Title: Multiwavelength locking method and apparatus using acousto-optic tunable filter
Abstract: Disclosed is a multiwavelength locking method and apparatus using an acousto-optic tunable filter in an optical communication system including optical transport networks, in which output wavelengths of light sources are monitored under the condition in which pilot signals are applied to the acousto-optic tunable filter, so as to lock the wavelengths of the light sources, thereby eliminating an wavelength instability of the light sources for an improvement in transmission characteristics. The acousto-optic tunable filter receives light beams of N different frequencies respectively outputted from N light sources, along with N pilot signals having different frequencies, and converts respective frequencies of beam components of the output beam corresponding to the N pilot signals, thereby outputting N frequency-converted output beams to be applied to a photo-detector. N electrical signals respectively corresponding to the frequencies of the pilot signals are outputted from the photo-detector, separated from one another while corresponding to the light sources, respectively, and then used to compensate for respective output wavelengths of the light sources, thereby enabling the corresponding light sources to output locked wavelengths, respectively.
Patent Number: 6,862,303 Issued on 03/01/2005 to Kim, et al.
| Inventors:
|
Kim; Bong Kyu (Daejon, KR);
Kim; Kwang Joon (Daejon, KR);
Kim; Byoung-Sung (Kangwon-do, KR);
Kim; Hae Geun (Daejon, KR)
|
| Assignee:
|
Electronics & Telecommunications Research Institute (KR)
|
| Appl. No.:
|
196465 |
| Filed:
|
July 15, 2002 |
Foreign Application Priority Data
| Dec 18, 2001[KR] | 2001-80895 |
| Current U.S. Class: |
372/32 |
| Intern'l Class: |
H01S 003//13 |
| Field of Search: |
372/13,18,32
398/1
250/214 R,227.23
|
References Cited [Referenced By]
U.S. Patent Documents
| 5208819 | May., 1993 | Huber | 372/32.
|
| 5825792 | Oct., 1998 | Villeneuve et al. | 372/32.
|
| 6094446 | Jul., 2000 | Tei et al. | 372/32.
|
| 6118562 | Sep., 2000 | Lee et al. | 398/1.
|
| 6233262 | May., 2001 | Mesh et al. | 372/32.
|
| 6240109 | May., 2001 | Shieh | 372/18.
|
| 6291813 | Sep., 2001 | Ackerman et al. | 250/214.
|
| 6548806 | Apr., 2003 | Chung et al. | 250/227.
|
| 6643060 | Nov., 2003 | Hashimoto et al. | 359/346.
|
Other References
Optics Letters, Jan. 1, 1996, vol. 21, No. 1, "All-fiber tunable filter and
laser based on two-mode fiber," pp. 27-29 Yun,S et al.
Optics Letters, Jun. 1986, vol. 11, No. 6, pp. 389-391 Kim et al "All-fiber
Acousto-optic FrequencyShifter" Jun. 1986.
|
Primary Examiner: Leung; Quyen
Attorney, Agent or Firm: Blakely Sokoloff Taylor & Zafman
Claims
What is claimed is:
1. A multiwavelength locking method using an acousto-optic tunable filter
being connected to output terminals of N light sources and outputting
beams of N different wavelengths outputted from the light sources in the
form of an output beam having N frequencies, the multiwavelength locking
method comprising the steps of:
(A) applying the output beam having the N frequencies outputted from a
multiplexer and N pilot signals to an acousto-optic tunable filter;
(B) splitting an output beam outputted from the acousto-optic tunable
filter, the output beam having the N frequencies, into two beams,
detecting an intensity signal from a first one of the split beams, and
detecting a wavelength signal and an intensity signal from a second one of
the split beams;
(C) detecting intensity signals of the first beam respectively
corresponding to the N frequencies, and detecting intensity signals and
wavelength signals of the second beam respectively corresponding to the N
frequencies; and
(D) detecting wavelength signals respectively corresponding to the light
sources on the basis of the intensity signals of the first beam and the
intensity signals and the wavelength signals of the second beam according
to the N frequencies, comparing each of the detected wavelength signals
with each of wavelength signals of the beams outputted from the light
sources, and compensating for a difference between the compared wavelength
signals.
