Multicore optical fiber

A multicore optical fiber comprises a plurality of optical fiber elements disposed in parallel, each optical fiber element being equipped with a covering layer, a common covering layer integrally covering the plurality of optical fiber elements, and a peel layer provided on the outermost layer of each optical fiber element.

BACKGROUND OF THE INVENTION 
The present invention relates to a novel multicore optical fiber suitable 
for high density optical communications and more particularly to a 
multicore optical fiber such as a tape-shaped optical coated fiber 
equipped with a covering layer which is capable of effectively protecting 
each optical fiber element and is easy to handle when such optical fibers 
are coupled together. 
With the recent advances in data communications, there has been a 
requirement for signal transmission at higher speed. As an example of 
materialization of such high speed signal transmission technology, the 
recent practical use of optical communications can be cited. The optical 
signal transmission has many advantages from the standpoint of 
communications in that the property of light itself is utilizable for 
realizing high speed transmission and that optical fibers used as 
transmission lines are lightweight and little affected by magnetic or 
electric fields. However, an optical fiber is still required which will 
increase the capacity of signal transmission and, there have been various 
kinds of optical fibers for signal transmission proposed. 
A multicore optical fiber is one of the proposed optical fibers for such 
circumstances. The multicore optical fiber, which is formed by integrating 
fiber elements as optical waveguides with a covering layer common to them, 
is now spotlighted as being of realizing high density signal transmission 
with simple care in handling. 
FIG. 1 is a sectional view showing the typical structure of a multicore 
optical fiber of the sort aforementioned, wherein glass fibers 1 for 
optical transmission respectively covered with covering layers 2 are 
further equipped with a common covering layer 3 to form a tape-shaped 
optical coated fiber. 
In the prior art, the covering layers 2 and 3 are formed of, for example, 
ultravoilet-curing urethane acrylate. 
When the multicore optical fiber is coupled to another member, e.g., an 
ordinary single core optical fiber or another multicore optical fiber, 
however, the form of a tape is difficult to handle and further the problem 
of transmission loss resulting from the connection of both also arises. In 
consequence, the common covering layer 3 must be removed to handle each 
element. 
On the other hand, the glass fibers without the covering layer suffer from 
insufficient strength and may break. Thus, when each element is a handled, 
it is prerequisite to remove the common covering layer 3 of the multicore 
optical fiber in such a manner that the covering layer 2 of each element 
perfectly remains as it is and is also undamaged. 
In the case of the conventional multicore optical fiber, however, the 
covering layers 2 and the common covering layer 3 are made of the same 
material. Even if they are not bonded chemically but bonded merely under 
pressure, it is not possible to remove only the common covering layer 3 
while the covering layer of each optical fiber is left unremoved 
completely. 
High degree of skill and many workhours have been required to couple 
multicore optical fibers. 
Another known arrangement is to form a covering layer 2 of thermosetting 
silicone resin on each optical fiber 1 and then form a covering layer 3 of 
nylon common to the optical fibers, in order to provide a multicore 
optical fiber. The operation of removing the common covering layer 3 from 
the multicore optical fiber of that type is considerably easy because the 
covering layer 3 is relatively readily peeled off the covering layers 2. 
However, because the mechanical strength of the thermosetting silicon resin 
used for the covering layer 2 of the optical fiber thus constructed is 
extremely low, the covering layer of each optical fiber element after the 
removal of the common covering layer can not withstand rubbing or 
scratching and are thus unsuitable for practical use. 
The normal way of removing the common covering layer 3 of the conventional 
multicore optical fiber is, for instance, to vertically tear the common 
covering layer 3 into two pieces from both left- and right-hand ends A, A' 
with respect to the section shown in FIG. 1. However, the thickness of the 
common covering layer 3, excluding its portions respectively penetrating 
into the gaps between the optical fiber elements 1 and the covering layers 
2, is practically uniform and therefore the common covering layer 3 will 
not readily be detached by simple pulling the halves thereof in certain 
directions. Even if it is attempted to tear the common covering layer into 
two pieces from cut points made in the edges thereof with a knife and the 
like, the common covering layer is not always torn in the longitudinal 
direction of the optical fiber. The removal of the common covering layer 
of the multicore optical fiber is indeed troublesome work. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a multicore optical fiber, 
e.g., a tape-shaped optical coated fiber equipped with a common covering 
layer readily removable and covering layers capable of respectively 
effectively protecting optical fiber elements contained therein. 
