CA1267801A

CA1267801A - Optical fiber junction devices

Info

Publication number: CA1267801A
Application number: CA000505926A
Authority: CA
Inventors: Brian S. Kawasaki; Guy Castonguay; Roger Tremblay
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1986-04-04
Filing date: 1986-04-04
Publication date: 1990-04-17

Abstract

OPTICAL FIBER JUNCTION DEVICES
Abstract of the Disclosure A pair of optical fiber terminating members or plugs, each comprising a precision ground frusto-conical plastic body are inserted into a precision molded seating member having correspondingly shaped frusto-conical recesses, the recesses opposed and coaxial. A
transmission path is formed between optical fibers terminated at the plugs. The cores of fibers terminated at the plugs are offset from the longitudinal axes of respective plugs. By relatively rotating the plugs about their longitudinal axes, light transmission between a fiber or fibers in one plug and a fiber or fibers in the other plug is varied. The device is implemented as a variable attenuator, as a switch or as a connector. A fiber is positioned within such a plug by forming a passage along the plug, the passage diameter being larger than the diameter of a fiber to be terminated and the passage having a curve along its length. This curve ensures that the resilient fiber, when fed into the passage locates at a particular angular position at which the fiber end is precisely offset from the plug axis. The fiber is fixed in position using an epoxy resin.

Description

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OPTICAL FIBER JUNCTION DEVICES
This invention relates to optical fiber terminating and junction devices for use in fiber optic systems.
A standard optical connector for use in fiber optic transmissions systems is that described by Runge (U.S. Patent 4,107,242). The fibers at such a connector are each terminated at precision ground frusto-conical plastic plugs. Each oF the fibers is accurately centered within the frustum so that when the terminations are pressed into a precision molded seating body having a passage formed by opposed frusto-conicdl apertures, the fibers at their ends are automatically rendered coaxidl so as to maximize light transmission between them. The plug is made using d transfer molding process employing a precision die whose interior surface deFines a frustum for forming the mating surface of the connector termination.
The fiber is inserted through the mold cavity and extends through an aperture in the end surface of the frustum and through an annulus disposed outside of the mold cavity imrnediately adjacent to the aperture. At the traosfer;molding temperature, the annulus e~xpands inward radially, simultaneously sealing the aperture and centering the fiber. After curing, the excess~fiber is removed, and a flexible, dome-shaped contacting member is formed over the fiber end. ~hen, while simultaneously displaying the magnified end of the termination, it is ground to remD~ve any inltial eccentricity of the fiber resulting after the molding process.
As is evident from the method for fabricating the connector plug, a critical stage in the process is ensuring that the fiber within the plug is accurately mounted on the longitudinal axis 67~

