Note: Descriptions are shown in the official language in which they were submitted.
:- 219462~ -
CHROMATIC DISPERSION CONPENSATOR AND CHROMATIC DISPERSION
COMPENSATING OPTICAL C~MMUNICATION SYSTEM
-BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chromatic dispersion
-compensator for compensating chromatic dispersion in a
transmission path of an optical fiber communication system and
relates to a chromatic dispersion compensating optical
communication system using such a chromatic dispersion
compensator.
2. DescriPtion of the Related Art
If there arises chromatic dispersion in an optical
fiber used as a transmission path in a optical digital
transmission system, lowering of transmission quality is
brought about. This is because chromatic dispersion and
chirping produced by direct modulation of a semiconductor laser
as a transmitter are coupled to generate waveform distortion.
The degradation of transmission quality caused by waveform
distortion becomes more remarkable as the bit rate increases.
For example, in the case of 10 Gbps, it is necessary that the
waveform distortion (spreading) is controlled to be
sufficiently smaller than the time width of one slot, that is,
about 10 ps which is one-tenth as much as 1/10 Gbps = 100 ps.
As adding to the recent impLove,-lent of the bit rate,
the distance of repeaterless transmission has been elongated
with the advent of an Er-doped optical fiber amplifier having
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2 1 94 628
an amplifying function in a 1.55 ~m band in which a silica
based optical fiber exhibits the lowest transmission loss, the
necessity of delicately controlling the value of chromatic
dispersion has arisen even in the case where a dispersion
shifted fiber (DSF) having zero dispersion wavelength in the
1.55 ~m band is used as a transmission path. For example,
there is used a chromatic dispersion compensating technique in
which accumulated chromatic dispersion is cancelled by a fiber
having chromatic dispersion with opposite sign at each relay
point.
Further, in the case where a 1.55 ~m-band optical fiber
amplifier is used in the transmission path of a 1.3 ~m-band
single mode fiber (1.3 SMF) having been already installed or in
the transmission path of a very low loss pure silica core
fiber, large positive chromatic dispersion of these fibers in
the 1.5S ~m band becomes a problem. Therefore, a dispersion
compensating fiber having large negative chromatic dispersion
in the 1.55 ~m wavelength band has been developed. Such a
dispersion compensating fiber is known by ELECTRONICS LETTERS,
Vol. 30, No. 2, (1994-1-20), pp. 161-162.
Considering further that the capacity will be increased
more and more in the future, a wavelength division multiplexing
transmission method (WDM) promises a bright future. In this
case, it is necessary that chromatic dispersion takes zero in
a wavelength range of a optical signal used. However,
chromatic dispersion itself has dependency on wavelength. In
.
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21 q4628
the case of a matched cladding type fiber, the slope of
dispersion, that is, the dispersion slope, is generally
positive. Accordingly, it is difficult to set the chromatic
dispersion zero in a wide wavelength range.
In order to solve this problem, a chromatic dispersion
flattened fiber in which chromatic dispersion is approximately
zero in a wide wavelength range is used as a transmission path.
Further, a dispersion compensating fiber having a negative
dispersion slope has been developed, for example, as known by
European Conference on Optical Communication '94, pp. 681-684.
However, it is difficult to produce these fibers because these
fibers are complex in the form of refractive index distribution
so as to be not controllable.
On the other hand, a chirped grating has been proposed
as means for compensating chromatic dispersion, for example, as
known by Optical Fiber Communication Conference '94,
postdeadline paper-2, PD2-1 to PD2-4. First, a fiber grating
will be described. The fact that the refractive index of a
core portion of a Ge-doped core optical fiber is increased when
ultraviolet rays of wavelength near 240 nm are radiated onto
the Ge-doped core optical fiber is known by Inoue et al,
~'Generation of Fiber Grating and Application thereof~,
SHINGAKU-GIHOU, OPE94-5, Institute of Electronics, Information
and Communication Engineers of Japan. A periodic refractive
index change is formed in the fiber core by using the
ultraviolet rays induced refractive index change, by which a
21 94628
diffraction grating can be obtained so that a specific
wavelength can be reflected by the diffraction grating.
Fig. 10 is an explanatory view for explaining the
chirped grating. In the drawing, the reference numeral 61
designates an optical signal of wavelength ~1; 62, an optical
signal of wavelength A2; 63, an optical signal of wavelength A3;
64, an optical signal of wavelength A4; and 65, an optical
fiber. The relations between the magnitudes of the wavelengths
are Al~12~A3~A4. The chirped grating operates so that the
wavelength reflected by the aforementioned diffraction grating
is shifted in the direction of the length of the fiber, that
is, chirped. Chromatic dispersion can be compensated by the
chirped grating. The optical fiber 65 has the core portion in
which the refractive index is changed by the ultraviolet rays
induced refractive index change. Respective optical signals 61
to 64 of wavelengths Al to A4 incident to the optical fiber from
the left in the drawing are reflected at intermediate portions
so as to return to the incident side.
