U.S. patent application number 10/126608 was filed with the patent office on 2002-10-31 for optical fiber transmission- line.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Onishi, Masashi, Sasaoka, Eisuke.
Application Number | 20020159689 10/126608 |
Document ID | / |
Family ID | 26550327 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020159689 |
Kind Code |
A1 |
Onishi, Masashi ; et
al. |
October 31, 2002 |
Optical fiber transmission- line
Abstract
A dispersion-managed optical fiber transmission-line with which
the formation of side bands around the optical signal wavelength
can be suppressed even when carrying out high-speed signal
transmission. An optical fiber transmission-line 1 constitutes one
repeater span installed between a transmitter (or repeater) 2 and a
receiver (or repeater) 3 and is made up of N sections 4.sub.1
through 4.sub.N in sequence from the transmitter 2 to the receiver
3. The chromatic dispersion at the wavelength 1.55 .mu.m is
positive in the sections 4.sub.n where the value of n is odd and is
negative in the sections 4.sub.n where the value of n is even. The
ratio between the maximum value and the minimum value of the
average chromatic dispersions D.sub.n of the sections 4.sub.n is at
least 1.3 and not greater than 10.0.
Inventors: |
Onishi, Masashi;
(Yokohama-shi, JP) ; Sasaoka, Eisuke;
(Yokohama-shi, JP) |
Correspondence
Address: |
McDermott, Will & Emery
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
Sumitomo Electric Industries,
Ltd.
Osaka
JP
|
Family ID: |
26550327 |
Appl. No.: |
10/126608 |
Filed: |
April 22, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10126608 |
Apr 22, 2002 |
|
|
|
09637872 |
Aug 15, 2000 |
|
|
|
6377740 |
|
|
|
|
Current U.S.
Class: |
385/24 ;
398/178 |
Current CPC
Class: |
G02B 6/29377 20130101;
H04B 10/25253 20130101 |
Class at
Publication: |
385/24 ;
359/179 |
International
Class: |
G02B 006/28; H04B
010/16 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 1999 |
JP |
272679/1999 |
Claims
What is claimed is:
1. An optical fiber transmission-line constituting a single
repeater span in which sections where the chromatic dispersion at a
predetermined wavelength is positive and sections where it is
negative are provided alternately, wherein the ratio between the
maximum value and the minimum value among the absolute values of
the average chromatic dispersions of the sections is not less than
1.3 and not greater than 10.0.
2. An optical fiber transmission-line constituting a single
repeater span in which sections where the chromatic dispersion at a
predetermined wavelength is positive and sections where it is
negative are provided alternately, wherein the number of sections
of which the absolute value of the average chromatic dispersion
differs by not less than 10% from that of an adjacent section is at
least half of the total number of sections.
3. An optical fiber transmission-line constituting a single
repeater span in which sections where the chromatic dispersion at a
predetermined wavelength is positive and sections where it is
negative are provided alternately, wherein the number of sections
of which the absolute value of the average chromatic dispersion
differs by not less than 0.5 ps/nm/km from that of an adjacent
section is at least half of the total number of sections.
4. An optical fiber transmission-line constituting a single
repeater span in which sections where the chromatic dispersion at a
predetermined wavelength is positive and sections where it is
negative are provided alternately, wherein for any two sections the
absolute value of the average chromatic dispersion of the section
nearer the optical signal input end of the repeater span is larger
than the absolute value of the average chromatic dispersion of the
section nearer the optical signal output end and the absolute value
of the average chromatic dispersion of the section at the output
end is not less than 1 ps/nm/km.
5. An optical fiber transmission-line according to claim 4, wherein
for any two sections the length of the section nearer the optical
signal input end of the repeater span is shorter than the length of
the section nearer the optical signal output end.
6. An optical fiber transmission-line according to any one of
claims 1 through 4, wherein the length of each section is not less
than 0.1 km and not greater than 10 km.
