U.S. patent application number 09/752561 was filed with the patent office on 2002-01-24 for wavelength division multiplexing optical fiber amplifier.
This patent application is currently assigned to NEC Corporation. Invention is credited to Akiyama, Koichi.
Application Number | 20020008902 09/752561 |
Document ID | / |
Family ID | 18530178 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008902 |
Kind Code |
A1 |
Akiyama, Koichi |
January 24, 2002 |
Wavelength division multiplexing optical fiber amplifier
Abstract
The control circuit controls the directional coupling type
optical switch, inputs the excited light from the excitation LD
light source mainly via the WDM coupler to the EDF, and thereby
intensifies forward excitation and controls the drive current of
the light source so that the signal light with a desired value is
output. On the other hand, when the signal light power with a
desired value is not obtained even if the drive current of the
light source reaches an upper limit, the control circuit gradually
increases the ratio of the excited light to be entered to the EDF
via the WDM coupler and at the same time controls the drive current
of the light source so that the signal light with the desired value
is output.
Inventors: |
Akiyama, Koichi; (Tokyo,
JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Assignee: |
NEC Corporation
|
Family ID: |
18530178 |
Appl. No.: |
09/752561 |
Filed: |
January 3, 2001 |
Current U.S.
Class: |
359/341.4 |
Current CPC
Class: |
H01S 3/094003 20130101;
H01S 3/13013 20190801; H01S 3/094011 20130101; H01S 3/06754
20130101; H01S 3/10015 20130101; H01S 3/1001 20190801 |
Class at
Publication: |
359/341.4 |
International
Class: |
H01S 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 6, 2000 |
JP |
2000-000907 |
Claims
What is claimed is:
1. A wavelength division multiplexing optical fiber amplifier
comprising: an optical fiber doped with rare-earth elements that
amplifies input signal light; an excitation LD light source that
outputs excited light of a predetermined wavelength; forward
excitation means for supplying said excited light from the signal
light input side of said optical fiber; backward excitation means
for supplying said excited light from the signal light output side
of said optical fiber; monitoring means for monitoring power of the
output signal light output from said optical fiber; and controlling
means for repeating a first operation that by maximizing the ratio
of forward excitation by said forward excitation means relative to
the ratio of backward excitation by said backward excitation means,
changes the drive current of said excitation LD light source based
on a monitor signal from said monitoring means until power of said
output signal light reaches a desired value and when power of said
output signal light does not reach said desired value, increases
said ratio of backward excitation by a predetermined amount
relative to said ratio of forward excitation, and a second
operation that changes the drive current of said excitation LD
light source based on a monitor signal from said monitoring means
until power of said output signal light reaches said desired value,
until power of said output signal light reaches the desired
value.
2. A wavelength division multiplexing optical fiber amplifier
comprising: an optical fiber doped with rare-earth elements that
amplifies input signal light; an excitation LD light source that
outputs excited light of a predetermined wavelength; forward
excitation means for supplying said excited light from the signal
light input side of said optical fiber; backward excitation means
for supplying said excited light from the signal light output side
of said optical fiber; monitoring means for monitoring power of the
output signal light output from said optical fiber; and controlling
means for repeating a first operation that by maximizing the ratio
of forward excitation by said forward excitation means relative to
the ratio of backward excitation by said backward excitation means,
changes the drive current of said excitation LD light source based
on a monitor signal from said monitoring means until power of said
output signal light reaches a desired value and when power of said
output signal light does not reach said desired value, increases
said ratio of backward excitation by a predetermined amount
relative to said ratio of forward excitation, and a second
operation that changes the drive current of said excitation LD
light source based on a monitor signal from said monitoring means
until power of said output signal light reaches said desired value,
until power of said output signal light reaches the desired value
said controlling means comprising: a directional coupling type
optical switch that variably controls the ratio of said excitation
LD light source branched to said forward excitation means and the
ratio of said excitation LD light source branched to said backward
excitation means according to a signal applied to the electrode;
and a control circuit that controls, in an initial state, a signal
applied to the electrode of said directional coupling type optical
switch so that most of said excited light is supplied to said
forward excitation means, compares power of said output signal
light detected based on a monitor signal from said monitoring means
and said desired value to see whether these two values match or
not, changes the drive current of said excitation LD light source
until the two values match and adjusts the signal applied to the
electrode of said directional coupling type optical switch so that
the ratio of said excited light supplied to said backward
excitation means increases gradually.