The refractive index change, that is, the period of the
grating is designed so as to be gradually reduced from the
incident side toward the right so that an optical signal of a
longer wavelength is reflected at a position nearer the
incident side. Further, by writing the grating so that the
percentage of the change of the period of the grating is
reduced as the wavelength becomes longer, the dispersion slope
can be selected to be negative. Incidentally, because
2194628
chromatic dispersion is a slope of propagation delay time with
respect to wavelength, and the dispersion slope is a slope of
the chromatic dispersion, the fact that the dispersion slope is
negative means the fact that the dependency of propagation
delay time on wavelength is convex upwards.
Fig. 11 is an explanatory view for explaining a WDM
transmission method. In the drawing, the reference numeral 71
designates an optical signal transmitter for transmitting an
optical signal of wavelength Al; 72, an optical signal
transmitter for transmitting an optical signal of wavelength
A2; 73, an optical signal transmitter for transmitting an
optical signal of wavelength A3; 74, an optical signal
transmitter for transmitting an optical signal of wavelength
A4; 75, a transmission path; 76, amplifiers; 77, a transmission
path; 78, a transmission path; 79, an optical receiver for
receiving an optical signal of wavelength Al; 80, an optical
receiver for receiving an optical signal of wavelength A2; 81,
an optical receiver for receiving an optical signal of
wavelength A3; and 82, an optical receiver for receiving an
optical signal of wavelength A4. Now, it is assumed that
amplified WDM transmission of four signal wavelengths by way of
example. In the transmitter side, the optical signal
transmitters 71 to 74 of wavelengths A " A2, A3 and A4 are
connected to one transmission-side transmission path 75 by a
multiplexer not shown. The transmission path 75 is connected
to final one amplifier 76 and the receiver side transmission
21 94628
path 78 through one pair of the relay amplifier 76 and the
transmission path 77 or a plurality of pairs of the relay
amplifiers 76 and the relay transmission paths 77. The
transmission path 78 is connected to the optical receivers 79
5to 82 of wavelengths Al, A2, ~3 and A4 by a demultiplexer not
shown.
Description will be made specifically by using
numerical values. The span of the transmission path 77 is
about 80 km. In the case where the transmission path 77 is
10comprised of a 1.3 ~m single mode fiber, chromatic dispersion
in wavelength of 1550 nm is 17 ps/nm/km, so that the quantity
of compensated chromatic dispersion of the overall relay
distance is 1360 ps/nm. Even if the amplification wavelength
range of the optical fiber amplifier is estimated to be 1550+10
15nm, that is, the width of the amplification wavelength range is
estimated to be 20 nm, the delay time difference of
(1360x20=27200ps=)27.2ns is required between the optical signal
of the longest wavelength and the optical signal of the
shortest wavelength in the amplification wavelength range.
20Conse~uently, the length of the optical fiber 65 giving the
chirped grating shown in Fig. 10 reaches 2.7 m unpractically.
Incidentally, the delay time difference means the
propagation time difference between the signal of the shortest
wavelength and the signal of the longest wavelength in the
25wavelength range of the optical signal as a subject. To set
the delay time difference to be A[ps], the grating length L[mm]
21 q4628
is selected to be L 3xl01l/1.5xAxl0~l2x(1/2)=Axl0~l[mm]. Here,
3xl01l[mm] is the velocity of light in vacuum, 1.5 is the
refractive index of glass, and (1/2) is a coefficient obtained
by taking into account the round trip of the optical signal.
Accordingly, a method in which gratings of narrow
wavelength widths near the respective optical signal
wavelengths Al to A4 are produced and the gratings thus produced
are arranged is known by the aforementioned Optical Fiber
Communication Conference '94, postdeadline paper-2, PD2-1 to
PD2-4. In this method, however, there arises a problem that
optical signal wavelengths Al to A4 in respective systems have
to be known in advance, etc.
SUMMARY OF THE INVENTION
It is an object of the present invention is to provide
a chromatic dispersion compensator for compensating the
chromatic dispersion of a transmission path in an optical
communication system and to provide a chromatic dispersion
compensating optical communication system using such a
chromatic dispersion compensator.
