U.S. patent application number 10/206904 was filed with the patent office on 2003-02-27 for reconfigurable multi-channel transmitter for dense wavelength division multiplexing (dwdm) optical communications.
This patent application is currently assigned to SRI International. Invention is credited to Cooper, David E., Vujkovic-Cvijin, Pajo.
Application Number | 20030039015 10/206904 |
Document ID | / |
Family ID | 24444526 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030039015 |
Kind Code |
A1 |
Vujkovic-Cvijin, Pajo ; et
al. |
February 27, 2003 |
Reconfigurable multi-channel transmitter for dense wavelength
division multiplexing (DWDM) optical communications
Abstract
The invention provides a high performance reconfigurable DWDM
transmitter incorporating low cost discrete optical components
which can be placed in v grooves etched in a silicon optical
micro-board or the like, keeping costs of manufacturing low. The
lasers can be packaged in modules based on the technology of meso
scale optics. The physical size of a multi channel module can be no
bigger than a conventional single laser module.
Inventors: |
Vujkovic-Cvijin, Pajo;
(Mountain View, CA) ; Cooper, David E.; (Palo
Alto, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
SRI International
Menlo Park
CA
|
Family ID: |
24444526 |
Appl. No.: |
10/206904 |
Filed: |
July 25, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10206904 |
Jul 25, 2002 |
|
|
|
09610312 |
Jul 5, 2000 |
|
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|
60212431 |
Jun 16, 2000 |
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Current U.S.
Class: |
398/197 |
Current CPC
Class: |
H04B 10/506 20130101;
H01S 5/06825 20130101; H01S 3/1303 20130101; H01S 5/4025 20130101;
H04B 10/572 20130101; H04J 14/02 20130101; H01S 5/026 20130101 |
Class at
Publication: |
359/187 ;
359/124 |
International
Class: |
H04J 014/02; H04B
010/04 |
Claims
What is claimed is:
1. An optical DWDM transmitter device comprising: a plurality of
lasers, each laser providing an optical channel; a plurality of
active control loops locking each of the plurality of lasers to an
associated frequency so as to enable an independent modulation
signal to be carried on each of the optical channels; and at least
one spare laser for transmitting on any of the optical channels
when the associated laser fails.
2. A device as in claim 1 wherein said lasers comprise diode
lasers, gas lasers, chemical lasers, or masers.
3. A device as in claim 1, further comprising a frequency comb
including a Fabry-Perot etalon, a spacing between the optional
channels being defined by the etalon.
4. A device as in claim 3, further comprising a stable reference
frequency defined by a gas absorption cell, the etalon being
frequency adjusted in response to the reference frequency.
5. A device as in claim 1, wherein the feedback loops comprise a
microprocessor/controller.
6. A device as in claim 1 wherein the optical channels have
frequencies separated according to fiber optic telecommunication
standards.
7. A device as in claim 1, wherein the spare laser takes over the
functioning of the replaced laser, transmitting the replaced
laser's frequency on a millisecond time scale so as to enable
restoration of DWDM optical layer communication with little
disruption to transmission of any modulated signals.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 09/610,312 filed Jul. 5, 2000, which is a
non-provisional patent application of and which claims the benefit
of priority from U.S. Provisional Patent Application No. 60/212,431
as filed Jun. 16, 2000, the full disclosures of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The field of the present invention relates in general to
optical communication using inexpensive, stable and accurate
devices for creating channels for wavelengths of light. More
particularly, the field of the invention relates to a wavelength
reconfigurable, multiple channel transmitter for optical Dense
Wavelength Division Multiplexed signals.
BACKGROUND OF THE INVENTION
[0003] Optical fiber has nearly unlimited bandwidth, yielding 20 to
50 times more bandwidth than copper cable. Optical fiber is now the
solution of choice at the wide area network (WAN) level.
Implementation of fiber optic communication at the local area
network (LAN) level will enable users to break current bottlenecks
in the last mile of information transfer. A further attraction of
fiber-optic technology is its scalability. Most current fiber LAN
products are Ethernet-based in a range of 100-Mbit/second up to
1-Gbit/s. Fiber optical communication is easily scalable up to 10
Gbits/s, and several equipment vendors have announced fiber-optic
links that can support transmission up to 1 terabit/s utilizing
more than 128 DWDM.
[0004] Optical telecommunications networks require significantly
increased bandwidth to handle current and projected communications
traffic. Most optical networks use time division multiplexing (TDM)
with a single laser transmitter as a means of combining many
separate transmissions, allowing data rates of up to 10 Gbits/sec.
The current market trend is toward systems that use many individual
transmitters, each of a different wavelength to increase channel
capacity (an approach known as wavelength division multiplexing,
WDM). For example, a transmitter consisting of 8 distinct
wavelengths provides 8.times. the capacity of a single channel
network (e.g. 80 Gbits/sec). Furthermore, WDM systems are scalable
to 16, 32, 64, 128 etc. distinct channels, and make the most
efficient use of the extremely high bandwidth of optical fiber
communication networks. Therefore, what is needed is a system for
providing a stable, rapidly reconfigurable, multi-channel source
for WDM and DWDM (dense-WDM) optical communications networks which
can meet anticipated bandwidth demand.