3. A wavelength division multiplexing optical fiber amplifier
comprising: an optical fiber doped with rare-earth elements that
amplifies input signal light; an excitation LD light source that
outputs excited light of a predetermined wavelength; forward
excitation means for supplying said excited light from the signal
light input side of said optical fiber; backward excitation means
for supplying said excited light from the signal light output side
of said optical fiber; monitoring means for monitoring power of the
output signal light output from said optical fiber; and controlling
means for repeating a first operation that by maximizing the ratio
of forward excitation by said forward excitation means relative to
the ratio of backward excitation by said backward excitation means,
changes the drive current of said excitation LD light source based
on a monitor signal from said monitoring means until power of said
output signal light reaches a desired value and when power of said
output signal light does not reach said desired value, increases
said ratio of backward excitation by a predetermined amount
relative to said ratio of forward excitation, and a second
operation that changes the drive current of said excitation LD
light source based on a monitor signal from said monitoring means
until power of said output signal light reaches said desired value,
until power of said output signal light reaches the desired value,
said signal light input to said optical fiber is of a 1580-nm range
and said excited light has a wavelength of approximately 1480
nm.
4. A wavelength division multiplexing optical fiber amplifier
comprising: an optical fiber doped with rare-earth elements that
amplifies input signal light; an excitation LD light source that
outputs excited light of a predetermined wavelength; forward
excitation means for supplying said excited light from the signal
light input side of said optical fiber; backward excitation means
for supplying said excited light from the signal light output side
of said optical fiber; monitoring means for monitoring power of the
output signal light output from said optical fiber; and controlling
means for repeating a first operation that by maximizing the ratio
of forward excitation by said forward excitation means relative to
the ratio of backward excitation by said backward excitation means,
changes the drive current of said excitation LD light source based
on a monitor signal from said monitoring means until power of said
output signal light reaches a desired value and when power of said
output signal light does not reach said desired value, increases
said ratio of backward excitation by a predetermined amount
relative to said ratio of forward excitation, and a second
operation that changes the drive current of said excitation LD
light source based on a monitor signal from said monitoring means
until power of said output signal light reaches said desired value,
until power of said output signal light reaches the desired value
said controlling means comprising: a directional coupling type
optical switch that variably controls the ratio of said excitation
LD light source branched to said forward excitation means and the
ratio of said excitation LD light source branched to said backward
excitation means according to a signal applied to the electrode;
and a control circuit that controls, in an initial state, a signal
applied to the electrode of said directional coupling type optical
switch so that most of said excited light is supplied to said
forward excitation means, compares power of said output signal
light detected based on a monitor signal from said monitoring means
and said desired value to see whether these two values match or
not, changes the drive current of said excitation LD light source
until the two values match and adjusts the signal applied to the
electrode of said directional coupling type optical switch so that
the ratio of said excited light supplied to said backward
excitation means increases gradually; and said signal light input
to said optical fiber is of a 1580-nm range and said excited light
has a wavelength of approximately 1480 nm.