7. An optical fiber transmission-line according to any one of
claims 1 through 4, wherein the absolute value of the average
chromatic dispersion of each section is at least 1 ps/nm/km.
8. An optical fiber transmission-line according to any one of
claims 1 through 4, wherein the absolute value of the average
chromatic dispersion of the whole repeater span at the
predetermined wavelength is not greater than 0.5 ps/nm/km.
9. An optical fiber transmission-line according to any one of
claims 1 through 4, wherein the polarization mode dispersion of the
whole repeater span at the predetermined wavelength is not greater
than 0.2 ps/km.sup.1/2.
10. An optical fiber transmission-line according to any one of
claims 1 through 4, wherein the transmission loss of the whole
repeater span at the predetermined wavelength is not greater than
0.3 dB/km.
11. An optical fiber transmission-line according to any one of
claims 1 through 4, wherein over the whole repeater span the
effective core area at the predetermined wavelength is at least 20
.mu.m.sup.2.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an optical fiber transmission-line
for transmitting multiple wavelength optical signals in a
wavelength division multiplexing transmission system.
[0003] 2. Related Background Arts
[0004] A wavelength division multiplexing (WDM) transmission system
can effect high speed, high capacity optical communication by
transmitting multiple wavelength optical signals. Because the
transmission loss of a silica optical fiber used as an optical
fiber transmission-line is small around the wavelength 1.55 .mu.m,
and optical amplifiers for amplifying optical signals in the
wavelength 1.55 .mu.m band are available, multiple wavelength
optical signals in the wavelength 1.55 .mu.m band are used in WDM
transmission systems.
[0005] When in an optical fiber transmission-line for transmitting
multiple wavelength optical signals there is chromatic dispersion
in the optical signal wavelength band, the pulse waveform of the
optical signal collapses and transmission quality deteriorates.
Therefore, from this point of view, it is desirable for the
chromatic dispersion in the optical signal wavelength band to be
small. On the other hand, when the chromatic dispersion in the
optical signal wavelength band is substantially zero, the nonlinear
optical phenomenon of four-wave mixing tends to occur, crosstalk
and noise arise, and transmission quality deteriorates. The
occurrence of four-wave mixing can be suppressed by making repeater
spans short and reducing optical signal power, but because this
makes it necessary to provide many optical amplifiers, it results
in a generally expensive optical transmission system.
[0006] To deal with such problems, dispersion-management has been
proposed, wherein sections where the chromatic dispersion at the
wavelength 1.55 .mu.m is positive and sections where it is negative
are provided alternately in the longitudinal direction of the
optical fiber transmission-line. If this kind of optical fiber
transmission-line is used, by making the average chromatic
dispersion in the optical fiber transmission-line as a whole
substantially zero, it is possible to suppress transmission quality
deterioration caused by chromatic dispersion. And because at most
points in the optical fiber transmission-line the absolute value of
the chromatic dispersion is not in the vicinity of zero, it is
thought to be possible also to suppress transmission quality
deterioration caused by four-wave mixing (see for example U.S. Pat.
No. 5,894,537 or 5,887,105).
[0007] However, the present inventors have recognized that in this
related art technology, when high-speed signal transmission with a
bit rate exceeding 10 Gb/s is carried out using an optical fiber
transmission-line wherein the alternating disposition of the
sections where the chromatic dispersion is positive and the
sections where it is negative is regular, side bands form around
the original wavelength of the optical signal. This formation of
side bands appears to be caused by the interaction in the same
pattern between the optical signal spectrum and the chromatic
dispersion as a result of the regularity of the alternating
disposition. And because this formation of side bands constitutes a
cause of transmission quality deterioration, it is important that
it be suppressed.
SUMMARY OF THE INVENTION
[0008] It is therefore an object of the present invention to
provide a dispersion-managed optical fiber transmission-line with
which the formation of side bands around the optical signal
wavelength can be suppressed even when carrying out high-speed
signal transmission.
