U.S. patent application number 10/114964 was filed with the patent office on 2002-12-05 for connection structure for dispersion compensating optical fiber.
Invention is credited to Aikawa, Kazuhiko, Himeno, Kuniharu, Shimizu, Shogo, Suzuki, Takaaki.
Application Number | 20020181885 10/114964 |
Document ID | / |
Family ID | 26613029 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020181885 |
Kind Code |
A1 |
Suzuki, Takaaki ; et
al. |
December 5, 2002 |
Connection structure for dispersion compensating optical fiber
Abstract
In fusion-splicing a dispersion compensating optical fiber
having a negative dispersion slope, with a connection optical fiber
having a different near field pattern from that of the dispersion
compensating optical fiber, if for the connection optical fiber,
one is selected such that a theoretical joint loss in a used
wavelength, obtained from an overlap integral of a near field
pattern of the dispersion compensating optical fiber after fusion
splicing and a near field pattern of the connection optical fiber
after fusion splicing is presumed to be 0.3 dB or less, in an
unconnected state, a construction enabling connection at a low loss
results.
Inventors: |
Suzuki, Takaaki;
(Sakura-shi, JP) ; Shimizu, Shogo; (Sakura-shi,
JP) ; Aikawa, Kazuhiko; (Sakura-shi, JP) ;
Himeno, Kuniharu; (Sakura-shi, JP) |
Correspondence
Address: |
BELL, BOYD & LLOYD, LLC
PO BOX 1135
CHICAGO
IL
60690-1135
US
|
Family ID: |
26613029 |
Appl. No.: |
10/114964 |
Filed: |
April 1, 2002 |
Current U.S.
Class: |
385/55 |
Current CPC
Class: |
G02B 6/02261 20130101;
G02B 6/2552 20130101; G02B 6/0365 20130101; G02B 6/03644 20130101;
G02B 6/03627 20130101 |
Class at
Publication: |
385/55 |
International
Class: |
G02B 006/38 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2001 |
JP |
P2001-104928 |
Mar 14, 2002 |
JP |
P2002-070833 |
Claims
What is claimed is:
1. A connection structure for a dispersion compensating optical
fiber obtained by fusion-splicing a dispersion compensating optical
fiber having a negative dispersion slope with a connection optical
fiber having a different near field pattern from that of the
dispersion compensating optical fiber, wherein said connection
optical fiber has a near field pattern such that a theoretical
joint loss in a used wavelength obtained from an overlap integral
of a near field pattern of said dispersion compensating optical
fiber after fusion splicing and a near field pattern of said
connection optical fiber after fusion splicing is 0.3 dB or less,
in an unconnected state.
2. A connection structure for a dispersion compensating optical
fiber according to claim 1, wherein an effective core area of said
connection optical fiber in the unconnected state is larger than
that of said dispersion compensating optical fiber in the
unconnected state.
3. A connection structure for a dispersion compensating optical
fiber according to claim 1, wherein an expansion rate of an
effective core area due to heating of said connection optical fiber
is smaller than that of an effective core area due to heating of
said dispersion compensating optical fiber.
4. A connection structure for a dispersion compensating optical
fiber according to claim 1, wherein said dispersion compensating
optical fiber is connected to one end of said connection optical
fiber, and an other end of said connection optical fiber is
connected to a single mode optical fiber having a positive
dispersion slope which is compensated by said dispersion
compensating optical fiber.
5. A connection structure for a dispersion compensating optical
fiber according to claim 4, wherein when an effective core area of
said dispersion compensating optical fiber in the unconnected state
is designated as A, an effective core area of said connection
optical fiber in the unconnected state is designated as B, and an
effective core area of said single mode optical fiber in the
unconnected state is designated as C, a relation thereof is
A<B<C.
6. A connection structure for a dispersion compensating optical
fiber according to claim 4, wherein when an expansion rate of an
effective core area due to heating of said dispersion compensating
optical fiber is designated as D, an expansion rate of an effective
core area due to heating of said connection optical fiber is
designated as E, and an expansion rate of an effective core area
due to heating of said single mode optical fiber is designated as
F, a relation thereof is F<E<D.
7. A connection structure for a dispersion compensating optical
fiber according to claim 4, wherein an effective core area in a
used wavelength of said single mode optical fiber is from 100 to
150 .mu.m.sup.2.
8. A connection structure for a dispersion compensating optical
fiber according to claim 4, wherein an effective core area in a
used wavelength of said single mode optical fiber is from 55 to 90
.mu.m.sup.2.
9. A connection structure for a dispersion compensating optical
fiber according to claim 1, wherein said connection optical fiber
comprises a core and a cladding provided on a circumference of said
core, and fluorine is added to said cladding.
10. A connection structure for a dispersion compensating optical
fiber according to claim 9, wherein the addition of the fluorine is
at least 0.6% by weight.
11. A connection structure for a dispersion compensating optical
fiber according to claim 9, wherein said cladding comprises at
least two layers, and an outermost layer of said cladding comprises
pure silica.
12. A connection structure for a dispersion compensating optical
fiber according to claim 1, wherein said dispersion compensating
optical fiber comprises a center core, a side core provided on a
circumference of said center core, and a cladding provided on a
circumference of said side core, and wherein a refractive index of
said center core is higher than that of said cladding, and a
refractive index of said side core is lower than that of said
cladding.
13. A connection structure for a dispersion compensating optical
fiber according to claim 12, wherein an effective core area in a
used wavelength of said dispersion compensating optical fiber is 16
.mu.m.sup.2 or larger.
14. A connection structure for a dispersion compensating optical
fiber according to claim 1, wherein said dispersion compensating
optical fiber comprises a center core, a side core provided on a
circumference of said center core, a ring core provided on a
circumference of said side core, and a cladding provided on a
circumference of said ring core, and wherein a refractive index of
said center core and said ring core is respectively higher than
that of said cladding, and a refractive index of said side core is
lower than that of said cladding.
15. A connection structure for a dispersion compensating optical
fiber according to claim 14, wherein an effective core area in a
used wavelength of said dispersion compensating optical fiber is 18
.mu.m.sup.2 or larger.
16. A connection structure for a dispersion compensating optical
fiber according to claim 15, wherein said dispersion compensating
optical fiber has characteristics shown in the following (a-1) to
(d-1): (a-1) a chromatic dispersion value in the used wavelength is
from -60 to -45 ps/nm/km; (b-1) a dispersion slope in the used
wavelength is from -0.180 to -0.135 ps/nm.sup.2/km; (c-1) the
effective core area in the used wavelength is from 20 to 26
.mu.m.sup.2; and (d-1) a transmission loss in the used wavelength
is 0.35 dB/km or less.
17. A connection structure for a dispersion compensating optical
fiber according to claim 15, wherein said dispersion compensating
optical fiber has characteristics shown in the following (a-2) to
(d-2): (a-2) a chromatic dispersion value in the used wavelength is
from -100 to -80 ps/nm/km; (b-2) a dispersion slope in the used
wavelength is from -0.300 to -0.230 ps/nm.sup.2/km; (c-2) the
effective core area in the used wavelength is from 18 to 24
.mu.m.sup.2; and (d-2) a transmission loss in the used wavelength
is 0.40 dB/km or less.
18. A connection structure for a dispersion compensating optical
fiber according to claim 15, wherein said dispersion compensating
optical fiber has characteristics shown in the following (a-3) to
(d-3): (a-3) a chromatic dispersion value in the used wavelength is
from -45 to -35 ps/nm/km; (b-3) a dispersion slope in the used
wavelength is from -0.150 to -0.100 ps/nm.sup.2/km; (c-3) the
effective core area in the used wavelength is from 26 to 35
.mu.m.sup.2; and (d-3) a transmission loss in the used wavelength
is 0.25 dB/km or less.
19. A transmission line having a connection structure for a
dispersion compensating optical fiber as disclosed in any one of
claim 1 through claim 18.
20. A dispersion compensator having a connection structure for a
dispersion compensating optical fiber as disclosed in any one of
claim 1 through claim 18.
21. A connection method for a dispersion compensating optical fiber
for obtaining a connection structure for a dispersion compensating
optical fiber as disclosed in any one of claim 1 through claim 18.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a connection structure
between a single-mode optical fiber and a dispersion compensating
optical fiber, and more specifically, relates to a connection
structure between a single-mode optical fiber and a dispersion
compensating optical fiber having a negative dispersion slope.
[0003] 2. Description of Related Art
[0004] A transmission line which combines a single mode optical
fiber such as one for use at 1.3 .mu.m and a dispersion
compensating optical fiber for compensating the chromatic
dispersion thereof, has heretofore been put to practical use.
[0005] The single mode optical fiber has a relatively large
effective core area (Aeff), and hence nonlinear effects are
suppressed. However, if a 1.55 .mu.m band or the like is designated
as the wavelength region for use, chromatic dispersion increases.
