Title: Apparatus and method for gain-spectrum-tilt compensation in long-wavelength band dispersion-compensating hybrid fiber amplifier
Abstract: The present invention provides an apparatus and method for compensating for the variation of a gain spectrum attributable to the temperature variation of a fiber amplifier, and a long-wavelength band dispersion-compensating hybrid amplifier equipped with the gain spectrum compensating apparatus. The apparatus includes a DCF located between a first amplification stage and a second amplification stage to compensate for dispersion of an optical signal output from the first amplification stage and perform Raman amplification of the optical signal using input pumping light; at least one pumping light provision means for providing forward or backward pumping light to the DCF; first and second temperature detection means for detecting temperature variations of the first and second amplification stages, respectively; and control means for controlling intensity of the pumping light of the pumping light provision means according to the detected temperature variations.
Patent Number: 6,992,816 Issued on 01/31/2006 to Chung, et al.
| Inventors:
|
Chung; Hee Sang (Daejeon, KR);
Lee; Won Kyoung (Busan, KR);
Chang; Sun Hyok (Daejeon, KR);
Lee; Hyun Jae (Daejeon, KR);
Chu; Moo Jung (Daejeon, KR);
Lee; Han Hyub (Daejeon, KR);
Lee; Dong Han (Daejeon, KR);
Lee; Yong Bae (Daejeon, KR)
|
| Assignee:
|
Electronics & Telecommunications Research Institute (KR)
|
| Appl. No.:
|
729088 |
| Filed:
|
December 4, 2003 |
Foreign Application Priority Data
| Jul 09, 2003[KR] | 10-2003-0046491 |
| Current U.S. Class: |
359/337.11 |
| Current Intern'l Class: |
H01S 3/00 (20060101) |
| Field of Search: |
359/3371,337.11,337.5
|
References Cited [Referenced By]
U.S. Patent Documents
| 6178038 | Jan., 2001 | Taylor et al.
| |
| 6335821 | Jan., 2002 | Suzuki et al.
| |
| 6411430 | Jun., 2002 | Ogino et al.
| |
| 6888670 | May., 2005 | Oh et al.
| |
| Foreign Patent Documents |
| 2004103599 | Apr., 2004 | JP.
| |
Other References
J. Nakagawa, et al.; 1580-nm Band Erbium-Doped Fiber Amplifier-Employing Novel
Temperature Compensation Technique, Information Technology R&D Center, Mitsubishi
Electric Corporation, Kamakura, Japan, 108-110.
|
Primary Examiner: Hellner; Mark
Attorney, Agent or Firm: Blakely, Sokoloff, Taylor & Zafman
Claims
What is claimed is:
1. An apparatus for compensating for the gain-spectrum-tilt due to a temperature
change of a fiber amplifier with a two-stage structure, comprising:
a dispersion-compensating fiber (DCF) located between a first amplification stage
and a second amplification stage to compensate for dispersion of an optical signal
output from the first amplification stage and perform Raman amplification of the
optical signal using input pumping light;
at least one pumping light provision means for providing forward or backward
pumping light to the DCF;
first and second temperature detection means for detecting temperature variations
of the first and second amplification stages, respectively; and
control means for controlling intensity of the pumping light of the pumping light
provision means according to the detected temperature variations;
wherein the gain-spectrum-tilt of the fiber amplifier is compensated for by controlling
the intensity of the pumping light.
2. The apparatus according to claim 1, further comprising at least one depolarization
means for reducing the degree of polarization of the pumping light provided by
the pumping light provision means.
3. The apparatus according to claim 2, wherein the depolarization means is a
polarization-beam combiner or a fiber-type depolarizer.
4. The apparatus according to claim 1, wherein the pumping light provision means
comprises a plurality of pumping light provision means, and the plurality of pumping
light provision means provide a plurality of rays of pumping light to boost up
the total pump power.
5. The apparatus according to claim 1, wherein the Raman amplification in the
DCF is controlled according to the intensity of the pumping light.
6. The apparatus according to claim 1, wherein the input optical signal has a
wavelength band of 1570 to 1605 nm.
7. The apparatus according to any of 6, wherein the pumping light has a wavelength
of 1500±10 nm.
8. The apparatus according to claim 1, wherein the DCF compensates for dispersion
of a single mode optical fiber, a non-zero dispersion shifted fiber, or other types
of transmission fiber.
