U.S. patent number RE42,368 [Application Number 12/816,084] was granted by the patent office on 2011-05-17 for reconfigurable optical add-drop multiplexers with servo control and dynamic spectral power management capabilities.
This patent grant is currently assigned to Capella Photonics, Inc.. Invention is credited to Tai Chen, Joseph E. Davis, Jeffrey P. Wilde.
United States Patent |
RE42,368 |
Chen , et al. |
May 17, 2011 |
**Please see images for:
( PTAB Trial Certificate ) ** |
Reconfigurable optical add-drop multiplexers with servo control and
dynamic spectral power management capabilities
Abstract
This invention provides a novel wavelength-separating-routing
(WSR) apparatus that uses a diffraction grating to separate a
multi-wavelength optical signal by wavelength into multiple
spectral channels, which are then focused onto an array of
corresponding channel micromirrors. The channel micromirrors are
individually controllable and continuously pivotable to reflect the
spectral channels into selected output ports. As such, the
inventive WSR apparatus is capable of routing the spectral channels
on a channel-by-channel basis and coupling any spectral channel
into any one of the output ports. The WSR apparatus of the present
invention may be further equipped with servo-control and spectral
power-management capabilities, thereby maintaining the coupling
efficiencies of the spectral channels into the output ports at
desired values. The WSR apparatus of the present invention can be
used to construct a novel class of dynamically reconfigurable
optical add-drop multiplexers (OADMs) for WDM optical networking
applications.
Inventors: |
Chen; Tai (San Jose, CA),
Wilde; Jeffrey P. (Morgan Hill, CA), Davis; Joseph E.
(Morgan Hill, CA) |
Assignee: |
Capella Photonics, Inc. (San
Jose, CA)
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Family
ID: |
44720436 |
Appl.
No.: |
12/816,084 |
Filed: |
June 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10005714 |
Nov 7, 2001 |
6687431 |
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09938426 |
Aug 23, 2001 |
6625346 |
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60277217 |
Mar 19, 2001 |
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Reissue of: |
10745364 |
Dec 22, 2003 |
6879750 |
Apr 12, 2005 |
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Current U.S.
Class: |
385/24; 385/37;
385/10; 398/83; 385/33 |
Current CPC
Class: |
G02B
6/29385 (20130101); G02B 6/29391 (20130101); G02B
6/3512 (20130101); G02B 6/2931 (20130101); G02B
6/29395 (20130101); G02B 6/29383 (20130101); G02B
6/29313 (20130101); G02B 6/3588 (20130101); G02B
6/34 (20130101); G02B 6/3586 (20130101); G02B
26/0833 (20130101); G02B 6/356 (20130101); G02B
6/3556 (20130101); G02B 6/32 (20130101); G02B
6/3592 (20130101) |
Current International
Class: |
G02B
6/28 (20060101); H04J 14/02 (20060101) |
Field of
Search: |
;385/24,11,10,37,34,33
;398/79,82,83,84,88,87 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Healy; Brian M
Attorney, Agent or Firm: Young; Barry N.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No.
10/005,714, filed Nov. 7, 2001 now U.S. Pat. No. 6,687,431, which
is a continuation of U.S. application Ser. No. 09/938,426, filed
Aug. 23, 2001, now U.S. Pat No. 6,625,346 which claims the benefit
of U.S. application Ser. No. 60/277,217, filed Mar. 19, 2001.
Claims
What is claimed is:
1. An optical add-drop apparatus comprising an input port for an
input multi-wavelength optical signal having first spectral
channels; one or more other ports for second spectral channels; an
output port for an output multi-wavelength optical signal; a
wavelength-selective device for spatially separating said spectral
channels; .[.and.]. a spatial array of beam-deflecting elements
positioned such that each element receives a corresponding one of
said spectral channels, each of said elements being individually
and continuously controllable .Iadd.in two dimensions .Iaddend.to
reflect its corresponding spectral channel to a selected one of
said ports .Iadd.and to control the power of the spectral channel
reflected to said selected port.Iaddend..
2. The optical add-drop apparatus of claim 1 further comprising a
control unit for controlling each of said beam-deflecting
elements.
3. The optical add-drop apparatus of claim 2, wherein the control
unit further comprises a servo-control assembly, including a
spectral monitor for monitoring power levels of selected ones of
said spectral channels, and a processing unit responsive to said
power levels for controlling said beam-deflecting elements.
4. The optical add-drop apparatus of claim 3, wherein said
servo-control assembly maintains said power levels at predetermined
values.
5. The optical add-drop apparatus of claim 2, wherein the control
unit controls said beam-deflecting elements to direct selected ones
of said first spectral channels to one or more of said second ports
to be dropped as second spectral channels from said output
multi-wavelength optical signal.
6. The optical add-drop apparatus of claim 2, wherein the control
unit controls said beam-deflecting elements to direct selected ones
of said second spectral channels to said output port to be added to
said output multi-wavelength optical signal.
7. The optical add-drop apparatus of claim 1 further comprising
alignment mirrors for adjusting alignment of said input and output
multi-wavelength optical signals and said second spectral channels
with said wavelength-selective device.
8. The optical add-drop apparatus of claim 7 further comprising
collimators associated with said alignment mirrors, and imaging
lenses in a telecentric arrangement with said alignment mirrors and
said collimators.
9. The optical add-drop apparatus of claim 1, wherein said
wavelength selective device further combines selected ones of said
spectral channels reflected from said beam-deflecting elements to
form said output multi-wavelength optical signal.
10. The optical add-drop apparatus of claim 1, wherein said one or
more other ports comprise an add port and a drop port for
respectively adding second and dropping first spectral
channels.
11. The optical add-drop apparatus of claim 1 further comprising a
beam-focuser for focusing said separated spectral channels onto
said beam deflecting elements.
12. The optical add-drop apparatus of claim 1, wherein said
wavelength-selective device comprises a device selected from the
group consisting of ruled diffraction gratings, holographic
diffraction gratings, echelle gratings, curved diffraction
gratings, and dispersing prisms.
13. The optical add-drop apparatus of claim 1, wherein said
beam-deflecting elements comprise micromachined mirrors.
