U.S. patent application number 10/310127 was filed with the patent office on 2004-06-03 for optical amplifier with a spectral gain monitor using a volume phase grating.
Invention is credited to Chen, Li, Levinson, Frank H., Liu, Wilson, Wang, Chase, Yang, William, Yu, Danny, Zhang, Charlie.
Application Number | 20040105144 10/310127 |
Document ID | / |
Family ID | 32325869 |
Filed Date | 2004-06-03 |
United States Patent
Application |
20040105144 |
Kind Code |
A1 |
Yang, William ; et
al. |
June 3, 2004 |
OPTICAL AMPLIFIER WITH A SPECTRAL GAIN MONITOR USING A VOLUME PHASE
GRATING
Abstract
An optical amplifier system for amplifying an input wavelength
division multiplexed (WDM) optical signal with a first optical
coupler to extract a portion of the power of the input signal, an
erbium-doped fiber amplifier to generate an output signal and a
second optical coupler to extract a portion of the power of the
output signal. A spectral monitoring unit having a volume phase
grating separates the extracted input and output signals into
spectral components. A photo-detector array of the spectral
monitoring unit determines the power level of the spectral
components. The system further includes a controller operative to
control the operation of the amplifier in response to the power
levels of the spectral components.
Inventors: |
Yang, William; (Fremont,
CA) ; Chen, Li; (Fremont, CA) ; Levinson,
Frank H.; (Palo Alto, CA) ; Yu, Danny;
(Fremont, CA) ; Zhang, Charlie; (Fremont, CA)
; Wang, Chase; (Fremont, CA) ; Liu, Wilson;
(Fremont, CA) |
Correspondence
Address: |
OPPENHEIMER WOLFF & DONNELLY
P. O. BOX 10356
PALO ALTO
CA
94303
US
|
Family ID: |
32325869 |
Appl. No.: |
10/310127 |
Filed: |
December 3, 2002 |
Current U.S.
Class: |
359/341.41 |
Current CPC
Class: |
H01S 3/06754 20130101;
H01S 3/1305 20130101; H01S 3/13013 20190801; H04B 10/2942 20130101;
H01S 3/10015 20130101; H04B 10/0731 20130101 |
Class at
Publication: |
359/341.41 |
International
Class: |
H04B 010/12 |
Claims
What is claimed is:
1. A system for amplifying an input wavelength division multiplexed
(WDM) optical signal, the system comprising: a first coupler
configured to receive the input WDM optical signal and extract a
portion therefrom; a first spectral monitoring unit having a volume
phase grating and optically connected to the first coupler, the
first spectral monitoring unit configured to detect the power level
of prescribed channels in the extracted portion of the input WDM
optical signal; an optical amplifier optically connected to the
first coupler and configured to amplify the input WDM optical
signal and generate an amplified output WDM optical signal; a
second coupler optically connected to the optical amplifier and
configured to extract a portion of the output WDM optical signal
therefrom; a second spectral monitoring unit having a volume phase
grating and optically connected to the second coupler, the second
spectral monitoring unit configured to detect the power level of
prescribed channels in the extracted portion of the output WDM
optical signal; and a controller electrically connected to the
first spectral monitoring unit, the second spectral monitoring unit
and the controller, the controller configured to receive the power
levels in the prescribed channels in the extracted portions of the
input and output signals and control the operation of the optical
amplifier in response thereto.
2. The system of claim 1 wherein the optical amplifier comprises an
erbium-doped fiber and a laser pump source.
3. The system of claim 1 wherein the first and second spectral
monitoring units comprise: an input fiber for receiving the optical
signal; a collimating lens in optical communication with the input
fiber; a volume phase grating in optical communication with the
collimating lens, the volume phase grating configured to separate
the optical signal from the focusing lens into the prescribed
channels; a focusing lens in optical communication with the volume
phase grating; and a photo-detector array in optical communication
with the focusing lens, the photo-detector array configured to
detect the power level of the prescribed channels transmitted
through the focusing lens.
4. The system of claim 3 wherein the photo-detector array has a
plurality of photo-detectors positioned to detect the power level
of the prescribed channels.
5. The system of claim 1 wherein the controller is configured to
control the amplification of input WDM signal in response to each
of the prescribed channels.
6. A optical signal amplifier system for amplifying a wavelength
division multiplexed (WDM) input signal, the amplifier system
comprising: a first optical coupler configured to receive the WDM
input signal and extract a portion thereof; an optical amplifier in
optical communication with the first optical coupler and configured
to amplify the WDM input signal and generate a WDM output signal; a
second optical coupler in optical communication with the optical
amplifier, the second optical coupler configured to extract a
portion of the WDM output signal; a spectral monitoring unit in
optical communication with the first optical coupler and the second
optical coupler, the spectral monitoring unit having a volume phase
grating and configured to determine the power level in prescribed
channels of the extracted input and output WDM signals; and a
controller in electrical communication with the spectral monitoring
unit and the optical amplifier, the controller configured to
operate the optical amplifier in response to the power levels of
the extracted input and output WDM signals.
