Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-06-02
2001-02-20
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S016000, C385S046000, C359S199200, C359S199200
Reexamination Certificate
active
06192173
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fiber optic networks and particularly to WDM optical networks which require continued usage of the main fiber path while the wavelength plan is changed.
BACKGROUND OF THE INVENTION
The public's increasing demand for bandwidth has contributed to an acceleration in the development of wavelength division multiplexing (WDM) technology. Using WDM, data can be transmitted at a high rate on each of several wavelengths of light sharing an optical fiber. Currently, systems exist in which a single fiber carries over 100 Gb/s of data using 40 or more wavelengths.
Because of the large number of wavelengths and the high data rates involved, it becomes increasingly necessary to find more flexible means for accessing individual wavelengths of light (optical channels) for purposes of reception at, or transmission from, a given node of a WDM network. Conventionally, individual optical channels may be “dropped” by inserting a filter in the main fiber path which reflects the desired wavelength towards equipment connected to the node where optoelectronic conversion and other processing may take place. Similarly, the addition of an optical channel is typically achieved by the insertion of a filter in the main fiber path, which filter transfers light arriving at the node at a desired wavelength back into the main optical path. When multiple wavelengths are to be dropped or added, multiple optical filters must be inserted in the main optical path at the location of the node.
Since the optical add and drop filters are tuned to specific wavelengths and intercept the optical flow along the main fiber path, it is crucial to provision a conventional WDM network with the correct number, location and operational wavelength of filters upon initial installation. Conventional WDM network architectures are therefore subject to eventual fiber interruptions and to the eventual addition or replacement of equipment in order to keep pace with the evolution of the network and that of the WDM technology used to transmit data through the network.
Given the high data rates involved, any disruption of the main fiber path leads to severe inconveniences for the operator and users of the network, while the operator is further burdened with the cost of adding or replacing equipment. Clearly, what is needed is a network architecture which satisfies current operational requirements while being sufficiently flexible to accommodate evolutionary changes in the network and in the wavelength plan.
SUMMARY OF THE INVENTION
According to a first broad aspect, the invention may be summarized as a node for connection to a first adjacent node and to a second adjacent node in a main optical traffic-carrying path. The node has a plurality of first ports and a plurality of second ports, at least one first port being connectable to the first adjacent node and at least one second port being connectable to the second adjacent node. The node also has an optical coupler which optically couples the first ports to the second ports and the second ports to the first ports. In addition, the node is equipped with filtering circuitry connected to at least one of another one of the first input ports and another one of the second input ports.
The filtering circuitry is adapted to add specified wavelengths to or drop specified wavelengths from the main optical path. Because the filtering circuitry does not intercept the main optical path, the wavelengths to be added or dropped by the filtering circuitry may be changed without interrupting optical traffic along the main optical path.
In accordance with another broad aspect, the present invention may be summarized as a node for use in an optical communications network as well as the network consisting of such nodes. The node comprises one or more optical couplers, such as broadband optical couplers, the couplers together having at least one input port and a plurality of output ports. The optical couplers are arranged to distribute the intensity of a first multi-channel input optical signal present at the first input port among the plurality of output ports, thereby to produce a multi-channel output optical signal present at each output port.
The node comprises either or both of drop circuitry connected to one of the output ports and add circuitry connected to another one of the input ports. The drop circuitry is used for isolating selected channels contained in the multi-channel output optical signal present at the output port connected to the drop circuitry while the add circuitry is used for providing an add signal having a selected number of wavelengths to the input port to which the add circuitry is connected. If add circuitry is used, then the wavelengths in the add signal should be distinct from those in the multi-channel input optical signal.
When optical paths are established between nodes of the invention, changes in the wavelengths to be dropped or added by any given node do not require interruption of the optical path.
Furthermore, the node may be bidirectional and the invention may therefore be summarized according to another broad aspect as a bidirectional node for use in an optical communications network, as well as a network consisting of such nodes. The node is equipped with an optical coupler having a plurality of bidirectional west ports and a plurality of bidirectional east ports. Bidirectional optical filtering circuitry is connected to at least one of the west ports. The wavelengths of optical signals coupled to the main optical path by the filtering circuitry and the optical coupler are selected to be substantially non-interfering with respect to wavelengths occupied by optical signals arriving at the node along the main optical path. Thus, the node is capable of receiving and transmitting optical information signals travelling in a bidirectional ring and any change to the wavelength plan advantageously requires no interruption of the ring.
A further feature of the invention is that when separate fibers are used for transmitting and receiving data in a ring, the through loss and the drop loss of the couplers in the nodes may be flexibly chosen to achieve certain advantages. Therefore, the invention may be summarized according to yet another broad aspect as a fiber optic network comprising a hub and a plurality N of nodes, where the hub and the N nodes are connected in a drop path for receiving optical signals transmitted by the hub.
Each node in a first group of N−1 nodes is equipped with a broadband coupler for accessing optical signals travelling in the drop path. The through loss and the branch loss of the couplers are selected such that the optical signals received at each of the N nodes from the hub advantageously have a substantially identical probability of error.
Similarly, a collect path may be provided, in which case the through loss and branch loss of selected couplers would be chosen such that the optical signals received at the hub from each of the N nodes advantageously have a substantially identical intensity.
The invention may be summarized according to still another broad aspect as a method of maintaining a target loss around a ring. The ring consists of nodes that are equipped with couplers characterized by a respective through loss. At each current node located between a respective previous node and a respective next node, the method may be summarized as: receiving information about the output power level P
OUT
—
PREV
of the previous node; measuring the optical power level P
IN
—
CURRENT
of an optical information signal received from the previous node; determining a gain as a function of P
OUT
—
PREV
, P
IN
—
CURRENT
, the through loss of the current node and a local target power level T
LOCAL
; and amplifying the optical information signal in accordance with the gain.
Due of the use of broadband couplers in the various nodes, this method advantageously allows accurate control of the loss around the ring, as the through loss of the couplers is generally known at each node.
The invention also exten
Solheim Alan Glen
Somani Azmina
Spagnoletti Robert
Nortel Networks Limited
Palmer Phan T. H.
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