A chromatic dispersion compensator according to the
present invention is comprised of: an optical signal directing
unit having a first, second, third and fourth ports, for
directing an optical signal inputted from one of the ports to
another port of the ports; a reflection-type compensating unit
including a dispersion compensating fiber, a reflecting portion
21 ~4628
and a changing unit for changing a polarization direction of a
reciprocating signal light, in which the dispersion
compensating fiber is connected to the reflecting portion via
the changing unit; an input transmission path which is
connected to the first port; an output transmission path which
is connected to the fourth port so that the signal light is
outputted from the fourth port; wherein the reflection-type
compensating unit is connected to one of the second and third
ports, and the chirped grating is connected to the other port.
In addition, the chromatic dispersion compensating
optical communication system of the present invention includes
a transmission path and the above described chromatic
dispersion compensator which is connected to an intermediate or
end portion of said transmission path.
According to the chromatic dispersion compensator of
the present invention, chromatic dispersion which cannot be
compensated only by the dispersion compensating fiber is
compensated by the chirped grating, so that not only chromatic
dispersion of a transmission path can be compensated in a wide
wavelength range but also the grating length can be reduced.
Further, by using the optical circulator, the loss of insertion
can be reduced. Alternatively, by using the directional
coupler, the length of the grating can be reduced.
According to the chromatic dispersion compensating
optical communication system of the present invention, the
chromatic dispersion of the transmission path of the 1.3 ~m-
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band or the fiber having the zero dispersion wavelength in
1.55~m-band can be compensated.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings;
Fig. lA is an explanatory view for explaining a ~irst
embodiment of the present invention showing a case where an
optical circulator is used;
Fig. lB is an explanatory view for explaining the first
embodiment of the present invention showing a case where a
directional coupler is used;
Fig. 2 is a graph for explaining the chromatic
dispersion characteristic of the first embodiment;
Fig. 3 is an explanatory view for explaining a first
modified example of the first embodiment of the present
invention;
Fig. 4 is an explanatory view for explaining a second
modified example of the first embodiment of the present
invention;
Fig. 5 is an explanatory view for explaining a second
embodiment of the present invention;
Fig. 6 is a structural view of a specific example of a
second embodiment of the present invention;
Fig. 7 is a structural view of a conventional one as a
'5 comparative example;
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2 1 ~4628
Fig. 8 is a structural view of an example of the first
embodiment of the present invention as a comparative example;
Fig. 9 is a graph showing a receving sensitivity of the
specific examples as shown in Figs. 6 to 8;
Fig. 10 is an explanatory view for explaining a chirped
grating;
Fig. 11 is an explanatory view for explaining the
wavelength multi relay transmission method.
PREFERRED EMBODIMENTS OF THE INVENTION
The preferred embodiments of the present invention will
be described referring to the accompanying drawings as follows.
Fig. lA is an explanatory view for explaining a first
embodiment of the present invention showing a case where an
optical circulator is used, and Fig. lB is an explanatory view
for explaining a first embodiment of the present invention
showing a case where a directional coupler is used. In the
drawing, the reference numeral 1 represents an input
transmission path; 2, an optical circulator; 3, an output
transmission path; 4, a dispersion compensating fiber; 5, a
chirped grating; 6, a nonreflective terminal; and 7, a
directional coupler.
In Fig. lA, the input transmission path 1 is connected
to a first port of the optical circulator 2, and a third port
of the optical circulator 2 is connected to the output
transmission path 3. Further, a second port of the optical
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21 ~4628
circulator 2 is connected to the dispersion compensating fiber
4. The chirped grating 5 is connected to an end of the
dispersion compensating fiber 4. The chirped grating 5 is
- preferably terminated at the nonreflective terminal 6. For
example, the transmission path is constituted by a 1.3 ~m
single mode optical fiber. The chirped grating 5 reflects an
optical signal so that chromatic dispersion in the transmission
path which cannot be compensated by the round trip's chromatic
dispersion characteristic of the dispersion compensating fiber
4 is compensated by a relatively wide wavelength range.
In Fig. lB, the directional coupler 7 such as, for
example, an optical fiber coupler is used instead of the
optical circulator 2 which is a non-reciprocal element shown in
Fig. lA. In Fig. lB, ports in the left of the directional
coupler 7 are referred to as a first port and a third ports,
and ports in the right of the directional coupler 7 are
referred to as a second port and a fourth port. The input
transmission path 1 is connected to the first port. The third
port is connected to the output transmission path 3. The
second port is connected to the dispersion compensating fiber
4. The fourth port is preferably terminated at the
nonreflective terminal 6. Although an insertion loss not lower
than 3 dB arises when the directional coupler 7 is used, the
loss can be compensated by an amplifier.