Therefore, a low loss transmission line can be built up by
compensating the chromatic dispersion with the dispersion
compensating optical fiber.
[0006] Japanese Patent No. 2,951,562 discloses a structure for
connecting a normal single mode optical fiber and a dispersion
compensating optical fiber having a so-called W-type refractive
index distribution shape for compensating the chromatic dispersion
of the single mode optical fiber, via an intermediate optical fiber
interposed therebetween.
[0007] In this structure, the mode field diameter of the
intermediate optical fiber is made to have substantially the same
value as the mode field diameter of the dispersion compensating
optical fiber. Also, the mode field diameter of this intermediate
optical fiber on the single mode optical fiber side is expanded so
as to match the mode field diameter of the single mode optical
fiber.
[0008] As a result, the joint loss between the intermediate optical
fiber, the dispersion compensating optical fiber and the single
mode optical fiber is reduced. Expansion of the mode field diameter
is performed by heating the end of the intermediate optical fiber
to diffuse the dopant such as germanium added to the core
thereof.
[0009] Recently, with the development of wavelength multiplex
transmission, a dispersion compensating optical fiber which can
compensate not only the chromatic dispersion of the single mode
optical fiber but also the dispersion slope has been developed.
[0010] The dispersion slope is an inclination in the graph when the
wavelength is plotted on the X-axis and the chromatic dispersion is
plotted on the Y-axis, and the normal single mode optical fiber has
a positive dispersion slope. Therefore, if the dispersion slope of
the single mode optical fiber is compensated by the dispersion
compensating optical fiber having a negative dispersion slope, a
flat chromatic dispersion characteristic can be obtained in a
relatively wide wavelength region.
[0011] The normal single mode optical fiber referred to herein is
assumed to be one normally used for propagating an optical signal,
such as one for use at 1.3 .mu.m and a dispersion shifted optical
fiber.
[0012] Moreover, the dispersion compensating optical fiber is
preferably one capable of single mode propagation.
[0013] However, the dispersion compensating optical fiber having
such a negative slope has a different refractive index distribution
shape from that of the dispersion compensating optical fiber of a
type for compensating only the chromatic dispersion which has been
heretofore proposed. Hence even if the connection structure is
formed based on the mode field diameter as disclosed in Japanese
Patent No. 2,951,562, the joint loss cannot be reduced.
Particularly, when fusion splicing is performed, there is a
tendency for the increase in joint loss to become conspicuous.
[0014] As the dispersion compensating optical fiber, there has been
provided one having an expanded effective core area, in order to
prevent deterioration in the transmission quality due to the
nonlinear effect which occurs therein. In this dispersion
compensating optical fiber having an expanded effective core area,
there is a tendency for the increase in joint loss due to fusion
splicing to further increase.
BRIEF SUMMARY OF THE INVENTION
[0015] In view of the above problems, it is an object of the
present invention to provide a structure which can connect at a low
loss a dispersion compensating optical fiber of a dispersion slope
compensation type having a negative dispersion slope, with an other
optical fiber such as a connection optical fiber.
[0016] It is an other object of the present invention to provide a
structure which can perform connection at a low loss, in the case
where a dispersion compensating optical fiber having a negative
dispersion slope is connected to one end of this connection optical
fiber, and a single mode optical fiber whose dispersion slope is
compensated by the dispersion compensating optical fiber is
connected to the other end thereof.
[0017] As a result of intensive study by the present inventors to
solve the above described problems, it has been found that by using
an optical fiber of which the near field pattern after fusion
splicing matches with the near field pattern after fusion splicing
of the dispersion compensating optical fiber having a negative
dispersion slope, the joint loss between this optical fiber and the
dispersion compensating optical fiber can be reliably reduced.
[0018] That is to say, a first aspect of the present invention is a
connection structure for a dispersion compensating optical fiber
obtained by fusion-splicing a dispersion compensating optical fiber
having a negative dispersion slope with a connection optical fiber
having a different near field pattern from that of the dispersion
compensating optical fiber, wherein the connection optical fiber
has a near field pattern such that a theoretical joint loss in a
used wavelength obtained from an overlap integral of a near field
pattern of the dispersion compensating optical fiber after fusion
splicing and a near field pattern of the connection optical fiber
after fusion splicing is 0.3 dB or less, in an unconnected
state.
[0019] A second aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber,
an effective core area of the connection optical fiber in the
unconnected state is larger than that of the dispersion
compensating optical fiber in the unconnected state.
[0020] A third aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber,
an expansion rate of an effective core area due to heating of the
connection optical fiber is smaller than that of an effective core
area due to heating of the dispersion compensating optical
fiber.
[0021] A fourth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber,
the dispersion compensating optical fiber is connected to one end
of the connection optical fiber, and an other end of the connection
optical fiber is connected to a single mode optical fiber having a
positive dispersion slope which is compensated by the dispersion
compensating optical fiber.
[0022] A fifth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to the fourth aspect, when an effective core area of the
dispersion compensating optical fiber in the unconnected state is
designated as A, an effective core area of the connection optical
fiber in the unconnected state is designated as B, and an effective
core area of the single mode optical fiber in the unconnected state
is designated as C, a relation thereof is A<B<C.
[0023] A sixth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to either one of the fourth and fifth aspects, when an
expansion rate of an effective core area due to heating of the
dispersion compensating optical fiber is designated as D, an
expansion rate of an effective core area due to heating of the
connection optical fiber is designated as E, and an expansion rate
of an effective core area due to heating of the single mode optical
fiber is designated as F, a relation thereof is F<E<D.
[0024] A seventh aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to any one of the fourth to the sixth aspects, an
effective core area in a used wavelength of the single mode optical
fiber is from 100 to 150 .mu.m.sup.2.
[0025] An eighth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to any one of the fourth to the sixth aspects, an
effective core area in a used wavelength of the single mode optical
fiber is from 55 to 90 .mu.m.sup.2.
[0026] A ninth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber,
the connection optical fiber comprises a core and a cladding
provided on a circumference of the core, and fluorine is added to
the cladding.
[0027] A tenth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to the ninth aspect, the addition of the fluorine is at
least 0.6% by weight.
[0028] An eleventh aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to either one of the ninth and tenth aspects, the
cladding comprises at least two layers, and an outermost layer of
the cladding comprises pure silica.
[0029] A twelfth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber,
the dispersion compensating optical fiber comprises a center core,
a side core provided on a circumference of the center core, and a
cladding provided on a circumference of the side core, and a
refractive index of the center core is higher than that of the
cladding, and a refractive index of the side core is lower than
that of the cladding.
[0030] A thirteenth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to the twelfth aspect, an effective core area in a used
wavelength of the dispersion compensating optical fiber is 16
.mu.m.sup.2 or larger.
[0031] A fourteenth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to any one of the first to the eleventh aspects, the
dispersion compensating optical fiber comprises a center core, a
side core provided on a circumference of the center core, a ring
core provided on a circumference of the side core, and a cladding
provided on a circumference of the ring core, and a refractive
index of the center core and the ring core is respectively higher
than that of the cladding, and a refractive index of the side core
is lower than that of the cladding.
[0032] A fifteenth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to the fourteenth aspect, an effective core area in a
used wavelength of the dispersion compensating optical fiber is 18
.mu.m.sup.2 or larger.
[0033] A sixteenth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to the fifteenth aspect, the dispersion compensating
optical fiber has characteristics shown in the following (a-1) to
(d-1):
[0034] (a-1) a chromatic dispersion value in the used wavelength is
from -60 to -45 ps/nm/km;
[0035] (b-1) a dispersion slope in the used wavelength is from
-0.180 to -0.135 ps/nm/km;
[0036] (c-1) the effective core area in the used wavelength is from
20 to 35 .mu.m.sup.2; and
[0037] (d-1) a transmission loss in the used wavelength is 0.35
dB/km or less.
[0038] A seventeenth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to the fifteenth aspect of the present invention, the
dispersion compensating optical fiber has characteristics shown in
the following (a-2) to (d-2):
[0039] (a-2) a chromatic dispersion value in the used wavelength is
from -100 to -80 ps/nm/km;
[0040] (b-2) a dispersion slope in the used wavelength is from
-0.300 to -0.230 ps/nm.sup.2/km;
[0041] (c-2) the effective core area in the used wavelength is from
18 to 24 .mu.m.sup.2; and
[0042] (d-2) a transmission loss in the used wavelength is 0.40
dB/km or less.