14. The optical add-drop apparatus of claim 1, wherein said
beam-deflecting elements comprise reflective membranes.
15. An optical add-drop apparatus, comprising an input port for an
input multi-wavelength optical signal having multiple spectral
channels; an output port for an output multi-wavelength optical
signal; one or more drop ports for selected spectral channels
dropped from said multi-wavelength optical signal; a
wavelength-selective device for spatially separating said multiple
spectral channels; and a spatial array of beam-deflecting elements
positioned such that each element receives a corresponding one of
said spectral channels, each of said elements being individually
and continuously controllable .Iadd.in two dimensions .Iaddend.to
reflect its corresponding spectral channel to a selected one of
said ports .Iadd.and to control the power of the spectral channel
reflected to said selected port.Iaddend., whereby a subset of said
spectral channels is directed to said drop ports.
16. An optical add-drop apparatus, comprising an input port for an
input multi-wavelength optical signal having multiple spectral
channels; an output port for an output multi-wavelength optical
signal; one or more add ports for selected spectral channels to be
added to said output multi-wavelength optical signal; a
wavelength-selective device for reflecting said multiple and said
selected spectral channels; and a spatial array of beam-deflecting
elements positioned such that each element receives a corresponding
one of said spectral channels, each of said elements being
individually and continuously controllable .Iadd.in two dimensions
.Iaddend.to reflect its corresponding spectral channel to a
selected one of said ports .Iadd.and to control the power of the
spectral channel reflected to said selected port.Iaddend., whereby
said spectral channels from said add ports are selectively provided
to said output port.
17. A method of performing dynamic add and drop in a WDM optical
network, comprising separating an input multi-wavelength optical
signal into spectral channels; imaging each of said spectral
channels onto a corresponding beam-deflecting element; and
controlling dynamically and continuously said beam-deflecting
elements .Iadd.in two dimensions .Iaddend.so as to combine selected
ones of said spectral channels into an output multi-wavelength
optical signal .Iadd.and to control the power of the spectral
channels combined into said output multi-wavelength optical
signal.Iaddend..
18. The method of claim 17, wherein said selected ones of said
spectral channels comprises a subset of said spectral channels,
such that other non-selected ones of said spectral channels are
dropped from said output multi-wavelength optical signal.
19. The method of claim 18, wherein said controlling comprises
reflecting said non-selected ones of said spectral channels to one
or more drop ports.
20. The method of claim 17 further comprising imaging other
spectral channels onto other corresponding beam-deflecting
elements, and controlling dynamically and continuously said other
beam-deflecting elements so as to combine said other spectral
channels with said selected ones of said spectral channels into
said output multi-wavelength optical signal.
21. The method of claim 17, wherein said imaging comprises focusing
said spectral channels onto said beam-deflecting elements.
22. The method of claim 17 further comprising monitoring a power
level in one or more of said selected ones of said spectral
channels, and controlling an alignment between said input
multi-wavelength optical signal and corresponding beam-deflecting
elements in response to said monitoring.
Description
FIELD OF THE INVENTION
This invention relates generally to optical communication systems.
More specifically, it relates to a novel class of dynamically
reconfigurable optical add-drop multiplexers (OADMs) for wavelength
division multiplexed optical networking applications.
BACKGROUND
As fiber-optic communication networks rapidly spread into every
walk of modern life, there is a growing demand for optical
components and subsystems that enable the fiber-optic
communications networks to be increasingly scalable, versatile,
robust, and cost-effective.
Contemporary fiber-optic communications networks commonly employ
wavelength division multiplexing (WDM), for it allows multiple
information (or data) channels to be simultaneously transmitted on
a single optical fiber by using different wavelengths and thereby
significantly enhances the information bandwidth of the fiber. The
prevalence of WDM technology has made optical add-drop multiplexers
indispensable building blocks of modern fiber-optic communication
networks. An optical add-drop multiplexer (OADM) serves to
selectively remove (or drop) one or more wavelengths from a
multiplicity of wavelengths on an optical fiber, hence taking away
one or more data channels from the traffic stream on the fiber. It
further adds one or more wavelengths back onto the fiber, thereby
inserting new data channels in the same stream of traffic. As such,
an OADM makes it possible to launch and retrieve multiple data
channels (each characterized by a distinct wavelength) onto and
from an optical fiber respectively, without disrupting the overall
traffic flow along the fiber. Indeed, careful placement of the
OADMs can dramatically improve an optical communication network's
flexibility and robustness, while providing significant cost
advantages.
Conventional OADMs in the art typically employ
multiplexers/demultiplexers (e.g, waveguide grating routers or
arrayed-waveguide gratings), tunable filters, optical switches, and
optical circulators in a parallel or serial architecture to
accomplish the add and drop functions. In the parallel
architecture, as exemplified in U.S. Pat. No. 5,974,207, a
demultiplexer (e.g., a waveguide grating router) first separates a
multi-wavelength signal into its constituent spectral components. A
wavelength switching/routing means (e.g., a combination of optical
switches and optical circulators) then serves to drop selective
wavelengths and add others. Finally, a multiplexer combines the
remaining (i.e., the pass-through) wavelengths into an output
multi-wavelength optical signal. In the serial architecture, as
exemplified in U.S. Pat. No. 6,205,269, tunable filters (e.g.,
Bragg fiber gratings) in combination with optical circulators are
used to separate the drop wavelengths from the pass-through
wavelengths and subsequently launch the add channels into the
pass-through path. And if multiple wavelengths are to be added and
dropped, additional multiplexers and demultiplexers are required to
demultiplex the drop wavelengths and multiplex the add wavelengths,
respectively. Irrespective of the underlying architecture, the
OADMs currently in the art are characteristically high in cost, and
prone to significant optical loss accumulation. Moreover, the
designs of these OADMs are such that it is inherently difficult to
reconfigure them in a dynamic fashion.
U.S. Pat. No. 6,204,946 to Askyuk et al. discloses an OADM that
makes use of free-space optics in a parallel construction. In this
case, a multi-wavelength optical signal emerging from an input port
is incident onto a ruled diffraction grating. The constituent
spectral channels thus separated are then focused by a focusing
lens onto a linear array of binary micromachined mirrors. Each
micromirror is configured to operate between two discrete states,
such that it either retroreflects its corresponding spectral
channel back into the input port as a pass-through channel, or
directs its spectral channel to an output port as a drop channel.