~5 Fig. 2 is a graph for explaining the chromatic
dispersion characteristic of the first embodiment. The
21 94628
horizontal axis represents wavelength of an optical signal, and
the vertical axis represents chromatic dispersion. The
reference numeral 11 represents chromatic dispersion
characteristic of the transmission path; 12, round trip's
chromatic dispersion characteristic of the dispersion
compensating fiber; 13, characteristic obtained by adding the
round trip's chromatic dispersion of the dispersion
compensating fiber to the chromatic dispersion of the
transmission path; and 14, chromatic dispersion characteristic
of the chirped grating which has a negative dispersion slope
characteristic. The dispersion compensating fiber 4 has
negative chromatic dispersion so that the round trip's
chromatic dispersion characteristic 12 thereof exhibits
chromatic dispersion which is equal in absolute value to but
has opposite sign to the chromatic dispersion exhibited by the
chromatic dispersion characteristic 11 of the transmission path
at a predetermined wavelength Ao. As a result, at the
predetermined wavelength Aot the chromatic dispersion of the
transmission path is compensated so that the value of chromatic
dispersion becomes zero. When almost the chromatic dispersion
characteristic 11 of the transmission path is compensated as
described above, only a wavelength-dependent component of
chromatic dispersion remains as a component which cannot be
compensated. This residual characteristic is the
~5 characteristic 13 obtained by adding the round trip's chromatic
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21 94628
dispersion of the dispersion compensating fiber to the
chromatic dispersion of the transmission path.
Therefore, only the wavelength-dependent component of
the chromatic dispersion is compensated by the chirped grating
having the negative dispersion slope equal in absolute value to
but with negative sign to the dispersion slope of the
wavelength-dependent component. Consequently, chromatic
dispersion after compensation can be set to be in a constant
value 0 regardless of the wavelength. That is, the chromatic
dispersion characteristic 14 of the chirped grating 5 is
provided as characteristic which exhibits a value equal in
absolute value to but with negative sign to the aforementioned
residual characteristic, that is, the characteristic 13
obtained by adding the round trip's chromatic dispersion of the
dispersion compensating fiber to the chromatic dispersion of
the transmission path.
Referring back again to Fig. lA, description will be
made. An optical signal, for example, having a 1.55 ~m band is
injected into the optical circulator 2 at the first port
thereof through the input transmission path 1 of a 1.3 ~m
single mode optical fiber, made to go out from the optical
circulator 2 at the second port thereof, led to the dispersion
compensating fiber 4 and reflected by the chirped grating 5.
The reflected optical signal is propagated in the dispersion
compensating fiber again, injected into the optical circulator
2 at the second port thereof and made to go out from the
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2 1 94 6~8
optical circulator 2 at the third port thereof toward the
output transmission path 3.
Incidentally, the dispersion compensating fiber 4 may
be connected to the first port or the third port of the optical
circulator 2. Alternatively, two or three dispersion
compensating fibers 4 may be used so as to be connected to
different ports among the first, second and third ports. When
the dispersion compensating fiber 4 is connected to the second
port as described above, there arises an advantage in that the
required length of the dispersion compensating fiber necessary
for obtaining a predetermined delay time can be reduced by
half.
Fig. 3 is an explanatory view for explaining a first
modified example of the first embodiment of the present
invention. In Fig. 3, elements the same as those in Fig. 1 are
referenced correspondingly, and the description of those
elements will be omitted. The reference numeral 21 and 22
represent an amplifier and an optical receiver, respectively.
In this modified embodiment, the amplifier 21 such as an
optical fiber amplifier, or the like, is inserted into the
first port of the optical circulator 2 to thereby compensate
the loss of insertion of the dispersion compensating fiber 4
and the chirped grating 5. The transmission path 3 or the
optical receiver 22 is connected to the third port of the
optical circulator 2.
2 1 ~ 4 628
Fig. 4 is an explanatory view for explaining a second
modified example of the first embodiment of the present
invention. In Fig. 4, elements the same as those in Figs. 1
and 3 are referenced correspondingly, and the description of
those elements will be omitted. The reference numeral 23
designates an optical signal receiver. In this modified
embodiment, the optical fiber amplifier 21 is inserted into the
third port of the optical circulator 2 to thereby compensate
the insertion loss of the dispersion compensating fiber 4 and
the chirped grating 5. The transmission path 3 or the optical
signal transmitter 23 is connected to the first port of the
optical circulator 2.
