Optical cable and its manufacturing method

An optical cable has a center member, around which a plurality of coated optical fibers are tightly stranded. Each optical fiber is given uneven strains in its longitudinal direction. Adhesive resin is applied to surround each of the coated optical fibers and binds each of them to the center member to make an integral optical unit when cured.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical cable suitable for a long 
distance communication and a manufacturing method of such an optical 
cable. 
2. Description of the Related Art 
In an optical communication system using an optical cable, an optical 
signal generated from an optical source is transmitted to a distant place 
through optical fibers within the optical cable and is received by an 
optical receiver. To transmit the optical signal for a long distance, the 
optical signal transmitted through the optical fiber must be large in 
power, the optical fiber, which is a transmitting medium, must be low in 
optical loss, and the optical receiver must be high in sensitivity. 
However, in the conventional optical cable, the optical power transmitted 
from the optical source through the optical fiber is reflected within the 
fiber due to stimulated Brillouin scattering (SBS), so that, even if the 
optical power is ever being increased, the optical power which can be 
transmitted through the optical fiber (hereinafter referred to as 
transmitted power) will soon reach at its upper limit. FIG. 1 shows an 
example of the experimental results of the transmitted power within the 
fiber, in which a laser beam having a spectral width (FWHM) of 73 kHz is 
introduced into a single mode optical fiber of a quartz group with a 125 
micron diameter. Even if the optical power of the optical source is ever 
being increased, the transmitted power will be saturated at about 1 mW and 
will not be increased any more. FIG. 1 also shows that the curve of a 
back-scattering power will sharply increase due to the transmitted power 
being saturated. The limitation imposed upon the transmitted power due to 
the stimulated Brillouin scattering will be a great hindrance in 
increasing a transmission distance. 
In the conventional optical cables for a long distance communication, the 
optical fibers are generally designed such that they will surely be 
prevented from being stretched or strained, because they are mechanically 
fragile. What follows is a literature which is written from a viewpoint of 
reducing strain: S. Hatano etc., "Multi-hundred-fiber cable composed of 
optical fiber ribbons inserted tightly into slots," Proceeding of IWCS, 
1986. This literature happens to show an optical cable with a double helix 
structure. The detail of the double helix structure is stated in the 
literature, W. Katsurashima "Characteristics of 1000-Fiber Optical Cable 
Composed of Tape-Slot Type Optical Fiber Units," Technical digest of 
IOOC'89, Paper No. 1983-10. According to this literature, the double helix 
structure in the conventional optical cable is designed such that the 
fibers move freely within the cable so as to relieve a bending strain 
which may occur within each of the fibers when laying the cable. 
In short, the prevention of strain is a generally accepted idea in the 
conventional optical cables, but it is not ever proposed to positively 
provide an optical cable with strain, which is uneven in the longitudinal 
direction of the optical cable, for increasing the Brillouin gain 
bandwidth, a critical input power and the transmission distance. 
In addition, it is also generally known to bind coated fibers together with 
adhesive resin so as to form an optical fiber unit. Such a technique is 
described, for instance, in the following literature: N. Yoshizawa et al., 
"Design and Characteristics of Optical Fiber Unit for Submarine Cable," 
IEEE, JLT, Vol. LT-3, No. 1, 1985. In this literature, the optical fibers 
and the cable core are closely adhered to one another with adhesive resin, 
so that the fibers and the core elongate or contract as an integral 
optical unit and water is prevented from entering into the optical fiber 
unit. The water propagation blocking property of this unit is described in 
the following literature: N. Yoshizawa et al., "Water Propagation Blocking 
Properties of Submarine Optical Fiber Cables," Electronics and 
Communications in Japan, Part 2, Vol. 70, No. 7, 1987. 
In contrast, in the present invention, adhesive resin is used to bind the 
coated optical fibers, each positively given strain, to a center member to 
prevent the strain having been given to each fiber from being averaged in 
the longitudinal direction of each fiber, which technique is not disclosed 
in any of the above references. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an optical cable which 
can greatly increase in transmitted power, the power being restricted in 
strength in the conventional optical cables, without changing a 
transmission system and/or optical fibers which belong to a transmitting 
medium, and to provide a manufacturing method of such an optical cable. 