[0005] A fiber optic communication link provides a virtually noise
free medium for transporting complex signals without distortion and
interference. Fiber optic cable has losses as low as 1-2 dB per
kilometer, which is much lower than the 1-2 dB per 100 ft. for
coaxial cable. Due to high frequency of laser light, fiber optic
cables provide a bandwidth in which many channels of information
can be transported across a single fiber cable. The laser diodes'
high efficiency, small size, high reliability, and low cost ($5-$10
for infrared, $40-$50 for visible light) make them the ideal choice
for communication devices. However, other components in these
devices escalate the cost of present optical DWDM transmitter
devices.
[0006] Low cost optical DWDM optical communication devices are not
available in the market due to the current trend toward integrating
costly solid state semiconductor lasers and associated optical
structures, and because optical solid state control structures are
new and changing rapidly. The latest integrated optical components
are created lithographically on a semiconductor chip substrate or
on glass. However, it is very difficult and expensive to make
multiple lasers lithographically (in VLSI) which are locked to a
reference grid.
[0007] Therefore, what is needed is a way to reduce the cost of
building state of the art DWDM optical communication devices. What
is also needed is a method for shifting the burden of providing a
high performance DWDM optical communication system from costly
precision optical components to inexpensive solid state control
structures. It would be desirable to achieve a high performance
DWDM transmitter using low cost optical components having a wider
range of tolerances than is presently possible
[0008] Conventional WDM/DWDM sources lack stabilized frequency
referencing and rapid reconfigurability in the event one or more
lasers fail or are disrupted. Fabry-Perot interferometers (FPIs)
have been used to attempt to lock a laser to a stable frequency.
Fabry-Perot Interferometers used as scanning interferometers can
sense extremely small wavelength shifts when piezo-electric
actuators (PZTs) are used for tuning the multi-pass dual mirror
optical cavity. However, these interferometers require adequate
wavelength references for long-term stability.
[0009] Molecular absorption line sources from various gases have
been tried, in particular acetylene, and offer a potential medium
for multi wavelength referencing. However, their unevenly spaced
absorption lines and absorptive operation makes them difficult to
tune for FPI-based spectral applications. FPIs with
narrowly--spaced uniform transmission peaks(.about.100 GHz) have
often been considered for an absolute frequency reference when
locked to atomic or molecular absorption lines. Glance, B. S. et
al. (1988), "Densely Spaced FDM coherent Star Network with Optical
Signals Confined to Equally Spaced Frequencies," IEEE J. Lightwave
Technol. LT-6:1770-1781. and others. While there have been some
applications for stabilizing laser arrays, such methods are
restricted in wavelength positioning and require active feedback
control in conjunction with costly precision optical structures,
thereby driving up costs.
[0010] Fiber Bragg gratings (FBGs) can produce a narrow band
response around a single wavelength. However, their narrow response
band and over wavelength span poses limitations. Some of the
current alternatives for optical transmitters provide only partial
solutions. Examples of some conventional solutions and their
disadvantages are described below. Most describe an optical
reference source. None use a gas spectral line reference
source.
[0011] U.S. Pat. No. 5,892,582 discloses a fiber Bragg grating
(FBG) source which provides spectral output at a selected
wavelength within a wavelength range. Thus, the FBG is used to
provide a reference wherein the spectral output of the FBG marks a
peak of a comb identifying its wavelength. The FBG comb is used as
the reference frequency source. This approach is taken because
atomic or molecular spectral lines are deemed unsuitable for the
purpose of providing a stable reference source due to unevenly
spaced absorption lines, and therefore are too restrictive in
wavelength positioning. While this is useful in identifying and
measuring wave-lengths of radiation from optical sources, it does
nothing to provide an inexpensive multi channel, reconfigurable
optical transmitter.
[0012] U.S. Pat. No. 5,646,762 discloses another conventional
approach to establishing a reference source comb using a voltage
source connected to a detector and tunable etalon comb of
frequencies. A digital processor connected to a photodetector and
to the voltage source controls the tunable etalon. The digital
processor further contains memory for storing tuning voltages for
wavelengths and for storing tuning voltages for temperatures.
However, this provides a costly partial approach limited to
providing only a comb of uniformly spaced optical channels from an
already presumed stable input reference frequency source.
Furthermore, there is no provision for establishing a stable
reference frequency source for the etalon comb, or to provide
stability for multiple channels or reconfiguration of channels.
[0013] U.S. Pat. No. 5,949,580 discloses a controllable light
amplitude divider for dividing light at a particular wavelength
into two portions, which together represent the amplitude of the
input light. The arrangement can be cascaded in a manner which
operates as a multiplexer or demultiplexer. The main thrust of the
'580 patent is to provide a fast multiplexer and demultiplexer
using an etalon. This arrangement requires costly precision optical
components and is not suitable for a high performance
reconfigurable DWDM application. Furthermore, the '580 patent is
not a solution for an optical transmitter, which has many other
functions, and the multiplexer/demultiplexer scheme is not
necessary for a most optical transmitters
[0014] U.S. Pat. No. 4,813,756 discloses a device for
interconnecting or for linking two optical fibers comprising a
mechanically rotatable etalon arrangement. The '756 patent
principally creates a device for interconnecting two optical
fibers. While multiple etalons are used in various configurations,
this does not provide a full multichannel optical transmitter, but
rather a partial and very expensive way to connect DWDM fiber. '756
also claims optical channel selection filter mounted for coupling
two single mode optical fibers.
