U.S. patent application number 12/574579 was filed with the patent office on 2010-01-28 for wavelength division multiplexed optical communication system having a reconfigurable optical switch and a tunable backup laser transmitter.
This patent application is currently assigned to MERITON NETWORKS US INC.. Invention is credited to Paul Bonenfant, Thomas Andrew Strasser, Jefferson L. Wagener.
Application Number | 20100021162 12/574579 |
Document ID | / |
Family ID | 26796226 |
Filed Date | 2010-01-28 |
United States Patent
Application |
20100021162 |
Kind Code |
A1 |
Strasser; Thomas Andrew ; et
al. |
January 28, 2010 |
WAVELENGTH DIVISION MULTIPLEXED OPTICAL COMMUNICATION SYSTEM HAVING
A RECONFIGURABLE OPTICAL SWITCH AND A TUNABLE BACKUP LASER
TRANSMITTER
Abstract
In a WDM optical communication system that includes a plurality
of nodes interconnected by communication links, a node is provided
that includes a reconfigurable optical switch having a plurality of
input ports and at least one output port. The node also includes a
plurality of transmitters that are each coupled to one of the input
ports of the optical switch. Each of the transmitters generate an
information-bearing optical signal at a different channel
wavelength from one another. The reconfigurable optical switch is
adaptable to receive at any of the input ports any of the channel
wavelengths at which the plurality of transmitters operate and
direct the channel wavelengths to the output port. At least one
backup transmitter is coupled to one of the input ports of the
optical switch. The backup transmitter includes a tunable laser
tunable to any of the channel wavelengths at which the plurality of
transmitters operate. The reconfigurable optical switch includes at
least one wavelength selective element that selects at least one
channel wavelength from among any of the channel wavelengths
received at any of the input ports. The switch also includes a
plurality of optical elements associated with the wavelength
selective elements, Each of the optical elements direct one of the
selected channel wavelengths selected by the associated wavelength
selective element to the output port independently from every other
channel wavelength. The selected channel wavelengths directed to
the output port are combined on the output port.
Inventors: |
Strasser; Thomas Andrew;
(Warren, NJ) ; Bonenfant; Paul; (Ocean, NJ)
; Wagener; Jefferson L.; (Morristown, NJ) |
Correspondence
Address: |
MAYER & WILLIAMS PC
251 NORTH AVENUE WEST, 2ND FLOOR
WESTFIELD
NJ
07090
US
|
Assignee: |
MERITON NETWORKS US INC.
Wilmington
DE
|
Family ID: |
26796226 |
Appl. No.: |
12/574579 |
Filed: |
October 6, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10099561 |
Mar 15, 2002 |
7599619 |
|
|
12574579 |
|
|
|
|
60276310 |
Mar 16, 2001 |
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Current U.S.
Class: |
398/49 |
Current CPC
Class: |
G02B 6/356 20130101;
H04Q 2011/0016 20130101; H04Q 2011/003 20130101; G02B 6/3512
20130101; G02B 6/3548 20130101; H04Q 2011/0024 20130101; H04J
14/0212 20130101; H04J 14/0295 20130101; H04J 14/021 20130101; H04Q
2011/0081 20130101; H04Q 2011/0009 20130101; H04J 14/0279 20130101;
G02B 6/29305 20130101; H04Q 2011/0018 20130101; G02B 6/4215
20130101; G02B 6/357 20130101; G02B 6/29395 20130101; G02B 6/3578
20130101; H04J 14/0297 20130101; G02B 6/29367 20130101 |
Class at
Publication: |
398/49 |
International
Class: |
H04J 14/00 20060101
H04J014/00 |
Claims
1. In a WDM optical communication system that includes a plurality
of nodes interconnected by communication links, a node comprising:
a reconfigurable optical switch having a plurality of input ports
and at least one output port; a plurality of transmitters each
coupled to one of the input ports of the optical switch, each of
said transmitters generating an information-bearing optical signal
at a different channel wavelength from one another, said
reconfigurable optical switch being adaptable to receive at any of
the input ports any of the channel wavelengths at which the
plurality of transmitters operate and direct said channel
wavelengths to said at least one output port; at least one backup
transmitter coupled to one of the input ports of the optical
switch, said backup transmitter including a tunable laser tunable
to any of the channel wavelengths at which the plurality of
transmitters operate; wherein said reconfigurable optical switch
includes; at least one wavelength selective element that selects at
least one channel wavelength from among any of the channel
wavelengths received at any of the input ports; and a plurality of
optical elements associated with said at least one wavelength
selective element, each of said optical elements directing one of
the selected channel wavelengths selected by the associated at
least one wavelength selective element to said output port
independently from every other channel wavelength, wherein said
selected channel wavelengths directed to said output port are
combined on said output port.