[0009] To achieve this object, a first optical fiber
transmission-line according to the invention is an optical fiber
transmission-line forming a single repeater span in which sections
where the chromatic dispersion at a predetermined wavelength is
positive and sections where it is negative are provided
alternately, wherein the ratio between the maximum value and the
minimum value among the absolute values of the average chromatic
dispersions of the sections is not less than 1.3 and not greater
than 10.0.
[0010] A second optical fiber transmission-line according to the
invention is an optical fiber transmission-line forming a single
repeater span in which sections where the chromatic dispersion at a
predetermined wavelength is positive and sections where it is
negative are provided alternately, wherein the number of sections
of which the absolute value of the average chromatic dispersion
differs by not less than 10% from that of an adjacent section is at
least half of the total number of sections.
[0011] A third optical fiber transmission-line according to the
invention is an optical fiber transmission-line forming a single
reperter span in which sections where the chromatic dispersion at a
predetermined wavelength is positive and sections where it is
negative are provided alternately, wherein the number of sections
of which the absolute value of the average chromatic dispersion
differs by not less than 0.5 ps/nm/km from that of an adjacent
section is at least half of the total number of sections.
[0012] A fourth optical fiber transmission-line according to the
invention is an optical fiber transmission-line forming a single
repeater span in which sections where the chromatic dispersion at a
predetermined wavelength is positive and sections where it is
negative are provided alternately, wherein for any two sections the
absolute value of the average chromatic dispersion of the section
nearer the optical signal input end of the repeater span is larger
than the absolute value of the average chromatic dispersion of the
section nearer the optical signal output end and the absolute value
of the average chromatic dispersion of the section at the output
end is not less than 1 ps/nm/km. For any two sections in this
fourth optical fiber transmission-line, the length of the section
nearer the optical signal input end of any repeater span is
preferably shorter than the length of the section nearer the
optical signal output end.
[0013] With any of the first through fourth optical fiber
transmission-lines described above, the formation of side bands
around the optical signal wavelength can be suppressed even when
carrying out high-speed signal transmission, and even when carrying
out high-speed signal transmission with a bit rate exceeding 10
Gb/s it is possible to realize WDM transmission of good
transmission quality.
[0014] Also, with the fourth optical fiber transmission-line
described above, it is not necessary for the sign of the chromatic
dispersion to be frequently alternated in regions where the
absolute value of the chromatic dispersion is large, and the
manufacturing productivity of the optical fiber transmission-line
is thereby improved.
[0015] And, in each of the first through fourth optical fiber
transmission-lines described above, when the length of each section
is not less than 0.1 km and not more than 10 km, the cumulative
chromatic dispersion does not become large, so the deterioration of
transmission quality caused by the interaction of cumulative
chromatic dispersion and nonlinear optical phenomena can be
suppressed. When the absolute value of the average chromatic
dispersion of each section is at least 1 ps/nm/km, transmission
quality deterioration caused by nonlinear optical phenomena can be
suppressed. When the absolute value of the average chromatic
dispersion of the whole repeater span is 0.5 ps/nm/km or less,
transmission quality deterioration caused by cumulative chromatic
dispersion can be suppressed. When the polarization mode dispersion
of the whole repeater span is not greater than 0.2 ps/km.sup.1/2,
transmission quality deterioration caused by polarization mode
dispersion can be suppressed. When the transmission loss is not
more than 0.3 dB/km, the distance of a transmission line without
repeaters can be made greater. And when the effective core area is
at least 20 .mu.m.sup.2 over the whole repeater span, transmission
quality deterioration caused by nonlinear optical phenomena can be
suppressed.
[0016] In this invention, a "predetermined wavelength" means the
center wavelength of an optical signal wavelength band, for
example, 1.55 .mu.m. And, when not otherwise specified, values of
polarization mode dispersion, transmission loss and effective core
area are values at the predetermined wavelength.
[0017] The above and further objects and novel features of the
invention will be more fully clarified from the following detailed
description when the same is read in connection with the
accompanying drawings. It is to be expressly understood, however,
that the drawings are for the purpose of illustration only and are
not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In order to more fully understand the drawings used in the
detailed description of the present invention, a brief description
of each drawing is provided.
