Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
1999-10-13
2003-09-09
Chan, Jason (Department: 2733)
Optical communications
Multiplex
Wavelength division or frequency division
C398S082000, C398S092000, C398S093000
Reexamination Certificate
active
06616351
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1998-42836, filed on Oct. 13, 1998, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bidirectional optical amplifying apparatus in a bidirectional WDM (wavelength division multiplexing) optical communication network, more particularly to a bidirectional optical amplifying apparatus which uniformly controls optical gain and gain flatness of a WDM channel in a bidirectional optical communication network that allows bidirectional communication using a single mode optical fiber (SMF).
2. Discussion of the Related Art
In general, WDM has been developed for increasing the channel capacity of an optical fiber. A WDM system adopts a plurality of optical signal channels, and a particular wavelength is allocated to each channel. Because each channel may transmit a plurality of information sources by TDM (Time Division Multiplexing), added optical channels increase their capacity in proportion to channels of single channel system. In the transmission end of the WDM system, a number of single channels are created. The single channels are multiplexed as WDM optical signals, then the multipexed WDM optical signals are transmitted through an optical line. At the receiving end, the WDM optical signals are demultiplexed to each channel in order to be transmitted through designated receiver paths. The multiplexed WDM optical signal may be directly amplified through an optical amplifier, for example a doped optical amplifier, at the same time. Such an optical amplifier is very useful for a WDM system in a long distance optical system.
A bidirectional WDM optical communication system is used for transmitting or receiving a plurality of optical signals via an optical fiber. Furthermore, the optical amplifier is a most important element adopted in the bidirectional WDM optical communication system.
FIG. 1
is a schematic diagram showing a general bidirectional WDM optical transmission system.
Referring to
FIG. 1
, a bidirectional optical amplifier
140
amplifies and transmits channel signals respectively of the forward transmission path
161
, which transmits signal from left optical line
151
to right optical line
152
, and the reverse or backward transmission path
162
, which transmits signals from right optical line
152
to left optical line
151
. Wavelengths for the forward transmission and the reverse or backward transmission are different from each other.
First (forward) optical sending-end
111
comprises a transmitter Tx
1
employing wavelength &lgr;
1
, and a transmitter Tx
2
employing wavelength &lgr;
2
. First optical receiving-end
112
for receiving signal from the first (forward) optical sending-end
111
comprises a receiver Rx
1
employing the wavelength &lgr;
1
, and a receiver Rx
2
employing the wavelength &lgr;
2
.
Second (reverse or backward) sending-end
121
comprises a transmitter Tx
3
employing wavelength &lgr;
3
, and a transmitter Tx
4
employing wavelength &lgr;
4
. Second optical receiving-end
122
for receiving signal from the second (reverse or backward) optical sending-end
121
comprises a receiver Rx
3
employing the wavelength &lgr;
3
, and a receiver Rx
4
employing the wavelength &lgr;
4
.
First optical multiplexer
131
multiplexes optical output signals of the first (forward) optical sending-end
111
, and transmits the multiplexed optical output signal to the first optical receiving-end
112
. The first optical multiplexer
131
demultiplexes an optical output signal of the second (reverse or backward) optical sending-end
121
and transmits the demultiplexed optical output signal to the second optical receiving-end
122
.
Second optical multiplexer
132
multiplexes the optical output signal of the second (reverse or backward) optical sending-end
121
and transmits the multiplexed optical output signal to the second optical receiving-end
122
. The second optical multiplexer
132
demultiplexes the optical output signal of the first (forward) optical sending-end
111
and transmits the demultiplexed optical output signal to first optical receiving-end
112
.
The bidirectional optical amplifier
140
should minimize any reduction of transmission quality caused by back reflections. To configure a bidirectional amplifying apparatus for minimizing reduction of transmission quality, the following method has been suggested. The wavelength(s) of WDM optical signal, which travels forward, and the wavelength(s) of WDM optical signal, which travels in a reverse or backward direction, are allocated with different wavelengths, respectively. And, an optical filter is provided to prevent the back reflected optical signal from passing through the optical amplifier.
FIG. 2
is a schematic diagram showing a bidirectional optical amplifying apparatus in which back reflections are removed, in a conventional optical transmission system. The bidirectional optical amplifying apparatus is comprised of optical connectors
211
,
251
, bidirectional optical amplifier
221
,
222
, circulators
231
,
232
and optical filters
241
,
242
.
The forward WDM transmission channel is comprised of the first optical connector
211
, the first bidirectional optical amplifier
221
, the first circulator
231
, first optical filter
241
, the second circulator
232
, the second bidirectional optical amplifier
222
, and the second optical connector
251
. The reverse WDM transmission channel is comprised of the second optical connector
251
, the second bidirectional optical amplifier
222
, the second circulator
232
, the second optical filter
242
, the first circulator
231
, the first bidirectional optical amplifier
221
, and the first optical connector
211
. Here, characteristics of the first bidirectional optical amplifier
221
and the second bidirectional optical amplifier
222
are the same.
The optical connectors
211
,
251
are elements used for transmitting output signals of the bidirectional optical amplifiers
221
,
222
to the optical line. The optical connectors
211
,
251
have the characteristic of reflecting back some of the optical signals by forming a reflective surface at a node.
The bidirectional optical amplifiers
221
,
222
are comprised of passive elements and a pump laser. The bidirectional optical amplifiers
221
,
222
are designed to be capable of bidirectional amplification by removing an isolator in a conventional unidirectional optical amplifier.
The circulators
231
,
232
are elements in which a unique output port is provided according to the input direction of an optical signal. For example, the circulator has the characteristic that an input signal at port
1
of the circulator should exit through port
2
, and an input signal at port
3
of the circulator should exit through port
1
. The other words, the output for each input port is the first port reached by rotating the ports of the circulator clockwise.
The bidirectional optical amplifying apparatus in
FIG. 2
operates as follows. An input signal (F_WDM_IN) in the forward WDM transmission line is represented with a dotted line in an upper part of the FIG.
2
. An input signal (R_WDM_IN) in the reverse WDM transmission line is represented with a dotted line in a lower part of the FIG.
2
. The wavelengths of the two input signals (F_WDM_IN and R_WDM_IN) are different from each other.
The forward WDM optical input signal (F_WDM_IN) is amplified in the first bidirectional optical amplifier
221
. The F_WDM_IN signal passes through the first optical circulator
231
, the first optical filter
241
, the second circulator
232
, and the second bidirectional optical amplifier
222
, and is output as a forward WDM optical output signal (F_WDM_OUT).
The reverse WDM optical input signal (R_WDM_IN) is amplified in the second bidirectional optical amplifier
222
. The R_WDM_IN signal passes through the second circulator
232
, the second optical filter
242
, the first circulator
231
, and the first bidirectional optical amplifier
221
, and
Chan Jason
LG Electronics Inc.
McKenna Long & Aldridge LLP
Payne David C.
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