2. The node of claim 1 wherein said at least one wavelength
selective element comprises a plurality thin film filters each
transmitting therethrough a different one of the channel
wavelengths and reflecting the remaining channel wavelengths.
3. The node of claim 1 wherein said optical elements are reflective
mirrors that are selectively tiltable in a plurality of positions
such that in each of the positions the mirrors reflect the channel
wavelength incident thereon to the output port.
4. The node of claim 2 wherein said optical elements are reflective
mirrors that are selectively tiltable in a plurality of positions
such that in each of the positions the mirrors reflect the channel
wavelength incident thereon to the output port.
5. The node of claim 3 wherein said reflective mirrors are part of
a micro-electromechanical (MEM) mirror assembly.
6. The node of claim 1 wherein said at least one wavelength
selective element comprises a bulk diffraction grating.
7. The node of claim 2 further comprising a free space region
disposed between the input ports and the wavelength selective
elements.
8. The node of claim 7 wherein said free space region comprises an
optically transparent substrate having first and second parallel
surfaces, said wavelength selective element includes a plurality of
wavelength selective elements arranged in first and second arrays
extending along the first and second parallel surfaces,
respectively.
9. The node of claim 8 wherein said first and second arrays are
laterally offset with respect to one another.
10. The node of claim 9 wherein each of said wavelength selective
elements arranged in the first array direct the selected wavelength
component to another of said wavelength selective elements arranged
in the second array.
11. In a WDM optical communication system that includes a plurality
of nodes interconnected by communication links, a node comprising:
a reconfigurable optical switch having (i) N input ports for
receiving a WDM optical signal having up to (N-1) channel
wavelengths (ii) at least one output port, where N is greater than
or equal to 2 and (iii) a switching fabric that includes at least
(N-1) optical elements each directing a selected one of the channel
wavelengths between the input ports and the at least one output
port N transmitters respectively coupled to the N input ports of
the optical switch, said transmitters each including a tunable
laser tunable to any of the (N-1) channel wavelengths, said
reconfigurable optical switch being adaptable to receive at any of
the input ports any of the channel wavelengths at which the
plurality of transmitters operate and direct each of the channel
wavelengths to said at least one output port by reconfiguration of
the optical element respectively directing the channel
wavelength.
12. The node of claim 11 further comprising at least (N-1)
wavelength selective elements that each select at least one channel
wavelength from among any of the channel wavelengths received at
any of the input ports and wherein the (N-1) optical elements are
respectively associated with said (N-1) wavelength selective
elements, each of said optical elements directing one of the
selected channel wavelengths selected by the associated at least
one wavelength selective element to said output port independently
from every other channel wavelength, wherein said selected channel
wavelengths directed to said output port are combined on said
output port.
13. The node of claim 12 wherein said wavelength selective elements
each comprise a plurality of thin film filters each transmitting
therethrough a different one of the channel wavelengths and
reflecting the remaining channel wavelengths.
14. The node of claim 11 wherein said optical elements are
reflective mirrors that are selectively tiltable in a plurality of
positions such that in each of the positions the mirrors reflect
the channel wavelength incident thereon to the output port.
15. The node of claim 12 wherein said optical elements are
reflective mirrors that are selectively tiltable in a plurality of
positions such that in each of the positions the mirrors reflect
the channel wavelength incident thereon to the output port.
16. The node of claim 14 wherein said reflective mirrors are part
of a micro-electromechanical (MEM) mirror assembly.
