Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-04-12
2001-11-27
Buczinski, Stephen C. (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S199200, C359S341200, C359S341410, C359S341420
Reexamination Certificate
active
06323994
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to Wavelength Division Multiplexed (WDM) systems and more specifically to equalization within a WDM system using Erbium-Doped Fiber Amplifier (EDFA) optical amplifiers.
BACKGROUND OF THE INVENTION
The use of erbium-doped fiber amplifier technology is increasing within optical fiber communication systems in a wide range of applications in which weak optical signals require amplification. These applications include, but are not limited to, preamplifiers, postamplifiers, and in-line repeaters in optical fiber communication systems.
Also within current optical fiber technology, there is a growing requirement to increase the capacity of the existing communication systems. According to current technology, an increase in capacity can be achieved by increasing the bit rate and/or by adding wavelength division multiplexed (WDM) channels. As a result of the need for more capacity, the use of WDM channels and further, an increased number of such channels are becoming increasingly popular.
FIG. 1
illustrates a typical unidirectional optical fiber communication system in which first and second optical fibers
102
,
104
couple a wavelength division multiplexer
106
at a first location to a wavelength division demultiplexer
108
at a second location which is remote from the first location. The multiplexer
106
is used to wavelength division multiplex a series of channels (&lgr;1-&lgr;N) and the demultiplexer
108
is used to subsequently demultiplex the WDM channels. As depicted within
FIG. 1
, coupled between the multiplexer
106
and the first fiber
102
is an EDFA postamplifier
110
and coupled between the second fiber
104
and the demultiplexer
108
is an EDFA preamplifier
112
. Further, as depicted within
FIG. 1
, coupled between the fibers
102
,
104
is an in-line repeater which comprises an EDFA optical amplifier
114
. One skilled in the art would understand that further line-repeaters could also be utilized in such an implementation. This setup is a well understood unidirectional optical fiber communication system.
One major problem in such an implementation as disclosed in
FIG. 1
is the non-uniform wavelength dependent gain profile of the EDFA amplifier
114
within the in-line repeater and further within any other EDFA optical fiber amplifiers that may be included between the multiplexer 106 and the demultiplexer
108
such as the post/preamplifiers
110
,
112
. These problems, inherent to the currently utilized EDFA optical fiber amplifiers, result in each channel within a particular WDM system having a different optical gain and a different resulting Optical Signal to Noise Ratio (OSNR). Hence, some channels could have a relatively low OSNR and low received power which, in turn, could result in an excessively high bit error rate.
Considerable efforts are being expended in order to equalize the received powers and OSNRs of the individual WDM channels at the demultiplexer
108
and therefore ensure that all channels have corresponding OSNRs that are above a predetermined allowable threshold level. One technique to equalize the received powers between the channels (&lgr;1-&lgr;N) is to add Variable Optical Attenuators (VOAs) for each channel directly after the demultiplexer
108
, so that, within a certain range, the received powers can be adjusted to a common value. Although effective in reducing the difference in received powers, the implementation of these VOAs does not reduce the differences between OSNRs of the individual channels (&lgr;1-&lgr;N).
A technique that is utilized to reduce the difference in received powers and OSNRs between the WDM channels at the demultiplexer
108
is disclosed in U.S. Pat. No. 5,225,922 entitled “Optical Transmission System Equalizer” by Chraplyvy et al, issued on Jul. 6, 1993 and assigned to AT&T Bell Laboratories of Murray Hill, NJ. With this technique, a controller detects the power of the optical signals of each individual channel at each amplifier with use of a series of power detectors and subsequently adjusts the transmission power corresponding to each of the channels at the multiplexer
106
with use of a series of transmission power adjusters. The controller, input with the detected powers, operates to adjust the transmission power for each channel in order to compensate for the non-uniform gain problems caused by the optical fiber amplifiers. Hence, any channels with a low OSNR will have their corresponding transmission power increased while any channels with a high OSNR will have their transmission power reduced. Eventually, this feedback technique will equalize the power corresponding to the received optical signals on all the channels, ensuring that all channels have satisfactory OSNRs and also limiting unnecessary transmission power.
There are a number of key problems with this technique for equalizing the OSNRs corresponding to the individual WDM channels. For one, this feedback technique typically requires numerous iterations, and therefore a considerable amount of time, to complete. This is especially true as the number of channels increase. Secondly, this technique must allow for the transmission power for the individual WDM channels to be adjustable over a large dynamic range. As the dynamic range increases, the complexity and cost of the transmission power adjusters required within the multiplexer
106
also increase.
It can be seen that the unidirectional system of
FIG. 1
can be expanded to a typical bidirectional optical fiber communication system as depicted in FIG.
2
. This system comprises first and second optical fibers
202
,
204
coupled between first and second WDM couplers
206
,
208
, each coupler operating as a red and blue band splitter. Further coupled to the first WDM coupler
206
is a blue band signal multiplexer
210
and a red band signal demultiplexer
212
, while further coupled to the second WDM coupler
208
is a red band signal multiplexer
214
and a blue band signal demultiplexer
216
. The multiplexers
210
,
214
are used, similar to that for the multiplexer
106
within
FIG. 1
, to wavelength division multiplex a series of respective channels (&lgr;b1-&lgr;N, &lgr;r1-&lgr;rN) and the demultiplexers
212
,
216
are used to subsequently demultiplex the channels.
As depicted within
FIG. 2
, coupled between the first WDM coupler
206
and the first fiber
202
is a blue post/red pre amplifier
218
and coupled between the second fiber
204
and the second WDM coupler
208
is a blue pre/red post amplifier
220
. Further, as depicted in
FIG. 2
, coupled between the fibers
202
,
204
is a bidirectional in-line repeater
222
. It can be seen from
FIG. 2
that there is a blue and red transmission path which respectively traverse blue multiplexer
210
, WDM coupler
206
, blue postamplifier
218
, fiber
202
, repeater
222
, fiber
204
, blue preamplifier
220
, WDM coupler
208
, and blue demultiplexer
216
; and traverse red mulitplexer
214
, WDM coupler
208
, red postamplifier
220
, fiber
204
, repeater
222
, fiber
202
, red preamplifier
218
, WDM coupler
206
, and red demultiplexer
212
. One skilled in the art would understand that the key differentiating feature between the red and blue paths is the transmission wavelengths of the corresponding WDM channels, those being in one sample case between 1528 to 1542 nm for the blue path and 1547 to 1561 nm for the red path.
One skilled in the art would understand that the bidirectional repeater
222
of
FIG. 2
has similar problems as discussed herein above with respect to the unidirectional repeater
114
, hence requiring an equalization technique to be implemented in the bidirectional system. The complexity of such an equalization technique in a bidirectional WDM system increases compared with that in a unidirectional WDM system.
Hence, an improvement in both unidirectional and bidirectional optical fiber communication systems is required that equalizes the OSNRs of the WDM channels in a more efficient manner. Preferably this improvement would reduce the number of
Li Jinghui
Yuen Suet
Buczinski Stephen C.
Nortel Networks Limited
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