OPTICAL FIBER PERMANENT CONNECTOR

Abstract: The connector has a capillary (1) with ends whose entrances taper to guide the inserted ends of the two optical fibres (3, 4) being joined. The capillary fits inside an outer protective tube (2) whose inside diameter is larger than the capillary so as to leave a gap (G) between the top of the capillary and the inside of the tube. The capillary rests on the bottom of the tubes inside). The capillarys inside diameter is less precise than the outside diameter of the fibre cores (3a, 4a). Both the capillary and the tube are made from glass that is partially transparent to UV radiation. A UV-hardened adhesive is injected into the connector which is enclosed in an opaque packaging material. Two UV-transparent glass sleeves (5) may fit over the fibres inside the tube (2) and adjoin each end of the capillary (1) so as to act adaptors increasing the size fibres whose coatings have small diameters.

Description: The present invention relates to a permanent connector for two optical fibers, and particularly to a permanent connector of simple construction for optical fibers with the use of an ultraviolet-curing type adhesive taken into consideration. Heretofore, permanent connectors for optical fibers using glass have commonly had a V-shaped groove or a like construction for optical fiber alignment (centering). Recently, a permanent connector for optical fibers including a glass sleeve in which optical fibers are concentrically butted against each other has been suggested (see Japanese Utility Model Application Disclosure Nos. 61505/1986 and 99806/1986). In the case where optical fibers are to be butted against each other, the use of a V-shaped groove has a risk of the ends of optical fibers deviating to the space opposite to the groove, and if this really happens, the connection loss increases. The conventional permanent connector for optical fibers for concentrically butting optical fibers against each other in a glass sleeve uses a specially shaped single sleeve of nonuniform thickness in order that optical fibers which are each composed of a core having a thickness of about 10 µm or about 50 µm and a clad having an outer diameter of about 125µm concentrically surrounding said core and which are each covered with a coat having an outer diameter of about 250 µm or about 900 µm may be butted against each other. Manufacture of glass sleeves of such shape is very difficult because of their small diameter. Further, they are low in strength and their nonuniform thickness tends to cause stress concentration and they have no reinforcing construction. Therefore, they have been of little practical use. EP-A-0171614 discloses a permanent connector for two optical fibers having a capillary tube for receiving the ends of the optical fibers to be connected. The capillary tube is provided with a transverse slot and is surrounded by a protective tube. JP-A-5625708 discloses a permanent connector for two optical fibers having a capillary tube for receiving the optical fibers. The capillary tube is surrounded by a protective tube and has a discharge port. An object of the present invention is to provide a permanent connector for optical fibers which can be produced relatively simply and with high accuracy and has sufficient strength and which, using a simple and inexpensive tool, can be operated in an explosion-protected place or a narrow working space for connecting optical fibers safely and quickly and with high accuracy. According to the present invention, there is provided a permanent connector for two optical fibers, comprising: a capillary tube made of ultraviolet-transmitting glass and capable of being filled with an ultraviolet curable adhesive agent, said capillary tube having an accurately produced inner diameter slightly greater than the outer diameter of the optical cladding of the two optical fibers to be connected, tapered portions on the opposite ends thereof, and a transverse slot formed in the middle portion thereof for discharging bubbles and some of the adhesive to be contained therein, said transverse slot having a depth which is not more than 1/2 of the tube inner diameter; and an ultraviolet-transmitting glass protective tube, which has an inner diameter greater than the outer diameter of the capillary tube, which surrounds said capillary tube, and to the inner wall of which the capillary tube is eccentrically joined, on its longitudinal side opposite to the side provided with the adhesive discharging slot, so as to define a clearance between said inner wall of said protective tube and said outer wall of said capillary tube on said side where the adhesive discharging slot is formed. Preferably, the permanent connector for optical fibers of the present invention has an ultraviolet-curing type adhesive agent injected thereinto and is wrapped in a light screening packaging material. Preferably, the permanent connector also incorporates two adapter tubes made of ultraviolet-transmitting glass and adapted to be applied to the ends of the optical fibers whose protective coatings have a small outer diameter, said adapter tube having an outer diameter slightly smaller than the inner diameter of the protective glass tube and an inner diameter slightly greater than the outer diameter of the protective coatings of the optical fibers, said adapter tube having tapered portions on their opposite ends. Since the opposite ends of the capillary tube are tapered, insertion of optical fibers is facilitated. When optical fibers are inserted into the capillary tube from its opposite ends, the remaining air in the capillary tube escapes through said slot and clearance under the piston action of the optical fibers, facilitating the insertion of optical fibers together with the adhesive agent sticking to the connected ends of optical fibers, preventing the remaining air in the capillary tube from remaining in the form of bubbles in the adhesive agent to increase connection loss. Since the depth of the transverse slot in the middle portion of