Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2000-12-05
2004-04-13
Negash, Kinfe-Michael (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S082000, C385S140000
Reexamination Certificate
active
06721509
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical communications network systems that carry wavelength division multiplexed (WDM) signals. More particularly, the present invention relates to WDM optical communications network systems that utilize optical add-drop multiplexers, wherein each such multiplexers add and/or remove one or more optical signals from the network.
BACKGROUND OF THE INVENTION
Optical add-drop multiplexer (OADM) technology substantially reduces the cost of Dense Wavelength Division Multiplexing (DWDM) optical networks. An example of a conventional OADM configuration within a DWDM system is shown in FIG.
1
. In the conventional system
100
shown in
FIG. 1
, a multi-channel optical signal
102
is delivered to the input port
103
of OADM device
120
. The optical signal comprises a plurality of channels, each comprising a different unique wavelength range, where each channel is denoted by its respective center wavelength, &lgr;
1
, &lgr;
2
, &lgr;
3
, etc. A first optical filter
106
a
is used to remove or “drop” one of the incoming multiple channels, e.g., &lgr;
1
, and to pass through the remaining “express” channels &lgr;
2
, &lgr;
3
, and &lgr;
4
as signals
104
. A second optical filter
106
b
is used to add a channel &lgr;
1
′ into the optical path containing the express channels. The express channels exit from OADM
120
apparatus together with the added channel &lgr;
1
′ as a single combined signal
112
at the output port
105
.
An example of an optical communications network system containing a conventional OADM is demonstrated in FIG.
2
. In the conventional network system
200
, a plurality of channels &lgr;
1
, &lgr;
2
, . . . , &lgr;
N
are transmitted between end locations
202
a
and
202
b
. The optical network comprises end locations
202
a
and
202
b
, a plurality of n intermediate locations, or “nodes”
206
.
1
-
206
.n disposed between the end locations, and a sequence of optical fiber spans
208
.
1
-
208
.(n+1) optically connecting the nodes
206
.
1
-
206
.n and the end locations
202
a
-
202
b
to one another in a single chain. The first end location
202
a
comprises a WDM multiplexer (MUX)
204
a
that combines the channels from separate input paths into a single combined signal that is delivered to a first span
208
.
1
of optical fiber. Likewise, the second end location
202
b
comprises a WDM de-multiplexer (DEMUX)
204
b
that receives a set of combined channels from the last span
208
.(n+1) of optical fiber and separates these channels into separate output paths.
Optical signals &lgr;
i
, &lgr;
j
, &lgr;
i
′, and &lgr;
j
′ are added and/or dropped from the chain of optical fiber spans
208
.
1
-
208
.(n+1) at each of the nodes
206
.
1
-
206
.n. Each node is disposed between two such consecutive spans of optical fiber and the optical fiber spans join the nodes to one another. Each of the nodes
206
.
1
-
206
.n comprises a respective one of a set of n OADMs
210
.
1
-
210
.n that performs the adding and dropping of channels at the node. The OADMs are required in order to allow each of the nodes access to a respective portion of the network traffic while, at the same time maintain the integrity of other channels. Without such OADMs, all channels would have to be terminated at each intermediate node even for a small portion of traffic exchange.
One characteristic of the conventional OADM structure shown in
FIG. 1
is that the added channel &lgr;′
1
generally comprises an optical power that is different from the powers of the express channels. This power difference arises because the added channel originates from a different optical path from those of the express channels and thus generally incurs a unique set of insertion loss along this path. This unequal-power characteristic does not impose any negative impact to the early local (e.g., “metropolitan” or “metro”) multi-channel OADM systems wherein no optical amplifiers are used. However, the trend of late is to widely deploy amplifiers in such metro OADM systems in order to stretch the link distance and reach more customers. If channels in such an optical network have differing power levels, the weak signals could quickly dissipate after passing through a chain of amplifiers, due to the gain competition of the amplifiers. Therefore, the use of conventional OADM apparatus within a metro optical network also comprising optical amplifiers presents some problems.
SUMMARY OF THE INVENTION
In order to overcome the aforementioned problems with conventional OADMs, and particularly, systems such as metro systems making use of such OADMs, the present invention discloses a novel optical network system that utilizes a new inventive design of self-adjusting OADM. The self-adjusting OADM in accordance with the invention automatically adjusts the power of an added channel with reference to the power of a dropped channel, so that the output channels of the OADM all have a comparable power level. The self-adjusting OADM of the present invention comprises a first optical filter or similar wavelength-selective component that removes a channel from a WDM (a wavelength division multiplexed) signal and passes through the other channels, a drop line that receives the removed channel, a beam combiner, such as a second optical filter, that adds a new channel to the other channels, an add line that delivers the added channel to the network, a first optical tap on the drop line, a second optical tap on the add line, a variable optical attenuator (VOA) on the add line, and a controller. The first and second optical taps divert known sample portions of the dropped and added channels, respectively, to the controller. The controller receives the sample portions, and, based upon these samples, controls the variable optical attenuator so that the added channel enters into the network at a suitable power level, preferably one that is substantially equal to those of the other channels.
The invention further comprises an optical ring network having a plurality of optical exchange nodes and a plurality of optical amplifiers disposed in a ring configuration, wherein each node includes a self-adjusting OADM. In accordance with one embodiment, the optical ring network is a two-fiber ring network having a pair of rings, one for signals propagating in what is referred to as a clockwise direction, and one for signals propagating in a counterclockwise direction. Each node includes a switching assembly which comprises a pair of OADMs each in optical communication with an associated one of the rings. The switching assembly is also provided with an optical switch which operates in one of two states in accordance with a first control signal from a controller also provided in the switching assembly. In one state, the optical switch propagates signals from one of the OADMs and associated ring to an optical receiver at a subscriber location, while in the other state the optical switch propagates signals from the other one of the OADMs and associated ring to the optical receiver. The first control signal is provided to the switch by a controller which outputs the control signal in accordance with the relative power of drop signals from the two OADMs. In accordance with a further aspect of the invention, the controller also issues second and third control signals, which are used to control the attenuation of an add signal provided by an optical transmitter to the OADMs for combination with the WDM signals in the two rings of the two-fiber ring network. The second and third control signals control the attenuation of first and second variable optical attenuators operating on the add signal. The self-adjusting OADM of the present invention enables the deployment of optical amplifiers within metropolitan optical network so that the link distance is significantly increased.
The present invention includes a method for controlling the power of an add channel of an optical add-drop multiplexer (OADM) used to add the add channel to a wavelength division multiplexed (
Cao Xiaofan
Mao Xiaoping
Xiao Guohua
Zhang Lintao
Avanex Corporation
Burns Doane , Swecker, Mathis LLP
Negash Kinfe-Michael
LandOfFree
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