U.S. patent number RE39,515 [Application Number 11/027,589] was granted by the patent office on 2007-03-13 for reconfigurable optical add-drop multiplexers employing polarization diversity.
This patent grant is currently assigned to Capella Photonics, Inc.. Invention is credited to Joseph E. Davis, Mark H. Garrett, Masud Mansuripur, Pavel G. Polynkin, Jeffrey P. Wilde.
United States Patent |
RE39,515 |
Garrett , et al. |
March 13, 2007 |
Reconfigurable optical add-drop multiplexers employing polarization
diversity
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 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 invention may
further employ a polarization diversity scheme, whereby
polarization-sensitive effects become inconsequential and insertion
loss is minimized. The WSR apparatus of the invention may
additionally be equipped with servo-control and channel
equalization capabilities. The WSR apparatus of the invention can
be used to construct a novel class of dynamically reconfigurable
optical add-drop multiplexers (OADMs) for WDM optical networking
applications.
Inventors: |
Garrett; Mark H. (Morgan Hill,
CA), Mansuripur; Masud (Tucson, AZ), Wilde; Jeffrey
P. (Morgan Hill, CA), Polynkin; Pavel G. (Tucson,
AZ), Davis; Joseph E. (Morgan Hill, CA) |
Assignee: |
Capella Photonics, Inc. (San
Jose, CA)
|
Family
ID: |
44720437 |
Appl.
No.: |
11/027,589 |
Filed: |
December 31, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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09938426 |
Aug 23, 2001 |
6625346 |
|
|
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60277217 |
Mar 19, 2001 |
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Reissue of: |
10076145 |
Feb 14, 2002 |
06760511 |
Jul 6, 2004 |
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Current U.S.
Class: |
385/24; 385/11;
385/34; 385/37 |
Current CPC
Class: |
G02B
6/2931 (20130101); G02B 6/29313 (20130101); G02B
6/29383 (20130101); G02B 6/29385 (20130101); G02B
6/29391 (20130101); G02B 6/29395 (20130101); G02B
6/32 (20130101); G02B 6/3512 (20130101); G02B
6/3588 (20130101); G02B 6/34 (20130101); G02B
6/3518 (20130101); G02B 6/3556 (20130101); G02B
6/356 (20130101); G02B 6/3586 (20130101); G02B
6/3592 (20130101) |
Current International
Class: |
G02B
6/28 (20060101) |
Field of
Search: |
;385/11,24,34,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Font; Frank G.
Assistant Examiner: El-Shammaa; Mary
Attorney, Agent or Firm: Young; Barry N.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent
application Ser. No. 09/938,426, filed on Aug. 23, 2001, .Iadd.now
U.S. Pat. No. 6,625,346 .Iaddend.and which claims priority from
U.S. Provisional Patent Application Ser. No. 60/277,217, filed on
Mar. 19, 2001.
Claims
What is claimed is:
1. An optical apparatus, comprising: fiber collimators providing an
input port for a multi-wavelength optical signal and a plurality of
output ports; a polarization-displacing unit that decomposes said
multi-wavelength optical signal into first and second polarization
components; a polarization-rotating unit that rotates a
polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; and an array of channel micromirrors
positioned to reflect said first and second sets of optical beams
such that said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
wherein said polarization-displacing unit comprises a
polarization-displacing element in optical communication with said
input port and said output ports, and wherein said
polarization-rotating unit comprises a polarization-rotating
element, in optical communication with said polarization-displacing
element.
2. The optical apparatus of claim 1, wherein said
polarization-displacing unit comprises a polarization-displacing
element in optical communication with said input port and said
output ports.
3. An optical apparatus, comprising: fiber collimators providing an
input port for a multi-wavelength optical signal and a plurality of
output ports; a polarization-displacing unit that decomposes said
multi-wavelength optical signal into first and second polarization
components; a polarization-rotating unit that rotates a
polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; and an array of channel micromirrors
positioned to reflect said first and second sets of optical beams
such that said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
wherein said polarization-displacing unit comprises a plurality of
polarization-displacing elements in correspondence with said input
port and said output ports.
4. The optical apparatus of claim 3, wherein said
polarization-displacing element comprises an element selected from
the group consisting of birefringent beam displacers and
polarizing-beam-splitting elements.
5. The optical apparatus of claim 3, wherein said
polarization-rotating unit comprises a plurality of
polarization-rotating elements in correspondence with said
polarization-displacing elements.
6. The optical apparatus of claim 5, wherein each
polarization-rotating element comprises an element selected from
the group consisting of half-wave plates, Faraday rotators, and
liquid crystal rotators.
7. An optical apparatus, comprising: fiber collimators providing an
input port for a multi-wavelength optical signal and a plurality of
output ports; a polarization-displacing unit that decomposes said
multi-wavelength optical signal into first and second polarization
components; a polarization-rotating unit that rotates a
polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; an array of channel micromirrors positioned
to reflect said first and second sets of optical beams such that
said reflected first mid second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
and a beam-modifying unit for providing anamorphic beam
magnification of said first and second polarization components and
anamorphic beam demagnification of said reflected first and second
sets of optical beams.
8. The optical apparatus of claim 7, wherein beam-modifying unit
comprises one or more cylindrical lenses.