To achieve the above object, an optical cable in the present invention 
comprises, a center member, coated optical fibers, each being given uneven 
strains in its longitudinal direction, stranded tightly round the center 
member, and adhesive resin surrounding each of the coated optical fibers 
and combining the coated optical fibers with the center member to make an 
integral optical unit when cured. 
The manufacturing method of an optical cable in the present invention 
comprises the steps of, drawing a center member out of a bobbin, drawing 
coated optical fibers out of their bobbins, which are located around a 
drawn out portion of the center member, with giving altering tension to 
each of the drawn out fibers to allow it to have uneven strain in its 
longitudinal direction, stranding the drawn out fibers round the drawn out 
center member, supplying and curing adhesive resin around each of the 
drawn out fibers to combine them with the drawn out center member to make 
an integral optical unit. 
Due to the above structure, it becomes possible in the optical cable and 
its manufacturing method in the present invention to increase an optical 
power which optical fibers can transmit and to increase a transmission 
distance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, preferred embodiments of the present invention will be explained below 
with reference to the attached drawings. 
An optical fiber has a critical input power I which occurs due to 
stimulated Brillouin scattering. The critical input power I may be 
expressed as follows: 
EQU I=A.alpha.V (1) 
in which A stands for the constant of proportion, .alpha. for the optical 
loss of the fiber, and V for the Brillouin gain bandwidth. This expression 
tells us that the input power increases with the increase in V. 
Now, let us suppose that the optical fiber has a fluctuation of strain in 
its longitudinal direction, which may be denoted by .+-..DELTA..epsilon. 
as shown in FIG. 3. Then, the strain has the following relation with V 
when V has the unit of MHz: 
EQU .DELTA..epsilon.=B(v-V.sub.0) (2) 
in which B stands for the constant of proportion, which is about 
1.times.10.sup.-5 in the Ge-doped fiber, and V.sub.0 stands for the 
Brillouin gain bandwidth when no strain exists. 
It is apparent from the above two expressions that the Brillouin gain 
bandwidth may increase by giving to each optical fiber within the cable 
the strain fluctuation .+-..DELTA..epsilon. which fluctuates in the 
longitudinal direction of respective fibers, and that as a result of the 
increase in Brillouin gain bandwidth the critical input power may finally 
increase. 
In the present invention, therefore, each optical fiber within an optical 
cable is positively given strain which alters in the longitudinal 
direction of the optical fiber to solve the above-mentioned task. 
It is a conventional practice when an optical cable is used on land that 
optical fibers are loosely received within the optical cable for the 
purpose of relieving any strains which may occur in the fibers when the 
cable is bent. A strain-holding structure such as shown in the present 
invention opposes to the above purpose, as everyone can soon understand. 
Therefore, the conventional optical cables do not have a strain-holding 
structure and optical fibers received within each conventional optical 
cable are not given any strains. In the cables which are used on sea-bed, 
the center member and optical fibers are tightly glued together to prevent 
water from entering into the cable. The structure itself allows the 
possibility of giving strain to each fiber, but there is no example that 
strain is intentionally given to each fiber as the present invention, or 
that strain which varies in the longitudinal direction of each fiber is 
given. 
In the first embodiment of the present invention, an optical cable is 
formed as follows: Coated optical fibers or assemblies of coated optical 
fibers are drawn out of their bobbins with strain, which alters in time, 
being given and are stranded round a center member which is much larger in 
bending and tensile rigidity than the optical fibers. Alternatively, the 
coated optical fibers or the assemblies of coated optical fibers thus 
drawn out of their bobbins are placed on both the upper and lower surfaces 
of the center member. A steel wire or a so called slotted rod (a 
polyethylene rod having on its surface spiral grooves) may be used a the 
center member. Then adhesive resin is applied to the resultant structure 
and is cured to form an optical unit. Since the center member is much 
larger in bending rigidity than the optical fibers, as stated above, and 
is integrally combined with the optical fibers by the adhesive resin, the 
strain which is given to the optical fibers when producing the optical 
cable remains in the optical fibers as longitudinally varying strain. 
