U.S. patent application number 10/855393 was filed with the patent office on 2004-11-04 for wavelength dispersion compensator and optical transmission line.
This patent application is currently assigned to NEC Corporation. Invention is credited to Ishii, Satoshi.
Application Number | 20040218862 10/855393 |
Document ID | / |
Family ID | 18771275 |
Filed Date | 2004-11-04 |
United States Patent
Application |
20040218862 |
Kind Code |
A1 |
Ishii, Satoshi |
November 4, 2004 |
Wavelength dispersion compensator and optical transmission line
Abstract
In the WDM optical transmission, optical signals propagated
through an optical transmission line are supplied to a circulator
of a wavelength dispersion compensator. The optical signals
supplied to an input terminal of the circulator are transmitted to
an input and output terminal of the circulator, and inputted to
fiber gratings of the reflection type. The optical signals having a
specified wavelength is reflected by one of the fiber gratings,
again inputted to the input and output terminal of the circulator,
and transmitted to the output terminal of the same. Since the
incident optical signals are reflected by the fiber gratings
situated at different positions, there arises the difference in the
time spent in traveling to and from the fiber grating between the
optical signals. Accordingly, the wavelength dispersions of the
optical signals can be compensated by suitably selecting the
positions of the fiber gratings.
Inventors: |
Ishii, Satoshi; (Tokyo,
JP) |
Correspondence
Address: |
MCGINN & GIBB, PLLC
8321 OLD COURTHOUSE ROAD
SUITE 200
VIENNA
VA
22182-3817
US
|
Assignee: |
NEC Corporation
Tokyo
JP
|
Family ID: |
18771275 |
Appl. No.: |
10/855393 |
Filed: |
May 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10855393 |
May 28, 2004 |
|
|
|
09956047 |
Sep 20, 2001 |
|
|
|
Current U.S.
Class: |
385/37 ;
385/27 |
Current CPC
Class: |
G02B 6/2932 20130101;
G02B 6/29374 20130101; G02B 6/29394 20130101 |
Class at
Publication: |
385/037 ;
385/027 |
International
Class: |
G02B 006/34 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 21, 2000 |
JP |
2000-287547 |
Claims
1. A wavelength dispersion compensator for compensating for
wavelength dispersions of wavelength division multiplexed (WDM)
optical signals which have been propagated through an input optical
transmission line, comprising: plural fiber gratings of a
reflection type which reflect said WDM optical signals at different
positions depending on wavelengths of said reflected optical
signals, and a gain equalizer having an input terminal for
receiving said reflected optical signals and an output terminal
connected to one of an output optical transmission line and an
output optical waveguide.
2. The wavelength dispersion compensator as defined in claim 1,
further comprising: a circulator comprising an input terminal, an
input and output terminal, an an output terminal, said input
terminal being connected with said input optical transmission line,
said input and output terminal being in communication with said
plural fiber gratings of a reflection type, and said output
terminal being connected with one of said output optical
transmission line and said output optical waveguide.
3. The wavelength dispersion compensator as defined in claim 2,
wherein said plural fiber gratings of a reflection type comprise a
chirped fiber grating of a reflection type.
4. The wavelength dispersion compensator as defined in claim 2,
wherein an input terminal of said gain equalizer is connected with
said output terminal of said circulator, and an output terminal of
said gain equalizer is connected with said output optical
transmission line or said output optical waveguide.
5-11. (Canceled)
12. The wavelength dispersion compensator as defined in claim 1,
wherein said plural fiber gratings are connected in series by one
of an optical fiber and an optical waveguide.
13. The wavelength dispersion compensator as defined in claim 12,
wherein said series comprises a cascading series.
14. The wavelength dispersion compensator as defined in claim 1,
wherein said plural fiber gratings comprise a chirped fiber grating
having a bandwidth of several to several tens of nanometers.
15. The wavelength dispersion compensator as defined in claim 2,
wherein said input terminal, said input and output terminal, and
said output terminal are arranged symmetrically around said
circulator, and a direction of propagation of said optical signals
is in a direction away from said output terminal and toward said
plural fiber gratings.
16. The wavelength dispersion compensator as defined in claim 2,
wherein said circulator comprises four or more terminals.
17. The wavelength dispersion compensator as defined in claim 2,
wherein said optical signals are input directly to one of said
plural fiber gratings from one of said circulator and another one
of said plural fiber gratings.
18. The wavelength dispersion compensator as defined in claim 1,
wherein refractive indices of said plural fiber gratings vary
periodically as a function of longitudinal distance, and a period
of variation is less than 1 .mu.m.
