Method and apparatus for measuring distribution of elongation in an optical cable

Elongation distortion at any location of an optical fiber cable is measured by using the principle of the stimulated Raman effect in which a stokes beam with a little frequency difference from that of a pumping beam is generated when a pumping beam and a seed beam with the same frequency as that of the stokes beam meet in a Raman material like an optical fiber cable. Due to the fact that the group refractive index of an optical fiber cable for a pumping beam is different from that for a stokes beam, elongation of a cable is measured by observing time difference of the arrival of pumping beam and stokes beam. The location that the pumping beam meets with the stokes beam can be adjusted merely by shifting the pumping pulse, then, elongation distortion at any location can be measured. According to the invention, the pumping pulse is applied to the cable to be tested from both the ends of the cable with one of the pumping pulse delayed according to the desired location for meeting of two pulses. At one end of the cable, the arrival time t.sub.1 and t.sub.2 is observed, where t.sub.1 is the arrival time of the stokes pulse and t.sub.1 +t.sub.2 is the arrival time of the pumping pulse. The distribution of elongation (d.DELTA.z/dz) is obtained by calculating the equations z=k.sub.1 t.sub.1 -k.sub.2 t.sub.2, and .DELTA.z=k.sub.3 t.sub.2 -k.sub.4 t.sub.1, where k.sub.1, k.sub.2, k.sub.3 and k.sub.4 are constants, z is the length between the end and the location to be tested, and .DELTA.z is the elongation of the cable of the length z.

BACKGROUND OF THE INVENTION 
The present invention relates to process for measuring elongation 
distribution of an optical fiber cable. 
An optical fiber cable has the nature that the bandwidth of the same is 
quite wide and the transmission loss is small, therefore, that cable is 
promising for transmission medium for a long distance digital transmission 
cable. However, the optical fiber cable has the disadvantage that it is 
mechanically weak, in particular, a submarine optical fiber cable 
elongates 0.3-0.4% when installed, and also elongates 0.6-0.7% when it is 
picked up on a deck of a boat for repairing. The characteristics of an 
optical fiber cable depends upon the elongation of the same, therefore, 
the measurement of the elongation of an optical fiber cable is essential 
to keep the high reliability of an optical fiber cable. 
A prior method for measuring elongation of an optical fiber cable is first 
described in accordance with FIG. 1, in which an optical pulse 2 is 
applied to an input end 11 of an optical fiber cable to be measured, and 
an output pulse is derived at the other end 12 of the cable, then, the 
delay time (.tau.) by the transmission of the optical pulse provides the 
elongation .DELTA.L of the cable. That value, elongation .DELTA.L, is 
calculated by the known equation (1) as follows. 
EQU (.tau.)=(N/c)L+(N'/c).DELTA.L (1) 
In the equation (1), N is the group refraction index of a cable to be 
measured, N' is the equivalent group refractive index which is defined by 
the optical elastic effect by the cable 1, c is the light velocity in free 
space, L is the original length of the cable. Other prior system for 
measuring elongation of an optical fiber cable is phase method, or 
interference method, which uses the same theoritical principle as that of 
FIG. 1. 
However, a prior method for measuring elongation of an optical fiber cable 
has the disadvantage that it can measure only the total elongation of the 
cable, but it can not provide an information where and how a cable is 
elongated. It should be noted that the characteristics of the optical 
fiber cable are not determined by an average elongation of the cable, but 
they depends upon the history of the elongation at various points of the 
cable. 
Therefore, it is essential to measure the distribution of the elongation of 
an optical fiber cable for the study and the design of an optical fiber 
cable system. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the disadvantages and 
limitations of a prior measuring system for an optical fiber cable by 
providing a new and improved process for measuring an elongation of an 
optical fiber cable. 
It is also an object of the present invention to provide a process for 
measuring elongation of an optical fiber cable so that an elongation at 
each designated portion of a cable can be measured. 