It is needless to say that an advantage of giving longitudinally uneven 
strains may be lost if the binding force of the adhesive resin is very 
weak. Because, in such a case, the optical fibers may longitudinally move 
within the cable and the strains may be leveled. 
In the second embodiment of the present invention, an optical cable has a 
structure that optical units are twisted round a cable core. Each optical 
unit is formed, as in the first embodiment, by twisting coated optical 
fibers or assemblies of coated optical fibers round a center member, or 
placing them on both the upper and lower surfaces of the center member, 
and applying adhesive resin to the resultant structure and curing the 
resin. In this embodiment, however, strain need not be given to the 
optical fibers at an optical unit forming process. Since the optical units 
are twisted round the cable core, the optical fibers twisted round the 
center member to form one optical unit are further twisted round the cable 
core along with the other optical fibers twisted round the other center 
members to form other optical units, which structure is hereinafter 
referred to as a double helix structure. As any optical fiber twisted 
round the center member in any optical unit is further twisted round the 
cable core in the double helix structure, a stretching strain and a 
compressing strain alternately occur within the optical fiber at a certain 
cycle determined by stranding pitches of fibers and units. 
FIG. 2 shows one example of the optical unit manufacturing method in the 
present invention. A stranding mechanism 8 draws coated optical fibers 3's 
out of their respective bobbins 2's, and strands the drawn out optical 
fibers 2's round a center member 1 which is drawn out of a separate 
bobbin. A resin applicator S applies adhesive resin 4 to the resultant 
structure. Then, a shaping die 5 shapes the structure and a curing device 
6 immediately cures the resin 4 to form an optical unit 9. The optical 
unit 9 thus formed is coiled round a reel 7. 
Each bobbin 2 has as shown in FIG. 4A a shaft 10 and a brake mechanism 11. 
The brake mechanism 11 controls the rotating movement of the shaft 10 to 
periodically give a braking force to the bobbin 2. The braking force acts 
on the coated optical fiber 3 as a back tension 12, because the coated 
optical fiber 3 is continuously fed from the bobbin 2. As a result, the 
coated optical fiber 3 fed from the bobbin 2 will be periodically 
stretched to have a periodic strain, which is kept remaining in the fiber 
3 by binding it to the center member 1 with using the adhesive resin 4. It 
should be noted therefore that the curing time of the adhesive resin 4 
must be shorter than the fluctuation cycle of the fiber stretching tensile 
strain is caused by the back tension (see FIG. 4B). An ultraviolet curing 
resin of a urethane group, that of an epoxy group, and that of a silicone 
group will satisfy this requirement so that one of them may be used as the 
adhesive resin 4 and an ultraviolet lamp is used as the curing device 6 as 
shown in FIG. 2. A heat curing resin or a thermoplastic resin may be used 
if they satisfy the above requirement. Note that it is possible to provide 
a further resin applicator and a further curing device along the center 
member 1 in addition to the resin applicator S and the curing device 6 
shown in FIG. 2. In such a case, six through twelve coated optical fibers 
3's twisted round the center member 1 may be embedded within a soft resin 
whose Young's modulus is lower than 1 Kg/mm.sup.2 and the resultant 
structure may be coated with a hard resin whose Young's modulus is 30-70 
Kg/mm.sup.2 to form an optical unit having a diameter of about 3 mm. The 
center member 1 must be much larger in bending rigidity than the optical 
fibers 3's. A steel wire, which has a diameter of 0.2-1 mm and is coated 
with resin, or a polyethylene rod, which has a diameter of 5-10 mm and is 
provided with spiral grooves which longitudinally extend on its surface 
may be used as the center member. A coated optical fiber, which has a 
coating diameter of 0.2-1 mm and is surrounded by two layers of 
ultraviolet curing resin, or a ribbon shaped fibers, in which 2-12 single 
optical fibers are arranged side by side and are integrally bound to one 
another with resin, may be used as the coated optical fiber 3. 