7. The system of claim 6 wherein the spectral monitoring unit is
configured to determine the power levels in the prescribed channels
of the extracted input and output WDM signals in a parallel
manner.
8. The system of claim 6 wherein the spectral monitoring unit is
configured to determine the power levels in the prescribed channels
of the extracted input and output WDM signals in a serial
manner.
9. The system of claim 6 wherein the spectral monitoring unit
comprises: a first input fiber for receiving the extracted input
WDM signal; a first collimating lens in optical communication with
the first input fiber; a second input fiber for receiving the
extracted output WDM signal; a second collimating lens in optical
communication with the second input fiber; a volume phase grating
in optical communication with the first collimating lens and the
second collimating lens, the volume phase grating configured to
separate the extracted input and output WDM signals into prescribed
channels; a focusing lens in optical communication with the volume
phase grating; and a photo-detector array in optical communication
with the focusing lens, the photo-detector array configured to
detect the power level of each of the prescribed channels.
10. The system of claim 9 wherein the photo-detector array
comprises a plurality of photo-detectors, each of the
photo-detectors configured to detect the power level of a
respective one of the prescribed channels.
11. The system of claim 6 further comprising an optical switch in
optical communication with spectral monitoring unit, the extracted
input WDM signal and the extracted output WDM signal, the optical
switch configured to switch the input of the spectral monitoring
unit between the extracted input WDM signal and the extracted
output WDM signal.
12. The system of claim 11 wherein the spectral monitoring unit
comprises: an input fiber for receiving the optical signal; a
collimating lens in optical communication with the input fiber; a
volume phase grating in optical communication with the collimating
lens, the volume phase grating configured to separate the optical
signal from the focusing lens into the prescribed channels; a
focusing lens in optical communication with the volume phase
grating; and a photo-detector array in optical communication with
the focusing lens, the photo-detector array configured to detect
the power level of respective ones of the prescribed channels
transmitted through the focusing lens.
13. The system of claim 12 wherein the optical switch is configured
to switch between the extracted input WDM signal and the extracted
output WDM signal by the controller.
14. A method of amplifying an input WDM optical signal with an
optical amplifier system having a first and a second optical
coupler, a first and a second spectral monitoring unit, an optical
amplifier, and a controller, the method comprising the steps of: a)
extracting a portion of the input WDM optical signal with the first
optical coupler; b) separating the extracted portion of the input
WDM optical signal into prescribed spectral components with the
first spectral monitoring unit; c) detecting the power level of
each of the spectral components of the extracted input WDM optical
signal with the first spectral monitoring unit; d) amplifying the
input WDM optical signal with the optical amplifier in order to
generate an output WDM optical signal; e) extracting a portion of
the output WDM optical signal with the second optical coupler; f)
separating the extracted portion of the output WDM optical signal
into prescribed spectral components with the second spectral
monitoring unit; g) detecting the power level of each of the
spectral components of the extracted output WDM optical signal with
the second spectral monitoring unit; and h) controlling the optical
amplifier with the controller in response to the power levels of
the spectral components of the extracted input WDM optical signal
and the extracted output WDM optical signal.
15. The method of claim 14 wherein the first spectral monitoring
unit comprises a volume phase grating and step (b) comprises
separating the extracted portion of the input WDM optical signal
into prescribed spectral components with the volume phase
grating.
16. The method of claim 14 wherein the second spectral monitoring
unit comprises a volume phase grating and step (f) comprises
separating the extracted portion of the input WDM optical signal
into prescribed spectral components with the volume phase
grating.
17. The method of claim 14 wherein the first spectral monitoring
unit comprises a photo-detector array and step (c) comprises
detecting the power level of each of the prescribed spectral
components with the photo-detector array.
18. The method of claim 17 wherein the photo-detector array
comprises a plurality of photo-detectors operative to detect the
power level of a respective one of the spectral components and step
(c) comprises detecting the power level of each prescribed spectral
component with a respective one of the photo-detectors.
19. The method of claim 14 wherein the second spectral monitoring
unit comprises a photo-detector array and step (g) comprises
detecting the power level of each of the prescribed spectral
components with the photo-detector array.
20. The method of claim 19 wherein the photo-detector array
comprises a plurality of photo-detectors operative to detect the
power level of a respective one of the spectral components and step
(c) comprises detecting the power level of each prescribed spectral
component with a respective one of the photo-detectors.
21. The method of claim 14 wherein in step (a) the first optical
coupler extracts about 2% of the power of the input WDM optical
signal.