The above and other objects are attained by using the principle of the 
stimulated Raman effect in which a stokes beam with a little frequency 
difference from that of a pumping beam is generated when a pumping beam 
and a seed beam with the same frequency as that of the stokes beam meet in 
a Raman material like an optical fiber cable. Since the group refractive 
index of an optical fiber cable for a pumping frequency is different from 
that for a stokes frequency, the time difference between the arrival of 
the pumping pulse to the end of the cable and the arrival of the stokes 
pulse to that end is observed, and that time difference implicites the 
elongation distribution. The present invention comprises the steps of 
applying optical pulses to the ends of an optical fiber cable to be tested 
from both the ends of the cable so that those optical pulses meet at 
desired location along the cable, measuring time t.sub.1 and time t.sub.2, 
where t.sub.1 is the arrival time of the faster pulse of the stokes pulse 
and the pumping pulse, and t.sub.1 +t.sub.2 is the arrival time of the 
other pulse, and providing distribution of elongation (d.DELTA.z/dz) using 
the equations z=k.sub.1 t.sub.1 -k.sub.2 t.sub.2 and .DELTA.z=k.sub.3 
t.sub.2 -k.sub.4 t.sub.1, where k.sub.1, k.sub.2, k.sub.3 and k.sub.4 are 
constants, z is the length of the cable between the end and the location 
to be tested, and .DELTA.z is the elongation of the cable of the length z.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The stimulated Raman effect which is used in the present invention, is 
described first for the easy understanding of the present invention. It 
has been known that when an optical beam (pumping beam) with a 
predetermined frequency (.nu..sub.0) illuminates a material which reflects 
irregularly or scatters the beam, it is observed not only the original 
pumping beam, but also the first stokes beam with the frequency 
(.nu..sub.0-.nu..sub.1), the second stokes beam with the frequency 
(.nu..sub.0 -2.nu..sub.i) and the third stokes beam with the frequency 
(.nu..sub.0 -3.nu..sub.i) et al. by observing the beam through a spectrum 
analyzer. That phenomenon is called the Raman effect. 
The strength of the Raman effect depends upon the product of the power 
density of an optical energy and the Raman gain which is defined by the 
transmission material itself (Raman material), and the transmission length 
which the optical beam transmits in the Raman material. The silica 
(S.sub.i O.sub.2) which is the material of an optical fiber is one of the 
Raman material, and the first stokes beam with the wavelength 1.12 micron 
is observed when a YAG laser with the wavelength 1.06 micron excites an 
optical fiber. The difference between the wavelengths of 1.12 micron and 
1.06 micron is defined by the Raman material. Although the Raman effect is 
not observed when the power density is small, it should be appreciated 
that the power density in an optical fiber is extremely large even when 
the power of an optical source is small (for instance, less than several 
watts), since the diameter of an optical fiber is very small (for instance 
that diameter is less than 10 microns). Further, when an optical beam 
transmits in an optical fiber which is longer than several kilo-meters, 
the Raman effect is easily observed. The observation of the Raman effect 
with the optical source less than several watts has been reported. 
The Raman effect is considerably strong when the original pumping beam (of 
the wavelength 1.06 micron in the present embodiment), and the first 
stokes beam with the wavelength .nu..sub.0 -.nu..sub.i (=1.12 micron) are 
applied simultaneously to a Raman material. That effect, that the pumping 
beam is converted to the first stokes beam with high efficiency when a 
seed beam of the wavelength of the first stokes beam exists, is called the 
stimulated Raman effect. The present invention uses that stimulated Raman 
effect in an optical fiber cable. 
FIG. 2 shows the basic concept of the present invention, and the generation 
of the stokes pulse 5 of the frequency .nu..sub.0 -.nu..sub.i at any 
location of the optical fiber cable 1. It is supposed that the pulse 3 
with the frequency .nu..sub.0 and the pulse 4 with the frequency 
.nu..sub.0 are applied to the optical fiber cable 1 from the opposite ends 
at the time t=0 and t=t.sub.d, respectively. Those pulses 3 and 4 meet 
with each other at the location z along the optical fiber cable 1, then, 
the stokes pulse 5 is generated at the location z. The stokes pulse 5 runs 
towards the end 11 of the cable 1 with the beam 4. The location z where 
the pulses 3 and 4 meet with each other depends upon the start time 
t.sub.d of the pulse 4. When the start time t.sub.d is changed between 
-N.sub.p L/c and N.sub.p L/c, the location z changes between 0 and L, 
where N.sub.p is the group refractive index of the optical pulses 3 and 4. 