2. The multiwavelength locking method according to claim 1, wherein the
step (A) comprises the step of performing a frequency conversion for the
output light having a specific frequency corresponding to the pilot
signal.
3. The multiwavelength locking method according to claim 2, wherein the
ratio of the frequency-converted beams to non-converted beams of the
output beams is controlled by respective intensities of the associated
pilot signals.
4. The multiwavelength locking method according to claim 2, wherein each of
the frequency-converted beams has a frequency corresponding to the sum of
the frequencies of the output beams and the frequencies of the pilot
signals associated with the beams.
5. The multiwavelength locking method according to claim 1, wherein the
partial beam of the output beam outputted from the multiplexer is inputted
to acousto-optic tunable filter at the step (A).
6. The multiwavelength locking method according to claim 1, wherein the
second beam is applied to a filter for the detection of the wavelength
signals and the intensity signals from the second beam at the step (B).
7. The multiwavelength locking method according to claim 1, wherein the
output beam is applied to a wavelength discriminator for the detection of
the intensity signal from the output beam at the step (B).
8. The multiwavelength locking method according to claim 7, wherein the
wavelength discriminator comprises an Etalon filter, an arrayed waveguide
grating filter, or a fiber Bragg grating filter, the filters having a
wavelength dependency.
9. The multiwavelength locking method according to claim 1, wherein the
wavelengths respectively outputted from the N light sources are
simultaneously locked.
10. A multiwavelength locking apparatus using an acousto-optic tunable
filter being connected to output terminals of N light sources and locking
output wavelengths of the light sources using respective light beams of N
different wavelengths outputted from the light sources, comprising an
optical splitter, a wavelength detecting filter, a wavelength
discriminator, a photo-detector, and a frequency filter adapted to allow
signals of a specific frequency to pass therethrough, further comprising:
an acousto-optic tunable filter for receiving the light beams having N
different frequencies respectively outputted from the light sources, along
with N pilot signals having N different frequencies respectively, and
converting each of the frequencies of output beams corresponding to the N
pilot signals,
whereby, the N frequency-converted output beams from the acousto-optic
tunable filter are inputted to the photo-detector, then N electrical
signals respectively corresponding to the N frequencies of the pilot
signals are outputted through the photo-detector, separated from one
another according to the light sources respectively, and then used to
compensate for each of the output wavelengths of the light sources,
thereby enabling the corresponding light sources to output locked
wavelengths respectively.
11. The multiwavelength locking apparatus according to claim 10, wherein
the acousto-optic tunable filter controls the ratio of the
frequency-converted beams to non-converted beams of the output beams by
controlling respective intensities of the corresponding pilot signals.
12. The multiwavelength locking apparatus according to claim 10, wherein
each of the frequency-converted beam has a frequency corresponding to the
sum of the frequency of the output beam associated with the beam the
frequency of the pilot signal associated with the beam in the
acousto-optic tunable filter.
13. The multiwavelength locking apparatus according to claim 10, wherein
each of the wavelengths outputted from the N light sources are
simultaneously locked by simultaneously inputting the pilot signals.
14. The multiwavelength locking apparatus according to claim 10, wherein
the magnitudes of the electrical signals outputted from the photo-detector
and those of the frequencies of the pilot signals are the same,
respectively.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multiwavelength locking method and
apparatus using an acousto-optic tunable filter, and more particularly to
a multiwavelength locking method and apparatus using an acousto-optic
tunable filter in an optical communication system including optical
transport networks, in which output wavelengths of light source are
monitored under the condition in which pilot signals are applied to the
acousto-optic tunable filter, so as to lock the output wavelengths of the
light sources, thereby eliminating an wavelength instability of the light
sources for an improvement in transmission characteristics.
2. Description of the Related Art
Recently, high-speed interchange services and broadband image services have
been made commercially available. As a result, an increase in the
transmission capacity of communication networks has been required.