17. The node of claim 12 wherein at least one of said wavelength
selective elements comprises a bulk diffraction grating.
18. The node of claim 12 further comprising a free space region
disposed between the input ports and the wavelength selective
elements.
19. The node of claim 18 wherein said free space region comprises
an optically transparent substrate having first and second parallel
surfaces, said wavelength selective element includes a plurality of
wavelength selective elements arranged in first and second arrays
extending along the first and second parallel surfaces,
respectively.
20. The node of claim 19 wherein said first and second arrays are
laterally offset with respect to one another.
Description
STATEMENT OF RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/099,561, filed Mar. 15, 2002, entitled
"Wavelength Division Multiplexed Optical Communication System
Having A Reconfigurable Optical Switch And A Tunable Backup Laser
Transmitter," which claims the benefit of U.S. Provisional Patent
Application No. 60/276,310, filed Mar. 16, 2001, entitled
"Reconfigurable Optical System."
FIELD OF THE INVENTION
[0002] The invention relates generally to wavelength division
multiplexed optical communication systems, and more particularly,
to wavelength division multiplexed optical communication systems
which include reconfigurable optical switches coupled to backup
transmitters that incorporate tunable lasers.
BACKGROUND OF THE INVENTION
[0003] Wavelength division multiplexing (WDM) has been explored as
an approach for increasing the capacity of fiber optic networks to
support the rapid growth in data and voice traffic applications. A
WDM system employs plural optical signal channels, each channel
being assigned a particular channel wavelength. In a WDM system,
signal channels are generated, multiplexed, and transmitted over a
single waveguide, and demultiplexed to individually route each
channel wavelength to a designated receiver. Through the use of
optical amplifiers, such as doped fiber amplifiers, plural optical
channels are directly amplified simultaneously, facilitating the
use of WDM systems in long-distance optical systems.
[0004] Proposed wavelength division multiplexed optical
communication systems typically include multiplexer and
demultiplexer switching elements which permit only a fixed number
of optical channels to be used in the optical system. In one
optical system configuration, for instance, the multiplexed signal
is broken down into its constituent optical signals through the use
of an integrated frequency router demultiplexer. The frequency
router uses silicon optical bench technology in which plural
phosphorus-doped silica waveguides are disposed on a silicon
substrate. An optical star outputs to an array of N waveguides
having adjacent optical path lengths which differ by q wavelengths;
this array in turn feeds an output N.times.N star. Such a frequency
router design for an optical communication system is described in
Alexander et al., J. Lightwave Tech., Vol. 11, No. 5/6, May/June
1993, p. 714. Using a 1.times.N configuration at the input, a
multiplexed optical signal containing light of different
frequencies is separated into its component frequencies at each
waveguide extending from the output N.times.N star. Although this
configuration adequately separates light of different frequencies,
the integrated optical design fixes both the number and the
respective wavelengths of the optical channels. Consequently,
adding or decreasing the number of optical channels or changing the
channel wavelength or spacing is not possible without providing a
completely new demultiplexing switching element to the optical
network. In other words, the scalability of such networks is
limited because of the switching element's lack of flexibility.
[0005] One area where this lack of flexibility manifests itself is
in connection with the provisioning of a backup path through the
network in the event of equipment failure. For example, in the
aforementioned WDM transmission system, since each channel
wavelength is assigned its own path through the switching element,
it is not possible to reroute a given channel wavelength along a
different path should a failure occur in the transmitter that
generates the given wavelength. In particular, it is not possible
to substitute for the failed transmitter a backup transmitter that
resides on another of the switching element's input ports unless
the backup transmitter operates on its own channel wavelength that
is different from the wavelength at which the failed transmitter
operates. As a result, when it becomes necessary to use the backup
transmitter a new path must be established through the entire
network to accommodate the change in channel wavelength.
Unfortunately, the provisioning of a backup path can be a slow
process requiring inter-node communication and processing, which
not only slows down the restoration process, but which may also
disturb other traffic in the system.
