U.S. patent application number 10/611778 was filed with the patent office on 2004-01-15 for optical fiber network having increased channel capacity.
Invention is credited to Hung, Henry.
Application Number | 20040008989 10/611778 |
Document ID | / |
Family ID | 24031740 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040008989 |
Kind Code |
A1 |
Hung, Henry |
January 15, 2004 |
Optical fiber network having increased channel capacity
Abstract
Optical communication system apparatus and methods of operating
an optical communications system is described. The system
advantageously may utilize existing optical fiber networks and
provide significantly increased channel capacity. In accordance
with one aspect of the invention the system apparatus provides for
a plurality of communications channels and a processor unit
receives requests for allocation of one or more channels from a
node coupled to the optical communications system. The system
apparatus dynamically allocates one or more channels selected from
unused channels.
Inventors: |
Hung, Henry; (Paradise
Valley, AZ) |
Correspondence
Address: |
Attn: IP Dept.
Squire, Sanders & Dempsey L.L.P.
Two Renaissance Square
40 North Central Avenue, Suite 2700
Phoenix
AZ
85004-4498
US
|
Family ID: |
24031740 |
Appl. No.: |
10/611778 |
Filed: |
June 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10611778 |
Jun 30, 2003 |
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09510685 |
Feb 23, 2000 |
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6587239 |
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Current U.S.
Class: |
398/69 |
Current CPC
Class: |
H04J 14/0201 20130101;
G02B 6/2821 20130101; H04J 14/02 20130101; H04J 14/0284 20130101;
H04J 14/0227 20130101; H04J 14/0241 20130101; H04J 14/0283
20130101; H04J 14/002 20130101; H01S 3/0675 20130101; H04J 14/0282
20130101; H01S 3/2383 20130101; H04J 14/0286 20130101 |
Class at
Publication: |
398/69 |
International
Class: |
H04J 014/00 |
Claims
What is claimed is:
1. A method of operating an optical network coupling a plurality of
nodes, comprising: providing a laser source as a network reference;
distributing optical reference signals to said network nodes from
said multiple wavelength laser source; providing a plurality of
channels each synchronized to said reference signals; and utilizing
one or more of said channels for communications from one of said
nodes to at least one other one of said nodes.
2. A method in accordance with claim 1, comprising: selecting each
channel by selecting one wavelength of a plurality of predetermined
optical wavelengths of said reference signals.
3. A method in accordance with claim 2, comprising: further
selecting each channel by selecting one modulation frequency of a
plurality of predetermined modulation frequencies.
4. A method in accordance with claim 3, comprising: further
selecting each channel by selecting one phase of a plurality of
predetermined phases.
5. A method in accordance with claim 2, comprising: further
selecting each channel by selecting by one phase of a plurality of
predetermined phases.
6. A method in accordance with claim 1, comprising: utilizing one
or more multiple wavelength lasers as said laser source.
7. A method in accordance with claim 6, comprising: selecting each
channel by selecting one wavelength of a plurality of predetermined
optical wavelengths of said reference signals.
8. A method in accordance with claim 7, comprising: further
selecting each channel by selecting one modulation frequency of a
plurality of predetermined modulation frequencies.
9. A method in accordance with claim 8, comprising: further
selecting each channel by selecting one phase of a plurality of
predetermined phases.
10. A method in accordance with claim 7, comprising: further
selecting each channel by selecting one phase of a plurality of
predetermined phases.
11. A method in accordance with claim 1, comprising: combining the
outputs of a plurality of lasers to provide said laser source.
12. A method in accordance with claim 11, comprising: utilizing a
plurality of multiple wavelength lasers as said plurality of
lasers.
13. A method of operating an optical communications system
comprising a plurality of nodes, said method comprising: providing
a plurality of channels; selecting each channel by selecting one
wavelength from a predetermined plurality of wavelengths and by
selecting one modulation frequency from a plurality of modulation
frequencies; assigning selected ones of said channels for
communications from one node of said plurality of nodes to at least
another node of said plurality of nodes; synchronizing said
channels to optical reference signals; and providing a said optical
reference signals from a source common to all of said plurality of
nodes.
14. A method in accordance with claim 13, comprising: providing a
laser source as said common source.
15. A method in accordance with claim 14, comprising: utilizing a
plurality of lasers as said laser source.
16. A method in accordance with claim 13, comprising: further
selecting each channel by selecting one phase from a plurality of
predetermined phases.
17. An optical communications system comprising: an optical
network; a plurality of nodes, each node of said plurality of nodes
being coupleable to said network for exchanging information with
other nodes of said plurality of nodes, said information being
transmitted over communication channels, each channel having a
wavelength selected from a predetermined plurality of wavelengths;
apparatus at each said node for synchronizing said channels to
optical reference signals; and a source of said optical reference
signals, said source being common to all of said plurality of
nodes.
18. An optical communication system in accordance with claim 17,
wherein: said source comprises a laser source.
19. An optical communication system in accordance with claim 18,
wherein: said laser source comprises a multiple wavelength laser
source.
20. An optical communication system in accordance with 19, wherein:
said laser source comprises a plurality of multiple wavelength
lasers.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to optical communications systems,
in general, and to an optical network and a method of operating an
optical network, in particular.
BACKGROUND OF THE INVENTION
[0002] As used herein, the term "optical network" relates to any
network that interconnects a plurality of nodes and conveys
information between nodes with optical signals. The term "optical
communications system" as used herein refers to any system that
utilizes optical signals to convey information between one node and
one or more other nodes. An optical communications system may
include one or more optical networks.
[0003] Telecommunications carriers began installing optical fiber
cable about 15 years ago. At the time the optical fiber cables were
installed, it was expected that the optical fiber infrastructure
would provide communications systems and networks with ample
capacity for the foreseeable future. However, the phenomenal growth
of data traffic on the Internet has taxed the capabilities of the
optical fiber infrastructure. In addition, new high bandwidth
applications are being developed and are being made available for
corporate applications. The result of this increased usage of the
fiber infrastructure is serious network congestion and exhaustion
of the fiber infrastructure. In the past, optical fiber systems
relied on time division multiplexing to route traffic through a
channel. Time division multiplexed systems add more capacity by
time multiplexing signals onto an optical fiber. A disadvantage of
time division multiplex systems is that data must be converted from
light waves to electronic signals and then back to light. The
system complexity is thereby increased.
[0004] As the demand for increasing traffic capacity continues, the
limitations of existing optical networks and optical communications
systems must be overcome. To do so, the capacity of the existing
optical networks and optical communications systems needs to be
increased. Capacity of existing optical infrastructure may be
expanded by the laying of more fiber optic cable, for example.
However, the cost of such expansion is prohibitive. Therefore a
need exists for a cost-effective way to increase the capacity of
existing optical infrastructure.
