Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2002-03-15
2004-07-06
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
Reexamination Certificate
active
06760150
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to an optical transmitting apparatus and an optical transmitting system. Particularly, the present invention relates to a technique for optimizing signal quality of an output optical signal of an optical amplifier in an optical transmitting apparatus or an optical transmitting system having the optical amplifier in a hybrid structure.
(2) Description of Related Art
With an increase in information communication quantity, development of optical fiber communication systems in large capacities and at low costs is lively in these years. For the purpose of increasing the capacity, there has been studied and developed a wavelength division multiplex (WDM: Wavelength Division Multiplex) transmission system which multiplexes optical signals at a plurality of wavelengths as a channel signal and transmits it. To realize a lower cost, there is a demand for a long-distance system, in which a distance (called a 3R section) between a terminal (terminal once converting an optical signal into an electric signal, and again regenerating the optical signal) to a terminal can be extended as much as possible.
In the latter, in order to extend the 3R section as much as possible, it is essential how low an optical noise level generating in an optical amplifier disposed in a regenerator can be suppressed. With regard to this point, distribution Raman amplifiers (DRAs) attract attention in recent years, which are tried to be introduced into real systems.
Optical signals are heretofore amplified intensively by an erbium doped fiber amplifier (EDFA) in a regenerator. However, the EDFA generates a relatively large amount of noise, although being able to amplify with a high gain, which is one of factors that limit a transmission distance of the whole system. Namely, even if an optical receiving terminal can receive an optical signal at a sufficient level (power) transmitted for a long distance, the terminal cannot normally demodulate the signal because of a poor optical signal to noise ratio (OSNR) representing quality of the received optical signal.
In order to avoid such phenomenon, a DRA is disposed in front of the EDFA, for example, as a structure of an optical amplifier (hereinafter referred as an amplifier structure) of a regenerator, a part of a transmission loss due to an optical transmission path (optical fiber) is compensated by the DRA, then the EDFA intensively amplifies the optical signal.
Advantage of this amplifier structure is that this amplifier structure is expected to improve the OSNR as compared with a system configured with only the EDFA, since the DRA can amplify with a lower noise than that of the EDFA because the DRA is an optical amplifier of a distribution amplifier type which distributively amplifies an optical signal using the optical transmission path although its gain is smaller than that of the EDFA.
FIG. 11
shows an example of WDM transmission system using a hybrid amplifier configured with a DRA and an EDFA as a regenerator. In the WDM transmission system shown in
FIG. 11
, optical signals each at a predetermined wavelength are generated by respecitve transponders (optical transmitters)
101
equal in number to multiplexed wavelengths, signal levels of the optical signals are adjusted by respective optical variable attenuators
102
each at a corresponding wavelength, and the optical signals are wavelength-multiplexed by an optical multiplexer
103
in an optical transmitting terminal
101
, then sent as a WDM signal to an optical transmission path
500
.
The WDM signal is transmitted to an optical receiving terminal
400
while being amplified by regenerators
200
and
300
. Namely, a DRA
301
complimentarily amplifies the WDM signal with a low noise by distribution amplification to compensate a part of a transmission loss due to the optical transmission path
500
, and EDFAs
201
and
302
intensively amplify the WDM signal to compensate a remaining transmission loss due to the opical transmission path
500
, whereby the WDM signal is transmitted through the optical transmission path
500
. Incidentally, the number of the regenerators
200
and
300
is determined according to a transmission distance between the optical transmission terminal
100
and the optical receiving terminal
400
, and regenerating (amplifying) capacities of the regenerators
200
and
300
.
When the above WDM signal is finally received by the optical receiving terminal
400
, the WDM signal is demultiplexed into optical signals at respective wavelengths by an optical demultiplexer
401
, and received by optical receivers
402
at respective wavelengths.
In the case of a hybrid optical amplifier using the DRA
301
and the EDFA
302
as described above, it is possible to arbitrarily set a set gain of the DRA
301
and a set gain of the EDFA
302
within certain ranges (input level operable ranges of the DRA
301
and the EDFA
302
) by changing pumping conditions of them.
Generally, there is used a method of estimating a DRA gain fluctuation due to variations in transmission path loss immediately before the DRA
301
and variations in fiber parameters of the optical transmission path
500
, and beforehand setting a pumping light power of the DRA
301
so that a range of the DRA gain fluctuation falls in the operating range (input dynamic range of the EDFA
302
) of the EDFA
302
.
An output level of the regenerator (hereinafter referred as a node)
200
or
300
is defined in consideration of a non-linear phenomenon generating on the transmission path, so that outputs of the EDFA
302
are required to be kept at a constant level (power). For this, as disclosed in Japanese Patent Laid-Open Publication No. 2000-98433, for example, there is proposed a technique (hereinafter referred as a known technique) of monitoring a level (power) of an output optical signal of the DRA or EDFA, feed-back-controlling a pumping light power of the DRA such that a level of the output optical signal (level of an input optical signal to the EDFA) is always constant, thereby controlling an output of the regenerator at a constant level.
However, operating conditions of the DRA
301
are determined by fiber parameters of the optical transmission path
500
in the above method of beforehand setting the pumping light power of the DRA
301
. When considering characteristics of a total node of the DRA
301
plus EDFA
302
, it is difficult to say that they operate in the optimum conditions from a viewpoint of OSNR.
The above described known technique is to feed-back-control the pumping light power of the DRA in order to control a level (power) of outputs of the regenerator at a defined constant level, which does not improve the OSNA. Accordingly, it is hardly said that the regenerator operates under conditions that the optimum OSNR of the whole node can be obtained, like the above pre-setting method.
In a WDM transmission system, a dispersion compensating fiber (DCF: Dispersion Compensating Fiber) is generally installed in a node, when it is necessary to compensate wavelength dispersion generating in a WDM signal due to a wavelength dependent transmission loss characteristic of the optical transmission path
500
. As a position at which the dispersion compensating fiber is to be disposed, it is said that between stages of the EDFAs (structure in which the EDFAs are in two stages, and the DCF is interposed between the stages) is suitable. This is to prevent degradation of the OSNR due to a DCF loss as much as possible by interposing the DCF having a relatively large loss between the stages of the EDFAs.
In a hybrid optical amplifier structure, there is used a DRA with an optical fiber being as an amplification medium. As compared with a known node structure including only an EDFA, a level diagram of an optical signal in the node is largely different, so that it is not always optimum from a viewpoint of OSNR that the DCF is interposed between stages of the EDFA.
SUMMARY OF THE INVENTION
In the light of the above problems, and object of the
Goto Ryosuke
Maeda Taizo
Yamane Kazuo
Black Thomas G.
Hughes Deandra M.
Staas & Halsey , LLP
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