Hybrid wavelength-interchanging cross-connect

Details Hybrid wavelength-interchanging cross-connect Hybrid wavelength-interchanging cross-connect

1998-01-06

2001-12-25

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200

Reexamination Certificate

active

06333799

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to optical communication networks. In particular, the invention relates to a combination of all-optical and opto-electronic elements for interchanging wavelengths at a node in the network.
BACKGROUND ART
Optical fiber is becoming the transmission medium of choice for communication networks. The bandwidth of well designed optical fiber is measured in the hundreds of terahertz, and the system capacity is limited not by the fiber but by the electronics at its ends. Fiber's attenuation can be reduced to a level allowing transmission over hundreds of kilometers without regeneration or amplification. Fiber further is largely immune to electronic noise.
Optical fiber was originally used as a point-to-point replacement for electrical cable, such as coaxial cable. In this architecture, at the transmitter end of the fiber, a data signal modulates a laser emitting its light of a well defined wavelength into the fiber. At the receiver end of the fiber, a detector detects the intensity envelope of the light, thus converting the transmission signal back from the optical domain to the electrical domain. It was early recognized that the fiber capacity could be significantly increased by wavelength-division multiplexing (WDM). A plurality W of lasers of differing emission wavelengths are modulated by separate data signals, and their outputs are combined (multiplexed) onto a single fiber. At the receiver end, an optical demultiplexer separates the W-wavelength signal into W separate optical paths according to wavelength. A detector is associated with each optical path so that the W detectors output W electrical signals. However, the described WDM optical transmission system without additional specialized elements is a point-to-point system having optical fibers connecting two nodes with an opto-electronic conversion being required at each node. If an optical signal is to be transmitted through the node, the node may act as a regenerator in detecting the received optical signal, converting it to an electrical signal, and using the electrical signal to generate a new optical signal for transmission, and the regeneration is required even if the optical signal will transit the node without change.
The principle advantage of fiber is its low-cost bandwidth. However, multiplexers, demultiplexers, opto-electronic converters, and high-speed electronics associated therewith are expensive, and regenerators are replete with such elements. Further, the electronic design of regenerators typically depends strongly upon the format of the signal and its bit rate. As a result, any upgrade in data rate or conversion to a different signal type requires significant changes at each node of the network, thus greatly increasing the cost of any such change.
Brackett et al. have suggested an all-optical network to solve some of these problems, as described in “A scalable multiwavelength multihop optical network: A proposal for research on all-optical networks,”
Journal of Lightwave Technology
, vol. 11, no. 5/6, 1993, pp. 736-753. In one type of an all-optical network
10
, illustrated in the network diagram of
FIG. 1
, a number of nodes
12
, designated respectively as A, B, C, D, E, transmit and receive respective multi-wavelength WDM signals onto and from the network
10
. Three wavelengths &lgr;
1
, &lgr;
2
&lgr;
3
are illustrated, but the number W of wavelengths may vary both between networks and over time on a single network
10
. The network
10
includes a web of optical fibers
14
between wavelength-selective cross-connects (WSXCs)
16
and between the wavelength-selective cross-connects
16
and the nodes
12
. The figure illustrates the WDM paths, not the fibers. Ignoring complexities like anti-parallel fibers for bidirectional transmission, very high-capacity links, and multi-fiber self-healing networks, two or more WDM signals are assumed to be carried in one direction between nodes
12
and WSXCs
16
on a single fiber. Importantly, the wavelength-selective cross-connects
16
can receive a W-wavelength WDM signal and switch its wavelength components in different directions without the need for converting the WDM optical signal to electrical form. For example, the A node
12
can transmit two signals of wavelengths &lgr;
1
, &lgr;
3
over a single fiber
14
. The wavelength-selective cross-connects
16
can switch the two wavelength signals at &lgr;
1
, &lgr;
2
separately to the B and E nodes
12
according to the wavelength. That is, the switching is all-optical, and the opto-electronic conversion is confined to the nodes
12
, not to the network
10
itself. This WDM network
10
can be characterized as transparent in the sense that an uninterrupted optical path exists between the transmitting and receiving nodes.
To preserve non-blocking transmission capability between nodes, the number W of WDM wavelengths needs to increase with the number of nodes
12
. However, this number W seems to be limited to a relatively small number because of the need to optically amplify the optical signals (the favored erbium-doped fiber amplifier has a limited flat-gain band) and because of the limited bandwidth of many of the preferred wavelength-selective cross-connects. Systems are being demonstrated with W equal to four. This number is planned to be increased to sixteen or twenty. The all-optical network as described, however, is inadequate for interlinking a substantially larger number of nodes. Since the required number of wavelengths grows with the number of nodes, such an architecture is not scalable to a significantly larger network size.
As recognized by Brackett et al. ibid. and by Bala et al. in “The case for opaque multiwavelength optical networks,” 1995
Digest of the LEOS Summer Topical Meetings
, Keystone, Colo., Aug. 7-11, 1995, pp. 58, 59, the number of interlinked nodes can be increased by wavelength reuse and wavelength conversion. The network of
FIG. 1
shows reuse of the wavelength &lgr;
1
in that the same wavelength is used between nodes AS and B and between nodes C and D. Wavelength reuse within a single network
10
can be extended if a node
12
can receive a data signal from another node at one wavelength and transmit that same data signal to yet another node at a second wavelength. This process is generally referred to as wavelength interchange or conversion. However, a more straightforward application of wavelength conversion occurs at the cross connect between two WDM networks.
As illustrated in the network diagram of
FIG. 2
, two all-optical networks
10
1
,
10
2
are connected by a wavelength-interchanging cross connect (WIXC)
20
. Only two networks are shown, but the concept scales to a large number of networks interconnecting a nearly arbitrarily large number of nodes
12
. It is assumed that enough WDM wavelengths are available within each all-optical network
10
1
,
10
2
to provide the wavelength-identified links between the nodes
12
, including the wavelength-interchanging cross-connect
20
, so that wavelength-selective switching suffices within each network
10
1
,
10
2
. On the other hand, it is likely that the number of WDM wavelengths is insufficient to provide the required number of such wavelength-identified links between the nodes
12
of both networks
10
1
,
10
2
.
The wavelength-interchanging cross-connect
20
alleviates this problem of insufficient number of WDM wavelengths with its capability of receiving a WDM signal from the first network
10
1
at a first wavelength &lgr;
1
and retransmitting it onto the second network
10
2
at another wavelength &lgr;
j
.
The network in
FIG. 1
can be characterized as implementing the architecture of a mesh network having a relatively large number of switching nodes
16
(more than the two illustrated) within the all-optical network
10
and being intra-connected within the network
10
by an irregular mesh of fibers. Each WSXC
16
may directly connect to multiple nodes
12
and to a number of other WSXCs
16
depending upon the network connectivity.
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