[0005] Wavelength Digital Multiplexing (WDM) and Dense Wavelength
Digital Multiplexing (DWDM) are being used and/or proposed for use
in long-haul telecom network applications for increasing the
capacity of existing fiber optic networks. The advantage of both
WDM and DWDM is that the conversion to electrical signals is not
necessary. The devices that handle and switch system traffic
process light and not electrical signals. WDM and DWDM would appear
to many to be the solution to optical network limitations. In WDM,
plural optical channels are carried over a single fiber optic, with
each channel being assigned to a particular wavelength. By using
optical amplifiers, multiple optical channels are directly.
amplified simultaneously thereby facilitating the use of WDM
systems in long-haul optical networks. DWDM is a WDM system in
which channel spacing is on the order of one nanometer or less. WDM
and DWDM expand the capacity of an optical fiber by multiple
wavelength channels into a single laser beam. Each wavelength is
capable of carrying as much traffic as the original. Thus in one
example set forth by Barry Greenberg in Special Report: new growth
markets/emerging OEMs: lighting the way to a network-capacity
solution, Electronic Buyer News, Apr. 19, 1999, pp. 50, "A fiber
carrying four 2.5-Gbit/s DWDM channels, for example, has its
capacity increased to 10 Gbit/s, without having to install
additional fiber of use higher-speed transmission equipment." With
WDM and DWDM, traffic passes from one node of the network to its
destination in the form of light waves without conversion to
electrical signals. DWDM and WDM will permit increase in the
capacity of the fiber infrastructure. Systems with up to 128 and
240 DWDM channels have been proposed and/or are being built.
However, DWDM and WDM are both limited by the non-linear cost
increase as the network is expanded. In each instance, expansion
beyond an incremental increase in traffic handling capacity may
trigger significant investment in new optical fiber and equipment
that is significantly in excess of the incremental increase in
network capacity. In addition, DWDM based systems are not scaleable
in expansion because equipment typically has to be replaced rather
than merely added to. Existing implementations of both WDM and DWDM
are too limited for solving the congestion problems of the existing
optical infrastructure. The present systems are limited in the
number of available channels. The slight increase in channel
occupancy in such systems will present severe restrictions on the
traffic handling capacity of the network. Additional difficulties
with present implementations of DWDM and WDM technology include
lack of flexibility; difficulty in handling packet switched
information, non-linear optical effects and the already noted lack
of incremental and scaleable upgrade capability.
[0006] It is therefor highly desirable to provide an optical
communication system that has increased channel capacity. It is
further desirable to provide an optical communication system that
provides bandwidth based upon user demand. It is further desirable
that any improved optical communication system is able to utilize
the existing fiber optic infrastructure. Such a solution will
prevent so called "fiber exhaust". To effectively utilize the
existing infrastructure in the face of the dramatic increases in
traffic that will be encountered, it is highly desirable to
increase channel capacity by a factor of 10 to 200 times that
provided by DWDM to permit up to 20,000 channels to be served. It
is also highly desirable that any improved optical communication
system has a low cost per channel.
SUMMARY OF THE INVENTION
[0007] In a method of operating an optical communications system in
accordance with the invention, a plurality of optical channels is
provided and the optical channels are utilized for communications
among a plurality of communications nodes. Each optical channel is
determined by at least two of three optical signal characteristics.
A first one of the optical signal characteristics is selected from
a plurality of predetermined optical wavelengths. A second one of
the optical signal characteristics is selected from a plurality of
predetermined optical phases, and a third one of said optical
signal characteristics is selected from a plurality of optical
modulation frequencies.
[0008] In one embodiment in accordance with the principles of the
invention, two of the optical signal characteristics are utilized
to determine the optical channels. In this first embodiment of the
invention, each channel is defined by one optical wavelength of a
plurality of optical wavelengths and by one modulation frequency of
a plurality of optical modulation frequencies.
[0009] In a second embodiment in accordance with the principles of
the invention, one wavelength of a plurality of optical
wavelengths, one frequency of a plurality of optical modulation
frequencies, and one phase of a plurality of optical signal phases
define each channel.
[0010] In a system in accordance with the principles of the
invention, an optical network having a plurality of nodes, each
node being coupled to the network, is provided with a laser source
that serves as a reference to synchronize operation of the network.
Still further in accordance with the principles of the invention,
the reference laser optical output is distributed to all nodes of
the network. In the illustrative embodiment of the invention, the
reference laser output is distributed via a separate fiber optic
path. By utilizing a distributed reference laser, a plurality of
channels may be defined by
[0011] In accordance with one aspect of the invention, the laser
reference is used to generate a plurality of channels for
communication paths through the network. In accordance with another
aspect of the invention, the laser reference is a multiple
wavelength laser.
[0012] Optical communication system apparatus and methods of
operating an optical communications system in accordance with the
invention may utilize existing optical fiber networks and provide
significantly increased channel capacity. In accordance with one
aspect of the invention the system apparatus provides for a
plurality of communications channels and a processor unit receives
requests for allocation of one or more channels from a node coupled
to the optical communications system. The system apparatus
dynamically allocates one or more channels selected from unused
channels.
[0013] Still further in accordance with the invention, any node
coupled to the communications system can be coupled to any other
node via any unused channel. In accordance with one aspect of the
invention, the selection of a channel is in accordance with a
predetermined algorithm. One algorithm is such that the channel
distance between the assigned channel and other channels in use is
maximized. Another algorithm is such that cross channel
interference between the assigned channel and channels in use is
minimized.
BRIEF DESCRIPTION OF THE DRAWING
[0014] The invention will be better understood from a reading of
the following detailed description in conjunction with drawing
figures, in which like reference designations are used to identify
like elements, and in which:
[0015] FIG. 1 depicts an optical communication system in accordance
with the principles of the invention;
[0016] FIG. 2 depicts representative optical signal power
distribution levels in a portion of the system of FIG. 1;
[0017] FIG. 3 illustrates multiplexing of optical signals in
accordance with the principles of the invention;
[0018] FIG. 4 is a block diagram of a system control unit in the
system of FIG. 1;
[0019] FIG. 5 is flow chart illustrating channel assignment in
accordance with the invention;
[0020] FIG. 6 is illustrates the wavelength multiplexing utilized
in the system of FIG. 1;
[0021] FIG. 7 illustrates use of interferometer technology in an
embodiment of the invention;
[0022] FIG. 8 illustrates a frequency multiplex/switch layer
implementation utilized in an embodiment of the invention;
[0023] FIG. 9 illustrates in block diagram form a frequency
modulator and frequency demodulator for use in a system in
accordance with the invention;
[0024] FIG. 10 illustrates the multiplexing of signals in a second
embodiment of the invention in accordance with the principles of
the invention;
[0025] FIG. 11 illustrates, in simplified form, the transmission of
data from one network node to a second network node;
[0026] FIG. 12 illustrates, in simplified form, the transmission of
data from the second network node to the first network node of FIG.
11;
[0027] FIG. 13 is a block diagram of a network node in accordance
with the invention;
[0028] FIG. 14 is a detailed block diagram of a portion of a first
optical network processor (ONP) for use with an embodiment of the
invention;
[0029] FIG. 15 is a detailed block of a second portion of the first
optical network processor useable in conjunction with the optical
network processor portion of FIG. 14;
[0030] FIG. 16 is a detailed block diagram of a second optical
network processor;
[0031] FIG. 17 is a detailed block diagram of a first portion of a
third optical network processor for use with the second embodiment
of the invention;
[0032] FIG. 18 is a detailed block diagram of a second portion of
the third optical network processor useable in conjunction with the
optical network processor portion of FIG. 17;
[0033] FIG. 19 is a detailed block diagram of a fourth optical
network processor;
[0034] FIGS. 20 through 23 depict multiple wavelength laser
reference sources;
[0035] FIGS. 24 and 25 depict Erbium doped fiber lasers (EDFLs)
utilized in the reference sources of FIGS. 20 through 23; and
[0036] FIG. 26 depicts an optical add/drop (OAD).