As such, the pass-through signal (i.e., the combined pass-through
channels) shares the same input port as the input signal. An
optical circulator is therefore coupled to the input port, to
provide necessary routing of these two signals. Likewise, the drop
channels share the output port with the add channels. An additional
optical circulator is thereby coupled to the output port, from
which the drop channels exit and the add channels are introduced
into the output port. The add channels are subsequently combined
with the pass-through signal by way of the diffraction grating and
the binary micromirrors.
Although the aforementioned OADM disclosed by Askyuk et al. has the
advantage of performing wavelength separating and routing in free
space and thereby incurring less optical loss, it suffers a number
of limitations. First, it requires that the pass-through signal
share the same port/fiber as the input signal. An optical
circulator therefore has to be implemented, to provide necessary
routing of these two signals. Likewise, all the add and drop
channels enter and leave the OADM through the same output port,
hence the need for another optical circulator. Moreover, additional
means must be provided to multiplex the add channels before
entering the system and to demultiplex the drop channels after
exiting the system. This additional multiplexing/demultiplexing
requirement adds more cost and complexity that can restrict the
versatility of the OADM thus-constructed. Second, the optical
circulators implemented in this OADM for various routing purposes
introduce additional optical losses, which can accumulate to a
substantial amount. Third, the constituent optical components must
be in a precise alignment, in order for the system to achieve its
intended purpose. There are, however, no provisions provided for
maintaining the requisite alignment; and no mechanisms implemented
for overcoming degradation in the alignment owing to environmental
effects such as thermal and mechanical disturbances over the course
of operation.
U.S. Pat. No. 5,906,133 to Tomlinson discloses an OADM that makes
use of a design similar to that of Aksyuk et al. There are input,
output, drop and add ports implemented in this case. By positioning
the four ports in a specific arrangement, each micromirror,
notwithstanding switchable between two discrete positions, either
reflects its corresponding channel (coming from the input port) to
the output port, or concomitantly reflects its channel to the drop
port and an incident add channel to the output port. As such, this
OADM is able to perform both the add and drop functions without
involving additional optical components (such as optical
circulators used in the system of Aksyuk et al.). However, because
a single drop port is designated for all the drop channels and a
single add port is designated for all the add channels, the add
channels would have to be multiplexed before entering the add port
and the drop channels likewise need to be demutiplexed upon exiting
from the drop port. Moreover, as in the case of Askyuk et al.,
there are no provisions provided for maintaining requisite optical
alignment in the system, and no mechanisms implemented for
combating degradation in the alignment due to environmental effects
over the course of operation.
As such, the prevailing drawbacks suffered by the OADMs currently
in the art are summarized as follows: 1) The wavelength routing is
intrinsically static, rendering it difficult to dynamically
reconfigure these OADMs. 2) Add and/or drop channels often need to
be multiplexed and/or demultiplexed, thereby imposing additional
complexity and cost. 3) Stringent fabrication tolerance and
painstaking optical alignment are required. Moreover, the optical
alignment is not actively maintained, rendering it susceptible to
environmental effects such as thermal and mechanical disturbances
over the course of operation. 4) In an optical communication
network, OADMs are typically in a ring or cascaded configuration.
In order to mitigate the interference amongst OADMs, which often
adversely affects the overall performance of the network, it is
essential that the power levels of spectral channels entering and
exiting each OADM be managed in a systematic way, for instance, by
introducing power (or gain) equalization at each stage. Such a
power equalization capability is also needed for compensating for
non-uniform gain caused by optical amplifiers (e.g., erbium doped
fiber amplifiers) in the network. There lacks, however, a
systematic and dynamic management of the power levels of various
spectral channels in these OADMs. 5) The inherent high cost and
heavy optical loss further impede the wide application of these
OADMs.
In view of the foregoing, there is an urgent need in the art for
optical add-drop multiplexers that overcome the aforementioned
shortcomings in a simple, effective, and economical
construction.
SUMMARY
The present invention provides a wavelength-separating-routing
(WSR) apparatus and method which employ an array of fiber
collimators serving as an input port and a plurality of output
ports; a wavelength-separator; a beam-focuser; and an array of
channel micromirrors.
In operation, a multi-wavelength optical signal emerges from the
input port. The wavelength-separator separates the multi-wavelength
optical signal into multiple spectral channels, each characterized
by a distinct center wavelength and associated bandwidth. The
beam-focuser focuses the spectral channels into corresponding
spectral spots. The channel micromirrors are positioned such that
each channel micromirror receives one of the spectral channels. The
channel micromirrors are individually controllable and movable,
e.g., continuously pivotable (or rotatable), so as to reflect the
spectral channels into selected ones of the output ports. As such,
each channel micromirror is assigned to a specific spectral
channel, hence the name "channel micromirror". And each output port
may receive any number of the reflected spectral channels.
A distinct feature of the channel micromirrors in the present
invention, in contrast to those used in the prior art, is that the
motion, e.g., pivoting (or rotation), of each channel micromirror
is under analog control such that its pivoting angle can be
continuously adjusted. This enables each channel micromirror to
scan its corresponding spectral channel across all possible output
ports and thereby direct the spectral channel to any desired output
port.
In the WSR apparatus of the present invention, the
wavelength-separator may be provided by a ruled diffraction
grating, a holographic diffraction grating, an echelle grating, a
curved diffraction grating, a dispersing prism, or other
wavelength-separating means known in the art. The beam-focuser may
be a single lens, an assembly of lenses, or other beam-focusing
means known in the art. The channel micromirrors may be provided by
silicon micromachined mirrors, reflective ribbons (or membranes),
or other types of beam-deflecting means known in the art. And each
channel micromirror may be pivotable about one or two axes. The
fiber collimators serving as the input and output ports may be
arranged in a one-dimensional or two-dimensional array. In the
latter case, the channel micromirrors must be pivotable
biaxially.
