Optical: systems and elements – Optical amplifier – Beam combination or separation
Reexamination Certificate
2001-10-26
2003-11-18
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Beam combination or separation
Reexamination Certificate
active
06650468
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical amplifier arrangements, which are for example provided at regular intervals along optical fibers within optical communication networks. These amplifier sites typically comprise optical nodes at which signal amplification takes place in addition to some sites at which signal routing functions are also preferred.
BACKGROUND OF THE INVENTION
When long haul, regional or metro optical communication networks are installed, amplification sites are required at intervals along the optical fiber spans. These sites provide amplification to compensate for fiber losses over the preceding span. Current technologies allow a maximum span between sites of approximately 80 km. The amplification sites may also compensate for other distortions arising in the preceding span, for example chromatic dispersion or polarization mode dispersion.
When the network is initially installed, some of the amplification sites will provide only the amplification and other compensation functions, and will not be required to perform any signal routing functions. Other sites, however, will require add/drop or signal routing capability to enable signals to be branched off the main span or to be provided to the span. These routing nodes require some form of routing arrangement to allow signals to be added or removed from the fiber span. A patch panel may be used for this purpose, or other manual routing arrangement, or else a switching core may operate in a wavelength-dependent manner. A switching core is the most expensive part of the node design, and sufficient switching capability will only be provided for the current or short term expected traffic requirements. Some redundancy in the switching capability of the communication system may be factored into the design at the outset, dependent on the requirements for availability at this node.
FIG. 1
shows an example of a known node architecture at which amplification and signal routing functions can take place. For the purposes of clarity, the components required for transmission from west to east are shown in
FIG. 1
, although it will be appreciated that the node will in fact be arranged for bi-directional flow of traffic. The node
10
receives an incoming WDM signal from the west and amplifies this signal using a first amplifier
12
. The amplified signal is provided to a dispersion slope compensation module
14
. A first booster amplifier
16
prepares the signal for the signal routing part
18
of the node.
There are two amplifiers
12
,
16
at the input side of the node because the loss which can be sustained between two amplifiers is limited. Typically, the dispersion compensation module
14
may introduce a loss of up to 10 dB and a switching core may introduce a loss of up to 15 dB. There are, however, numerous other possible configurations for the amplifier stages.
There is a maximum power per wavelength at which light can be launched into a fiber before the non-linear distortions make the signal unusable. This limits the power of the booster
16
, but the power of the booster amplifier can be increased if it is followed by a linear loss element (such as a switch). The signal routing part
18
has a wavelength de-multiplexer
20
which divides the incoming WDM signal into individual channels or groups of channels
22
. These channels
22
are switched by a switching arrangement
24
which, in addition to routing signals across the node
10
, also provides add and drop capability, not shown. The switching arrangement is commonly termed “switching core”. The output signals
26
from the switching core
24
are provided to a bank
28
of variable optical attenuators which are provided for channel balancing. The balanced channels are then combined by a multiplexer
30
to define the output of the signal routing portion
18
. This output is then amplified by a second booster amplifier
32
to define the east bound output of the node
10
.
This node configuration will be well known to those skilled in the art. In such an arrangement, the switching core
24
can provide per-channel routing of signals. However, this switching core
24
is an expensive component and will not be installed at every amplifier node where this level of switching capability is not initially required. However, subsequent changes to the network may require the switching capability at a node to be upgraded. Increasing the switching capability also increases the loss of the signal routing portion
18
so that the booster amplifier
16
will also require upgrading to support the increased switching capability.
The node illustrated in
FIG. 1
is shown in simplified schematic form. For clarity,
FIG. 2
shows the node architecture in which the node can perform signal routing operations between three fiber spans, to the east, west and south of the node. Thus, the node architecture shown in
FIG. 2
implements a Y-branch. The incoming fibers from the east, west and south each undergo amplification, dispersion compensation, first stage boosting and de-multiplexing using the same components as described in connection with FIG.
1
. In the example shown in
FIG. 2
, each de-multiplexer
20
provides five channels on different respective wavelengths. In the example shown in
FIG. 2
, the switching arrangement
24
has individual switching planes
25
for each of the different wavelengths. For example, the switching plane
25
a
receives as input the first channel
22
a
from each of the multiplexers
20
, and each of these channels
22
a
are on the same carrier frequency. This enables the switching arrangement
24
to be designed as a number of separate switching planes
25
, each designed for a specific wavelength. Furthermore, the node can be arranged to add or drop signals on predetermined wavelengths by modification to one of the switching planes only. The outputs of the switching arrangement
24
are again provided to a bank
28
of variable attenuators before being combined by multiplexers
30
to form the individual east bound, west bound and south bound signals.
In order to avoid the need to provide full switching capability when a network is installed, the node architecture needs to be designed to enable upgrades to be performed. There is also a need to provide protection/duplication of equipment to enable repair or servicing of components within the node.
SUMMARY OF THE INVENTION
The inventors have firstly recognised the need to provide protection for the switching arrangement
24
, as it may require servicing or repair, and it may also be desirable to change some of the switching planes, or to add new switching planes, to allow different signal routing capability. One possible way of providing this protection is shown in FIG.
3
. In this arrangement, two switching cores
24
a
,
24
b
are provided. An array of splitters
40
provides the individual channels or groups of channels
22
on two different paths, each leading to a different switching arrangement
24
. The outputs from the two switching arrangements
24
a
,
24
b
are then applied to a bank
42
of two-way switches which enable one or other of the inputs to be routed to the bank of variable optical attenuators
28
. This arrangement enables the switching core
24
to be replaced for upgrade or maintenance and provides duplication only of the switching arrangement itself. By providing two separate paths for the different switching arrangements
24
protection is provided, but this protection is not provided for the bank
28
of attenuators, and this arrangement does not allow upgrade of the de-multiplexer
20
and multiplexer
30
without interrupting normal service.
One way to overcome these disadvantages is to provide two full signal routing portions
18
a
,
18
b
between the splitter
40
and the switch
42
as shown in FIG.
4
. In this way, the whole signal routing portion
18
is protected so that the failure of any component within the signal routing portion
18
is protected and all components can be upgraded without interrupting service through the node. However
Agnew Martin
Bruce Paul A
Bryant Andrew J
Gibbon Mark A
Barnes & Thornburg
Hellner Mark
Nortel Networks Limited
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