Dispersion compensating optical fiber for wavelength multiplex transmission and method using same

The present invention is intended to provide a dispersion compensating optical fiber for wavelength division multiplex optical communication. A refractive index distribution of the dispersion compensating optical fiber is set to be W-shaped, an outside of a core is formed as an internal clad layer and the outside of the internal clad layer is formed as an outermost clad layer made of pure silica. Germanium for raising the refractive index by 2.8% in a specific refractive index difference is doped in the core, and fluorine is uniformly doped in the internal clad layer so that the refractive index is reduced by 0.45% in the specific refractive index difference. A diameter ratio of the core and the internal clad layer is set to be in a range of 1:1.5 to 1:4.0, a wavelength dispersion slope is set to be in a negative area, and dispersion is controlled to a negative high dispersion structure of -100 ps/km-nm or under.

FIELD OF THE INVENTION 
The present invention relates to dispersion compensating optical fiber for 
wavelength division multiplexed transmission to compensate wavelength 
dispersion in a multiplexed transmission, and more particularly, relating 
to optical transmission signals having a plurality of wavelengths at or 
about 1550 nm. 
BACKGROUND OF THE INVENTION 
A 1300 nm zero dispersion fiber network is known to be used providing 
highly reliable optical communications. At a receiving side of a network 
zero dispersion of optical signals having a wavelength of 1300 nm is 
obtainable. 
As of late, wavelength division multiplex communications for transmitting 
optical transmission signals (optical pulse transmission signals) having a 
plurality of wavelengths is being implemented by using this existing 1300 
nm zero dispersion fiber network. In wavelength division multiplex 
communication using a wavelength of approximately 1550 mm through the 
existing 1300 mn zero dispersion transmission network, wavelength 
dispersion of approximately 17 ps/km-nm occurs causing a disturbance in 
long range transmission. Generally, wavelength dispersion includes 
positive dispersion and negative dispersion; negative dispersion refers to 
a phenomenon that, as the wavelength becomes larger, the group index of 
the optical transmission fiber lessens and the group velocity of the 
transmission signal increases and the pulse width increases in response to 
this increase of the group velocity; positive dispersion refers to a 
phenomenon that, as the wavelength increases, the group index of the 
optical transmission fiber increases and the group velocity of the 
transmission signal lessens and the pulse width increases in response to 
this decrease of group velocity. 
An ordinary existing 1300 nm zero dispersion transmission network has a 
dispersion of approximately 17 ps/km-nm at a wavelength of approximately 
1500 nm as described above. In a long range transmission of, for example, 
100 km distance, there is a problem that dispersion of approximately 1700 
ps/km-nm occurs at the receiving side of an optical transmission and, even 
though high density/high speed communication is attempted by minutely 
dividing the wavelength at approximately 1550 nm, a signal of one side 
wavelength is superposed with a signal of the other side wavelength and 
separation of signals is difficult since dispersion is large as described; 
therefore due to this overlapping of adjacent channels, optical 
communication performance is worsened. 
In the prior art, a dispersion compensating optical fiber for compensating 
a quantity of chromatic dispersion of a specific wavelength is inserted 
into the optical transmission path to prevent the increase in the quantity 
of chromatic dispersion as is described above. 
This type of dispersion compensating optical fiber has negative dispersion 
and the increase of the quantity of dispersion of a specific wavelength in 
an optical transmission is lessened by offsetting positive dispersion of a 
1300 nm zero dispersion transmission network with negative dispersion by 
utilizing this dispersion compensating optical fiber. 
Dispersion compensating optical fiber includes five types of refractive 
index profiles as disclosed in Japanese Patent Application Disclosure HEI 
6-11620. These five types of refractive index profile are shown in FIG. 5. 