DETAILED DESCRIPTION
[0037] Optical networks are increasingly being utilized to support
communication of various content between nodes. These optical
networks form part of optical communication systems. These optical
communication systems are, in some applications, characterized as
"long-haul", "metro" and "short haul" networks. In "metro" network
applications the bandwidth requirements of the networks has doubled
every ten months. New communications applications require high
bandwidth of the type found in Metro networks. However, the present
traffic capacity of Metro networks is limited.
[0038] In accordance with the principles of the invention a
solution to the problems with existing optical network and optical
communication systems combines bandwidth and channel requirements
into a single complete architecture.
[0039] One objective of the present invention is to be able to
provide for bandwidth upgrade to the existing optical fiber
networks and optical communications systems. In accordance with the
present invention, increasing the available channel count and speed
increases the bandwidth of existing optical networks. The number of
concurrently available channels is increased over WDM systems by a
factor of 10 to 200 permitting serving up to 20,000 channels
simultaneously. As will be appreciated by those skilled in the art,
the number of users that may be served by a multi-channel
communications system is determined by the number of channels and
an occupancy factor. In accordance with even a modest occupancy
factor and bandwidth demand, the system of the present invention
may be used to provide communications for in excess of 200,000
users concurrently. In addition, the system and method of the
present invention provides for dynamic, on-demand bandwidth
allocation and the capability of establishing communication between
two random nodes, i.e., any node coupled to the communications
system can communicate with any other node coupled to the
communications system.
[0040] FIG. 1 depicts an optical communication system 1000 in
accordance with the principles of the invention. Optical
communication system 1000 includes a Metro Network 1100 coupled to
a Long Haul Network 1200 via an optical add/drop module 1203 and
optical cross connect module 1205. Metro Network 1100 couples one
or more local access networks 1301, 1303, 1305 to each other and to
Long Haul Network 1200. Long Haul Network 1200 interconnects plural
Metro Networks 1100. For purposes of clarity in the drawings and
simplicity in the description, only one Metro Network 1100 is
shown. Metro Networks 1100 are typically located at widespread
geographic locations. However, it is not intended to limit
applicability of the present invention to arrangements in which
networks are dispersed geographically. The present invention is
applicable to networks that are overlapping in geographic areas or
even to networks that are in the same geographic area.
[0041] Metro Network 1100 is, in the illustrative embodiment,
depicted as a ring-based metropolitan network system. Metro
Networks are intended to provide high bandwidth to end customers
directly and/or via local loop access networks. In the illustrative
embodiment depicted in FIG. 1, Metro Network 1100 is depicted as a
ring based network having a fiber optic ring 1101. It will be
understood by those skilled in the art that the invention is not
limited to use in networks that are of a ring based structure. The
principles of the invention are equally applicable to other network
architecture structures including, not by way of limitation but by
way of example, star network structures, mesh network structures,
and point-to-point structures. For purposes of clarity and brevity,
those additional network architecture structures are not shown in
the drawing. In addition, although the illustrative embodiment of
the invention depicts a Metro Network coupled to plural Access
Networks 1300, the principles of the invention are not so limited.
In addition, those skilled in the art will realize that the
particular nomenclature used to describe the illustrative
embodiment is also not intended to be limiting of the invention in
any manner. For example, it is not intended to limit any aspect of
the invention to so-called "Metro Network" applications. Those
skilled in the art are familiar with the specific terminology
utilized to describe the illustrative embodiment and will
realize
[0042] In a first embodiment of the invention, a multiple
wavelength laser is utilized to provide a reference optical signals
for generation and assignment of optical channels that are
determined from selecting for each channel one wavelength of a
plurality of optical wavelengths and one frequency of a plurality
of optical modulation frequencies. In the illustrative system, the
number of wavelengths that are obtainable from a multiple
wavelength laser source is M wavelengths, where M is 32. The number
of optical modulation frequencies is O, where O is 128. Thus the
system of the first embodiment of the invention has a channel
capacity of 32.times.128=4096 channels. Each channel in the system
of the illustrative embodiment has a bandwidth of 155 mbs. In other
embodiments of the invention higher or lower speed and bandwidths
may be used. Also, in other embodiments of the invention, different
numbers of channels, different numbers of wavelengths and different
numbers of optical modulation frequencies may be utilized.
[0043] Turning to FIG. 3, the functionality of multiplexing and
switching channels identified by wavelength and frequency is
illustrated. For each wavelength, .lambda..sub.1 through
.lambda..sub.M, frequencies F.sub.1 through F.sub.O, are
multiplexed by multiplexor/demultiplexors 201. The frequency
multiplexed wavelengths at the outputs of multiplexors 201 are
multiplexed together at wavelength multiplexor/demultiplexor 203.
The multiplexed optical output of multiplexor/demultiplexor 203 is
coupled to optical network 1101. The multiplexor/demultiplexor
functions are changed to demultiplexing for received optical
signals. The optical signals received over optical network 1101 are
first wavelength demultiplexed by multiplexor/demultiplexor 203 to
wavelengths .lambda..sub.1 through .lambda..sub.M. For each
wavelength, a multiplexor/demultiplexor 201 demultiplexes the
frequencies F.sub.1 through F.sub.O. The multiplexor/demultiplexor
201, 203 provide switched multiplexing. For example,
multiplexor/demultiplexor 203 to the left in FIG. 3 can switch any
number of wavelengths onto optical network 1101
[0044] FIG. 4 illustrates a system control unit 1360 in block
diagram form. System control unit 1360 includes multiple wavelength
laser 1362 that is coupled to optical amplifier 1363. Optical
amplifer 1363 couples wavelength laser 1362 to laser reference ring
network 1370. A network processing unit 1364 is provided to control
and monitor operation of the supply of reference optical signals to
the reference ring network 1370. A wavelength sensing circuit 1366
is coupled to the output of optical amplifier 1363. Optical
amplifier 1363 provides sensing signals to network processing unit
1364 that permit network processing unit 1364 to adjust the output
level of optical amplifier 1363 and to control multiple wavelength
laser 1362. Network processing unit 1364 is coupled to network 1100
via an optical network processor 1368, an optical amplifier 1369
and an optical add/drop 1367. Network processing unit 1364 receives
requests for bandwidth and channel assignments from nodes coupled
to the network 1101 and responds with the address of one or more
allocated channels. The number of channels allocated to a node
depends upon the bandwidth needed for handling the traffic. Network
processing unit 1364 includes one or more processors and associated
memory. The processor units may be commercially available
processors. Memory associated with the processor unit or units may
be any commercially available memory. Programs stored in memory are
utilized to control the operation of network processing unit
1364.
