Description
OPTICAL FIBER CABLE AND METHOD OF MID-SPAN
ACCESS THEREOF
Technical Field
[0001] Apparatuses consistent with the present invention relate to optical fiber cables
enclosing fibers, in which enclosed fibers are easily accessible but prevented from
damage, and a method of mid-span access of the optical fiber cables.
Background Art
[0002] An optical fiber cable in some cases includes plural fibers for the purpose of in-
creasing the capacity or the number of devices linking via the cable. These fibers may
be enclosed with a slotted core and the slotted core along with the fibers may be further
enclosed with a sheath.
[0003] After being laid, some optical fiber cables are often subject to a work named "mid-
span access" to make the enclosed optical fibers branch off In the mid-span access
work, the sheath and the core are cut and split to enable access to one or more of the
enclosed fibers. As the fibers exposed to the exterior are susceptible to damage, any
proper measure for protection may be required.
[0004] Japanese Patent Unexamined Application Publications Nos. S62-291608, S63-5313,
H06-50009 and H08-211261 disclose related arts of optical fiber cables.
Disclosure of Invention
Technical Problem
[0005] Some circumstances cause damage to properties of the optical fibers. For example, as
the slotted core is likely to move relative to the sheath, projection of the slotted core
out of one end of the sheath may occur. The projection will lead to damage to the
optical fibers at the projecting part. Further, curving or meandering may generate com-
pression or tensile stress on the optical fibers, which causes increase in transmission
loss. Certain embodiments of the present invention provide optical fiber cables
enclosing fibers, in which enclosed fibers are easily accessible but prevented from
damage.
Technical Solution
[0006] An optical fiber cable according to an aspect of the present invention has an axis. The
optical fiber cable is comprised of: a slotted core elongated along the axis, the slotted
core including a slot running in parallel with the axis and a groove accessible through
the slot; one or more optical fibers placed in the groove; a sheath enclosing the slotted
core and the optical fibers; a bonding portion where the slotted core is bonded with the
sheath; and two or more strength members embedded in the slotted core, the strength
member running in parallel with the axis and being aligned on a plane including the
axis.
[0007] According to another aspect of the present invention, a method of mid-span access of
the optical fiber cable is comprised of separating a part of a sheath from the optical
fiber to expose the slot in part; covering the exposed slot in part with the separated part
of the sheath; and pulling one or more of the optical fibers out of the slotted core.
Brief Description of the Drawings
[0008] [fig.1]FIG. 1 shows a cross section of an optical fiber cable according to a first em-
bodiment of the present invention.
[fig.2]FIGS. 2(A) through 2(E) are drawings explaining a process of mid-span access,
[fig. 3] FIG. 3 is a schematic drawing explaining a draw test method.
[fig.4]FIG. 4 shows a cross section of an optical fiber cable according to a second em-
bodiment of the present invention.
[fig.5]FIG. 5 shows a cross section of an optical fiber cable according to a fourth em-
bodiment of the present invention.
[fig.6]FIG. 6 shows a cross section of an optical fiber cable according to a fifth em-
bodiment of the present invention.
[fig.7]FIG. 7 shows a cross section of an optical fiber cable according to a sixth em-
bodiment of the present invention.
[fig.8]FIG. 8 shows a cross section of an optical fiber cable according to a seventh em-
bodiment of the present invention, which further applies to eighth and ninth em-
bodiments of the present invention.
[fig.9]FIG. 9 shows a cross section of an optical fiber cable according to a tenth em-
bodiment of the present invention.
[fig.10]FIG. 10 shows a longitudinal section of the optical fiber taken along the Y-axis
of FIG. 9.
[fig. 11]FIG. 11 shows a cross section of an optical fiber cable according to an eleventh
embodiment of the present invention.
[fig.12]FIGs. 12(A) through 12(C) are partial sectional views to show variations of
markers for indicating where a slotted core is fixed with a sheath.
[fig. 13]FIG. 13 shows a cross section of an optical fiber according to an embodiment
of the present invention, which is replaceable with that of the first embodiment.
[fig.14]FIG. 14 shows a cross section of an optical fiber according to a fourteenth em-
bodiment of the present invention.
[fig. 15]FIGs. 15(A) through 15(D) are drawings explaining a process of mid-span
access explained in relation to the optical fiber cable according to the fourteenth em-
bodiment.