In the refractive index distributions shown in FIGS. 5a and 5b, the 
dispersion slopes (the derivative of dispersion with respect to 
wavelength) respectively have a positive value and, in the use of such 
dispersion compensating fiber, dispersion compensation can be carried out 
for a specific wavelength; however this dispersion compensating optical 
fiber is unsuitable for other wavelengths as a compensation optical fiber 
for wavelength division multiplex transmission since the quantity of 
dispersion increases with wavelength. The three types of optical fibers 
relating to FIGS. 5c to 5e may have a refractive index having a negative 
dispersion slope. Though the W-shaped refractive index profile shown in 
FIG. 5c has long been examined, conventional W-shaped optical fiber has 
been able to provide a negative dispersion slope but has required an 
extremely long fiber length necessary for a dispersion compensation since 
the quantity of negative dispersion of the conventional W-shaped optical 
fiber has been small, and therefore it has been unsuitable for practical 
use. It is better to reduce the diameter of the core to increase the 
quantity of negative dispersion; if the core diameter of the optical fiber 
is reduced and the quantity of negative dispersion is increased, the 
dispersion slope in the W- shaped refractive index profile is inverted 
from a negative slope to a positive slope and therefore the W-shaped 
optical fiber is unsuitable to wavelength multiplex division transmission. 
It is an object of the invention to overcome the above-described problem of 
the prior art by providing a structure capable of simultaneously having a 
negative dispersion slope with an effective size and a negative dispersion 
with an appropriate magnitude in the W-shaped refractive index 
distribution to provide a dispersion compensating optical fiber for 
wavelength division multiplex transmission which enables compensation of 
dispersion in a wide range of wavelengths at or about approximately 1550 
nm, and wavelength division multiplex transmission by using the existing 
1300 nm zero dispersion transmission network. 
A dispersion compensating optical fiber for wavelength division multiplex 
communication wherein wavelength dispersion .sigma. in range of 
a0.ltoreq.a.ltoreq.a1 is controlled in a range of .sigma..ltoreq.-100 
ps/km-nm when it in assumed that a core radius with which a wavelength 
dispersion slope (d.sigma./d.lambda.) is zero is a0 and a core radius with 
which the wavelength dispersion slope (d.sigma./d.lambda.) is -0.28 
ps/km-nm.sup.2 is al in a that a core radius of an optical fiber is a, 
wavelength dispersion is .sigma., and a wavelength of optical transmission 
signal is .lambda.. 
The present invention is adapted as described below to attain the above 
object. Specifically, the present invention is characterized in that the 
wavelength dispersion .sigma. in range of a0.ltoreq.a.ltoreq.a1 is set to 
be within in a range of .sigma..ltoreq.-100 ps/km-nm when it in assumed 
that a core radius in a case that the wavelength dispersion slope 
(d.sigma./d.lambda.) is zero is a0 and a core radius in the case that the 
wavelength dispersion slope (d.sigma./d.lambda.) is 0.28 ps/km-nm.sup.2 is 
a 1 if the core radius of an optical fiber is a, wavelength dispersion is 
.sigma., and a wavelength of optical transmission signal is .lambda.. 
The present invention is also characterized in that the refractive index 
structure of the above optical fiber has the W-shaped refractive index 
profile, an internal clad layer is formed outside the core, an outermost 
clad layer is formed on the outside of the internal clad layer, a dopant 
for reducing the refractive index is doped in the internal clad layer so 
that a specific refractive index difference is -0.45%, the outermost clad 
layer being made of pure silica and the dopant for raising the refractive 
index is doped in the core so that the specific refractive index 
difference is +2.8%, the diameter ratio of the core to the internal clad 
layer is determined to be within the range of 1:1.5 to 1:4.0, the 
dispersion having a negative slope at a wavelength of about 1550 nm, and 
the wavelength dispersion at optical wavelengths of about 1550 nm is 
smaller than -100 ps/km-nm and larger than -170 ps/km-nm in a small range 
wherein the core diameter is larger than 2.1 .mu.m and smaller than 2.3 
.mu.m. 
In the above configuration according to the present invention, if the 
dispersion compensating optical fiber for wavelength division multiplex 
transmission according to the present invention is inserted into, for 
example, an existing 1300 nm zero dispersion transmission network and 
wavelength division multiplex communication is carded out with at a 
wavelength of approximately 1550 nm, optical signals of respective 
wavelengths which have reached the terminal through the 1300 nm zero 
dispersion transmission network have large quantities of wavelength 
dispersion. However, since the dispersion compensating optical fiber 
according to the present invention simultaneously has the high negative 
chromatic dispersion and a negative dispersion slope, this offsets a large 
unwanted positive dispersion quantity, which occurs through the 1300 nm 
zero dispersion transmission network; Effectively optical signals of 
respective wavelengths which have passed through the dispersion 
compensating optical fiber according to the present invention have 
dispersion values almost equal to zero. Consequently, separation of 
signals with respective wavelengths is certainly carried out at the 
receiving side to enable high density/high speed wavelength division 
multiplex communication in high reliability. 
