident light has relatively high energy, the one end face is heated to a relatively high temperature so that the core may be thermally expanded or deteriorated and even burnt at the one end face, leading to a risk of fire. The core end face can be clouded white in the presence of moisture, also leading to a loss of transmittance. There is a demand for overcoming these problems. When the core undergoes thermal expansion due to direct exposure to light and changes in the ambient atmosphere, the core can project out of the cladding because of the difference in coefficient of thermal expansion between the core and the cladding, especially when the cladding is made of a fluoro resin. The position of light incidence then changes to alter the quantity of incident light. Also the end face of the core and the outer periphery of its projecting portion can be deformed to alter transmission capability. These undesirably change the efficiency of light incidence. Particularly when the core is formed of flexible plastic materials such as acrylic resins, the core is more likely to project out of the cladding. Such core projection leads to losses of light incidence efficiency and light quantity transmitted. Once the core projects out of the cladding, the tube does not always resume the original state when the temperature is restored. The tube can be fixed with the core projected. This is undesirable especially at the incident side. US-A-3,858,577 discloses an optical waveguide tube comprising a tubular cladding, a solid core within the cladding having a higher refractive index than the cladding, the core having opposed end faces, and a light transmissive window member disposed in close contact with an end face of said core over its entire surface, and means disposed around said cladding and said window member of securing said window member to said cladding. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical waveguide tube of the solid single core type which prevents the core end face from being directly heated upon exposure to light, and which prevents the core from projecting out of the cladding. Broadly stated, the present invention is directed to an optical waveguide tube comprising a tubular cladding filled with a solid core having a higher refractive index than the cladding. The tube and hence, the core has axially opposed end faces on light incident and emergent sides. The object of the invention is achieved by disposing a light transmissive window member in close contact with at least the end face of the core on the light incident side over its entire surface, preferably with each of the end faces of the core over its entire surface and means disposed around said cladding and said window member for securing said window member to said cladding, wherein said securing means comprises a tape wrapped on the outer periphery of said cladding and said window member with an adhesive intervening therebetween and a sleeve fitted over the wrapped tape and heat shrunk thereto for tightly surrounding said cladding and said window member. The close contact is preferably attained by bonding the window member to the core end face with an adhesive. The optical waveguide tube of the invention becomes more advantageous when the core is formed of a flexible plastic material and the window member is formed of an inorganic glass material. The optical waveguide tube of the invention is of the solid single core type wherein light from a light source directly reaches the core for light entry. In such application, various inconvenient problems arose with conventional waveguide tubes. In the optical waveguide tube of the invention wherein a light transmissive window member is disposed in close contact with at least the incident end face of the core, light from a light source reaches the window member and enters the core through the window member. This prevents heat generation by direct entry of incident light to the core end face due to light absorption by fine irregularities and deposits on the core end face, and thus prevents the core from being directly heated by light irradiation. This, in turn, prevents the core from thermal expansion and deterioration and eliminates the risk that the core end face is heated by high energy light to a temperature high enough to burn to cause a fire. Also the window member prevents dust and contaminants from depositing on the core end face and shuts off moisture from the ambient atmosphere so that the core may experience a minimal loss of transmittance. Ultraviolet radiation which otherwise enters the core is absorbed by the window member so that only a reduced quantity of ultraviolet radiation enters the core, minimizing deterioration of the core by direct incidence of ultraviolet radiation on the core end face. These advantages become more prominent when the window member is made of inorganic glass. The present invention is very effective to optical waveguide tubes wherein the core is made of a flexible plastic material which is relatively less resistant to heat, has a high efficient of thermal expansion, and is likely to undergo thermal deterioration. In the optical waveguide tube according to the invention, a securing means is disposed