[0043] An eighteenth aspect of the present invention is that in the
connection structure for a dispersion compensating optical fiber
according to the fifteenth aspect of the present invention, the
dispersion compensating optical fiber has characteristics shown in
the following (a-3) to (d-3):
[0044] (a-3) a chromatic dispersion value in the used wavelength is
from -45 to -35 ps/nm/km;
[0045] (b-3) a dispersion slope in the used wavelength is from
-0.150 to -0.100 ps/nm.sup.2/km;
[0046] (c-3) the effective core area in the used wavelength is from
26 to 35 .mu.m.sup.2; and
[0047] (d-3) a transmission loss in the used wavelength is 0.25
dB/km or less.
[0048] A nineteenth aspect of the present invention is a
transmission line having the above described connection structure
for a dispersion compensating optical fiber.
[0049] A twentieth aspect of the present invention is a dispersion
compensator having the above described connection structure for a
dispersion compensating optical fiber.
[0050] A twenty-first aspect of the present invention is a
connection method for a dispersion compensating optical fiber for
obtaining the above described connection structure for a dispersion
compensating optical fiber.
[0051] According to the above aspects, by using the connection
optical fiber whose near field pattern after fusion splicing is
matched, the dispersion compensating optical fiber having a
negative dispersion slope can be connected with the connection
optical fiber at low loss.
[0052] As a result, the dispersion compensating optical fiber can
be connected with a single mode optical fiber which is compensated
by this dispersion compensating optical fiber via the connection
optical fiber, at low loss.
[0053] Also in the cladding of the connection optical fiber, the
joint loss with a single mode optical fiber having a particularly
large effective core area can be reduced, by forming at least a
layer adjacent to the core from a fluorine-added silica glass.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0054] FIG. 1 is a graph showing changes in the near field pattern
of a dispersion compensating optical fiber due to heating;
[0055] FIG. 2 is a graph showing a W-type refractive index
distribution shape, as one example of a single mode optical fiber
suitable for the connection structure of the present invention;
[0056] FIG. 3 is a graph showing a W-type refractive index
distribution shape with a segment core, as one example of a
dispersion compensating optical fiber suitable for the connection
structure of the present invention; and
[0057] FIG. 4 is a graph showing near field patterns of various
optical fibers.
DETAILED DESCRIPTION OF THE INVENTION
[0058] One example of the present invention will be described
below.
[0059] In the present invention, for the used wavelength, a
suitable wavelength region is preferably selected from 1.53 to 1.63
.mu.m, from the standpoint of transmission characteristics. In the
case of performing wavelength multiplex transmission, a relatively
wide wavelength region is selected.
[0060] FIG. 1 is a graph showing changes in the near field pattern
at the time of fusion splicing of a dispersion compensating optical
fiber having a negative dispersion slope. The near field pattern
can be measured by a method described in G.650 specified in the
ITUT Standard. Specifically, for example, the near field pattern
can be obtained by measuring a far field pattern and subjecting
this measurement result to an inverse Fourier transform. The near
field pattern is expressed by an optical power distribution.
[0061] In this graph, the X-axis plots a radius of the optical
fiber, and the Y-axis plots the optical power observed at the time
of measurement. The scale in the Y-axis is dBm, which is normalized
by the optical power.
[0062] In the graph, 0, 1000 ms (milli-seconds) and 1800 ms show
heating time at the time of fusion splicing. Conditions such as the
temperature, other than at the time of heating, are constant. Here
0 shows a near field pattern before fusion splicing.
[0063] As shown in this graph, in the dispersion compensating
optical fiber having a negative dispersion slope, the near field
pattern is easily changed due to heating at the time of fusion
splicing.
[0064] In the normal optical fiber, such a change in the near field
pattern occurs. However, the change in the dispersion compensating
optical fiber having the negative dispersion slope is
conspicuous.
[0065] FIG. 2 and FIG. 3 are explanatory diagrams respectively
showing examples of refractive index distribution shapes of
dispersion compensating optical fibers having negative dispersion
slopes.
[0066] FIG. 2 shows a so-called W-type refractive index
distribution shape, and in this refractive index distribution
shape, a core 13 is formed of a center core 11 provided in a center
and a side core 12 provided in a concentric circular shape on the
circumference thereof, and a cladding 15 is provided in a
concentric circular shape on the circumference of the core.
[0067] The relation between refractive indexes of these is such
that the refractive index of the center core 11 is higher than that
of the cladding 15, and the refractive index of the side core 12 is
lower than that of the cladding 15.
[0068] The refractive index is adjusted by the addition of a dopant
such as germanium having an action of raising the refractive index,
or fluorine having an action of lowering the refractive index.
[0069] For example, the center core 11 comprises a germanium-added
silica glass, the side core 12 comprises a fluorine-added silica
glass, and the cladding 15 comprises a pure silica glass or a
fluorine-added silica glass.
[0070] The dispersion compensating optical fiber having a negative
chromatic dispersion and a negative dispersion slope can be
obtained by adjusting the relative index difference .DELTA..sub.1
of the center core 11 based on the cladding 15, the relative index
difference .DELTA..sub.2 of the side core 12 based on the cladding
15, and the ratio between the radius r.sub.1 of the center core 11
and the radius r.sub.2 of the side core 12.
[0071] FIG. 3 shows a so-called W-type refractive index
distribution shape with a segment, and this refractive index
distribution shape is formed by a core 23 comprising a central
center core 21, a side core 22, and a ring core 24 provided in a
concentric circular shape in order, and a cladding 25 provided in a
concentric circular shape on the circumference thereof.
[0072] The refractive index of the center core and the ring core 24
is respectively set higher than that of the cladding 25, and the
refractive index of the side core 22 is set lower than that of the
cladding 25.
[0073] In this example, the refractive index of the ring core 24 is
lower than that of the center core 21.
[0074] For example, the center core 21 and the ring core 24
comprise a germanium-added silica glass, the side core 22 comprises
a fluorine-added silica glass, and the cladding 25 comprises a pure
silica glass or a fluorine-added silica glass.
[0075] The dispersion compensating optical fiber having a negative
chromatic dispersion and a negative dispersion slope can be
obtained by adjusting the relative index difference .DELTA..sub.11
of the center core 21 based on the cladding 25, the relative index
difference .DELTA..sub.12 of the side core 22 based on the cladding
25, the relative index difference .DELTA..sub.13 of the ring core
24 based on the cladding 25, the ratio between the radius r.sub.11
of the center core 21 and the radius r.sub.12 of the side core 22
and the ratio between the radius r.sub.11 of the center core 21 and
the radius r.sub.13 of the ring core 24.
[0076] As a result of the study by the present inventors, in the
dispersion compensating optical fiber having a negative dispersion
slope, the reason why the change in the near field pattern due to
heating is conspicuous is as described below.
[0077] A dopant having an action of raising the refractive index,
such as germanium, is generally added not only to the dispersion
compensating optical fiber having a negative dispersion slope but
also to the core of the optical fiber. Since germanium is generally
used preferably, the description below will be made taking
germanium as an example.
[0078] Also, when an optical fiber is drawn from the base material
of the fiber, and cooled and solidified, a stress resulting from a
difference in the glass transition point of the constituent
materials for each layer which constitutes the optical fiber, is
frozen in the optical fiber (mainly in the core). This is referred
to as residual stress.
[0079] Then, if the optical fiber is heated, germanium added to the
core diffuses towards the cladding. The residual stress is also
released due to softening of the constituent material by
heating.
[0080] As described above, since germanium has an action of raising
the refractive index, the effective refractive index of the core
decreases due to the diffusion of germanium. The effective
refractive index of the core also decreases due to release of the
residual stress. As a result, confinement of light in the core
becomes weak, thereby expanding the effective core area and also
expanding the near field pattern.
[0081] In the dispersion compensating optical fiber having a
negative dispersion slope, the relative index differences
.DELTA..sub.1, .DELTA..sub.11 of the center cores 11 and 21 shown
in FIGS. 2 and 3 are respectively for example 1.0% or higher, which
is relatively high. Therefore, at the time of drawing the optical
fiber from the base material of the fiber, it is necessary to
suppress an increase in the transmission loss by increasing the
drawing tension, and this tension becomes the residual stress.
Accordingly, the residual stress increases.
[0082] On the circumference of the center cores 11 and 21, there
are generally provided side cores 12, 22 added with fluorine. If
fluorine is added in a layer adjacent to a layer comprising a
germanium-added silica glass, diffusion of germanium is
promoted.
[0083] Since the large residual stress is released by heating at
the time of fusion splicing, and germanium in a relatively large
amount is readily diffused, a decrease in the effective refractive
index becomes large.
[0084] When the effective refractive index of the center cores 11,
21 decreases, confinement of the light in the cores 13, 23 is
weakened, thereby expanding the effective core area and the near
field pattern outwards of the cores 13, 23, respectively. When
heating is continued, and expansion of the near field pattern
progresses, and coupling between the propagation mode propagating
the cores 13, 23 and the cladding mode occurs, thereby further
increasing the joint loss.