5. A wavelength division multiplexing optical fiber amplifier in a
two-stage configuration with a first optical fiber amplifier and a
second optical fiber amplifier connected via a gain equalizer, said
first and second optical fiber amplifiers each comprising: an
optical fiber doped with rare-earth elements that amplifies input
signal light; an excitation LD light source that outputs excited
light of a predetermined wavelength; forward excitation means for
supplying said excited light from the signal light input side of
said optical fiber; backward excitation means for supplying said
excited light from the signal light output side of said optical
fiber; monitoring means for monitoring power of the output signal
light output from said optical fiber; and controlling means for
changing the relative proportion between forward excitation by said
forward excitation means and backward excitation by said backward
excitation means so that power of said output signal light detected
based on a monitor signal from said monitoring means matches a
desired value and controlling the drive current of said excitation
LD light source, wherein said controlling means of said first
optical fiber amplifier controls so that the ratio of said forward
excitation becomes relatively greater than the ratio of said
backward excitation and said controlling means of said second
optical fiber amplifier controls said relative proportion so that
power of said output signal light of said second optical fiber
amplifier matches a desired value.
6. A wavelength division multiplexing optical fiber amplifier in a
two-stage configuration with a first optical fiber amplifier and a
second optical fiber amplifier connected via a gain equalizer, said
first and second optical fiber amplifiers each comprising: an
optical fiber doped with rare-earth elements that amplifies input
signal light; an excitation LD light source that outputs excited
light of a predetermined wavelength; forward excitation means for
supplying said excited light from the signal light input side of
said optical fiber; backward excitation means for supplying said
excited light from the signal light output side of said optical
fiber; monitoring means for monitoring power of the output signal
light output from said optical fiber; and controlling means for
changing the relative proportion between forward excitation by said
forward excitation means and backward excitation by said backward
excitation means so that power of said output signal light detected
based on a monitor signal from said monitoring means matches a
desired value and controlling the drive current of said excitation
LD light source, wherein said controlling means of said first
optical fiber amplifier controls so that the ratio of said forward
excitation becomes relatively greater than the ratio of said
backward excitation and said controlling means of said second
optical fiber amplifier controls said relative proportion so that
power of said output signal light of said second optical fiber
amplifier matches a desired value, said signal light input to the
optical fibers of said first and second optical fiber amplifiers is
of a 1580-nm range and said excited light has a wavelength of
approximately 1480 nm.
7. The wavelength division multiplexing optical fiber amplifier
according to claim 1, wherein said optical fiber is an erbium-doped
fiber.
8. The wavelength division multiplexing optical fiber amplifier
according to claim 2, wherein said optical fiber is an erbium-doped
fiber.
9. The wavelength division multiplexing optical fiber amplifier
according to claim 3, wherein said optical fiber is an erbium-doped
fiber.
10. The wavelength division multiplexing optical fiber amplifier
according to claim 4, wherein said optical fiber is an erbium-doped
fiber.
11. The wavelength division multiplexing optical fiber amplifier
according to claim 5, wherein said optical fiber is an erbium-doped
fiber.
12. The wavelength division multiplexing optical fiber amplifier
according to claim 6, wherein said optical fiber is an erbium-doped
fiber.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a wavelength division
multiplexing optical fiber amplifier, and more particularly, to a
wavelength division multiplexing optical fiber amplifier that
amplifies a wavelength division multiplexing light signal using a
fiber doped with rare-earth elements.
PRIOR ART
[0002] As a wavelength division multiplexing optical fiber
amplifier, EDFA (Erbium Doped Fiber Amplifier) is conventionally
known. This EDFA changes output signal optical power according to
the number of input signals to keep the input/output level per 1
wavelength constant.
[0003] On the other hand, in the field of wavelength division
multiplexing optical fiber amplifiers, wavelength division
multiplexing optical fiber amplifiers using a wavelength range of
1580 nm (1570 nm to 1600 nm) instead of the previous 1550 nm range
have been developed in recent years. When a high-level
(multi-wavelength) signal of approximately -5 dBm (32 waves), etc.
is input, the efficiency of this wavelength division multiplexing
optical fiber amplifier using the 1580-nm range for forward
excitation only or for backward excitation only is as low as 10%
and achieving output of +20 dBm would require such high excitation
power as 1 W. For this reason, improving the efficiency requires
bi-directional excitation.