22. The method of claim 14 wherein in step (e) the second optical
coupler extracts about 2% of the power of the output WDM optical
signal.
23. The method of claim 14 wherein the optical amplifier is a laser
pump source in optical communication with an erbium-doped fiber and
step (d) comprises amplifying the input WDM optical signal with the
erbium-doped fiber and laser pump source.
24. A method of amplifying an input WDM optical signal with an
optical amplifier system having a first and a second optical
coupler, a spectral monitoring unit, an optical amplifier and a
controller, the method comprising the steps of: a) extracting a
portion of the input WDM optical signal with the first optical
coupler; b) amplifying the input WDM optical signal with the
optical amplifier in order to generate an output WDM optical
signal; c) extracting a portion of the output WDM optical signal
with the second optical coupler; e) separating the extracted
portion of the input WDM optical signal and the extracted portion
of the output WDM optical signal into respective spectral
components with the spectral monitoring unit; f) detecting the
power level of the spectral components with the spectral monitoring
unit; and g) controlling the amplification of the input WDM optical
signal with the optical amplifier and the controller in response to
the power level of the spectral components detected by the spectral
monitoring unit.
25. The method of claim 24 wherein the optical amplifier has a
laser pump source and an erbium-doped fiber and step (b) comprises
amplifying the input WDM optical signal with the laser pump source
and the erbium-doped fiber.
26. The method of claim 24 wherein the spectral monitoring unit
comprises a volume phase grating and step (e) comprises separating
the extracted portion of the input WDM optical signal and the
extracted portion of the output WDM optical signal with the volume
phase grating.
27. The method of claim 24 wherein the spectral monitoring unit
comprises a photo-detectors array and step (f) comprises detecting
the power level of the spectral components with the photo-detector
array.
28. The method of claim 27 wherein the photo-detector array has a
plurality of photo-detectors corresponding to the spectral
components and step (f) comprises detecting the power level of each
of the spectral components with a respective one of the
photo-detectors.
29. The method of claim 24 wherein in step (a) the first optical
coupler extracts about 2% of the power of the input WDM optical
signal.
30. The method of claim 24 wherein in step (c) the second optical
coupler extracts about 2% of the power of the output WDM optical
signal.
31. The method of claim 24 wherein the spectral monitoring unit has
an optical switch and a volume phase grating and step (e) comprises
switching between the input WDM optical signal and the output WDM
optical signal with the optical switch in order to separate the
spectral components with the volume phase grating.
32. An optical amplifier system for amplifying a wavelength
division multiplexed (WDM) optical signal, the system comprising:
first coupling means for extracting a portion of the input WDM
optical signal; first monitoring means having a volume phase
grating for detecting the power level of spectral components of the
extracted input WDM optical signal; amplifying means for amplifying
the input WDM optical signal and generating an amplified output WDM
optical signal; second coupling means for extracting a portion of
the output WDM optical signal; second monitoring means having a
volume phase grating for detecting the power level of spectral
components of the extracted output WDM optical signal; and control
means for controlling the operation of the amplifying means in
response to the power level of the spectral components of the
extracted input WDM optical signal and the extracted output WDM
optical signal.
33. The system of claim 32 wherein the first coupling means and the
second coupling means are respective first and second optical
couplers.
34. The system of claim 32 wherein the first monitoring means and
the second monitoring means are respective first and second
spectral monitoring units.
35. The system of claim 32 wherein the amplifying means is a laser
pump source and an erbium-doped fiber.
36. A system for amplifying an input optical signal, the system
comprising: first coupling means for extracting a portion of the
power of the input optical signal; amplifying means for amplifying
the input optical signal; second coupling means for extracting a
portion of the power of the output optical signal; monitoring means
having a volume phase grating for separating spectral components of
the extracted input and output optical signals and determining the
power level of the spectral components; and control means for
controlling the operation of the amplifying means in response to
the power level of the spectral components.
37. The system of claim 36 wherein the first and second coupling
means are respective first and second optical couplers.
38. The system of claim 36 wherein the monitoring means is a
spectral monitoring unit.
39. The system of claim 36 wherein the amplifying means is a laser
pump source and an erbium-doped fiber.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to an optical
amplifier with spectral gain monitoring functions and in particular
to a compact erbium-doped fiber amplifier (EDFA) with gain spectrum
and optical performance dynamically controlled.
