Optical: systems and elements – Light interference – Electrically or mechanically variable
Reexamination Certificate
1999-06-28
2002-12-24
Chang, Audrey (Department: 2872)
Optical: systems and elements
Light interference
Electrically or mechanically variable
C359S889000, C359S885000, C385S031000, C385S033000
Reexamination Certificate
active
06498682
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an electromagnetic filter whose frequency may be quickly and easily altered.
Electromagnetic filters may filter a stream of electromagnetic energy by separating certain frequencies from the stream and/or by adding certain frequencies to the stream. For example, an optical filter may subtract a band of frequencies from a beam of light containing multiple frequencies of light. Such filters may be used in communication networks transmitting information using beams of electromagnetic information.
A communication network transports information from a source to a destination. The source and destination may be in close proximity, such as in an office environment, or thousands of miles apart, such as in a long-distance telephone system. The information, which may be, for example, computer data, voice transmissions, or video programming, known as “traffic”, usually enters and leaves a network at nodes, and is transported through the network via links and nodes. Nodes, sometimes termed offices, are devices or structures that direct traffic into, out of, and through the network. Links connect nodes and transmit data between nodes.
Modem communication networks may transmit information in digital form by light waves using links of optical fiber cable. Multiple wavelengths of light may be transmitted on one optical fiber line, each wavelength carrying a separate channel of information. One wavelength of light may carry 2.5 gigabits of information per second in one direction, and current optical fiber lines may carry 16 wavelengths at the same time. Data may be sent in two directions at the same time on one link. A network using optical fiber cable carrying multiple wavelengths is called a wavelength division multiplexed (“WDM”) optical network.
The wavelength and the frequency of electromagnetic radiation are related in a fixed manner; thus electromagnetic energy and filters for electromagnetic energy may be characterized using both measures interchangeably.
Specific wavelengths carried on an optical fiber line may be added to the line or dropped (i.e., removed) from the line using an add/drop filter. Such a filter accepts as an input an optical fiber line transmitting a beam of electromagnetic energy carrying multiple frequencies, including the “target” or “tuned” frequency for which the filter is tuned (the frequency at which the filter operates, or the frequency centered in the band of frequencies at which the filter operates). The filter selects the frequency for which the filter is tuned (the “drop frequency”) from the beam on the optical fiber line and provides two outputs to two optical fiber output lines. A first optical fiber output line receives the original beam from the input optical fiber line, with the tuned frequency removed, and a second optical fiber output line receives the drop frequency, separated from the original beam. The filter may accept as an input an optical fiber line carrying a frequency to be added (“add frequency”), which corresponds in frequency to the drop frequency. In such a case the first output line receives the original beam with the add frequency replacing the drop frequency. It is not necessary that the drop frequency exist in the original beam: the filter may be used to add a frequency, add a frequency and drop a frequency, or drop a frequency.
When used herein, a frequency may include a range of frequencies covering a bandwidth (a range of frequencies covering a portion of the electromagnetic spectrum). When used herein, a frequency or wavelength may refer to a beam, or a component of a beam, containing a frequency or a band of frequencies surrounding a certain frequency. A data signal or channel may be carried on a band of frequencies surrounding a certain frequency. A multiple frequency beam is a beam of electromagnetic energy containing different channels which use different frequencies.
The frequencies added and dropped from a line may carry data. Frequencies may be dropped because a node requires access to the portion of the data carried on the optical fiber beam. A frequency may be added after a node alters the information on the frequency, which was dropped, or if the frequency does not exist on the beam. A node may need to add, subtract, monitor or modify data on one or more frequencies on a beam carried on a fiber, and may need to add or drop more than one frequency. Typically, one filter is used for each frequency for which access is desired. Filters may be used to multiplex multiple frequencies of data onto one optical fiber line. Filters are used to selectively add (multiplex) or drop (demultiplex) frequencies from a fiber.
When used herein, “multiplexing” may include demultiplexing, and “multiplexer” may include a device having demultiplexing capabilities. A filter adding and/or removing a wavelength of light from a link may be termed a multiplexer, an add/drop filter, or an add/drop multiplexer (“ADM”). At each node one ADM is required for add/drop capability for each of the multiple wavelengths that may be carried on an optical fiber cable.
One known network is organized as a mesh.
FIG. 1
is a block diagram illustrating a simplified portion of a mesh network. Referring to
FIG. 1
, mesh network
300
comprises nodes (e.g. nodes
304
,
306
,
308
and
310
) connected by links (e.g. links
305
,
307
, and
309
) transmitting traffic between nodes. For example, nodes
304
and
306
are connected by, and may transmit traffic via, link
305
. For clarity, not all nodes and links in
FIG. 1
are identified with reference numerals. Each node in network
300
may access some or all of the frequencies carried by the links to which it is connected. An add/drop filter is required at a node if traffic is to be added or dropped from a link on a certain frequency.
Typically, a node may add, drop and reroute traffic which originates or terminates at that node in order to allow customers connecting to that node access to that traffic or to route traffic to other nodes. For example, a customer connecting to node
310
may transmit traffic to a customer connecting to node
304
via links
309
and
305
and node
306
, using a certain frequency. In such a case, both nodes
310
and
304
require add/drop filters tuned to that frequency. At some point the traffic on the frequency may need to be rerouted to flow to node
308
rather than
304
; in such a case node
306
requires an add/drop filter to be able to access the frequency and, using equipment such as a cross connect, route the frequency to link
307
and node
308
.
Networks employing architectures other than mesh configurations are also known. Ring networks, for example, interconnect nodes, using links, in a circular fashion to form rings. Multiple rings may be interconnected to form a network.
FIG. 2
is a block diagram illustrating a simplified portion of a ring network. Referring to
FIG. 2
, network
330
includes nodes
332
,
334
,
336
,
338
and
340
. Nodes are connected by links
331
,
333
,
335
,
337
, and
339
. Nodes may use add/drop filters to add or drop a frequency from a line. For example, node
332
may send data to node
336
using a frequency of 2·10
14
Hz via links
331
and
333
and node
334
. Node
336
receives a beam of light on link
333
which contains multiple frequencies, including 2·10
14
Hz, and transmits most of those frequencies unaltered on to link
335
. Node
336
also receives a beam of light containing multiple frequencies on link
335
and transmits most or all of those frequencies unaltered on link
333
.
To access data sent by node
332
, node
336
uses an add/drop filter tuned to a target frequency of 2·10
14
Hz. The filter removes electromagnetic radiation at or near a frequency of 2·10
14
Hz (the “dropped” frequency) from link
333
; all other surrounding frequencies are unaltered by the filter and node
336
, and are placed on link
335
. Node
336
may accept the data sent on the dropped frequency and transmit this data to, for example, customers serviced by network
300
. Node
AT&T Corp.
Chang Audrey
Kenyon & Kenyon
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