[0015] U.S. Pat. No. 4,707,061 discloses an optical communications
system using a resonant cavity for supporting a set of resonant
modes and introducing predetermined reference wavelengths. A means
for controlling the resonant cavity tunes one resonant mode to the
wavelength of a fixed wavelength light source. Semiconductor laser
sources are used in conjunction with resonant cavities which must
be present at the transmitter as well as the receiver, thereby
adding expense and complexity to an all ready cumbersome scheme to
communicate over an optic network.
[0016] U.S. Pat. No. 5,673,129 discloses a plurality of optical
transmitters for outputting optical signals, at least one optical
wavelength selector communicating with the optical transmission.
The wavelength selector includes a Bragg grating member with a
wavelength band of high reflectivity. The wavelength band of high
reflectivity for each Bragg grating member corresponds to an
optical channel output. The '129 patent provides a closed loop
optical system and uses semiconductor lasers, thereby resulting in
a costly approach and complex system using gain bands, amplifier
stages and pumps.
[0017] U.S. Pat. No. 6,014,237 discloses a multi wavelength
mode-locked (MWML) laser source including a semiconductor optical
amplifier (SOA) disposed in a cavity of the MWML laser source. The
SOA is actively driven by a radio frequency (RF) signal and emits
periodic pulses within a plurality of discrete wavelength bands
simultaneously. The semiconductor optical amplifier and radio
frequency (RF) signal driver make this a relatively expensive
solution. The '237 patent also requires that input signals be
multiplexed by a high speed electronic time domain multiplexer
(ETDM) to a higher bit-rate electronic data stream for coding by an
optical modulator in the optical pulse stream emitted by the
MWML-DWDM. This results in complex design and interfacing
requirements which are not suited to a practical, low cost
implementation.
[0018] U.S. Pat. No. 6,044,189 discloses a temperature compensating
fiber Bragg grating contained in an optical fiber. The '189 patent
is concerned with the improvement of control of a fiber Bragg
grating, only one possible component of a DWDM system.
[0019] U.S. Pat. No. 6,028,881 discloses a pump source tunable
among a plurality of pumping wavelengths; a plurality of waveguide
lasers responsive to respective pumping wavelengths for emitting
light. The '881 patent introduces a method of combining solid state
waveguide lasers with semiconductor lasers and enhancing electrical
tunability. Components include intra-cavity pumping and pump
reflectors. These require expensive integrated optics to
manufacture.
[0020] U.S. Pat. No. 5,953,139 discloses an analog light wave
communication system having at least two optical transmitters. The
first WDM receives optical information signals from the optical
transmitters and multiplexes the optical information signals to a
composite optical signal at an output. Each input of the WDM
comprises at least one optical resonant cavity; an oscillator
circuit providing a single tone modulation signal and a phase
modulator having an optical input coupled to the output of the WDM.
The single tone modulation signal drives a composite optical signal
which is too restrictive for most optical transmitter uses.
[0021] Conventional WDM/DWDM sources fail to provide the stabilized
spectral frequency referencing or the rapid optical channel
reconfigurability necessary for a high performance, low cost
optical DWDM system. The design of conventional optical
transmitters is directed toward the use of integrated optics
wherein all optical components are created lithographically on a
semiconductor chip substrate or on glass. These can be difficult
and expensive to manufacture, as multiple lasers must be locked to
a reference grid lithographically, using VLSI techniques.
[0022] Conventional DWDM approaches are not cost-competitive in the
metro markets where they must compete with lower cost all
electronic products which are more dynamic and operate at lower
bandwidth demands. Most conventional DWDM systems operate at OC-48.
As demand for higher bandwidth increases, these data rate demands
will migrate to Metro markets. Therefore, what is needed are low
cost optical devices which are dynamic from a standpoint of
configurability and can meet higher bandwidth demands.
[0023] DWDM optical devices demand very large bandwidths. A failure
of one optical channel affects multiple protocol stack layers and
thus large numbers of users. Therefore, in the event of a failure
at the channel level, restoration must occur at multiple stack
levels. The speed of restoration is critical. Bandwidth reservation
is an option for these slower layers, but this requires that excess
idle capacity must be built in at the optical device level.
[0024] In the protocol layer scheme for optical channels, the
Internet Protocol (IP) layer is carried by the ATM layer below. IP
over DWDM presents topology node architecture issues. There is a
virtual mapping between the physical and logical topology of IP
over ATM, which leads to scaling challenges. One of the solutions
is to make every switch into a router. The current cost of optical
routers makes this a very expensive solution. What is needed are
inexpensive optical switches and routers.
[0025] In high reliability networks, reconfigurability options are
necessary. However, conventional designs are very expensive. For
example, each channel has a fixed-frequency laser, in addition to
the laser that carries the data, the "active" laser. The fixed
frequency laser is a spare, identical to the active laser that
initially carries the modulated signal. The spare laser must be
prepared to carry the modulated signal on the active laser's
specific carrier frequency. Both lasers are generally physically
located in the same rack.
[0026] Thus, in the event of a laser failure, the spare
fixed-frequency laser takes functional control on only the
designated frequency and signal. However, this approach forces the
manufacturer to fabricate a redundant number of lasers, at least
one spare for each active laser, to ensure a high reliability
device. Therefore, what is needed is an optical channel device
which requires fewer lasers, lower production cost and provides
high reliability. What is also needed is a DWDM transmitter which
is reconfigurable, not only with respect to lasers, but also across
channels.