[0003] 2. Status of the Prior Art
[0004] The past decade has witnessed a rapid growth in the volume
of high-speed data traffic carried over national and international
communication networks. This growth has been driven principally by
the dramatic increase in the wide use of the Internet and
commercial data networks. This tremendous amount of worldwide data
traffic volume requires fiber-optic communications networks having
multi-gigabit transmission capacities with highly efficient
cross-connect links. To this end, in the field of fiber-optic
technology, products have been developed for multi-carrier
transmission over a single fiber thereby multiplying the amount of
information capacity transmitted over a single carrier system. By
assembling several individual data signals of different wavelengths
into a composite multi-channel signal transmitted on a single
fiber(i.e., wavelength division multiplexing (WDM)), it is possible
for multiple users to share a common fiber-optic link and thereby
realize high throughput.
[0005] To assemble the multi-channel signals, a multiplexing device
(MUX) is employed at the transmitting end that combines the
multiple light-wave signals from several sources or channels of
different wavelengths into a single composite signal. The center
wavelengths of the signals must be properly spaced and have pass
bands well defined in order to avoid cross-talk between channels.
For example, the well-accepted industrial standard is a channel
spacing of 100 GHz (0.8 nm in 1.55 .mu.m window) centered at the
ITU grid wherein each signal channel has a pass bandwidth of 0.3 nm
at 0.5 dB down power level. The multiplexed signal is then
transmitted on a single fiber-optic communications link. At the
receiving end, a demultiplexing device (DEMUX) separates the
composite signal received from the fiber link into the original
channel signals, each of which is a single signal channel centered
at the ITU grid. Such dense wavelength division multiplexing (DWDM)
technology dramatically increases the information-carrying capacity
that is transmitted on a single carrier fiber. For example, a
40-channel 100 GHz DWDM system with a 10 Gb/s transmission rate can
transmit 400 Gb/s data in the C-band (1528-1563 nm). The number of
channels deployed in long-haul DWDM systems is rapidly increasing
to beyond 100 channels over the C-band and L-band (1575-1610
nm).
[0006] In optical networks having a large number of channels, the
stability of the channels (both in terms of the amplitude and
wavelength) is critical. The stability of channels in optical
networks is largely dependent on the operational characteristics of
the optical amplifiers, optical transmitters, and network
architecture.
[0007] As the multi-wavelength signals propagate along the optical
fibers, the powers of the signals are gradually decayed due to the
presence of insertion, distribution, and transmission losses. To
boost the signal powers, optical amplifiers are employed
periodically to compensate for the power loss. Optical amplifiers
receive one or more optical signals and simultaneously amplify all
wavelengths. This is a significant advantage of multi-wavelength
fiber systems over regenerators. However, not all channels are
amplified by the same factor because the gain spectrum of the
optical amplifier is not uniform. For example, the gain spectrum of
an EDFA has well-known asymmetrical twin peaks due to a luminescent
spectrum caused by the fine structure of the energy levels. Because
the gain spectrum is not flat, a power deviation exists between the
amplified signals that corresponds to the different wavelengths.
Though a gain flattening technique can resolve this, it is
important to monitor power fluctuations of individual channels,
rather than aggregate power.
[0008] It is also well known that the wavelength and amplitude of
the light emitted by the lasers tends to vary as the lasers age and
as the operational temperature of the lasers changes. As the number
of channels deployed in a WDM optical network increases, wavelength
drifts are more likely to result in interference between channels
because the channel spacing is narrower. As a result, wavelength
drifts and amplitude variations are more likely to cause data error
or transmission failures. These variations of optical performance
will inevitably lead to fluctuations of the amplification
characteristics of optical amplifiers.
[0009] The presence or absence of individual channels across the
whole gain band has an important influence on the characterization
of optical performance of optical amplifiers. In some cases, for
example, a channel may be absent such that extra amplification of
the other existing channels will result. It is obvious that as more
channels are absent, channel amplification becomes a more serious
problem.
[0010] It is therefore important to monitor the performance of an
optical amplifier in an optical network, and in particular the
individual channels. To do so, external channel performance
monitors have been used in conjunction with optical amplifiers. A
compact channel performance monitor is described in U.S. patent
application Ser. No. 09/715,765 filed Nov. 17, 2000 titled COMPACT
OPTICAL PERFORMANCE MONITOR, the contents of which are incorporated
herein by reference. The channel performance monitor can be
tailored and integrated into an optical amplifier.
SUMMARY OF THE INVENTION
[0011] The present invention provides a method and system of
integrating optical amplifiers with a spectral monitor. The
spectral gain monitor is a compact module having a low-cost volume
phase grating (VPG) optical element, a compact photo-detector array
and a micro-processor controller. It is initially designed for
EDFAs, but not limited to.
[0012] A primary object of the present invention is to provide a
compact design of a low-cost optical amplifier system with spectral
gain monitoring capabilities based on erbium-doped fiber amplifiers
and VPG technology. The present invention provides a method for
designing optical amplifiers with spectral gain monitoring
capabilities for Raman amplifiers and other semiconductor optical
amplifiers. Accordingly, a method is provided for designing a
multi-channel device with spectral gain monitoring
capabilities.