9. A method for compensating for the gain-spectrum-tilt due to a temperature
change of a fiber amplifier with a two-stage structure, comprising the steps of:
detecting the temperatures of a first amplification stage and a second amplification stage;
controlling intensity of pumping light input to a DCF located between the first
and second amplification stages according to temperature variations of the first
and second amplification stages; and
controlling Raman gain of the DCF using the pumping light with intensity thereof controlled.
10. The method according to claim 9, wherein the step of detecting the temperature
is performed by detecting temperature at a plurality of locations of each of the
first and second amplification stages.
11. The method according to claim 9, wherein the DCF compensates for dispersion
of an optical signal output from the first amplification stage and performs Raman
amplification of an input optical signal.
12. The method according to claim 11, wherein the input optical signal has a
wavelength band of 1570 to 1605 nm.
13. The method according to claim 9, wherein the step of controlling the Raman
gain comprises the step of providing at least one of forward or backward pumping light.
14. The method according to claim 9, wherein the step of controlling the intensity
of the pumping light comprises the step of reducing the degree of polarization
of two or more pumping light inputted to the DCF.
15. The method according to any of 14, wherein the pumping light has a wavelength
of 1500±10 nm.
16. A Long-wavelength band (L-band) dispersion-compensating hybrid fiber amplifier
(DCHFA), comprising:
a first amplification stage for first amplifying an input optical signal using
first pumping light;
dispersion-compensating Raman amplification means for compensating for dispersion
of first amplified optical signal output from the first amplification stage and
performing Raman amplification of the first amplified optical signal using second
pumping signal;
a second amplification stage for second amplifying an optical signal output from
the dispersion-compensating Raman amplification means using third pumping light;
first and second temperature detection means for detecting temperature variations
of the first and second stages, respectively; and
control means for controlling intensity of the second pumping light according
to the detected temperature variations of the first and second amplification stages;
wherein a gain-spectrum-tilt of a fiber amplifier attributable to a change of
temperature is compensated by controlling the intensity of the pumping light.
17. The L-band DCFHA according to claim 16, wherein the first amplification stage
comprises a first erbium-doped fiber (EDF) for first amplifying the input optical
signal using the first pumping signal, the second amplification stage comprises
a second EDF for second amplifying the optical signal output from the dispersion-compensating
Raman amplification means using the third pumping light, and the first EDF obtains
gain equal to or greater than 8 dB over the whole wavelength band of 1570 to 1605 nm.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the optical fiber amplifiers used in the
optical communication systems and, more specifically, to an apparatus and method
for compensating for the gain-spectrum-tilt of a long-wavelength band dispersion-compensating
hybrid amplifier due to an environmental temperature change, and a long-wavelength
band dispersion-compensating hybrid fiber amplifier equipped with the gain-spectrum-tilt
compensating apparatus.
2. Description of the Prior Art
In the optical communication using wavelength-division multiplexed (WDM) systems,
a plurality of channels are transmitted simultaneously; the conventional band (C-band)
of 1530 to 1565 nm, the long-wavelength band (L-band) of 1570 to 1605 nm, or both
of them are normally used. In these systems, the optical signals become small through
the transmission fibers and recover their launching powers by the optical amplifiers
at the end of each span. However, the amplified signal qualities deteriorate due
to the noises, mainly amplified spontaneous emission (ASE) noise. Therefore, it
is desirable to minimize the ASE noises from the optical amplifiers for the successful
signal transmission. Among various optical amplifiers the erbium-doped fiber amplifiers
(EDFAs) are widely used in the WDM transmissions these days because of their good
gain and noise characteristics. In principle, they can have noise figures close
to the quantum limit, 3 dB, if designed well. Meanwhile, most high-speed core networks
based on 2.5, 10, or 40 Gbit/s per channel require dispersion compensation to keep
the transmitted channels from becoming distorted in time domain, where the dispersion-compensating
fibers (DCFs) are normally used. Then, the extra losses of DCFs have to be compensated
by the EDFAs, which, in turn, cause the decrease of optical signal-to-noise ratios
(OSNRs) of the transmitted channels. An effective way to retain high OSNRs is to
incorporate the DCF into the inter-stage of a two-stage EDFA. Though, the insertion
of DCF raises the noise figure of the EDFA.
