Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2001-10-03
2003-05-27
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Particular active medium
C359S199200, C359S344000
Reexamination Certificate
active
06570703
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifying apparatus which is used in an optical communication system or the like and, more particularly, to an optical amplifying apparatus and an optical amplifying method which can amplify light at a predetermined level, even when a predetermined optical signal is rejected from a wavelength-division multiplexed optical signal in an optical repeater station of the optical communication system. Further, it relates to an optical communication system utilizing the optical amplifying apparatus.
2. Description of the Related Art
FIG. 16
is a view showing the structure of a conventional optical communication system.
As in
FIG. 16
, the optical communication system is structured by including an optical transmitting station
501
which generates a WDM optical signal in which a plurality of optical signals in the number of m with different wavelengths from each other are wavelength-multiplexed, an optical transmission line
502
through which the WDM optical signal outputted from the optical transmitting station
501
is transmitted, and an optical receiving station
503
into which the transmitted WDM optical signal is inputted to be received and processed. Moreover, in the optical communication system, optical repeater stations
504
are connected in the optical transmission line
502
. A plurality of the optical repeater stations
504
are provided in the optical transmission line
502
as necessary, and each of the optical repeater stations
504
may include an optical amplifier
531
which amplifies the WDM optical signal to a predetermined optical level in order to compensate transmission loss occurring in the optical transmission line
502
, or it may include an optical add/drop multiplexer (hereinafter abbreviated to “OADM”)
532
for dropping/adding an optical signal corresponding to a predetermined channel (hereinafter abbreviated to “ch.”) from/to the WDM optical signal.
The optical transmitting station
501
is structured by including, for example, a plurality of optical senders (hereinafter abbreviated to “OS”)
511
-
1
to
511
-
m
in the number of m, each of which generates an optical signal corresponding to the respective channels of the WDM optical signal, an optical multiplexer (hereinafter abbreviated to “MUX”)
512
which multiplexes wavelengths of the respective optical signals outputted from the OSs
511
-
1
to
511
-
m,
and an optical amplifier
513
which amplifies the WDM optical signal outputted from the MUX
512
.
The optical receiving station
503
is structured by including, for example, an optical amplifier
521
, an optical demultiplexer (hereinafter abbreviated to “DEMUX”)
522
and optical receivers (hereinafter abbreviated to “OR”)
523
-
1
to
523
-
m.
The WDM optical signal which is inputted from the optical transmission line
502
into the optical amplifier
521
is amplified therein and outputted to the DEMUX
522
, in which its wavelength is demultiplexed to each of the optical signals corresponding to the respective channels. The demultiplexed optical signals of the respective channels are inputted into the ORs
523
-
1
to
523
-
m,
respectively, each of which is structured by including a photo diode, a demodulator and so on, to be received and processed therein.
The optical amplifying apparatus
531
of the optical repeater station
504
is structured by including a first optical fiber amplifier doped with an rare earth element, an optical attenuator, and a second optical fiber amplifier doped with an rare earth element. The rare earth element is selected corresponding to an amplification wavelength band as necessary and, for example, an erbium element (elemental symbol: Er) is used in amplifying a 1550 nm band. The optical fiber amplifier is designed to obtain an appropriate gain with a predetermined wavelength multiplexing number so that a gain deviation becomes 0, and it is controlled to obtain the constant gain (constant gain control). The optical attenuator controls the optical amplifying apparatus
531
to control its output constantly.
The OADM
532
of the optical repeater station
504
is structured by including, for example, an optical coupler, an optical filter and an optical multiplexer/demultiplexer. The inputted WDM optical signal is divided into two in the optical coupler, and one of these is inputted into the optical filter and the other is used for receiving/processing a predetermined optical signal corresponding to a channel to be dropped in the OADM. The optical filter filters the inputted WDM optical signal to reject the predetermined optical signal. The optical multiplexer/demultiplexer multiplexes wavelengths of the WDM optical signal from which the predetermined optical signal is rejected and an optical signal to be newly added in the optical repeater station
504
.
How the WDM optical signal is transmitted in the optical communication system like the above is explained as follows. When, for example, m is 4, that is, when a 4-wave WDM optical signal is transmitted, it is generated in the optical transmitting station
501
and repeated/amplified in an optical repeater station
504
-
1
, an optical signal corresponding to, for example, ch.
3
is dropped/added therefrom/thereto in an optical repeater station
504
-
2
, and it is repeated/amplified in an optical repeater station
504
-
3
. Thus, it is repeated/amplified and dropped/added in the optical repeater stations
504
in sequence, to be received in the optical receiving station
503
. In this case, a cutoff wavelength of the optical filter of the OADM
532
-
1
is set so as to filter the ch.
3
.
FIG. 17
are views showing states of the 4-wave wavelength-division multiplexed optical signal being amplified.
FIG. 17A
shows a state of the 4-wave WDM optical signal after being amplified in the optical repeater station
504
-
1
,
FIG. 17B
shows a state in which the optical signal corresponding to the ch.
3
is dropped therefrom and then newly added thereto in the optical repeater station
504
-
2
, and
FIG. 17C
shows a state of the WDM optical signal, to which the ch.
3
is added/dropped thereto/therefrom, after being amplified in the optical repeater station
504
-
3
. Lateral axes of
FIG. 17
show a wavelength (ch.) and vertical axes show an optical level.
As shown in
FIG. 17B
, when the predetermined optical signal (ch.
3
, in
FIG. 17B
) is dropped/added from/to the WDM optical signal, the optical filter of the OADM
532
rejects the predetermined optical signal including ASE. Therefore, as shown in
FIG. 17C
, the WDM optical signal after the predetermined optical signal is dropped/added therefrom/thereto has the different optical levels of the ASE between the respective optical signals, after being amplified in the optical repeater station
504
.
Supposing that, for example, the 4-wave WDM optical signal is amplified in an optical repeater station
1
, the ch.
3
is dropped/added therefrom/thereto in an optical repeater station
2
and it is amplified in an optical repeater station
3
. As to an output from the optical repeater station
3
, as shown in
FIG. 17C
, the ASEs of ch.
1
, ch.
2
and ch.
4
have a noise level from the two optical repeater stations, but the ASE of the ch.
3
has a noise level from one optical repeater station. When the ch.
3
is dropped/added therefrom/thereto during the transmission, optical levels of the ASEs differ between the ch.
1
, ch.
2
, ch.
4
and the ch.
3
.
It should be mentioned that the optical amplifying apparatuses
513
,
531
,
521
which are provided in the optical transmitting station
501
, optical repeater station
504
and the optical receiving station
503
are normally controlled so that outputs from the optical amplifying apparatuses become constant (output constant control). This is because an input optical level of the optical transmission line
502
is limited in order to prevent nonlinear optical effects such as a self-phase modulation (SPM), a cross-phase modulation (XPM) and the lik
Murakami Makoto
Saito Hideto
Sakamoto Takeshi
Sekiya Motoyoshi
Tomofuji Hiroaki
Moskowitz Nelson
Staas & Halsey , LLP
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