Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2001-07-30
2004-04-27
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S341330, C359S199200, C359S199200
Reexamination Certificate
active
06728028
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical communication systems and more particularly to systems and methods for amplifying wavelength-division-multiplexed (WDM) signals.
The explosion of communication services, ranging from video teleconferencing to electronic commerce has spawned a new era of personal and business interactions. As evident in the rapid growth of internet traffic, consumers and businesses have embraced broadband services, viewing them as a necessity. However, this enormous growth in traffic challenges the telecommunication industry to develop technology that will greatly expand the bandwidth of communication networks. Further improvements in optical communications hold great promise to meet the demand for greater bandwidth.
Wavelength division multiplexing (WDM) technology permits the concurrent transmission of multiple channels over a common optical fiber, thus expanding available bandwidth and providing other advantages in implementation. In a WDM link between two points, it may be necessary to amplify the WDM signal at various locations. For example, amplification may be required at the transmitter, the receiver, or at intermediate points along the link.
It is desirable in certain situations to amplify and control power of sub-bands of the WDM signal independently. To assure effective automatic power control operation on each of the sub-bands, it is important that there be low crosstalk between the sub-bands. Furthermore, to assure optimal communication performance, it is desirable that the optical amplification system exhibit a low noise figure, i.e., output signal to noise ratio over input signal to noise ratio. It is further desirable that loss of amplifier input be independently detected for each sub-band so that automatic shutdown safety features may be correctly implemented. Other desirable features include ease of expandability to accommodate adding extra groups of WDM channels to existing systems and minimum volume consumption for the amplification system packaging.
FIG. 1
depicts an optical amplification system
100
according to one prior art approach. The design of
FIG. 1
is intended for a C-band WDM system where there are multiple WDM channels to be amplified. This C-band is divided into two sub-bands, which require separate power control: a “red” band (1540-1560 nm) and a “blue” band (1529-1535 nm). Both the red and blue bands are amplified within a first amplification stage
102
that is pumped in a co-propagating mode by a pump laser
104
. The red band is separated from the blue band by a band separator
106
and then amplified within a second amplification stage
108
. Second amplification stage
108
is also pumped by pump laser
104
but in a counter-propagating mode.
Monitoring of the input signal is achieved by photodiode
110
. Monitoring of the blue band and red band amplified outputs is achieved by photodiodes
112
and
114
respectively.
Other components of optical amplification system
100
are included to appropriately filter and direct the various optical signals. A wavelength selective filter
116
separates the LSM (Line Service Modem) signals at 1480 nm and 1510 nm from the C band signals. The LSM signals carry various telemetry information. A tap coupler
118
removes a small portion of the C band signal for monitoring by photodiode
110
. An isolator
120
suppresses undesired oscillation within first amplification stage
102
.
A wavelength-selective filter
122
combines the pump laser energy with the C-band signal for input to the active fiber that implements first amplification stage
102
. A tap coupler
124
taps off a small portion of the output of pump laser
104
for monitoring by photodiode
126
. A wavelength-selective filter
128
separates the pump energy from the amplified C band signal.
The C band signal passes through an isolator
130
into band separator
106
while the pump energy is coupled into the active fiber implementing second optical amplification stage
108
by a wavelength-selective filter
132
. A small portion of the blue band signal is tapped off by a coupler
134
. A splitter
136
divides the blue band monitor signal between photodiode
112
and a monitor output. Similarly for the red band, a tap coupler
138
separates out of a portion of the red band signal for monitoring purposes. The red band monitor signal is split by a splitter
140
into one component that is input to photodiode
114
and another component that is presented at a red band monitor output.
This design fails to achieve many of the objectives given above for a WDM optical amplification system. The only way to regulate output power for either the red band or the blue band is by controlling pump current to pump laser
104
. However, reducing the output of pump laser
104
to control red band output power will also have the effect of reducing blue band output power since pump laser
104
is also the pump energy source for first amplification stage
102
that amplifies both bands. Furthermore, if pump laser
104
is adjusted to regulate the blue band output power not only will red band output power be affected but also red band noise figure will be changed because of the reduction of gain in the first stage. Another obstacle to correct automatic power control is that band separator
106
is insufficient to provide good isolation between the red band and blue band signals so that in fact the blue path incorporates some red band energy or vice versa due to the limitations of filter technology. Thus, blue band power regulation will be based in part on extraneous red band energy and vice versa.
Another drawback is that photodiode
110
cannot separately detect the failure of the blue and red bands. It has also been found that optical amplification system
100
provides insufficient noise figure performance for certain applications.
What is needed are systems and methods for optical amplification that meet all of the objectives described above while permitting implementation within a small package.
SUMMARY OF THE INVENTION
Improved systems and methods for optical amplification of WDM signals are provided by virtue of one embodiment of the present invention. Multiple sub-bands of a WDM signal are amplified in a first common amplification stage. The sub-bands are separated from one another and amplified by independent parallel amplification stages. Each of the independent parallel amplification stages is equipped with a corresponding optical pump energy source. Furthermore, all of these optical pump energy sources together also provide the pump energy for the first common amplification stage. This architecture provides low noise figure and independent power regulation for each of the sub-bands while employing only N−1 pump energy sources for N amplification stages, thus greatly reducing space requirements and cost.
A first aspect of the present invention provides apparatus for amplifying a WDM signal. The apparatus includes: a first amplification stage that amplifies the WDM signal, an optical filter structure that separates the WDM signal into at least first and second sub-bands, a second amplification stage that amplifies the first sub-band and not other components of the WDM signal and that is pumped by a first pump, and a third amplification stage that amplifies the second sub-band and not other components of the WDM signal and that is pumped by a second pump. The first pump and the second pump contribute pump energy to the first amplification stage.
A second aspect of the present invention provides apparatus for amplifying a WDM signal. The apparatus includes N amplification stages. The N amplification stages include: a first amplification stage that amplifies a plurality of components of the WDM signal and N−1 amplification stages each associated with a sub-band of the WDM signal, each amplifying only the associated sub-band of the WDM signal after amplification by the first amplification stage. The apparatus further includes N−1 pumps where each provides pump energy to an associat
Gabba Sara
Mazzini Marco
Rodriguez Fransisco Martinez
Cisco Technology Inc.
Moskowitz Nelson
Ritter Lang & Kaplan LLP
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