Next, the reason of the generation of the stokes pulse 5 in FIG. 2 is 
described in accordance with FIGS. 3 and 4. When the optical pulse 4 
transmits in the optical fiber cable 1, a stokes pulse is generated 
according to the Raman effect, and the generated stokes pulse is 
transmitted with the original optical pulse 4. FIG. 3 shows the 
transmission of the pulse 4 and the stokes beam 6 when the group 
refractive index N.sub.s of the stokes beam is smaller than the group 
refractive index N.sub.p of the original beam 4. The head end (a) of the 
stokes beam 6 is generated when the original beam 4 passes the input end 
12, and the rear end (b) of the stokes beam 6 is generated when the 
original beam 4 passes the present location in the cable 1. It should be 
noted that the level of the stokes beam 6 is considerably small when the 
power density of the original beam 4 is not large. 
The stimulated Raman effect amplifies the stokes beam 6 when the stokes 
beam 6 which is generated by the beam 4 meets with another beam 3 from the 
opposite direction. The amplification of the stokes pulse is effected only 
when the stokes pulse 6 meets with the pulse 3, and finishes when the 
pulse 3 meets with the rear end (b) of the beam 4, as no stokes pulse 
towards the end 11 exists after the beam 4. The amplification of the 
stokes pulse 4 is shown in FIGS. 4 and 5. 
The rear falling edge (c) of the stokes beam 6 (see FIG. 4 is generated 
just when the first pulse 4 meets with another pulse 3. Since that rear 
falling edge (c) is steep, that rear edge is called a pulse (pulse 5), and 
the pulse 5 is defined by the rear edge of the stokes pulse 4. On the 
other hand, when the group refractive index N.sub.s of the stokes beam 6 
is larger than the group refraction index N.sub.p of the original beam, 
the stokes pulse 5 is defined by the front edge of the stokes beam 6. 
Accordingly, the stokes pulse 5 is obtained when the first beam 4 meets 
with the second beam 3 from the opposite direction irrespective of the 
group refractive index N.sub.s of the stokes beam 6. 
The principle of the measurement of the distribution of an elongation of an 
optical fiber cable according to the present invention based upon the 
generation of a stokes pulse as described is now described in accordance 
with FIG. 6. In FIG. 6, the curve (a) shows the total (accumulated) 
elongation .DELTA.z of the cable 1 between the end 11 and the location z, 
and the curve (b) shows the elongation distortion .epsilon.(z) at the 
location z of the cable 1. That elongation distortion shows the 
distribution of the elongation at any location of a cable. 
It is supposed that a pair of optical beams 3 and 4 are applied to the 
opposite ends 11 and 12 of the cable 1 at time t=0. Then, those beams 3 
and 4 meet with each other at the location z along the cable 1, and the 
stokes pulse 5 is generated. The generated stokes pulse 5 and the original 
pumping pulse 4 propagate through cable 1, and reach the end 11 of the 
cable 1. It is assumed that the stokes pulse 5 reaches the end 11 at time 
t=t.sub.1, and the time difference between the arrival of the stokes pulse 
5 and the original pulse 4 to the end 11 is t=t.sub.2 (that is to say, the 
original pumping pulse 4 reaches the end 11 at time t=t.sub.1 +t.sub.2). 
It should be appreciated that the stokes pulse 5 and the original pumping 
pulse 4 do not reach the end 11 of the cable 1 simultaneously, although 
those pulses 4 and 5 coincide with each other at the location z, because 
of the difference of the group refractive index of the wavelength of those 
pulses 4 and 5. 
The relations between t.sub.1, t.sub.2, z and .epsilon.(z) are shown by the 
following equations (2). 
##EQU1## 
where N.sub.p and N.sub.s are group refractive indices of the pumping beam 
4 and the stokes beam 5 in a material itself of a transmission medium, 
respectively, N.sub.p ' and N.sub.s ' are equivalent group refractive 
indices of the pumping beam 4 and the stokes pulse 5, respectively, in an 
optical fiber cable 1, and .DELTA.z is the elongation of the short length 
at the location z. It should be noted that the values N.sub.p ' and 
N.sub.s ' for an optical fiber cable are different from the values N.sub.p 
and N.sub.s for the material itself of an optical fiber, and that 
difference depends upon the photo elastic effect of the optical pulses 4 
(or 3) and the stokes pulse 5, the temperature of the cables, and/or the 
pressure applied to the cable. 