More specifically, the multicore optical fiber according to the present 
invention comprises a plurality of parallel optical fiber elements 
respectively equipped with covering layers, a common covering layer used 
for integrally covering the plurality of optical fiber elements, and a 
thin peel layer as the outermost layer of each optical fiber element for 
preventing both covering layers from adhering or pressure welding to each 
other. 
Moreover, the multicore optical fiber according to the present invention is 
arranged so that the substantial thickness of the common covering layer is 
made small locally over the whole length of the optical fiber at symmetric 
positions with respect to the cross section thereof. 
The novel multicore optical fiber according to the present invention has a 
peel layer provided as the outermost layer of each optical fiber 
constituting the multicore optical fiber, so that the common covering 
layer can readily be peeled from the covering layer of each optical fiber. 
Accordingly, the common covering layer can readily be removed from the 
covering layer of each optical fiber without damaging the latter. 
Moreover, the peel layer sandwiched between each optical fiber and the 
common covering layer functions so as to prevent both from press welding 
to each other and each optical fiber can sufficiently maintain its 
property by the covering layer of each fiber element. 
Furthermore, the multicore optical fiber prepared according to the present 
invention is provided with thin portions locally in the common layer. 
Accordingly, when the common covering layer is removed from the multicore 
optical fiber, if tensile force is applied in the direction perpendicular 
to a line connecting the thinned portions of the common covering layer, 
each optical fiber element is easily exposed since the common covering 
layer naturally tears from the thinned portions. 
In view of the function of the common covering layer caused to be torn in 
its thinned portions first, the thinned portions may be located anywhere 
to attain the intended purpose as long as the common covering layer is 
halved with respect to the section. However, since the actual optical 
fiber is relatively as thin as several millimeters in the maximum 
dimension and, for better workability during the operation, the common 
covering layer should be preferably tore at both ends of the plane on 
which the optical fiber elements are disposed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Resin material for use in forming a covering layer on each single core 
optical fiber should be strong enough to protect the optical fiber and 
various materials conventionally used to form the protective layer of the 
optical fiber may be employed. Typical materials include thermoplastic 
resin such as nylon and ultraviolet-curing resin such as urethane 
acrylate. 
As materials for use in forming a peel layer, use can be made of various 
resins having thermosetting or photo-setting properties such as 
ultraviolet-curing properties and easy moldability, and further properties 
which prevent its adherence and press-welding to a common covering layer 
or the covering layer of each optical fiber. 
The material resins used as the peel layer include peeling agents prepared 
from ultraviolet-curing or thermosetting silicon resin, or 
ultraviolet-curing or thermosetting fluorocarbon resin. The silicon or 
fluorocarbon resin for use in the embodiment discussed herein is an 
organic compound containing silicon (Si) or fluorine (F) atoms in each 
molecule and can be hardened when exposed to heat or light, the hardened 
resin has excellent peel properties. 
The peel layer is generally less than 20 .mu.m and preferably less than 10 
.mu.m thick. The reason for this is that, because the peel layer does not 
protect protective function to the fiber element, it should preferably be 
as thin as possible. Also, if the peel layer is excessively thick, it will 
exert an unfavorable influence on the transmission characteristics of the 
fiber element and moreover may be damaged or peeled off as a result of 
friction or the like. The peel layer however should preferably be thick 
enough to separate the common covering layer from the covering layer of 
each optical fiber element. 
With respect to the bond strength of the peel layer relative to an 
adjoining layer, i.e., the covering layer of each fiber element or common 
layer, it does not cause inconvenience whether one is greater than or 
equal to the other but the bond strength thereof relative to the former 
should preferably be, if anything, greater. The reason for this is 
attributed to the fact that, because the peel layer has to be destroyed 
for the removal of the common covering layer in case the peel layer clings 
to the common layer, undesirable stress acts on the element then. Also, 
part of the peel layer thus damaged is allowed to remain on the element 
side, which will necessitate the additional operation of removing the 
remnants of the peel layer from the element deprived of the common 
covering layer. 
It should further be understood that the formation of multi-covering layers 
or peel layers are within the technical scope of the present invention. 
Referring now to the accompanying drawings, a concrete description will be 
given to a multicore optical fiber embodying the present invention, which 
should not be construed as limiting the scope of the present invention. 
FIG. 2 is a sectional view showing the construction of a multicore optical 
fiber according to the first embodiment of the present invention. 