[0013] In the preferred embodiments of the present invention, an
optical amplifier with spectral gain monitoring capabilities is
provided wherein individual channel powers (including the presence
or absence of some channels) are monitored. Feedback control to
stabilize variation of optical performance is also provided.
[0014] In accordance with the present invention, there is provided
a system for amplifying an input wavelength division multiplexed
(WDM) optical signal with a first optical coupler operative to
receive the input WDM optical signal and extract a portion of the
signal therefrom. The system further includes a first spectral
monitoring unit having a volume phase grating optically connected
to the first coupler. The first spectral monitoring unit separates
the input WDM optical signal into input spectral components (i.e.,
prescribed channels) and detects the power levels thereof. An
optical amplifier is optically connected to the first coupler and
amplifies the input WDM optical signal to generate an amplified
output WDM optical signal. The optical amplifier may be a laser
pump source optically connected to an erbium-doped fiber. A second
optical coupler is optically connected to the optical amplifier and
extracts a portion of the output WDM optical signal. The system has
a second spectral monitoring unit with a volume phase grating
optically connected to the second optical coupler. The second
spectral monitoring unit separates the output WDM optical signal
into output spectral components (i.e., prescribed channels) and
detects the power levels thereof. A controller is electrically
connected to the first spectral monitoring unit, the second
spectral monitoring unit and the optical amplifier. The controller
dynamically operates the amplifier in response to the power levels
of the input and output spectral components. In this regard, it is
possible for the amplifier to dynamically adjust the amplification
of the input optical signal in response to the power in the
channels.
[0015] The first and second spectral monitoring units separate and
detect the power level in the spectral components of the extracted
input and output signals. Accordingly, the spectral monitoring
units each have an input fiber for receiving the optical signal and
a collimating lens optically connected to the input fiber. The
collimating lens emits the optical signal onto the volume phase
grating which separates the optical signal into spectral
components. Each of the first and second spectral monitoring units
further include a focusing lens for focusing the spectral
components onto a photo-detector array which detects the power
level of each of the spectral components. The photo-detector array
has a plurality of photo-detectors wherein each of the
photo-detectors correspond to one of the spectral components. In
this regard, each of the photo-detectors detects the power level of
a respective one of the spectral components.
[0016] It will be recognized by those of ordinary skill in the art
that the amplifier system may operate with only a single spectral
monitoring unit. In this regard, the spectral monitoring unit will
determine the power levels of each of the spectral components by
processing the extracted input and output optical signals either in
a serial manner or parallel manner. For instance, if the signals
are processed in a serial manner, an optical switch will be used to
switch between the extracted input and output signals. If the
signals are processed in a parallel manner, the volume phase
grating, as well as the photo-detector array, will be configured to
receive both the extracted input and output optical signals
simultaneously.
[0017] In accordance with the present invention, there is provided
a method of amplifying an input optical signal with an optical
amplifier system having a first and second optical coupler, a
spectral monitoring unit, an optical amplifier, and a controller.
The method starts by extracting a portion of the input WDM optical
signal with the first optical coupler. Next, the input WDM optical
signal is amplified with the optical amplifier in order to generate
an output WDM optical signal. A portion of the amplified output WDM
optical signal is extracted with the second optical coupler. The
spectral monitoring unit separates the spectral components of the
extracted input and output WDM signals and detects the power levels
of the spectral components. The controller dynamically operates the
optical amplifier in response to power levels of the spectral
components. In this regard, the controller can control the
amplification of the input WDM optical signal in order to provide
uniform amplification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These, as well as other features of the present invention,
will become more apparent upon reference to the drawings
wherein:
[0019] FIG. 1 is a system block diagram of a prior art optical
amplifier having an erbium-doped fiber with a single forward
pumping source.
[0020] FIG. 2 is a system block diagram of a second prior art
optical amplifier having two erbium-doped fibers with one forward
and one backward pumping source.
[0021] FIG. 3 is a system block diagram showing a first embodiment
of an optical amplifier having two spectral gain monitoring units
with volume phase grating dispersion elements.
[0022] FIG. 4 is an illustration of the spectral gain monitoring
unit of FIG. 3.
[0023] FIG. 5 is a system block diagram showing a second embodiment
of an optical amplifier using one spectral gain monitoring unit to
measure input and output power distributions.
[0024] FIG. 6 is an illustration of the spectral gain monitoring
unit of FIG. 5.
[0025] FIG. 7 is a system block diagram showing a spectral gain
monitoring unit utilizing a 1.times.2 optical switch.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Referring now to the drawings wherein the showings are for
purposes of illustrating preferred embodiments of the present
invention only, and not for purposes of limiting the same, FIG. 1
illustrates an optical amplifier system 100 for increasing the gain
of an optical signal. Most optical amplifier systems used in DWDM
fiber-optic communications networks use erbium-doped fiber
amplifiers (EDFAs) that boost optical power across the C-band. FIG.