19. The wavelength dispersion compensator as defined in claim 1,
wherein an optical signal reflected by a fiber grating in said
plural fiber gratings has a wavelength, .lambda..sub.B, given by
Equation (1),.lambda..sub.B=2n.sub.effA(.mu.m) (1)wherein n.sub.eff
comprises an effective refractive index of an optical fiber in said
fiber grating, and A comprises a coefficient determined by a
variation of the refractive index of said optical fiber.
20. A method of compensating for wavelength dispersions of
wavelength division multiplexed (WDM) optical signals, said method
comprising: reflecting said WDM optical signals at different
positions depending on wavelengths of said reflected optical
signals, using plural fiber gratings, wherein a number of said
plural fiber gratings which is at least equivalent to a number of
said optical signals are connected in series.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a wavelength dispersion compensator
and an optical transmission line, and especially to a wavelength
dispersion compensator and an optical transmission line using the
aforementioned wavelength dispersion compensator.
BACKGROUND OF THE INVENTION
[0002] In the wavelength division multiplexed (WDM, hereinafter)
optical communication, optical signals propagated through an
optical transmission line (optical fibers) respectively undergo
wavelength dispersions. The wavelength dispersion is a kind of
dispersion, and means that a shape of an optical pulse propagated
through the optical fiber is deformed or broadened in various ways
depending on a slight difference in the wavelength of the optical
pulse, and group velocities of the respective optical signals take
different values, chiefly because the refractive index of the
optical fiber varies as a function of the wavelength of a
light.
[0003] Accordingly, the wavelength dispersion is a primary factor
of a limitation of the transmission distance of the optical
communication system, or of a deterioration of the quality of the
optical transmission caused by the distortion of the optical pulse.
Especially, in the long distance transmission system using
Erbium-doped fiber amplifiers, since the optical signal is
propagated through the optical fiber without being converted into
an electrical signal from the sending end to the receiving end, the
wavelength dispersion caused in the total length of the
transmission line is accumulated on the optical signal.
[0004] Moreover, in a case of the long distance transmission system
such as the submarine optical transmission system or of a high bit
rate transmission system, the effect of the accumulation of the
wavelength dispersion becomes a serious problem. Accordingly, the
effect of the wavelength dispersion must be compensated by taking
some measures, and explaining concretely, the wavelength dispersion
has been thus far compensated by inserting the dispersion
compensation fiber into the optical transmission line, where the
wavelength dispersion of the dispersion compensation fiber should
have the same absolute value as and a sign opposite to that of the
optical transmission line.
[0005] The structure of the optical transmission line using the
dispersion compensation fiber is shown in FIG. 1. In general, the
wavelength dispersion of the dispersion compensation fiber 110 and
that of the optical fiber for the signal transmission 120 change in
accordance with the wavelength of the optical signal. Accordingly,
if the wavelength dispersion of the dispersion compensation fiber
11 has the same absolute value as and a sign opposite to that of
the optical fiber for the signal transmission 120 independently of
the wavelength, the wavelength dispersion of the optical signal
must be compensated perfectly at any wavelength.
[0006] However, although it is possible to theoretically cancel the
wavelength dispersion of the optical fiber for the signal
transmission 120 by the wavelength dispersion compensation fiber
110 having the aforementioned characteristic, it has been
impossible to actually provide the wavelength dispersion
compensation fiber 110 which cancels the wavelength dispersions of
the optical fiber for the signal transmission throughout all the
channels of the WDM optical signals.
[0007] Herein, FIG. 2 shows a dispersion map of the optical signals
in case that the dispersion compensation fibers having the positive
wavelength dispersion are inserted into the optical transmission
line composed of optical fibers for the signal transmission having
the negative wavelength dispersion. As shown in FIG. 2, the
wavelength dispersions of the optical signal .lambda..sub.1 to
.lambda..sub.5 are compensated by the dispersion compensation
fibers, and approach zero at a certain interval of the transmission
distance.
[0008] As seen from FIG. 2, although only the wavelength dispersion
of the optical signal .lambda..sub.3 return to zero whenever it is
compensated by the dispersion compensation fibers, those of the
other optical signals .lambda..sub.1, .lambda..sub.2,
.lambda..sub.4, .lambda..sub.5, do not return to zero even when
they are compensated by the same, because the wavelength
dispersions of the optical signals vary as functions of the
wavelengths of the optical signals, and thereby the wavelength
dispersions are accumulated on the optical signals depending on the
transmission distance.
[0009] That is to say, although the conventional dispersion
compensation fiber can compensate the accumulated wavelength
dispersion satisfactory only when it is limited within a certain
value, it cannot return the accumulated wavelength dispersion
exceeding the certain limit to zero. Namely, the range of the
accumulated wavelength dispersion which can be satisfactorily
compensated by the conventional dispersion compensation fiber is
limited.