[0019] FIG. 1 is a schematic view illustrating an optical fiber
transmission-line according to any of first, second and third
preferred embodiments;
[0020] FIG. 2 is a schematic view illustrating an optical fiber
transmission-line according to a fourth preferred embodiment;
[0021] FIG. 3 is a graph illustrating another version of an optical
fiber transmission-line according to the invention;
[0022] FIG. 4 is a graph illustrating another version of an optical
fiber transmission-line according to the invention; and
[0023] FIG. 5 is a graph illustrating another version of an optical
fiber transmission-line according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] In the following, the preferred embodiments of the present
invention will be explained in detail with reference to the
accompanying drawings. To facilitate comprehension of the
explanation, the same reference numerals denote the same parts,
where possible, throughout the drawings, and a repeated explanation
will be omitted. The dimensions in the drawings are partly
exaggerated and do not always correspond to actual ratios of
dimensions.
[0025] (First Preferred Embodiment)
[0026] A first preferred embodiment of an optical fiber
transmission-line according to the invention will now be described.
FIG. 1 is a view illustrating an optical fiber transmission-line 1
according to this first preferred embodiment. This optical fiber
transmission-line 1 constitutes one repeater span installed between
a transmitter (or repeater) 2 and a receiver (or repeater) 3, and
is made up of N(N.gtoreq.2) sections 4.sub.1 through 4.sub.N in
sequence from the transmitter 2 to the receiver 3. At a
predetermined wavelength the chromatic dispersion is positive in
the sections 4.sub.n where the value of n is odd and is negative in
the sections 4.sub.n where the value of n is even.
[0027] The optical fiber transmission-line 1 may be made by
connecting together section by section with optical fibers having
predetermined chromatic dispersions, or the whole repeater span may
be made from a single optical fiber. In the latter case, the
optical fiber transmission-line 1 can be obtained, for example, by
the diameter of a core region being modulated to vary the chromatic
dispersion while the diameter of the cladding region is kept
constant along the longitudinal direction. In this case the optical
fiber transmission-line 1 can be manufactured by making a preform
in which the diameter of a core part varies in the longitudinal
direction and the diameter of the cladding part is fixed in the
longitudinal direction and drawing from this preform an optical
fiber whose cladding diameter is fixed. Or, the optical fiber
transmission-line 1 can be obtained by the diameter of the core
region and the diameter of the cladding region being modulated to
adjust the chromatic dispersion in the longitudinal direction. In
this case the optical fiber transmission-line 1 can be manufactured
by making a preform in which the respective diameters of the core
part and the cladding part are fixed in the longitudinal direction
and varying the cladding diameter in the longitudinal direction
while drawing an optical fiber from this preform. In either case,
the larger the diameter of the core region is, the larger the
chromatic dispersion can be made.
[0028] The average chromatic dispersion of a section 4.sub.n will
be written D.sub.n. Also, the maximum value among the absolute
values of D.sub.n will be written Dmax, and the minimum value among
the absolute values of D.sub.n will be written Dmin. It is a
characteristic feature of this preferred embodiment that the ratio
R=Dmax/Dmin is at least 1.3 and not more than 10.
[0029] If R is at least 1.3, then even when carrying out high-speed
signal transmission it is possible to suppress the formation of
side bands around the optical signal wavelength. And, because if
the absolute value of D.sub.n is at least 2 ps/nm/km in each
section the occurrence of nonlinear optical phenomena is suppressed
and if the absolute value of D.sub.n is not greater than 20
ps/nm/km in each section the interaction of cumulative chromatic
dispersion and nonlinear optical phenomena is suppressed, the upper
limit value of R is 10.
[0030] Table 1 shows Dmax (ps/nm/km), Dmin (ps/nm/km), R and
transmission characteristics for each of five different optical
fiber transmission-lines A through E.