[fig.16]FIG. 16 is a cross sectional view of the optical fiber cable for explanation of the
process of the odd-span access.
[fig.17]FIG. 17 is a cross sectional view of the optical fiber cable for illustrating a state
after splitting.
Best Mode for Carrying Out the Invention
[0009] Exemplary embodiments of the present invention will be described hereinafter with
reference to the appended drawings. While optical fiber cables according to the em-
bodiments are elongated along a central axis C thereof, FIGS. 1, 4-9, 11-13 only show
cross sections thereof taken along a plane perpendicular to the central axis. The
following descriptions and the appended drawings often refer rectangular coordinates
represented by X- and Y-axes on these sectional planes for convenience of ex-
planation. These X- and Y-axes and elements related thereto sometimes represent
planes and bodies elongated along the central axis C.
[0010] Referring to FIG. 1, an optical fiber cable 1 according to a first embodiment of the
present invention is comprised of optical fibers 3, a slotted core 7 having a groove 5
for housing the optical fibers 3, and a sheath 9 enclosing the slotted core 9 along with
the optical fibers 3. Needless to say, all the fibers 3, the groove 5, the core 7, the sheath
9 and the slot 11 run in parallel with the central axis C of the optical fiber cable 1.
[0011] The slotted core 7 is further comprised of a slot 11 opened linearly along the slotted
core 7 for enabling access to the interior of the groove 5. Therefore the slotted core 7
has a C-letter cross sectional shape. The wall of the slotted core 7 gradually becomes
thicker toward the side opposite to the slot 11. The groove 5 is eccentric from the outer
profile of the slotted core 7. When the center of the slot 11 and the side just opposite to
the slot 11 are made aligned on the Y-axis, the eccentricity is also in a direction along
the Y-axis.
[0012] The sheath 9 preferably consists of any proper resin such as polyethylene. The sheath
9 in comprised of a nonuniform wall which gradually becomes thinner from a thickest
wall portion 13 toward a thinnest wall portion 15, both of which are aligned on the Y-
axis. Thereby eccentricity in the direction along the Y-axis is given to a hollow defined
by the wall relative to the outer profile of the sheath 9. The thickest wall portion 13
covers the slot 11.
[0013] As the eccentricity of the groove 5 relative to the slotted core 7 is just reverse to the
eccentricity of the hollow of the sheath 9, the groove 5 is resultantly substantially
concentric with the central axis C of the optical fiber cable 1. Alternatively, the groove
5 may be slightly eccentric from the central axis C in either direction along the Y-axis.
[0014] The slotted core 7 is further comprised of a pair of strength members 17 embedded
therein. Both the strength members 17 are aligned on the plane including both the Y-
axis and the central axis C of the cable 1. Further, the strength members 17 are in
nature opposite to the slot 11 with respect to the central axis C. The strength members
17 may be formed in various shapes such as a line, a strip, an elongated multilateral
prism or a column. The number of the strength members 17 is not limited to two and
may be three or more. A slight interval is preferably held between the strength
members 17.
[0015] The strength members 17 are made of any material reinforcing the optical fiber cable
1 against tensile force, such as steel or FRP (Fiber Reinforced Plastic), and in general
have greater stiffness than those of the other members. As the strength members 17
having such stiffness are aligned on the plane, when the optical fiber cable 1 is curved,
this plane functions as a neutral surface in a meaning of mechanics (a surface along
which material is neither compressed nor extended).
[0016] In any case, the strength members 17 may be aligned on another plane. Even then, if
the optical fibers 3 are disposed around the plane, increase in transmission loss can be
suppressed as will be discussed later.
[0017] Although the cross sectional shape of the groove 5 is illustrated as a circle in FIG. 1,
the shape is not limited thereto and may be an ellipse or any irregular shape instead.
Further, the interior of the groove 5 may be either vacant except the optical fibers 3 or
filled with any buffer members. In any case, the optical fibers 3 are preferably disposed
around the central axis C.
[0018] The optical fibers 3 may be any of bare optical fibers, optical fiber cords, and optical
fiber ribbons.