An effective method for solving such a problem is to use a dispersion-compensating
hybrid fiber amplifier (DCHFA). This consists of a two-stage EDFA and an actively
pumped DCF where the optical signals experience Raman amplification. Unlike the
C-band DCHFA, however, the L-band DCFHA has a temperature-dependent gain-spectrum
tilt, which comes from the inherent temperature dependences of the absorption and
emission cross sections of erbium-doped fibers (EDFs).
FIG. 1 illustrates the variation of gain spectrum according to the temperature
variation of the EDFs in a conventional two-stage L-band EDFA. The gain spectrum
is flat over the signal wavelengths of the L-band at 25° C. However, the gain
profiles become slanted with positive gain slopes at the lower temperatures, and
vice versa at the higher temperatures.
There are a few known methods to suppress these gain tilts. First, keeping
the temperature of the EDF coils constant is the easiest method. However, since
this method needs thermal insulation, the complete EDFA module has to become more
bulky, which is against the current tendency for compact size.
The other methods are related to the control of parameters that can affect the
gain spectrum, i.e., the signal powers or pump powers. As an example, U.S. Pat.
No. 6,335,821 discloses a method of measuring the temperature of an EDF using a
temperature sensor and controlling the pump-driving currents according to the measured
temperature. In this method, the pump powers in the EDFA to get a flat gain spectrum
are set. Another example is to employ a variable optical attenuator (VOA) in a
two-stage EDFA. The VOA can adjust the launching signal power into the second stage,
which is usually in a deep saturation regime. Since the gain spectrum of a deeply
saturated EDFA changes with the input signal power, the inter-stage VOA can adjust
the overall gain-spectrum tilt.
In principle, keeping the temperature of an overall optical amplifier constant
can fix the temperature-dependent gain tilts in an L-band DCHFA as well as an L-band
EDFA. However, the size problem of a relatively bulky amplifier still remains.
On the other hands, the methods of the parameter control cannot be directly applied
to the DCHFA since it has the pumped DCF section where the Raman amplification
process occurs. Understanding its impact on the overall gain spectrum of an L-band
DCHFA under the temperature changes should be preceded to compensate accurately
the gain-spectrum tilts.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for compensating for the
variation of the gain spectrum in an L-band DCHFA due to the variation of temperature
by controlling the magnitude and slope of the Raman gain in the inserted DCF. In
addition, the present invention provides an L-band DCHFA equipped with the gain-tilt
compensating apparatus.
In order to accomplish the above object, the present invention provides an apparatus
that consists of a two-stage optical amplifier and a DCF between each stage to
compensate for the dispersion of the incoming high-speed modulated signals. At
the same time, the DCF is optically pumped by pump laser diodes (LDs) to obtain
Raman gains. It also includes; at least one pumping light provision means for forward
or backward pumping light injection to the DCF, first and second temperature detection
means for detecting the temperature variations of the first and second amplification
stages, respectively; and control means for controlling the optical power of the
pumping light of the pumping light provision means according to the detected temperature
variations, wherein the variation of the gain spectrum of the optical fiber amplifier
is compensated by controlling the intensity of the pumping light.
In an embodiment of the present invention, the apparatus may further include
at
least one means to reduce the degree of polarization of the pumping light. This
can be done, for example, by the use of a depolarizer or a polarization-beam combiner.
Both can suppress the polarization dependence of the Raman gain in the DCF.
In addition, the present invention provides a method of compensating for the
variation
of the gain spectrum due to the temperature variation of the optical fiber amplifier
that has a two-stage structure, which includes the steps of detecting the temperature
of the first and second amplification stages, controlling the optical power of
pumping light inputted to the DCF located between them, and thereby controlling
the Raman gain.
In an embodiment of the present invention, the step of controlling the Raman
gain
may include the step of providing at least one of forward or backward pumping light.
Additionally, the step of controlling the intensity of the pumping
light may includes the step of reducing the degree of polarization of the pumping
light of a wavelength of 1500±10 nm inputted to the DCF.