The WSR apparatus of the present invention may further comprise an
array of collimator-alignment mirrors, in optical communication
with the wavelength-separator and the fiber collimators, for
adjusting the alignment of the input multi-wavelength signal and
directing the spectral channels into the selected output ports by
way of angular control of the collimated beams. Each
collimator-alignment mirror may be rotatable about one or two axes.
The collimator-alignment mirrors may be arranged in a
one-dimensional or two-dimensional array. First and second arrays
of imaging lenses may additionally be optically interposed between
the collimator-alignment mirrors and the fiber collimators in a
telecentric arrangement, thereby "imaging" the collimator-alignment
mirrors onto the corresponding fiber collimators to ensure an
optimal alignment.
The WSR apparatus of the present invention may further include a
servo-control assembly, in communication with the channel
micromirrors and the output ports. The servo-control assembly
serves to monitor the power levels of the spectral channels coupled
into the output ports and further provide control of the channel
micromirrors on an individual basis, so as to maintain a
predetermined coupling efficiency of each spectral channel in one
of the output ports. As such, the servo-control assembly provides
dynamic control of the coupling of the spectral channels into the
respective output ports and actively manages the power levels of
the spectral channels coupled into the output ports. (If the WSR
apparatus includes an array of collimator-alignment mirrors as
described above, the servo-control assembly may additionally
provide dynamic control of the collimator-alignment mirrors.)
Moreover, the utilization of such a servo-control assembly
effectively relaxes the requisite fabrication tolerances and the
precision of optical alignment during assembly of a WSR apparatus
of the present invention, and further enables the system to correct
for shift in optical alignment over the course of operation. A WSR
apparatus incorporating a servo-control assembly thus described is
termed a WSR-S apparatus, thereinafter in the present
invention.
Accordingly, the WSR-S (or WSR) apparatus of the present invention
may be used to construct a variety of optical devices, including a
novel class of dynamically reconfigurable optical add-drop
multiplexers (OADMs), as exemplified in the following
embodiments.
One embodiment of an OADM of the present invention comprises an
aforementioned WSR-S (or WSR) apparatus and an optical combiner.
The output ports of the WSR-S apparatus include a pass-through port
and one or more drop ports, each carrying any number of the
spectral channels. The optical combiner is coupled to the
pass-through port, serving to combine the pass-through channels
with one or more add spectral channels. The combined optical signal
constitutes an output signal of the system. The optical combiner
may be an N.times.1 (N.gtoreq.2) broadband fiber-optic coupler, for
instance, which also serves the purpose of multiplexing a
multiplicity of add spectral channels to be coupled into the
system.
In another embodiment of an OADM of the present invention, a first
WSR-S (or WSR) apparatus is cascaded with a second WSR-S (or WSR)
apparatus. The output ports of the first WSR-S (or WSR) apparatus
include a passthrough port and one or more drop ports. The second
WSR-S (or WSR) apparatus includes a plurality of input ports and an
exiting port. The configuration is such that the pass-through
channels from the first WSR-S apparatus and one or more add
channels are directed into the input ports of the second WSR-S
apparatus, and consequently multiplexed into an output
multi-wavelength optical signal directed into the exiting port of
the second WSR-S apparatus. That is to say that in this embodiment,
one WSR-S apparatus (e.g., the first one) effectively performs a
dynamic drop function, whereas the other WSR-S apparatus (e.g., the
second one) carries out a dynamic add function. And there are
essentially no fundamental restrictions on the wavelengths that can
be added or dropped, other than those imposed by the overall
communication system. Moreover, the underlying OADM architecture
thus presented is intrinsically scalable and can be readily
extended to any number of the WSR-S (or WSR) systems, if so desired
for performing intricate add and drop functions in a network
environment.
Those skilled in the art will recognize that the aforementioned
embodiments provide only two of many embodiments of a dynamically
reconfigurable OADM according to the present invention. Various
changes, substitutions, and alternations can be made herein,
without departing from the principles and the scope of the
invention. Accordingly, a skilled artisan can design an OADM in
accordance with the present invention, to best suit a given
application.
All in all, the OADMs of the present invention provide many
advantages over the prior art devices, notably: 1) By
advantageously employing an array of channel micromirrors that are
individually and continuously controllable, an OADM of the present
invention is capable of routing the spectral channels on a
channel-by-channel basis and directing any spectral channel into
any one of the output ports. As such, its underlying operation is
dynamically reconfigurable, and its underlying architecture is
intrinsically scalable to a large number of channel counts. 2) The
add and drop spectral channels need not be multiplexed and
demultiplexed before entering and after leaving the OADM
respectively. And there are not fundamental restrictions on the
wavelengths to be added or dropped. 3) The coupling of the spectral
channels into the output ports is dynamically controlled by a
servo-control assembly, rendering the OADM less susceptible to
environmental effects (such as thermal and mechanical disturbances)
and therefore more robust in performance. By maintaining an optimal
optical alignment, the optical losses incurred by the spectral
channels are also significantly reduced. 4) The power levels of the
spectral channels coupled into the output ports can be dynamically
managed according to demand, or maintained at desired values (e.g.,
equalized at a predetermined value) by way of the servo-control
assembly. This spectral power-management capability as an integral
part of the OADM will be particularly desirable in WDM optical
networking applications. 5) The use of free-space optics provides a
simple, low loss, and cost-effective construction. Moreover, the
utilization of the servo-control assembly effectively relaxes the
requisite fabrication tolerances and the precision of optical
alignment during initial assembly, enabling the OADM to be simpler
and more adaptable in structure, lower in cost and optical loss. 6)
The underlying OADM architecture allows a multiplicity of the OADMs
according to the present invention to be readily assembled (e.g.,
cascaded) for WDM optical networking applications.
The novel features of this invention, as well as the invention
itself, will be best understood from the following drawings and
detailed description.
BRIEF DESCRIPTION OF THE FIGURES
FIGS. 1A-1D show a first embodiment of a
wavelength-separating-routing (WSR) apparatus according to the
present invention, and the modeling results demonstrating the
performance of the WSR apparatus;
FIGS. 2A-2C depict second and third embodiments of a WSR apparatus
according to the present invention;
FIG. 3 shows a fourth embodiment of a WSR apparatus according to
the present invention;
FIGS. 4A-4B show schematic illustrations of two embodiments of a
WSR-S apparatus comprising a WSR apparatus and a servo-control
assembly, according to the present invention;
FIG. 5 depicts an exemplary embodiment of an optical add-drop
multiplexer (OADM) according to the present invention; and
FIG. 6 shows an alternative embodiment of an OADM according to the
present invention.