FIG. 5 shows another example of the optical unit manufacturing method of 
the present invention. In this example, what is rotated for stranding the 
fibers round the center member is not the stranding mechanism 8 but the 
winding mechanism 7. This example is particularly suitable for receiving a 
multitude of ribbon shaped fibers 3's within the slotted rod. Each bobbin 
2 is periodically given a back tension as in the first example. 
In the optical cables in the present invention, a large tensile strain of 
about 1-2 % will act on each optical fiber. Therefore, if the conventional 
optical fibers are used, they may be broken. The carbon-coated optical 
fibers recently developed are improved in fatigue resistance in comparison 
with the conventional optical fibers, and thus may be used in the present 
invention. The withstand characteristics of the carbon-coated optical 
fibers are described in the following paper: K. E. Lu et. al., "Recent 
Developments in Hermetically Coated Optical Fiber," IEEE, JLT Vol. 6, No. 
2, 1988. 
FIG. 6A through FIG. 6D respectively show structural examples of the 
optical unit 9 in the present invention. They are commonly characterized 
by a strain-holding structure that the optical fibers are integrally bound 
to the center member, which is large in bending rigidity, with the use of 
adhesive resin. 
In FIG. 6A, a resin coated steel wire is used as the center member 1. A 
plurality of single optical fibers 3's, each having a coating diameter of 
about 0.2-1 mm, are stranded round the center member 1 and are tightly 
bound to it by an ultraviolet curing resin, which is used as the adhesive 
resin 4, to form a cylindrical optical unit having a diameter of 2-3 mm. 
In FIG. 6B, a slotted rod is used as the center member 1. A plurality of 
single optical fibers 3's are inserted into the corresponding grooves of 
the slotted rod and bound there by the ultraviolet curing resin 4. The 
slotted rod may be made of polyethylene or copper or aluminum. Each 
optical fiber 3 is held where it is by both the binding force of the 
applied adhesive resin and the frictional force between the optical fiber 
itself and the groove where it is inserted, so that the strain exerted in 
the fiber will stably remain in it for a long time. The diameter of the 
slotted rod is 2-3 mm. 
In FIG. 6C, a plurality of ribbons, each made of optical fibers 3's, are 
placed one upon another to form a stratified ribbon structure. A plurality 
of the stratified ribbon structures are then inserted into the 
corresponding grooves of the slotted rod and are bound there by the 
ultraviolet curing resin 4. The ribbon has a width of about 1.1 mm and a 
thickness of about 0.4 mm, if it is made of four optical fibers. 
Therefore, if five ribbons are inserted into each of five grooves to form 
a hundred optical fiber structure, the slotted rod will have a diameter of 
about 10 mm. Note that ribbons 3's and adhesive bodies 13's are 
alternately placed one upon another in each stratified ribbon structure in 
FIG. 6C. The adhesive bodies 13's are a tape having adhesive material on 
its both sides. They are used for bonding the adjacent ribbons together. 
In FIG. 6D, a stainless steel tape or a steel tape is used as the center 
member 1, and a plurality of ribbons, each made of optical fibers 3's, are 
placed on the upper and lower surfaces of the center member 1. 
Each of the above optical units shown in FIG. 6A through FIG. 6D may be 
received in a polyethylene sheath or an LAP sheath to form a cable. When 
each of the above optical units is used on sea-bed, it may be inserted 
into a pressure resistant pipe integrally consisting of the steel wire and 
the welded metal pipe. 
Now, the relationship between the optical cable manufacturing method in the 
present invention and largeness in periodically changing strain which 
remains in one optical fiber in an optical cable will be explained below. 