In addition, the present invention provides an L-band DCHFA, including a first
amplification stage for first amplifying an input optical signal using first pumping
light; dispersion-compensating Raman amplification means both for compensating
for dispersion of the optical signal and performing Raman amplification of the
first amplified optical signal using second pumping light; a second amplification
stage for second amplifying the optical signal from the dispersion-compensating
Raman amplification means using third pumping light; first and second temperature
detection means for detecting temperature variations of the first and second stages,
respectively; and control means for controlling intensity of the second pumping
light according to the detected temperature variations of the first and second
amplification stages; wherein the variation of the gain spectrum of the optical
fiber amplifier due to the variation of temperature is compensated by controlling
the intensity of the Raman-pumping light.
In an embodiment of the present invention, the input optical signal may have a
wavelength of 1570 to 1605 nm, and the Raman pumping light may have a wavelength
of 1500±10 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present invention
will be more clearly understood from the following detailed description taken in
conjunction with the accompanying drawings, in which:
FIG. 1 is a graph showing the variation of gain spectrum according to the variation
of the temperature of the EDFs in a conventional L-band EDFA;
FIG. 2 is a configuration diagram of an L-band DCHFA equipped with a gain-spectrum-tilt
compensating apparatus in accordance with an embodiment of the present invention;
FIG. 3 is a graph showing the variation of gain spectrum according to the environmental
temperature variation of the L-band DCHFA of the present invention;
FIG. 4 is a graph illustrating the variation of the Raman gain in the DCF according
to the environmental temperature variation of the L-band DCHFA of the present invention;
FIG. 5 is a graph illustrating the variation of the Raman gain when the intensity
of the Raman pumping light is varied according to the environmental temperature
variation of the L-band DCHFA of the present invention;
FIG. 6 is a graph illustrating the variation of gain spectrum of the overall
L-band DCHFA when the intensity of the DCF pumping is varied according to the temperature
variation of the L-band DCHFA of the present invention;
FIG. 7 is a graph showing a relationship between the environmental temperature
and the intensity of the light of a DCF pumping LD; and
FIG. 8 is a flowchart showing a process of compensating for the variation of
the gain spectrum due to the temperature variation of the optical fiber amplifier
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference now should be made to the drawings, in which the same reference
numerals are used throughout the different drawings to designate the same or similar components.
A preferred embodiment of the present invention is described in detail with reference
to the accompanying drawings.
FIG. 2 is a configuration diagram of an L-band DCHFA equipped with a gain-tilt
compensating apparatus in accordance with an embodiment of the present invention.
The configuration shown in the drawing is an example of the gain-tilt compensating
apparatus applied to an optical fiber amplifier and the L-band DCHFA equipped with
the gain-tilt compensating apparatus. Reference numerals
10 to
13
designate wavelength-division multiplexers (WDMs), which combine the L-band optical
signals and pumping light into a single optical fiber. In this case, reference
numeral
10 preferably designates a WDM that combines 980-nm pumping light
with the L-band signals and reference numerals
11 to
13 preferably
designate WDMs that combine 1480 to 1500-nm pumping light with the L-band signals.
Reference numerals
20 to
22 designate passive components, in which
reference numeral
20 designates an optical isolator for suppressing the
reflection of the signal, reference numeral
21 designates a gain equalizing
filter for equalizing gains over the signal bandwidth, and reference numeral
22
designates a polarization-beam combiner for combining two pumping lasers of the
same wavelength. Reference numerals
30 to
34 designate pumping LDs,
in which reference numeral
30 designates a 980-nm LD for supplying pump
energy to the first amplification stage
100, reference numerals
31
and
32 designate 1500-nm LDs used for the Raman amplification in a DCF,
and reference numerals
33 and
34 designate 1480-nm LDs for supplying
pump energy in the second amplification stage
300. Reference numerals
40
and
41 designate EDFs, which are gain media allowing gain to be achieved
in the first and second amplification stage. Reference numeral
50 designates
the DCF used to compensate for the dispersion of the optical signal from the first
amplification stage
100, which is also the Raman gain medium. Reference
numerals
60 and
61 designate temperature measurement sensors that
detect the temperatures of the EDFs
40 and
41, respectively. Reference
numeral
70 designates a control means that controls the driving currents
for 1500-nm LDs
31 and
32 based on the temperatures detected by the
temperature measurement sensors
60 and
61. The control means
70
controls the Raman gain in the DCF
50 by controlling the intensity of the
pumping light of the 1500-nm LDs
31 and
32 and, thus, compensates
for the variation of the gain spectrum of the overall optical amplifier that comes
from the variation of the environmental temperature. Reference numerals
80
and
81 designate optical connectors that connect the first amplification
stage
100 to the DCF
50 and the DCF
50 to the second amplification
stage
300, respectively.