Transmission of a large quantity of data has also been required in the
construction of ultra-high speed communication networks, due to desires of
consumers for convenience, and development of techniques and information
communications. In order to meet such requirements, various methods for
achieving an increase in communication speed have been proposed. Among
these methods, a wavelength division multiplexing (WDM) technique adapted
to simultaneously transmit different wavelengths through a signal optical
fiber is being most actively studied because it allows communication over
broad bandwidths.
Where such a WDM transmission scheme is applied to an optical communication
system, channels are arranged at a certain wavelength interval, and
signals are carried through those channels. These channels are transmitted
through a single optical fiber after being optically multiplexed. In order
to stabilize power intensities of diverse light sources used in this
system, thereby achieving an improvement in transmission characteristics,
locking of multiwavelengths is performed. As a method for simultaneously
locking wavelengths of diverse light sources, a method adapted to directly
apply a pilot signal to light sources has been mainly used. On the other
hand, as a method for locking the wavelength of a single light source,
wavelength locking techniques using a filter having a wavelength
dependency such as a Fabry-Perot filter, a fiber Bragg grating, or an
arrayed waveguide grating have been mainly used.
Now, a typical wavelength locking method for locking a wavelength of a
single light source having a single wavelength will be described in brief.
First, beam outputted from the light source is split into two beams. And
then each of the beams is passed through optical elements having different
transmittances each other according to wavelength. In this case, when two
optical elements which have different transmitting wavelength
characteristics in terms of a transmitting wavelength peak value or the
gradient of a variation in transmittance depending on a variation in
wavelength are used, the ratio between the intensities of the beams passed
through the two optical elements is varied each other depending on a
variation in wavelength. Accordingly, wavelength locking can be achieved
by controlling two light beams to have the same intensity or a desired
ratio, thereby causing the light beam emitted from the light source to
have a constant wavelength. (U.S. Pat. No. 6,094,446. 2000: D. Tei, et
al., "Wavelength Stabilization Apparatus of Laser Source").
Meanwhile, dense wavelength division multiplexing (DWDM) systems use a
method for multiplexing beams outputted from light sources having
different wavelengths distributed at a certain wavelength interval, and
transmitting the multiplexed light beams. In this case, locking of the
wavelength of each light source causes an increase in costs and an
increase in system size because a number of light sources are used. In
order to solve this problem, active research efforts have been made to
simultaneously lock a number of different wavelengths. As a method for
simultaneously locking diverse wavelengths, a method using pilot signals
has been mainly used. In accordance with this method, pilot signals of
different frequencies are applied to each of the light sources. An output
signal having diverse multiplexed wavelengths is then observed at
respective frequencies of the pilot signals. Thus, output characteristics
of each of the light sources can be determined. For example, pilot signals
having frequencies of f1, f2, . . . , and fn are applied to each of the
light sources 1, 2, . . . , and n. Using a photo-detector, an optical
signal obtained by multiplexing the output signals from the light sources
1, 2, . . . , and n by a DWDM system is detected at respective frequencies
of the pilot signals. Based on the detected results, each of the
wavelengths of the light sources can be locked in accordance with the
wavelength locking method for a single light source. However, the
wavelength locking method using pilot signals has a problem in that errors
may be generated in optical signals because a variation in the power
intensity of each light source may occur. (U.S. Pat. No. 6,118,562. 2000:
H. J. Lee, et al., "Wavelength Aligning Apparatus Using Arrayed Wavelength
Grating").
Meanwhile, an acousto-optic frequency converter has been developed which
utilizes characteristics of an optical signal varying in wavelength or
frequency when acoustic waves interfere with the optical signal. Such an
acousto-optic frequency converter is used as an optical modulator or a
wavelength filter. In the case of an acousto-optic wavelength filter using
optical fibers, a variation in transmitting wavelength occurs depending on
a variation in the frequency of acoustic waves applied to the filter. That
is, wavelength shift occurs in proportion to a variation in frequency. For
this reason, it has been required to develop a method for simultaneously
locking multiwavelengths using an acousto-optic wavelength filter
exhibiting a variation in transmitting wavelength and a variation in
frequency conversion depending on a variation in the frequency of acoustic
waves applied thereto.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a multiwavelength
locking method and apparatus using an acousto-optic tunable filter, in
which a pilot signal is not directly applied to a light source, but
applied to the acousto-optic tunable filter of which a transmitting
wavelength and a frequency conversion are different through the filter
according to the frequency of acoustic waves, so that it is possible to
reduce the level of signal noise caused by the pilot signal in a
multiwavelength locking method in which a pilot signal is directly applied
to a light source.