[0006] Accordingly, it would be desirable to provide an optical
communication system in which a backup path can be provisioned
through the system in the event of a transmitter or receiver
failure that allows restoration to be accomplished in a more rapid
and less disruptive manner than in the aforementioned system.
SUMMARY OF THE INVENTION
[0007] In a WDM optical communication system that includes a
plurality of nodes interconnected by communication links, the
present invention provides a node that includes a reconfigurable
optical switch having a plurality of input ports and at least one
output port. The node also includes a plurality of transmitters
that are each coupled to one of the input ports of the optical
switch. Each of the transmitters generates an information-bearing
optical signal at a different channel wavelength from one another.
The reconfigurable optical switch is adaptable to receive at any of
the input ports any of the channel wavelengths at which the
plurality of transmitters operate and direct the channel
wavelengths to the output port. At least one backup transmitter is
coupled to one of the input ports of the optical switch. The backup
transmitter includes a tunable laser tunable to any of the channel
wavelengths at which the plurality of transmitters operates. The
reconfigurable optical switch includes at least one wavelength
selective element that selects at least one channel wavelength from
among any of the channel wavelengths received at any of the input
ports. The switch also includes a plurality of optical elements
associated with the wavelength selective element. Each of the
optical elements direct one of the selected channel wavelengths
selected by the associated wavelength selective element to the
output port independently from every other channel wavelength. The
selected channel wavelengths directed to the output port are
combined on the output port.
[0008] In accordance with one aspect of the invention, the
wavelength selective element includes a plurality of thin film
filters each transmitting therethrough a different one of the
channel wavelengths and reflecting the remaining channel
wavelengths.
[0009] In accordance with another aspect of the invention, the
optical elements are reflective mirrors that are selectively
tiltable in a plurality of positions such that in each of the
positions the mirrors reflect the channel wavelength incident
thereon to the output port.
[0010] In accordance with yet another aspect of the invention, the
wavelength selective elements may be bulk diffraction gratings.
[0011] In accordance with yet another aspect of the invention, a
free space region is located between the input ports and the
wavelength selective elements.
[0012] In accordance with another aspect of the invention, a node,
which is situated in a WDM optical communication system that
includes a plurality of nodes interconnected by communication
links, includes a reconfigurable optical switch. The reconfigurable
optical switch has (i) N input ports for receiving a WDM optical
signal having up to (N-1) channel wavelengths (ii) at least one
output port, where N is greater than or equal to 2 and (iii) a
switching fabric that includes at least (N-1) optical elements each
directing a selected one of the channel wavelengths between the
input ports and the output port. The node also includes N
transmitters respectively coupled to the N input ports of the
optical switch. The transmitters each include a tunable laser
tunable to any of the (N-1) channel wavelengths. The reconfigurable
optical switch is adaptable to receive at any of the input ports
any of the channel wavelengths at which the plurality of
transmitters operate and direct each of the channel wavelengths to
the output port by reconfiguration of the optical element
respectively directing the channel wavelength. A similar
reconfigurable switch arrangement is present at the receiving end
of the optical signal to direct the received signal to the backup
transponder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIGS. 1 and 2 are schematic representations of a wavelength
division multiplexed optical communication system in accordance
with the present invention.
[0014] FIG. 3 shows an exemplary reconfigurable optical switch that
may be employed in the present invention.
DETAILED DESCRIPTION
[0015] In accordance with the present invention, a WDM optical
transmission system is provided which employs reconfigurable
switching elements that can dynamically change the path along which
a given wavelength is routed. By employing such switching elements,
the present invention provides a restoration arrangement for a
failed transmitter that is more responsive and less disruptive to
other traffic than the conventional arrangement that employs a
backup transmitter operating at a different channel wavelength from
that of the failed transmitter.