BRIEF SUMMARY OF THE INVENTION
[0027] In order to overcome the foregoing deficiencies in
conventional optical DWDM systems, an aspect of the invention
provides a high performance reconfigurable DWDM transmitter
incorporating low cost discrete optical components which can be
placed in v grooves etched in a silicon optical micro-board or the
like, keeping costs of manufacturing low. The lasers are packaged
in modules based on the technology of meso scale optics. The
physical size of a multi channel module is no bigger than a
conventional single laser module.
[0028] In another aspect of the invention, direct wavelength
monitoring is achieved by using a wavelength modulation locking
technique applied independently to a gas absorption line and to
etalon fringes. The frequency stability and resolution achieved
thereby make it possible to pack channels closely and achieve
spacing up to the modulation limit, filling the available
bandwidth. This now enables high density DWDM to populate as many
channels as desired to the modulation limit.
[0029] An aspect of the invention uses a set of n lasers and k
spare sources, wherein each laser is actively locked to a set of
equally spaced wavelengths according to the ITU frequency grid, and
simultaneously to a stable spectral reference wavelength. The set
of equally spaced frequencies is generated by an etalon, acting as
a frequency comb generator. The absolute wavelength standard is
provided by a gas absorption cell. The wavelength of each channel
can be changed on a millisecond (msec) time scale under
microprocessor control in the event that any channel should fail,
thereby enabling substantially instantaneous reconfigurability.
[0030] According to this aspect of the invention, a separate (fixed
frequency) spare laser is not needed for each active (fixed
frequency) laser. The invention enables a single laser to be used
as a substitute for a number of fixed-frequency lasers, and a
number of fixed-frequency spare lasers as well.
[0031] In addition, any "active" laser (i.e. a laser already
assigned to a particular channel, not a spare) can be reassigned to
a different channel, if necessary, which further improves network
reliability. If all spare lasers within a module should fail, and
an active laser at the most valuable channel fails as well, the
system still can carry the traffic over the most valuable channel
by reassigning a laser from one of a lesser used or less valuable
channels to the most valuable channel, until physical replacement
of lasers is made. The modulation signal is switched electronically
to modulate the spare laser instead of the laser that failed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] These and other features and advantages of the present
invention will become more apparent to those skilled in the art
from the following detailed description in conjunction with the
appended drawings in which:
[0033] FIG. 1 is a diagram showing basic components of a frequency
stabilized laser for one channel according to an aspect of the
invention.
[0034] FIG. 2 is a schematic diagram showing a multi-channel
re-configurable DWDM transmitter according to an aspect of the
invention.
[0035] FIG. 3 is a diagram showing a simple configuration of a
multi-channel reconfigurable transmitter according to an aspect of
the invention.
[0036] FIG. 4 is a graphical display illustrating the output
produced by a multi-channel reconfigurable transmitter according to
an aspect of the invention.
[0037] FIG. 5 is a block diagram illustrating operative connection
of components according to an aspect of the invention.
[0038] FIG. 6 is a drawing showing an example implementation of a
low cost multichannel re-configurable transmitter according to an
aspect of the invention.
[0039] FIG. 7 is a flow chart of the control logic which enables
reconfigurability according to an aspect of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] FIG. 1 shows the basic components of a frequency stabilized
diode laser wherein stabilization occurs through a spectral line.
An overview of a simple mode of operation for one channel starts
with a laser source 102, in this case a tunable diode laser. The
laser light is directed into a collimating lens 106 and into a gas
absorption cell 108, comprising a chamber holding gas of known
spectral line characteristics. This gas is used to lock onto a
reference frequency (wavelength) using wavelength modulation
spectroscopy (WMS), also known as derivative spectroscopy. A beam
splitter 110 can be used to forward a part of the beam through a
lens 131 and to a photodetector 132 which enables the laser
frequency to be compared and tuned to a spectral line frequency of
the known gas in the reference gas cell 108.
[0041] This part of the process is designed to be a self-tuning
procedure: the diode laser 102 is turned on, and locked to a gas
absorption line from gas absorption cell 108 using a standard WMS
technique. Detector 132 is used to monitor light transmission
through the cell 108, and a reference frequency control loop 528
(FIG. 5) includes a microprocessor (530 shown in FIG. 5) for
feedback in tuning laser 102 to the reference frequency. The gas
absorption cell 108 contains a suitable gas, for example acetylene,
which has numerous well-known narrow absorption lines in the 1550 m
optical communications window. Other gases may be used for other
windows as needed. Wavelength modulation spectroscopy is
implemented with diode lasers having the advantages of small size,
non-intrusiveness, speed, and ease of use.
[0042] Next, the reference frequency stabilized diode laser is
frequency locked to a sequential fringe frequency of a
(temperature) tunable stable Fabry-Perot etalon 112. The beam
proceeds through to the etalon 112 which is tuned to a position
where an etalon transmission fringe coincides with the laser
wavelength locked to the reference spectral line derived from gas
absorption cell 108. Passing through a lens 113 and into Detector 2
114, the transmission is monitored for tuning. The free-spectral
range of the tunable etalon 112 is chosen to be equal to the
required ITU grid spacing (e.g. 100 GHz for the current ITU
standard), thus the etalon 112 transmission spectrum consists of a
series of regularly spaced transmission peaks. Each of these
transmission peaks will constitute a channel when fully initialize,
tuned and configured.