Another object of the invention is to provide a simultaneous
multiwavelength locking method and apparatus in which multiple different
wavelengths are simultaneously locked using filters under the condition in
which electrical pilot signals having different frequencies are
simultaneously applied to an acousto-optic frequency converter.
In accordance with one aspect, the present invention provides a
multiwavelength locking method using an acousto-optic tunable filter being
connected to output terminals of N light sources and outputting beams of N
different wavelengths outputted from the light sources in the form of an
output beam having N frequencies, the multiwavelength locking method
comprising the steps of: (A) applying the output beam having the N
frequencies outputted from a multiplexer and N pilot signals to an
acousto-optic tunable filter; (B) splitting an output beam outputted from
the acousto-optic tunable filter, the output beam having the N
frequencies, into two beams, detecting an intensity signal from a first
one of the split beams, and detecting a wavelength signal and an intensity
signal from a second one of the split beams; (C) detecting intensity
signals of the first beam respectively corresponding to the N frequencies,
and detecting intensity signals and wavelength signals of the second beam
respectively corresponding to the N frequencies; and (D) detecting
wavelength signals respectively corresponding to the light sources on the
basis of the intensity signals of the first beam and the intensity signals
and the wavelength signals of the second beam according to the N
frequencies, comparing each of the detected wavelength signals with each
of wavelength signals of the beams outputted from the light sources, and
compensating for a difference between the compared wavelength signals.
In accordance with another aspect, the present invention provides a
multiwavelength locking apparatus using an acousto-optic tunable filter
being connected to output terminals of N light sources and locking output
wavelengths of the light sources using respective light beams of N
different wavelengths outputted from the light sources, comprising an
optical splitter, a wavelength detecting filter, a wavelength
discriminator, a photo-detector, and a frequency filter adapted to allow
signals of a specific frequency to pass therethrough, further comprising:
an acousto-optic tunable filter for receiving the light beams having N
different frequencies respectively outputted from the light sources, along
with N pilot signals having N different frequencies respectively, and
converting each of the frequencies of output beams corresponding to the N
pilot signals,
whereby, the N frequency-converted output beams from the acousto-optic
tunable filter are inputted to the photo-detector, then N electrical
signals respectively corresponding to the N frequencies of the pilot
signals are outputted through the photo-detector, separated from one
another according to the light sources respectively, and then used to
compensate for each of the output wavelengths of the light sources,
thereby enabling the corresponding light sources to output locked
wavelengths respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
The above objects, and other features and advantages of the present
invention will become more apparent after a reading of the following
detailed description when taken in conjunction with the drawings, in
which:
FIG. 1 is a diagram illustrating output characteristics of an acousto-optic
tunable filter according to an embodiment of the present invention;
FIG. 2 is a diagram illustrating output characteristics exhibited when
pilot signals of diverse frequencies are applied to the acousto-optic
tunable filter according to the embodiment of the present invention; and
FIG. 3 is a diagram illustrating a multiwavelength locking method using the
acousto-optic tunable filter according to the embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the present invention will be described in detail with reference to
the annexed drawings illustrating an embodiment of the present invention.
FIG. 1 is a diagram illustrating output characteristics of an acousto-optic
tunable filter according to an embodiment of the present invention. When
input beam having an optical frequency of .omega.1 is applied to the
acousto-optic tunable filter 14 along with a pilot signal having a
frequency of f1, a part of the input beam is converted in its optical
frequency from ".omega.1" to ".omega.1+f1", and the remaining beam part,
and the remaining part of the input beam maintains the optical frequency
of .omega.1 as the input beam passes through the acousto-optic tunable
filter 14. In this case, the ratio between the frequency-converted beam
and the non-converted beam is adjustable by controlling the intensity of
the pilot signal applied to the acousto-optic tunable filter 14. The pilot
signal is preferably an electrical signal. Accordingly, an electrical
signal having a frequency of f1 is outputted from the photo-detector 18 in
accordance with an interference between the frequency-converted beam and
non-converted beam exhibited when the ratio between the
frequency-converted beam and non-converted beam is properly adjusted. The
magnitude of the outputted electric signal is related to the intensities
of the frequency-converted beam and non-converted beam.