[0010] Moreover, following disadvantages are pointed out on the
conventional dispersion compensation fiber. That is to say, an
insertion loss of the conventional dispersion compensation fiber
becomes large as the wavelength dispersion to be compensated
becomes large. The dispersion compensation fiber necessitates a
length which is proportional to the length of the optical
transmission line, and the weight thereof becomes large.
Accordingly, it becomes difficult to make the dispersion
compensator using the conventional wavelength dispersion
compensation fiber and devices concerned therewith compact and
lightweight, and to reduce consumed electric power and the cost
price thereof.
[0011] Although it can be considered that the accumulated
wavelength dispersions are compensated in the lump at the
transmitting and receiving terminals (not shown), since the
accumulated wavelength dispersion exceeding a certain value cannot
be compensated similarly to the conventional dispersion
compensation fiber, it is impossible to compensate the accumulated
wavelength dispersions throughout all the cannels of the WDM
optical signals.
SUMMARY OF THE INVENTION
[0012] Accordingly, it is an object of the invention to provide a
wavelength dispersion compensator and an optical transmission line
in which a wavelength dispersion can be compensated throughout a
wide range by a wavelength dispersion compensator having a simple
structure, a wavelength dispersion compensator can be made compact,
consumed electric power can be reduced, and a quality of a
transmission of an optical transmission line can be heightened.
[0013] According to the first feature of the invention, a
wavelength dispersion compensator for compensating wavelength
dispersions of WDM optical signals which have been propagated
through an input optical transmission line, comprises:
[0014] plural fiber gratings of a refection type which reflect the
WDM optical signals at different positions depending on wavelengths
of the reflected optical signals.
[0015] The wavelength dispersion compensator mentioned in the above
corresponds to claim 1.
[0016] If the wavelength dispersion compensator having the
aforementioned structure is adopted, since the incident optical
signals are reflected by the fiber gratings of the reflection type
at different positions depending on the wavelengths of the
reflected optical signals, there arises a difference in the time
spent in travelling to and from the fiber grating of the reflection
type between the reflected optical signals, and thereby the
wavelength dispersions of the optical signals can be compensated.
Accordingly, since the plural optical signals, the wavelength
dispersions of which are satisfactorily compensated, are
transmitted on the optical transmission line, the quality of the
signal transmission of the optical transmission line can be
heightened. Moreover, since the weight of the fiber gratings of the
reflection type is lighter than that of the conventional wavelength
dispersion compensation fiber, and the length of the former is
shorter that of the latter, the wavelength dispersion compensator
according to the invention can be made compact and lightweight, and
the cost price thereof can be cut down.
[0017] In the wavelength dispersion compensator according to claim
2, the wavelength dispersion compensator further comprises a
circulator, an input terminal of which is connected with the input
optical transmission line, an input and output terminal of which is
communicated with the plural fiber gratings of a reflection type,
and an output terminal of which is connected with an output optical
transmission line or an output optical waveguide.
[0018] If the wavelength dispersion compensator having the
aforementioned structure is adopted, the wavelength dispersion
compensator can be fabricated simply by combining the circulator
with the plural fiber gratings of the reflection type, and thereby
the wavelength dispersions of the plural optical signals propagated
throughout the input optical transmission line can be
compensated.
[0019] In the wavelength dispersion compensator according to claim
3, each of the plural fiber gratings of the reflection type is
formed of a chirped fiber grating of the reflection type.
[0020] If the wavelength dispersion compensator having the
aforementioned structure is adopted, since the chirped fiber
grating of the reflection type which can be used at any wavelength
or can compensate any amount of the wavelength dispersion can be
fabricated, and the chirped fiber grating of the reflection type
has such a feature that the wavelength of the reflected optical
signal varies continuously as a function of a longitudinal
distance, the wavelength dispersion compensator according to the
invention can compensated the wavelength dispersions throughout a
wide range of the wavelength.
[0021] In the wavelength dispersion compensator according to claim
4, the wavelength dispersion compensator further comprises a gain
equalizer, wherein:
[0022] an input terminal of the gain equalizer is connected with
the output terminal of the circulator, and an output terminal of
the gain equalizer is connected with the output optical
transmission line or the output optical waveguide.
[0023] If the wavelength dispersion compensator having the
aforementioned structure is adopted, unevenness of the levels of
the optical signals reflected from the plural fiber gratings of the
reflection type can be equalized. Accordingly, the accumulated
wavelength dispersion of the optical signals can be compensated and
the signal levels of the same can be equalized by the wavelength
dispersion compensator according to the invention.