[0045] Operation of system control unit 1360 in processing requests
for channel assignments is shown in FIG. 5. System control unit
1360 constantly identifies which channels have been allocated and
which channels are idle. System control unit 1360 responds
dynamically to requests for channels by selecting channels from the
idle channels and allocating the channels as needed. When
communication between users over a channel is complete, the channel
is returned to the designated idle channel pool. In the
illustrative embodiment of the invention, system control unit 1360
selects an idle channel to achieve maximum isolation with used
channels, i.e., the channel is selected to have the maximum
separation from channels in use. In other embodiments of the
invention, the manner in which channels are selected may utilize a
selection algorithm or a weighting selection or other scheme for
channel assignment. In operation, a system node that needs to
transmit information via the network 1100 transmits a request to
system control unit 1360 as indicated at step 501, for a channel.
The request also identifies the destination node or nodes. After
receiving the request, network processing unit 1364 selects a
channel from the pool of available channels, as indicated at step
503. The channel address is assigned. Wavelength and frequency
identify the channel address. At step 505, network processing unit
1364 provides the designated channel identity to the transmitting
node and to the receiving node. Network processing unit 1364
identifies the assigned channel as in use at step 507. Transmission
and reception of information occurs at step 509. Upon completion of
transmission by transmitting node, network processing unit 1364
reclaims the channel and again assigns it to the pool of available
channels at step 511. Communication of channel assignments to
system nodes may be accomplished in any one of a number of
conventional channel assignment methods. In the illustrative
embodiment of the invention, communication of channel assignments
to nodes from SCU 1360 and from node to SCU 1360 is accomplished by
use of dedicated control and communication channels.
[0046] FIG. 6 illustrates the operability of the multiplexing and
switching provided in improved network of the invention. In FIG. 6,
an optical fiber network such as network 1101 is illustrated as a
ring. At the system control unit 1360, multiple wavelengths optical
signals, .lambda..sub.1 through .lambda..sub.M, are multiplexed
together and distributed via reference laser ring network 1370 to
network nodes. Chart 602 indicates the wavelengths that are
available on reference laser ring network 1370. A network node,
identified as node 603 has requested that a channel be assigned.
System control unit 1360 allocates a channel. The allocated channel
includes wavelength .lambda..sub.Z. At network node 603, a tuned
optical wavelength filter 605 is utilized to select the wavelength
.lambda..sub.Z assigned by the system control unit 1360. Filter 605
couples optical channel signals at the wavelength .lambda..sub.Z
over the optical fiber network 1101. Chart 604 indicates that the
output of the output of node 603 presented to network 1101 is a
single wavelength. Other nodes likewise transmit different
wavelength channels over the network 1101 as indicated by the
additional inputs to optical fiber network 1101. Wavelength chart
606 illustrates that although each node may provide an optical
signal at a single wavelength, optical fiber network 1101 carries
multiple wavelengths. System control unit 1360 has informed node
607 that it is assigned to receive communications from node 603 at
wavelength .lambda..sub.Z. At a node 607, a tunable optical
wavelength filter 609 is adjusted to select wavelength
.lambda..sub.Z and provide the signal to a detector 611 that is
used to extract information carried by the optical signals. Chart
608 indicates that the output of tunable optical wavelength filter
609 provides a single wavelength output. Tunable output wavelength
filters 605, 609 may be of a design described in the
literature.
[0047] In a second embodiment of the invention, advantageous use is
made of the properties of optical signals to further enhance the
channel capacity of optical communication systems. A phase
modulated optical signal may be characterized in terms its
wavelength .lambda., its phase .phi., and its modulation frequency
f. Recognizing this, the second embodiment of the invention
utilizes phase modulated and delayed optical signals and defines
each optical channel by a wavelength multiplex, a phase delay or
coherence multiplex and a frequency multiplex. In the second
illustrative embodiment of the invention, the number of wavelength
multiplexed is identified as "M". The number of phase or coherence
multiplexed channels is identified as "N". The number of frequency
multiplexed channels is "O". With M=32, N=8, and O=64, the number
of available channels that may be multiplexed together is
32.times.8.times.64 or 16,000 channels. Each channel has a
bandwidth of 155 mbs. Thus the total bandwidth is 16,000
channels.times.155 mbs=2.5 tbs. In other embodiments of the
invention higher or lower speed and bandwidths may be used. Also,
in other embodiments of the invention, different numbers of
channels, different numbers of wavelengths and different numbers of
optical modulation frequencies may be utilized.
[0048] The architecture of the second embodiment of the invention
is the same as shown in FIGS. 1 and 2. The system control unit 1360
as shown in FIG. 4 and its operation as set forth with respect to
FIG. 5 are substantially the same in the second embodiment. The
system and method of the second embodiment of the invention utilize
a phase multiplex/switch layer, a frequency multiplex/switch layer
and a wavelength multiplex layer in addition to the wavelength
multiplex/switch layer described in conjunction with FIG. 6.
[0049] The phase multiplex/switch layer makes advantageous use of
the fact that a single wavelength optical signal in optical fiber
can carry multiple phases. At the transmission end phase separation
is provided through delay of the channel with respect to the
reference channel. By creating a phase delay that is larger than
the coherence length of the laser, multiple phase channels can be
multiplexed into a single wavelength. At the receive end, phase
recovery is provided. By reversing the phase delay and interfering
with the reference signal, the phase-multiplexed channel can be
separated and detected with an interferometer.
[0050] Turning to FIG. 7, the manner in which interferometer
technology may be utilized to provide phase multiplex/switching is
illustrated. In the illustration, four phase channels are
illustrated. It will be understood by those skilled in the art that
the number of phase channels shown in FIG. 7 is merely illustrative
and is not to in any way be considered as limiting. In FIG. 7, the
transmit end is illustrated at 700 and the receive end is
illustrated at 710. Interconnecting transmit end 700 and receive
end 710 is the optical fiber network 1101. Light source 701
generates optical signals. Phase delays are created as shown at 703
to produce four phase multiplexed data channels. The undelayed
optical signal 704 provides a reference. The phase delayed signals
702 are phase modulated at 705 to encode data onto the signals. The
phase delayed optical signals appear on the optical network 1101 as
shown at 707. At receive end 710, a phase delay reversal is
provided at 709. By utilizing reference signals the phase
multiplexed reference signal is demultiplexed. Interferometer
techniques 711 are utilized to demodulate and decode the data that
was transmitted via the optical signals.
[0051] In the phase interferometer multiplex/switching portion,
signals are detected by interferometer based on phase amplitude,
not by intensity, to get better sensitivity. Amplifiers can
compensate for system losses thereby leading to a system that
tolerates more loss. In addition, by providing signals in
difference phases, multiple wavelength channels are carried in the
same wavelength. Switching between phase channels can be done
electro-optically within less than 0.1 microsecond to allow for
fast packet switching. Channel isolation is enhanced by properly
selecting phase, wavelength and modulation frequency.
[0052] FIG. 8 illustrates the frequency multiplex/switch layer
implementation utilized in the invention. In the illustrative
embodiment, O optical frequencies, F.sub.1 through F.sub.O, are
utilized as carriers. Modulators 801 produce modulated optical
signals at the individual optical carrier frequencies, F.sub.1
through F.sub.O. Combiner 803 combines the individual carrier
frequencies onto the optical network 1100. As illustrated in
spectral chart 804, combiner 803 combines all the carrier
frequencies onto the network optical fiber 1101. At the receive end
a divider 805 separates the frequency components, F.sub.1 through
F.sub.O. Demodulators 807 demodulate the optical signals.
