Optical communication network

Details Optical communication network Optical communication network

1999-12-06

2003-09-30

Pascal, Leslie (Department: 2633)

Optical communications

Multiplex

Optical local area network

C398S082000, C398S083000, C398S009000, C398S045000, C398S050000, C398S091000, C398S079000, C398S087000

Reexamination Certificate

active

06626590

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength division multiplexing network, and relates in particular to a communication network where multiplexed signal transmission lines are connected in a ring through a plurality of nodes that can be used to process multiple wavelengths. This technology enables to reduce the number of hardwares needed inside the node and simplify the system configuration, and enables to switch optical paths to bypass a fault, thereby enabling to continue operating the network even when the failure is within the node. The technology is particularly useful when a two-fiber bi-directional ring network has been serviced to its capacity, which can be increased by transforming the network into a 4-fiber directional ring network, without stopping the normal operation of the existing network.
2. Description of the Related Art
Conventional Technology 1
FIG. 15
is a schematic diagram of a WDM ring network, which is an example of the conventional wavelength multiplexing optical communication network. The WDM ring network is comprised by: nodes
901
a
~
901
e
; optical fibers
902
serving as WDM transmission lines, an optical path
903
a
for providing normal optical communication through the optical fiber
902
, and an optical path
903
b
contained in the optical fiber
902
, which is used when problems develop in the optical path provided in the optical fiber
902
. Here, the logical connection between each node is conducted using wavelengths as routing information, and these signal channels are called optical paths.
During the normal communication in this WDM ring network, WDM signals are input in the optical path
903
a
. In other words, optical signals input in node
901
a
are output from node
901
c
by propagating clockwise by way of node
901
b.
Suppose that a fault
904
develops between the nodes
901
a
and
901
b
in the optical path
901
, as shown in
FIG. 16
, signals cannot be propagated between the nodes
901
a
,
901
b
. Therefore, WDM signals entering node
901
a
are first propagated counter-clockwise through the nodes
901
e
,
901
d
,
901
c
and
901
b
, and are then propagated clockwise in the optical path
903
b
through the nodes
901
b
,
901
c
to be output from the node
901
b.
FIG. 17
is a schematic diagram of an example of the general configuration of the WDM optical communication network, in which the node structure of a two-fiber unidirectional ring, that allows extraction/insertion (adding/dropping) of any wavelength, is applied to a two-fiber bi-directional ring.
This type of WDM optical communication network is reported, for example, in L. Berthelon et. al., Proc. GLOBECOM 96, pp. 311-315, 1996, or A Mariconda et. al., Proc. ECOC 96, ThD. Jan. 10, 1996. These articles describe a general structure for the application of the node structure of a two-fiber unidirectional ring, that allows processing of any wavelength, to a two-fiber bi-directional ring.
This type of WDM optical communication network is operated using single wavelength 2×2 optical switches that may include wavelength filters to enable extraction or insertion of waves, and the spectral source for different wavelengths is a fixed-wavelength source, and the system does not include a device for wavelength conversion. Also, in general, switching of optical path during circuit problems is considered in such ring networks, but in this discussion, switching is not considered for simplification. An example of switching is described later in Conventional Technology 2.
Node B (
1000
) in such a WDM optical network is connected to two adjacent nodes A and B having the same structure as the node B through optical fibers
911
~
914
, and supplies M-channels (or channels) of a required wavelength to the optical paths in a full mesh configuration between the nodes. This WDM network is comprised by: optical add/drop circuits
1001
,
1002
for processing at least N−1 waves of a given wavelength; and the optical add/drop circuits
1001
,
1002
are provided with wavelength de-multiplexers
1003
,
1004
for de-multiplexing M input waves of WDM signals; 2×2 optical switches
1005
1
to
1005
M/2
; and optical couplers (or wavelength multiplexers)
1007
,
1008
.
Also, this WDM network is provided with optical path (op) termination circuits (transmit end and receive end) for selecting the optical paths, and the transmit end
1009
of the op termination circuit is provided with M pieces of fixed-wavelength light source
1010
1
~
1010
M
; M pieces of modulators
1011
1
~
1101
M
for superimposing electrical signals on optical signals; and M lines of electrical input
1012
1
~
1012
M
and the receive end
1013
of the op termination circuit is provided with M lines of photo-electric converter
1014
1
~
1014
M
for converting optical signals of respective wavelengths to electrical signals; and M lines of electrical signal output
1015
1
~
1015
M
.
Here, optical fiber
911
contains optical signals input from node A, and optical fiber
912
contains optical signals input from other node C, and optical fiber
913
contains optical signals output to node C, and optical fiber
914
contains signals output to node A.
Bi-directional communication between node B and the other node is carried out in the following manner.
Here, the direction of nodes are defined such that A B C is clockwise (clock) and C B A is counter-clockwise. Also, for the counter-clockwise direction, the waves are used in the ascending order of refractive index stating from the lowest index using M/2 waves, and for the counter-clockwise direction, the waves are used in the descending order of refractive index starting from the highest index using M/2 waves. If the same wavelength is used in both directions, M/2 waves are sufficient number of waves required, but, for use in public networks, it is necessary to consider protection circuits, and in such cases, the remaining M/2 waves in each fiber is used generally for emergency use. Therefore, in this discussion, it is left as M-channels. Also, the reason for using different wavelengths for clockwise and counter-clockwise directions is to prevent wave collision for lines at the insertion circuit during switching operations, and this aspect of the circuit will be discussed later in the section related to Technology 2.
In a clock optical path from node B to another node, for example node C, one wave of the &lgr;
1
~&lgr;
M/2
modulated by one of the electrical signal input
1012
1
~
1012
M/2
is input in the optical insertion circuit
1001
, and is output to optical fiber
913
through one of the optical switches
1005
1
~
1005
M/2
. On the other hand, an optical path from node C to node B is a counter-clockwise path, so that one wave of the &lgr;
M/2+1
~&lgr;
M
is allocated, and it is input in optical fiber
912
into node B, and is output to the receiving end
1013
of the op termination circuit through one of the optical switches
1006
1
~
1006
M/2
in the optical add/drop circuit
1002
.
In this type of WDM network, to enable insertion/extraction of any wavelength at a node, it is necessary to be able to process each M-channels in the optical add/drop multiplexing circuit (OADM), as well as to couple all the 2M-channels multiplexed by the two wavelength de-multiplexers to the op termination circuit. Therefore, in order to produce an optical path using any wavelength, it is necessary to provide a modulator in each of the transmit ends of the op termination circuit of all the 2M-channels, and in order to receive any wavelength of the 2M-channels, it is necessary for each of the receive ends of the op termination circuit to have an op termination circuit.
The above configuration has an advantage of offering logical connectivity between the nodes, that is, it does not restrict the traffic distribution pattern, however, assuming that the network is in a full mesh configuration, which is a typical logical connectivity between the nodes N, each node needs to process N−1 channels of the 2M-channels,

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