[0004] FIG. 4 shows a block diagram of the above-described
conventional example of a wavelength division multiplexing optical
fiber amplifier using the 1580-nm range. In the figure, signal
light to be amplified is introduced from an optical connector 10
through an optical isolator 11a to a wavelength division
multiplexing (WDM) coupler 12a. On the other hand, excited light
output from an excitation LD (laser diode) light source 13 is
bifurcated with a division ratio of 1:1 by a 1.times.2 division
coupler 14 having a division ratio of 1:1.
[0005] The signal light is combined with the excited light from the
1.times.2 division coupler 14 by the WDM coupler 12a, amplified by
an erbium-doped fiber (EDF) 15, combined with the excited light by
a WDM coupler 12b and output through an optical isolator 11b to an
optical connector 16. In this way, this conventional wavelength
division multiplexing optical fiber amplifier provides
bidirectional excitation by connecting the 1.times.2 division
coupler 14 to the WDM couplers 12a and 12b provided with on the
input side and output side, respectively of the EDF 15 and
simultaneously executing forward excitation by which the excited
light is input from the input side of the EDF 15 in the same
direction as that of the signal light and backward excitation by
which the excited light is input from the output side of the EDF 15
in the direction opposite to that of the signal light.
[0006] However, a number of wavelength sometimes lacks in the
wavelength of the conventional wavelength division multiplexing
optical fiber amplifier described above, for example, there becomes
inputting of approximately -20 dBm (1 wave). In this case,
relatively high efficiency is obtained when bi-directional
excitation is applied to obtain output of approximately +5 dBm, and
therefore output is obtained with a small amount of excitation
power. However, since the EDF 15 is a lengthy fiber, the EDF 15 has
a poor inverted population due to backward excitation at the
incidence end of the signal light, causing possible deterioration
of the noise figure.
[0007] On the other hand, another conventional optical fiber
amplifier is also known (Japanese Patent Application Laid-Open No.
HEI 10-209540) that supplies excited laser light to wave combiners
connected on both ends of a rare-earth-doped fiber, directly
amplifies a light signal input to the one wave combiner and output
from the other wave combiner, and includes one excitation laser
that outputs excited laser beam and a light division circuit that
divides the laser beam input from this excitation laser and
supplies the divided laser beams to the wave combiner on the input
side and the wave combiner on the output side.
[0008] This conventional optical fiber amplifier adjusts, through
the light division circuit, the excitation power output ratio of
forward excitation to backward excitation to approximately 1:1, but
since the EDF is a lengthy fiber as described above, the problem is
that making such an adjustment to 1580-nm range signal light will
deteriorate the noise figure.
SUMMARY OF THE INVENTION
[0009] The present invention has been implemented taking into
account the points described above and it is an object of the
present invention to provide a wavelength division multiplexing
optical fiber amplifier capable of providing characteristics such
as low noise and high output by basically intensifying forward
excitation, and in the case where the output is not obtained by
gradually increasing the ratio of backward excitation.
[0010] In order to attain the above object, the present invention
adopts a configuration comprising:
[0011] an optical fiber doped with rare-earth elements that
amplifies input signal light;
[0012] an excitation LD light source that outputs excited light of
a predetermined wavelength;
[0013] forward excitation means for supplying the excited light
from the signal light input side of the optical fiber;
[0014] backward excitation means for supplying the excited light
from the signal light output side of the optical fiber;
[0015] monitoring means for monitoring power of the output signal
light output from the optical fiber; and
[0016] controlling means for repeating a first operation that by
maximizing the ratio of forward excitation by the forward
excitation means relative to the ratio of backward excitation by
the backward excitation means, changes the drive current of the
excitation LD light source based on a monitor signal from the
monitoring means until power of the output signal light reaches a
desired value and when power of the output signal light does not
reach the desired value, increases the ratio of backward excitation
by a predetermined amount relative to the ratio of forward
excitation, and a second operation that changes the drive current
of the excitation LD light source based on a monitor signal from
the monitoring means until power of the output signal light reaches
the desired value, until power of the output signal light reaches
the desired value.