DETAILED DESCRIPTION
In this specification and appending claims, a "spectral channel" is
characterized by a distinct center wavelength and associated
bandwidth. Each spectral channel may carry a unique information
signal, as in WDM optical networking applications.
FIG. 1A depicts a first embodiment of a
wavelength-separating-routing (WSR) apparatus according to the
present invention. By way of example to illustrate the general
principles and the topological structure of a
wavelength-separating-routing (WSR) apparatus of the present
invention, the WSR apparatus 100 comprises multiple input/output
ports which may be in the form of an array of fiber collimators
110, providing an input port 110-1 and a plurality of output ports
110-2 through 110-N (N.gtoreq.3); a wavelength-separator which in
one form may be a diffraction grating 101; a beam-focuser in the
form of a focusing lens 102; and an array of channel micromirrors
103.
In operation, a multi-wavelength optical signal emerges from the
input port 110-1. The diffraction grating 101 angularly separates
the multi-wavelength optical signal into multiple spectral
channels, which are in turn focused by the focusing lens 102 into a
spatial array of distinct spectral spots (not shown in FIG. 1A) in
a one-to-one correspondence. The channel micromirrors 103 are
positioned in accordance with the spatial array formed by the
spectral spots, such that each channel micromirror receives one of
the spectral channels. The channel micromirrors 103 are
individually controllable and movable, e.g., pivotable (or
rotatable) under analog (or continuous) control, such that, upon
reflection, the spectral channels are directed into selected ones
of the output ports 110-2 through 110-N by way of the focusing lens
102 and the diffraction grating 101. As such, each channel
micromirror is assigned to a specific spectral channel, hence the
name "channel micromirror". Each output port may receive any number
of the reflected spectral channels.
For purposes of illustration and clarity, only a selective few
(e.g., three) of the spectral channels, along with the input
multi-wavelength optical signal, are graphically illustrated in
FIG. 1A and the following figures. It should be noted, however,
that there can be any number of the spectral channels in a WSR
apparatus of the present invention (so long as the number of
spectral channels does not exceed the number of channel mirrors
employed in the system). It should also be noted that the optical
beams representing the spectral channels shown in FIG. 1A and the
following figures are provided for illustrative purpose only. That
is, their sizes and shapes may not be drawn according to scale. For
instance, the input beam and the corresponding diffracted beams
generally have different cross-sectional shapes, so long as the
angle of incidence upon the diffraction grating is not equal to the
angle of diffraction, as is known to those skilled in the art.
In the embodiment of FIG. 1A, it is preferable that the diffraction
grating 101 and the channel micromirrors 103 are placed
respectively at the first and second (i.e., the front and back)
focal points (on the opposing sides) of the focusing lens 102. Such
a telecentric arrangement allows the chief rays of the focused
beams to be parallel to each other and generally parallel to the
optical axis. In this application, the telecentric configuration
further allows the reflected spectral channels to be efficiently
coupled into the respective output ports, thereby minimizing
various translational walk-off effects that may otherwise arise.
Moreover, the input multi-wavelength optical signal is preferably
collimated and circular in cross-section. The corresponding
spectral channels diffracted from the diffraction grating 101 are
generally elliptical in cross-section; they may be of the same size
as the input beam in one dimension and elongated in the other
dimension.
It is known that the diffraction efficiency of a diffraction
grating is generally polarization-dependent. That is, the
diffraction efficiency of a grating in a standard mounting
configuration may be considerably higher for P-polarization that is
perpendicular to the groove lines on the grating than for
S-polarization that is orthogonal to P-polarization, especially as
the number of groove lines (per unit length) increases. To mitigate
such polarization-sensitive effects, a quarter-wave plate 104 may
be optically interposed between the diffraction grating 101 and the
channel micromirrors 103, and preferably placed between the
diffraction grating 101 and the focusing lens 102 as is shown in
FIG. 1A. In this way, each spectral channel experiences a total of
approximately 90-degree rotation in polarization upon traversing
the quarter-wave plate 104 twice. (That is, if a beam of light has
P-polarization when first encountering the diffraction grating, it
would have predominantly (if not all) S-polarization upon the
second encountering, and vice versa.) This ensures that all the
spectral channels incur nearly the same amount of round-trip
polarization dependent loss.
In the WSR apparatus 100 of FIG. 1A, the diffraction grating 101,
by way of example, is oriented such that the focused spots of the
spectral channels fall onto the channel micromirrors 103 in a
horizontal array, as illustrated in FIG. 1B.
Depicted in FIG. 1B is a close-up view of the channel micromirrors
103 shown in the embodiment of FIG. 1A. By way of example, the
channel micromirrors 103 are arranged in a one-dimensional array
along the x-axis (i.e., the horizontal direction in the figure), so
as to receive the focused spots of the spatially separated spectral
channels in a one-to-one correspondence. (As in the case of FIG.
1A, only three spectral channels are illustrated, each represented
by a converging beam.) Let the reflective surface of each channel
micromirror lie in the x-y plane as defined in the figure and be
movable, e.g., pivotable (or deflectable) about the x-axis in an
analog (or continuous) manner. Each spectral channel, upon
reflection, is deflected in the y-direction (e.g., downward)
relative to its incident direction, so to be directed into one of
the output ports 110-2 through 110-N shown in FIG. 1A.