The magnitude of the electrical signal having a frequency of f1, detected
by the photo-detector 18, is proportional to the intensity of the beam
having an optical frequency of .omega.1. Based on the electrical signal,
the intensity of the beam having an optical frequency of .omega.1, which
corresponds to the pilot signal having a frequency of f1, is measured.
Accordingly, it is possible to measure the intensity of the beam inputted
to the acousto-optic tunable filter 14, based on the magnitude of the
pilot signal outputted from the photo-detector 18 via the acousto-optic
tunable filter 14.
Here, it should be noted that the acousto-optic tunable filter 14 does not
frequency-convert beam components of all optical frequencies (or
wavelengths), but frequency-converts a beam component of a specific
optical frequency while allowing frequency components having optical
frequencies other than the specific optical frequency to pass therethrough
without being frequency-converted. In the illustrated embodiment, only
when a pilot signal having a frequency of f1, that is, a specific
frequency in association with the optical frequency of .omega.1, is
inputted, the input beam is partially frequency-converted. Also, a pilot
signal having the specific frequency of f1 associated with the frequency
of .omega.1 is outputted.
When pilot signals of diverse frequencies are simultaneously applied to the
acousto-optic tunable filter 14 in accordance with the above described
principle, as shown in FIG. 2, frequency conversion for a specific optical
frequency associated with each pilot signal occurs. This will now be
described in more detail.
FIG. 2 is a diagram illustrating output characteristics exhibited when
pilot signals of diverse frequencies are applied to the acousto-optic
tunable filter 14 according to the illustrated embodiment of the present
invention. FIG. 2 is different from FIG. 1 in that beam having n optical
frequencies is inputted to the acousto-optic tunable filter 14. As shown
in FIG. 2, when light beams having optical frequencies of .omega.1,
.omega.2, . . . , and .omega.n are inputted to the acousto-optic tunable
filter 14, along with pilot signals respectively having frequencies of f1,
f2, . . . , and fn, they are partially frequency-converted into optical
frequencies of ".omega.1 +f1", ".omega.2 +f2", . . . , and ".omega.n+fn",
respectively, as they pass through the acousto-optic tunable filter 14.
Respective remaining parts of the light beams are outputted from the
acousto-optic tunable filter 14 while having their original optical
frequencies of .omega.1, .omega.2, . . . , and .omega.n. In this case,
electrical signals having frequencies of f1, f2, . . . , and fn can be
obtained at the photo-detector 18 by appropriately adjusting the ratio
between the frequency-converted beam and the non-converted beam in the
same fashion as that of FIG. 1. In accordance with the same principle as
that of FIG. 1, respective intensities of the light beams having optical
frequencies of .omega.1, .omega.2, . . . , and .omega.n can be measured by
measuring respective magnitudes of corresponding signals respectively
having frequencies of f1, f2, . . . , and fn outputted from the
photo-detector 18.
Accordingly, the magnitudes of the pilot signals having frequencies of f1,
f2, . . . , and fn, detected by the photo-detector 18, are proportional to
the intensities of the light beams having optical frequencies of .omega.1,
.omega.2, . . . , and .omega.n, respectively. Thus, the intensities of the
light beams having optical frequencies of .omega.1, .omega.2, . . . , and
.omega.n can be measured by calculating the magnitudes of the
corresponding signals having frequencies of f1, f2, . . . , and fn,
respectively.
Now, a method for measuring wavelength information carried by an optical
signal, based on the characteristics of the acousto-optic tunable filter
14 and photo-detector 18, and performing a frequency locking operation,
based on the measured wavelength information will be described in FIG. 3.
FIG. 3 is a diagram illustrating a multiwavelength locking method using the
acousto-optic tunable filter according to the illustrated embodiment of
the present invention. In accordance with this method, light beams emitted
from a plurality of light sources 11 while having n different optical
frequencies of .omega.1, .omega.2, . . . , and .omega.n are multiplexed
using a multiplexer 12 so that they are utilized in a WDM system 10.