In the aforementioned first and second modified
examples, the directional coupler 7 shown in Fig. lB may be
used instead of the optical circulator 2 shown in Fig. lA. An
amplifier function for compensating the loss of insertion of
the directional coupler 7 may be given to the amplifier 21.
The chromatic dispersion compensator described above
with reference to Figs. lA through 4 can be set in an arbitrary
portion on transmission paths 75, 77 and 78 in a amplified WDM
transmission system shown in Fig. 11. Typically, the chromatic
dispersion compensator is provided in the front, inside, or
rear of an amplifier 76 so as to be adjacent thereto, so that
chromatic dispersion of a transmission path 77 is compensated.
The function of the amplifier 21 and/or the function of an
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21 94628
amplifier for compensating the loss of insertion of the
directional coupler 7 may be given to the amplifier 76.
Referring to Figs. 11 and lA, a specific example of the
present invention will be explained. First, the case where the
transmission path 77 in the four wavelength multiplexed
amplified transmission system shown in Fig. 11 is constituted
by a 1.3 ~m single mode fiber will be described. It is assumed
that the span of the transmission path 77 is 80 km, and the
wavelength of the amplification band of the optical fiber
amplifier 46 is 1550ilO nm, that is, the band width is 20 nm.
In the case of wavelength lo=1540 nm, the chromatic dispersion
of the 1.3 ~m single mode fiber is about 16.5 ps/nm/km and the
dispersion slope thereof is about 0.05 ps/nmZ/km. Further, in
the case of the wavelength 1540 nm, the chromatic dispersion of
the dispersion compensating fiber 4 is about -80 ps/nm/km and
the dispersion slope thereof is about 0.10 ps/nm2/km. In order
to set the chromatic dispersion zero in the case of the
wavelength Ao=1540 nm, the required length of the dispersion
compensating fiber 4 is made to be (16.5x80/80=)16.5 km. If
the optical signal is used so as to make a round trip, the
required length of the dispersion compensating fiber 4 is made
to be a half of the aforementioned value. In this case, the
dispersion slope of the overall transmission path inclusive of
the transmission path 77 of the 1.3 ~m single mode fiber and
~5 the compensation due to the dispersion compensating fiber 4 is
made to be (0.05x80+0.10x16.5=)5.65 ps/nmZ.
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Accordingly, the delay time difference of
{(l/2)x5.65x202=}1130 ps is required to be generated in the
chirped grating 6. Accordingly, the required grating length
becomes 11.3 cm. In the conventional method described above,
the required grating length is 27.2m in which the chirped
grating 6 is only used. As a result, the required grating
length can be reduced to 1/20 or less.
To put the required length of the dispersion
compensating fiber 4 in a practical range, it is preferable
that the chromatic dispersion of the dispersion compensating
fiber 4 is set to be not larger than -40 ps/nm/km. When the
chromatic dispersion of the dispersion compensating fiber 4 is
-40 ps/nm/km, the required length of the dispersion
compensating fiber 4 becomes (16.5x80/40=)33 km. In the case
where the optical signal is used so as to make a round trip,
however, the required length of the dispersion compensating
fiber 4 is reduced to a half of this value, that is, the
required length of the dispersion compensating fiber 4 becomes
equal to the length of the aforementioned dispersion
compensating fiber 4.
Next, the case where the transmission path 77 is
constituted by a dispersion shifted fiber will be described.
It is assumed that the transmission path 77 is constituted by
a dispersion shifted fiber of 80 km. In order to avoid the
'5 four wave mixing generated in the ~ispersion shifted fiber, the
zero dispersion wavelength is set, for example, to 1570 nm.
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21 94628
The dispersion slope is about 0.08 ps/nm2/km. The wavelength
of the amplification band of the optical fiber amplifier is set
to 1550~10 nm, that is, the band width is set to 20 nm.
First, the case where no dispersion compensating fiber
4 is used will be described as a comparative example. In the
case of the wavelength 1550 nm, the chromatic dispersion of the
dispersion shifted fiber is {0.08x(-20)x80=}-128 ps/nm.
Accordingly, the required delay time difference in the chirped
grating 6 becomes {128x20=}2560 ps. As a result, the required
grating length necessary for setting the chromatic dispersion
to zero is 25.6 cm.