1 shows a prior art EDFA system 100 having three stages: 1)
pump-amplification stage 130; 2) monitoring stage; and 3)
controlling electronics stage 180. The system 100 includes a first
and second coupling device 120 and 150 respectively to receive an
optical WDM input signal 110 and generate an amplified optical WDM
output signal 160. The system 100 also has first and second optical
isolators 132 and 138 optically connected to respective coupling
devices 120 and 150. The first optical isolator 132 is optically
coupled to the output of the first coupler 120 and ensures that the
optical signal from the coupler 120 travels in the desired
direction. Similarly, the second optical isolator 138 is connected
to the input of the second coupler 150. The system 100 also has a
WDM coupler 134 with an input optically connected to the output of
the first isolator 132. An erbium-doped fiber 136 is optically
connected to the output of the WDM coupler 134 and receives the
optical signal therefrom. A pump laser 140 is coupled to another
input of the first WDM coupler 134 to amplify the optical input
signal 110. The system 100 further includes first and second
photo-detectors 170 and 175 and controlling circuitry 180. The
first photo-detector 170 is optically connected to an output of the
first coupler 170 and the controller 180. Similarly, the second
photo-detector 170 is optically connected to the output of second
coupler 150 and the controller 180. The controller 180 is also
electrically connected to the pump laser 140 in order to control
the operation thereof.
[0027] In the operation of the system 100, the EDFA 136 is pumped
by the higher optical frequency laser source (980 nm or 1480 nm)
140. The EDFA 136 and the pump laser 140 are optically coupled via
the WDM coupler 134. The first isolator 132 is used to prevent
light from reflecting into the incoming fiber and the second
isolator 138 is used to suppress reflection from the outgoing
fiber. The first coupler 120 extracts a small fraction of the
incoming power (typically 2%) from the input signal 110 for power
monitoring purposes. The input power level is detected by the
photo-detector 170 and the value thereof is sent to the controller
180. Similarly, the second coupler 150 extracts a small fraction of
the output power (typically 2%) from the amplified output signal
160 in order to measure the aggregate output power. The amplified
power level is detected by the photo-detector 175 and is also sent
to the controller 180. The gain of the system 100 is defined as
G=P.sub.out/P.sub.in, where P.sub.in is the power measured at the
first photo-detector 170 and P.sub.out is the power measured at the
second photo-detector 175. Two drawbacks of the system 100 shown in
FIG. 1 are: 1) the gain G is estimated according to the total power
and spectral details are hidden; and 2) the pump efficiency is
low.
[0028] Pump efficiency can be improved by using a bi-directional
pumping scheme. A prior art bi-directionally-pumped EDFA system 200
is shown in FIG. 2. The system 200 has a first coupler 215 for
receiving an input optical signal 210 from an input fiber. The
first coupler 215 has a first output optically connected to a first
photo-detector 265 which receives a small fraction of the power
(about 2%) from the first coupler 215. The first photo-detector 265
is electrically connected to a controller 290 in order to measure
the power level detected by the first photo-detector 265. A second
output of the first coupler 215 is optically connected to an input
of a first optical isolator 220 that prevents reflection back into
the input optical fiber. The output of the first optical isolator
220 is fed into an input of a first WDM coupler 225.
[0029] The system 200 further includes a first pump laser 270 which
is operated by the controller 290. The first pump laser 270 has an
output optically connected to an input of the first WDM coupler
225. The output of the first WDM coupler 225 is optically connected
to a first erbium-doped fiber 230. A second optical isolator 235
connects the output of the first erbium-doped fiber 230 to the
input of a second erbium-doped fiber 240.
[0030] The system 200 also has a second pump laser 275 operating at
the wavelength of 1480 nm that is optically coupled to the second
erbium-doped fiber 240 via a second WDM coupler 245. The output of
the second WDM coupler 245 is optically connected to a third
optical isolator 250 which has an output coupled to an input of a
second coupler 255. An input of a second photo-detector 280 is
optically connected to an output of the second coupler 255. In this
regard, the second photo-detector 280 receives a small fraction of
the power (about 2%) from the second coupler 255. The second
photo-detector 280 is electrically connected to the controller 290
in order to measure the power level detected by the second
photo-detector 280. The amplified output signal 260 is available at
an output fiber at an output of the second coupler 255.