of the trustum.
In fact, other types of fiber optic device can be made using this basic connector design if the frusto-conical elements are manufactured so ~hat ~he fiber core has a small but precise offset relative to the longitudinal axis of the frustum.
According to one aspect of the invention, there is provided an optical fiber terminating device comprising a frusto-conical plug having an optical fiber anchored therein and extending therethrough to an end surface at a narrow end of the frusto-conical plug, an end of the fiber at said end of the plug being marginally offset from a longitudinal axis of said plug.
An optical junction device can comprise two of the plugs and a biconical seating member having a first frusto-conical recess dimensioned to snugly receive a first of said plugs on insertion thereof into said first recess, the seating member having a second frusto-conical recess dimensioned to snugly receive a second of said plugs on insertion thereof into the second recess, sald recesses being opposed and coaxial, means for locking the plugs into respectlve recesses within the seating member, and rotation means for relatively rotating the plugs thereby controllably to alter light transmission between said opticat ~1bers.
By relatively rotating the plugs, respective fiber end faces can be moved from a position in which they are substantially coincident and light transmission is at a maximum to a posltion ln which they do not overlap and there is no light transmission. Between the two positions, the magnitude of light transmission depends on the area of overlap. Thus by accurate1y controlling the relati~e angular :az~o~
positioning of the plugs about their longitudinal axis, the junction device functions as a variable optical attenuator.
Alternatively, if more than one fiber is terminated at one or both plugs and the fiber end faces have identical offsets relative to respective longitudinal axes, the optical junction device can be implemented as a switchO Such a switch junction device preferably further comprises a means for locking the two plugs within the seating member at a first relative angular position at which light transmission between at least one pair of flbers exists and at a second relative angular position at which light transmission between a different pair of fibers results.
Preferably the plugs and the seating member have index marks displayed thereon to show to a user a leYel of attenuation or a switch position depending on the particular implementation of the junction device.
Preferably the plugs are made from an epoxy compound molded onto a metal former.
The frusto-conical terminating member can ha~e d passage extending therethrough somewhat ~Jreater in diameter than the didmeter of a fiber to be terminated thereat, the passdge being curved along its length whereby when a fiber is fed approximately along the passage then owing to the naturdl resilience of the fiber, the fiber end is forced towards the outside of the curve. In an optical device according to the invention, the fiber is fixed in its o~f~set position during manufacture or field installation by injecting epoxy resin into the passage to surround the fiber and then curing the epoxy. The oversized passage is formed by initially molding the terminating ~i7~301 member around a curved wire.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 is a longitudinal cross section sh~wing an5 optical device according to the invention;
Figure lA shows a detail of Figure 1 to a larger scale;
Figure 2 is a schemdtic view showing the device when implemented as a variable attenuator;
Figure 3 is a view similar to Figure 2 but showing the Figure 1 device implemented as a 1 x 2 switch;
Figure 4 ls a view similar to Figure 2 but showing the Figure 1 device implemented as~ a 2 x 2 switch;
Flgure 5 shows one mechanism for relatively rotatlng device plugs, the mechanism adapted for use in a variable attenuator;
Figure 6 shows another mechanism for relatively rotating device plugs, the mechanism adapted for use in an optical ~ ;
switch, Figure 7 shows a longitudinal sectional view through a terminating member;
Figure 8 shows a magnified end view of the member of Figure 7 in which the dimensions of various sizes oF fiber are ~::
indicated; and Figure 9 shows a holder for a device according to the invention.
Referring ln detail to Figure 1, there is shown a longitudinal section through a variable optical attenuator. The attenuator has two terminating members or plugs~10 and 12, which have :

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opposed frusto-conical ends 11. The plugs each have a central metal former 14 surrounded by a molded plastic part 16. Extending from a central bore 18 to an end face 20 of each plug is a passage 22.
Located within the passages 22 are stripped and portions o~ respective optical fibers 24 and 26, the fibers being anchored within respective passages by epoxy adhesive. The conical plug ends have raised seat portions 2~ with the fiber ends projecting slightly beyond the respective seats to terminate at raised pips 30 at which the fiber ends are polished. The plugs bear against opposed conical recesses 32 and 34 in an accurately molded seating member 36.
Referring to Figure 2, there are shown views (a) and (b) of the ends of respective fibers looking in opposite axial directions from a junction zone of the Figure 1 device. A third view (c) shows the two fiber end surfaces superimposed~ The views on the left of the Figure show one attenuator position and those to the right show a different attenuator position. As shown in Figure 29 the fibers terminating at the narrow ends of respective frusto conical plug ends are offset from iongitudinal axes 38 of the plugs, the offset of core 40 of fiber 24 within plug 10 being identical to the offset of the ccre 42 of fiber 26 within plug 12.
Consequently, when the plug 10 is rotated within recess 32, the core end face of the fiber 24 is moved laterally closer to or farther from the core end face of the fiber 26. When the core end faces are coincident as shown in the left-hand superimposed view, then maximum light transmission between the two fibers 24 and 26 results.
As the relative angular position of the plugs 10 and 12 is changed as shown (right-hand superimposed view) transmission across the junction s 126~7~