In each of the manufacturing methods shown in FIG. 2 and FIG. 5, the center 
member 1 drawn out of its bobbin is given back a tension by the braking 
force of its bobbin, and each coated optical fiber 3 drawn out of its 
bobbin is given a back tension by the braking force of its bobbin. Let us 
suppose that .epsilon..sub.T stands for the tensile strain which occurs in 
the center member 1 due to the back tension acting on it, and that 
.epsilon..sub.f stands for the tensile strain which occurs in one optical 
fiber 3 due to the back tension acting on it. The optical unit is 
manufactured under the condition that the center member 1 and each optical 
fiber 3 have their own strain in them. However, once the optical unit has 
been manufactured, the center member 1 is relieved from its back tension 
and the strain .epsilon..sub.T of the center member 1 becomes almost zero. 
What remains is the strain .epsilon..sub.f ' acting on each optical fiber 
in the optical unit. Therefore, the strain .epsilon..sub.f ' which remains 
in one optical fiber will be explained below taking into consideration 
three separate cases as shown in FIG. 7A through FIG. 7C. In the following 
explanation, .epsilon..sub.f is an average of .epsilon..sub.f. 
Case 1: If .epsilon..sub.T &gt;.epsilon..sub.f, then .epsilon..sub.f 
'=.epsilon..sub.f ".epsilon..sub.T &lt;0. Therefore, the compressive strain 
remains in the optical fiber at an average. If the compressive strain 
becomes 0.2 % or more, the optical fiber buckles, and the bending loss of 
the optical fiber increases. Namely, the transmission loss increases. It 
is, therefore, necessary to control the tension when manufacturing so that 
the compressing strain will be lower than 0.2 %. 
Case 2: If .epsilon..sub.T =.epsilon..sub.f, then .epsilon..sub.f '=0. 
Therefore, the tensile strain and the compressive strain will remain 
periodically in the optical fiber. 
Case 3: If .epsilon..sub.T &lt;.epsilon..sub.f, then .epsilon..sub.f 
'=.epsilon..sub.f -.epsilon..sub.T &gt;0. Therefore, the tensile strain will 
remain in the opt fiber. The presence of the tensile strain in the optical 
fiber reduces the breakage strength in a long period of time. The tensile 
strain which can be tolerated by the optical fiber used in the optical 
cable is determined by a proof strain and a fatigue coefficient value n. 
In an optical fiber of a SiO.sub.2 group in which the n value is about 20, 
the tensile strain which can be tolerated for about twenty years is about 
1/3 through 1/4 of the proof strain. If the proof strain, for instance, is 
2 %, then the tolerable tensile strain is 0.5 %. If the proof strain is 
0.5 %, then the tolerable stretching strain is about 0.2 %. It must be 
kept in mind that .epsilon..sub.f ' must not exceed the value thus 
determined. If a hermetically coated fiber, such as a carbon coated fiber, 
is used, the tensile strain which can be tolerated for about twenty years 
can be enlarged to about 90 % of the proof strain. For instance, if the 
proof strain is 2 %, the tolerable tensile strain will be about 1.8 %. 
Since a large tensile strain can be thus tolerated, the stimulated 
Brillouin scattering will be largely suppressed. 
The relationship between the amplitude of the strain remaining in the fiber 
and the amount of the critical input power will be shown below. In the 
expression (3), I.sub.0 stands for the critical input power obtained by 
the Brillouin gain bandwidth V.sub.0 under the strain-free condition, and 
I stands for the critical input power where the periodical strain of 
.+-..DELTA..epsilon. is given to the optical fiber. 
The following expression will be obtained from the relation between the 
expression (1) and the expression (2) 
EQU I/I.sub.0 =(.DELTA..epsilon./BV.sub.0)+1 (3) 
FIG. 8 shows the calculation results of the expression (3). Let us suppose, 
for instance, that V.sub.0 =20 MHz, then the application of an amplitude 
which the strain of .+-.1 % has increases the critical input power by 
fifty times. 