[0024] In the wavelength dispersion compensator according to claim
5, the wavelength dispersion compensator further comprises a series
connection of an optical repeater and a gain equalizer,
wherein:
[0025] an input terminal of the optical repeater is connected with
the output terminal of the circulator, an input terminal of the
gain equalizer is connected with an output terminal of the optical
repeater, and an output terminal of the gain equalizer is connected
with the output optical transmission line or the output optical
waveguide.
[0026] If the wavelength dispersion compensator having the
aforementioned structure is adopted, some of the optical signals
reflected from the fiber gratings of the reflection type, powers of
which are lower than their rated powers assigned by the level
diagram, can be amplified by the optical repeater to achieve their
regular values.
[0027] Moreover, since the gain equalizer is connected in series
with the optical repeater, the accumulated wavelength dispersions
of the respective optical signals are compensated, the powers of
the same are amplified, and the signal levels of the same are
equalized. Accordingly, the quality of the signal transmission of
the optical transmission line can be heightened.
[0028] According to the second feature of the invention, the
optical transmission line according to claim 6 comprises:
[0029] plural wavelength dispersion compensators, each of which
compensates a wavelength dispersion of a single optical signal,
[0030] a demultiplexing arrayed waveguide grating (AWG,
hereinafter) which demultiplexes WDM optical signals into plural
optical signals, and supplies them to the plural wavelength
dispersion compensators respectively, and
[0031] a multiplexing AWG which multiplexes the plural optical
signals respectively outputted from the plural wavelength
dispersion compensators,
[0032] wherein each of the plural wavelength dispersion
compensators comprises:
[0033] a fiber grating of the reflection type for reflecting one of
the plural optical signals, and
[0034] a circulator, an input terminal of which is communicated
with the demultiplexing AWG, an input and output terminal of which
is communicated with the fiber grating of the reflection-type, and
an output terminal of which is communicated with the multiplexing
AWG,
[0035] wherein optical distances between the fiber gratings of the
reflection type and the corresponding circulators take different
values depending on wavelengths of the reflected optical signals in
the plural wavelength dispersion compensators.
[0036] If the optical transmission line having the aforementioned
structure is adopted, since the plural wavelength dispersion
compensators are respectively provided for the plural optical
signals obtained by demultiplexing the WDM optical signals, the
wavelength dispersions can be compensated throughout a wide range
of the wavelength.
[0037] In the optical transmission according to claim 7, the fiber
grating of the reflection type of each of the plural wavelength
dispersion compensators is formed of a chirped fiber grating of the
reflection type.
[0038] The advantage of this structure is similar that of the
wavelength dispersion compensator according to claim 3.
[0039] In the optical transmission line according to claim 8, the
optical transmission line further comprises gain equalizers,
wherein:
[0040] input terminals of the gain equalizers are respectively
connected with the output terminals of the circulators, and output
terminals of the gain equalizers are respectively communicated with
the multiplexing AWG.
[0041] The advantage of this structure is similar to that of the
wavelength dispersion compensator according to claim 4.
[0042] In the optical transmission line according to claim 9, the
optical transmission line further comprises optical repeaters
connected with gain equalizers in series, wherein:
[0043] input terminals of the optical repeaters are respectively
connected with the output terminals of the circulators, output
terminals of the optical repeaters are respectively connected with
input terminals of the gain equalizers, and output terminals of the
gain equalizers are respectively communicated with the multiplexing
AWG.
[0044] The advantage of this structure is similar to that of the
wavelength dispersion compensator according to claim 5.
[0045] In the optical transmission line according to claim 10,
further comprises Co doped fibers inserted between the wavelength
dispersion compensators and the multiplexing AWG.
[0046] If the optical transmission line having the aforementioned
structure is adopted, since some of the optical signals outputted
from the plural wavelength dispersion compensators, each having a
high signal level, are propagated through the Co fibers and undergo
insertion losses, the levels of the respective optical signals are
equalized.
[0047] In the optical transmission line according to claim 11,
further comprises optical amplifiers respectively inserted between
the wavelength dispersion compensators and the multiplexing
AWG.