An optical signal obtained in accordance with the above described
multiplexing operation is applied to a fiber-optic directional coupler 13
so that it is partially transmitted to a transport system (not shown)
while being applied to the acousto-optic tunable filter 14 for a
multiwavelength locking operation according to the present invention. The
reason why a part of the optical signal obtained in accordance with the
multiplexing operation of the WDM system 10 is applied from the
fiber-optic directional coupler 13 to the acousto-optic tunable filter 14
is to achieve a feed-back control for allowing the light beam outputted
from each of the n light sources 11 to have a fixed frequency without any
variation in wavelength by applying electrical signals, corresponding to
respective wavelengths of the multiplexed optical signal, to the
associated light sources 11 in accordance with the principle described in
conjunction with FIG. 2. The feed-back control should be achieved because
the transport system may have degraded characteristics due to a variation
in the wavelength of each light source 11 caused by a variation in
environmental conditions such as temperature or other factors.
Accordingly, the characteristics of the transport system can be stabilized
by compensating for a variation in the wavelength of each light source 11
in accordance with the feed-back control, thereby allowing the light
source 11 to output light beam at a fixed frequency.
As described above, n light beams split by the fiber-optic directional
coupler 12 while having optical frequencies of .omega.1, .omega.2, . . . ,
and .omega.n are inputted to the acousto-optic tunable filter 14, along
with pilot signals respectively having frequencies of f1, f2, . . . . ,
and fn while corresponding to those optical frequencies of .omega.1,
.omega.2, . . . , and .omega.n. As the light beams pass through the
acousto-optic tunable filter 14, they are partially frequency-converted at
a desired rate, as shown in FIG. 2. The light beam emerging from the
acousto-optic tunable filter 14 is applied to an optical splitter 15
included in the wavelength discriminator 17, and split into two light
beams. One light beam from the optical splitter 15 is applied to a first
photo-detector 18, and the other light beam from the optical splitter 15
is applied to a second photo-detector 19 via an Etalon filter 16. In the
first photo-detector 18, respective intensities of the light beams having
optical frequencies of .omega.1, .omega.2, . . . , and .omega.n
corresponding to the magnitudes of the pilot signals having frequencies of
f1, f2, . . . , and fn are measured in accordance with the principle
described in conjunction with FIG. 2.
Meanwhile, the Etalon filter 16 has transmittance characteristics varying
in accordance with a variation in wavelength. Accordingly, where light
beams of different wavelengths are inputted to the Etalon filter 16, even
when they have the same intensity, the Etalon filter 16 outputs light
beams of different intensities. Thus, the intensity of light beam
outputted from the Etalon filter 16 is related to the wavelength and
intensity of light beam inputted to the Etalon filter 16. Therefore, if
the intensity of the light beam inputted to the Etalon filter 16 is known,
it is possible to measure the wavelength of that light beam. Thus, it is
possible to detect the intensity and wavelength of the light beam inputted
to the second photo-detector 19.
As apparent from the above description, one of the two light beams
outputted from the optical splitter 15 is used to measure the intensity of
the input light beam, whereas the other light beam is inputted to the
Etalon filter 16 so as to measure the intensity and wavelength of the
input beam. When two electrical signals respectively detected by the first
and second photo-detectors 18 and 19 are compared with each other, it is
possible to determine information about respective optical frequencies.
The electrical signals detected by the photo-detectors 18 and 19 are
applied to band-pass filters 22, respectively. Each band-pass filter 22
allows a particular one of multiple frequency components, contained in the
input light beam, to pass therethrough while removing the remaining
frequency components other than the particular frequency component. There
are a number of band-pass filters 22 so that each of the band-pass filters
22 outputs a signal corresponding to the optical frequency of an
associated one of the light sources. This will be described in more
detail. The light beam inputted to each of the photo-detectors 18 and 19
has n wavelengths. Accordingly, each of the photo-detectors 18 and 19
outputs n frequencies in association with each of the n pilot signals
applied to the acousto-optic tunable filter 14. Where the n frequencies
are in a mixed state, it is impossible to distinguish those frequencies
from one another, so that the wavelength information associated with a
selected frequency cannot be determined. In order to measure a particular
frequency component, the band-pass filters 22 are used. In accordance with
the use of the band-pass filters 22, information about the wavelength and
intensity of light beam emitted from each light source can be obtained.