1 TABLE 1 Transmission D max D min R Characteristics Waveguide A 10
2 5 .largecircle. Waveguide B 20 2 10 .largecircle. Waveguide C 2.4
1.8 1.3 .largecircle. Waveguide D 2.0 1.8 1.1 X Waveguide E Each
absolute value is X identical.
[0031] As shown in this table, in the optical fiber
transmission-line A, Dmax was 10 ps/nm/km, Dmin was 2 ps/nm/km, and
R was 5, and there was no formation of side bands. In the optical
fiber transmission-line B, Dmax was 20 ps/nm/km, Dmin was 2
ps/nm/km, and R was 10, and there was no formation of side bands.
In the optical fiber transmission-line C, Dmax was 2.4 ps/nm/km,
Dmin was 1.8 ps/nm/km, and R was 1.3, and there was no formation of
side bands. However, in the optical fiber transmission-line D, in
which Dmax was 2.0 ps/nm/km, Dmin was 1.8 ps/nm/km, and R was 1.1,
side bands formed and the transmission characteristics
deteriorated. And also in the optical fiber transmission-line E, in
which the absolute value of D.sub.n was the same in all the
sections and R was 1.0, side bands formed and the transmission
characteristics deteriorated.
[0032] Thus, in this preferred embodiment, as a result of R being
between 1.3 and 10.0, the formation of side bands around the
optical signal wavelength can be suppressed and good transmission
quality can be obtained.
[0033] In this preferred embodiment, the length L.sub.n of each of
the sections 4.sub.n is preferably at least 0.1 km and not more
than 10 km, and in this case, because the cumulative chromatic
dispersion does not become large, transmission quality
deterioration caused by interaction between cumulative chromatic
dispersion and nonlinear optical phenomena can be suppressed. The
absolute value of D.sub.n is preferably at least 1 ps/nm/km, and in
this case transmission quality deterioration caused by nonlinear
optical phenomena can be suppressed. The absolute value of the
average chromatic dispersion of the whole repeater span is
preferably not greater than 0.5 ps/nm/km, and in this case
transmission quality deterioration caused by cumulative chromatic
dispersion can be suppressed. The polarization mode dispersion of
the whole repeater span is preferably not greater than 0.2
ps/km.sup.1/2, and in this case transmission quality deterioration
caused by polarization mode dispersion can be suppressed. The
transmission loss of the whole repeater span is preferably not
greater than 0.3 dB/km, and in this case transmission distances
without repeater can be made long. And the effective core area is
preferably 20 .mu.m.sup.2 or more over the whole repeater span; in
this case, transmission quality deterioration caused by nonlinear
optical phenomena can be suppressed.
[0034] (Second Preferred Embodiment)
[0035] Next, a second preferred embodiment of an optical fiber
transmission-line according to the invention will be described. The
construction of this second preferred embodiment is the same as
that shown in FIG. 1.
[0036] It is a characteristic feature of this preferred embodiment
that the number of sections of which the absolute value of the
average chromatic dispersion differs by not less than 10% from that
of an adjacent section is at least half of the total number N. That
is, among the N sections 4.sub.1 through 4.sub.N in the repeater
span, the number of sections of which the absolute value of D.sub.n
differs by at least 10% from the absolute value of D.sub.n-1 or the
absolute value of D.sub.n+1 is at least N/2.
[0037] Table 2 shows chromatic dispersions D.sub.n (units:
ps/nm/km) in each of two different optical fiber transmission-lines
F and G.
2TABLE 2 n 1 2 3 4 5 6 7 8 9 10 chromatic +11 -10 +8 -8 +6 -6 +4 -4
+2 -3 dispersions D.sub.n of Waveguide F chromatic +5 -5 +3 -3 +2.5
-2.5 +2 -2.5 +2.5 -2 dispersions D.sub.n of Waveguide G
[0038] As shown in this table, in both of the optical fiber
transmission-lines F and G, the number of sections of which the
absolute value of the average chromatic dispersion differs by at
least 10% from that of an adjacent section is at least half of the
overall number N (=10), and even when carrying out high-speed
signal transmission it is possible to suppress the formation of
side bands around the optical signal wavelength and good
transmission quality can be obtained.