around the cladding and the window member for securing the window member to the cladding. The securing means is a tape wrapped around the cladding and the window member as well as a sleeve fitted over the wrapped tape and heat shrunk thereto. The tape is wrapped on the outer periphery of the cladding and the window member with an adhesive, typically pressure-sensitive adhesive intervening therebetween. This may be simply accomplished by using a pressure-sensitive adhesive tape. The sleeve is a heat shrinkable sleeve which is heat shrunk to cover the outer periphery of the cladding and the window member via the tape wrapping. In addition to the advantages described above, the optical waveguide tube of the invention has the advantage that if the window member is secured to the cladding by wrapping a pressure-sensitive adhesive tape and fitting a heat shrinkable sleeve, the window member restricts axial motion of the core within the cladding, eliminating the inconvenience that the core might project out of the cladding due to thermal expansion when the ambient atmosphere is hot. Even in the optical waveguide tube wherein the window member is in close contact with the core end face for protection, if the core can project out of the cladding, then dust and contaminants will deposit on the outer periphery of the projecting core portion, the core will absorb moisture through the projecting portion, and light and heat will directly act on the projecting core portion. The embodiment of the invention wherein the core is firmly secured to the cladding is effective for preventing the core from projecting out of the cladding, eliminating the inconvenient problems described just above. This ensures that the core is protected by the window member. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmental cross-sectional view of an optical waveguide tube according to a Reference embodiment. FIG. 2 is a fragmental cross-sectional view of another optical waveguide tube according to a Reference embodiment. FIG. 3 is a fragmental cross-sectional view of one optical waveguide tube according to a Reference embodiment. FIG. 4 is a fragmental cross-sectional view of another optical waveguide tube according to a Reference embodiment. FIG. 5 is a fragmental cross-sectional view of an exemplary optical waveguide tube according to the invention. BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention as well as Reference embodiments are described below in conjunction with the figures. Optical waveguide tubes have the same basic structure throughout the figures. That is, an optical waveguide tube generally designated at 1 is illustrated as comprising a tubular transparent cladding 2 which is filled with a solid core 3 having a higher refractive index than the cladding 2. The tube 1 and hence, the core 3 has axially opposed end faces on light incident and emergent sides although only the light incident side is shown in the figures. In a Reference embodiment, as shown in FIG. 1, the optical waveguide tube 1 further includes a light transmissive window member 4 disposed in close contact with at least the end face of the core on the light incident side, preferably with each of the end faces of the core. The window member 4 is in close fit with the core end face over its entire surface. In the illustrated Reference embodiment, the close contact is attained by bonding the window member 4 to the core 3 end face with an adhesive 5. The window member 4 may be closely contacted or bonded to either only the end face of the core 3 as shown in FIG. 1 or both the end faces of the core 3 and the cladding 2 as shown in FIG. 2 although the latter is preferred. The window member and the core are in close contact so that no air layer intervenes therebetween, thereby increasing the efficiency of light incidence from the window member to the core. The adhesive bonding of the window member to the core, preferably to both the core and the cladding at the end face thereof minimizes projection or retraction of the core relative to the cladding even when the core is thermally expanded or contracted as a result of a temperature variation of the service atmosphere and the presence or absence of light irradiation. The hollow tubular cladding is preferably made of plastic and elastomeric materials which are flexible, moldable into tubing, and have a relatively low refractive index. Exemplary cladding materials include polyethylene, polypropylene, polyamide, polystyrene, ABS resins, polymethyl methacrylate, polycarbonate, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, ethylene-vinyl acetate copolymers, polyvinyl alcohol, ethylene-vinyl alcohol copolymers, fluoro-resins, silicone resins, natural rubber, polyisoprene rubber, polybutadiene rubber, styrene-butadiene copolymers, butyl rubber, halogenated butyl rubber, chloroprene rubber, acryl rubber, EPDM, acrylonitrile-butadiene copolymers, fluoro-rubber, and silicone rubber. Preferred among others are silicone polymers and fluorinated polymers having a low refractive index. Examples include silicone polymers such as polydimethylsiloxane polymers, polymethylphenylsiloxane polymers, and fluorosilicone polymers; polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkoxy ethylene copolymers (PFE), polychlorotrifluoroethylene (PCTFE), ethylene tetrafluoride-ethylene copolymer (ETFE), polyvinylidene fluoride, polyvinyl fluoride, vinylidene fluoridetrifluorochloroethylene copolymers, vinylidene fluoridehexafluoropropylene copolymers, vinylidene fluoridehexafluoropropylene-tetrafluoroethylene terpolymers, tetrafluoroethylene-propylene rubber, and fluorinated thermoplastic elastomers. These materials may be used alone or in admixture of two or more. The solid material of which the core is formed may be selected from various materials which have a higher refractive index than the cladding. Flexible plastic materials are preferred in the practice of the invention. More particularly, homopolymer and copolymer resins of (meth)acrylic monomers are preferably used. More preferred are copolymers of at least one monomer selected from Groups A and B shown below and homopolymers and copolymers of a monomer selected from Group B. Group A: Esters of (meth)acrylic acid with lower alcohols having 1 to 5 carbon atoms. Group B: Monomers of general formulae (1) and (2) shown below. In formula (1), R 1 is a hydrogen atom or methyl group, R 2 is an alkyl group having 8 to 20 carbon atoms. In formula (2), R 3 is a hydrogen atom or methyl group, R 4 is an alkyl or aryl group, letter n is an integer of 1 to 10, and m is an integer of 1 to 10. The light transmissive material of which the window member is formed includes inorganic glass materials such as quartz glass, Pyrex® glass, multicomponent glass, sapphire, and rock crystal, and organic glass and plastic transparent materials such as polyethylene, polypropylene, ABS resins, acrylonitrile-styrene copolymers, styrene-butadiene copolymers, acrylonitrile-EPDM-styrene terpolymers, styrenemethyl methacrylate copolymers, (meth)acrylic resins, epoxy resins, polymethylpentene, allyl diglycol carbonate resins, spiran resins, amorphous polyolefins, polycarbonates, polyamides, polyarylates, polysulfones, polyallylsulfones, polyether sulfones, polyether imides, polyimides, polyethylene terephthalate, diallyl phthalate, fluoro-resins, polyester carbonates, and silicone resins. Among these, inorganic glass materials such as quartz glass, Pyrex® glass, and multicomponent glass offer superior performance since they are not only highly transparent, but also heat resistant and chemically stable. Where the window member is adhesively bonded to the core, the adhesive may be selected from transparent adhesives, for example, adhesives based on acrylic resins, epoxy resins, phenolic resins, cyanoacrylate resins, silicone resins, and chloroprene rubber. In the practice of the invention, the window member is preferably formed as a disc having an axial thickness of 0.1 to 5 cm, especially 0.3 to 2 cm. The incident end face of the window member is preferably finished to a smooth surface for increasing the efficiency of light incidence on that end face. Although the optical waveguide tube of the invention favors to use an adhesive as means for providing close contact between the window member and the core, it is acceptable, for example, to interpose grease between the core and the window member and securing the window member to the cladding by suitable coupling means. In either case, it is preferred that the window member and the core are in close contact so that no air layer intervenes therebetween. Heat fusing is also acceptable. Owing to the window member attached to the end face of the core, the optical waveguide tube prevents heat generation by direct entry of incident light to the core end face, thereby minimizing thermal expansion and deterioration by such heat generation and eliminating the risk of the core burning and catching fire. It also prevents deterioration of the core end face by deposition of contaminants from the ambient atmosphere and penetration of moisture. The tube thus maintains a superior light transmitting function. In an embodiment of the invention, the optical waveguide tube having a light transmissive window member disposed in close contact with at least the end face of the core on the light incident side over its entire surface includes securing means disposed around the cladding and the window member for securing the window member to the cladding. The securing means is a tape continuously wrapped around the cladding and the window member as well as a sleeve fitted over the wrapped tape and least shrunk thereto for surrounding the cladding and the window member. In one Reference example, as illustrated in FIG. 3, a heat shrinkable sleeve or tubing 6 is fitted so that it may cover the outer periphery of an end portion of the cladding 2 and the entire outer periphery of the window member 4 whereupon the sleeve 6 is heat shrunk to tightly fit on the cladding 2 and the window member 4 for firmly securing the window member 4 to the cladding 2. In another Reference example, as illustrated in FIG. 4, a pressure-sensitive adhesive tape 7 is tightly wound on the outer periphery of an end portion of the cladding 2 and the entire outer periphery of the window member 4 for firmly securing the window member 4 to the cladding 2. In an example of the invention, as