[0085] Therefore, in the dispersion compensating optical fiber
having a negative dispersion slope and the connection optical fiber
connected thereto, even in the case where the mode field diameter
before the fusion splicing is relatively small, since the near
field pattern of the dispersion compensating optical fiber largely
changes after the fusion splicing, the joint loss increases.
[0086] The present inventors therefore have tried to reduce the
joint loss between the dispersion compensating optical fiber having
a negative dispersion slope and the connection optical fiber, by
matching the near field patterns after fusion splicing, to each
other.
[0087] As a result, it has been found that even in the case where
the dispersion compensating optical fiber and the connection
optical fiber having different near field patterns from each other
are to be connected, the joint loss can be reduced by matching the
near field patterns after fusion splicing, with each other, as
described below.
[0088] For example, connection at a low loss can be realized by
using a connection optical fiber in which the theoretical joint
loss obtained from the overlap integral of the near field pattern
of the dispersion compensating optical fiber after fusion splicing
and the near field pattern of the connection optical fiber after
fusion splicing, is presumed to be 0.3 dB or less, and preferably,
0.1 dB or less. If the theoretical joint loss exceeds 0.3 dB, there
is a problem in that the joint loss increases. The theoretical
joint loss is preferable as small as possible, but from the
standpoint of joint loss with the single mode optical fiber
described below, which is compensated by the dispersion
compensating optical fiber, the theoretical joint loss is
substantially 0.05 dB or higher.
[0089] The theoretical joint loss can be estimated from the
refractive index distribution shape, the diffusion rate of the
dopant, and the size of the residual stress. Actually, it is
preferable to confirm the theoretical joint loss by performing a
preliminary experiment.
[0090] The theoretical joint loss can be obtained in the following
manner.
[0091] At first, one end of the dispersion compensating optical
fiber is heated in the unconnected state under the same conditions
as at the time of fusion splicing. The connection optical fiber
connected to this dispersion compensating optical fiber is
similarly heated in the unconnected state.
[0092] With respect to the dispersion compensating optical fiber
and the connection optical fiber, the power distribution of the
light which enters from one end and exits from the other end is
measured by a far field pattern measuring device, using a method
specified in the above described ITUT Standard, and the measurement
result is subjected to an inverse Fourier transform to thereby
obtain a near field pattern.
[0093] By multiplying the thus obtained near field pattern by 1/2,
the electric field intensity distribution can be obtained.
[0094] If the electric field intensity is substituted in expression
1 described below, the theoretical joint loss obtained from the
overlap integral of these near field patterns can be
calculated.
[0095] Generally, if the near field patterns are the same, the
theoretical joint loss is zero, and as the near field pattern
becomes similar, the theoretical joint loss becomes smaller. 1 - 10
log 10 { E 1 ( r ) E 2 ( r ) r r } 2 E 1 2 ( r ) r r E 2 2 ( r ) r
r ( dB ) ( 1 )
[0096] In expression 1, E.sub.1(r) and E.sub.2(r) respectively
denote the electric field intensity of the dispersion compensating
optical fiber and the connection optical fiber.
[0097] As for the near field pattern of the connection optical
fiber, a connection optical fiber in which the near field pattern
does not change largely due to heating, compared to the near field
pattern of the dispersion compensating optical fiber is preferably
selected.
[0098] In such a connection optical fiber, the conditions for the
theoretical joint loss can be easily satisfied, by selecting a
connection optical fiber having a near field pattern before fusion
splicing which is close to the near field pattern after fusion
splicing of the dispersion compensating optical fiber, or a
connection optical fiber having a near field pattern slightly
smaller than the near field pattern after fusion splicing of the
dispersion compensating optical fiber, taking into consideration
that the dopant such as germanium added to the core diffuses due to
heating to thereby expand the near field pattern slightly.
[0099] In order to easily satisfy such conditions for the
theoretical joint loss, the dispersion compensating optical fiber
and the connection optical fiber preferably satisfy the following
conditions.
[0100] That is to say, the effective core area of the connection
optical fiber in the unconnected state is preferably larger than
that of the dispersion compensating optical fiber in the
unconnected state.
[0101] The difference thereof is, for example, from 1 to 35
.mu.m.sup.2, and preferably, from 2 to 25 .mu.m.sup.2. If the
difference is less than 1 .mu.m.sup.2, sufficient effect cannot be
obtained, and if the difference exceeds 35 .mu.m.sup.2, there is
the possibility that the joint loss increases.
[0102] Also, the expansion rate of the effective core area of the
connection optical fiber due to heating is preferably smaller than
that of the effective core area of the dispersion compensating
optical fiber due to heating. If the connection optical fiber has a
larger expansion rate than the dispersion compensating optical
fiber, bonding to the cladding mode is likely to occur before the
near field patterns are matched, and hence the joint loss tends to
increase.
[0103] The expansion rate of the effective core area due to heating
can be measured in the following manner. That is, the size of the
effective core area is obtained by the far field pattern measuring
device as described above, using the heating time as a parameter,
under a predetermined heating temperature condition.
[0104] This expansion rate is preferably such that when values of
the expansion rate under certain heating conditions are compared,
the ratio of the expansion rate of the dispersion compensating
optical fiber to the expansion rate of the connection optical fiber
is from 1.1 to 8.0, and more preferably, from 1.2 to 7.0. If it is
below 1.1, sufficient effect cannot be obtained, and if it exceeds
8.0, control of the heating time at the time of connection may be
difficult.
[0105] Since the expansion rate largely depends on the heating
conditions (particularly, on the heating temperature), it is
necessary to compare values under the same heating conditions.
[0106] From the standpoint of reducing the joint loss, it is
preferable to use a connection optical fiber in which even if it is
heated for a long period of time at the time of fusion splicing,
and the dopant such as germanium added to the core diffuses, the
propagation mode propagating the core is hardly bonded with the
cladding mode. Therefore, it is preferable to select one having a
large difference in the propagation constant (.DELTA..beta.)
between the propagation mode and the cladding mode. .DELTA..beta.
is set to be, for example, 8000 (rad/m) or larger, and preferably,
9000 (rad/m) or larger.
[0107] The connection optical fiber includes one comprising for
example a center core, a side core provided on the circumference
thereof, and a cladding provided on the circumference of the side
core and having a so-called dual type refractive index distribution
shape in which the refractive index decreases in order from the
center core, the side core and the cladding. In this dual type
refractive index distribution shape, for example, the center core
and the side core are formed of a germanium-added silica glass or
the like. The cladding will be described later.
[0108] Since the connection optical fiber is appropriately selected
corresponding to the dispersion compensating optical fiber,
conditions such as the structural parameters are not particularly
limited. However, the relative index difference of the center core
based on the cladding is preferably 1.0% or less, and substantially
at least 0.5%, from the standpoint of suppressing changes in the
near field pattern due to heating.
[0109] Also the relative index difference of the side core based on
the cladding is preferably from 0.07 to 0.2%, from the standpoint
of having a suitable cutoff wavelength, having a suitable near
field pattern and reducing bend loss.
[0110] Moreover, the ratio of the radius of the side core to the
radius of the center core is preferably from 2.5 to 4.0, from the
standpoint of having a suitable cutoff wavelength, having a
suitable near field pattern and reducing bend loss.
[0111] Furthermore, there can be exemplified a connection optical
fiber having a refractive index distribution shape similar to that
shown in FIG. 3, and comprising a center core, a side core provided
on the circumference of the center core, a ring core provided on
the circumference of the side core, and a cladding provided on the
circumference of the ring core, wherein the refractive index of the
center core and the ring core is respectively higher than that of
the cladding, and the refractive index of the side core is higher
than that of the cladding and lower than that of the center core
and the ring core.
[0112] In this refractive index distribution shape, for example,
the center core and the ring core are formed of a germanium-added
silica glass, and the side core comprises a pure silica glass, a
germanium-added silica glass, or a fluorine-added silica glass. The
cladding will be described later.
[0113] Since the connection optical fiber is appropriately selected
corresponding to the dispersion compensating optical fiber,
conditions such as the structural parameters are not particularly
limited. However, the relative index difference of the center core
based on the cladding is preferably 1.2% or less, and substantially
0.8% or more, from the standpoint of suppressing changes in the
near field pattern due to heating.
[0114] Also the relative index difference of the side core based on
the cladding is preferably from +0.05 to +0.10%, from the
standpoint of having a suitable cutoff wavelength, having a
suitable near field pattern and reducing bend loss.
[0115] The relative index difference of the ring core based on the
cladding is preferably from 0.2 to 0.4%, from the standpoint of
having a suitable cutoff wavelength, having a suitable near field
pattern and reducing bend loss.
[0116] The ratio of the radius of the side core to the radius of
the center core is preferably from 3.2 to 3.8, from the standpoint
of having a suitable cutoff wavelength, having a suitable near
field pattern and reducing bend loss.