[0019] An elongate tape 19 preferably made of non-woven fabric or any resin such as PET
(PolyEthylene Terephthalate) is attached on the slotted core 7 to cover the slot 11. The
elongate tape 19 is not wrapped around the slotted core 7 and leaves a lower part of a
surface of the slotted core 7 uncovered. Therefore the sheath 9 may be directly in
contact with this lower part of the slotted core 7 while the elongate tape 19 intervenes
between the upper part of the slotted core 7 and the sheath 9.
[0020] At this uncovered part, the slotted core 7 has a bonding portion 23 where the slotted
core 7 is bonded to the sheath 9. The bonding portion 23 longitudinally ranges over the
slotted core 7 to form a continuous line or a row of separate portions at intervals.
Thermal fusion boding may be applied to bonding at the bonding portion 23. In the
present embodiment, a projecting rib 21 projecting from the slotted core 7 is formed in
advance of bonding. The projecting rib 21 facilitates thermal fusion bonding with the
sheath 9 and, after bonding, becomes the bonding portion 23 fitting in and bonding
with a complementary recess of the sheath 9. In any proper case, thermal fusion
bonding or any other bonding treatment can be omitted and the projection rib 21 fitting
in the recess by itself functions as bond. Preferably the projecting rib 21 does not
project out of the sheath 9.
[0021] The optical fiber cable 1 may include a rip cord to facilitate splitting the sheath 9.
[0022] As already discussed, the plane on which the strength members 17 are aligned,
shown as the Y-axis in FIG. 1, functions as a neutral surface in a meaning of
mechanics when the optical fiber cable 1 is curved in any directions perpendicular to
the plane (namely, in the direction of the X-axis). Moreover the optical fiber cable 1
can substantially freely rotate or twist with very small stress, because the cable 1 is in
general subject to very small restriction. Thus, even if one would curve the optical
fiber cable 1 in a direction deviated from the X-axis, the optical fiber cable 1 would be
slightly reoriented to have itself curved in the X-axis and then the plane including the
central axis C still functions as a neutral surface. Further, as the optical fibers 3 are
disposed around the central axis C (included in the neutral surface), the optical fibers 3
are substantially neither compressed nor extended. Therefore transmission loss caused
by compression or tensile stress can be suppressed in a very low level. It is ad-
vantageous in view of suppression of transmission loss particularly when some cir-
cumstances force a laid optical fiber cable to curve or meander.
[0023] As the sheath 9 is free from a strength member which may be obstructive to splitting
or cutting out the sheath 9, the optical fiber cable 1 provides facility for the mid-span
access work.
[0024] As the sheath 9 has a nonuniform wall in which the thickest wall portion 13 covers
the slot 11, mechanical strength in this part is reinforced. This is advantageous in view
of prevention of damage to the enclosed optical fibers 3 when external force is applied
to the sheath 9, in particular over the slot 11. This effect becomes remarkable when the
thickness of the thickest wall portion 13 is 1.5 times or more of the thickness of the
thinnest wall portion 15.
[0025] Without the bonding portion 23, the slotted core 7 is likely to move in its longitudinal
direction because temperature change after laying the optical fiber cable 1 may cause
thermal expansion or contraction. Further, some manners of handling of the optical
fiber cable 1 may cause rotational displacement of the slotted core 7 relative to the
sheath 9. As the sheath 9 and the slotted core 7 are bonded together at the bonding
portion 23, the slotted core 7 is prevented from displacement relative to the sheath 9 in
both the longitudinal and rotational directions. The bond at the bonding portion 23 ef-
fectively prevents projection, retraction and rotational displacement of the slotted core
7. As the bond at the bonding portion 23 prevents such displacement, the optical fiber
cable 1 provides prominent facility for handling.
[0026] The bond between the slotted core 7 and the sheath 9 is limited in the bonding
portion 23. This fact provides facility for the mid-span access work because peeling of
the sheath 9 is easily carried out as compared with a case where the core and the sheath
are entirely bonded together. In particular, while a cutter is put into the sheath at the
beginning of the mid-span access work, the cutter may cut out the projecting rib 21 and
therefore simultaneously break the bond between the slotted core 7 and the sheath 9 at
the bonding portion 23. Thus workability about the mid-span access work is
prominently improved.