As described above, a unique feature of the present invention is
that the motion of each channel micromirror is individually and
continuously controllable, such that its position, e.g., pivoting
angle, can be continuously adjusted. This enables each channel
micromirror to scan its corresponding spectral channel across all
possible output ports and thereby direct the spectral channel to
any desired output port. To illustrate this capability, FIG. 1C
shows a plot of coupling efficiency as a function of a channel
micromirror's pivoting angle .theta., provided by a ray-tracing
model of a WSR apparatus in the embodiment of FIG. 1A. As used
herein, the coupling efficiency for a spectral channel is defined
as the ratio of the amount of optical power coupled into the fiber
core in an output port to the total amount of optical power
incident upon the entrance surface of the fiber (associated with
the fiber collimator serving as the output port). In the
ray-tracing model, the input optical signal is incident upon a
diffraction grating with 700 lines per millimeter at a grazing
angle of 85 degrees, where the grating is blazed to optimize the
diffraction efficiency for the "-1" order. The focusing lens has a
focal length of 100 mm. Each output port is provided by a
quarter-pitch GRIN lens (2 mm in diameter) coupled to an optical
fiber (see FIG. 1D). As displayed in FIG. 1C, the coupling
efficiency varies with the pivoting angle .theta., and it requires
about a 0.2-degree change in .theta. for the coupling efficiency to
become practically negligible in this exemplary case. As such, each
spectral channel may practically acquire any coupling efficiency
value by way of controlling the pivoting angle of its corresponding
channel micromirror. This is also to say that variable optical
attenuation at the granularity of a single wavelength can be
obtained in a WSR apparatus of the present invention. FIG. 1D
provides ray-tracing illustrations of two extreme points on the
coupling efficiency vs. .theta. curve of FIG. 1C: on-axis coupling
corresponding to .theta.=0, where the coupling efficiency is
maximum; and off-axis coupling corresponding to .theta.=0.2
degrees, where the representative collimated beam (representing an
exemplary spectral channel) undergoes a significant translational
walk-off and renders the coupling efficiency practically
negligible. All in all, the exemplary modeling results thus
described demonstrate the unique capabilities of the WSR apparatus
of the present invention.
FIG. 1A provides one of many embodiments of a WSR apparatus
according to the present invention. In general, the
wavelength-separator is a wavelength-separating means that may be a
ruled diffraction grating, a holographic diffraction grating, an
echelle grating, a dispersing prism, or other types of
spectral-separating means known in the art. The beam-focuser may be
a focusing lens, an assembly of lenses, or other beam-focusing
means known in the art. The focusing function may also be
accomplished by using a curved diffraction grating as the
wavelength-separator. The channel micromirrors may be provided by
silicon micromachined mirrors, reflective ribbons (or membranes),
or other types of beam-deflecting elements known in the art. And
each micromirror may be pivoted about one or two axes. What is
important is that the pivoting (or rotational) motion of each
channel micromirror be individually controllable in an analog
manner, whereby the pivoting angle can be continuously adjusted so
as to enable the channel micromirror to scan a spectral channel
across all possible output ports. The underlying fabrication
techniques for micromachined mirrors and associated actuation
mechanisms are well documented in the art, see U.S. Pat. No.
5,629,790 for example. Moreover, a fiber collimator is typically in
the form of a collimating lens (such as a GRIN lens) and a
ferrule-mounted fiber packaged together in a mechanically rigid
stainless steel (or glass) tube. The fiber collimators serving as
the input and output ports may be arranged in a one-dimensional
array, a two-dimensional array, or other desired spatial pattern.
For instance, they may be conveniently mounted in a linear array
along a V-groove fabricated on a substrate made of silicon,
plastic, or ceramic, as commonly practiced in the art. It should be
noted, however, that the input port and the output ports need not
necessarily be in close spatial proximity with each other, such as
in an array configuration (although a close packing would reduce
the rotational range required for each channel micromirror). Those
skilled in the art will know how to design a WSR apparatus
according to the present invention, to best suit a given
application.
A WSR apparatus of the present invention may further comprise an
array of collimator-alignment mirrors, for adjusting the alignment
of the input multi-wavelength optical signal and facilitating the
coupling of the spectral channels into the respective output ports,
as shown in FIGS. 2A-2B and 3.
Depicted in FIG. 2A is a second embodiment of a WSR apparatus
according to the present invention. By way of example, WSR
apparatus 200 is built upon and hence shares a number of the
elements used in the embodiment of FIG. 1A, as identified by those
labeled with identical numerals. Moreover, a one-dimensional array
220 of collimator-alignment mirrors 220-1 through 220-N is
optically interposed between the diffraction grating 101 and the
fiber collimator array 110. The collimator-alignment mirror 220-1
is designated to correspond with the input port 110-1, for
adjusting the alignment of the input multi-wavelength optical
signal and therefore ensuring that the spectral channels impinge
onto the corresponding channel micromirrors. The
collimator-alignment mirrors 220-2 through 220-N are designated to
the output ports 110-2 through 110-N in a one-to-one
correspondence, serving to provide angular control of the
collimated beams of the reflected spectral channels and thereby
facilitating the coupling of the spectral channels into the
respective output ports according to desired coupling efficiencies.
Each collimator-alignment mirror may be rotatable about one axis,
or two axes.
The embodiment of FIG. 2A is attractive in applications where the
fiber collimators (serving as the input and output ports) are
desired to be placed in close proximity to the collimator-alignment
mirror array 220. To best facilitate the coupling of the spectral
channels into the output ports, arrays of imaging lenses may be
implemented between the collimator-alignment mirror array 220 and
the fiber collimator array 110, as depicted in FIG. 2B. By way of
example, WSR apparatus 250 of FIG. 2B is built upon and hence
shares many of the elements used in the embodiment of FIG. 2A, as
identified by those labeled with identical numerals. Additionally,
first and second arrays 260, 270 of imaging lenses are placed in a
4-f telecentric arrangement with respect to the
collimator-alignment mirror array 220 and the fiber collimator
array 110. The dashed box 280 shown in FIG. 2C provides a top view
of such a telecentric arrangement. In this case, the imaging lenses
in the first and second arrays 260, 270 all have the same focal
length f. The collimator-alignment mirrors 220-1 through 220-N are
placed at the respective first (or front) focal points of the
imaging lenses in the first array 260. Likewise, the fiber
collimators 110-1 through 110-N are placed at the respective second
(or back) focal points of the imaging lenses in the second array
270. And the separation between the first and second arrays 260,
270 of imaging lenses is 2f. In this way, the collimator-alignment
mirrors 220-1 through 220-N are effectively imaged onto the
respective entrance surfaces (i.e., the front focal planes) of the
GRIN lenses in the corresponding fiber collimators 110-1 through
110-N. Such a telecentric imaging system substantially eliminates
translational walk-off of the collimated beams at the output ports
that may otherwise occur as the mirror angles change.