[0053] FIG. 9 illustrates in block diagram form a modulator 801 and
a demodulator 807. Data 903 to be transmitted is combined in a
mixer 905 with an IF signal produced by a RF source 901. The
resulting RF signal is applied to a RF driver filter 907 that
provides appropriate filtering and driver buffering. The output of
RF driver filter 907 is supplied to modulator 909 to modulate an
optical signal from a light source 911. The optical signal from
light source 911 is modulated by an RF signal at the modulation
frequency corresponding to the channel assigned for communication
to the node at which the modulator 801 is located. At the receiver
node that is intended to receive data from the node at which
modulator 801 is located, demodulator 807 receives optical signals.
Demodulator 807 includes a detector circuit 913. Detector circuit
913 is set to detect optical signals at the channel frequency
designated for communication from the node at which modulator 801
is located. The output of detector 913 is coupled to a RF driver
filter 915. The output of the RF driver 915 is combined with an IF
signal provided by RF source 917 in a mixer 919 to recover the
transmitted data at terminal 921. The IF signal is at the
modulation frequency assigned to the particular channel.
[0054] Turning to FIG. 10, the functionality of multiplexing and
switching channels identified by wavelength, phase and frequency is
illustrated. For each phase, .phi..sub.1 through .phi..sub.N, of
each wavelength, .lambda..sub.1 through .lambda..sub.M, frequencies
F.sub.1 through F.sub.O, are multiplexed by
multiplexor/demultiplexors 201. The frequency multiplexed signals
for each of the phases at the outputs of multiplexors 201 are
multiplexed together at phase multiplexor/demultiplexors 1021. The
frequency and phase-multiplexed signals for each wavelength are
applied to wavelength multiplexor/demultiplexor 203. The
multiplexed optical output of multiplexor/demultiplexor 203 is
coupled to optical network 1101. The multiplexor/demultiplexor
functions are changed to demultiplexing for received optical
signals. The optical signals received over optical network 1101 are
first wavelength demultiplexed by multiplexor/demultiplexor 203 to
wavelengths .lambda..sub.1 through .lambda..sub.M. For each
wavelength, a corresponding phase multiplexor/demultiplexor 1021
demultiplexes phases and for each phase a multiplexor/demultiplexor
201 demultiplexes the frequencies F.sub.1 through F.sub.O. Each
multiplexor/demultiplexor is bi-directional in that it will switch
or multiplex one or more signals into a single stream and that it
will demultiplex or switch signals out of a combination stream.
[0055] Since each channel has a unique wavelength, phase and
modulation frequency correlation, it can be identified by a unique
address that references its wavelength, phase and frequency. For M
wavelengths, N phases, and O modulation frequencies each channel
may be particularly identified by a channel identity in which the
wavelength is assigned a number of from 1 to M, each phase is
assigned a number of from 1 to N and each modulation frequency is
assigned a number from 1 to O. The channel identity for each
channel may be referred to as .lambda..sub.m.PHI..sub.n- f.sub.o,
where "m" is the wavelength number, "n" is the phase number and "o"
is the frequency number. This channel identity is selected for
convenience and clarity in description only and is not in any way
intended to limit the invention.
[0056] FIGS. 11 and 12 illustrate the exchange of data between two
network nodes as represented by optical network processors ONP#1
and ONP#50. Initially, ONP#1 request a channel allocation from
system control unit 1360. System control unit 1360 makes the
selection of an idle channel and as a result allocates a channel
identified as .lambda..sub.2.phi..sub.8F.- sub.4 for transmission
of data from ONP#1 to ONP#50. As shown in FIG. 11, ONP#1 inserts
data, D.sub.TX into the designated channel. ONP#50 receives the
modulated signal and extracts the data
[0057] D.sub.RX. Upon completion of the data transmission to
ONP#50, system control unit 1360 returns the channel assignment of
channel .lambda..sub.2.phi..sub.8F.sub.4 to the pool of unassigned
channels for reassignment. Later, the node at which ONP#50 is
located requests a channel assignment from system control unit
1360. System control unit 1360 assigns a channel form the pool of
available idle channels. In this instance channel
.lambda..sub.4.phi..sub.3F.sub.6 is assigned. ONP#50 transmits and
ONP#1 receives data in the assigned channel. Upon completion of the
data transmission, the channel is reassigned by system control unit
1360 to the pool of idle channels.
[0058] As shown in FIG. 13, each network node includes an optical
network processor ONP that includes a modulator and a demodulator
as described above. Each ONP is coupled to the laser reference
source 1362 via the laser reference network 1370 as shown in FIG.
1. Each ONP is coupled to the optical fiber network 1101 via an
optical add/drop OAD and an optical amplifier EDFA.
[0059] Optical Network Processor
[0060] FIGS. 14 and 15 depict a transmitter portion and a receiver
portion of an optical network processor particularly well adapted
for use with the above-described first embodiment of the invention.
FIGS. 16 and 17 depict a transmitter portion and a receiver portion
of an optical network processor particularly well adapted for use
with the above-described second embodiment of the invention.
[0061] Each optical network processor includes a transmit function
and a receive function. The receive function decodes data from a
systems communications channel assigned for communications to a
node coupled to the optical network processor to control the
associated wavelength multiplex/switch, phase multiplex/switch and
frequency multiplex/switch. The transmit function converts data
from an associated node to an assigned system communications
channel by controlling the associated wavelength multiplex/switch;
phase multiplex/switch and frequency multiplex/switch.
[0062] Turning to FIG. 14, a transmitter portion of an optical
network processor for use in a first embodiment of the invention is
shown. Transmitter portion 1400 of an optical network processor
includes one or more processors or micro controllers 1401 that
provides program control of operation of the optical network
processor. For clarity only one processor is shown for each optical
network processor, but more than one processor may be used.
Transmitter portion 1400 is coupled to laser reference network 1370
and receives signals from the multiple wavelength signals from
laser reference source 1360. A polarization controller 1403 under
control of micro controller 1401 selects polarization of the
received laser signals. The output of polarization controller 1403
is coupled to tunable filter 1407. In an alternate embodiment of
the invention, a depolarizer replaces polarization controller 1403.
Micro controller 1401 receives channel allocation information and
utilizes the channels allocation information to select a wavelength
and frequency for its associated node to transmit data. Micro
controller 1401 via wavelength tuning module 1405 operates tunable
filter 1407. Wavelength tuning module 1405 selects a wavelength in
response to micro controller 1401 providing a wavelength select
signal. Tunable filter 1407 is tuned to the selected wavelength.
Tunable filter 1407 thereby selects the wavelength optical signal
for transmitting data under control of micro controller 1401. The
output of tunable filter 1407 is coupled to a Mach-Zehnder
interferometer 1413. Interferometer 1413 includes two legs coupled
at the input to a coupler 1409 and at the output by coupler 1419. A
first leg includes dc bias module 1412 and a phase modulator 1416.
A second leg includes dc bias module 1414 and a phase modulator
1418. Microcontroller 1401 provides quadrature control of
interferometer 1413 via bias control module 1411. Quadrature
control ensures stable linear operation of the interferometer 1413.