With reference to the preferred embodiment of the present invention shown in
FIG. 2, the operations of the gain-tilt compensating apparatus and the L-band DCHFA
equipped with the apparatus according to the variation of temperature are described
below. Although, in this drawing, the L-band DCHFA has been illustrated as an embodiment
of the present invention, the gain-tilt compensating apparatus of the present invention
may be applied to various types of two-stage fiber amplifiers.
The fiber amplifier of the present invention has a two-stage amplification structure.
The WDM
10 of the first amplification stage
100 receives the L-band
optical signal and forward pumping light provided by the first LD
30, and
couples them into the first EDF
40. The first LD
30 provides 980-nm
pumping light to the WDM
10, and the first EDF
40 is a medium that
allows gain to be achieved in the first amplification stage
100. The optical
signal passed through the EDF
40 is inputted to the DCF
50 located
in the dispersion-compensating Raman amplification means
200 through an
optical isolator
20 for suppressing the reflection of the signal and the
gain-equalizing filter
21 for equalizing gain over the signal bandwidth.
The operation of the dispersion-compensating Raman amplification means
200
is described below. The WDM
11 receives the L-band optical signal to pass
it through the next stage
300, and also receives the backward pumping light
in the wavelength range of 1500±10 nm provided by at least one of the LDs
31 and
32 to couple it into the DCF
50. The backward pumping
light preferably has a 1500-nm wavelength. Although two LDs
31 and
32
are illustrated in the drawing as an example, one or more LDs may be installed
to provide the pumping light. In the case where two LDs
31 and
32
are provided as shown in the drawing, the polarization-beam combiner
22
is provided so that they are combined with orthogonal polarization directions each
other. This polarization-beam combining is to reduce the pump-polarization dependence
of the Raman amplification in the DCF
50. As another embodiment, a depolarization
means may be provided to reduce the degree of polarization of the pumping light.
A fiber-type depolarizer may be an example of the depolarization means. Furthermore,
although the backward pumping light is illustrated as being provided to the DCF
50, it is also possible to provide the forward pumping light to the DCF
50 in another embodiment of the present invention.
The Raman gain in the DCF
50 is obtained by controlling the LDs
31
and
32. That is, by controlling the intensity of the pumping light provided
by the LDs
31 and
32, the Raman gain in the DCF
50 can be varied.
Thereafter, the WDM
12 of the second amplification stage
300
receives the L-band signal and forward pumping light provided by the LD
33,
and couples them into the second EDF
41. The LD
33 provides 1480-nm
pumping light to the gain medium, EDF
41 via the WDM
12, and allows
the signal gain to be achieved in the amplification stage
300. The second
amplification stage
300, as depicted in the drawing, is provided with another
pumping LD
34, so that the second amplification stage
300 is constructed
to enable the forward, backward or bi-directional provision of the pumping light.
The EDF
40 to which the present invention is applied has an appropriate
length that allows a gain that is equal to or higher than at least 8 dB over the
whole L-band to achieve low noise figures of the whole DCHFA. The DCF
50
has a length that allows the dispersion compensation of single-mode fiber, non-zero
dispersion-shifted fiber, or other types of transmission fibers.
Temperature detection sensors
60 and
61 detect the temperatures
of the first and second amplification stages
100 and
300, respectively.
Preferably, the temperature detection sensors
60 and
61 detect the
temperatures of the first and second EDFs
40 and
41, respectively.
The control means
70 controls the intensity of the pumping light of at least
one of the LDs
31 and
32 to control the Raman gain of the DCF
50
according to the detected temperatures. That is, the intensity of the output pumping
light is controlled by controlling the driving circuit of the LDs
31 and
32 according to the detected temperatures and, accordingly, the Raman gain
in the DCF
50 is changed. As a result, by controlling the magnitude and
slope of Raman gain in the DCF
50, the gain-spectrum tilts of the L-band
DCHFA depending on the temperatures can be effectively suppressed to get a fairy
flat gain spectrum over the L-band.