The present invention provides a new dispersion compensating optical fiber 
which has a negative dispersion slope and a negative high dispersion, that 
effectively offsets large dispersion quantities caused in the optical 
transmission path and receives signals with small-wavelength dispersion at 
the receiving side by utilizing optical fiber having high negative 
dispersion according to the present invention. 
Thus the reliability of high density/high speed wavelength division 
multiplex communication can be substantially raised. 
The dispersion compensating optical fiber according to the present 
invention has negative high dispersion and, even when large positive 
dispersion occurs in optical transmission signals which have passed 
through the optical transmission path, the positive dispersion can be 
compensated with a short length of optical fiber. Accordingly, the 
dispersion compensating optical fiber can be housed in a small compact 
package and therefore excellent in practical use. 
In addition, in the wavelength division multiplex communication using the 
wavelength of approximately 1550 nm with an existing 1300 nm zero 
dispersion transmission network, wavelength dispersion of transmission 
optical signals of various wavelengths can be effectively offset and 
compensated at the receiving side by inserting the dispersion compensating 
optical fiber according to the present invention into the optical 
transmission path, thereby achieving high density/high speed wavelength 
division multiplex communication with high reliability at wavelength of or 
about approximately 1550 nm. 
The optical fiber is subject to a condition for effective propagation of 
light. This light propagating condition depends on the effective 
refractive index (.beta./k), where .beta. is a propagation constant within 
the waveguide and k is the number of waves in the media space. 
In the optical fiber having the W-shaped refractive index profile, the 
effective refractive index of light signals depend on the values of 
specific refractive index difference .DELTA.+ of the core and specific 
refractive index difference .DELTA.- of the internal clad and it is 
necessary to find an optimum combination of these specific refractive 
index differences .DELTA.+and .alpha.-. 
According to the studies of the present inventor, the propagation 
conditions tend to be satisfied with a larger specific refractive index 
difference .DELTA.+ of the core and a smaller specific refractive index 
difference .DELTA.- of the internal clad and particularly the optimum 
propagation conditions are obtained --from a-- combination of the specific 
refractive index difference .DELTA.+ of +2.8% of the core and the specific 
refractive index difference .DELTA.- of 0.45% of the internal clad. The 
light propagation performance of the optical fiber deteriorates as the 
above refractive index differences deviate from the optimum propagation 
conditions. for example, in case of the optical fiber with .DELTA.+=+2.8% 
and .DELTA.-=-0.7%, .DELTA.- is too large to deteriorate and in case of 
the optical fiber with .DELTA.+=2.1% and .DELTA.-=-0.35%, the -light 
propagation performance similarly deteriorates since .DELTA.+ is 
excessively small. 
As in the present invention, the optimum refractive index for propagation 
of light can be obtained by applying .DELTA.+=+2-8% and .DELTA.-=-0.45%.

DETAILED DESCRIPTION 
The embodiments of the present invention are described below referring to 
the drawings. FIG. 1 shows a structure of an embodiment of a dispersion 
compensating optical fiber for wavelength division multiplex transmission 
according to the present invention. 
The dispersion compensating optical fiber of the present embodiment has a 
W-shaped refractive index profile and germanium Ge for raising the 
refractive index so that .DELTA.+=2.8% is obtained as a specific 
refractive index difference .DELTA. is doped in the core 1. A clad layer 2 
is formed outside the core I and fluorine F for reducing the refractive 
index so that the value of the specific refractive index difference 
.DELTA. is -0.45% is uniformly doped in this clad layer 2. An outermost 
clad layer (not shown) made of pure silica is formed on the outside of the 
internal clad layer 2. The diameter ratio a/D of the core and the internal 
dad layer is set in the range of 1:1.5 to 1:4.0. 
FIG. 4 shows a conventional typical simple step type optical fiber 
structure as a comparative example. In this comparative example of the 
optical fiber, germanium for raising the refractive index so that the 
specific refractive index difference of 2.8% is obtained is doped in the 
core 1 and fluorine F for reducing the refractive index to obtain the 
specific refractive index difference of -0.45% is uniformly doped in the 
clad layer on the outside of the core 1. 
Dispersion values .sigma. and dispersion slopes .delta. with respect to 
respective core diameters in the optical fiber structure shown in FIG. 1 
are calculated with the diameter ratio of the core and the internal clad 
layer as a parameter as shown in Tables 1 to 3. 