[0016] Recently, switching elements that provide a degree of
reconfigurability have become available. These reconfigurable
optical elements can dynamically change the path along which a
given wavelength is routed to effectively reconstruct the topology
of the network as necessary to accommodate a change in demand or to
restore services around a network failure. Examples of
reconfigurable optical elements include optical Add/Drop
Multiplexers (OADM) and Optical Cross-Connects (OXC). OADMs are
used to separate or drop one or more wavelength components from a
WDM signal, which is then directed onto a different path. In some
cases the dropped wavelengths are directed onto a common fiber path
and in other cases each dropped wavelength is directed onto its own
fiber path. OXCs are more flexible devices than OADMs, which can
redistribute in virtually any arrangement the components of
multiple WDM input signals onto any number of output paths.
Unfortunately, current OXC's generally employ a digital
cross-connect at their cores, thus requiring optical-to-electrical
interfaces into and out of the cross-connect. Such an arrangement
gives rise to a number of limitations, including a relatively high
insertion loss because the optical signals must pass through three
discrete components. In addition, the components are relatively
expensive while still not providing a completely flexible switch
that can transfer light between any two subsets of the ports.
Finally, because of their high loss as well as the need to provide
channels with equal power, such OXC's typically employ
optoelectronic regenerators on at least their output side, and in
many instances on their input side as well. While these
regenerators overcome the problem of insertion loss and effectively
allow wavelength conversion of the signal as it traverses the
switch fabric, they substantially add to the cost of an already
expensive switch fabric because a regenerator is required for each
and every wavelength that is used in the network.
[0017] More flexible still are all-optical reconfigurable switches
which have much lower insertion losses and are less expensive than
the aforementioned OXC's. Various examples of all-optical
reconfigurable optical switches are disclosed in U.S. patent
application Ser. No. 09/571,833, which is hereby incorporated by
reference in its entirety, and in particular FIGS. 2-4 of that
reference. The switching elements disclosed therein can selectively
direct any wavelength component from any input port to any output
port, independent of the routing of the other wavelengths without
the need for any electrical-to-optical conversion. Another
all-optical reconfigurable optical switch that provides additional
functionality is disclosed in U.S. patent application Ser. No.
09/691,812, which is hereby incorporated by reference in its
entirety. This reference discloses an optical switching element in
which each and every wavelength component can be directed from any
given port to any other port without constraint. More specifically,
unlike most optical switches, this switch is not limited to
providing connections between a subset of input ports and a subset
of output ports, or vice versa. Rather, this switch can also
provide a connection between two ports within the same subset
(either input or output). While the present invention may employ
any of the aforementioned reconfigurable optical switches, the
optical switch disclosed in U.S. patent application Ser. No.
09/691,812 will serve as an exemplary reconfigurable optical
switch, and accordingly, additional details concerning this switch
will be presented below.
[0018] Turning now to the drawings in detail in which like numerals
indicate the same or similar elements, FIG. 1 schematically depicts
a wavelength division multiplexed (WDM) optical communication
system 10 according to one embodiment of the present invention.
Optical communication system 10 includes a plurality of optical
transmitters 20, each optical transmitter emitting an
information-bearing optical signal at an optical channel wavelength
that differs from transmitter to transmitter. The expression
"information-bearing optical signal," as used herein, refers to an
optical signal which has been coded with information, including,
but not limited to, audio signals, video signals, and computer
data. The WDM optical communication systems of the present
invention include N channels, where N is a whole number greater
than or equal to 2. Exemplary values for N are 4, 8, and 16 optical
channels. In the optical system of FIG. 1, N is depicted as 4 for
ease of illustration.
[0019] It should be noted at the outset that the present invention
is not limited to WDM systems such as shown in FIG. 1, which have a
point-to-point configuration consisting of end terminals or nodes
spaced from each other by one or more segments of optical fiber.
For example, in metropolitan areas, WDM systems having a ring or
loop configuration are currently being developed. Such systems
typically include a plurality of nodes located along the ring. At
least one optical add/drop element, associated with each node, is
typically connected to the ring with optical connectors. The
optical add/drop element permits both addition and extraction of
channels to and from the ring. One of the nodes, referred to as a
hub or central office node, typically has a plurality of associated
add/drop elements for transmitting and receiving a corresponding
plurality of channels to/from other nodes along the ring. Of
course, the present invention is equally applicable to other
network topologies in addition to rings such as a mesh
topology.