FIG. 9 shows one structural embodiment of the optical cable in the present 
invention. In this embodiment, the optical units each having a 
strain-holding structure as shown in FIG. 6A are stranded round the cable 
core 14. The sheath 15 may be provided as the needs of the case demands. A 
polyethylene rod having a steel wire in its center is generally used as 
the cable core. The main purpose of the sheath is the protection and 
fixation of the optical units 9's. There is no need to closely combine the 
cable core 14 with the optical units 9's. It is not necessarily needed to 
have periodically changing strain remained within each of the optical 
units. In this structure, the bending strain which occurs in each optical 
fiber due to the double stranding operation will be expressed by the 
following expression, wherein the symbols used are shown in FIG. 10: 
##EQU1## 
The symbol .theta. stands for a parameter indicating a longitudinal 
position z of the cable, and is given by .theta.=2.pi.Z/p.sub.1. The 
symbol .theta. stands for a phase difference between a stranding pitch 
p.sub.2 and a stranding pitch p.sub.1. The symbol R stands for a bend 
radius, or a distance between the center of the cable core 14 and the 
center of the drum around which the cable is wound. In the expression (4), 
the first term stands for the strain which occurs when the optical units 
are stranded round the cable core, and the second and third terms stand 
for the strain which occurs when the cables are bent or coiled. 
An optical cable which has a structure shown in FIG. 11 and has a length of 
4 km was made. Fiber loops were made by splicing the fibers at the cable 
end. Brillouin gain bandwidth distribution in the cable was measured from 
the other end of the cable. The results are shown in FIG. 12. FIG. 13 
shows the measuring results of the original optical fibers, wound loosely 
around a fiber bobbin before cabling. It is apparent from these figures 
that the Brillouin gain bandwidth of the optical fiber is about 130 MHz 
when it is double stranded to form the cable, whereas the Brillouin gain 
bandwidth the optical fiber is 60 MHz when it is not double stranded to 
form the cable. Therefore, strain having an amplitude of 0.07 %, which can 
be calculated from the expression (2), remains in the optical fiber due to 
the formation of the optical cable. The dimensions and the materials of 
the optical cable is shown in Table 1. 
TABLE 1 
______________________________________ 
Materials and Dimensions for Optical Cable 
Dimensions and Materials 
______________________________________ 
Coated Optical Fiber 
UV Curable Urethane Coating 
with Outer Diameter of 
0.25 mm 
Center Member Steel Having Outer Diameter 
of 0.45 mm and Covered with 
UV Curable Urethane, 
Resultant Structure Having 
Outer Diameter of 0.8 mm 
First Adhesive Resin 
UV curable Urethane Having 
Outer Diameter of 1.75 
(Young's Modulus: 3-5 kg/mm.sup.2) 
Second Adhesive Resin 
UV Urethane Having Outer 
Diameter of 2.15 mm (Young's 
Modulus: 40 kg/mm.sup.2) 
Stranding Pitch 230 mm 
of Fibers 
Cable Core Steel Having Outer Diameter 
of 0.6 mm and Covered with 
UV curable urethane, 
Resultant Structure Having 
Outer Diameter of 0.9 mm 
Buffer Layer UV Curable Urethane Having 
Outer Diameter of 5.2 mm 
Jacket Layer UV Curable Urethane Having 
Outer Diameter of 5.5 mm 
Stranding Pitch of 
180 mm 
Optical Fiber Unit 
______________________________________ 
NOTE: UV stands for ultraviolet 
FIG. 14 shows strain calculated from the expression (4). It is apparent 
that the strain which has an amplitude of about 0.08% and is nearly 
identical with the measuring results will periodically occur. 
What follows is the result of the study on the periodic strain given to 
optical fibers. As shown in the first term of the expression (4), the 
shorter pitches P.sub.1 and P.sub.2 will be, the larger strain will be. 
Therefore, the critical input power will be larger. However, strain 
consists of compressive strain and tensile strain both having the same 
amplitude within one pitch. Therefore, it is possible to cancel each other 
to be zero. What prevents these strains from cancelling each other is the 
binding function of the coating material of the optical fiber. The binding 
function depends on the binding force of the coating material itself and 
the stranding pitch. If the binding force of the material itself is 
extremely large, however, the extraction of the fibers out of the unit and 
the fusion splicing will be difficult. Therefore, what is suitable to the 
coating material may be the resin of a urethane group or a silicone group. 