[0039] In this preferred embodiment also, all of the lengths
L.sub.n are preferably at least 0.1 km and not more than 10 km; the
absolute value of every D.sub.n is preferably at least 1 ps/nm/km;
the absolute value of the average chromatic dispersion of the whole
repeater span is preferably not greater than 0.5 ps/nm/km; the
polarization mode dispersion of the whole repeater span is
preferably not greater than 0.2 ps/km.sup.1/2; the transmission
loss of the whole repeater span is preferably not greater than 0.3
dB/km; and the effective core area is preferably 20 .mu.m.sup.2 or
more over the whole repeater span.
[0040] (Third Preferred Embodiment)
[0041] A third preferred embodiment of an optical fiber
transmission-line according to the invention will now be described.
The construction of the optical fiber transmission-line of this
third preferred embodiment is also the same as that shown in FIG.
1.
[0042] It is a characteristic feature of this preferred embodiment
that the number of sections of which the absolute value of the
average chromatic dispersion differs by at least 0.5 ps/nm/km from
that of an adjacent section is at least half of the total number of
sections. That is, among the N sections 4.sub.1 through 4.sub.N in
the repeater span, the number of sections of which the absolute
value of D.sub.n differs by at least 0.5 ps/nm/km from the absolute
value of D.sub.n-1 or the absolute value of D.sub.n+1 is at least
N/2.
[0043] Table 3 shows chromatic dispersions D.sub.n (units:
ps/nm/km) in an optical fiber transmission-line H.
3TABLE 3 n 1 2 3 4 5 6 7 8 9 10 chromatic +8 -8 +7.5 -7.5 +8.3 -6.0
+5.0 -7.3 +3.0 -3.0 dispersions D.sub.n of Waveguide H
[0044] As shown in this table, in the optical fiber
transmission-line H, the number of sections of which the absolute
value of the average chromatic dispersion differs by at least 0.5
ps/nm/km from that of an adjacent section is at least half of the
overall number of sections N (=10), and even when carrying out
high-speed signal transmission it is possible to suppress the
formation of side bands around the optical signal wavelength and
good transmission quality can be obtained.
[0045] In this preferred embodiment also, all of the lengths
L.sub.n are preferably at least 0.1 km and not more than 10 km; the
absolute value of every D.sub.n is preferably at least 1 ps/nm/km;
the absolute value of the average chromatic dispersion of the whole
repeater span is preferably not greater than 0.5 ps/nm/km; the
polarization mode dispersion of the whole repeater span is
preferably not greater than 0.2 ps/km.sup.1/2; the transmission
loss of the whole repeater span is preferably not greater than 0.3
dB/km; and the effective core area is preferably 20 .mu.m.sup.2 or
more over the whole repeater span.
[0046] (Fourth Preferred Embodiment)
[0047] Next, a fourth preferred embodiment of an optical fiber
transmission-line according to the invention will be described.
FIG. 2 is a schematic view of an optical fiber transmission-line
according to this fourth preferred embodiment. This optical fiber
transmission-line 1 constitutes one repeater span installed between
a transmitter (or repeater) 2 and a receiver (or repeater) 3, and
is made up of N(N.gtoreq.2) sections 4.sub.1 through 4.sub.N in
sequence from the transmitter 2 to the receiver 3. At a
predetermined wavelength the chromatic dispersion is positive in
the sections 4.sub.n where the value of n is odd and is negative in
the sections 4.sub.n where the value of n is even. D.sub.n and
L.sub.n are defined in the same way as in the first preferred
embodiment.