[0117] Also, the ratio of the radius of the ring core to the radius
of the center core is preferably from 4.0 to 5.0, from the
standpoint of having a suitable cutoff wavelength, having a
suitable near field pattern and reducing bend loss.
[0118] The conditions for the fusion splicing between the
connection optical fiber and the dispersion compensating optical
fiber are not particularly limited, but for example, a temperature
of from 1800 to 2300.degree. C. and a period of from 0.8 to 3
seconds are preferable. It is also preferable to perform fusion
splicing, while shining light thereon and monitoring the optical
characteristic. Values of the expansion rate are compared under the
same heating conditions, as described above.
[0119] One end of the connection optical fiber is connected to the
dispersion compensating optical fiber, but the other end thereof is
preferably fusion spliced with a single mode optical fiber having a
positive chromatic dispersion and dispersion slope compensated by
this dispersion compensating optical fiber. The used length of the
connection optical fiber at this time is, for example, 50 cm or
more, and preferably not larger than 20 m. If the length is less
than 50 cm, the length is not sufficient, and hence the operability
of the fusion splice may drop, or light corresponding to the loss
occurring at the joint may be combined with the mode propagating
the core to cause noise. If the length exceeds 20 m, a problem may
occur from the standpoint of the transmission characteristics.
[0120] For this single mode optical fiber, there is used one having
positive chromatic dispersion and dispersion slope, and having an
effective core area of preferably at least 55 .mu.m.sup.2, and more
preferably at least 80 .mu.m.sup.2, in the used wavelength. Normal
single mode optical fibers such as one for use at 1.3 .mu.m, and a
dispersion shifted optical fiber, which have an effective core area
of from 55 to 90 .mu.m.sup.2, may be used, but one having an
effective core area of from 100 to 150 .mu.m.sup.2, and preferably
from 120 to 140 .mu.m.sup.2 is particularly preferable. A single
mode optical fiber having an effective core area in this range
hardly causes any nonlinear effects, and contributes to an
improvement of the transmission characteristics.
[0121] A single mode optical fiber having a large effective core
area includes an optical fiber having a so-called W-type refractive
index distribution shape, which comprises, for example, a center
core, a side core provided on the circumference of the center core
and having a refractive index lower than that of the center core,
and a cladding provided on the circumference of the side core and
having a refractive index higher than that of the side core.
[0122] In order to expand the effective core area, it is preferable
that the relative index difference of the center core based on the
cladding is from 0.2 to 0.25%, the relative index difference of the
side core based on the cladding is from -0.02 to -0.07%, and the
ratio of the diameter of the side core to the diameter of the
center core is from 3.5 to 4.5.
[0123] Which one of the single mode optical fiber having an
effective core area of from 55 to 90 .mu.m.sup.2, or the single
mode optical fiber having an effective core area of from 100 to 150
.mu.m.sup.2 is to be used, is appropriately judged, depending on
the application thereof and the required characteristics.
[0124] In the dispersion compensating optical fiber, the connection
optical fiber and the single mode optical fiber, when the effective
core area of the dispersion compensating optical fiber in the
unconnected state is designated as A, the effective core area of
the connection optical fiber in the unconnected state is designated
as B, and the effective core area of the single mode optical fiber
in the unconnected state is designated as C, the relation thereof
is preferably A<B<C.
[0125] Also, when the expansion rate of the effective core area due
to heating of the dispersion compensating optical fiber is
designated as D, the expansion rate of the effective core area due
to heating of the connection optical fiber is designated as E, and
the expansion rate of the effective core area due to heating of the
single mode optical fiber is designated as F, the relation thereof
is preferably F<E<D. The measurement method of the expansion
rate is as described above. Values of the expansion rate should be
compared under the same heating conditions, as described above.
[0126] As described above, in order to connect the dispersion
compensating optical fiber and the connection optical fiber at a
low loss, it is necessary that A<B and E<D. Generally, the
relation of A<C is established herein. Therefore, in order to
connect the dispersion compensating optical fiber, the connection
optical fiber and the single mode optical fiber all at a low loss,
it is necessary that B<C and F<E.
[0127] The difference between B and C is designated herein as from
15 to 130 .mu.m.sup.2, and preferably from 20 to 120 .mu.m.sup.2.
If the difference is less than 15 .mu.m.sup.2, control of the
heating conditions at the time of connection becomes difficult, and
if it exceeds 130 .mu.m.sup.2, the joint loss between the
connection optical fiber and the single mode optical fiber may not
be reduced sufficiently. Moreover, the ratio of E to F (E/F) is
designated as from 2 to 15, and preferably from 2.5 to 10. If the
ratio is less than 2, bonding with the cladding mode occurs in the
connection optical fiber, before the joint loss between the
connection optical fiber and the single mode optical fiber is
reduced. Hence, connection at a low loss may not be realized. If
the ratio exceeds 15, control of the heating conditions at the time
of connection may be difficult.
[0128] There is a tendency for the connection optical fiber to have
an effective core area similar to that of the dispersion
compensating optical fiber, in order to satisfy the above described
near field pattern condition, and hence it has a large difference
in the effective core area from that of the single mode optical
fiber of the nonlinear effect suppression type (effective core area
expansion type), as described above. The effective core area of the
dispersion compensating optical fiber having a negative dispersion
slope is, for example, approximately from 16 to 35 .mu.m.sup.2.
[0129] Therefore, at the time of fusion splicing with the single
mode optical fiber, it is desirable that the connection optical
fiber be heated for a relatively long period of time to diffuse the
dopant such as germanium added to the core as much as possible, to
thereby expand the effective core area, in order to reduce the
joint loss between the connection optical fiber and the single mode
optical fiber.
[0130] At this time, in the case where the outer layer adjacent to
the core (on the circumference of the core) comprises a
fluorine-added silica glass, diffusion of the dopant such as
germanium can be promoted, thereby enabling rapid expansion of the
effective core area.
[0131] The added amount of fluorine is preferably at least 0.6% by
weight, and more preferably, from 0.9 to 1.5% by weight. If the
amount is less than 0.6% by weight, the diffusion promotion effect
cannot be obtained. On the other hand, if it exceeds 1.5% by
weight, the near field pattern is likely to change at the time of
fusion splicing with the dispersion compensating optical fiber, and
hence a problem may occur.
[0132] However, if the dopant is added, the melting point of the
silica glass decreases. Therefore, there is a problem in that the
outer shape of the connection optical fiber may be deformed due to
heating for a long period of time.
[0133] Hence, it is preferable that the cladding is formed by at
least two layers, with the layer adjacent to the core being formed
of a fluorine-added silica glass, and the outermost layer being
formed of a pure silica glass, so as to promote diffusion of the
dopant and prevent deformation of the outer shape.
[0134] The outer diameter of each layer constituting the cladding
can be appropriately changed by means of the effective core areas
of the connection optical fiber and the single mode optical fiber.
However, it is generally preferable that the outer diameter of the
first layer of the cladding comprising a fluorine-added silica
glass, which is adjacent to the core, is generally from 45 to 70
.mu.m, and the outer diameter of the outermost layer of the
cladding is about 125 .mu.m.
[0135] The heating conditions for fusion splicing and the expansion
of the core are not particularly limited, but preferably, for
example, a temperature of from 1800 to 2300.degree. C. and a period
of from 10 to 30 seconds is preferable.
[0136] In the dispersion compensating optical fiber, it is
preferable from the standpoint of improving the transmission
characteristics that the effective core area is as large as
possible.
[0137] In the dispersion compensating optical fiber having a W-type
refractive index distribution shape as shown in FIG. 2, the
effective core area is preferably at least 16 .mu.m.sup.2
(substantially not larger than 20 .mu.m.sup.2).
[0138] In the dispersion compensating optical fiber having a W-type
refractive index distribution shape with a segment as shown in FIG.
3 there is a tendency for the effective core area to be expanded
more than the dispersion compensating optical fiber having the
W-type refractive index distribution shape as described above. The
effective core area of this dispersion compensating optical fiber
is preferably at least 18 .mu.m.sup.2 (substantially not larger
than 35 .mu.m.sup.2).
[0139] Moreover, in the dispersion compensating optical fiber used
in the present invention, the value of the dispersion slope changes
depending on other characteristics such as the chromatic dispersion
and the effective core area. Therefore, the value of the dispersion
slope is appropriately selected corresponding to the chromatic
dispersion and the dispersion slope of the single mode optical
fiber to be compensated by this dispersion compensating optical
fiber.
[0140] Specifically, in the present invention, for example a
dispersion compensating optical fiber having characteristics
described below is preferably used.