[0027] Referring to FIGS. 2(A) through 2(E), a process of mid-span access wiU be described
hereinafter. First, a sharp edge of a cutter 25 is put into the sheath 9 and made advance
around the circumference thereof to carry out cutting around of the sheath 9. Further
the cutter 25 is made advance along the bonding portion 23 to carry out splitting of the
sheath 9 and subsequently cutting around is carried out again at another part as shown
in FIG. 2(B). As the bonding portion 23 is formed at the projecting rib 21, the bond
between the slotted core 7 and the sheath 9 is easily broken in the course of the afore-
mentioned step. Further, while movement of the cutter 25 along the longitudinal
direction gives rise to damage to the enclosed optical fibers 3 as the slot 11 runs in this
direction, such movement is made at the bonding portion 23 opposite to the slot 11 and
therefore the optical fibers 3 are not subject to damage.
[0028] Then a part of the sheath 9 is separated from the remainder of the sheath 9 and
further a longitudinal slit 27 is formed in this part as shown in FIG. 2(C). This part
becomes removable from the slotted core 7 and the optical fibers 3.
[0029] After removal of the part of the sheath 9, the slot 11 is exposed as shown in FIG.
2(D) and therefore the optical fibers 3 housed in the groove 5 become accessible
through the slot 11. One or more of the optical fibere 3 are pulled out of the slotted
core 7 as shown in FIG. 2(E) and then subject to a branching process.
[0030] Table 1 demonstrates test results of some examples in regard to a drawing test,
projection length of the slotted core at the end of the sheath, workability about the mid-
span access work, and transmission loss. The drawing test had been carried out in a
manner shown in FIG. 3, in which a slotted core 7 of a test piece 29 is drawn from a
sheath 9 having a length of 400mm in a speed of 100mm/min in a direction indicated
by an arrow therein and a maximum value of force of drawing is measured.
[0031] Meanwhile, the force of drawing is preferably 98N or more in view of prevention of
displacement of the slotted core relative to the sheath.
[0032] The working example 1 is produced in accordance with the present embodiment.
Comparative examples 1-5 are different from the present embodiment in structural pa-
rameters as summarized in this table.
[0033] As being understood from Table 1, the working example 1 in accordance with the
present embodiment has satisfactory properties in that the force of drawing is 98N or
more, the projection length is 1mm or less, and the transmission loss is only
0.21dB/km while workability about the mid-span access work is excellent.
[0034] The comparative example 1 is different from the working example 1 in that the
slotted core 7 and the sheath 9 are totally bonded together. Workability about the mid-
span access work is inferior to that of the working example 1 because it is considerably
laborious to peel off the sheath 9 totally bonded with the slotted core 7.
[0035] The comparative example 2 is different from the working example 1 in that no bond
is formed between the slotted core and the sheath. This structure results in relatively
small force of 10N or less required to draw the slotted core out of the sheath and a
relatively large projection length of 55mm of the slotted core out of the sheath. This
means that the slotted core is susceptible to displacement relative to the sheath.
[0036] The comparative example 3 is different from the working example 1 in that fixation
of the slotted core with the sheath depends only on pressure of the sheath onto the
slotted core. This structure results in a relatively large projection length of 5rmn of the
slotted core out of the sheath. Further, workability about the mid-span access work is
inferior to that of the working example 1. Transmission loss increases up to 0.45dB/km
which is considerably larger than 0.21dB/km of the working example 1.
[0037] The comparative example 4 is different from the working example 1 in that no bond
is formed between the slotted core and the sheath and a wrapping made of a tape is
wound around the slotted core in a spiral shape. As the wrapping serves for friction
against displacement of the slotted core, force of drawing is relatively high, 85N.
However, projection length of the slotted core out of the sheath reaches about 5mm.
Further, workability about the mid-span access work is inferior to that of the working
example 1 as extra work to remove the wrapping is required. Transmission is relatively
low, 0.23dB/km, although this value is slightly larger than that of the working example
1.
[0038] The comparative example 4 is different from the working example 1 in that no bond
is formed between the slotted core and the sheath and further a wrapping made of a
yam is wound around the slotted core along with the elongate tape along the slot.
While the wrapping serves for friction against displacement of the slotted core, force of
drawing the slotted core is only 20N and projection length of the slotted core out of the
sheath reaches 36mm. Further, workability about the mid-span access work is inferior
to that of the working example 1 as extra work to remove the wrapping is required.
Transmission loss is fairly low, 0.21dB/km.