[0020] Returning to FIG. 1, each optical transmitter 20 generally
includes a laser, such as a DFB semiconductor laser, a laser
controller, and a modulator for creation of an information-bearing
optical signal. In an exemplary embodiment, the transmitter laser
is a DFB semiconductor diode laser, generally comprising one or
more III-V semiconductor materials, commercially available from a
wide variety of suppliers. The laser outputs an optical carrier
signal at a particular wavelength assigned to its channel. The
laser controller provides the required laser bias current as well
as thermal control of the laser. Using thermal control, the precise
operating wavelength of the laser is maintained, typically to
within a one-angstrom bandwidth or less.
[0021] The optical transmitter typically includes a modulator for
imparting information to the optical carrier signal. An exemplary
modulator is an external modulator, such as a Mach-Zehnder
modulator, employing a waveguiding medium whose refractive index
changes according to the applied electrical field, i.e., a material
exhibiting an electro-optic effect. In the Mach-Zehnder
configuration, two optical interferometer paths are provided. An
incoming optical carrier signal is split between the two optical
paths. At least one path of the interferometer is phase modulated.
When the signal is recombined at the output, the light from the
paths either constructively or destructively interferes, depending
upon the electrical field applied to the surrounding electrodes
during the travel time of the carrier. This recombination creates
an amplitude-modulated output optical signal. The optical carrier
signal can alternatively be directly modulated for some system
applications. It is noted that while the above-described
transmitters are exemplary, any transmitting elements capable of
producing optical signals for use in an optical communication
system can be employed in the WDM systems of the present
invention.
[0022] Typically, the wavelengths emitted by optical transmitters
20 are selected to be within the 1500 nanometer range, the range in
which the minimum signal attenuation occurs for silica-based
fibers. More particularly, the wavelengths emitted by the optical
transmitters are selected to be in the range from 1540 to 1560
nanometers. However, other wavelengths, such as those in the
1300-1500 nm range and the 1600 nm range, can also be employed in
the WDM systems of the present invention.
[0023] Each information-bearing optical signal produced by an
optical transmitter constitutes a channel in optical system 10. In
a WDM system, each channel is generally associated with a unique
wavelength. As depicted in FIG. 1, four optical transmitters
20.sub.1, 20.sub.2, 20.sub.3, and 20.sub.4 are provided to create a
four-channel wavelength division multiplexed optical communication
system. The optical transmitters 20.sub.1, 20.sub.2, 20.sub.3, and
20.sub.4 operate at channel wavelengths of .lamda..sub.1,
.lamda..sub.2, .lamda..sub.3, and .lamda..sub.4, respectively.
These optical signal channels are output from transmitters 20 and
are brought together in optical switch 30 for conveyance to optical
waveguide 40 via output port 26.
[0024] Optical switch 30 combines plural optical channels from
transmitters 20 onto a single output to create a multiplexed
optical signal. Optical switch 30 has four input ports that are
optically coupled to the four transmitters 20 through optical
waveguides 22. The combination of channels forms a multiplexed
optical signal which is output to optical transmission path 40
through output port 26. Optical transmission path 40 is typically
an optical waveguide and is the principal transmission medium for
the optical communication system. While the optical waveguide is
generally selected from single-mode optical, any optical
waveguiding medium which is capable of transporting multiple
optical wavelengths can be employed as waveguide 40 in optical
system 10.
[0025] Optionally, one or more optical amplifiers 50 are interposed
along optical transmission path 40. Optical amplifiers 50 are
selected from any device which directly increases the strength of
plural optical signals without the need for optical-to-electrical
conversion. In general, optical amplifiers 50 are selected from
optical waveguides doped with a material which can produce laser
action in the waveguide. Such materials include rare earth dopants
such as erbium, neodymium, praseodymium, ytterbium, or mixtures
thereof Pumping of the doped waveguide at a specific pump
wavelength causes population inversion among the electron energy
levels of the dopant, producing optical amplification of the
wavelength division multiplexed optical signals. For doped fiber
amplifiers employing erbium as the dopant, a wavelength band
between approximately 1500 nm and approximately 1590 nm provides
gain to optical signals when the doped fiber is pumped.