[0141] The dispersion compensating optical fiber of a first
embodiment has a W-type refractive index distribution shape with a
segment as shown in FIG. 3, wherein the chromatic dispersion value
is from -60 to -45 ps/nm/km, the dispersion slope is from -0.180 to
-0.135 ps/nm.sup.2/km, the effective core area is from 20 to 26
.mu.m.sup.2, and the transmission loss is not larger than 0.35
dB/km (substantially not smaller than 0.25 dB/km).
[0142] This dispersion compensating optical fiber has a large
chromatic dispersion value, and as a result, the transmission loss
is also small. It has also a characteristic that the dispersion
slope and the effective core area are large.
[0143] Preferably, r.sub.12/r.sub.11 is from 2.5 to 5.0,
r.sub.13/r.sub.11 is from 4.0 to 5.5, .DELTA..sub.11 is from 0.8 to
1.5%, .DELTA..sub.12 is from -0.3 to -0.45%, and .DELTA..sub.13 is
from 0.4 to 1.0%. It is preferable to combine and select values
satisfying the above described preferable characteristics from
these numerical ranges.
[0144] The dispersion compensating optical fiber in a second
embodiment has a W-type refractive index distribution shape with a
segment as shown in FIG. 3, wherein the chromatic dispersion value
is from -45 to -35 ps/nm/km, the dispersion slope is from -0.150 to
-0.100 ps/nm.sup.2/km, the effective core area is from 26 to 35
.mu.m.sup.2, and the transmission loss is not larger than 0.25
dB/km (substantially not smaller than 0.20 dB/km).
[0145] This dispersion compensating optical fiber has a smaller
chromatic dispersion value than the chromatic dispersion value of
the dispersion compensating optical fiber of a first embodiment,
and as a result, the transmission loss is further small.
Furthermore, it has also a characteristic that the dispersion slope
and the effective core area are further large.
[0146] In wavelength multiplex transmission, since it is important
that small transmission loss and large effective core area are
achieved, the above second embodiment is preferably adopted.
[0147] Preferably, r.sub.12/r.sub.11 is from 2.5 to 5.0,
r.sub.13/r.sub.11 is from 4.0 to 5.5, .DELTA..sub.11 is from 0.8 to
1.5%, .DELTA..sub.12 is from -0.3 to -0.45%, and .DELTA..sub.13 is
from 0.4 to 1.0%. It is preferable to combine and select values
satisfying the above described preferable characteristics from
these numerical ranges.
[0148] The dispersion compensating optical fiber in a third
embodiment has a W-type refractive index distribution shape with a
segment as shown in FIG. 3, wherein the chromatic dispersion value
is from -100 to -80 ps/nm/km, the dispersion slope is from -0.300
to -0.230 ps/nm.sup.2/km, the effective core area is from 18 to 24
.mu.m.sup.2, and the transmission loss is not larger than 0.40
dB/km (substantially not smaller than 0.31 dB/km).
[0149] This dispersion compensating optical fiber has a large
chromatic dispersion value, and as a result, the transmission loss
is relatively large. Moreover, in this dispersion compensating
optical fiber there is a tendency for the dispersion slope and the
effective core area to be slightly smaller, compared to the
dispersion compensating optical fiber of the first embodiment.
[0150] Preferably, r.sub.12/r.sub.11 is from 2.5 to 4.0,
r.sub.13/r.sub.11 is from 2.7 to 8.0, .DELTA..sub.11 is from 1.2 to
1.7%, .DELTA..sub.12 is from -0.25 to -0.45%, and .DELTA..sub.13 is
from 0.2 to 1.1%. It is preferable to combine and select values
satisfying the above described preferable characteristics from
these numerical ranges.
[0151] FIG. 4 is a graph showing an example of a near field pattern
of each optical fiber used for the connection structure of the
present invention.
[0152] No. 1 is a normal single mode optical fiber such as one for
use at 1.3 .mu.m (the effective core area being 80
.mu.m.sup.2).
[0153] No. 2 is a single mode optical fiber having the effective
core area of 135 .mu.m.sup.2.
[0154] No. 3 is a dispersion compensating optical fiber heated
under the same conditions as that of the fusion splice.
[0155] No. 4 is a near field pattern of a connection optical fiber
heated under the same conditions as that of the fusion splice.
[0156] The near field patterns of No. 3 and No. 4 coincide well
with each other, and the theoretical joint loss in this example is
0.08 dB.
[0157] In a transmission line having the connection structure of
the present invention, the dispersion compensating optical fiber
and the single mode optical fiber can be connected at a low loss
via the connection optical fiber, thereby enabling improvement of
the transmission characteristics.
[0158] The connection structure of the present invention can be
also applied to a dispersion compensator.
[0159] That is to say, the dispersion compensator is for providing
the dispersion compensating optical fiber in a module. For example,
this module accommodates the dispersion compensating optical fiber
wound in a cylindrical body inside a hexahedron housing or the
like.
[0160] In the present invention, for example a dispersion
compensator can be constructed in such a manner that connection
optical fibers as lead fibers are respectively fusion-spliced to
the opposite ends of the dispersion compensating optical fiber in a
housing, and these lead fibers are respectively pulled out from two
holes provided in the housing. By connecting a single mode optical
fiber used for a transmission line to these pulled out lead fibers,
a low-loss transmission line can be constructed.
[0161] The cylindrical body and the housing used for the dispersion
compensator are formed of, for example, a metal or ceramics, and
the size thereof can be appropriately changed depending on the
length of the dispersion compensating optical fiber.
[0162] In the transmission line and the dispersion compensator, the
dispersion compensating optical fiber, the single mode optical
fiber and the connection optical fiber (lead fiber) are used in the
form of an optical fiber strand in which a covering layer
comprising an ultraviolet hardening-type resin is provided on the
circumference of the outermost layer (cladding) consisting of a
silica glass, or in the form of an optical fiber core in which a
covering layer comprising a nylon or the like is further provided
on the circumference of the optical fiber strand.
EXAMPLES
[0163] The present invention will now be described in detail by way
of examples.
Example 1
[0164] A dispersion compensating optical fiber having a negative
dispersion slope and a single mode optical fiber compensated by
this dispersion compensating optical fiber were fusion-spliced to
the opposite ends of a connection optical fiber, and the near field
pattern and the joint loss were measured. The used wavelength was
1.55 .mu.m, and the outer diameters of the used optical fibers
(outer diameter of cladding) were respectively about 125 .mu.m.
Since the expansion rate of the effective core area due to heating
largely depends on the heating temperature, the values in this
example are shown as reference values. The heating conditions were
substantially the same.
[0165] The dispersion compensating optical fiber had a W-type
refractive index distribution shape with a segment as shown in FIG.
3, and the structural parameters and characteristics thereof were
as follows.
[0166] The center core and the ring core were made of a
germanium-added silica glass, the side core was made of a
fluorine-added silica glass, and the cladding was made of a pure
silica glass.
[0167] r.sub.11: 1.8 .mu.m
[0168] r.sub.12: 5.8 .mu.m
[0169] r.sub.13: 7.1 .mu.m
[0170] .DELTA..sub.11: 1.65%
[0171] .DELTA..sub.12: -0.35%
[0172] .DELTA..sub.13: 0.5%
[0173] Effective core area: 22 .mu.m.sup.2
[0174] Mode field diameter: 5.3 .mu.m
[0175] Chromatic dispersion: -93 ps/nm/km
[0176] Dispersion slope: -0.28 ps/nm.sup.2/km
[0177] Transmission loss: 0.33 dB/km
[0178] Cutoff wavelength: 1.7 .mu.m
[0179] Expansion rate of the effective core area due to heating:
7.0 .mu.m.sup.2/sec
[0180] The connection optical fiber had a W-type refractive index
distribution shape with a segment, and one having a near field
pattern different from that of the dispersion compensating optical
fiber before fusion splicing was used (theoretical joint loss
before fusion splicing was 0.7 dB). The cladding had a one-layer
structure, and was formed of a pure silica glass. The structural
parameters and characteristics thereof were as follows.
[0181] The center core was made of a germanium-added silica glass,
the side core was made of a silica glass in which fluorine and
germanium were co-added, and the ring core was made of a
germanium-added silica glass.
[0182] Relative index difference of the center core based on the
cladding: 1.3%
[0183] Relative index difference of the side core based on the
cladding: 0.01%
[0184] Relative index difference of the ring core based on the
cladding: 0.35%
[0185] Radius of the center core: 1.2 .mu.m
[0186] Radius of the side core: 8.3 .mu.m
[0187] Radius of the ring core: 9.4 .mu.m
[0188] Mode field diameter: 6.2 .mu.m
[0189] Effective core area: 24.5 .mu.m.sup.2
[0190] Expansion rate of the effective core area due to heating:
3.0 .mu.m.sup.2/sec
[0191] The single mode optical fiber had a W-type refractive index
distribution shape with a segment as shown in FIG. 3, and the
structural parameters and characteristics thereof were as
follows.