Frequency selection is provided via microcontroller 1401
controlling voltage-controlled oscillator 1415 that in turn
provides a selected modulation frequency to mixer/driver module
1417. Mixer/driver module 1417 mixes the modulation frequency
output of voltage controlled oscillator 1415 with Transmit data
D.sub.TX. The outputs of interferometer 1413 are provided to
tunable filter 1421 which is tuned by wavelength tuning module 1405
to the wavelength selected by micro controller 1401. The output of
tunable filter 1421 is coupled to network 1101. In addition,
coupler 1419 has an output coupled to photo detector 1423. The
output of photo detector 1423 is coupled to micro controller
1401.
[0063] FIG. 15 depicts optical network processor receive portion
1500. Receive portion 1500 of an optical network processor includes
a processor or micro controller 1501 that provides program control
of operation of the optical network processor. Receive portion 1500
is coupled to network 1101 and receives signals from another
network node. Micro controller 1501 receives channel assignment
information from SCU 1360 and utilizes the channel assignment to
select the wavelength and frequency of a channel carrying data for
its associated node. A polarization controller 1503 under control
of micro controller 1501 selects polarization of the received laser
signals. In an alternate embodiment, a depolarizer replaces
polarization controller 1503. The output of polarization controller
1503 is coupled to tunable filter 1507. Micro controller 1501 via
wavelength tuning module 1505 operates tunable filter 1507.
Wavelength tuning module 1505 selects a wavelength in response to
micro controller 1501 providing a wavelength select signal. Tunable
filter 1507 selects the wavelength of a receive channel under
control of micro controller 1501. A coupler 1509 couples the output
of tunable filter 1507 to a Mach-Zehnder Interferometer 1513.
Interferometer 1513 includes two legs. A first leg includes dc bias
module 1512 and a phase modulator 1516. A second leg includes dc
bias module 1514 and a phase modulator 1518. Interferometer 1513 is
not used as an interferometer in the receiver. Only the dc bias
modules 1512 and 1514 are used in the receive finction. Phase
modulators 1516, 1518 are left unused in this receiver
implementation. Micro controller 1501 provides quadrature control
via bias control module 1511. Frequency selection is provided via
micro controller 1501 controlling voltage-controlled oscillator
1515 that in turn provides a selected frequency to mixer/driver
module 1517. The outputs of interferometer 1513 are applied to
coupler 1519. The output of coupler 1519 is in turn applied to
tunable filter 1521 which is controlled by micro controller 1501
via wavelength tuning module 1505. The wavelength-selected output
of tunable filter 1521 is in turn applied to detector 1523.
Detector 1523 provides a quadrature dc output, which is provided to
micro controller 1501 for use in controlling bias control circuit
1511. An RF output of detector 1523 is provided to amplifier 1525.
Output of amplifier 1525 is coupled to a second input of
mixer/driver 1517. An output of mixer/driver 1517 is applied to low
pass filter 1529. The output of low pass filter 1529 provides data
output signals D.sub.RX that are provided to an network node such
as user 1331.
[0064] As can easily be seen from a comparison of FIGS. 14 and 15,
the design of the optical network processor receive portion and
transmit portion share similar basic design components in the
implementations shown. The transmit portion and receive portions in
one embodiment are implemented on tow separate chips for full
duplex operation. In another embodiment of the invention, a
bi-directional, half-duplex design combines both transmit and
receive portions in a single integrated optic chip using reflective
design. Advantages of the second embodiment are that the length of
the integrated optic chip is shortened by 1/2; cost is reduced; and
transmit and receive portions are combined into one design. In
addition, performance of the wavelength filter is greatly enhanced
for double pass operation. Sidelobe suppression of 15 dB for one
pass through the filter increases to 30 dB with double pass
operation. Still further, the drive voltage of the modulator is
reduced 50%. A further significant advantage is that integration
onto a single chip allows creation of a large sized phase
detector.
[0065] FIG. 16 depicts a transceiver 1600 in which a single
integrated optic chip 1670 is utilized advantageously. Transceiver
1600 is coupled to network 1101 and laser reference ring 1370. A
circulator 1604 and an isolator interposed in the reference laser
ring connect transceiver 1600 to both. Circulator 1604 is coupled
to integrated optic chip 1603 via a polarization controller or
scrambler 1603. Integrated optic chip 1670 includes a TM polarizer
1651 coupled to a tunable filter 1652. Micro controller 1601
receives transmit and receive channel assignment information from
system control unit 1360 and utilizes the channel assignment
information to select wavelength and frequency for transmit or
receive functions. Micro controller 1601 via a wavelength-tuning
module 1605 controls tunable filter 1652. A TE polarizer 1653
follows tunable filter 1652 to remove unwanted signals. A 2.times.2
coupler 1654 is disposed between TE polarizer 1654 and optical bias
modulator 1656. Optical bias modulators 1612, 1614 are followed by
phase modulators 1616, 1618. Reflection mirrors 1662, 1660 are
provided on the end of integrated optic chip 1670. The operation of
the various circuit elements shown in FIG. 16 is substantially
identical to the operation of the elements in FIG. 14 for receive
operation and to the elements in FIG. 15 for receive operation.
There is a one to one correspondence to the elements of FIGS. 14,
15, and 16 and the operation is identical.
[0066] FIG. 17 depicts a transmitter portion 1700 of an optical
network processor for use in the above described second embodiment
of the invention. Transmitter portion 1700 includes a processor or
micro controller 1701 that provides program control of operation of
transmitter portion 1700. Micro controller 1701 receives channel
assignment information from system control unit 1360 and utilizes
that information to select wavelength, phase and frequency of
assigned channels. Transmitter portion 1700 is coupled to laser
reference network 1370 and receives multiple wavelength signals
from laser reference source 1360. A polarization controller 1703
under control of micro controller 1701 selects polarization of the
reference laser signals. The output of polarization controller 1703
is coupled to tunable filter 1707. Micro controller 1701 via
wavelength tuning module 1705 controls tunable filter 1707.
Wavelength tuning module 1705 selects a wavelength in response to
micro controller 1701 providing a wavelength select signal. Tunable
filter 1707 selects the wavelength optical signal for transmitting
data under control of micro controller 1701. A coupler 1709 couples
the output of tunable filter 1707 to phase selector for selecting
one out of "N" phases. The phase selector includes a 1.times.n
switch 1771 that is controlled by micro controller 1701. Each of
the N outputs of switch 1771 is coupled to a corresponding phase
modulator 1775. Frequency selection is provided by micro controller
1701 controlling a voltage controlled oscillator 1715. The selected
frequency output of voltage controlled oscillator 1715 is combined
with data to be transmitted D.sub.TX by mixer/driver 1717. The data
D.sub.TX to be transmitted is received from a user node 1331. A
filter/switch module 1770 under control of micro controller 1701
provides the output of mixer/driver 1717 t the N phase modulators
1775. Each phase modulator 1775 is coupled to a phase delay module
1777. The outputs of the phase delay modules are the N phases
.PHI.1 through .PHI.N. Switch 1779 under control of micro
controller 1701 selects the output phase. The output of switch 1779
and the wavelength-selected reference are combined in coupler 119
and filtered by tunable wavelength filter 1721. Micro controller
1701 via wavelength tuning module 1705 controls tunable filter
1721. The output of filter 1721 is the wavelength/frequency/phase
selected optical signal modulated with transmit data and is coupled
to optical network 1101. A portion of the output is coupled to a
detector 1723 that provides a dc feedback signal to micro
controller 1701.