[0048] If the optical transmission line having the aforementioned
structure is adopted, since the optical signals, the accumulated
wavelength dispersions of which are respectively compensated by the
plural wavelength dispersion compensators, are so amplified by the
optical amplifiers that the peak levels of the optical signals are
equalized, and the outputs of the optical amplifiers are supplied
to the multiplexing AWG, the deflections of the respective signal
levels can be corrected easily.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] The invention will be explained in more detail in
conjunction with appended drawings, wherein:
[0050] FIG. 1 shows a structure of a conventional optical
transmission line schematically,
[0051] FIG. 2 is a diagram for showing accumulated wavelength
dispersions of optical signals in a conventional optical
transmission line as functions of a transmission distance,
[0052] FIG. 3 shows a structure of a wavelength dispersion
compensator according to the invention schematically,
[0053] FIG. 4 shows a structure of another wavelength dispersion
compensator according to the invention schematically,
[0054] FIG. 5 shows a structure of the other wavelength dispersion
compensator according to the invention schematically,
[0055] FIG. 6 is a diagram for showing wavelength dispersions of
optical signals in an optical transmission line using a wavelength
dispersion compensator according to the invention as functions of a
transmission distance,
[0056] FIG. 7 shows a structure of an optical transmission line
according to the invention schematically,
[0057] FIG. 8 shows a structure of another optical transmission
line according to the invention schematically,
[0058] FIG. 9 shows a structure of the other optical transmission
line according to the invention schematically.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0059] Hereafter, preferred embodiments of the invention will be
explained referring to the appended drawings.
Structure of Wavelength Dispersion Compensator
[0060] First, a wavelength dispersion compensator according to the
preferred embodiment of the invention will be explained referring
to FIG. 3. This drawing shows an internal structure of the
wavelength dispersion compensator according to the invention.
[0061] As shown in FIG. 3, the wavelength dispersion compensator 10
according to the invention is composed of plural fiber gratings of
the reflection type 11 and a circulator 12. Herein, the fiber
grating of the reflection type 11 is formed of an optical fiber,
the refractive index of which varies periodically as a function of
the longitudinal distance, and the period of variation of the
refractive index is less than 1 .mu.m. According to the
aforementioned structure, the fiber grating of the reflection type
11 functions as a band rejection filter having an extremely narrow
band width. That is to say, the fiber grating of the reflection
type 11 shown in FIG. 3 reflects an incident optical signal having
a specified wavelength in the opposite direction, and the other
optical signals pass therethrough without hindrance.
[0062] The wavelength of the optical signal reflected by the fiber
grating of the reflection type 11 is given by the following
equation.
.lambda..sub.B=2n.sub.effA(.mu.m),
[0063] wherein
[0064] .lambda..sub.B: the wavelength of the reflected optical
signal,
[0065] n.sub.eff: the effective refractive index of the optical
fiber forming the fiber grating, and
[0066] A: a coefficient determined by a variation of the refractive
index of the optical fiber forming the fiber grating (.mu.m).
[0067] In the above equation, when the fiber grating of the
reflection type 11 is used in the 1.55 .mu.m band, A is about 0.5
.mu.m. Accordingly, the fiber grating of the reflection type 11
serves as a band rejection filter having a center wavelength of
.lambda..sub.B and a band width of about 1 nm or below.
[0068] The number of the fiber gratings of the reflection type 11
is the same as that of the optical signals. The fiber gratings of
the reflection type 11 are connected in cascade by an optical fiber
16 or an optical wave guide, and an input end (the left end) of the
optical fiber 16 is connected with a port 12-2 (an input and output
terminal) of the circulator 12. As seen from FIG. 3, since the
optical signals are respectively reflected by the fiber gratings of
the reflection type 11 which are situated at different positions
depending on the wavelengths of the optical signals to be
reflected, in other words, since the length of time in which the
optical signal travels from the fiber grating of the reflection
type 11 to the circulator 12 varies depending on the wavelength of
the reflected optical signal, the wavelength dispersions
accumulated on the plural optical signals can be compensated
satisfactorily by suitably determining the positions of the fiber
gratings of the reflection type 11.
[0069] Furthermore, in case that the fiber gratings of the
reflection type 11 is used in the wavelength dispersion compensator
10, it is necessary to design the amount of variation of the
refractive index, the period of the same, and the total length of
this device should be determined so that the fiber grating of the
reflection type 11 meets the characteristics required thereto.
[0070] Moreover, a chirped fiber grating (not shown) may be applied
to the fiber grating of the reflection type 11. The chirped fiber
grating is a kind of the fiber grating of the reflection type 11,
in which the period of variation of the refractive index of the
fiber varies as a function of the longitudinal distance. According
to the aforementioned structure, since the center wavelength of the
reflection band varies continuously depending on the period of
variation of the refractive index, the band width of the chirped
fiber grating becomes wider than that of the ordinary fiber
grating, and the band width of several to several tens nm can be
obtained.
[0071] In case that the chirped fiber grating is applied to the
wavelength dispersion compensator, the number of the chirped fiber
grating is the same as that of the optical signals, and the chirped
fiber gratings are connected in cascade by an optical fiber or an
optical waveguide similarly to the ordinary fiber gratings of the
reflection type 11. Each optical signal is reflected by the
corresponding chirped fiber grating of the reflection type situated
at an appropriate position, and its wavelength dispersion is
compensated.