Next, the case where a 1.3 ~m single mode fiber is used
as the dispersion compensating fiber 4 will be described. A
pure silica core fiber of very low loss is preferably used as
the 1.3 ~m single mode fiber. The chromatic dispersion of the
dispersion shifted fiber at the wavelength of Ao=1560 nm is
{0.08x(-10)=}-0.8 ps/nm/km because the dispersion slope is
about 0.08 ps/nm2/km, whereas the chromatic dispersion of the
1.3 ~m single mode fiber at the wavelength of Ao=1560 nm is
17.5 ps/nm/km and the dispersion slope is about 0.05 ps/nm2/km.
In order to set the chromatic dispersion to zero at the
wavelength of lo=1560 nm, the required length of the dispersion
compensating fiber 4 is made to be {0.8x80/17.5=}3.66 km.
However, in the case where the optical signal is used so as to
make a round trip, the required length is reduced to a half of
this value. In this case, the dispersion slope of the overall
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2 1 94628
transmission path inclusive of the transmission path 77 of the
dispersion shifted fiber and the compensation due to the
dispersion compensating fiber 4 is made to be
{0.08x80+0.05x3.66=}6.58 ps/nm2.
Accordingly, the delay time difference of
{(l/2)x6.58x202=}1316 ps is required to be generated in the
chirped grating 6. As a result, the required grating length
becomes 13.2 cm which is about a half of 25.6 cm in the case
where no dispersion compensating fiber 4 is used as described
above. That is, the required grating length becomes a value
adapted for practical use.
The required grating length in the case where the
transmission path 77 is constituted by a 1.3 ~m single mode
fiber is substantially equal to the required grating length in
the case where the transmission path 77 is constituted by a
dispersion shifted fiber.
In the above described one example of the first
embodiment, the second port of the optical circulator is
connected to the chirped grating 5 via the dispersion
compensating fiber 4 so as to reciprocate the signal light in
the dispersion compensating fiber 4, thereby reducing the
necessary length of the dispersion compensating fiber 4. The
above structure is not always necessary to reciprocate the
signal light in the dispersion compensating fiber 4. As
described in OPTICAL FIBER TECHNOLOGY 1, (1995), p.p. 162-166
which is explained as the conventional art, the reflec~ion type
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21 94628
structure in which the dispersion compensating fiber is
connected to the Faraday rotator mirror can be applied.
Fig. 5 is an explanatory view for explaining a second
embodiment of the present invention. In the drawing, elements
the same as those in Fig. 1 are referenced correspondingly, and
the description of those elements will be omitted. Reference
numeral 31 represents an optical circulator; 32, Faraday
rotator; and 33, a reflector. One of this embodiment is that
the dispersion compensating fiber 4 is connected to the
i0 reflector 33 via the Faraday rotator 32, and the other one is
that the chirped grating 5 is connected to the intermediate
port of individual optical circulator 31. The Faraday rotator
32 changes the polarization direction of the reciprocating
signal light.
~5 The input transmission path 1 is connected to the first
port which is an input port of the optical circulator 31. The
output transmission path 3 is connected to a fourth port which
is an output port thereof. The Faraday rotator 32 and the
reflector 33 is sequentially connected so that the reflector 33
~0 is connected to a second port which is one of the intermediate
port of the four-terminal optical circulator 31. The chirped
grating 5 is connected to a third port which is the other of
the intermediate port.
The signal light from the input transmission path 1 is
'5 inputted to the first port of the optical circulator 31, is
outputted from the second port, passes through the Faraday
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21 94628
rotator 32 and the dispersion compensating fiber 4, and is
inputted to the second port, sequentially. The signal light
inputted to the second port is outputted from the third port,
reciprocates the chirped grating 5, is inputted to the third
port, and is outputted from the fourth port to the output
transmission path 3.
The chirped grating 5 and the dispersion compensating
fiber 4 give an opposite chromatic dispersion characteristic of
the transmission path to the signal light, and reduce the
necessary length of the dispersion compensating fiber 4 to half
extent. At this time, the Faraday rotator 32 changes the
polarization direction of the signal light reciprocating in the
dispersion compensating fiber 4. For example, if the
polarization direction is changed to the substantially vertical
direction, it is possible to suppress the interference between
the signal light and the Reyleigh dispersing light of the
signal light reciprocating in the dispersion compensating fiber
4 to thereby suppress the induction of noise. Also, in the
above described dispersion compensating fiber 4 having a large
negative chromatic dispersion in 1.55~m-band and the dispersion
compensating fiber 4 using a 1.3~m single mode fiber, the
interference is generated to thereby induce noise.