[0043] FIG. 2 is a block diagram showing a multi-channel
re-configurable DWDM transmitter. The channel shown above in FIG. 1
is just one of n channels in a bank. Referring to FIG. 2, one
common gas cell 208 comprising a gas of a known spectral line
frequency and one etalon, also called the grid generator 212
establish a stable frequency grid for a plurality (n) number of
independent active lasers 202 and therefore (n) channels. All
lasers 202 have the same reference data structures since they all
lapse through the same gas cell 208 and are locked onto successive
fringes produced by common etalon 212. The reference frequency
control loop 228 enables initialization and tuning of each laser in
the bank to the reference spectral frequency derived from gas cell
208 in a well known manner. The grid positioning control loop 226
sequentially tunes each independent active laser 202 in the bank to
one of the fringe frequencies of grid generator 212 using the grid
positioning control loop 226 for stabilizing and controlling the
fringe count and fringe frequency tuning.
[0044] Direct monitoring of output wavelength within grid
positioning control loop 226 and reference frequency control loop
228 is used to stabilize lasers 202. Direct wavelength monitoring
is achieved by using well known laser spectroscopy wavelength
modulation locking techniques applied independently to a gas
absorption line derived from gas cell 208 and to etalon fringes
derived from the grid generator. This is far superior to using
"blind" control loops to stabilize a laser's temperature and
current 212 as is done in many conventional DWDM systems. Both
control loops 226 and 228 are microprocessor controlled and provide
two essential stages in the system initialization to lock and
maintain lasers at stable frequencies. Thus, the initialization
provides self-stabilizing and self-calibrating optical
channels.
[0045] These two processes are under the control of two independent
frequency control loops, the first one tunes the laser and controls
the laser injection current; the second one tunes the etalon by
controlling its temperature. The two control loops could be merged
into the same electronic digital control circuit as well. The two
independent control loops 226, 228 also enable independent control
of a plurality (n) lasers 202 to be connected in parallel in a
bank. This makes possible a configurable feature of the optical
transmitter, which is covered in detail below.
[0046] Reference positions are now firmly defined and each laser
202 in the bank is locked to a fringe frequency. These dense
wavelengths (refer to FIG. 4) are then multiplexed into the optical
communication fiber 224. That is, the fiber optic cable is
physically hardwired to the output of the transmitter module. This
is an advantage in reconfiguration in that the optical paths of
alternate or spare lasers are always connected to the output of
transmitter module 225. The optical fiber output carries different
wavelength light 232 at uniform channel spacing 230.
[0047] FIG. 3 is a diagram showing another configuration of a
multi-channel reconfigurable transmitter according to an aspect of
the invention. Referring to FIG. 3, a plurality (n) of laser diodes
302 are connected in parallel in a bank. Although many types of
laser sources can be used, the lasers are preferably Distributed
Bragg Reflector (DBR) lasers with a characteristic tunability of
10-30 nanometers in a single mode output or narrow band output.
Such lasers are typically low cost and tunable over a wide range in
comparison to other lasers. DBR lasers offer the widest wavelength
coverage. Distributed Feedback (DFB) lasers are acceptable as an
alternative if each laser is required to a small wavelength range,
i.e., a small number of channels. The laser beams are collimated in
corresponding GRIN lenses 306 and directed into the gas absorption
cell 308. A common gas absorption cell 308 provides a molecular
absorption line stabilized to a molecular transition, which is the
reference frequency for all lasers. The reference stabilized beams
then pass to a first set of photodetectors 332 which enable
frequencies to be mapped to voltages which provide the feedback
loop means for tuning the lasers to the stabilized molecular
absorption line. Upon initialization, the lasers 302 become locked
to an absolute reference frequency.
[0048] The stable reference tuned lasers are further directed into
the etalon 312 and on to the second set of detectors 314, where
they are each tuned to an etalon comb frequency fringe. The etalon
312 is a solid etalon comprising a glass plate with coated surfaces
and is temperature tunable, as is well known. The output
transmission spectrum from the etalon 312, that is, the distance
between peaks and valleys in the corresponding wave form produced
by the etalon, depends on the index of refraction, temperature and
thickness of the etalon. The detectors 314 and 332 are photo diodes
having a spectral range of (for example) 155 nanometers and are
gallium arsenide photo diodes. However, other lasers with suitable
characteristics may be used. Examples are: diode lasers, gas
lasers, chemical lasers, masers, or the like. The feedback loops
are shown in FIG. 2. The bank of tuned laser diodes 302 and their
corresponding channels are optically coupled to the fiber optic
transmission line 318: The etalon 312 (e.g. 10-100 GHz free
spectral range) serves as the generator of an optical frequency
comb with e.g. 10-100 GHz separation. Each of the lasers 302 is
tuned to one of the fringes produced by the etalon 312, and the
etalon grid is locked there by its respective frequency control
loop (228 in FIG. 2).