[0072] The function of the circulator 12 is used in the optical
circuit shown in FIG. 3 is similar to that used in a microwave
circuit, and the port 12-1 (an input terminal), the port 12-2 (an
input and output terminal), and the port 12-3 (an output terminal)
are arranged symmetrically therearound. In case that the optical
signal is transmitted between the adjacent terminals, if the
direction of the propagation of the optical signal coincides with
an arrow on the circulator 12 (from the port 12-1 to the port 12-2,
from the port 12-2 to the port 12-3, from the port 12-3 to the port
12-1), the insertion loss of the optical signal is quite low. On
the other hand, if the direction of the propagation of the optical
signal is opposite to the arrow, the insertion loss of the optical
signal is extremely high. Although the circulator 12 having the
three terminals is applied to the wavelength dispersion compensator
shown in FIG. 3, the circulator having the four or more terminals
may be applied thereto. In FIG. 3, the optical signal reflected
from the waveguide gratings of the reflection type 11 again in
inputted to the port 12-2 (the input and output terminal) of the
circulator 12, and transmitted to the output optical transmission
line via the port 12-3 (the output terminal) of the circuit 12.
[0073] In the wavelength dispersion compensator 10 shown in FIG. 4,
a gain equalizer 14 may be connected with the port 12-3 (the output
terminal) of the circulator 12 via an optical fiber or an optical
waveguide. The gain equalizer 14 equalizes unevenness of the
optical signal levels, which take different values depending on the
wavelengths of the optical signals. Since the respective optical
signals undergo different transmission losses which take different
values depending on the wavelengths of the optical signals while
they propagate through the optical fiber for transmission 30, the
levels of the optical signals are lowered ununiformaly.
[0074] Accordingly, if the gain equalizer 14 is added to the
wavelength dispersion compensator 10, unevenness of the levels of
the optical signals which are propagated through the optical fiber
for the signal transmission 30 and reflected by the fiber gratings
of the reflection type 11 are equalized by the gain equalizer 14,
and the optical signals outputted form the gain equalizer 14 can be
supplied to the optical transmission line 1.
[0075] As mentioned in the above, the wavelength dispersion
compensator 10 shown in FIG. 4 completes equalization of the levels
of the optical signals as well as the dispersion compensations of
the same. Since the aforementioned wavelength dispersion
compensator 10 can supply the optical signals, in which unevenness
of the levels thereof are equalized and the wavelength dispersions
accumulated thereon are compensated, to the optical transmission
line 1, the quality of the signal transmission of the optical
transmission line 1 can be heightened.
[0076] Moreover, as shown in FIG. 5, a series connection of an
optical repeater 15 and a gain equalizer 14 may be added to the
wavelength dispersion compensator 10 at the port 12-3 (the output
terminal) of the circulator 12 via an optical fiber or an optical
waveguide. By providing the optical repeater 15 for the wavelength
dispersion compensator 10, some of the optical signals reflected
from the fiber gratings of the reflection type 11, powers of which
are lower than their rated values assigned by the level diagram,
are amplified by the optical repeater 15 to achieve their regular
values.
[0077] Accordingly, in the wavelength dispersion compensator 10,
the wavelength dispersions of the optical signals can be
compensated, the powers of the optical signals are compensated to
achieve their regular values assigned by the level diagram, and
their signal levels are equalized. Moreover, since the optical
signals in which their powers are compensated and their signal
levels are equalized can be transmitted, the quality of the signal
transmission of the optical transmission lines can be
heightened.
[0078] Next, the operation of the wavelength dispersion compensator
will be explained referring to FIG. 3. In the WDM optical
transmission, the optical signals propagated through the optical
transmission line 1 mentioned later are supplied to the circulator
12 of the wavelength dispersion compensator 10.
[0079] The optical signals inputted to the port 12-1 of the
circulator 12 are transmitted to the port 12-2, and supplied to the
fiber gratings of the reflection type 11. In the wavelength
dispersion compensator 10 shown in FIG. 3, the optical signal
having a specified wavelength is reflected by one of the plural
fiber gratings of the reflection type 11, again inputted to the
port 12-2 (the input and output terminal), and outputted from the
port 12-3 (the output terminal) to the optical transmission line.
Since the fiber gratings of the reflection type 11 are situated at
the different positions, there arises a difference in the time
spent in traveling to and from the fiber grating of the reflection
type 11 between the optical signals. Accordingly, the wavelength
dispersions of the optical signals can be satisfactorily
compensated by suitably determining the positions of the fiber
gratings of the reflection type 11.