[0031] The operation of the system 200 is similar to the operation
of system 100 shown in FIG. 1. Specifically, the controller 290
determines the aggregate power from both the first photo-detector
265 and the second photo-detector 280 in order to control the
amplification process. The controller 290 operates both the first
and second pump lasers 270 and 275 in order to produce the desired
amplified output signal 260. It is highly desirable to know the
power levels of individual channels in the input and output signals
rather than simply measuring the aggregate power over the whole
amplified band so that a constant gain can be provided. To this
end, prior art channel performance monitors may be used. One
channel monitor may be positioned before the EDFA while another
channel monitor is positioned after the EDFA. The performance
monitors are linked to a center controller to compute the gain of
each channel. However, the performance monitors are expensive and
not economic to use.
[0032] Referring back to FIG. 1, the two photo-detectors 170 and
175 can be replaced by two spectral-resolved units. One of the
preferred embodiments of the present invention uses a pair of
volume phase grating (VPG) based spectral monitor so that detailed
power levels of individual channels can be obtained. Referring to
FIG. 3, an amplifier system 300 constructed according to a first
embodiment of the present invention is shown. An input signal 310
from an input fiber is divided into two parts by a first coupler
320 with a power ratio of 98:2. A majority of the power (about 98%)
enters an amplifier unit 330 having an erbium-doped fiber and a
pump laser source (i.e., EDFA). The power of the remaining weak
signal (about 2%) separated by the coupler 320 is sent to a
spectral monitoring unit 360 which measures the power distribution
of the input signal. The spectral monitoring unit 360 is
electrically connected to a master controller 380 in order to
transmit the power distribution of the input signal 310 to the
master controller 380. A second spectral monitoring unit 370 is
employed to provide the amplified power levels of the output
signal. Specifically, the second spectral monitoring unit 370
receives the output signal from a second coupler 340 that is
optically connected to the amplifier unit 330. The second coupler
340 divides the output signal into two parts with a ratio of 98:2.
A majority of the output power (about 98%) is outputted in the
output signal 350. The remaining power of the signal (about 2%) is
transmitted to the second spectral monitoring unit 370. The second
spectral monitoring unit 370 detects the spectral distribution of
the outgoing signal 350. The power distribution of the output
signal measured by the second spectral monitoring unit 370 is then
transmitted to the master controller 380 that is electrically
connected therewith.
[0033] The gain for a certain channel can be specified and defined
as the ratio between the output power and the input power
corresponding to the desired wavelength channel (i.e., spectral
component). By determining the power of each particular channel,
the aggregate power and hence the aggregate gain can be obtained
accordingly. The master controller 380 can use the power level
information detected by the first and second spectral monitoring
units 360 and 370, together with the specified gain, to dynamically
adjust the pump rate. The erroneous setting of pump parameters is
therefore avoided.
[0034] The first and second spectral gain monitoring units 360 and
370 are similar to a channel performance monitor but emphasize
spectral power detection capabilities. A VPG-based diffraction
element and detector array can be used as the spectral gain
monitoring units 360 and 370. Specifically, referring FIG. 4, a
spectral gain monitoring unit 400 that can be used as the first and
second spectral gain monitoring units 360 and 370 is shown. The
spectral gain monitoring unit 400 has a receiving fiber 410, a
collimating lens 430, a transmission volume phase grating (VPG)
440, a focusing lens 460, a detector array 470, and an electrical
link 480. The receiving fiber 410 receives the incoming optical
signal from the coupler 320 shown in FIG. 3 and emits an input beam
420 onto the collimating lens 430. The collimated beam after the
lens 430 is incident upon the VPG 440 at a preferable angle so that
the Bragg condition is satisfied for the grating. The VPG 440 is
characterized by its grating constant, thickness and modulation
depth of the refractive index. The VPG 440 separates the optical
signal into spectral components (i.e., prescribed channels). After
the VPG 440, each spectral component 450 of the input light signal
propagates in a particular direction in space. The focusing lens
460 directs a narrow band of each desired channel signal to a
corresponding photo-detector of the detector array 470. All the
photo-detectors of the detector array 470 are arranged in such a
way that the two adjacent units precisely correspond to two
adjacent wavelength channels. The dependence of the polarization of
the light signal is less important because only power values are
relevant. The power level of each channel detected by each
photo-detector and is transmitted to the master controller 380 with
electrical link 480. In this regard, it is possible to determine
the power level for each channel with the spectral gain monitoring
unit 400.