The Faraday rotator has to be disposed ad~acently to
the reflector to suppress the interference. It may be
considered that the polarization condition changes during the
Faraday rotator 32 and the reflector 33. A known Faraday
-- 2 1 9~628
rotator with a mirror in which the functions of the Faraday
rotator 32 and the reflector 33 are integrated is preferably
used to obtain a large effect of suppression of the
interference. Incidentally, it is desirable that the Faraday
rotator vertically changes the polarization direction of the
signal light reciprocating in the dispersion compensating fiber
4. However, it is acceptable that the Faraday rotator changes
the polarization direction at an extent which the interference
can be suppressed within an allowable range for practical use.
Similarly, the dispersion compensating fiber 4 which is
connected to the reflector 33 via the Faraday rotator 32 is
connected to the third port, and the chirped grating 5 is
connected to the second port. Further, when an optical
circulator having 5 or more ports is used, a plurality of
chirped gratings are used in place of one chirped grating, and
the chirped gratings are connected to respective intermediate
ports.
In this case, the integration of respective chirped
gratings can be replaced with one chirped grating.
Accordingly, the length of respective chirped grating can be
shorten one by one, and chirped gratings having different
characteristics can be combined for use. Generally, the short
chirped grating can be easily produced. Incidentally, this
replacement can be also made in the first embodiment explained
referring to Fig. 1 by using an optical circulator having four
or more ports.
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21 94628
As similar to the first embodiment, the chromatic
dispersion of the transmission path can be made by the
dispersion compensating fiber 4 and the chirped grating 5. In
the wavelength range used, the chromatic dispersion
characteristic of the transmission path is desirably
compensated as much as possible. However, it is possible to
compensate the chromatic dispersion characteristic of the
transmission path with them by connecting other dispersion
compensating fibers to the first and fourth ports of the
optical circulator 31. Two pairs of ones in which the
dispersion compensating fiber 4 is connected to the reflector
33 via the Faraday rotator 32 may be used so that they are
individually connected to the intermediate ports. Further, a
plurality of chromatic dispersion compensator as shown in Fig.
1 or Fig. 2 is used so that the combination of the plurality of
chromatic dispersion compensator compensate the chromatic
dispersion of the transmission path.
In the above description, an example in which the
optical circulator has four or more terminals is explained.
However, a plurality of optical circulator having three
terminals are connected in parallel to thereby obtain the
optical circulator having four or more optical circulator. For
example, the third port of a three-terminal optical circulator
is connected to the first port of the other three-terminal
'5 optical circulator to thereby obtain the four-terminal optical
circulator. Needless to say, one optical circulator can
21 94628
realize its function to thereby suppress the cost and reduce
its arrangement space.
An amplifier such as an optical fiber amplifier has
generally a single directivity. Accordingly, the amplifier can
disposed at a place where the signal light is transmitted in
one direction, for example, the amplifier can be connected to
at least one of the first port which is the input port of the
optical circulator 31 and the fourth port which is the output
port thereof. Additionally, the directional coupler such as
the optical fiber coupler can be used in place of the optical
circulator 31.
Finally, an experimental result with respect to the
function of the Faraday rotator 32 which changes the
polarization direction of the signal light reciprocating in the
dispersion compensating fiber 4 will be explained. As the
dispersion compensating fiber 4, an optical fiber having large
negative Fhromatic dispersion in l.S5~m-band was used, and an
optical signal transmission experimentation of l.55~m-band was
carried out in a transmission path for a 1.3~m single mode
optical fiber.
Fig. 6 is a structural view of a specific example of a
second embodiment according to the present invention. Fig. 7
is a conventional structural view as a comparative example.
Fig. 8 is a structural view of a specific example of the first
'5 embodiment according to the present invention as a comparative
example. In the drawings, elements the same as those in Figs.
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21 ~4628
1 and 5 are referenced correspondingly, and the description of
those elements will be omitted. In the drawings, reference
numeral 41 represents a signal generator; 42, an optical
transmitter; 43, a 1.3~m-band single mode optical fiber
(1.3SMF); 44, a variable optical attenuator; 45, a optical
receiver; and 46, an error ratio measuring device. The signal
generator 41 generates a 2.488 Gbps signal, and the optical
receiver output a signal light having a wavelength of A = 1.551
~m by receiving the output of the signal generator 41. The
chromatic dispersion of the 1.3~m single mode optical fiber at
the wavelength of 1.55 ~m is 16.5 ps/nm/km and its length is 67
km. The chromatic dispersion of the dispersion compensating
fiber (DCF) at the wavelength of 1.55 ~m is -102.4 ps/nm/km,
and its length is 5.5 km in Figs. 6 and 8, but is 11 km in Fig.
7.