the wall of the capillary tube is less than 1/2 of the inner diameter of the capillary tube, the ends of the optical fibers butted against each other adjacent the position where the slot is formed are surrounded by the inner wall of the capillary tube over at least more than half the circumference. Optical fibers can be butted against each other concentrically while retaining the connected end surface of the optical fibers in parallel relation. To give a supplementary explanation, quartz glass optical fibers having an outer diameter of 125 µm are now in popular use and expected to be an international standard. When optical fibers each having an outer diameter of 125 µm, or as thick as a hair, are to be joined together end to end, the transverse slot is formed after the capillary tube has been formed. If this slot has a depth equal to not less than 1/2 of the inner diameter of the capillary tube, then, at the position where the slot is formed, it is impossible to prevent optical fibers from escaping to the outside of the slot. And the connected end surfaces of the optical fibers do not become parallel but are bent and inclined, or the optical axes are off-centered and in this state the optical fibers are butted against each other, thus increasing connection loss. However, this can be prevented by making the depth of the slot equal to less than 1/2 of the inner diameter of the capillary tube. Since the protective glass tube has received no processing which will cause stress concentration, it protects the capillary tube without decreasing the glass strength. And since its inner diameter is greater than the outer diameter of the capillary tube and the capillary tube is deviated to the side opposite to the slot and joined thereto, the formation of a clearance for escapement of air and adhesive agent discharged through the adhesive-passing slot in the middle portion of the capillary tube is facilitated. Since the protective glass and capillary tube are made of ultraviolet-transmitting glass, optical fiber connection can be made simply and quickly by filling an ultraviolet-curable type adhesive agent therein and irradiating them by an ultraviolet lamp. If said adhesive is charged into the protective glass tube in advance and the whole is wrapped in a light-screening packaging material, optical fiber connection can be made soon after the package is unwrapped in the field; thus, it is possible to prevent entry of dust and foreign matter into the adhesive agent, a phenomenon which is liable to occur when an adhesive agent is injected at a site of poor job environment. Further, for optical fibers whose protective coatings have a small outer diameter, an adapter tube may be used, whereby connection can be made in the same manner. Fig. 1 is a longitudinal sectional view of a first embodiment of a permanent connector for optical fibers according to the invention; Fig. 2 is a sectional view taken along II-II; Fig. 3 is a longitudinal sectional view, showing an optical fiber connecting operation; Fig. 4 is a longitudinal sectional view showing another embodiment of a permanent connector according to the invention. In the attached drawings, 1 denotes a capillary tube; 2 denotes a protective glass tube; 3 and 4 denote optical fibers; and 5 denotes an adapter tube. The capillary tube 1 has an inner diameter slightly greater than the outer diameter of the optical claddings 3a and 4a of the optical fibers 3 and 4, e.g., an inner diameter which is about 1 µm greater than the clad outer diameter which is 125 µm, and an outer diameter equal to the outer diameter (e.g., 125 µm). The capillary tube is made of ultraviolet ray-transmitting glass, e.g., borosilicate glass into a cylinder of uniform thickness having tapered portions 1a and 1b at the opposite ends and a transverse slot 1c formed in the middle portion and having a depth which is less than 1/2 of the capillary tube inner diameter. The protective glass tube 2 has an inner diameter slightly greater (by several µm-tens of µm) than the outer diameter of the capillary tube 1. It is made of ultraviolet-transmitting glass similar to that for the capillary tube 1 into a cylinder of uniform wall thickness (a wall thickness corresponding to a required strength). It is greater in length than the capillary tube 1; for example, it is about three times as long as the capillary tube 1 and is chamfered at the opposite ends of the inner diameter to provide tapered portions 2a and 2b. In the middle portion of the protective glass tube 2, the capillary tube 1 is eccentrically joined thereto to form a clearance G between the outer diameter of the capillary tube 1 and the inner diameter of the protective glass tube 2. The eccentric joint is opposite to the position of the transverse slot 1c in the capillary tube, thereby bringing the transverse slot into communication with the clearance G . The capillary tube 1 and the protective glass tube 2 are joined together along a line contact region between the two tubes 1 and 2; thus, the use of an ultraviolet-transmitting adhesive agent is desirable, but other adhesive agents may be used. As for the basic construction of the optical fibers 3 and 4, they are each composed of a passage for optical signals, made of an optical core and an optical cladding 3a, 4a concentrically surrounding the core. Since the refractive index of the cores is about 1% greater than that of the claddings 3a and 4a, there is no possibility of an optical signal escaping to the outside through the claddings 3a and 4a. Protective coatings 3b and 4b are formed on the optical claddings 3a and 4a. The operation of connecting the optical fibers 3 and 4 comprises the steps of stripping predetermined lengths or more of the coatings 3b and 4b, wiping the surfaces of the claddings 3a and 4a and cutting them to a predetermined length suited to the length of the capillary tube 1 by a optical fiber cutting machine (not shown). An ultraviolet-curing type adhesive agent 6 is injected into the protective glass tube 2 in advance. This adhesive