Although the temperature detection sensors
60 and
61 are illustrated
as being provided in the amplification stages
100 and
300, respectively,
a plurality of temperature detection sensors may be provided in each of the amplification
stages
100 and
300. In this case, the control means
70 receives
temperature values detected at various locations of each amplification stage, calculates
the average temperature of each amplification stage, and controls the intensity
of the pump LDs
31 and
32 according to the calculated average temperatures.
As described above, by controlling the magnitude and slope of gain in the DCF
50, the overall gain-spectrum tilt of the fiber amplifier can be maintained
unchanged within a small error bound.
FIG. 3 is a graph showing the variation of gain spectrum according to the temperature
variation of the L-band DCHFA of the present invention, where the environmental
temperature of the EDFs
40 and
41 is changed from 30 to 60°
C. The pump powers from LDs
30 to
34 are kept constant during the
measurement. In particular, the total pump power of the 1500-nm LDs
31 and
32 required for the Raman gain is maintained at 365 mW. For 30° C.,
the gain is flat within 0.5 dB over the wavelength band of 33 nm. As the temperature
increases, the gains at the short wavelengths are increased and the gains at the
long wavelengths are decreased. For 60° C., the gain variation becomes 1.4
dB. The tendency of the variation is the same as that of the conventional L-band
EDFA shown in FIG. 1.
FIG. 4 is a graph illustrating the variation of Raman gain in the DCF according
to the temperature variation of the L-band DCHFA of the present invention. Referring
to FIG. 4, for the temperature range from 30° C. to 60° C., almost the
same gain spectra are obtained. In this case, the total pump power of the 1500-nm
LDs
31 and
32 is also maintained at 365 mW. Therefore, the variation
of gain spectrum shown in FIG. 3 comes from the amplification stage of the EDF
regardless of the Raman gain in the DCF
50.
FIG. 5 is a graph illustrating the variation of Raman gain spectrum when the
total optical power of the Raman pumps varies with the temperature of the L-band
DCHFA of the present invention. The launching pump power to the DCF varies from
365 to 395 mW corresponding to the environmental temperature of from 30 to 60°
C. The Raman gain is increased as a whole, but the increases are different for
the signal wavelengths. That is, the increment is large on the 1605-nm wavelength
side, while it is small on the 1570-nm wavelength side. Since this change is opposite
to that of the entire DCHFA, the overall variation of the gain spectrum can be
compensated by the present invention. The reason why the Raman gain of the DCF
50 has such a tendency according to the intensity of the Raman pumping light
is that the Raman gain peak is in the vicinity of a wavelength of 1605 nm which
is 105-nm apart from 1500-nm pump. For example, if the Raman pump of 1465 nm is
used, the Raman gain peak is achieved in the vicinity of a wavelength of 1570 nm.
If the intensity of the Raman pumping light is increased, the inclination of the
variation of the gain spectrum similar to that in the EDFA due to the temperature
variation will be achieved. Conversely, if the intensity of the Raman pumping light
is decreased, the inclination of the variation of the gain spectrum opposite to
that in the EDFA due to the temperature variation may be achieved, but the intensity
of the signal inputted to the second amplification stage
300 is decreased.
This decrease of intensity of the signal input to the second amplification stage
300 induces the gain tilt similar to that from the temperature variation
of the EDF
41 of the second amplification stage
300, and then it
is impossible to compensate for the gain variation from the temperature changes.
Therefore, the wavelength of the pumping light used to achieve Raman gain is preferably
1500±10 nm.
FIG. 6 is a graph illustrating the variation of gain spectrum of the whole L-band
DCHFA when the intensity of the DCF pumping light varies with the temperature of
the L-band DCHFA of the present invention. FIG. 6 illustrates the gain spectrum
for the temperatures from 30° C. to 60° C., where the pump powers of
the first and second stages
100 and
300 are kept constant and those
of the Raman pumps to the DCF
50 varies from 365 to 395 mW. Referring to
FIG. 6, there is no variation of gain spectrum attributable to the variation of temperature.
FIG. 7 is a graph showing a relationship between the temperature and the optical
power of Raman pump LD. In this drawing, there are shown the optical power of the
Raman pump LD that is required to maintain the gain spectrum of the L-band DCHFA
according to the temperature change. From FIG. 7, it can be appreciated that the
gain-tilt compensation can be achieved by measuring and storing the appropriate
pump powers with various temperatures in advance, and driving the Raman pump LDs
according to the measured and stored data.