TABLE 1 
______________________________________ 
Core Diameter 
Dispersion Value 
Dispersion Slope 
______________________________________ 
1.6 -262 +0.86 
1.7 -267 +0.97 
1.8 -248 +0.92 
1.9 -220 +0.61 
2.0 -185 +0.29 
2.1 -157 0.00 
2.2 -127 -0.19 
2.3 -107 -0.24 
2.4 -88 -0.26 
2.5 -71 -0.27 
2.6 -55 -0.28 
2.7 -41 -0.25 
2.8 -36 -0.23 
2.9 -27 -0.20 
3.0 -18 -0.18 
3.1 -5 -0.14 
3.2 +3 -0.11 
______________________________________ 
TABLE 2 
______________________________________ 
Core Diameter 
Dispersion Value 
Dispersion Slope 
______________________________________ 
1.83 -208.6 +0.374 
2.00 -170.15 +0.099 
2.17 -129.97 -0.083 
2.33 -93.94 -0.161 
2.5 -64.44 -0.193 
2.67 -40.6 -0.188 
______________________________________ 
TABLE 3 
______________________________________ 
Core Diameter 
Dispersion Value 
Dispersion Slope 
______________________________________ 
1.9 -211.6 +0.4498 
2.0 -190.9 +0.3028 
2.1 -166.9 +0.1621 
2.2 -144.05 -0.056 
2.3 -101.15 -0.1612 
2.4 -40.6 -0.188 
2.5 -83.07 -0.177 
______________________________________ 
Equations for obtaining the dispersion value s and the dispersion slope d 
are as given below. 
EQU .sigma.=(K/c) dM2/dK+(K/c) (d(M1-M2)/dK) d(V.multidot.b)/dV+{(M1-M2)/c}V 
d2(V.multidot.b)/dV.sup.2 (1) 
EQU .delta.=d.sigma./d.lambda. (2) 
The symbols used in the equations denote as follows. c: Velocity of light, 
K: the wave number in the media space, n1: Refractive index of the core, 
n2: Refractive index of the external clad layer, A: Specific refractive 
index difference between the core and the external clad layer, 
V=(K.multidot.n1.multidot.a(2.DELTA.)).sup.1/2, a: Core radius, M1: 
d(K.multidot.n1)/dK, M2: d(K.multidot.n2)/dK, b: Normalized variable. 
Table 1 shows the results of calculations in the case that the diameter 
ratio of the core and the internal clad layer is 1:2.5, Table 2 shows the 
results of calculations in the case that the diameter ratio of the core 
and the internal dad layer is 1:1.5, and Table 3 shows the results of 
calculations in the case that the diameter ratio of the core and the 
internal clad layer is 1:4.0. 
As known from these results of the calculations, the dispersion is inverted 
from having a the positive slope to a negative slope with a certain core 
diameter as the border. The dispersion is inverted with the core diameter 
of 2.1 .mu.m as the border in the data shown in Table 1, the dispersion 
slope is inverted with the core diameter between 2.00 and 2.1 .mu.m as the 
border in the data shown in Table 2, and the dispersion slope is inverted 
with the core diameter between 2.1 and 2.2 .mu.m as the border in the data 
shown in Table 3. Accordingly, the structure of the dispersion 
compensating optical fiber having a dispersion slope in the negative range 
and negative high dispersion can be specified according to these 
calculation data. 
The present embodiment with a key emphasis on this point is intended to 
provide a dispersion compensating optical fiber having a negative 
dispersion slope and a negative high dispersion. The present embodiment is 
also intended to obtain a dispersion compensating optical fiber whose 
wavelength dispersion .sigma. becomes .sigma..ltoreq.-100 ps/km-nm when it 
in assumed that a core radius in a case that the wavelength dispersion 
Mope (d.sigma./d .lambda.) is zero is a0 and a core radius in the case 
that the wavelength dispersion slope (d.sigma./d.lambda.) is 0.28 ps/km 
/nm.sup.2 is al if the core radius of an optical fiber is a, wavelength 
dispersion is .sigma., and a wavelength of optical transmission signal is 
80 . The conditions are satisfied respectively with the core diameter of 
2.1 to 2.3 in the data shown in Table 1, the core diameter of 2.17 .mu.m 
in the data shown in Table 2, and the core diameter of 2.2 to 2.4 .mu.m in 
the data shown in Table 3. 