between the fibers is reduced. The device is used to introduce attenua~ion into an optical transmission lineO The remote ends of the fibers 24 and 26 are termina-ted by connector elements (not shown) such as that described in Runge, and attenuation is introduced into a transmission line after breaking the line at a connector site.
Referring to Figure 3, as an alternative to a single offset fiber, plug 10 has two separate fibers which, at the plug end surface, present two polished fiber ends having cores 44, 46 which are offset from the longitudinal axis 38 by an identical distance. Using a turning mechanism such as that shown in Figures 5 or 6, one or other of the plugs 10, 12 is rotated From a position in which core end 46 is axially coincident with core end face 48 of a fiber ternlinated within plug 12 (left-hand superimposed view) through an angle ~ to a position in which the cores 44 and 48 are axially coincident. The device is thus operable as a 1 x 2 switch.
The required fiber core axis:frusto-conical axis offset~
ls dependent both on the fiber dlameter and the required rotation to change from maximum to minimum attenuation or from one switch position to another. For a 1 x 2 switch a 240 micron offset between axes is suitable for a 30 rotation device using 125 micron diameter fiber.
Referring to Figure 4 and in contrast to Figure 3, both of the plugs ha~e tw terminated Fibers. The fiber core positions are diametrically oppos1te to one another in both of the plugs. A
ro-tating mechanism is used to ensure a relative rotation of exactly 180 to produce a 2 x 2 connector.
When positioning an optical fiber into a plug body, the fiber is stripped of its jacket over an end portion. As shown in ~26'7~3~

Figure 1, successive leng-ths of hea-t shrink tubing 50, flexible tubing 52 and a plastic fiber holding tube 54 are then slid over the bared fiber end and the fiber end portion is coated in liquid epoxy. After sliding the stripped fiber portion through the plug body, the end of the plug is terminated with the plastic holder 54 inside the plug by crimping 56 the metal former 14 at the rear end of the plug. The heat shrink tubing 50 is then heated to cause it to shrink down onto the flexible tube 52 and the trailing end of the plug.
A worm gear mechanism for rotating one of the plugs relative to the other is shown in Figure 5. Plug 12 and seating member 36 are attached to a driven gear 58 and a driving gear 60 is then rotated to cause relative rotation of the plugs lO, 12 at a lubricated junction between the plug 10 and the seating member 36.
An alternative electrically operated mechanism for relatively moving the plugs is shown in Figure 6. Again the seating member 36 is attached to, or made integral with, plug 12. The seating member 36 has an actuating lever 62 extending from it. ~hen a solenoid 64 is actuated, the lever 62 is moved to rotate the seatlng member 36 and integral plug 12 from a first stop position at which there is maximum transmission, against a compression spring bias (not shown) to a second stop position at which there is minimum transmission. When the solenoid 64 is released, the lever 62 returns to the first stop position.
Referring to Figures 7 and 8, there is shown a method of terminating a fiber with a frusto-conical terminating member in such a way that a small, but precise and reproducible fiber end offset is obtained at the end surface of the member.

In contrast wi-th known connector plugs such as that described in U.S. Patent 4,107,242 (Runge), the frusto-conical plug end 11 of Figure 7 is manufactured wi-th a curved passage 72 having a radius of curvature of about 3 centimeters. The curvature is made by molding the frusto-conical plug end 11 around a curved wire (not shown) and then, when cured, drawing the wire out along the passage.
When subsequently an optical fiber 74, stripped of its plastic jacket, is fed into the passage 72, the natural resilience of the fiber ensures that the fiber end locates at the outer side of the curve. By ensuring that the diameter of the passage 72 is greater by a prescribed extent than the diameter of the fiber, a predetermined offset relative to the plug longitudinal axis is obtained. The end of the passage 72 is itself centered accurately on the longitudinal axis of the plug 10 by grinding the frusto-conical part using a fiber core exposed at the end of passage 72 as a grinding center~
The particular conical member of FigurP 8 is useful both in manufacturing attenuator, switch and connector plugs of the type described and also in field installing an optical connector.
Conventionally, as previously indicated with reference to Runge a plug having a straight passage has been used in connectors. Since fiber diameters can vary quite considerably, for example, in a fiber nominally of 125 micron diameter, there may be a diameter variation from 122 to 128 microns, conical plug ends for field connection purposes have been made with a corresponding range in central passage diameter. During field termination of fibers, a match-up between fiber diameter and plug passage diameter is made. By using the design of Figure 7, the dimensions of the plug 10 can be ;'7~