[0192] The center core was made of a germanium-added silica glass,
the side core was made of a fluorine-added silica glass, and the
cladding was made of a pure silica glass.
[0193] Relative index difference of the center core based on the
cladding: 0.25%
[0194] Relative index difference of the side core based on the
cladding: -0.05%
[0195] Radius of the center core: 6.8 .mu.m
[0196] Radius of the side core: 27 .mu.m
[0197] Mode field diameter: 12.7 .mu.m
[0198] Effective core area: 135 .mu.m.sup.2
[0199] Chromatic dispersion: 20 ps/nm/km
[0200] Dispersion slope: 0.06 ps/nm.sup.2/km
[0201] Transmission loss: 0.19 dB/km
[0202] Cutoff wavelength: 1.6 .mu.m
[0203] Expansion rate of the effective core area due to heating:
2.0 .mu.m.sup.2/sec
[0204] A fusion splicing machine was used for connection of the
connection optical fiber with the dispersion compensating optical
fiber, to heat the connection optical fiber at about 2200.degree.
C. for 2 seconds. The mode field diameter of the dispersion
compensating optical fiber after fusion splicing was 5.9 .mu.m. The
mode field diameter of the connection optical fiber after fusion
splicing hardly changed.
[0205] A fusion splicing machine was used for connection of the
connection optical fiber with the single mode optical fiber, to
heat the connection optical fiber at about 2200.degree. C. for 2
seconds for effecting fusion splicing, and thereafter, the end of
the connection optical fiber was further heated for 30 seconds to
thereby diffuse the germanium.
[0206] The theoretical joint loss obtained from the overlap
integral of the near field pattern of the dispersion compensating
optical fiber and the near field pattern of the connection optical
fiber was 0.11 dB. The actual measurement values of the joint loss
are shown in Table 1.
Example 1-A
[0207] As another single mode fiber, a single mode optical fiber
had a step type refractive index distribution shape, and the
structural parameters and characteristics thereof were as
follows.
[0208] Relative index difference of the core based on the cladding:
0.31%
[0209] Radius of the core: 4.5 .mu.m
[0210] Mode field diameter: 10.1 .mu.m
[0211] Effective core area: 83 .mu.m.sup.2
[0212] Chromatic dispersion: 16.5 ps/nm/km
[0213] Dispersion slope: 0.057 ps/nm.sup.2/km
[0214] Transmission loss: 0.196 dB/km
[0215] Cutoff wavelength: 1.2 .mu.m
[0216] Expansion rate of the effective core area due to heating:
1.1 .mu.m.sup.2/sec
[0217] A fusion splicing machine was used for connection of the
connection optical fiber with the single mode optical fiber, to
heat the connection optical fiber at about 2200.degree. C. for 2
seconds for effecting fusion splicing, and thereafter, the end of
the connection optical fiber was further heated for 20 seconds to
thereby diffuse the germanium. The actual measurement values of the
joint loss are shown in Table 1.
Example 1-B
[0218] As another single mode fiber, a single mode optical fiber
had a step type refractive index distribution shape, and the
structural parameters and characteristics thereof were as
follows.
[0219] Relative index difference of the core based on the cladding:
0.33%
[0220] Radius of the core: 4.78 .mu.m
[0221] Mode field diameter: 10.8 .mu.m
[0222] Effective core area: 90 .mu.m.sup.2
[0223] Chromatic dispersion: 18.2 ps/nm/km
[0224] Dispersion slope: 0.06 ps/nm.sup.2/km
[0225] Transmission loss: 0.196 dB/km
[0226] Cutoff wavelength: 1.34 .mu.m
[0227] Expansion rate of the effective core area due to heating:
1.1 .mu.m.sup.2/sec
[0228] A fusion splicing machine was used for connection of the
connection optical fiber with the single mode optical fiber, to
heat the connection optical fiber at about 2200.degree. C. for 2
seconds for effecting fusion splicing, and thereafter, the end of
the connection optical fiber was further heated for 25 seconds to
thereby diffuse the germanium. The actual measurement values of the
joint loss are shown in Table 1.
Example 1-C
[0229] As another single mode fiber, a single mode optical fiber
had a W-type refractive index distribution shape with a segment,
and the structural parameters and characteristics thereof were as
follows.
[0230] Relative index difference of the center core based on the
cladding: 0.5%
[0231] Relative index difference of the side core based on the
cladding: -0.11%
[0232] Relative index difference of the ring core based on the
cladding: 0.18%
[0233] Radius of the center core: 3.5 .mu.m
[0234] Radius of the side core: 5.9 .mu.m
[0235] Radius of the ring core: 7.9 .mu.m
[0236] Mode field diameter: 8.5 .mu.m
[0237] Effective core area: 55 .mu.m.sup.2
[0238] Chromatic dispersion: 3 ps/nm/km
[0239] Dispersion slope: 0.05 ps/nm.sup.2/km
[0240] Transmission loss: 0.210 dB/km
[0241] Cutoff wavelength: 1.25 .mu.m
[0242] Expansion rate of the effective core area due to heating:
1.1 .mu.m.sup.2/sec
[0243] A fusion splicing machine was used for connection of the
connection optical fiber with the single mode optical fiber, to
heat the connection optical fiber at about 2200.degree. C. for 2
seconds for effecting fusion splicing, and thereafter, the end of
the connection optical fiber was further heated for 10 seconds to
thereby diffuse the germanium. The actual measurement values of the
joint loss are shown in Table 1.
Example 2
[0244] A dispersion compensating optical fiber having a negative
dispersion slope and a single mode optical fiber compensated by
this dispersion compensating optical fiber were fusion-spliced to
the opposite ends of a connection optical fiber, and the near field
pattern and the joint loss were measured. The used wavelength was
1.55 .mu.m, and the outer diameters of the used optical fibers
(outer diameter of cladding) were respectively about 125 .mu.m.
Since the expansion rate of the effective core area due to heating
largely depends on the heating temperature, the values in this
example are shown as reference values. The heating conditions were
substantially the same.
[0245] The dispersion compensating optical fiber had a W-type
refractive index distribution shape with a segment as shown in FIG.
3, and the structural parameters and characteristics thereof were
as follows.
[0246] The center core and the ring core were made of a
germanium-added silica glass, the side core was made of a
fluorine-added silica glass, and the cladding was made of a
fluorine-added silica glass.
[0247] r.sub.11: 2.0 .mu.m
[0248] r.sub.12: 5.7 .mu.m
[0249] r.sub.13: 6.9 .mu.m
[0250] .DELTA..sub.11: 0.8%
[0251] .DELTA..sub.12: -0.37%
[0252] .DELTA..sub.13: 0.4%
[0253] Effective core area: 29 .mu.m.sup.2
[0254] Mode field diameter: 6.1 .mu.m
[0255] Chromatic dispersion: -40 ps/nm/km
[0256] Dispersion slope: -0.21 ps/nm.sup.2/km
[0257] Transmission loss: 0.228 dB/km
[0258] Cutoff wavelength: 1.5 .mu.m
[0259] Expansion rate of the effective core area due to heating:
8.3 .mu.m.sup.2/sec
[0260] The cladding in the connection optical fiber had a two-layer
structure, with the first layer adjacent to the core being formed
of a silica glass added with fluorine in an amount of 2% by weight
and the radius thereof was 25 .mu.m. The outermost layer (the
second layer) was formed of a pure silica glass.
[0261] Relative index difference of the center core based on the
first layer of the cladding: 1.9%
[0262] Relative index difference of the side core based on the
first layer of the cladding: 0.05%
[0263] Relative index difference of the ring core based on the
first layer of the cladding: 0.38%
[0264] Radius of the center core: 1.9 .mu.m
[0265] Radius of the side core: 6.9 .mu.m
[0266] Radius of the ring core: 8.5 .mu.m
[0267] Mode field diameter: 6.3 .mu.m
[0268] Effective core area: 35 .mu.m.sup.2
[0269] Expansion rate of the effective core area due to heating:
4.3 .mu.m.sup.2/sec
[0270] The single mode optical fiber had a W-type refractive index
distribution shape with a segment as shown in FIG. 3, and the
structural parameters and characteristics thereof were as
follows.
[0271] The center core was made of a germanium-added silica glass,
the side core was made of a fluorine-added silica glass, and the
cladding was made of a pure silica glass.
