METHOD FOR PRODUCING AN OPTICAL SENSOR

Abstract: An optical sensor comprising a polymeric core (2) acting as an optical waveguide part, a polymeric cladding surrounding the core and at least one image fiber (1) inserted in at least on bore formed in the core is conveniently produced by a method comprising forming an optical wave­ guide comprising a core (2) in which at least one bore (6) extends along its entire length and a cladding (4, 5) surrounding the core (2) and inserting the image fiber (1) in the the bore, which is characterized in that the optical waveguide is produced by: a method comprising coextruding core (2) and cladding (4, 5) materials whilst blowing air in the core material (fig. 5, 6) to form the bore (6) in the core (2), or a method comprising coextruding core (2) and cladding (4, 5) materials and suppplying at least one metal wire (31, fig. 7) in the core material, removing the metal wire from the core after stretching the wire to form the bore (6) in the core (2) and drawing the waveguide at a temperature higher than a glass transition temperature of the core material, or a method comprising coextruding core (2) and cladding (4, 5) materials and supplying at least one hollow fiber (41, fig. 9) in the core to form the bore (6) in the core (2).

Description: FIELD OF THE INVENTION The present invention relates to a method for producing an optical sensor. More particularly, it relates to a method for producing an optical sensor comprising a core acting as an optical waveguide part for illumination, a cladding surrounding the core and having a refractive index lower than that of the core and an image fiber which is inserted in an elongate bore formed in the core along the entire length of the sensor, or embedded in the core. BACKGROUND OF THE INVENTION An optical sensor is a light- or image-transmit­ ting elongate element comprising an optical waveguide, an image fiber and optionally an elongate hollow tube member for transporting a fluid. Such the optical sensor is often used as a light transmitting element of an endoscope for inspecting an inte­ rior of an organ such as an abdomen, a heart or a blood vessel. The optical sensor is introduced in a human or mammal body to be inspected together with an image pick up adapter which comprises a lens and forms an image of the organ interior on a front end of the image fiber. To the other end near an operator, other elements of the endoscope are attached. The endoscope comprises an image-receiving apparatus for monitoring the image propagated through the image fiber. The image is directly observed by means of an orthoptic adaptor or projected on a monitoring television. Further, a light source attached at the near end of the optical waveguide emits light, which is propagated through the optical waveguide and illuminates the organ interior. The conventional optical waveguide comprises bundled transparent plastic optical fibers each having a round cross section. However, gaps are formed between the adjacent optical fibers, and light is not propagated through the gaps. This reduces an effective volume of the optical waveguide. A hollow tube is additionally assembled in the optical sensor to remove a fluids such as blood from the organ interior or to inject physiologic saline in a small balloon attached to the front end of the sensor so as to inflate the balloon and exclude the blood from a part to be observed. Air in the balloon is exhausted through an addi­ tional tube provided in the sensor. Fig. 1 shows a conven­ tional sensor having an image fiber 1, plural optical fibers 2 as optical waveguides and two tubes 3 all of which are enclosed in a cladding 4. Such the sensor also has gaps between the waveguides and/or the tubes. It has been proposed to increase the effective volume of the optical waveguide by utilizing all the cross sectional area of the sensor except the cross sections of the image fiber and/or the tubes as the optical waveguide (cf. Japanese Patent Kokai Publication (unexamined) Nos. 54830/1985 and 54833/1985). For example, the sensor containing the optical waveguide and the image fiber has a cross section as shown in Fig. 2, 3 or 4. The sensor of Fig. 2 has one bore 6 having a cladding layer 5 through which an image fiber is inserted, that of Fig. 2 has three bores 6 each having a cladding layer 5 through one of which an image fiber is inserted, and that of Fig. 4 has one image fiber 1 and two vacant bores 6. In the methods of the above Publications, the bore is formed in the waveguide by extruding a core material from a coextrusion die with supplying at least one metal wire, simultaneously extruding a cladding material around the core material to form a cylindrical waveguide with the metal wire embedded and removing the metal wire after stretching it to form a bore in the core. These methods, however, have some drawbacks. For example, the inner diameter of the die and the outer dia­ meter of the metal wire should be extremely precise since the former determines the outer diameter of the waveguide and the latter determines the inner diameter of the bore. The methods include many steps including the removal of the metal wire. Further, marks are generated on the inner surface of the bore