[0035] The first and second spectral gain monitoring units 360 and
370 shown in FIG. 3 can be integrated into a single unit. Referring
to FIG. 5, an amplifier unit 500 constructed according to a second
embodiment of the present invention and using a single spectral
gain monitoring unit 570 is shown. An input signal 510 from an
input fiber is divided into two parts by a first coupler 520 with a
power ration of 98:2. A majority of the power (about 98%) from the
input signal 510 enters amplifier unit 530 that has an erbium-doped
fiber and pump laser source (i.e., EDFA). The power of the
remaining weak signal (about 2%) from the first coupler 520 is
directed to a spectral monitoring unit 570. After amplification by
the amplifying unit 530, the optical signal is passed through a
gain flattening filter 540 that is either static or dynamic. For
EDFAs, a static gain flattening filter is sufficient. After passing
through the gain flattening filter 540, the optical signal is
inputted into a second coupler 550 which divides the signal into
two parts. A majority of the power (about 98%) is outputted from
the second coupler 550 as output signal 560. The power of the
remaining weak signal (about 2%) from the second coupler 550 is
inputted into the spectral monitoring unit 570. The input power of
the input signal 510 extracted by the first coupler 520 and the
output power of the output signal 560 extracted by the second
coupler 550 are sent to the spectral monitoring unit 570. The two
power distributions are processed either in parallel or in series
by the spectral monitoring unit 570 as described below in order to
provide a low-cost compact design.
[0036] A parallel processing scheme for the spectral monitoring
unit 570 is shown in FIG. 6. Referring to FIG. 6(a), two incident
signals 525 and 555 from respective first and second couplers 520
and 550 enter the spectral gain monitoring unit 570 at the same
time. Referring FIG. 6(b), the spectral gain unit 570 for parallel
processing of the two incident signals 525 and 555 has first and
second input fibers 610 and 615, first and second collimating
lenses 620 and 625, a VPG 630, a focusing lens 640, and a detector
array 650 with electrical link 660. The first input fiber 610
receives the optical signal 525 from the first coupler 520 (FIG.
5). The first input fiber 610 emits the optical signal 525 onto the
first collimating lens 620. Similarly, the second input fiber 615
receives the optical signal 555 from the second coupler 550 and
emits the signal onto collimating lens 625. The beams collimated
after collimating lenses 620 and 625 are incident upon the VPG 630
at an angle so that the Bragg condition is satisfied for the
grating. The VPG 630 is characterized by its grating constant,
thickness and modulation depth of the refractive index. The VPG 630
separates the optical signals into each spectral component which
are then incident upon the focusing lens 640. Accordingly, the
focusing lens 640 directs each spectral component of the desired
channels onto a corresponding photo-detector of the detector array
650. The photo-detectors of the detector array 650 are arranged in
such a way that two adjacent units precisely correspond to the two
adjacent wavelength channels. The VPG 630, focusing lens 640, and
detector array 650 are configured such that an upper portion of the
VPG 630 and detector array 650 detect the power of the input signal
525. Similarly, the VPG 630, focusing lens 640, and detector array
650 are configured such that a lower portion of the VPG 630 and
detector array 650 detect the power of the output signal 555. In
this regard, it is possible to monitor the power of each signal 525
and 555 with a single spectral monitor 570. The electrical link 660
transmits an electrical signal to the master controller 580 (FIG.
5) in proportion to the power of each channel in order to adjust
the amplification of the amplifier unit 530. The spectral
monitoring unit 570 requires a double-size VPG 630 and detector
array 650 than that shown in FIG. 4 in order to detect the power in
both of the signals 525, 555 simultaneously.
[0037] In addition to the foregoing, it is also possible to detect
the power in both signals 525 and 555 using a serial processing
configuration. Referring to FIG. 7, a serial spectral gain
monitoring unit 700 for serial processing of two signals has a
1.times.2 optical switch 710 and a spectral gain monitoring unit
730 that is similar to the spectral gain monitoring unit 400 (FIG.
4). The switch 710 generates a time-division switching operation
between the two incoming signals 525 and 555. The optical switch
710 connects either signal 525 or 555 to the monitoring unit 730
through transmission line 720. The switching operation can be
managed by the master controller 580 (FIG. 5). Because only one
signal, either 525 or 555, is optically processed, the spectral
gain monitoring unit 730 can be the same as shown in FIG. 4. The
monitoring unit 730 generates an electrical signal in proportion to
the power of the optical signal on output line 740. The master
controller 580 (FIG. 5) receives the signal from the output line
740 in order to determine the power in the signal. Because the
master controller 580 can control the switching operation, the
controller 580 can determine which signal corresponds to the power
of either signal 525 or 555.
[0038] In summary, the present invention provides a new optical
amplifier module having spectral monitoring capabilities. The
optical amplifiers can provide precise power distribution across
all wavelength channels before and after the multi-channel signal
is amplified through the use of the spectral gain monitoring units
having VPG elements. Furthermore, by using rugged VPG and detector
array elements, a low-cost compact amplifier module design can be
provided.
[0039] Additional modifications and improvements of the present
invention may also be apparent to those skilled in the art. Thus,
the particular combination of parts described and illustrated
herein is intended to represent only certain embodiments of the
present invention, and is not intended to serve as limitations of
alternative devices within the spirit and scope of the
invention.
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