The longer the stranding pitch is, the smaller the number of the 
cancelling of strains within one pitch is. Now, this relation will be 
quantitatively demonstrated based on the experimental results. Three 
units, each having an outer diameter of 2.15 mm and a length of 1 km, were 
made out of the same materials as shown in Table 1 to have the same 
structure as shown in Table 1. The pitch P.sub.2 of the first unit was 30 
mm, that of the second unit was 50 mm, and that of the third unit was 100 
mm. These units were wound around a bobbin having an outer diameter of 400 
mm in such a manner that they were arranged side by side with one another. 
The Brillouin gain bandwidth was measured, and the fiber strain was 
calculated from the expression (2). The calculation results showed that 
the first unit had strain having an amplitude of 0.02 %, the second unit 
had strain having an amplitude of 0.06 %, and the third unit had strain 
having an amplitude of 0.12 %. Let us suppose that there was no 
cancellation in strain. Then, the strain of 0.26 % should have occurred in 
consideration of the expression (4). Therefore, the shorter the stranding 
pitch of optical fibers becomes, the easier the cancellation of strains 
occurs. It can be understood from this experimental result that the pitch 
or the period of strain should be about 100 mm or more. This fact tells us 
that it is impossible to give large strain by making equal to or less than 
100 mm the fiber stranding pitch P.sub.2 in the first term of the 
expression (4 ). 
FIG. 15 shows another embodiment of the optical cable in the present 
invention. This embodiment is different from the embodiment shown in FIG. 
11 in that optical units shown in FIG. 6A are respectively inserted into 
the spiral grooves of the rod. The dimensions, materials and other data 
used in this embodiment are shown in Table 2. 
TABLE 2 
______________________________________ 
Materials, Dimensions, and Data for Deep-sea Optical 
Cable with 100 Optical Fibers 
Materials, Dimensions, and Data 
______________________________________ 
Coated Optical Fiber 
Outer Diameter: 0.25 mm 
Including UV Curable Urethane 
Coating 
Center Member Steel Wire Having Outer 
Diameter of 0.5 mm and 
Covered with UV Curable 
Urethane Coating, Resultant 
Structure Having Outer 
Diameter of 1.35 mm 
First Adhesive Resin 
UV curable Urethane Having 
Outer Diameter of 2.35 mm 
(Young's Modulus: 
3-5 kg/mm.sup.2) 
Second Adhesive Resin 
UV Curable Urethane Having 
Outer Diameter of 2.85 mm 
(Young's Modulus: 40 kg/mm.sup.2) 
Stranding Pitch 180 mm 
of Fibers 
Slotted Rod Outer Diameter: 18 mm, Groove 
Depth: 3.5 mm, Five Grooves, 
Groove Pitch: 130 mm 
Filling Resin Synthetic Jelly 
Tension Stranding 7 Steel Wires Each 
Resisting Member 
Having Diameter of 2.5 mm 
Winding for Holding 
Mylar tape Having Thickness 
of 0.2 mm 
Pressure Welded Copper Pipe Having 
Resistant Pipe Wall Thickness of 0.6 mm 
Outer Sheath Polyethylene, Outer Diameter: 
29 mm 
Cable Weight In Air: 1.2 kg/m, In Water: 
0.58 kg/m 
Cable Breakage 7 t 
Tension: 
.increment..epsilon. 
0.9% 
______________________________________ 
Five optical units, each having an outer diameter of 2.85 mm, were made by 
stranding twenty optical fibers round a steel wire having a diameter of 
0.5 mm with a stranding pitch of 180 mm and then binding the stranded 
fibers to the steel wire with adhesive resin. The optical units were 
respectively inserted into five spiral grooves, each having a spiral pitch 
of 130 mm and formed on a polyethylene rod having a diameter of 18 mm. A 
pressure resisting pipe 22 made of a welded copper pipe of 0.6 mm in 
thickness was provided through the holding windings, and an outer sheath 
15 of polyethylene was injection-molded to form a deep-see cable having an 
outer diameter of 29 mm. Carbon-coated optical fibers which were excellent 
in fatigue resistance property were used as the optical fibers. 