There are some appreciable differences between the results of calculations 
shown in the above tables and the actual values of measurement of the 
actually manufactured dispersion compensating optical fiber. For example, 
the actually measured values of the actually manufactured dispersion 
compensating optical fiber with the diameter ratio of the core diameter 
and the internal clad layer of 1:2.5 are such that the dispersion value is 
-163.3 ps/km-nm and the dispersion slope is -0.129 ps/km-nm.sup.2 in case 
of the core diameter of 2.124 .mu.m and the dispersion value is -152 
ps/km-nm and the dispersion slope is -0.249 ps/km-nm.sup.2 in case of the 
core diameter of 2.184 .mu.m. An optical fiber having the negative high 
dispersion that is -100 ps/km-nm or less in the range of the negative 
dispersion slope can be made by referring to the values obtained from 
calculations (while adjusting the data as required with the calculated 
values as the targets). 
FIG. 2 shows the results of actual measurements of the dispersion slopes 
and the dispersion values in reference to the core diameters of the 
dispersion compensating optical fiber in the present embodiment with the 
diameter ratio of the core and the internal clad layer as a parameter. 
According to the data of actual measurements, the range of the core 
diameter, where negative high dispersion with the wavelength dispersion of 
-100 ps/km-nm or less occurs in the range that the dispersion slope is 
zero or under, is obtained, the diameter of the internal clad layer is 
determined by specifying the core diameter, and the dispersion 
compensating optical fiber provided with a fiber structure shown in FIG. 1 
and an excellent dispersion compensating function can be obtained. The 
data of actual measurements of the conventional comparative example fiber 
shown in FIG. 4 is shown in FIG. 2 for reference. 
By inserting the dispersion compensating optical fiber having a chromatic 
dispersion with a negative slope and the negative high dispersion with 
wavelength dispersion of -100 ps/km-nm or less into the optical 
transmission path having positive dispersion, a large positive dispersion 
which is caused in the optical transmission path can be effectively 
compensated and the dispersion of respective wavelengths can be 
compensated to a small value (preferably, dispersion of almost zero) at 
the receiving side. Though the ordinary optical transmission path has a 
positive dispersion slope, the dispersion compensating optical fiber 
having the negative dispersion slope of the present embodiment can be used 
to obtain an effect that the variations of wavelength dispersion of 
respective wavelengths at the receiving side can be prevented and 
wavelength dispersion can be limited to the range of small variations. 
In wavelength multiplex division transmission at approximately 1550 nm with 
the existing 1300 nm zero dispersion optical fiber network, an optical 
fiber having the negative high dispersion in which the dispersion value at 
the wavelength of 1550 nm is smaller than -100 ps/km-nm is selected by 
setting the conditions for selecting the dispersion compensation optical 
fiber in a range where the chromatic dispersion has a negative slope at 
approximately 1550 nm and the core diameter is larger than 2.1 .mu.m and 
smaller than 2.3 .mu.m. Dispersion of the optical transmission path can be 
effectively compensated and optical signals having a small dispersion with 
respect to wavelengths can be readily received by inserting this optical 
fiber as the dispersion compensating optical fiber into the existing 1300 
nm zero dispersion optical fiber network and carrying out wavelength 
division multiplex transmission using the wavelength of approximately 1550 
nm. 
FIG. 3 shows the data of actual measurement of the induced Brillouin 
scattering of the dispersion compensating optical fiber made in the 
present embodiment. The measuring apparatus for this induced Brillouin 
scattering is shown in the graph for reference. In FIG. 3, the horizontal 
axis indicates the input power level and the vertical axis indicates the 
power level of the backscatter. As known from the experimental data, the 
induced Brillouin scattering occurs when the input power level is 8 dBm, 
and the threshold value of 8 dBm at which the induced Brillouin scattering 
will occur is similar to that of the conventional typical dispersion 
compensating optical fiber having the positive dispersion slope. It is 
also known that the induced Brillouin scattering of the dispersion 
compensating optical fiber in the present embodiment is not more worsened 
than the conventional, the input power can be sufficiently increased in a 
range where the backscatter is not caused, and practical applicability is 
fully satisfied. 
Numerous other emobidiments may be envisaged without departing from the 
spirit and scope of the invention.