[0272] Relative index difference of the center core based on the
cladding: 0.25%
[0273] Relative index difference of the side core based on the
cladding: -0.05%
[0274] Radius of the center core: 6.8 .mu.m
[0275] Radius of the side core: 27 .mu.m
[0276] Mode field diameter: 12.7 .mu.m
[0277] Effective core area: 135 .mu.m.sup.2
[0278] Chromatic dispersion: 20 ps/nm/km
[0279] Dispersion slope: 0.06 ps/nm.sup.2/km
[0280] Transmission loss: 0.19 dB/km
[0281] Cutoff wavelength: 1.6 .mu.m
[0282] Expansion rate of the effective core area due to heating:
2.0 .mu.m.sup.2/sec
[0283] A fusion splicing machine was used for connection of the
connection optical fiber with the dispersion compensating optical
fiber, to heat the connection optical fiber at about 2200.degree.
C. for 2 seconds. The mode field diameter of the dispersion
compensating optical fiber after fusion splicing was 6.4 .mu.m. The
mode field diameter of the connection optical fiber after fusion
splicing hardly changed.
[0284] A fusion splicing machine was used for connection of the
connection optical fiber with the single mode optical fiber, to
heat the connection optical fiber at about 2200.degree. C. for 2
seconds for effecting fusion splicing, and thereafter, the end of
the connection optical fiber was further heated for 30 seconds to
thereby diffuse the germanium.
[0285] The theoretical joint loss obtained from the overlap
integral of the near field pattern of the dispersion compensating
optical fiber and the near field pattern of the connection optical
fiber was 0.11 dB. The actual measurement values of the joint loss
are shown in Table 1.
Example 3
[0286] The connection structure was constructed in the same manner
as in Example 1, except that the construction of the cladding of
the connection optical fiber was changed.
[0287] That is to say, the cladding in the connection optical fiber
had a two-layer structure, with the first layer adjacent to the
core being formed of a silica glass added with fluorine in an
amount of 1.2% by weight and the radius thereof was 25 .mu.m. The
outermost layer (the second layer) was formed of a pure silica
glass.
[0288] The structural parameters and characteristics of this
connection optical fiber were as follows.
[0289] Relative index difference of the center core based on the
first layer of the cladding: 1.0%
[0290] Relative index difference of the side core based on the
first layer of the cladding: 0.05%
[0291] Relative index difference of the ring core based on the
first layer of the cladding: 0.38%
[0292] Radius of the center core: 1.8 .mu.m
[0293] Radius of the side core: 6.7 .mu.m
[0294] Radius of the ring core: 8.2 .mu.m
[0295] Mode field diameter: 5.7 .mu.m
[0296] Effective core area: 24.7 .mu.m.sup.2
[0297] Expansion rate of the effective core area due to heating:
5.6 .mu.m.sup.2/sec
[0298] The mode field diameter of the dispersion compensating
optical fiber after fusion splicing was 5.9 .mu.m. The mode field
diameter of the connection optical fiber after fusion splicing also
changed, and was 5.9 .mu.m.
[0299] The theoretical joint loss obtained from the overlap
integral of the near field pattern of the dispersion compensating
optical fiber and the near field pattern of the connection optical
fiber was 0.07 dB. The actual measurement values of the joint loss
are shown in Table 1.
Comparative Example 1
[0300] A dispersion compensating optical fiber having a negative
dispersion slope and a single mode optical fiber compensated by
this dispersion compensating optical fiber were fusion-spliced to
the opposite ends of a connection optical fiber, in the same manner
as in Example 1, and the near field pattern and the joint loss were
measured. The used wavelength was 1.55 .mu.m, and the outer
diameters of the used optical fibers were respectively about 125
.mu.m.
[0301] The dispersion compensating optical fiber had a W-type
refractive index distribution shape with a segment as shown in FIG.
3, and the structural parameters and characteristics thereof were
as follows.
[0302] The center core and the ring core were made of a
germanium-added silica glass, the side core was made of a
fluorine-added silica glass, and the cladding was made of a pure
silica glass.
[0303] r.sub.11: 2.0 .mu.m
[0304] r.sub.12: 5.8 .mu.m
[0305] r.sub.13: 6.8 .mu.m
[0306] .DELTA..sub.11: 1.0%
[0307] .DELTA..sub.12: -0.4%
[0308] .DELTA..sub.13: 0.9%
[0309] Effective core area: 26 .mu.m.sup.2
[0310] Mode field diameter: 6.0 .mu.m
[0311] Chromatic dispersion: -54 ps/nm/km
[0312] Dispersion slope: -0.15 ps/nm.sup.2/km
[0313] Transmission loss: 0.3 dB/km
[0314] Cutoff wavelength: 1.6 .mu.m
[0315] Expansion rate of the effective core area due to heating:
8.5 .mu.m.sup.2/sec
[0316] The connection optical fiber had a step type refractive
index distribution shape, and one having a near field pattern
different from that of the dispersion compensating optical fiber
before fusion splicing was used (theoretical joint loss before
fusion splicing was 0.6 dB). The step type is one having a
two-layer structure consisting of a core and a cladding provided on
the circumference thereof, wherein the refractive index of the core
is higher than that of the cladding.
[0317] The cladding had a two-layer structure, with the first layer
being formed of a fluorine-added silica glass in which fluorine was
added in an amount of 0.4% by weight, and the radius thereof was 30
.mu.m. The outermost layer (the second layer) was formed of a pure
silica glass.
[0318] The structural parameters and characteristics of this
connection optical fiber were as follows.
[0319] The core was made of a germanium-added silica glass.
[0320] Relative index difference of the core based on the first
layer of the cladding: 1.0%
[0321] Radius of the core: 2.4 .mu.m
[0322] Mode field diameter: 6.2 .mu.m
[0323] Effective core area: 29 .mu.m.sup.2
[0324] Expansion rate of the effective core area due to heating:
7.2 .mu.m.sup.2/sec
[0325] A single mode optical fiber the same as in Example 1 was
used herein.
[0326] The theoretical joint loss obtained from the overlap
integral of the near field pattern of the dispersion compensating
optical fiber and the near field pattern of the connection optical
fiber after the connection was 0.95 dB. The actual measurement
values of the joint loss are shown in Table 1.
Comparative Example 2
[0327] The connection optical fiber had a step type refractive
index distribution shape, and one having a near field pattern
different from that of the dispersion compensating optical fiber
before fusion splicing was used (theoretical joint loss before
fusion splicing was 0.68 dB). The cladding had a two-layer
structure, with the first layer being formed of a fluorine-added
silica glass in which fluorine was added in an amount of 0.3% by
weight, and the radius thereof was 30 .mu.m. The outermost layer
(the second layer) was formed of a pure silica glass.
[0328] The structural parameters and characteristics of this
connection optical fiber were as follows.
[0329] The core was made of a germanium-added silica glass.
[0330] Relative index difference of the core based on the first
layer of the cladding: 1.2%
[0331] Radius of the core: 3.0 .mu.m
[0332] Mode field diameter: 6.0 .mu.m
[0333] Effective core area: 27 .mu.m.sup.2
[0334] Expansion rate of the effective core area due to heating:
7.1 .mu.m.sup.2/sec
[0335] A dispersion compensating optical fiber and a single mode
optical fiber the same as in Comparative Example 1 was used
herein.
[0336] The theoretical joint loss obtained from the overlap
integral of the near field pattern of the dispersion compensating
optical fiber and the near field pattern of the connection optical
fiber after the connection was 0.90 dB. The actual measurement
values of the joint loss are shown in Table 1.
1 TABLE 1 Joint loss with dispersion Joint loss with single Total
joint loss compensating mode optical fiber (dB) optical fiber (dB)
(dB) Example 1 0.71 0.18 0.53 Example 1-A 0.33 0.18 0.15 Example
1-B 0.36 0.18 0.18 Example 1-C 0.33 0.18 0.15 Example 2 0.37 0.13
0.24 Example 3 0.38 0.14 0.24 Com. Ex. 1 1.08 0.93 0.15 Com. Ex. 2
1.09 0.92 0.17
[0337] Examples 1, 1-A to 1-C, and 2 are compared with Comparative
Example 1.
[0338] The difference in the mode field diameter between the
dispersion compensating optical fiber and the connection optical
fiber before fusion splicing in Examples 1, 1-A to 1-C, and 2 was
larger than that for Comparative Examples 1 and 2.
[0339] However, as the results shown in Table 1, in Examples 1, 1-A
to 1-C, and 2, the joint loss could be reduced more than for the
Comparative Examples 1 and 2. Particularly, in Examples 2 and 3
where fluorine was added to the cladding respectively, the joint
loss with the single mode optical fiber could be reduced
considerably. Therefore, it is apparent that it is important to
match the near field patterns after fusion splicing with each
other, in the connection between the dispersion compensating
optical fiber having a negative dispersion slope and the connection
optical fiber.
[0340] For comparison sake, the dispersion compensating optical
fiber and the single mode optical fiber used in each of the
Examples and Comparative Examples were directly fusion-spliced. The
joint loss thereof had a value exceeding 1.5 dB.
* * * * *