[0026] Following transmission and amplification of the multiplexed
optical signals along waveguide 40, each channel must be
demultiplexed and routed to the receiver designated for the
particular optical signal channel. The multiplexed signal is input
to optical switch 80. In a preferred embodiment of the invention,
optical switch 80 is also a reconfigurable optical switch. Optical
switch 80 receives the multiplexed optical signal through input
port 96 and provides the individual channels on output ports 92.
Output ports 92 are each coupled to receivers 120 over optical
waveguides 122. Receivers 120 generally detect the optical signal
and converts it to an electrical signal, typically through the use
of a photodiode device.
[0027] As previously mentioned, in a conventional WDM optical
communication system optical switches 30 and 80 are generally based
on multiplexers and demultiplexers that are fixed
wavelength-dependent elements in which a given wavelength is always
routed along the same path. However, in the present invention,
instead of fixed-wavelength dependent elements, more flexible
optical switches are employed. Such optical switches are
reconfigurable elements that can dynamically change the path along
which a given wavelength is routed. As discussed below, the use of
reconfigurable optical switches that allow the path along which a
given wavelength is routed to be dynamically changed, effectively
reconstructing the topology of the network, is particularly
advantageous in the event of a failure in one or more transmitters
or receivers.
[0028] As previously mentioned, for purposes of illustration only
the present invention will be depicted in connection with the
reconfigurable optical switch disclosed in the aforementioned U. S.
Apple. Serial No. [PH01-00-01], which is shown in FIG. 3. Of
course, those of ordinary skill in the art will recognize that the
invention is equally applicable to a communication system that
employs any reconfigurable optical switch in which any wavelength
component received on any input port can be selectively directed to
any output port, independent of the routing of the other
wavelengths. In FIG. 5, the optical switch 300 comprises an
optically transparent substrate 308, a plurality of dielectric thin
film filters 301, 302, 303, and 304, a plurality of collimating
lens pairs 321.sub.1 and 321.sub.2, 322.sub.1 and 322.sub.2,
323.sub.1 and 323.sub.2, 324.sub.1 and 324.sub.2, a plurality of
tiltable mirrors 315, 316, 317, and 318 and a plurality of output
ports 340.sub.1, 340.sub.2, . . . 340.sub.n. A first filter array
is composed of thin film filters 301 and 303 and a second filter
array is composed of thin film filters 302 and 304. Individual ones
of the collimating lens pairs 321-324 and tiltable mirrors 315-318
are associated with each of the thin film filters. Each thin film
filter, along with its associated collimating lens pair and
tiltable mirror effectively forms a narrow band, free space switch,
i.e. a switch that routes individual wavelength components along
different paths. The tiltable mirrors are micro mirrors such as the
MEMS (microelectromechanical systems) mirrors. Alternatively, other
mechanisms may be employed to control the position of the mirrors,
such as piezoelectric actuators, for example.
[0029] In operation, a WDM optical signal composed of different
wavelengths .lamda..sub.1, .lamda..sub.2, .lamda..sub.3 and
.lamda..sub.4 is directed from the optical input port 312 to a
collimator lens 3 14. The WDM signal traverses substrate 308 and is
received by thin film filter 301. According to the characteristics
of the thin film filter 301, the optical component with wavelength
.lamda..sub.1 is transmitted through the thin film filter 301,
while the other wavelength components are reflected and directed to
thin film filter 302 via substrate 308. The wavelength component
.lamda..sub.1, which is transmitted through the thin film filter
301, is converged by the collimating lens 321.sub.1 onto the
tiltable mirror 315. Tiltable mirror 315 is positioned so that
wavelength component .lamda..sub.1 is reflected from the mirror to
a selected one of the output ports 340.sub.1-340.sub.n via thin
film filters 302-304, which all reflect wavelength component
.lamda..sub.1. The particular output port that is selected to
receive the wavelength component will determine the particular
orientation of the mirror 315.