agent has a refractive index which is nearly equal to that of the light transmitting portions of the optical fibers and has high environmental durability. And, as shown in Fig. 3, the optical fibers 3 and 4 are inserted into the protective glass tube 2 through the opposite ends of the latter until the fiber end faces are butted against each other, whereupon ultraviolet rays UV are radiated to cure the adhesive agent 6. Fig. 4 shows the use of adapter tubes 5 in permanently connecting optical fibers 3 and 4 whose protective coatings 3b and 4b have an outer diameter considerably smaller than the outer diameter of the capillary tube 1. The adapter tubes 5 have an outer diameter slightly smaller than the inner diameter of the protective glass tube 2 and an inner diameter slightly larger than the outer diameter of the coatings 3b and 4b and tapered portions 5a and 5b at the opposite ends of the inner diameter, said adapter tubes being made of the same glass material as that of the capillary tube 1. When it is desired to use the adapter tubes 5, a predetermined amount of adhesive agent 6 is injected into the protective glass tube 1. The adapter tubes 5 may be inserted in advance or after the optical fibers 3 and 4 have been inserted. Ultraviolet rays UV are then radiated to cure the adhesive agent 6. If said adhesive agent 6 is injected into the protective glass tube 2 in advance and the whole is wrapped in a light screening packaging material, connection of the optical fibers 3 and 4 can be made soon after the package is unwrapped in the field; thus, it is possible to prevent entry of dust and the like into the adhesive agent 6 can be prevented, a phenomenon which is liable to occur when an adhesive agent is injected at a site of poor job environment. Further, a portable ultraviolet lamp can be used as the means for radiating ultraviolet rays UV. According to the invention, optical fibers can be permanently connected together simply and quickly in a short time using an ultraviolet-curing type adhesive agent and an ultraviolet lamp. Particularly, the centering of optical fibers at their butt end can be effected with high accuracy by forming a high precision glass capillary tube with an adhesive-passing groove to a depth of less than 1/2 of the inner diameter; thus, it is possible to use a reliable adhesive agent in a glass capillary tube with the clearance between it and optical fibers being minimized. It is possible to increase the reliability and butting precision of the permanent connector while decreasing connection loss to about 0.2 dB. Further, since the protective glass tube has received no processing which causes stress concentration, the glass strength is not decreased, and because of the formation of the clearance between the protective glass tube and the capillary tube, a passage for the adhesive agent is obtained to reduce the resistance to insertion of optical fibers, preventing air from remaining in the form of bubbles. Connection of optical fibers can be made using inexpensive tools such as a portable ultraviolet source, an optical fiber connection end finishing tool and a connecting jig and at a connection loss of only 0.2 dB and at an explosion-protected site or narrow job site in such applications as emergency connection, temporary connection and the like. By injecting an adhesive agent into the protective glass tube in advance and wrapping the whole in a light screening packaging material, preservation is attained and connection of optical fibers can be made soon after the package is unwrapped in the field, making it possible to prevent entry of dust and the like into the adhesive agent during injection of the adhesive agent at a site of poor job environment. For optical fibers whose protective coatings have a small diameter, the permanent connector can be used in the same manner by using adapter tubes.

Claims: A permanent connector for two optical fibers, comprising: a capillary tube (1) made of ultraviolet-transmitting glass and capable of being filled with an ultraviolet adhesive agent, said capillary tube having an accurately produced inner diameter slightly greater than the outer diameter of the optical cladding (3a, 4a) of the two optical fibers (3, 4) to be connected, tapered portions (1a, 1b) on the opposite ends thereof, and a transverse slot (1c) formed in the middle portion thereof for discharging bubbles and some of the adhesive to be contained therein, said transverse slot having a depth which is not more than 1/2 of the tube inner diameter; and an ultraviolet-transmitting glass protective tube (2), which has an inner diameter greater than the outer diameter of the capillary tube, which surrounds said capillary tube, and to the inner wall of which the capillary tube is eccentrically joined, on its longitudinal side opposite to the side provided with the adhesive discharging slot, so as to define a clearance between said inner wall of said protective tube and said outer wall of said capillary tube on said side where the adhesive-discharging slot is formed. A permanent connector as set forth in claim 1, characterized by having an ultraviolet-curable type adhesive agent (6) injected thereinto, the whole being wrapped in a light screening packaging material. A permanent connector as set forth in claim 1 or 2, characterized by further incorporating two adapter tubes (5) made of ultraviolet-transmitting glass and adapted to be applied to the ends of the optical fibers whose protective coatings (3b, 4b) have a small outer diameter, said adapter tubes having an outer diameter slightly smaller than the inner diameter of the protective glass tube and an inner diameter slightly greater than the outer diameter of said protective coatings of said optical fibers, said adapter tubes having tapered portions on their opposite ends.

Inventors: FUJIKAWA JUNJI, TAMURA FUMIAKI, FUJIKAWA, JUNJI, TAMURA, FUMIAKI
Applicants: NIPPON ELECTRIC GLASS CO, NIPPON ELECTRIC GLASS COMPANY, LIMITED
Application year: 1989
Main classification: G02B6
IPC codes: G02B6