illustrated in FIG. 5, a pressure-sensitive adhesive tape 7 is wound on the cladding 2 and the window member 4 as in the Reference embodiment of FIG. 4 and a heat shrinkable sleeve 6 is fitted over the tape wrapping 7 and heat shrunk for tight fit. The pressure-sensitive adhesive tape used herein may be selected from those adhesive tapes prepared by coating substrates such as fabric, nonwoven fabric, plastic films and metal films with pressure-sensitive adhesives of acrylic, butyl rubber, natural rubber, hot melt, and silicone systems, for example. The substrates include films of polyamide resins, polyethylene terephthalate resins, fluoro-resins, polypropylene resins, polyethylene resins, polyimide resins (e.g., Kapton®), glass fibers, brass, aluminum, copper, and stainless steel as well as metallized plastic films. The heat shrinkable sleeve used herein may be selected from tubes of Teflon®, PFA, FEP, dual TFE/FEP, synthetic rubber, polyethylene, silicone rubber, and polyvinyl chloride systems. Since the window member closely abuts the core end face with the window member coupled to the cladding, the optical waveguide tubes illustrated in FIGS. 3 to 5 are effective for restricting axial motion of the core, thereby preventing the core from projecting out of the cladding due to thermal expansion. The example according to the invention of FIG. 5 is especially advantageous since the window member is more firmly and integrally coupled to the cladding. In addition to the advantages mentioned in conjunction with the Reference embodiment, the optical waveguide tube according to the invention has the advantage of eliminating the inconvenience that the core projects out of the cladding when the core undergoes thermal expansion. EXAMPLE A Reference example and an example of the present invention are given below by way of illustration and not by way of limitation. Reference Example 1 An optical waveguide tube having a diameter of 10 mm was prepared using a FEP tube with a gage of 0.6 mm as the cladding and a 4/6 copolymer of Acrylate M and Acrylate SL (manufactured by Mitsubishi Rayon K.K.) as the core. As shown in FIG. 2, a window disc of quartz having a thickness of 1 cm was bonded to one end face of the tube on an incident side using an acrylic adhesive (Photobond 300, Meisei Sunrise K.K.). A similar optical waveguide tube without a window disc was prepared as a control. Both the optical waveguide tubes were coupled to 150-W focus halogen lamps for fiber optics (manufactured by Hayashi Watch K.K.) whereupon a continuous lighting test of burning the lamp for 8 hours a day was carried out for several days. The tube with a window disc remained intact. In contrast, the control tube without a window disc started smoking on the fifth day. At the end of the test, the incident end face of the control tube was observed to find a carbonized area of about 2 mm in diameter. Example 1 An optical waveguide tube having a diameter of 13.6 mm was prepared using a FEP tube with a gage of 0.6 mm as the cladding and a 4/6 copolymer of Acrylate M and Acrylate SL (manufactured by Mitsubishi Rayon K.K.) as the core. As shown in FIG. 5, a window disc of quartz having a thickness of 1 cm was bonded to each end face of the tube using an acrylic adhesive (Photobond 300, Meisei Sunrise K.K.). A pressure-sensitive adhesive tape in the form of a Teflon film of 2 cm wide having an acrylic adhesive coated thereon was wrapped around the tube end and the window disc. A heat shrinkable PFA sleeve of 4 cm long was fitted over the wrapped tape. On the optical waveguide tube having the tape and sleeve mounted thereon, a thermal cycling test consisting of cooling at -30°C for 1 hour and heating at 80°C for 1 hour was continuously carried out for 7 days. Projection of the core from the cladding was not observed during the test. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: An optical waveguide tube comprising a tubular cladding (2), a solid core (3) within the cladding having a higher refractive index than the cladding, the core having opposed end faces, a light transmissive window member (4) disposed in close contact with at least the end face of said core on a light incident side over its entire surface, and means disposed around said cladding and said window member for securing said window member to said cladding, characterized in that said securing means comprises a tape (7) wrapped on the outer periphery of said cladding and said window member with an adhesive intervening therebetween and a sleeve (6) fitted over the wrapped tape and heat shrunk thereto for tightly surrounding said cladding and said window member. The optical waveguide tube of claim 1, wherein said window member (4) is adhesively bonded to the core end face. The optical waveguide tube of claim 1, wherein the core (3) is formed of a flexible plastic material and the window member (4) is formed of an inorganic glass material.

Inventors: HASHIMOTO TORAO, HOTTA ATSUSHI, ISHIHARADA MINORU, SUGIMACHI MASATO, SUGIYAMA HIDEO, TANUMA ITSUO, HASHIMOTO, TORAO, HOTTA, ATSUSHI, ISHIHARADA, MINORU, SUGIMACHI, MASATO, SUGIYAMA, HIDEO, TANUMA, ITSUO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: G02B6
IPC codes: G02B6