[0017] Since the present invention maximizes the ratio of forward
excitation relative to the ratio of backward excitation and changes
the drive current of the excitation LD light source in that state,
it is possible to provide light amplification focused on forward
excitation when input power is low and also provide light
amplification focused on forward excitation even when power of the
output signal light does not reach a desired value even if forward
excitation is intensified, by gradually increasing the ratio of
backward excitation.
[0018] In order to attain the above object, the present invention
is characterized by configuring the above-described controlling
means to include:
[0019] a directional coupling type optical switch that variably
controls the ratio of the excitation LD light source branched to
the forward excitation means and the ratio of the excitation LD
light source branched to the backward excitation means according to
a signal applied to the electrode; and
[0020] a control circuit that controls, in an initial state, a
signal applied to the electrode of the directional coupling type
optical switch so that most of the excited light is supplied to the
forward excitation means, compares power of the output signal light
detected based on a monitor signal from the monitoring means and
the desired value to see whether these two values match or not,
changes the drive current of the excitation LD light source until
the two values match and adjusts the signal applied to the
electrode of the directional coupling type optical switch so that
the ratio of the excited light to the backward excitation means
increases gradually.
[0021] In order to attain the above object, the present invention
further provides a wavelength division multiplexing optical fiber
amplifier in a two-stage configuration with a first optical fiber
amplifier and a second optical fiber amplifier connected via a gain
equalizer, the first and second optical fiber amplifiers each
comprising:
[0022] an optical fiber doped with rare-earth elements that
amplifies input signal light;
[0023] an excitation LD light source that outputs excited light of
a predetermined wavelength;
[0024] forward excitation means for supplying the excited light
from the signal light input side of the optical fiber;
[0025] backward excitation means for supplying the excited light
from the signal light output side of the optical fiber;
[0026] monitoring means for monitoring power of the output signal
light output from the optical fiber; and
[0027] controlling means for changing the relative proportion
between forward excitation by the forward excitation means and
backward excitation by the backward excitation means so that power
of the output signal light detected based on a monitor signal from
the monitoring means matches a desired value and controlling the
drive current of the excitation LD light source,
[0028] characterized in that the controlling means of the first
optical fiber amplifier controls so that the ratio of the forward
excitation becomes relatively greater than the ratio of the
backward excitation and the controlling means of the second optical
fiber amplifier controls the relative proportion so that power of
the output signal light of the second optical fiber amplifier
matches a desired value.
[0029] The present invention provides an optical fiber amplifier in
two-stage configuration, which can, through the first optical fiber
amplifier in the first stage that controls the overall noise
figure, perform light amplification focused on forward excitation
and, through the second optical fiber amplifier in the second
stage, perform optimal bi-directional excitation control according
to power of input signal light.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The objects and features of the present invention will
become more apparent from the consideration of the following
detailed description taken in conjunction with the accompanying
drawings, in which:
[0031] FIG. 1 is a block diagram of a first embodiment of the
present invention;
[0032] FIG. 2 is a flow chart to explain operation of the control
circuit of FIG. 1;
[0033] FIG. 3 is a block diagram of a second embodiment of the
present invention; and
[0034] FIG. 4 is a block diagram showing a conventional
example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] With reference now to the attached drawings, embodiments of
the present invention will be explained below. FIG. 1 is a block
diagram of a first embodiment of the wavelength division
multiplexing optical fiber amplifier of the present invention. In
the figure, a signal light to be amplified is inputted from an
optical connector 20, led through a 1.times.2 division coupler 22a,
an optical isolator 23a, a wavelength division multiplexing (WDM)
coupler 24a, an EDF 30, a wavelength division multiplexing (WDM)
coupler 24b, an optical isolator 23b, a 1.times.2 division coupler
22b and is outputted from an optical connector 32.