[0080] As mentioned later, the optical signal .lambda..sub.1 to
.lambda..sub.5 are transmitted from the optical transmitter (OS) 20
situated at the sending end, and the wavelength dispersions of the
optical signals are compensated by the wavelength dispersion
compensator 10 in the lump. FIG. 6 shows a dispersion map of the
aforementioned system, and the wavelength dispersions of the
optical signals .lambda..sub.1 to .lambda..sub.5 are represented as
functions of the transmission distance. As shown in FIG. 6, zigzag
lines representing the wavelength dispersions of the respective
optical signals coverage at a point A, which corresponds to the
position of the wavelength dispersion compensator 10. It should be
noted that the wavelength dispersions of the all the optical
signals .lambda..sub.1 to .lambda..sub.5 become zero at the point
A.
[0081] When the dispersion map shown in FIG. 2 is compared with
that shown in FIG. 6, it can be clearly understood that the
wavelength dispersion compensator 10 according to the invention
keeps the wavelength dispersions of the optical signals within a
far narrower range than that of the conventional optical
transmission system throughout a wide range of the wavelength.
Accordingly, when the wavelength dispersion compensator 10 is
applied to the optical transmission system, the optical signals can
be transmitted under the low wavelength dispersions, and the
quality of the signal transmission can be heightened.
Optical Transmission Lines According to Referred Embodiments of
Invention
[0082] Next, an optical transmission line according to the
invention will be explained referring to FIG. 7. FIG. 7 shows a
structure of the optical transmission line according to the
preferred embodiment of the invention schematically. As shown in
this drawing, the optical transmission line 1 is composed of an
optical transmitter (OS) 20, an optical fiber for a signal
transmission 30, optical repeaters 40, and an optical receiver (OR)
50. Furthermore, a wavelength dispersion compensator 10 is added
thereto.
[0083] In the optical transmission line 1, dispersion compensation
fibers 60 may be inserted into the optical fiber for the signal
transmission 30 in series at predetermined positions on the route
thereof. In the optical transmission line 1, the wavelength
dispersions of the optical signals can be compensated surely by
using the wavelength dispersion compensator 10 in combine with the
dispersion compensation fibers 60.
[0084] Moreover, the optical signals transmitted from the optical
transmitter (OS) 20 can be transmitted to the optical receiver (OR)
50 after the wavelength dispersions thereof have been compensated
in the wavelength dispersion compensator 10 by providing the
wavelength dispersion compensator 10 for the optical transmission
line 1. Accordingly, the quality of the signal transmission of the
optical transmission line 1 can be heightened. The structure of the
wavelength dispersion compensator 10 can be selected from those
shown in FIGS. 3 to 5.
[0085] Moreover, as shown in FIG. 8, the optical transmission line
1 may be provided with a demultiplexing arrayed waveguide grating
(AWG, hereinafter) 70a and a multiplexing AWG 70b. The plural
wavelength dispersion compensators 10-1 to 10-n connected in
parallel are inserted between the demultiplexing AWG 70a and the
multiplexing AWG 70b.
[0086] Herein, each of the demultiplexing AWG 70a and the
multiplexing AWG 70b is composed of the waveguides which are
arranged thereon by depositing SiO.sub.2 on a Si substrate and
arranged thereon to form an array. According to the aforementioned
structure, since the diffraction angle of the optical signal in the
slab waveguide varies as a function of the wavelength of the
optical signal similarly to the diffraction gratings, the
demultiplexed optical signals can be taken out from the output
waveguides.
[0087] Each of the wavelength dispersion compensators 10-1 to 10n
corresponds to one of the WDM optical signals diffracted by the
demultiplexing AWG 70a. Accordingly, the wavelength dispersions of
the WDM optical signals can be compensated throughout a wide range
of the wavelength by the wavelength dispersion compensators 10-1 to
10-n as a whole. It should be noted that, although each of the
dispersion compensators 10-1 to 10-n is provided with a single
fiber grating of the reflection type 11, optical distances between
the fiber gratings of the reflection type 11 and the corresponding
circulators 12 take different values depending on the wavelengths
of the reflected optical signals in the plural wavelength
dispersion compensators 10-1 to 10-n.
[0088] In the optical transmission line 1 provided with the
demultiplexing AWG 70a and the multiplexing AWG 70b, Co doped
fibers 80 may be inserted between the wavelength dispersion
compensators 10-1 to 10-n and the multiplexing AWG 70b. The Co
doped fibers 80 are inserted between the wavelength dispersion
compensators 10-2 to 10-(n-1) outputting the optical signals having
high signal levels and the multiplexing AWG 70b.