Lubricant jelly was filled into the grooves to prevent water from entering 
into the cable and to reduce frictional coefficient so as to allow the 
optical units to freely move within the grooves. The loose contact between 
the optical units and the grooves is also one of the characteristic 
features of the present invention. Its advantage will be quantitatively 
explained below. 
In this structure, a.sub.1 =6.6 mm, a.sub.2 =0.8 mm, p.sub.1 =130 mm, and 
p.sub.2 =180 mm in the expression (4). Therefore, the strain expressed by 
the first term in the expression (4), i.e., the strain contributing to the 
enlargement in the critical input power of light, will be 0.9 %. Let us 
suppose that V.sub.0 =20 MHz from FIG. 8, then I/I.sub.0 becomes as large 
as about 40 times. Namely, it is possible to increase the critical input 
power by about 16 dB. Let us suppose that the loss in optical fiber is 0.2 
dB/km, then the non-repeated transmission distance will increase by 80 km. 
The strains expressed by the second and third terms in the expression (4) 
occur when the cable is bent. Therefore, they do not act on the optical 
fibers when the cable is actually in use, because the cable is straightly 
laid for use. That is to say, they do not contribute to the enlargement in 
the critical input power. It must be noted, however, that the cable must 
pass through the sheave of the cable ship when the cable is drawn out of 
the cable ship to the sea-bed. If the radius of the sheave is 500 mm, the 
value of the second term and that of the third term will respectively be 
1.32 % and 0.16 %. These strains act on the fibers along with the strain 
of the first term, so that the optical fibers must be strong enough to 
withstand a large strain of about 2.4 %. However, in this embodiment, the 
optical units are loosely inserted in the respective grooves, as stated 
before, and the grooves are filled with jelly which acts as a lubricant. 
Therefore, the optical units move to and fro within the grooves when the 
cable is bent and the strain indicated by the second term will not occur. 
As a result, the bending strain actually acts on the optical units is 
restricted to the small strain indicated by the third term. 
If the compressive strain becomes 0.2 % or more, the optical fiber will 
buckle and transmission loss will increase. To prevent such an occasion, 
each optical fiber is given a back tension when an optical unit is 
manufactured, so that the strain expressed by the first term, the tensile 
strain of 0.9 % in this example, is remained in the fiber after the 
optical unit is made. When the unit is inserted into the groove, the 
optical fiber is given the tensile strain of 0.9.+-.0.9 %, so that the 
compressive strain which is the cause of buckling may be removed. 
FIG. 15 shows an example in which the unit shown in FIG. 6A is used. 
However, it is possible to use the units shown in FIGS. 6B to 6D. 
If the number of optical fibers stranded together is small, the grooves may 
be much slender, so that the strains expressed by the second and third 
terms may also be small. In such a case, adhesive resin which effectively 
prevents water from entering may be filled into the grooves. In this 
embodiment, a welded copper pipe is used for a deep-see cable so that the 
cable can be used on a sea-bed which is 2000 m or more in depth. However, 
if the cable is used much shallower areas of water, an aluminum laminated 
tape or stainless laminated tape, which is much easier in manufacture, may 
be used instead of the welded copper pipe. A water absorption tape may be 
used instead of the jelly. 
In the above explanation, the optical fiber is given strain which changes 
periodically. However, it is only necessary that the strain changes in the 
longitudinal direction of the optical fiber, but it is not necessarily 
needed that the strain changes periodically. The length of the optical 
fiber in which stimulated Brillouin scattering effectively occurs is given 
by the following expression: 
EQU L.sub.e =1/.alpha. (5) 
wherein .alpha. stands for a loss coefficient of the optical fiber 
(neper/m). Let us suppose, for instance, that .alpha.=4.6.times.10.sup.-5 
(0.2 dB/km), then L.sub.e =22 km. Therefore, the strain given to the 
optical fiber must be changing at a spatial frequency of 1/(22 km) or more 
.