Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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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
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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
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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
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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
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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
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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,