[0039] As being understood from the aforementioned comparisons, the working example 1
in accordance with the present embodiment provides beneficial results as compared
with the comparative examples, such as prevention of displacement of the slotted core
relative to the sheath, low transmission loss, and excellent workability about the mid-
span access work.
[0040] The aforementioned embodiment will be modified in various ways. Some of such
modifications will be exemplarily described hereinafter. In the following descriptions,
differences compared with the aforementioned embodiment will be mainly described
and descriptions about elements substantially identical to those of the aforementioned
embodiment will be omitted or simplified.
[0041] Referring to FIG. 4 which illustrates a second embodiment, the slotted core 7 is in
part given roughness in advance of bonding and the rough surface of the slotted core 7
is subject to thermal fusion bonding to form a bonding portion 23 with the sheath 9.
The bonding portion 23 is composed of a thermal fusion bonding portion 31 produced
by the thermal fusion bonding, where the slotted core 7 and the sheath 9 are fused
together and thereby locally form a unitary body.
[0042] Alternatively, in a third embodiment, the slotted core 7 is in part heated up to a tem-
perature sufficiently close to, or higher than, that of the sheath 9 in advance of bonding
so as to cause softening of the slotted core 7, and then thermal fusion bonding is
carried out.
[0043] Refening to FIG. 5 which illustrates a fourth embodiment, instead of thermal fusion
bonding, a bonding material 33 such as adhesive may be used to form bond between
the slotted core 7 and the sheath 9. Therefore the bonding portion 23 is composed of
the bonding material 33.
[0044] Referring to FIG. 6 which illustrates a fifth embodiment, instead of a projecting rib, a
recess 35 receding into the slotted core 7 is formed in advance of bonding and the
sheath 9 has a complementary projection. After fitting the projection in the recess 35,
thermal fusion bonding is carried out to form a bonding portion 23 therebetween. As
with the rib of the first embodiment, the recess 35 may be either a continuous line of a
concavity or a row of separate concavities, which longitudinally ranges over the slotted
core 7.
[0045] Referring to FIG. 7 which illustrates a sixth embodiment, a rip cord 37 having a
bonding material such as adhesive is interposed between the slotted core 7 and the
sheath 9. The rip cord 37 is preferably disposed on or close to the plane on which the
strength members 17 are aligned. By means of the bonding material of the rip cord 37
instead of thermal fusion bonding, the slotted core 7 is bonded with the sheath 9. When
the rip cord 37 is drawn, it helps to split the sheath as an ordinary rip cord does, and
further simultaneously breaks bond between the slotted core 7 and the sheath 9. As in
this way removal of the sheath 9 is further facilitated, one can further easily execute
mid-span access work as compared with the first embodiment.
[0046] Referring to FIG. 8 which illustrates a seventh embodiment, one or more absorptive
yams 39 may be housed in the groove 5 of the slotted core 7. The absorptive yams 39
improve quality of being waterproof of the optical fiber cable 1.
[0047] Alternatively, an absorptive tape 41 having capacity to absorb water may be applied
instead of, or along with, the elongate tape 19 of the first embodiment. The absorptive
tape 41 also improves quality of being waterproof
[0048] Further alternatively, both the absorptive yams 39 and the absorptive tape 41 may be
used. This combination of the absorptive yams 39 and the absorptive tape 41 further
improves quality of being waterproof
[0049] Referring to FIGS. 9 and 10 which illustrate a tenth embodiment, anchors 43 are
provided in the groove 5 of the slotted core 7. The anchors 43 support one or more of
the optical fibers 3 in place. Preferably, the anchors 43 are disposed at intervals in a
direction along the central axis C. This structure prevents undesuable force acting on
the optical fibers 3 even when the optical fiber cable 1 is deformed. Moreover, the
anchore 43 are preferably made of any soft viscous material. Preferably the material is
a UV-setting resin having a Young's modulus of 800MPa or less and a viscosity of
500cps or more at the normal temperamre, whereby preventing undesirable force
acting on the optical fibers 3, which may increase transmission loss. Preferably, the
intervals of the respective anchors 43 are in a range from 100mm to 2000mm, whereby
preventing undesirable force acting on the optical fibers 3. Preferably, the support of
the optical fibers 3 by the anchors 43 is regulated so that a force required to draw out
the supported optical fibers are 5N/10m or more.
[0050] Installation of the anchors 43 is executed in, but not limited to, the following way.