[0036] The 1.times.2 division coupler 22a is provided, by means of
fusion connecting (fusion splicing), with a photodiode (PD) 21 to
monitor input signal light power. The WDM couplers 24a and 24b are
provided, by means of fusion connecting, with a directional
coupling type optical switch 29 and the directional coupling type
optical switch 29 is provided, by means of fusion connecting, an
excitation LD light source 28. Furthermore, the 1.times.2 division
coupler 22b is provided, by means of fusion connecting, a PD 31 to
monitor amplified signal light power.
[0037] This embodiment is further provided with a control circuit
25 to control the directional coupling type optical switch 29 and
the excitation LD light source 28 (pumping LD light source 28) so
that output signal light is constant, based on reception data of
the PD 31. The control circuit 25 is provided with a directional
coupling type optical switch control circuit 26 that controls the
directional coupling type optical switch 29 and an excitation LD
drive control circuit 27 that controls the excitation LD light
source 28.
[0038] The excitation LD light source 28 is a high-output
semiconductor laser diode to excite the EDF 30. Regarding
amplification of a 1580-nm range, the quantum efficiency of a
1550-nm range, which is used for the excitation light source, for
excitation of a 1480-nm range is greater than that for excitation
of a 980-nm range, and therefore this embodiment uses an
InGaAsP/InP laser diode with an oscillation wavelength of
approximately 1480 nm band.
[0039] The optical isolators 23a and 23b are used to lead light in
only one direction and provided to reduce influences of reflection.
Moreover, the WDM couplers 22a and 22b are used to combine light
beams of different wavelengths and are designed, according to this
embodiment, to combine and output the excitation LD light source of
a 1480 nm range and the signal light.
[0040] The directional coupling type optical switch 29 is a device
that can switch between optical paths by externally changing
propagation constants of light waveguides. According to this
embodiment, the directional coupling type optical switch 29 has a
function capable of dividing a light beam output from the
excitation LD light source 28 with a desired division ratio because
the index of refraction of the directional coupling section changes
when a directional coupler is formed with a light guide, an
electrode is formed on top of the coupling section and an
appropriate voltage is applied to the electrode.
[0041] Next, operation of this embodiment will be explained. The
signal light introduced through the optical connector 20 is
bifurcated (divided) by the 1.times.2 division coupler 22a, 5% of
the divided light is introduced to the PD 21 to monitor input
signal light power and the remaining 95% of the divided light is
introduced through the optical isolator 23a to the WDM coupler 24a
as signal light. On the other hand, the excited light from the
excitation LD light source 28 is divided by the directional
coupling type optical switch 29 with a division ratio under the
control of the control circuit 25, which will be described later
and combined with the signal light or divided by the WDM couplers
24a and 24b provided before and after the EDF 30.
[0042] The signal light output from the WDF 24a is amplified by the
EDF 30 and introduced to the WDF coupler 24b. The amplified signal
light output from the WDF coupler 24b is led through the optical
isolator 23b, divided by the 1.times.2 division coupler 22b and 5%
of the divided light is introduced to the PD 31 to monitor power of
the amplified signal light and the remaining 95% of the divided
light is output from the optical connector 32 as the signal
light.
[0043] Then, control operation by the control circuit 25 of the
excitation LD light source 28 and directional coupling type optical
switch 29 will be explained with reference to the flow chart of
FIG. 2. The directional coupling type optical switch control
circuit 26 in the control circuit 25 adjusts a voltage applied to
the electrode of the directional coupling type optical switch 29 so
that the ratio of forward excitation to backward excitation becomes
10:0 or close to that value (step 101).
[0044] Then, the excitation LD drive control circuit 27 in the
control circuit 25 converts a detection current with a value
corresponding to the amplified signal light power obtained through
photoelectric conversion by the PD 31 to a voltage Z.sub.OUTPUT
(step 102), compares with an externally set reference voltage
Z.sub.0 (step 103), and when the two voltages do not match, decides
whether drive current (excitation current) lop of the excitation LD
light source 28 is smaller than upper limit I.sub.limit, or not
(step 104), and when I.sub.op is smaller than upper limit
I.sub.limit, increases the excitation current I.sub.op and repeats
processing in steps 102 and 103.