In Fig. 6, the optical transmitter 42 receives the
signal from the signal generator 41 and transmits the signal
light to the first port of the optical circulator 31 of the
chromatic dispersion compensator. The signal light from the
fourth port o~ the optical circulator 31 passes through the
1.3~m single mode optical fiber and the variable optical
attenuator 44 to thereby be inputted into the optical receiver
45. The output of the optical receiver 45 is inputted into the
error ratio measuring device 46 to measure the error ratio of
the signal light. As the Faraday rotator 32 and the reflector
33, the Faraday rotator with mirror in which the rotation angle
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2 1 ~4628
of the polarization plane of the signal light is 90~ at the
wavelength of 1.55 ~m in the reciprocation is used. In
practical use, the rotation angle is preferably in the range of
85~ to 95O
In Fig. 7, the optical transmitter 42 transmits the
signal light to the 1.3 ~m single mode fiber 43 via the
dispersion compensating fiber 4. In Fig. 8, the optical
transmitter 42 transmits the signal light to the first port of
the optical circulator 2, and the signal light outputted from
the third port of the optical-circulator 2 is transmitted to
the 1.3 ~m single mode optical fiber 43.
Fig. 9 is a graph showing a receiving sensitivity of
the specific examples shown in Figs. 6 to 9. In the graph, the
horizontal axis represents a receiving power [dBm] and the
vertical axis represents a bit error ratio. Reference numeral
51 represents an error ratio of the so-called BACK to BACK
structure in which the optical transmitter 42 is directly
connected to the optical receiver 45. Reference numeral 52 is
an error ratio of the conventional structure shown in Fig. 7
and the specific example of the second embodiment according to
the present invention shown in Fig. 6; and 53, an error ratio
of the specific example of the first embodiment according to
the present invention shown in Fig. 8.
The bit error ratio is reduced as the receiving power
~5 increases. The error ratio 51 of the BACK to BACK structure is
the smallest one. The error ratio of the specific example of
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21 94628
the first embodiment according to the present invention as
shown in Fig. 8 tends to increase and has a dispersion. In the
conventional structure as shown in Fig. 7 and the specific
example of the second embodiment according to the present
invention, the error ratio slightly increases but has no
dispersion.
The dispersion of the receiving sensitivity does not
depend on the chirped grating 5. This is because the same
phenomenon occurs even when the chirped grating 5 is replaced
with the whole reflection mirror as the reflector 33 in Fig. 8.
The dispersion is decreased or increased, when a polarization
controller disposed just before the optical circulator 2 is
controlled so as to change the polarization condition of the
light inputted into the optical circulator 2. It is considered
that the dispersion of the receiving sensitivity is caused by
the interference between one signal light and the Reyleigh
dispersing light of the other signal light which reciprocate in
the dispersion compensating fiber. This is because the
receiving sensitivity when the dispersion compensating fiber 4
is removed so that the signal light outputted from the signal
light is immediately reflected by the chirped grating is
consistent with that of the above described conventional
structure and become stable. Further, if the dispersion
compensating fiber 4 is replaced with a dispersion shift fiber
(DSF) in which the transmission loss and the dispersion are
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21 94628
small, the width of the dispersion of the receiving sensitivity
is made small.
In Fig. 8, although the receiving sensitivity was
measured in a condition where the Faraday rotator 32 was
inserted between the dispersion compensating fiber 4 and the
chirped grating 5 and disposed about 30 cm before the chirped
grating 5, the receiving sensitivity was dispersed. However,
when the Faraday rotator 32 and the reflector 33 are separately
provided, the receiving sensitivity disperses if the distance
iO between them is about 30 cm. If there is such a problem that
the error ratio is dispersed and become large, it is possible
to solve the problem by increasing the output power of the
signal light is increased, raising the correcting ability of an
error correction symbol during transmission, or the like.
i5 As is apparent from the above description, in
accordance with the present invention, chromatic dispersion
which cannot be compensated by the dispersion compensating
fiber is compensated by the chirped grating so that not only
the chromatic dispersion of the transmission path in an optical
~O fiber communication system can be reduced but also the grating
length of the chirped grating can be reduced greatly in a wide
wavelength band of the optical fiber amplifier. As a result,
waveform distortion caused by combination between chirping
produced by direct modulation of a semiconductor laser used as
'5 a transmitter and chromatic dispersion becomes low. There
arises an effect that lowering of transmission quality can be
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2 1 ~4628
prevented. Particularly, the present invention is preferably
used in a case where the chromatic dispersion of the respective
signal lights in the WDM transmission using optical fibers are
substantially simultaneously compensated.
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