during the removal of the metal wire. Since the inner surface of the bore should be coated with a material having a smaller refractive index than that of the waveguide (core) material so as to reflect light to be transmitted, the metal wire should be coated by such the material before extrusion of the core material. This makes the structure of the die more complicate. SUMMARY OF THE INVENTION One object of the present invention is to provide an improved method for producing an optical sensor compri­ sing a core acting as an optical waveguide part, a cladding surrounding the core and an image fiber inserted in a bore formed in the core. Another object of the present invention is to provide an improved method for producing an optical sensor having an enlarged effective cross sectional area of the optical waveguide. Further object of the present invention is to provide a novel optical sensor comprising a core acting as an optical waveguide part and having a bore extending along the entire length of the sensor, a cladding surrounding the core, and an image fiber embedded in the core. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a cross section of the conventional optical sensor comprising plural optical fibers as optical waveguides, an image fiber and two hollow tubes and a clad­ ding surrounding all these elongate elements, Figs. 2, 3 and 4 show cross sections of embodi­ ments of an optical sensor to be produced according to the present invention, Fig. 5 shows a cross section of a die to be used in the first embodiment of the method according to the present invention, Fig. 6 schematically shows an equipment used in the first embodiment of the method according to the present invention, Figs. 7 and 8 schematically show equipments used in the second embodiment of the method according to the present invention, Fig. 9 schematically shows an equipment used in the third embodiment of the method according to the present invention, Fig. 10 shows a cross section of a die to be used in the third embodiment of the method according to the present invention, and Fig. 11 shows a cross section of an optical sensor produced by the third embodiment of the method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The core of the sensor to be produced by the method of the invention is usually made of a transparent amorphous polymeric material optionally added with a plasti­ cizer. Specific examples of the core material are homo- or co-polymer of styrene or methacrylate (e.g., polymethyl methacrylate, n-butyl methacrylate/isobutyl methacrylate copolymer and the like), polymethyl methacrylate plasticized with adipate, and polycarbonate. The core is surrounded by a cladding to confine the propagated light in the core. The cladding is made of a material having a smaller refractive index than that of the core material such as fluororesin (e.g., homo- or co-polymer of vinylidene fluoride or tetrafluoroethylene), silicone resin and ethylene/vinyl acetate copolymer. Preferably, the optical sensor has an outer dia­ meter of 0.25 to 2.2 mm, the cladding has a thickness of 0.01 to 0.04, and the bore has a diameter up to 1.5 mm. In the first embodiment of the method according to the present invention, the optical waveguide is produced by means of a coextrusion die shown in Fig. 5. The die 11 has an aperture 13 and an inlet 14 for the core and cladding materials coaxially surrounding the aperture 13. At the front end of the inlet 14, a frustoconical slant channel 15 is provided with. The tip of the channel 15 opens near the front end of the aperture 13. The core material 22 and the cladding material 23 are fed into the inlet 14. Simultaneously, a suitable volume of air is injected through the aperture 13 at suitable pressure. The core and cladding materials are coextruded in the form of a tube a central hollow portion of which is formed by injected air. The thus produced optical waveguide has a cross section as shown in Fig. 2, wherein the inner and outer surfaces of the core 2 are coated by the cladding material 4,5. Through the bore 6, an image fiber is inserted to form an optical sensor. Fig. 6 schematically shows an equipment for produ­ cing the optical waveguide of Fig. 2. The core and cladding materials are fed in a co­ extrusion die part 17 having the die 11 of Fig. 5 from extruders 16 and 18, respectively. A fiber 19 extruded from the die is cooled in a cooling zone 20, and drawn by a capstan puller 21 and finally wound by a winder 22. The outer diameter of the fiber 19 is controlled by adjusting the puling rate of the capstan 21 and the fed amounts of the core and cladding materials. A waveguide having two or more bores in the core can be produced by means of a suitable die having two or more apertures for injecting air. In the second embodiment of the method of the present invention, the optical waveguide is produced by coextruding the core and cladding materials with supplying at least one metal wire in the core material to form an optical waveguide having the metal wire embedded, removing the wire from the fiber after stretching it and drawing the waveguide at a temperature higher than a glass transition temperature of the core material. Such the method will be described with