The tape 19 is uncoiled and then made to run. An uncured UV-setting resin is inter-
mittently injected onto the running tape 19 substantially at the center thereof Then the
tape 19 along with the uncured UV-setting resin is exposed to UV light so as to cure
the UV-setting resin and is subsequently turned around upside down. Thereby the
anchors 43 made of the UV-setting resin are disposed at intervals on the lower face of
the tape 19. On the other hand, the optical fibers 3 are put in the groove 5 of the slotted
core 7 and the slot 11 is made oriented upward. The tape 19 along with the anchors 43
is attached on the slotted core 7 so as to cover the slot 11, whereby the anchors 43 are
inserted in the groove 5 to support the optical fibers 3. An extruder may be used to
enclose the slotted core 7 with the sheath 9.
[0051] Table 2 demonstrates test results of some examples in regard to transmission loss, a
drawing test and workability about the mid-span access. The examples 1-8 are in
general manufactured in accordance with the aforementioned tenth embodiment and
vary in kinds of resin, Young's moduli thereof, and viscosities thereof, as summarized
in Table 2.
[0052] Provided that a target level of performance is set such that a transmission loss is
0.25dB/km or less, a force required to draw out the slotted core from the sheath is
greater than 5, and workability about the mid-span access work is beyond that of
existing cables, what meet the target level among the examples are the example 3, 4
and 6, which are commonly comprised of anchors made of the UV-setting resin. Both
the example 7 having anchors made of hot-melt adhesive and the example 8 in which
yams filled in the groove fix the optical fibers do not meet the target level.
[0053] In more detail, the examples 2, 5 and 8 do not have sufficiently low transmission loss
which meets the target level as the Young's moduli of the anchore of these examples
reach 1000MPa, In contrast, the examples 1, 3, 4 and 6 meet the target transmission
loss, in which the Young's moduli of the anchors are 800MPa or less. More
specifically, anchor's Young's moduli of 800MPa or less provide beneficial results in
view of suppression of transmission loss.
[0054] Further, the examples 1 and 2 in which the viscosities of the anchors are 300cps do
not meet the target force of drawing, whereas the examples 3, 4, 5 and 6 in which the
viscosities are 500cps or more meet the target force of drawing. More specifically,
anchor's viscosities of 500cps or more provide beneficial results in view of prevention
of displacement of the slotted core.
[0055] Further modification of the above embodiments with occur. Referring to FIG. 11
which illustrate an eleventh embodiment, widths of the slot 11 in a proper range also
beneficial results. A plane emanating from the central axis C in contact with an edge in
the right of the slot 11 is shown as a line L in Fig. 11 and another plane emanating
from the central axis C in contact with another edge in the left of the slot 11 is shown
as a line L'. These planes make an angle "theta" as shown in FIG. 11. When the angle
theta is larger than 30 degrees, workability about the mid-span access work becomes
easy. Further, when the angle theta is less than 90 degrees, the sheath 9 is prevented
from falling into the groove 5 and therefore does not have undesirable influence on
transmission loss. More specifically, the angles theta in a range from 30 degrees to 90
degrees provide beneficial results.
[0056] Further, widths of the tape 19 in a proper range also beneficial results. A plane
emanating from the central axis C in contact with an edge in the right of the tape 19 is
shown as a line T in FIG. 11 and another plane emanating from the central axis C in
contact with another edge in the left of the tape 19 is shown as a line T. These planes
make an angle "gamma" as shown in FIG. 11. Beneficial results provided by angles
gamma larger than the angle theta would be needless to say. When the angle gamma is
less than four times the angle theta, the slotted core 7 is securely fixed with the sheath
9 as the slotted core 7 and the sheath 9 ensure sufficient contact area. More
specifically, the angles gamma in a range from the angle gamma to four times the
angle gamma provide beneficial results.
[0057] Referring to FIGs. 12(A)-12(C) which illustrate a twelfth embodiment, the optical
fiber cable 1 may be further comprised of a marker for indicating a position of the
bonding portion 23. The maker may be a projection 45 projecting from the sheath 9,
which is just aligned to the bonding portion 23 as shown in FIG. 12(A). Alternatively,
the marker may be a colored bar 47 on the sheath 9 as shown in FIG. 12(B). Further al-
ternatively, the marker may be a concave portion 49 as shown in FIG. 12(C). Existence
of the marker helps one who would cany out the mid-span access work to fmd out
where to cut.