[0049] Each laser 302 is passively stabilized so that, at the
starting point, it finds itself within the capture range of the
loop, centered around a reference wavelength .lambda..sub.0. Each
laser 302 is then brought to a wavelength .lambda..sub.n by either
temperature or injection current tuning, or both. Starting from the
reference wavelength .lambda..sub.0, each laser 302 is in the next
step brought to any position on the etalon grid (400 in FIG. 4)
produced by etalon 312 by tuning its wavelength in the selected
direction (direction from the reference frequency fringe to a
selected fringe), and monitoring the laser output as it passes
through etalon fringes produced by etalon 312. The position of the
laser's frequency on the etalon grid 400 is determined by counting
the fringes. In this way, each laser 302 can be tuned to any of n
wavelengths ("channels"). The channels are independently modulated
(data impressed on the laser carrier) with either an external or an
internal (electro-absorption) modulator, integrated in the diode
laser chip.
[0050] The device operates and initially tunes itself under
standard microprocessor control techniques which are well known,
which is simple and inexpensive. The entire system operates under
the microprocessor control, which performs the auto-calibration
procedure described above. In this way, an aspect of the invention
provides a multi-wavelength source for DWDM that is
self-calibrating, self stabilizing and self tunable. This aspect of
the invention provides an advantage over conventional systems in
that if one of the lasers should fail, resulting in a temporary
channel loss, an alternate laser can be tuned to the desired
wavelength.
[0051] FIG. 4 is a graphical representation of a wave form
illustrating the output produced by a multi-channel re-configurable
transmitter. The transmission spectrum is shown as optical power
401 as a function of wavelength 408. The reference source
wavelength 404 provides an anchor for a uniform spacing output
grid, such as ITU grid 410. (ITU grid 410 is an etalon grid with a
specific channel spacing.) For example, the grid 410 consists of
frequencies with channel spacings 402 which comply with ITU
standards.
[0052] An aspect of the invention is shown in FIG. 2 wherein laser
tuning is accomplished using two independent control loops, the
first control loop capable of locking an entire comb. The second
loop locks any laser in a plurality of lasers to a given point on
the comb. This thereby provides superior frequency stability and
resolution. The frequency stability and resolution achieved with
this aspect of the invention makes it possible to pack the channels
closely and achieves stability and spacing reduction possible down
to the modulation limit, thereby filling the available bandwidth.
This now enables high density DWDM to populate as many channels as
desired to the modulation limit.
[0053] The ability to populate as many channels as desired up to
the modulation frequency may provide enabling technology for a low
cost switchless network; in particular, a high density metropolitan
switchless network.
[0054] FIG. 5 is a block diagram illustrating the operation of the
control of the laser path through its optical components. It will
be appreciated that the optical path is decoupled from its
electronic control elements. This decoupling enables production of
a high performance, fully reconfigurable DWDM transmitter through
the use of inexpensive electronic components. It shifts the burden
of providing high density DWDM from costly precision optical
components integrated in VLSI to inexpensive electronic control
circuitry. In FIG. 5, the control elements and circuits are
depicted in broken lines and the optical paths are drawn in solid
lines.
[0055] In operation, when laser 502 is turned on, first detector
532 looks at output using standard wavelength modulation
spectroscopy. Laser 502 is tuned under microprocessor control in a
known manner over a wavelength range in which it finds the
absorption line of the gas in the gas absorption cell. Some of the
beam can be diverted through the use of a beam splitter 510 and
directed to a first detector 532.
[0056] First detector 532 provides control information to the
reference frequency control loop which is applied in real time to
keep the laser 502 wavelength locked to the absorption line. To
accomplish this, first detector 532 measures the intensity of the
beam, detects the peak of absorption line, a control mechanism
provides information on laser wavelength coincident with the
absorption line, determines whether laser wavelength is coincident
with the absorption line or not then makes a correction signal to
502 corresponding laser.
[0057] Correction is applied so the first detector 532 always sees
whether the laser wavelength is coincident with gas absorption
line. Once the microprocessor/controller 530 recognizes the laser
wavelength is stable and locked to a reference source 508,
microprocessor/controller 530 tunes the etalon 512 by changing the
temperature and monitoring the output of second detector 514. The
detector 514 is maximum amplitude seeking and detects when a local
peak or comb tooth wavelength is found. The microprocessor 530
receives the signal from the second detector 514. Etalon 512 has a
transmission spectrum consisting of a fringe or comb. As
microprocessor/controller 530 tunes the etalon, it moves the comb
in wavelength space and makes one fringe of the comb coincident
with laser wavelength. Since the reference frequency is known from
the reference frequency control loop 528 and coincident with one of
the comb frequencies, microprocessor/controller 530 can set any
laser frequency by counting the fringes while changing the
wavelength of the tunable laser 502 by changing the injection
current to the laser 502 or the etalon temperature 532 or both.
[0058] In this fashion, microprocessor/controller 530 can tune any
of the lasers 502 to any of the channel frequencies 406. For
example, to tune laser 502 to fringe 406, microprocessor/controller
530 addresses laser 502 by first tuning to absorption line. Once
the reference point is known on the etalon grid 512,
microprocessor/controller 530 counts the fringes while changing the
wavelength of the laser by changing the injection current or the
etalon temperature or both. The second detector 514 provides a
correction signal to the etalon stabilization loop 526 which locks
the tunable etalon to the wavelength of the laser wavelength
desired. By reading the output of the second detector 514,
microprocessor/controller 530 can use this information to make
light to go through tunable etalon fringes and lock on to any
desired frequency.
[0059] The foregoing process establishes a reference grid with
stable defined wavelengths at any point, which can be described as
a reference wavelength+/-integer number of free spectral range
spaces. This equals distance between fringes or channel spacing and
is shown in FIG. 4.