As shown in FIG. 2, a plurality of signal core optical fibers 10 are 
disposed in parallel on the same plane, each optical fiber being equipped 
with a covering layer 20, and incorporated in a covering layer 30 common 
to them in order to form a multicore optical fiber. An elliptic common 
covering layer may be used as is shown in FIG. 2. Each optical fiber 10 is 
equipped with a peel layer 40 on the outer periphery of the covering layer 
20. The optical fiber elements are arranged so that the adjacent peel 
layers 40 make contact with each other. 
The optical fiber 10 is a glass fiber with 125 .mu.m diameter for optical 
transmission and the covering layer 20 is made of ultraviolet-curing 
urethane acrylate, so that a signal core optical fiber with 245 .mu.m 
diameter is formed. 
In one embodiment of optical fiber according to the present invention, the 
optical fiber 10 equipped with the covering layer 20 was further passed 
through a coating die filled with ultraviolet-curing peel layer material 
to form the peel layer 40. The die used had a hole diameter of 260 .mu.m, 
whereas ultraviolet-curing silicone acrylate was used as the peel layer 
material. As for the ultraviolet curing, a 120 W/cm high pressure mercury 
vapor lamp was used to apply ultraviolet ray irradiation for about five 
second and an optical fiber element with 251 .mu.m diameter equipped with 
the peel layer 40 was obtained. 
Five optical fiber elements 10 respectively equipped with covering layers 
20 and the peel layers 40 were arranged in parallel and incorporated in 
the common covering layer 30 to form a multicore optical fiber. The common 
covering layer 30 was formed by applying ultraviolet-curing urethane 
acrylate and providing ultraviolet ray irradiation. 
The common covering layer 30 of the multicore optical fiber thus 
constructed could be removed easily by tearing the left- and right-hand 
ends of the section shown in FIG. 2 and the covering layer free from 
damage and having sufficient protective strength was left on each optical 
fiber element deprived of the common covering layer. 
FIG. 3(a) is a sectional view showing the structure of a multicore optical 
fiber according to the second embodiment of the present invention, wherein 
like reference characters designate like parts of FIG. 2. The optical 
fiber in this embodiment was formed of the same material and through the 
same process as in the case of the multicore optical fiber shown in FIG. 
2, except that the upper and lower thickness T.sub.A of the common 
covering 30 was 0.05 mm, whereas the thickness T.sub.B at the left- and 
right-hand ends was 0.03 mm. 
In the multicore optical fiber thus constructed, since the peel layer 40 is 
sandwiched between the protective covering layer 20 of each optical fiber 
element and the common covering layer 30, both were prevented from 
adhering or press-welding to each other and the removal of the common 
covering layer 30 was easy. Moreover, the complete protective layer free 
from defects was left on each optical fiber deprived of the common 
covering layer. 
FIG. 3(b) shows another modification, wherein the sectional shape of the 
common covering layer 30 was made rectangular, and wherein the thickness 
of a portion forming a face perpendicular to the direction in which the 
inner optical fiber elements were disposed was made substantially thinner 
than a portion forming a face parallel to the direction in which the inner 
optical fiber elements were disposed. 
Although FIGS. 3(a) and 3(b) shows the multicore optical fibers with the 
adjoining optical fiber elements in contact with each other, the present 
invention is applicable to any multicore optical fiber having the common 
covering layer. More specifically, if the common covering layer 30 is 
discontinuous at the gap 50 as shown in FIG. 4(a) or even if it is 
continuous between the optical fiber elements but sufficiently thin as 
shown in Fib. 4(b), it can readily be separated and removed. Further, as 
shown in FIG. 4(c), the gap 60 may be formed in portions between the 
optical fiber element 10 and the common covering layer 30. 
As set forth above, the multicore optical fiber according to the present 
invention is supplied with the peel layer for preventing the covering 
layer provided for each optical fibers and the common covering layer for 
integrating the optical fibers to form the multicore optical fiber from 
press-welding or adhering to each other. Accordingly, even though the 
common covering layer is removed for connecting purposes, the complete 
covering layer on each optical fiber is allowed to remain, and the 
connecting operation can be conducted easily for a short time. 
Moreover, the multicore optical fiber according to the present invention is 
equipped with the peel layer and the common covering layer, a part of 
which is made substantially thin. Accordingly, even though the common 
covering layer is removed for connecting purposes, the complete covering 
layer on each optical fiber is allowed to remain, and the connecting 
operation can be conducted easily for a short time. 
Consequently, it becomes possible to enlarge the applicability of the 
multicore optical fiber having numerous merits and such a demerit that a 
high degree of skill is required for handling. That is, the range of 
applications of optical communication technology and consequently the 
practicability of a large volume of data transmission can be further 
advanced according to the present invention.