[0058] Referring to FIG. 13 which illustrates a thirteenth embodiment, a pair of rectangular-
prism-shape strength members 20 are embedded in the slotted core 7, instead of the
columnar strength members 17 of the first embodiment.
[0059] Referring to FIG. 14 which illustrates a fourteenth embodiment, the optical fiber
cable 1 is comprised of two notches 51 formed on an outer surface of the sheath 9,
which run along tlie central axis C. The notches 51 facilitate splitting of the sheath 9 at
the time of the mid-span access process. The number of the notches 51 is not limited to
two but may be three or more. Further, the sections of the notches 51 are not limited to
a V-letter shape but may be any proper shape. The other aspects of the current em-
bodiment may be identical to those of the first embodiment or the other.
[0060] The notches 51 (or, outermost two of the notches 51 in a case where three or more
notches are provided) may be disposed symmetrically with respect to the thinnest wall
portion 15 and leave a properly narrow interval therebetwen. Thus positioned notches
51 are located at relatively thin portions of the sheath 9 and relatively close to each
other, thereby sufficiently facilitating splitting of the sheath 9. A part of the sheath 9
separated from the remaining sheath 9 may be beneficially reused for the purpose of
covering the exposed slot 11 as will be described later.
[0061] The breadth between the notches 51 affects easiness of re-enclosure of the optical
fibers and easiness of ripping of the sheath. The test results summarized in Table 3
demonstrate this feature. In these tests, test pieces 1-6 respectively having various
breadth were tested. In Table 3, breadths are indicated by angles alpha, where an angle
alpha is defined as an angle NCN' formed by planes CN, CN' emanating from the
central axis C and respectively passing the notches 51.
[0062] As will be understood from Table 3, two or more notches on a sheath facilitate
splitting of the sheath, much more do those having alpha-angles greater than 30
degrees (test pieces 3-6). Meanwhile, the test pieces 3-5 having alpha-angles in a range
in a range from 30 degrees to 160 degrees are more advantageous in light of easiness
of re-enclosure of optical fibers than the test pieces 1, 2 and 6 having alpha-angles out
of the range.
[0063] The depth t of the notches 51 may be set in a particular range in relation to the
thickness D of the sheath 9 at the part of the notches 51. Relatively small depths t in a
range satisfying an inequality of t < D/5 are disadvantageous in light of easiness of
splitting, and too large depths t in a range satisfying an inequality of t > 4D/5 are disad-
vantageous in light of toughness of the sheath 9. Therefore the depth t in a range from
D/5 to 4D/5 may be preferable.
[0064] Referring to FIGs. 15 and 16, a process of mid-span access will be described in
relation to the optical fiber cable according to the fourth embodiment.
[0065] Referring to FIG. 15(A), a sharp edge of a cutter 25 is put into the sheath 9 and made
advance around the circumference thereof to carry out cutting around of the sheath 9.
Movement of a sharp end of the cutter 25 may be on a track illustrated as a round
broken line in FIG. 16.
[0066] Next, as shown in FIG. 15(B) and FIG. 17, the sheath 9 is made split with the help of
the notches 51. Then the sheath 9 is partly separated into a larger part and a smaller
part, thereby the slotted core 7 is partly exposed in between these parts.
[0067] Referring to FIG. 15C, the smaller part of the sheath 9 is allowed to be cut out but the
larger part is preferably left. One or more of the optical fibers 3 are pulled out of the
slotted core 7 and then subject to a branching process. The rest of the optical fibers 3
are returned to and re-enclosed with the slotted core 7.
[0068] Then the uncut part of the sheath 9 can be reused for the purpose of covering the
exposed slot 11 as shown in FIG. 15D. While the optical fiber(s) 3 branching off the
optical fiber cable 1 is led out of the sheath 9, the remaining optical fibers 3 are re-
enclosed with the sheath 9. This process does not require any specific jigs and can be
executed by hands and fingers.
[0069] The fourteenth embodiment provides such reusability of the split sheath 9 as a cover
of the exposed optical fibers 3, while such reuse may be made possible even in the
other embodiments when some additional labors are added. In contrast, in the prior
arts, an exposed core requires any tape wrapped around the core or such protection
means so as to avoid damage to the optical fibers. Wrapping a tape on the core is con-
siderably laborious. Therefore, the fourteenth embodiment provides both excellent
protection and easy workability.
