Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-05-11
2002-10-01
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337400
Reexamination Certificate
active
06459527
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier apparatus and a controlling method thereof, and an optical transmission system using the optical amplifier apparatus, and it relates, in particular to the optical amplifier apparatus and the optical transmission system being suitable to be applied to a wavelength multiplexing optical transmission system.
2. Description of Related Art
In recent years, accompanying with requirement of a low cost optical system, an optical transmission system of so-called a wavelength multiplexing optical transmission system has been studied, in which a plurality of optical signals different in wavelength thereof are multiplexed to be transferred through a single optical transmission fiber.
On the other hand, an optical amplifier apparatus, since it has a wide range in wavelength of optical signal to be amplified therewith and it has an ability of amplifying with low noise, is suitable for use as an amplifier apparatus in the wavelength multiplexing optical transmission system. Optical fiber added with a rare-earth material or metal therein or a semiconductor amplifier, which can construct the optical amplifier apparatus, has a dependency on the wavelength in gain thereof, therefore, a difference is occurred in an optical output or in the gain, for each wavelength, after amplification therewith.
The above-mentioned difference in the wavelength is added up or integrated, in particular in multistage in-line amplification with the optical amplifier apparatuses, thereby increasing the difference in optical power for the each wavelength. As a result of this, a maximum transmission distance in a total system is restricted by deterioration in a S/N ratio of the optical wavelength having the lowest power among the multiplexed wavelengths. Accordingly, it is very important to provide an optical amplifier apparatus having an ensured characteristic of flatness, i.e., no difference in the optical output for every wavelength, in the gain thereof.
Therefore, as a conventional method, there has been already known a method, “Flattening of characteristic in collective amplification of multi-wavelengths with an optical fiber amplifier using a control of amplification factor of fiber”, in Institute of Electronics, Information and Communication Engineers of Japan, Technical Paper OCS94-66, OPE94-88(1944-11), for example.
In the conventional method mentioned in the above, a characteristic curve of wavelength—optical power which is complex and variable in the shape thereof with respect to changes in an input power is made a constant or flat under a predetermined condition, thereby the characteristic curve in the gain is compensated under the predetermined condition.
Namely, an optical signal which is multiplexed with four waves in wavelengths of −11 dBm is inputted into an optical amplifier, and an optical output as a total of the amplified optical signal is monitored, wherein a fiber gain controller (it is called as “AFGC” hereinafter) for controlling fiber gain is used so as to make a level of that output a constant value. In this manner, the fiber gain can be controlled at the constant value of 12 dB, thereby minimizing the difference for each wavelength.
Or, by use of an automatic power controller (called as “APC” hereinafter) with an optical attenuator, optical loss is adjusted while maintaining the fiber gain at the constant value of 12 dB, thereby inhibiting the changes in spectrum of the fiber gain if the amplification factor of the in-line amplifier is changed.
SUMMARY OF THE INVENTION
In an actual application of the optical amplifier apparatus to the optical transmission system, it is conceivable that a transmission span length is not always constant. One example of such the application is explained with referring to FIG.
9
.
FIGS.
9
(
a
)shows a block diagram in a case when the optical amplifier apparatus is applied to the optical transmission system in which the transmission span is not constant. In the figure, a reference numeral
1
denotes an optical receiver,
2
an optical amplifier apparatus,
3
an optical multiplexer,
4
a
,
4
b
,
4
c
optical transmitters. The distance from the optical transmitter
4
a
to the optical multiplexer
3
, that from the optical transmitter
4
b
to the optical multiplexer
3
, and that from the optical transmitter
4
c
to the optical multiplexer
3
are different from one another.
As shown in FIG.
9
(
a
), wavelength multiplexed transmission optical signals from the optical transmitter
4
a
, the optical transmitter
4
b
and the optical transmitter
4
c
, passing through the optical multiplexer
3
, are amplified by the optical amplifier apparatus
2
, thereafter they are distributed to the optical receiver
1
.
As shown in FIG.
9
(
b
), time bands, during which the wavelength multiplexed transmission optical signals from those optical transmitters
4
a
,
4
b
and
4
c
are distributed to the optical receiver
1
, are pre-assigned, respectively. Namely, this shows a transmission method of so-called a time division multiplex (hereinafter, called as only “TDM”), in which the optical receiver
1
receives the optical signals in a time sequence from the predetermined one of the optical transmitters.
In the method mentioned in the above, the transmission distance from the optical transmitter to the optical amplifier apparatus is not constant, therefore, a level of the optical input at the optical amplifier is low, during the time band when the wavelength multiplexed optical signals from the optical transmitter at long transmission distance are distributed. On the contrary, during the time band when the wavelength multiplexed optical signals from the optical transmitter at the short transmission distance are distributed, the level of the optical input at the optical amplifier is high.
There is a drawback that it is necessary to apply an optical amplifier apparatus having a wide dynamic range for input, in order to achieve equal optical amplification for all of such the optical input signals.
Further, a system of such construction is also conceivable that a plurality of optical amplifiers are located at positions where they are not necessarily constant in the transmission span. If the transmission span differs, the span loss also differs, then the input level of the optical signals at the optical amplifier differs depending on the location where it is positioned. Therefore, there is a drawback that it is desired to apply an optical amplifier which possesses the input dynamic range being able to cope with any length of the transmission span, in order to construct a transmission system of high reliability with ease and with certainty.
However, the conventional art mentioned in the above has studied only the case where the input level is fixed at −11 dBm. And, in the method mentioned in the above conventional art, only if the dynamic range from −30 dBm to 0 dBm can be secured with respect to the input, for example, then the fiber gain is made constant at 12 dBm.
Therefore, the output level of the fiber changes from −18 dBm up to +12 dBm depending on the level of the signals. At this time, when the optical output is controlled at constant all over the input dynamic range by use of the APC, the optical output mentioned above must be below −18 dBm, i.e., about one-hundredth ({fraction (1/100)}) of the ordinal optical transmission power, thereby causing a problem in practical use.
For dissolving such the problem as mentioned in above, it becomes a necessary object to increase the gain above 12 dB, however, in general, the optical fiber which is added with the rare-earth metal therein has such a problem, that the higher in the gain thereof, the more difficult to realize the flatness in wavelength. In the conventional method mentioned in the above, though it studied only the case of the small gain amplification of 12 dB, but it fails to study a method or countermeasure for ensuring a gain over 30 dB and for realizing the flatness in t
Antonelli Terry Stout & Kraus LLP
Hellner Mark
Hitachi , Ltd.
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