For example, the first band-pass filter outputs the intensity and
wavelength of light beam associated with the frequency of f1, whereas the
second band-pass filter outputs the intensity and wavelength of light beam
associated with the frequency of f2. Thus, wavelength information about
the frequency of each light source is obtained. Although n frequencies are
separated from one another using the band-pass filters, other devices may
be used, in so far as they can separate an optical signal into components
of different frequencies. For example, a radio frequency (RF) spectrum
analyzer may be used.
N band-pass filters 22 should be used to achieve a filtering operation for
n beams outputted from the first photo-detector 18. In order to achieve a
filtering operation for n beams outputted from the second photo-detector
18, n band-pass filters 22 are also required. Accordingly, 2n band-pass
filters 22 are required. Of course, the present invention is not limited
to this arrangement. Each band-pass filter 22 may be used for both the
first and second photo-detectors 18 and 19.
Using each band-pass filter 22, it is possible to measure the value of a
particular frequency associated with the band-pass filter 22, from the
electrical signal detected by the first photo-detector 18. Thus,
respective optical intensities corresponding to n optical frequencies can
be measured. Similarly, it is possible to measure the value of a
particular frequency from the electrical signal detected by the second
photo-detector 19 after being filtered by the Etalon filter 16, using an
associated one of the band-pass filters 22. Thus, respective optical
intensities and optical wavelengths corresponding to the n optical
frequencies can be measured in accordance with an addition of the
characteristics of the Etalon filter 16.
Thereafter, wavelength information about the optical frequency of each
light source 11 is detected by a wavelength controllers 21. In order to
detect the wavelength information, the wavelength controller 21 uses
electrical signals representing respective wavelengths detected by the
photo-detectors 18 and 19. The detected wavelength information is compared
with a wavelength desired by the associated light source 11. When there is
a difference between the compared wavelengths, the wavelength controller
21 applies a control signal to the associated light source 11 so as to
vary the environmental conditions (mainly, temperature) of that light
source 11, thereby allowing the light source 11 to output a desired
wavelength. Thus, the light source 11 outputs light beam at a locked
frequency. The frequency locking can be achieved in a conventional manner
by adjusting the wavelength of the beam emitted from the light source in
such a fashion that the ratio between the signal magnitudes respectively
detected by the first and second photo-detectors 18 and 19 is constant at
all optical frequencies.
In accordance with the present invention, a variation in the output
wavelength of a light source depending on a variation in the wavelength of
the light source is monitored under the condition in which no pilot signal
is directly applied to the light source. Based on the monitored result,
the varied wavelength of the light source is compensated for. Thus, a
locked wavelength is outputted. Although a pilot signal may be directly
applied to the light source, a degradation in transmission characteristics
may occur because the wavelength or output intensity of the light source
may vary due to the pilot signal.
Thus, the multiwavelength locking method of the present invention may be
very useful for an optical communication system including optical
transport networks, as compared to conventional locking methods.
There is no limitation on the numbers and wavelengths of light sources,
directional couplers, band-pass filters, wavelength controllers,
photo-detectors, and wavelength discriminators used in the present
invention. Those numbers and wavelengths may be appropriately determined
in accordance with the system, to which the present invention is applied.
Although the apparatus carrying out the multiwavelength locking method of
the present invention has been described as being applied to an optical
communication system using optical transport networks, it may be applied
to other systems. Although the preferred embodiments of the 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.
As apparent from the above description, the present invention provides a
multiwavelength locking method and apparatus capable of locking diverse
wavelengths without directly applying pilot signals to light sources,
thereby eliminating influence of noise on pilot signals. Accordingly, the
multiwavelength locking method and apparatus can be very useful for
optical communication systems including optical transport networks.
Although the preferred embodiments of the 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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