standardized. Thus the design ensures that regardless of whether the fiber is of diameter D1, D2 or D3 (Figure 8) the fiber seats at the same angular position within the oversized passage 72. If a fiber of identical diameter is anchored within -the opposed plug 12 and if the field engineer ensures that the plane of the curve of one plug is co-planar with the plane of curve within the other plug, then the other fiber automatically seats within d coincident angular position and good transmission between the two fibers results.
Referring to Figure 9, a mounting arrangement for the plugs 10, 12 and the seating member 36 includes a barrel portion 74 and respective holders 76. Each plug is locked within its respective holder 76 by a pair of spaced C-rings 80 which lock into grooves within the plugs 10 and 12. Compression springs 82 which surround each plug and extend between forward C-clips 80 and internally projecting flanges 84, serve to bias the plugs towards the junction position. Each holder 76 has an externally threaded part enabling it to be screwed into the barrel portion 74 and a knurled surfdce to facilitate gripping.
The holder arangement shown is particularly adopted for use in a connector such as that described with reference to Figures 7 and 8. However, if used in attenuator or switch arrangements such as those of Figures 2, 3 and 4, modification is necessary to introduce a rotation mechanism such as those o~ Figures 5 and 6,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An optical junction device comprising first and second frusto-conical plugs each having an optical fiber anchored therein and extending therethrough to an end surface at a narrow end of each respective frusto-conical plug, an end of each fiber at said end of the plug being marginally offset from a longitudinal axis of that plug;
a biconical seating member having a first frusto-conical recess for snugly receiving the first plug on insertion thereof into said first recess, the seating member having a second frusto-conical recess for snugly receiving the second plug on insertion thereof into the second recess, said recesses being opposed and coaxial;
means for locking the plugs into respective recesses within the seating member; and rotation means for relatively rotating the plugs thereby controllably to alter light transmission between said optical fibers.

2. An optical junction device as claimed in claim 1 in which the plugs are controllably movable to and fixable in any position between a first position in which the fiber ends are substantially aligned and a second position in which the fiber ends are misaligned, so that the device functions as a variable attenuator.

3. An optical junction device as claimed in claim 1 in which the first plug has two such optical fibers extending therethrough to said end face at the narrow end thereof, said two fibers ar said end surface being angularly spaced about the axis of said first plug, the fibers at the end face having identical offsets from the longitudinal axis so that the device functions as a fiber optic switch.

4. An optical junction device as claimed in claim 3 wherein both of the plugs have at least two fibers extending therethrough to respective end faces at the narrow ends thereof, the fibers at the end faces having identical offsets relative to respective longitudinal axes.

5. An optical junction device as claimed in claim 1 further comprising means for locking the two plugs within the seating member at a preset orientation of the first plug relative to the second plug about said longitudinal axis.

6. An optical device as claimed in claim 1 wherein one of the plugs forms a structure integral with the seating member, a worm gear arrangement driven gear is mounted on the integral structure and a worm gear arrangement driving gear is mounted in driving relationship to said driven gear, the worm gear arrangement operable to rotate said integral structure relative to the other plug.

7. An optical device as claimed in claim 1 wherein one of the plugs forms a structure integral with the seating member, a leer mounted on the integral structure and a solenoid energizable to move the lever and the integral structure against a resilient bias from a first to a second position to change optical transmission between the plugs.

8. An optical device as claimed in claim 2 further comprising an indicator mark on one or both of the plugs and a corresponding mark on the seating member thereby to show the relative orientation of the plugs.

9. An optical termination device as claimed in claim 2 wherein the optical fiber is anchored within a passage extending through the plug, the passage being close to a longitudinal axis of the plug, the passage being oversized in comparison to the diameter of the fiber and being curved, the fiber extending along the passage and at said end surface, occupying a position on the outside of the curve.