[0048] It is a characteristic feature of this preferred embodiment
that for any two sections the absolute value of the average
chromatic dispersion of the section nearer the optical signal input
end of the repeater span is larger than the absolute value of the
average chromatic dispersion of the section nearer the optical
signal output end of the repeater span. That is, of any two
adjacent sections 4.sub.n, 4.sub.n+1, the absolute value of the
average chromatic dispersion D.sub.n of the section 4.sub.n nearer
the input end is larger than the absolute value of the average
chromatic dispersion D.sub.n+1 of the section 4.sub.n+1 nearer the
output end. It is also a characteristic feature of this preferred
embodiment that the absolute value of the average chromatic
dispersion D.sub.N of the section 4.sub.N at the output end of the
repeater span is at least 1 ps/nm/km.
[0049] In an optical fiber transmission-line having the chromatic
dispersion of its sections set in this way, in sections nearer the
transmitter 2, where the power of the optical signal is relatively
large as a result of the absolute value of the chromatic dispersion
being relatively large, the occurrence of nonlinear optical
phenomena is suppressed. In sections nearer the receiver 3, where
the power of the optical signal is relatively small, because
nonlinear optical phenomena do not readily occur, the absolute
value of the chromatic dispersion can be made small and the
cumulative chromatic dispersion can thereby also be made small.
And, by the absolute value of D.sub.N being made at least 1
ps/nm/km, the occurrence of nonlinear optical phenomena can be
suppressed. In this preferred embodiment also, even when carrying
out high-speed signal transmission, it is possible to suppress the
formation of side bands around the optical signal wavelength and
good transmission quality can be obtained.
[0050] Also, preferably, for any two sections the length of the
section nearer the optical signal input end of the repeater span is
shorter than the length of the section nearer the optical signal
output end of the repeater span, that is, in any two sections
4.sub.n and 4.sub.n+1, L.sub.n<L.sub.n+1. When this is done, in
sections nearer the transmitter 2, as a result of the section
length being relatively small, the cumulative chromatic dispersion
can be made small. And in sections nearer the receiver 3, as a
result of the section length being relatively long, manufacturing
productivity can be improved.
[0051] In this preferred embodiment also, all of the lengths
L.sub.n are preferably at least 0.1 km and not more than 10 km; the
absolute value of every D.sub.n is preferably at least 1 ps/nm/km;
the absolute value of the average chromatic dispersion of the whole
repeater span is preferably not greater than 0.5 ps/nm/km; the
polarization mode dispersion of the whole repeater span is
preferably not greater than 0.2 ps/km.sup.1/2; the transmission
loss of the whole repeater span is preferably not greater than 0.3
dB/km; and the effective core area is preferably 20 .mu.m.sup.2 or
more over the whole repeater span.
[0052] (Modification Examples)
[0053] This invention is not limited to the preferred embodiments
described above, and these preferred embodiments can be changed in
various ways within the scope of the invention. For example,
although the chromatic dispersion in each section of the optical
fiber transmission-line may be uniform, as shown in FIG. 3,
alternatively it may change within the section, as shown in FIG. 4.
In this latter case, for example if the sections 4.sub.n (of length
L.sub.n) where the chromatic dispersion is negative, are divided
into sub-sections 4.sub.ni (i=1,2,3) of chromatic dispersion
D.sub.ni and length L.sub.ni, the average chromatic dispersion
D.sub.n of each such section 4.sub.n can be obtained from the
formula D.sub.n=(D.sub.n1L.sub.n1+D.sub.n2L.sub.n2+D.su-
b.n3L.sub.n3)/L.sub.n. Indeed, provided that the average chromatic
dispersion conforms to the provisions of the invention, inside the
sections the chromatic dispersion may be changed freely as long as
its sign does not change.
[0054] Also, preferably, of any two adjacent sections 4.sub.2m-1
and 4.sub.2m (m=1 to N/2), it is preferable that the relationship
D.sub.2m-1L.sub.2m-1+D.sub.2mL.sub.2m=0 holds. In this case,
because the cumulative chromatic dispersion over any two adjacent
sections 4.sub.2m-1 and 4.sub.2m is zero, waveform deterioration is
suppressed.
* * * * *