[0067] FIG. 18 depicts optical network processor receive portion
1800 for the above described second embodiment. Receive portion
1800 includes a processor or micro controller 1801 that provides
program controlled operation of optical network processor receive
portion. In addition, micro controller 1801 receives channel
assignment information from system control unit 1360 and utilizes
that information to select channel wavelength, phase and frequency
to select a desired channel for recovery of received data. The
received data is provided to a node 1331. Micro controller 1801
generates wavelength select, phase select and frequency select
signals. The frequency select signals control a voltage-controlled
oscillator 1815 to provide a frequency selected signal to a
mixer/driver circuit 1817. The output of mixer/driver 1817 is
filtered by filter 1840 to provide output data signals D.sub.TX.
Receive portion 1800 is coupled to network 1101 and receives
optical signals carrying data D.sub.TX from another node coupled to
network 1101. A depolarizer 1803 depolarizes the optical signals
received via network 1101. As those skilled in the art will
appreciate, depolarizer 1803 may be replaced with a polarization
controller controlled by micro controller 1801. The output of
depolarizer 1803 is coupled to tunable filter 1807. Micro
controller 1801 via wavelength tuning module 1805 operates tunable
filter 1807. Wavelength tuning module 1805 selects a wavelength in
response to micro controller 1801 and tunes filter 1807 to the
selected wavelength. Phase selection is accomplished by micro
controller 1801 providing phase select signals to control switches
1871 and 1879. Switches 1871 and 1879 are used to select one phase
delay path from a group of "n" phase delay, where "n" is the number
of selectable phases. Each phase delay path includes a phase
modulator 1875 and a phase delay circuit 1877. Micro controller
1871 via bias control 1811 controls phase modulators 1875. The
output of the selected phase path is coupled via switch 1879 to
coupler 1819. A phase reference signal is coupled from signals
received from network 1101 from coupler 1871 to coupler 1919 via
optical connection 1873. Coupler 1819 combines the phase reference
signal from connection 1873 with the output of phase switch 1879.
The combined output is applied to wavelength filter 1821 that is
tuned to the wavelength selected by micro controller 1801. The
output of tunable filter 1821 is coupled to detector 1823 that
separates an RF signal and a dc servo feedback signal. The RF
signal is applied to mixer/driver 1817 via pre amplifier 1880. All
of the components shown within box 1881 may be fabricated on a
single integrated optic chip using reflective design.
[0068] A comparison of transmit portion 1700 of FIG. 17 and receive
portion 1800 of FIG. 18 shows that much of the functionality of the
transmit portion and receive portion is similar. FIG. 19 is a block
diagram of a transceiver 1900 in which economies are achieved by
utilizing the commonality of receive and transmit portions, 1800,
1700. Transceiver 1900 receives data D.sub.TX from a node 1331 and
provides data D.sub.RX to a node 1331. Transceiver 1900 is coupled
to optical network 1101 and reference network 1370 by circulator
1940. A micro controller 1901 provides program controlled operation
of transmit and receive functions. In addition, micro controller
1901 provides wavelength, phase and frequency selection to select a
desired channel for recovery of received data and providing the
received data to a node 1331 and for receipt of transmit data from
node 1331 for transmission over network 1101. Micro controller 1901
generates wavelength select, phase select and frequency select
signals for transmit and receive. The frequency select signals
control a voltage-controlled oscillator 1915a to provide a
frequency selected signal to a mixer/driver circuit 1917a. The
output of mixer/driver circuit 1917a is filtered by low pass filter
1940 to provide output data signals D.sub.RX.
[0069] Frequency select signals from micro controller 1901 are used
for transmission of data from a node 1331 over network 1101.
Frequency select signals control voltage controlled oscillator 1915
to select a desired transmit channel frequency. A mixer/driver 1917
combines the output of voltage-controlled oscillator 1915 and
D.sub.TX. The modulated frequency signals are applied to filter
switch 1970. Micro controller 1901 also controls phase and
wavelength selections. Phase selection is provide by micro
controller 1910 providing phase selection signals to a phase
control module 1972, bias control signals to bias control circuit
1911 and filter control signals to filter switch 1970. For transmit
data, filter switch 1970 is active but bias control 1911 is not.
Integrated optical chip assembly 1981 provides wavelength selection
and phase multiplex selections. Integrated optical chip assembly
1981 utilizes reflective multiplex technology. Double pass
operation of the integrated optical chip assembly 1981 greatly
enhances performance of the wavelength filter operation. Sidelobe
suppression is increased, for example, from 15 dB to 30 dB. Input
signals received from network 1101 via circulator 1940 are applied
to depolarizer 1903. Outputs of depolarizer 1903 are applied to a
TE polazer 1982. Polarizer 1982 is coupled to tunable wavelength
filter 1983. Tunable filter 1983 is coupled to TM polarizer 1984.
TM polarizer 1984 is coupled to a phase selection circuit including
2.times.2 coupler 1909, a 1.times.4 optical switch 1985, bias
modulator array 1986, phase modulator array 1987 and phase delay
and recovery reflection mirror 1988. In the embodiment shown,
selection of four phase channels may be accomplished. The phase
selection circuit may be expanded to more phase channels, but for
purposes of drawing clarity, only a four phase channel selection
structure is shown. For both transmit and receive, micro controller
1901 provides wavelength selection signals to wavelength tuning
module 1905. Wavelength tuning module 1905 controls tunable filter
1983 to select the wavelength channel for transmit and receive.
[0070] For transmit functionality, micro controller 1901 controls
filter switch 1970 to control the phase modulator array 1987. For
receive functionality, micro controller 1901 controls bias control
1911 to in turn control bias modulator array 1986. For transmit
functionality, filter switch 1970 is used to select a phase and
couple the output of mixer/driver 1917 via coupler 1909 through
polarizer 1983, wavelength filter 1983, polarizer 1982 to
depolarizer 1903 and to network 1101 via circulator 1940. For
receive functionality, optical signals received from network 1101
are coupled via circulator 1940 through depolarizer 1903 to
polarizer 1982, tunable filter 1983, polarizer 1984 to the phase
selector. Bias control module 1911 under control of micro
controller 1901 sets the bias to a quadrature point to stabilize
the receive phase channel. The output of the phase selector is
coupled to detector 1923. Detector 1923 provides an RF output to
preamplifier 1980. Preamplifier 1980 is coupled to mixer/driver
1917, and its output is filtered by lowpass filter 1940 to provide
output data to node 1331. The invention has been described in
conjunction with specific embodiments.