[0060] The tuned laser is directed into the output coupling 518 by
optical fiber connection which is then multiplexed into the optical
fiber transmitter output 522. The optical path is preserved by
having all lasers multiplexed into the communication fiber 522 at
all times. That is, individual channels 518 are physically
hardwired (optically connected) to the output fiber 522. All
lasers, even spares or alternates 504 not currently active, are
connected through the use of couplers, to the output of the
transmitter multiplexer 522 module, and the lasers go online
"active" as needed during reconfiguration. Thus, an aspect of the
invention provides the capability to reconfigure individual tunable
lasers to maintain optical network integrity.
[0061] Each of the lasers 502 is tunable over any number of
channels (e.g. 10-20). Therefore, lasers 502 are wavelength
selectable, as well as wavelength re-configurable. Since each laser
502 in the bank can be tuned to work on a multiplicity of
wavelengths or "channels" and we also have a dynamically selectable
wavelength multi-channel transmitter.
[0062] The microprocessor/controller 530 manages a lookup table.
The look up table is produced during the initial calibration
process and stored in the memory of a controller 530. The
parameters in the lookup table include individual laser operating
points, and parameters, such as injection currents, Bragg reflector
control currents, substrate temperatures and individual wavelength
modulation signal output addresses, comb fringe count and other
data needed to reconfigure (under software control) laser operation
in the event of a channel failure. Replacing a laser that has
failed without manual intervention by microprocessor/controller 530
which incorporates a look up table comprising operational
parameters of all currently active lasers. When a failure is
sensed, microprocessor/controller 530 elects an alternate laser,
initializes the new laser to the failed channel specifications,
attaches the failed laser's modulated signal to the new laser
channel and continues operation within 20-50 milliseconds or less.
This operation is seamless and transparent to a user due to the
tolerances in communication protocol.
[0063] Upon an active laser 502 failure, the detector 514 senses
the failure of the down fringe, and notifies the
microprocessor/controller 530 of the failed comb number. Other
convenient ways of sensing laser failure are well known and also
may be employed. The microprocessor/controller 530 initializes an
alternate laser 504 to the properties of the failed laser 502 found
in the lookup table. The spare 504 is tuned to the wavelength of
the failed channel 502 and the modulated signal is vectored to the
spare 504 laser for transport.
[0064] The foregoing aspects of the invention provides a
reconfigurable transmitter, whereby any tunable spare laser can be
brought on line substantially instantaneously and automatically,
upon an active laser failure, regardless of the failed laser's
carrier wavelength. In addition, if the spare fails immediately or
before another active laser fails, another spare can be brought on
line to substitute for the spare, thus providing an additional back
up. This chain of redundancy can progress to exhaust all spares,
thereby providing an extremely high reliability and reconfigurable
device, which is not possible with conventional DWDM transmitters.
With the exhaustion of all spares, active channel lasers can be
substituted for the highest demand or load carrying channels to
continue the operation. This is not currently possible with the
optical transmitters. This aspect of the invention provides a
method requiring far fewer spare lasers, while still increasing
transmitter and therefore network reliability, simply by virtue of
the reconfigurability for any number of fixed-frequency lasers.
[0065] FIG. 6 is a drawing of a low cost implementation of
multi-channel re-configurable transmitter according to an aspect.
of the invention This aspect of the invention provides a modular,
meso scale approach which decouples the performance of an optical
DWDM transmitter from the performance of its optical components. It
eliminates the need for costly lithographic integration of optical
components, while at the same time increasing channel capacity to
the modulation limit as set forth above.
[0066] FIG. 6 shows a low cost meso scale implementation which uses
discrete optical elements mounted on v grooves etched in a silicon
642 optical micro-board. Laser diode 602 is disposed between lens
606 and lens 620 as shown. Gas cell 608 and etalon plate 614 are
all discrete, components mounted in the v grooves 642. This is in
direct contrast to conventional integrated optics, wherein all
optical components are created lithographically on a semiconductor
chip substrate or on glass. Discrete optical components according
to an aspect of the invention enable system to be modularized.
[0067] This embodiment eliminates the need for difficult to
manufacture and expensive to make multiple lasers which are locked
to a reference grid lithographically, using VLSI techniques. An
aspect of the invention provides a solution by using low cost
discrete optical components which can be placed in v grooves etched
in silicon optical micro board. Lasers 602 are packaged in modules
based on the technology of meso scale optics. The physical size of
a multi channel module is not bigger than the current single laser
module, on the order of millimeters.
[0068] Referring to FIG. 6, a DBR laser 602 is mounted on a silicon
optical microboard 642. A DBR laser is low cost relative to a fixed
frequency distributed feedback laser DFB, currently used. The
entire assembly is fixed to a ceramic base 640 or other suitable
substrate.
[0069] The individual signal input circuitry converts the digital
signal to the modulated analog signal via D/A converter. The
intensity of the DBR laser 602 diode is modulated by adding AC
current to the laser diodes' bias current. Many other means to do
this are possible and well known to those skilled in the art. This
still enables precise and simple control of the quiescent current.
Bias current is necessary to keep the laser diode operating in a
region where the relationship between the AC current and the change
in intensity of the output is semi-linear.
[0070] The optical section transmits the signal from the lasers to
the detectors through a Gradient Index (GRIN) 606. The GRIN lenses
606 and 620 are mounted in the grooves 642 on a silicon optical
micro board, or other suitable substrate for mounting modular
optical components.