reference to Figs. 7 and 8. In Fig. 7, a metal wire 31 is supplied from a supplier 30 to a die 32, and the core and cladding materials are supplied from an extruder 32' and coated around the metal wire 31. Two or more metal wires may be supplied to finally produce an optical waveguide having two or more bores. An optical waveguide 33 with the metal wire is wound by a winder 34. In a subsequent step, the metal wire is removed from the optical waveguide. In Fig. 8, the optical waveguide 34 without the wire is unwound from a supplier 35 and wound by a winder 39 through a roll 35 for controlling a supply rate, a heating means 37 and a roll 38 for controlling a winding rate. The heating means 37 may be a furnace, a water or oil bath or a heating roll. The optical waveguide is heated at a tempera­ ture higher than the glass transition temperature of the core material by 10 to 50°C. The drawn rate of the optical waveguide is controlled by adjusting the supply rate and the winding rate. Usually, the waveguide is drawn by a factor of 2 to 3. The metal wire is made of a rigid flexible metal such as annealed copper wire. In the third embodiment of the method of the present invention, the waveguide is produced by substan­ tially the same manner as in the second embodiment but using a hollow fiber in place of the metal wire and not drawing the waveguide. In this embodiment, the hollow fiber forms the bore of the optical waveguide. The hollow fiber is made of a thermoplastic resin (e.g, polyamide, polysulfone, polyethersulfone, polyether­ imide, polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene/hexafluoropropylene copolymer and the like), a thermoset resin (e.g., polyesterimide, polyamide­ imide, polyimide and the like), quartz glass, multicomponent glass, metal (e.g., gold, silver, copper, aluminum, stain­ less steel and the like). Because of flexibility, the resins are preferable. In this embodiment, an image fiber may be supplied together with the hollow fiber. This method is carried out by an equipment schema­ tically shown in Fig. 9. Two hollow fibers 41 are continuously supplied from corresponding hollow fiber suppliers 42 to a coext­ rusion die 40. An image fiber 1 is continuously supplied from an image fiber supplier 43 to the coextrusion die 1. Distance between the two hollow fibers 7 is controlled by means of a pair of guide rolls 44. The coextrusion die is attached to a head of an extruder consisting of extruders 45,46 for the core material and the cladding material, respectively. Fig. 10 shows an enlarged cross section of a die 10, which has apertures 51 for forwarding the respective hollow fiber and an aperture (not shown) for forwarding the image fiber 1. In the die 10, the core material A is cylin­ drically extruded around the hollow fibers 41 and the image fiber 1 and then a cladding material B is applied around the core material to form an optical waveguide comprising a core 52 and a cladding 53. Then, the cladding is sized to form an optical sensor of Fig. 11. The optical sensor extruded from the die 10 is cooled in a cooling zone 47 to solidify the cladding mate­ rial. In the downstream of the cooling zone, a capstan puller 48 and a winder 49 are provided with. Extrusion conditions are so selected that the thermal deformation of the hollow fiber is prevented during extrusion of the core and cladding materials. For example, the hollow fiber is made of a material having a deformation temperature higher than the extrusion temperature of the core material preferably by at least 10°C. This method can be modified by replacing the image fiber with a third hollow fiber. In this case, the image fiber is inserted in one of three hollow fibers. This modi­ fication is preferred since the image fiber is not subjected to a high temperature so that it is not thermally damaged. The present invention will be explained by follow­ ing Examples. EXAMPLE 1 By means of the die of Fig. 5, an optical wave­ guide shown in Fig. 2 having one bore was produced. As the core and cladding materials, polymethyl methacrylate and vinylidene fluoride/tetrafluoroethylene copolymer were used, respectively. The optical waveguide was produced under following conditions: Extrusion rate of the resin: 20-40 cm³/min. Pulling rate: 5-10 m/min. Extrusion temperature: 240°C The produced waveguide had an outer diameter of 2.2 mm, a bore diameter of 1.5 mm and cladding thickness of 20µm. EXAMPLE 2 In the same manner as in EXAMPLE 1 but using, as a core material, n-butyl methacrylate/isobutyl methacrylate copolymer or polymethyl methacrylate plasticized with adipate and, as a cladding material, polymer of fluorine-­ containing alkyl methacrylate, an optical waveguide was produced under substantially the same conditions but the extrusion temperature being 150-200°C. EXAMPLE 3 A core material of polymethyl methacrylate (glass transition temperature, 105°C) and a cladding material of vinylidene fluoride/tetrafluoroethylene copolymer were extruded and applied around annealed copper wire having an outer diameter of 1.06 mm. Then, the copper wire was removed to form an optical waveguide having an outer diameter of 1.70 mm and a bore diameter of 1.06 mm. The produced optical waveguide was drawn with a draw