[0045] The excitation current I.sub.op is increased toward the
upper limit I.sub.limit in this way and when it is decided in step
103 that the detected voltage (output light power) Z.sub.OUTPUT
matches the reference voltage (output set value) Z.sub.0, the
processing ends (step 106). This means that the signal light with a
desired value is output from the 1.times.2 division coupler
22b.
[0046] Thus, this embodiment allows the signal light with a desired
value to be output by intensifying forward excitation first. Since
required output power with low input power such as 1-wave input is
small, this allows the ratio of forward excitation above to be set
to a maximum or close to a maximum, thus suppressing the noise
figure to a low level.
[0047] However, depending on power of the output signal light,
power of the signal light with a desired value may not be obtained
even if the drive current of the excitation LD light source 28
reaches the upper limit. Thus, when it is decided in step 104 that
excitation current I.sub.op has reached the upper limit
I.sub.limit, the excitation LD drive control circuit 27 notifies
this information to the directional coupling type optical switch
control circuit 26, and then the directional coupling type optical
switch control circuit 26 adjusts the voltage applied to the
directional coupling type optical switch 29 to change the division
ratio so that the ratio of backward excitation increases by a
predetermined amount (step 105).
[0048] Then, the excitation LD drive control circuit 27 converts
the detection current with a value corresponding to the amplified
power of the signal light obtained through photoelectric conversion
by the PD 31 to voltage Z.sub.OUTPUT (step 102), compares with
externally set reference voltage Z0 (step 103), and if the two
voltages do not match, decides whether drive current (excitation
current) I.sub.op of the excitation LD light source 28 is smaller
than upper limit I.sub.limit or not (step 104), and if I.sub.op is
smaller than upper limit I.sub.limit, increases the excitation
current I.sub.op and repeats the processing in steps 102 and 103.
Hereinafter, the processing of above steps 102, 103 and 104 is
repeated in the same way as shown above until Z.sub.OUTPUT=Z.sub.0
is reached and when I.sub.op=I.sub.limit, the excitation LD drive
control circuit 27 adjusts the voltage applied to the directional
coupling type optical switch 29 again, changes the division ratio
so that the ratio of backward excitation increases by a
predetermined amount and repeats the processing in steps 102, 103
and 104.
[0049] While gradually increasing the ratio of backward excitation,
the excitation LD drive control circuit 27 increases excitation
current I.sub.op toward upper limit I.sub.limit and finishes the
processing when it is decided in step 103 that detection voltage
(output light power) Z.sub.OUTPUT matches reference voltage (output
set value) Z.sub.0 (step 106). This allows the signal light to be
output with a desired value from the 1.times.2 division coupler
22b.
[0050] Thus, when the signal light with a desired value cannot be
output even if forward excitation is intensified, this embodiment
gradually increases the ratio of backward excitation making it
possible to obtain power of the output signal light with a desired
value in the end. Though with high input power such as 32-wave
input, which requires high output power, desired excitation power
cannot be obtained through forward excitation alone, this
embodiment can minimize the noise figure by gradually increasing
the ratio of backward excitation, and moreover obtain power of the
output signal light with a desired value.
[0051] Next, a second embodiment of the present invention will be
explained. FIG. 3 is a block diagram of a second embodiment of the
wavelength division multiplexing optical fiber amplifier of the
present invention. In the figure, the same components as those in
FIG. 1 are assigned the same reference numerals. In FIG. 3, a
signal light to be amplified is entered from an optical connector
20 and amplified by a first optical fiber amplifier including a
1.times.2 division coupler 22a, an optical isolator 23a, a WDM
coupler 24a, an EDF 30a, a WDM coupler 24b, an optical isolator
23b, a 1.times.2 division coupler 22b, led through a gain equalizer
34 and after being amplified by a second optical fiber amplifier
including an optical isolator 23c, a WDM coupler 24c, an EDF 30b, a
WDM coupler 24d, an optical isolator 23d and a 1.times.2 division
coupler 22c, output from an optical connector 32.