[0071] A GRIN 620 is also used to introduce the laser light into
the fiber on the output end. The light propagates in the fiber
where signals multiplexed onto the output transmitter fiber. Since
the beam of a diode laser diverges severely from its source, a lens
must be used to focus the light onto the fiber 622. To minimize the
loss between the photo diode and the fiber a GRIN lens is used.
GRIN lenses 620 collimate a beam with minimal loss and aberration
because their index of refraction varies radially due to dopants
infused into the glass of the lens. By using a GRIN lens with an
uneven fractional pitch length, the photo light can be focused on
the tip of the fiber.
[0072] The light then travels from the GRIN 606 through the gas
absorption cell 608 and impinging on the photodiode 632. The signal
from a photodiode 632 (in the absorption case) is fed to the
reference control loop for tuning the laser to the gas spectral
line frequency. This signal is used in real time to lock the light
to the reference frequency. This allows the remaining light
traveling through the etalon 612 impinging on the second detector
614 to have a base reference frequency from which to tune to a comb
frequency. The signal from the photodiode 614 is fed to the etalon
grid control loop for tuning to an individual comb frequency.
[0073] In another aspect of the invention, by using active control,
high performance DWDM is achieved with low cost optical components
of much looser specifications than is presently possible. In other
words, this aspect of the invention shifts the burden of
maintaining high performance from expensive optical components to
inexpensive electronic control components.
[0074] FIG. 7 shows the control logic steps in the reconfiguring of
a failed optical channel. A laser failure signal starts the
reconfiguration processes 702. The network has 20-50 milliseconds
to activate another laser to take over the functioning of the
failed laser for this to be seamless to light router protocol.
[0075] For example, laser no. 3 operates at 1552 nm and drops out,
thereby producing a failure signal 702. The micro controller now
must go to a spare or alternate laser and obtain all characteristic
parameters from the incorporated lookup table. The lookup table
contains all the parameters for the failed laser 705 which are to
be used in the spare laser.
[0076] All the necessary parameters for the failed channel are
stored in the controller lookup tables 705. These parameters
contain, among other data, individual laser operating points,
injection currents, Bragg reflector control currents, substrate
temperatures and individual wavelength modulation signal output
addresses, comb fringe count.
[0077] The lookup table provides the injection current and the
specific substrate temperature. The micro controller then selects
an available spare laser or less loaded alternate laser 708. There
are typically two spare lasers for every eight active lasers.
However, any convenient ratio of spares to active lasers may be
employed. To tune the spare/alternate laser to a reference spectral
line frequency 711, the micro controller applies an injection
current of about 35 mA to the active region and any correction, as
the laser control loop is open while tuning.
[0078] Next, the micro controller tunes the spare/alternate laser
to fringe channel n of failed laser m 714. There are several ways
to do this and a combination may be the best. Micro controller
counts the fringes while changing the wavelength of the laser by
changing the injection current or the temperature or both. The
micro controller increases the injection current on the selected
alternate/spare laser and monitors the second detector to see if
signal of the selected laser is coincident with fringe count of the
failed laser. The micro controller keeps count of the fringes as it
increments to match the lookup table fringe number of the failed
laser.
[0079] Another method may simply use the Bragg reflector control
current from the lookup table to tune to the desired fringe, for
example, 25 ma to the Bragg region. When the micro controller has
tuned the spare to the failed laser fringe frequency, it has
established a carrier for the channel.
[0080] The modulated signal must then be applied to the spare laser
carrier channel. This can also be done in several ways. A simple
way is to keep the modulated signal output circuit address in the
lookup table for each laser and transfer this information to the
spare laser upon laser failure. The laser modulation signal is
switched electronically to modulate the spare laser instead of the
laser that failed. The modulation may be direct, or by means of a
built in electro absorption modulator. If the modulator is an
external device, nothing needs to be switched, the external
modulator does not care where the carrier light comes from.
[0081] As is well known, quality control is implemented to insure
that the reconfigured channel is indeed carrying the interrupted
signal 718. If the modulated signal is back online, a have a valid
reconfiguration has been established. The foregoing steps are then
repeated with an alternate laser until the micro controller
reestablishes a reconfigured channel.
[0082] Conventional high speed Internet Protocol (IP) routing is
challenged by greatly increasing volumes of traffic. The capability
to re-configure channels rapidly and automatically in accordance
with the foregoing aspects of the invention, provides a basis for
the accommodation of advanced routing protocols such as resource
reservation RSVP or multicasting. MPLS (Multiple Protocol Lambda
Switching) over DWDM depends heavily on restoration through
redundancy. Thus, the foregoing aspects of the invention could be
applied to provide improved MPLS.
[0083] The frequency stability and resolution achieved by the
foregoing aspects of the invention make it possible to pack
communication channels closely and maintain stability and spacing
up to the modulation limit, thereby filling the available
bandwidth. This would enable a high density DWDM system to populate
as many channels as desired up to the modulation limit, and may
support a switchless optical network or enable the implementation
of fault tolerant optical routers.
[0084] While this invention has been described in connection with
what are presently considered to be the most practical and
preferred embodiments, it is to be understood that the invention is
not limited to the disclosed embodiments, but on the contrary, it
is intended to cover various modifications and equivalent
arrangements which are included within the spirit and scope of the
appended claims.
* * * * *