Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2003-07-08
2004-12-21
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337400
Reexamination Certificate
active
06833948
ABSTRACT:
FIELD OF THE TECHNOLOGY
The present invention relates generally to an optical power equalization method for every channel in a Dense Wavelength Division Multiplexing (DWDM) system, and more particularly to an implementing method for improving transmission gain spectrum flatness of a DWDM system.
BACKGROUND OF THE INVENTION
Along with the rapid development of digital communication, modem telecommunication system has a relentless demand for networks of higher capacities. In optical communication area, capacities of optical fibers are tremendous. In traditional transmission networks, not matter it is space-division multiplex (SDM) or time-division multiplex (TDM) for expanding capacity, it is only a single wavelength transmission of optical signals. The bandwidth of optical fiber comparing with the single wavelength transmission is almost unlimited. In order to deploy the tremendous bandwidth resource of optical fiber and to increase capacity of optical fibers transmission, a new generation optical fiber technology, taking Dense Wavelength Division Multiplexing (DWDM) technology as a core, has been naturally developed.
Deploying the widthband and low loss properties of a single mode optical fiber, DWDM technology uses multiple wavelength optics as carriers, and allows optical signals with different wavelengths propagates simultaneously in an optical fiber. Conventionally, optical channels kept farther apart (larger spacing) and even multiplex at different windows of a fiber is called Wavelength Division Multiplexing (WDM), but channels kept less apart (smaller spacing) and multiplex at same window is called Dense Wavelength Division Multiplexing (DWDM). At present, wavelength spacing of multiplex can be nanometer level or even several tenth of one nanometer. Comparing with single channel system, DWDM technology not only greatly increases network capacity and thoroughly uses fiber bandwidth, but also has advantages such as simplicity of expanding capacity and reliability etc. Especially, the DWDM is capable of accessing multiple services directly, so it will have a bright application future.
Appearance of Erbium-Doped Fiber Amplifier (EDFA) makes that the wavelength division multiplexing technology develops rapidly. With this technology, increasing communication traffic needs only increasing more multiplexing wavelengths. Nevertheless, more multiplexing wavelengths need that amplifier has wide and flat gain spectrum, but gain spectrum of EDFA is not so flat as expected. At present, increasing multiplexing wavelengths is mainly restricted by gain bandwidth of EDFA used in a system. In 1545~1560 nm wavelength band, gain spectrum of EDFA are flatter, so in general, there is no need to flatten with Gain Flattened Filter (GFF). However, for wider wavelength band (such as 1530~1560 nm wavelength band), because of gain spectrum characteristic of EDFA, it is needed to flattened the gain with GFF. As a filter, it is required that GFF inserting loss is different for different wavelengths. If loss spectrum curve of GFF coincides with gain spectrum curve of EDFA, then wider and flatter gain spectrum can be obtained. Of course, this flattening is cost by power loss.
FIG. 1
shows a present DWDM system, which mainly includes an optical multiplexer
101
, an optical power booster amplifier module
102
, a transmission fiber
103
, an optical link amplifier unit
104
, a transmission fiber
105
, an optical preamplifier unit
106
and an optical demultiplexer
107
. Among them, the optical preamplifier unit
106
and the optical link amplifier unit
104
are basically the same, which mainly include an optical preamplifier (PA) module
108
, a dispersion-compensating module (DCM)
109
and an optical power booster amplifier (BA) unit
110
. The multichannel signals are combined at optical multiplexer
101
, then pass optical power booster amplifier module
102
, and enter transmission fiber
103
. After some distances along the fiber, the signal enters optical link amplifier unit
104
, because of compensation for power fading and dispersion. In general, the optical link amplifier unit is consisted of PA
108
, DCM
109
and BA
110
, among them DCM
109
mainly is a dispersion compensating fiber and is optional. After signal power has been amplified and dispersion has been compensated, the signal enters transmission fiber
105
again. After several stages of similar link, the signal enters optical preamplifier unit
106
for power amplify and dispersion compensation.
The disadvantages of present technology are: the design of GFF of EDFA used in DWDM system, only considers absorption spectrum of EDFA itself is non-flatness, without considering loss spectrum of transmission fiber or dispersion compensating fiber used in higher than 10 Gb/s speed system. If the loss spectrums of these fibers have more different under different wavelengths, the difference will be accumulated along with increasing of length, and will affect power equalization of a system. The main reason is that every channel optical power difference affected by optical fiber will increase along with length increasing. Without considering this phenomenon, optical power equalization between every channel is getting worse, when passing links are increased. Taking Leaf fiber of Corning Co. as an example for measuring, the results are as follow.
FIG. 2
shows loss spectrums measured for 25 km single mode fiber.
FIG. 3
shows loss spectrums measured for dispersion compensating fiber of 60 km single mode fiber. It can be seen from
FIG. 2
that maximum power difference, inserted by 25 km single mode fiber, between channels is greater than 0.3 dB. Suppose a system has 8*22 dB distances with a 640 km single mode fiber; for 32 channels, the maximum difference of loss inserted by the 640 km single mode fiber will be 7.68 dB.
FIG. 3
shows that, for 32 channels, insertion loss difference of dispersion compensating fiber, used to compensate 60 km single mode fiber, is approximately 1 dB. Similarly, for 32 channels in 8*22 dB system, insertion loss difference of dispersion compensating fiber, used to compensate 640 km single mode fiber, will reach 10 dB.
SUMMARY OF THE INVENTION
The invention proposes a method to raise effectively optical power flatness for a DWDM system, in order to guarantee optical power equalization of every channel.
A method for implementing power equalization for a DWDM system comprises the steps of:
a) Measure and calculate, respectively, a gain spectrum characteristic curve of an optical power booster amplifier unit and a loss spectrum characteristic curve of a loss device, having related wavelength with the optical power booster amplifier unit, in a DWDM system.
b) Subtract the loss spectrum characteristic curve from the gain spectrum characteristic curve to obtain a difference curve. Then, complement the difference curve to obtain a complementary curve. The complementary curve is defined as a loss characteristic target curve of a GFF.
c) Set a GFF having loss characteristic curve coinciding with the loss characteristic target curve in the optical power booster amplifier unit.
The method further comprises that divide, in advance, the whole DWDM system into more than one independent unit which includes at least the optical power booster amplifier unit. Then, calculate the loss characteristic target curves for GFF of every independent unit itself, respectively.
The method also further comprises: having defined an EDFA as the optical power booster amplifier unit; having defined a transmission fiber, or a dispersion compensating module, or their combination as the loss device with relating wavelength; having defined the dispersion compensating module is consisted of dispersion compensating fibers.
The invention considers gain spectrum characteristics and loss spectrum characteristics synthetically. The invention not only considers gain spectrum non-flatness of optical power booster amplifier units, but considers loss spectrum non-flatness of transmission fibers and dispersion compensating modules as well. The invention ma
Chen Juan
Li Changchun
Harness & Dickey & Pierce P.L.C.
Hellner Mark
Huawei Technologies Co. Ltd.
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