FIG. 3 shows a fourth embodiment of a WSR apparatus according to
the present invention. By way of example, WSR apparatus 300 is
built upon and hence shares a number of the elements used in the
embodiment of FIG. 2B, as identified by those labeled with
identical numerals. In this case, the one-dimensional fiber
collimator array 110 of FIG. 2B is replaced by a two-dimensional
array 350 of fiber collimators, providing for an input-port and a
plurality of output ports. Accordingly, the one-dimensional
collimator-alignment mirror array 220 of FIG. 2B is replaced by a
two-dimensional array 320 of collimator-alignment mirrors, and
first and second one-dimensional arrays 260, 270 of imaging lenses
of FIG. 2B are likewise replaced by first and second
two-dimensional arrays 360, 370 of imagining lenses respectively.
As in the case of the embodiment of FIG. 2B, the first and second
two-dimensional arrays 360, 370 of imaging lenses are placed in a
4-f telecentric arrangement with respect to the two-dimensional
collimator-alignment mirror array 320 and the two-dimensional fiber
collimator array 350. The channel micromirrors 103 must be
pivotable biaxially in this case (in order to direct its
corresponding spectral channel to any one of the output ports). As
such, the WSR apparatus 300 is equipped to support a greater number
of the output ports.
In addition to facilitating the coupling of the spectral channels
into the respective output ports as described above, the
collimator-alignment mirrors in the above embodiments also serve to
compensate for misalignment (e.g., due to fabrication and assembly
errors) in the fiber collimators that provide for the input and
output ports. For instance, relative misalignment between the fiber
cores and their respective collimating lenses in the fiber
collimators can lead to pointing errors in the collimated beams,
which may be corrected for by the collimator-alignment mirrors. For
these reasons, the collimator-alignment mirrors are preferably
rotatable about two axes. They may be silicon micromachined
mirrors, for fast rotational speeds. They may also be other types
of mirrors or beam-deflecting elements known in the art.
To optimize the coupling of the spectral channels into the output
ports and further maintain the optimal optical alignment against
environmental effects such as temperature variations and mechanical
instabilities over the course of operation, a WSR apparatus of the
present invention may incorporate a servo-control assembly, for
providing dynamic control of the coupling of the spectral channels
into the respective output ports on a channel-by-channel basis. A
WSR apparatus incorporating a servo-control assembly is termed a
WSR-S apparatus, thereinafter in this specification.
FIG. 4A depicts a schematic illustration of a first embodiment of a
WSR-S apparatus according to the present invention. The WSR-S
apparatus 400 comprises a WSR apparatus 410 and a servo-control
assembly 440. The WSR 410 may be in the embodiment of FIG. 1A, or
any other embodiment in accordance with the present invention. The
servo-control assembly 440 includes a spectral monitor 460, for
monitoring the power levels of the spectral channels coupled into
the output ports 420-1 through 420-N of the WSR apparatus 410. By
way of example, the spectral monitor 460 is coupled to the output
ports 420-1 through 420-N by way of fiber-optic couplers 420-1-C
through 420-N-C, wherein each fiber-optic coupler serves to tap off
a predetermined fraction of the optical signal in the corresponding
output port. The servo-control assembly 440 further includes a
processing unit 470, in communication with the spectral monitor 460
and the channel micromirrors 430 of the WSR apparatus 410. The
processing unit 470 uses the power measurements from the spectral
monitor 460 to provide feedback control of the channel micromirrors
430 on an individual basis, so as to maintain a desired coupling
efficiency for each spectral channel into a selected output port.
As such, the servo-control assembly 440 provides dynamic control of
the coupling of the spectral channels into the respective output
ports on a channel-by-channel basis and thereby manages the power
levels of the spectral channels coupled into the output ports. The
power levels of the spectral channels in the output ports may be
dynamically managed according to demand, or maintained at desired
values (e.g., equalized at a predetermined value) in the present
invention. Such a spectral power-management capability is essential
in WDM optical networking applications, as discussed above.
FIG. 4B depicts a schematic illustration of a second embodiment of
a WSR-S apparatus according to the present invention. The WSR-S
apparatus 450 comprises a WSR apparatus 480 and a servo-control
assembly 490. In addition to the channel micromirrors 430 (and
other elements identified by the same numerals as those used in
FIG. 4A), the WSR apparatus 480 further includes a plurality of
collimator-alignment mirrors 485, and may be configured according
to the embodiment of FIGS. 2A, 2B, 3, or any other embodiment in
accordance with the present invention. By way of example, the
servo-control assembly 490 includes the spectral monitor 460 as
described in the embodiment of FIG. 4A, and a processing unit 495.
In this case, the processing unit 495 is in communication with the
channel micromirrors 430 and the collimator-alignment mirrors 485
of the WSR apparatus 480, as well as the spectral monitor 460. The
processing unit 495 uses the power measurements from the spectral
monitor 460 to provide dynamic control of the channel micromirrors
430 along with the collimator-alignment mirrors 485, so to maintain
the coupling efficiencies of the spectral channels into the output
ports at desired values.
In the embodiment of FIG. 4A or 4B, the spectral monitor 460 may be
one of spectral power monitoring devices known in the art that is
capable of detecting the power levels of spectral components in a
multi-wavelength optical signal. Such devices are typically in the
form of a wavelength-separating means (e.g., a diffraction grating)
that spatially separates a multi-wavelength optical signal by
wavelength into constituent spectral components, and one or more
optical sensors (e.g., an array of photodiodes) that are configured
such to detect the power levels of these spectral components. The
processing unit 470 in FIG. 4A (or the processing unit 495 in FIG.
4B) typically includes electrical circuits and signal processing
programs for processing the power measurements received from the
spectral monitor 460 and generating appropriate control signals to
be applied to the channel micromirrors 430 (and the
collimator-alignment mirrors 485 in the case of FIG. 4B), so to
maintain the coupling efficiencies of the spectral channels into
the output ports at desired values. The electronic circuitry and
the associated signal processing algorithm/software for such
processing unit in a servo-control system are known in the art. A
skilled artisan will know how to implement a suitable spectral
monitor along with an appropriate processing unit to provide a
servo-control assembly in a WSP-S apparatus according to the
present invention, for a given application.