[0070] The aforementioned first through fourteenth embodiments are compatible with each
other. Therefore, any combination of these embodiments will occur. Further, additional
rip cords may be interposed between the slotted core 7 and the sheath 9.
[0071] Although the invention has been described above by reference to certain exemplary
embodiments of the invention, the invention is not limited to the exemplary em-
bodiments described above. Modifications and variations of the embodiments
described above will occur to those skilled in the art, in light of the above teachings.
Industrial Applicability
[0072] Optical fiber cables enclosing fibers, in which enclosed fibers are easily accessible
but prevented from damage, are provided.
Claims
[1] An optical fiber cable having an axis, the optical fiber comprising:
a slotted core elongated along the axis, the slotted core including a slot running
in parallel with the axis and a groove accessible through the slot;
one or more optical fibers placed in the groove;
a sheath enclosing the slotted core and the optical fibers;
a bonding portion where the slotted core is bonded with the sheath; and
two or more strength members embedded in the slotted core, the strength
member running in parallel with the axis and being aligned on a plane including
the axis.
[2] The optical fiber cable of claim 1, wherein the strength members include one
selected from the group of steel and FRP.
[3] The optical fiber cable of claim 1, wherein the bonding portion includes one
selected from the group consisting of a projecting rib projecting from the slotted
core, a binding element interposed between the slotted core and the sheath, a
recess receding in the slotted core, and a string having adhesive interposed
between the slotted core and the sheath.
[4] The optical fiber cable of claim 1, further comprising:
an absorptive yam placed in the groove.
[5] The optical fiber cable of claim 1, further comprising:
an elongate tape attached on the slotted core to cover the slot.
[6] The optical fiber cable of claim 5, wherein the elongate tape includes an ab-
sorptive tape.
[7] The optical fiber cable of claim 5, wherein the bonding portion is left uncovered
by the elongate tape and aligned with the slot and the strength members on the
plane.
[8] The optical fiber cable of claim 5, wherein the slot and the elongate tape are so
dimensioned that a theta-angle formed by planes emanating from the axis and re-
spectively in contact with edges of the slot of the slotted core is in a range from
30 degrees to 90 degrees, and a gamma-angle formed by another planes
emanating from the axis and respectively in contact with both edges of the
elongate tape is greater than the theta-angle and smaller than four times the theta-
angle.
[9] The optical fiber cable of claim 1, further comprising:
one or more anchors configured to support one or more of the optical fibers, the
anchors being disposed at intervals in a direction along the axis.
[10] The optical fiber cable of claim 9, wherein each of the anchors includes a UV-
setting resin having a Young's modulus of 800MPa or less and a viscosity of
500cps or more at a normal temperature, each of the intervals between the
anchors is in a range from 100mm to 2000mm, and a force required to draw out
the supported optical fiber is 5N/10m or more.
[11] The optical fiber cable of claim 1, wherein the sheath includes a nonuniform wall
so that a largest thickness of the wall is 1.5 times or more of a smallest thickness
of the wall.
[12] The optical fiber cable of claim 1, further comprising:
a marker formed on the sheath, the marker indicating a position of the bonding
portion.
[13] The optical fiber cable of claim 1, further comprising:
two or more notches formed on an outer surface of the sheath and running along
the axis.
[14] The optical fiber cable of claim 13, wherein the notched are disposed so as to
have an alpha-angle formed by planes emanating from the axis and respectively
passing two of the notches in a range from 30 degrees to 160 degrees.
[15] A method of mid-span access of the optical fiber cable of claim 1, the method
comprising:
splitting the sheath in part to expose the slot in part;
pulling one or more of the optical fibers out of the slotted core; and
covering the exposed slot in part with a split part of the sheath.

An optical fiber cable is comprised of: a
slotted core (7) elongated along an axis of the optical fiber
cable, the slotted core including a slot (11) running in parallel
with the axis and a groove (5) accessible through the
slot; one or more optical fibers (3) placed in the groove; a
sheath (9) enclosing the slotted core and the optical fibers;
a bonding portion (15) where the slotted core is bonded
with the sheath; and two or more strength members (17)
embedded in the slotted core, the strength member running
in parallel with the axis, and being aligned on a plane
including the axis.