[0071] Reference Laser
[0072] Multiple lasers may be assembled together to provide a laser
reference source useable in the optical networks and optical
communication system of the invention. Various laser sources may be
employed; however, each laser source must have specific
characteristics. In particular, multiple wavelength lasers that
have high launch power are desirable. In particular, it is
desirable that the reference provides optical signals for each
wavelength channel at levels greater than 20 mw and each laser
source should desirably meet this requirement. It is also desirable
that nonlinear effects such as self phase modulation (SPM),
stimulated Brillion scattering (SBS) and four wave mixing be
minimized. A short coherence length of less than 5 mm should be
provided for phase multiplex/switching operation. To ensure proper
wavelength multiplexing, wavelength stability is to be controlled
within 20 picometers. It is desirable that spurious spectral
components be minimized between wavelength channels. In
particularly advantageous embodiments of the invention, 16 to 32
wavelengths are provided by the reference laser source.
[0073] FIG. 20 depicts one embodiment of a multiple wavelength
reference laser source in which multiple distributed feedback (DFB)
lasers are used. A separate DFB laser is used to generate each
wavelength .lambda..sub.1 through .lambda..sub.M. There are two
limitations on DFB lasers that need to be accommodated. First, the
output of each DFB laser 2001 typically has a narrow linewidth of
less than 50 MHz. This spectral width is too narrow for use in the
embodiments of the invention described above. Second, the coherence
length of the DFB output is too large for application in the
embodiments of the present invention. Phase modulating the output
of the DFB laser with an RF signal broadens the spectral width of
the output and further can reduce the coherence length. In other
words, for optimum performance, the laser signals can not be too
coherent and can not have too narrow a line width in the
above-described embodiments. A phase modulator 2003 is coupled to
each DFB laser 2001. Modulation is with an RF signal having
multiple frequency components that are selected in the RF range of
0.01 to 10.0 GHz. Modulation with a multiple component RF signal
produces a laser signal having a broad line width output of greater
than 20 GHz. In addition, the phase modulation reduces the
coherence length. Each modulated laser output is filtered to remove
sidelobes by utilizing fiber gratings 2005 to shape the modulated
laser output spectrum. A DWDM multiplexer 2007 is utilized to
combine the outputs of each of the DFB lasers. Amplifier 2009
amplifies the resulting multiple wavelength laser output. Amplifier
2009 is an EDFA.
[0074] FIG. 21 illustrates a modification to the multiple
wavelength laser source of FIG. 20. In the design of FIG. 21, the
phase modulated laser signals at the different wavelengths are each
amplified by EDFA amplifiers 2101 prior to being shaped by fiber
gratings 2005. By amplifying each wavelength component prior to
combining the wavelength components, it I possible to achieve a
combined output in which the components are more uniform and a
higher output level may b achieved.
[0075] FIG. 22 illustrates a third embodiment of a multiple
wavelength laser source that may be used in accordance with the
invention. Separate Erbium Doped Fiber Lasers (EDFL) 2201 are used
as sources. Wavelength control technology is used to control EDFL
emission wavelength. Each EDFL provides a single wavelength output.
Fiber gratings 2203 are used to provide output spectrum shaping and
coherence function. EDFAs 2205 amplify each output and a DWDM
multiplexer 2207 is used to combine the outputs to produce a
multiple wavelength laser output. By using EDFLs, phase modulation
is not necessary because the EDFLs have a broader line width and
the coherence length is not too short. Through selection of
appropriate fiber gratings the desired spectral response is
achieved.
[0076] An alternative EDFL based design for the multiple wavelength
laser reference is illustrated in FIG. 23. In the reference source
of FIG. 23, rather than separately amplify each wavelength, a
single EDFA amplifier 2301 is utilized to amplify the combined
output. A filter 2303 is used to shape the amplified multiplexed
output.
[0077] FIGS. 24 and 25 illustrate EDFLs suitable for application to
the laser reference sources depicted in FIGS. 22 and 23. In the
EDFLs of both FIGS. 23 and 24, an erbium doped fiber 2401 is pumped
from a laser pump source 2407. The fiber 2401 is coupled at either
end to a fiber grating. In the embodiment of FIG. 24, both gratings
2403 and 2405 are reflecting narowband gratings at the same
wavelength. In the embodiment of FIG. 25 the narrow band fiber
grating 2403 is replaced with a broadband reflecting grating 2501
or alternatively, a mirror. WDM 2409 couples the pump source 2407
output to fiber grating 2405. An isolator 2411 is used at the
output of the EDFL
[0078] FIG. 26 depicts an optical add/drop 1307 that is utilized to
particular advantage in the embodiments of the invention described
above. In addition, FIG. 26 also shows further details of a typical
EDFA construction, such as EDFA 1313. The design shown is for a
reciprocal optical add/drop inserted into optical link network
1101. Optical add/drop 1307 utilizes three couplers 2603, 2605,
2607 and two isolators 2609, 2611 all of which are known in the art
and are commercially available. Optical add/drop 1307 includes a
first bi-directional port P1, a second bi-directional port P2 and a
third bi-directional port P3. Bi-directional ports P1 and P2 are
connected to optical link network 1101 and bi-directional port P3
is coupled to an optical network processor or coupler via
bi-directional amplifier 1313. Drop signals from optical link
network 1101 are coupled from coupler 2605 to coupler 2607 and to
isolator 2611. Isolator 2611 couples the optical signals to
amplifier 1313. Add signals from amplifier 1313 are supplied to
isolator 2609. From isolator 2609, the transmit signals are
supplied to coupler 2607 which in turn is connected to coupler 2603
and from coupler 2603 to optical link network 1101. A through path
couples the couplers 2603 and 2605. Coupler 2607 is utilized to
permit the bidirectional drop and add of optical signals. Each of
couplers 2603 and 2605 are chosen in the illustrative embodiment
such that 5% of the optical signal is coupled to an add/drop path
and 95% of the optical signal is passed on the through path of the
coupler. Coupler 2607 is chosen such that 50% of the signal is
coupled from one path to the other. Isolators 2609, 2611 are used
to provide directionality for the add and drop paths to the ONP or
coupler. Amplifier 1331 comprises an EDFA 1313a for amplifying
output signals and an EDFA 1313b for amplifying input signals. A
circulator 1313c having three ports c1, c2, c3 is used to couple
both EDFAs 1313a, 1313b to the optical network processor or
coupler. Drop signals from P1 are extracted via coupler 2603 and
are coupled via coupler 2607 to isolator 2611, amplified by EDFA
1313b, applied to circulator 1313c at its port c2 and extracted
from circulator at port c3 which is connected to an optical network
processor at port P3.
[0079] Optical signals at port P2 are coupled by coupler 2605 to
coupler 2607 and processed as described above. Optical signals
received at port P3 are provided by circulator 1313c to EDFA 1313
and applied to isolator 2609. The output in this add path is
applied to coupler 2607 provides 50% of the add signal to each of
couplers 2603, 2605. Because the same level of signals are achieved
in transmission of signals from ports P1 to P2 as are achieved from
ports P2 to P1 and between any combination of pairs of the three
ports P1, P2, P3, the optical add/drop in this embodiment may be
characterized as a reciprocal add/drop.
[0080] It will be appreciated by those skilled in the art that
various changes and modifications may be made to the various
embodiments without departing from the spirit or scope of the
invention. It is intended that those various changes and
modifications be included within the scope of the invention. It is
further intended that the invention not be limited to the various
embodiments shown and described herein nor limited to those
embodiments that would be apparent as of the filing date of this
application. It is intended that the invention be limited in scope
only by the claims appended hereto.
* * * * *