The incorporation of a servo-control assembly provides additional
advantages of effectively relaxing the requisite fabrication
tolerances and the precision of optical alignment during initial
assembly of a WSR apparatus of the present invention, and further
enabling the system to correct for shift in the alignment over the
course of operation. By maintaining an optimal optical alignment,
the optical losses incurred by the spectral channels are also
significantly reduced. As such, the WSR-S apparatus thus
constructed is simpler and more adaptable in structure, more robust
in performance, and lower in cost and optical loss. Accordingly,
the WSR-S (or WSR) apparatus of the present invention may be used
to construct a variety of optical devices and utilized in many
applications.
For instance, by directing the spectral channels into the output
ports in a one-channel-per-port fashion and coupling the output
ports of a WSR-S (or WSR) apparatus to an array of optical sensors
(e.g., photodiodes), or a single optical sensor that is capable of
scanning across the output ports, a dynamic and versatile spectral
power monitor (or channel analyzer) is provided, which would be
highly desired in WDM optical networking applications. Moreover, a
novel class of optical add-drop multiplexers (OADMs) may be built
upon the WSR-S (or WSR) apparatus of the present invention, as
exemplified in the following embodiments.
FIG. 5 depicts an exemplary embodiment of an optical add-drop
multiplexer (OADM) according to the present invention. By way of
example, OADM 500 comprises a WSR-S (or WSR) apparatus 510 and an
optical combiner 550. An input port 520 of the WSR-S apparatus 510
transmits a multi-wavelength optical signal. The constituent
spectral channels are subsequently separated and routed into a
plurality of output ports, including a pass-through port 530 and
one or more drop ports 540-1 through 540-N (N.gtoreq.1). The
pass-through port 530 may receive any number of the spectral
channels (i.e., the pass-through spectral channels). Each drop port
may also receive any number of the spectral channels (i.e., the
drop spectral channels). The pass-through port 530 is optically
coupled to the optical combiner 550, which serves to combine the
pass-through spectral channels with one or more add spectral
channels provided by one or more add ports 560-1 through 560-M
(M.gtoreq.1). The combined optical signal is then routed into an
existing port 570, providing an output multi-wavelength optical
signal.
In the above embodiment, the optical combiner 550 may be a
K.times.1 (K.gtoreq.2) broadband fiber-optic coupler, wherein there
are K input-ends and one output-end. The pass-through spectral
channels and the add spectral channels are fed into the K
input-ends (e.g., in a one-to-one correspondence) and the combined
optical signal exits from the output-end of the K.times.1
fiber-optic coupler as the output multi-wavelength optical signal
of the system. Such a multiple-input coupler also serves the
purpose of multiplexing a multiplicity of add spectral channels to
be coupled into the OADM 500. If the power levels of the spectral
channels in the output multi-wavelength optical signal are desired
to be actively managed, such as being equalized at a predetermined
value, two spectral monitors may be utilized. As a way of example,
the first spectral monitor may receive optical signals tapped off
from the pass-through port 530 and the drop ports 540-1 through
540-N (e.g., by way of fiber-optic couplers as depicted in FIG. 4A
or 4B). The second spectral monitor receives optical signals tapped
off from the exiting port 570. A servo-control system may be
constructed accordingly for monitoring and controlling the
pass-through, drop and add spectral channels. As such, the
embodiment of FIG. 5 provides a versatile optical add-drop
multiplexer in a simple and low-cost assembly, while providing
multiple physically separate drop/add ports in a dynamically
reconfigurable fashion.
FIG. 6 depicts an alternative embodiment of an optical add-drop
multiplexer (OADM) according to the present invention. By way of
example, OADM 600 comprises a first WSR-S apparatus 610 optically
coupled to a second WSR-S apparatus 650. Each WSR-S apparatus may
be in the embodiment of FIG. 4A or 4B. (A WSR apparatus of the
embodiment of FIG. 1A, 2A, 2B, or 3 may be alternatively
implemented.) The first WSR-S apparatus 610 includes an input port
620, a pass-through port 630, and one or more drop ports 640-1
through 640-N (N.gtoreq.1). The pass-through spectral channels from
the pass-through port 630 are further coupled to the second WSR-S
apparatus 650, along with one or more add spectral channels
emerging from add ports 660-1 through 660-M (M.gtoreq.1). In this
exemplary case, the pass-through port 630 and the add ports 660-1
through 660-M constitute the input ports for the second WSR-S
apparatus 650. By way of its constituent wavelength-separator
(e.g., a diffraction grating) and channel micromirrors (not shown
in FIG. 6), the second WSR-S apparatus 650 serves to multiplex the
pass-through spectral channels and the add spectral channels, and
route the multiplexed optical signal into an exiting port 770 to
provide an output signal of the system.
In the embodiment of FIG. 6, one WSR-S apparatus (e.g., the first
WSR-S apparatus 610) effectively performs dynamic drop function,
whereas the other WSR-S apparatus (e.g., the second WSR-S apparatus
650) carries out dynamic add function. And there are essentially no
fundamental restrictions on the wavelengths that can be added or
dropped (other than those imposed by the overall communication
system). Moreover, the underlying OADM architecture thus presented
is intrinsically scalable and can be readily extended to any number
of cascaded WSR-S (or WSR) systems, if so desired for performing
intricate add and drop functions. Additionally, the OADM of FIG. 6
may be operated in reverse direction, by using the input ports as
the output ports, the drop ports as the add ports, and vice
versa.
Those skilled in the art will recognize that the aforementioned
embodiments provide only two of many embodiments of a dynamically
reconfigurable OADM according to the present invention. Those
skilled in the art will also appreciate that various changes,
substitutions, and alternations can be made herein without
departing from the principles and the scope of the invention as
defined in the appended claims. Accordingly, a skilled artisan can
design an OADM in accordance with the principles of the present
invention, to best suit a given application.
Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions, and alternations can be made herein without
departing from the principles and the scope of the invention.
Accordingly, the scope of the present invention should be
determined by the following claims and their legal equivalents.
* * * * *