[0052] The WDM couplers 24a and 24b are provided, by means of
fusion connecting, with a directional coupling type optical switch
29a and the directional coupling type optical switch 29a is
provided, by means of fusion connecting, a 1480-nm excitation LD
light source 28a. Furthermore, the 1.times.2 division coupler 22b
is provided, by means of fusion connecting, a PD 31a to monitor
amplified signal light power. Likewise, the WDM couplers 24c and
24d are provided, by means of fusion connecting, with a directional
coupling type optical switch 29b and the directional coupling type
optical switch 29b is provided, by means of fusion connecting, a
1480-nm excitation LD light source 28b. Furthermore, the 1.times.2
division coupler 22c is provided, by means of fusion connecting, a
PD 31b to monitor amplified signal light power.
[0053] This embodiment is further provided with control circuits
25a and 25b to control the directional coupling type optical
switches 29a and 29b and the excitation LD light sources 28a and
28b so that output signal light is constant, based on reception
data of the PD 31a and PD 31b. The control circuits 25a and 25b are
provided with directional coupling type optical switch control
circuits 26a and 26b that control the directional coupling type
optical switches 29a and 29b and excitation LD drive control
circuits 27a and 27b that control the excitation LD light sources
28a and 28b and perform the same control operation as that of the
control circuit 25 of the first embodiment.
[0054] This embodiment forms an optical fiber amplifier in a
two-stage configuration with a first optical fiber amplifier
consisting of an amplification section from the 1.times.2 division
coupler 22a to 22b, PD 21, PD 31a, excitation LD light source 28a,
directional coupling type optical switch 29a and control circuit
25a, and a second optical fiber amplifier consisting of an
amplification section from the optical isolator 23c to 1.times.2
division coupler 22c, PD 31b, excitation LD light source 28b,
directional coupling type optical switch 29b and control circuit
25b, connected via the gain equalizer 34, and both the first and
second optical fiber amplifiers are provided with the directional
coupling type optical switches 29a and 29b as the excitation
systems.
[0055] As a result, the first optical fiber amplifier controls the
noise figure of the entire amplifier, and therefore suppresses the
noise figure of the EDF 30a through the control circuit 25a focused
on forward excitation, while the second optical fiber amplifier
adjusts the division ratio through the control circuit 25b so that
an optimal efficiency is obtained according to input power, thus
focused on output power. This is because the two-stage
configuration prevents deterioration of the noise figure of the
second optical fiber amplifier in the second stage from influencing
the noise figure of the entire amplifier a great deal. This
suppresses deterioration of the noise figure and allows a 1580-nm
range amplified wavelength division multiplexing signal light with
desired output power to be obtained from the optical connector
32.
[0056] The present invention is not limited to the embodiments
above, but is also applicable to optical fibers other than EDF,
doped with other rare-earth elements such as praseodymium-doped
fiber.
[0057] As described above, when input power is low, the present
invention performs light amplification focused on forward
excitation, thus making it possible to suppress the noise figure to
a low level. On the other hand, when power of the output signal
light does not reach a desired value even if forward excitation is
intensified, the present invention performs light amplification
focused on forward excitation by gradually increasing the ratio of
backward excitation, thus making it possible to obtain desired
high-output light power and secure required efficiency while
minimizing deterioration of the noise figure when input power is
high.
[0058] Furthermore, the present invention provides an optical fiber
amplifier in a two-stage configuration with the first optical fiber
amplifier that controls the overall noise figure performing light
amplification focused on forward excitation and the second optical
fiber amplifier performing optimal bi-directional control according
to power of the input signal light, making it possible to
efficiently obtain desired output light power with deterioration of
the noise figure suppressed to a minimum.
* * * * *