FIG. 8 is a flowchart showing a process of compensating for the variation of
the gain spectrum attributable to the temperature variation of the fiber amplifier
of the present invention. As described above, the fiber amplifier to which the
method of the present invention is applied has a two-stage structure, and is preferably
the L-band DCHFA with the two-stage structure shown in FIG. 2. With reference to
the DCHFA of FIG. 2 according to the preferred embodiment of the present invention,
the method of compensating for the variation of a gain spectrum attributable to
the temperature variation of the fiber amplifier is described below.
The fiber amplifier to which the gain spectrum compensating method of the present
invention is applied has a two-stage structure. The temperatures of the first amplification
stage
100 and the second stage
300 are detected at step S
81.
In this case, the detection of temperature may be performed at various locations
of each stage. Additionally, the average temperature of each stage is calculated
using the temperature values detected at the various locations, and it is used
as the detected temperature of each amplification stage
100 or
300.
Subsequently, the temperature variation of each amplification stage
100
or
300 is detected at step S
82.
Forward or backward pumping light is provided from at least one of the LDs
31 and
32 to the DCF
50 of the optical fiber amplifier at
step S
83. The DCF
50 compensates for the dispersion of the optical
signal amplified in the first amplification stage
100 and performs the amplification
of the input optical signal through Raman amplification using the provided pumping light.
Subsequently, the driving circuits of the LDs
31 and
32
are controlled according to the temperature variations of the first and second
amplification stages
100 and
200 detected at step S
84 and,
thus, the intensity of pumping light provided by the LDs
31 and
32
is controlled at step S
85. That is, the intensity of the pumping light provided
to the DCF
50 is controlled according to the temperature variations of the
first and second amplification stages
31 and
32. The reason for this
is that the intensity of the pumping light needs to be controlled according to
the temperature variations so that the Raman amplification in the DCF
50
is controlled accordingly because the overall gain spectrum is varied according
to the temperature variations in the fiber amplifier as described above. As a result,
by controlling the intensity of the pumping light according to the temperature
variations, the variation of the gain spectrum of the fiber amplifier is compensated.
At the same step, the degree of polarization of the pumping light provided to the
DCF
50 is reduced. This reduces the pump-polarization dependence of the
Raman amplification in the DCF
50, so that the uniform Raman amplification
can be performed regardless of the pump polarization. As described above, the variation
of the overall gain spectrum of the fiber amplifier is compensated at step S
86.
The pumping light preferably has a wavelength of 1500±10 nm.
As described above, in the present invention, the gain-spectrum-tilt of the L-band
DCHFA due to the temperature variation is compensated by selecting the appropriate
wavelength and intensity of Raman pumping light, which causes the slope of Raman
gain to be oppositely produced with respect to the variation of gain attributable
to the temperature variation, thus compensating for the variation of gain spectrum
attributable to the temperature variation.
Meanwhile, although the gain-tilt compensating apparatus applied to the
L-band DCHFA has been described in the above-described embodiment, the gain-spectrum-tilt
compensating apparatus of the present invention is not applied only to the fiber
amplifier, but may be modified without departing from the spirit of the present
invention. Furthermore, although the gain spectra are illustrated as being uniform
and flat in the specified temperature range from 30 to 60° C. in FIGS. 3 to
7, the variation of gain can also be compensated in ranges from 0 to 30° C.
and higher than 70° C. according to the same principle. This scheme can be
implemented by detecting the temperature of amplification modules by temperature
sensors and using a computer program for controlling the intensity of the light
of the Raman pumping LD, without additional parts or circuits.
As described above, the present invention provides the simple and effective gain-spectrum-tilt
compensating apparatus that compensates for the variation of the gain spectrum
attributable to the temperature variation of the L-band DCHFA by controlling only
the intensity of the light of the Raman pumping LD.
Furthermore, since the present invention can be implemented by detecting
the temperature of amplification modules by temperature sensors and using a computer
program for controlling the intensity of the light of the Raman pumping LD, without
additional parts or circuits, the present invention is easy to implement, and the
apparatus and DCHFA of the present invention can be miniaturized, and the apparatus
of the present invention is inexpensive and effective.
Although the preferred embodiments of the present invention have been disclosed
for illustrative purposes, those skilled in the art will appreciate that various
modifications, additions and substitutions are possible, without departing from
the scope and spirit of the invention as disclosed in the accompanying claims.
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