ratio of two in a furnace kept at 140°C to produce the waveguide having an outer diameter of 1.2 mm and a bore diameter of 0.75 mm. Its physical properties are shown in Table. EXAMPLE 4 In the same manner as in EXAMPLE 3 but drawing a waveguide having an outer diameter of 2.08 mm and a bore diameter of 1.30 mm with a draw ratio of three at 150°C, an optical waveguide having an outer diameter of 1.2 mm and a bore diameter of 0.75 mm was produced. Its physical proper­ ties are shown in Table. COMPARATIVE EXAMPLE In the same manner as in EXAMPLE 3 but not drawing the waveguide after removing the wire, an optical waveguide having an outer diameter of 1.2 mm and a bore diameter of 0.75 mm was produced. Its physical properties are shown in Table. In these examples, an image sensor could be pro­ duced by inserting an image fiber in the bore of the wave­ guide. EXAMPLE 5 An optical sensor was produced by means of the die of Fig. 10. From the die, supplied were two hollow fibers made of polysulfone (manufactured by UCC) each having an outer diameter of 0.6 mm and an inner diameter of 0.5 mm and an image fiber having 3,000 picture elements, a diameter of the picture element bundle of 0.5 mm and an outer diameter of 0.95 mm. Simultaneously, n-butyl methacrylate/isobutyl methacrylate copolymer as a core material and a blend of vinylidene fluoride/tetrafluoroethylene copolymer and polyn­ butyl methacrylate were extruded to form an optical sensor having an outer diameter of 2.5 mm. EXAMPLE 6 In the same manner as in EXAMPLE 5 but using two hollow fiber made of polyamideimide having an outer diameter of 0.6 mm and an inner diameter of 0.5 mm and a hollow fiber made of polyamideimide having an outer diameter of 1.1 mm and an inner diameter of 1.0 mm, an optical waveguide having three bores was produced. Then, an image fiber was inserted in the hollow fiber having the inner diameter of 1.0 mm to produce an optical sensor. EXAMPLE 7 In the same manner as in EXAMPLE 5 but using two hollow fiber made of tetrafluoroethylene/hexafluoroethylene copolymer having an outer diameter of 0.6 mm and an inner diameter of 0.5 mm and a hollow fiber made of the same copolymer having an outer diameter of 1.1 mm and an inner diameter of 1.0 mm, an optical waveguide was produced. Then, an image fiber was inserted in the hollow fiber having the inner diameter of 1.0 mm to produce an optical sensor.

Claims: 1. A method for producing an optical sensor comprising a core acting as an optical waveguide part, a cladding surrounding the core and at least one image fiber inserted in at least one bore formed in the core, which method comprising forming an optical waveguide comprising a core in which at least one bore is extended along its entire length and a cladding surrounding the core and inserting the image fiber in the bore, characterized in that the optical waveguide is produced by: a method comprising coextruding core and cladding materials with blowing air in the core material to form the bore in the core, or a method comprising coextruding core and cladding materials with supplying at least one metal wire in the core material, removing the metal wire from the core after stretching the wire to form the bore in the core and drawing the waveguide at a temperature higher than a glass transition temperature of the core material, or a method comprising coextruding core and cladding materials with supplying at least one hollow fiber in the core to form the bore in the core. 2. A method according to claim 1, wherein the optical waveguide is produced by a method comprising coextruding core and cladding materials with blowing air in the core material to form the bore in the core. 3. A method according to claim 1, wherein the optical waveguide is produced by a method comprising coextruding core and cladding materials with supplying the metal wire in the core material, removing the metal wire from the core after stretching the wire to form the bore in the core and drawing the waveguide at a temperature higher than the glass transition temperature of the core material. 4. A method according to claim 3, wherein the drawing temperature is higher than the glass transition temperature of the core material by 10 to 50°C. 5. A method according to claim 3, wherein the draw ratio of the waveguide is 2 to 3. 6. A method according to claim 1, wherein the waveguide is produced by a method comprising coextruding core and cladding materials with supplying the hollow fiber in the core to form the bore in the core. 7. An optical sensor comprising a core acting as an optical waveguide part and having at least one bore extending along the entire length of the sensor, a cladding surrounding the core, at least one image fiber embedded in the core or inserted in at least one bore. 8. A method for producing an optical sensor according to claim 7, which comprises coextruding core and cladding materials with supplying at least one image fiber and at least one hollow fiber in the core to form the bore in the core.

Inventors: MATSUBARA, HIRONAGA C/O OSAKA WORKS OF, MATSUMIYA, NORIFUMI C/O OSAKA WORKS OF, UEBA, YOSHINOBU C/O OSAKA WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1985
Main classification: B29C47
IPC codes: B29C47, A61B1