[0030] As mentioned, the remaining wavelength components
.lamda..sub.2, .lamda..sub.3, and .lamda..sub.4 are reflected by
thin film filter 301 through lens 321.sub.2 back into substrate 308
and directed to thin film filter 302. Wavelength component
.lamda..sub.2 is transmitted through thin film filter 302 and lens
322.sub.1 and directed to a selected output port by tiltable mirror
316 via thin film filters 303-304, which all reflect wavelength
component .lamda..sub.2. Similarly, all other wavelength components
are separated in sequence by the thin film filters 303-304 and
subsequently directed by tiltable mirrors 317-318 to selected
output ports. By appropriate actuation of the tiltable mirrors,
each wavelength component can be directed to an output port that is
selected independently of all other wavelength components.
[0031] Returning to FIG. 1, as previously noted, optical
transmitters 20.sub.1, 20.sub.2, 20.sub.3, and 20.sub.4 operate at
channel wavelengths of .lamda..sub.1, .lamda..sub.2, .lamda..sub.3,
and .lamda..sub.4, respectively. To ensure system reliability in
the event that one of the transmitters should fail, an additional
transmitter is sometimes reserved as a spare transmitter that can
serve as a backup until the failed transmitter can be repaired or
replaced. In a conventional communication system employing
fixed-wavelength dependent switching elements, the backup
transmitter operates at a different channel wavelength from the
failed transmitter, requiring that an end-to-end backup path be
established through the system. For example, if a backup
transmitter were to be employed in the network shown in FIG. 1, it
could operate at a channel wavelength of .lamda..sub.5.
Consequently, the original data path must be reconfigured for a
backup path operating at a different channel wavelength. As
previously mentioned, one problem with this approach is that path
reconfiguration can be a slow process because it often requires
inter-node communication and processing. Moreover, in some cases
path reconfiguration may disturb other traffic in the system.
[0032] The present inventors have recognized that rather than
reconfigure the path for a backup channel in the event of a
transmitter failure, it will often be preferable to maintain the
original path while only reconfiguring equipment at the switch
associated with the failed transmitter. While such a
reconfiguration procedure is not possible with fixed-wavelength
dependent optical switches, it can be readily accomplished with any
of the aforementioned reconfigurable optical switches that allow
any wavelength to be selectively routed between any two ports.
[0033] FIG. 2 shows a WDM system that includes a backup transmitter
20.sub.5 having a tunable laser that can be tuned to any of the
channel wavelengths at which transmitters 20.sub.1-20.sub.4
operate. In this way backup transmitter 20.sub.5 can be readily
substituted for any of the primary transmitters. Since optical
switch 30 can receive any wavelength at any input port, optical
switch 30 can be reconfigured so that any of the wavelengths
.lamda..sub.1-.lamda..sub.4 can be received at the input port
24.sub.5 to which backup transmitter 20.sub.5 is coupled. For
example, assume transmitter 202 fails. In response to the failure,
backup transmitter 20.sub.5 is tuned to channel wavelength
.lamda..sub.2. In turn, optical switch 30 is internally
reconfigured so that it can accept wavelength .lamda..sub.2 from
input port 24.sub.5 and direct it to output port 26. In this way
the transmitter failure is transparent to the remainder of the
network so that no reconfiguration of the path through the network
is required. While FIG. 2 shows only a single backup transmitter,
those of ordinary skill in the art will recognize that additional
backup transmitters may be employed in the relatively unlikely
event that two or more transmitters fail at the same time.
[0034] In the event of a failure in one of the receivers 120, the
same reconfiguration problems arise as with a failed transmitter.
Accordingly, the present invention may also be advantageously used
to redirect a channel wavelength from the failed receiver to a
backup receiver if the receivers are in communication with a
reconfigurable optical switch. That is, assuming, for example, that
receiver 120.sub.3 fails, switch 80 can be internally reconfigured
so that channel wavelength .lamda..sub.3 is redirected from output
port 92.sub.3 to the port 92.sub.5, which is coupled to the backup
receiver 120.sub.5. Unlike the backup transmitter, however, a
tunable receiver will generally not be necessary because the
receivers can typically detect all the channel wavelengths that are
available to the network.
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