[0089] If the optical transmission line 1 having the structure
shown in FIG. 8 is adopted, since some of the optical signals
outputted from the wavelength dispersion compensators 10-1 to 10-n,
each having a high signal level, are propagated through the Co
doped fibers 80 and undergo insertion losses, the levels of the
optical signals to be multiplexed by the multiplexing AWG 70b can
be equalized.
[0090] Moreover, in the optical transmission line 1 shown in FIG.
9, the optical amplifiers 90-1 to 90-n may be inserted between the
output terminals of the wavelength dispersion compensators 10-1 to
10-n and the multiplexing AWG 70b respectively.
[0091] If the optical transmission line is constructed in the
structure shown in FIG. 9, the optical signals outputted from the
wavelength dispersion compensators 10-1 to 10-n can be so amplified
by the optical amplifiers 90-1 to 90-n that the peak levels of the
optical signals are equalized. According to the aforementioned
structure, deflections of the signal levels in the respective
channels can be corrected easily.
[0092] As mentioned in the above, according to the invention, in
the fiber gratings of the reflection type provided for the
wavelength dispersion compensator, since the optical signals are
reflected from the different positions depending on the wavelengths
of the incident optical signals, the wavelength dispersions of the
respective optical signals can be compensated because of the
difference in the time spent in travelling to and from the fiber
gating of the reflection type between the optical signals.
Accordingly, since the optical signals, the wavelength dispersions
of which are satisfactorily compensated, are transmitted to the
optical receiver, the quality of the signal transmission of the
optical transmission line can be heightened.
[0093] Since the weight of the fiber gratings of the reflection
type is less than that of the conventional dispersion compensation
fiber and the length thereof can be made short, the wavelength
dispersion compensator becomes compact and lightweight, and the
cost price thereof can be reduced. The wavelength dispersions of
the optical signals can be compensated by a simple structure by
providing the fiber gratings of the reflection type and the
circulator for the wavelength dispersion compensator.
[0094] The fiber grating of the reflection type can be fabricated
for the optical signal having a wavelength selected at will and a
wavelength dispersion to be compensated having any desired value.
Moreover, the wavelength dispersion extending over a wide range can
be compensated by adopting the chirped fiber grating, in which the
wavelength of the reflected optical signal varies continuously in
the longitudinal direction.
[0095] Unevenness of the levels of the optical signals reflected
from the fiber gratings of the reflection type can be equalized by
inserting the gain equalizer between the output terminal of the
circulator and the optical transmission line. Moreover, some of the
optical signals reflected from the fiber gratings of the reflection
type, the power levels of which are lower than their rated values
assigned by the level diagram, can be compensated by the optical
repeater inserted between the circulator and the gain
equalizer.
[0096] Since the wavelength dispersion compensator having the fiber
gratings of the reflection type is provided for the optical
transmission line, the insertion losses caused by increases of the
dispersion compensations are lower than those of the conventional
dispersion compensation fiber, and thereby the loss compensation
amplifier becomes unnecessary, hence the cost prices of the optical
transmission line and the wavelength dispersion compensator can be
cut down.
[0097] Since the optical transmission system is composed of the
plural wavelength dispersion compensators corresponding to the
respective optical signals which are propagated through the optical
transmission line and undergo the wavelength dispersions, the
demultiplexing AWG which demultiplexes the WDM optical signals into
the plural optical signals and supply them to the plural wavelength
dispersion compensators respectively, and the multiplexing AWG
which multiplexes the respective optical signals outputted from the
plural wavelength dispersion compensators, the wavelength
dispersions of the optical signals are compensated throughout a
wide range of the wavelength by the plural wavelength dispersion
compensators, each of which is suitably designed to serve as a
compensator for an optical signal inputted thereto.
[0098] Moreover, since the Co doped fibers are inserted between the
wavelength dispersion compensators and the multiplexing AWG, some
of the optical signals outputted from the wavelength dispersion
compensators, each having a high signal level, are propagated
through the Co doped fibers, and undergo level losses, hence the
signal levels of the respective optical signals can be
equalized.
[0099] Moreover, since the optical amplifiers are inserted between
the wavelength dispersion compensators and the multiplexing AWG,
the optical signals, the wavelength dispersions accumulated on
which have been compensated by the wavelength dispersion
compensators, are so amplified by the optical amplifiers that the
peak levels thereof are equalized, and transmitted to the
multiplexing AWG, hence the deflections of the levels of the
optical signals can be corrected easily.
[0100] Although the invention has been described with respect to
specific embodiment for complete and clear disclosure, the appended
claims are not to be thus limited but are to be construed as
embodying all modification and alternative constructions that may
be occurred to one skilled in the art which fairly fall within the
basic teaching herein set forth.
* * * * *