The above equation (2) is modified, and the following equation (3) is 
derived from the above equation (2). 
##EQU2## 
where k.sub.1, k.sub.2, k.sub.3 and k.sub.4 are constants. 
Accordingly, the elongation distortion .epsilon.(z) at any location along 
an optical fiber cable is measured by observing the values t.sub.1 and 
t.sub.2 while designating the location z by sweeping the start time 
t.sub.d of the pumping pulse 4. 
Further, it should be appreciated that the measurement of the elongation 
distortion with the use of the only a single end 11 or 12 of the cable may 
be possible by using a reflection mirror at the other end 12 or 11. 
FIG. 7 shows a block diagram of the apparatus for measuring an elongation 
distribution of an optical fiber cable according to the present invention. 
In FIG. 7, the pulse oscillator 7 provides an electrical output pulse which 
is divided to the pulses 8 and 9, each applied to the delay circuits 10 
and 11, respectively. The first delay circuit 10 delays the pulse 8 by the 
time t=t'+t.sub.d, and the second delay circuit 13 delays the pulse 9 by 
the time t=t'. When t'=0, the second delay circuit 13 can of course be 
removed. The delay time t.sub.d in the first delay circuit 10 is sweeped 
or shifted so that the location z covers the whole length of a cable to be 
tested. The delayed outputs of the delay circuits 10 and 13 excite lasers 
14 and 15, respectively, so that a pair of optical pulses with the time 
interval t.sub.d are provided. Thus, the optical pulse 4 by the laser 14 
is delayed by the time t.sub.d as compared with the optical pulse 3 of the 
laser 15. Those lasers 14 and 15 are for instance implemented by a YAG 
laser. The optical pulse 3 by the laser 15 is separated to two beams by 
the beam splitter 16, and the offset beam 3a is applied to an 
oscillo-scope 20 through the optical-electrical converter 18. The signal 
21 converted from the offset beam 3a is used as a time basis in the 
oscillo-scope 20. 
On the other hand, the optical pulse 3 of the laser 15 is applied to the 
optical fiber cable 1 at the input 11 of the cable 1 through the other 
beam splitter 17, and the delayed optical pulse 4 is applied to the other 
end 12 of the cable 1. Then, a stokes pulse is generated at the location 
where the optical pulses 3 and 4 meet, and both the optical pulse 4 and 
the generated stokes pulse 5 are provided at the end 11 of the cable 1, as 
described in detail in accordance with the previous figures. The output 
optical pulses 4 and 5 are offset by the beam splitter 17, and are applied 
to the oscillo-scope 20 through the optical-electrical converter 19. 
On the screen of the oscillo-scope 20, the reference pulse 21, the stokes 
pulse 5 and the pumping pulse 4 are indicated, and the time t.sub.1 is the 
duration between the reference pulse 21 and the stokes pulse 5, and the 
time t.sub.2 is the duration between the stokes pulse 5 and the pumping 
pulse 4. It is supposed in FIG. 7 that the oscillo-scope 20 accepts the 
synchronization signal from the output of the pulse oscillator 7 through 
the line 7a. 
With those observed values t.sub.1 and t.sub.2, the location z and the 
total elongation .DELTA.z between the location z and the end of the cable 
are obtained by the equation (3), and thus the curve (a) of FIG. 6 is 
obtained. Then, the curve (b) which shows the elongation distortion is 
obtained by differentiating the curve (a). The curve (b) is the final 
result of the present invention. The calculation of the equation (3) and 
the differentiation calculation are performed by using a programmed 
computer which is coupled with the outputs of the optical-electrical 
converters 18 and 19. 
As described above in detail, according to the present invention, the 
distribution of elongation of an optical fiber cable can be measured by 
using the principle of the stimulated Raman effect. 
From the forgoing, it will now be apparent that a new and improved process 
for measuring distribution of elongation of an optical fiber cable has 
been found. It should be understood of course that the emboidment 
disclosed is merely illustrative and is not intended to limit the scope of 
the invention. Reference should be made to the appended claims, therefore, 
rather than the specification as indicating the scope of the invention.