9. The optical apparatus of claim 7, wherein beam-modifying unit
comprises one or more prisms.
10. An optical apparatus, comprising: fiber collimators providing
an input port for a multi-wavelength optical signal and a plurality
of output ports; a polarization-displacing unit that decomposes
said multi-wavelength optical signal into first and second
polarization components; a polarization-rotating unit that rotates
a polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; an array of channel micromirrors positioned
to reflect said first and second sets of optical beams such that
said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
and an array of collimator-alignment mirrors in optical
communication with said fiber collimators and said
polarization-displacing unit for adjusting an alignment of said
multi-wavelength optical signal from said input port and for
directing said reflected spectral channels into said output
ports.
11. The optical apparatus of claim 10, wherein each
collimator-alignment mirror is rotatable about at least one
axis.
12. An optical apparatus, comprising: fiber collimators providing
an input pod for a multi-wavelength optical signal and a plurality
of output pods; a polarization-displacing unit that decomposes said
multi-wavelength optical signal into first and second polarization
components; a polarization-rotating unit that rotates a
polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; and an array of channel micromirrors
positioned to reflect said first and second sets of optical beams
such that said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
wherein said polarization-displacing unit comprises a polarizing
beam splitter and a first beam-deflecting unit.
13. The optical apparatus of claim 12, wherein said first
beam-deflecting unit comprises an array of first mirrors that are
individually adjustable to control positions of said second
polarization component and said reflected first set of optical
beams.
14. The optical apparatus of claim 13, further comprising a second
beam-deflecting unit, in optical communication with said first
polarization component and said reflected second set of optical
beams, said second beam-deflecting unit comprising an array of
second mirrors that are individually adjustable.
15. An optical apparatus, comprising: fiber collimators providing
an input port for a multi-wavelength optical signal and a plurality
of output ports; a polarization-displacing unit that decomposes
said multi-wavelength optical signal into first and second
polarization components; a polarization-rotating unit that rotates
a polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; an array of channel micromirrors positioned
to reflect said first and second sets of optical beams such that
said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
and a servo-control assembly, including a spectral monitor for
monitoring optical power lever of said reflected spectral channels
and a processing unit responsive to said optical power levels for
controlling said channel micromirrors.
16. The optical apparatus of claim 15, wherein said servo-control
assembly controls said channel micromirrors to maintain said
optical power levels at a predetermined value.
17. An optical apparatus, comprising: fiber collimators providing
an input port for a multi-wavelength optical signal and a plurality
of output ports; a polarization-displacing unit that decomposes
said multi-wavelength optical signal into first and second
polarization components; a polarization-rotating unit that rotates
a polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; and an array of channel micromirrors
positioned to reflect said first and second sets of optical beams
such that said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit arid said polarization-displacing unit;
wherein each channel micromirror is pivotable about two axes; and
wherein said fiber collimators are arranged in a two-dimensional
array.
18. An optical apparatus, comprising: fiber collimators providing
an input port for a multi-wavelength optical signal arid a
plurality of output ports; a polarization-displacing unit that
decomposes said multi-wavelength optical signal into first and
second polarization components; a polarization-rotating unit that
rotates a polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; and an array of channel micromirrors
positioned to reflect said first and second sets of optical beams
such that said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
wherein said array of channel micromirrors reflects said first and
second sets of optical beams so as to couple said beams into
selected output ports.
19. An optical apparatus, comprising: fiber collimators providing
an input pod for a multi-wavelength optical signal and a plurality
of output ports; a polarization-displacing unit that decomposes
said multi-wavelength optical signal into first and second
polarization components; a polarization-rotating unit that rotates
a polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets optical beams; and an array of channel micromirrors positioned
to reflect said first and second sets of optical beams such that
said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
wherein said fiber collimators are arranged in a one-dimensional
array.
20. An optical apparatus, comprising: fiber collimators providing
an input port for a multi-wavelength optical signal and a plurality
of output ports; a polarization-displacing unit that decomposes
said multi-wavelength optical signal into first and second
polarization components; a polarization-rotating unit that rotates
a polarization of the second polarization component to be
substantially parallel to a polarization of the first polarization
component; a wavelength-separator that separates said first and
second polarization components by wavelength into first and second
sets of optical beams; an array of channel micromirrors positioned
to reflect said first and second sets of optical beams such that
said reflected first and second sets of optical beams are
recombined by wavelength into reflected spectral channels by said
polarization-rotating unit and said polarization-displacing unit;
and a beam-focuser for focusing said first and second sets of
optical beams onto said channel micromirrors.
21. A method of dynamic routing of a multi-wavelength optical
signal in a polarization diversity arrangement comprising:
decomposing said multi-wavelength optical signal into first and
second polarization components; providing an anamorphic beam
magnification to said first and second polarization components,
respectively; rotating a polarization of said second polarization
component to be substantially parallel to a polarization of the
first polarization component; separating said first and second
polarization components by wavelength respectively into first and
second sets of optical beams; focusing said first and second sets
of optical beams onto an array of micromirrors; dynamically
controlling said micromirrors to reflect said first and second sets
of optical beams into selected output ports; rotating a
polarization of said reflected First, set of optical beams of
approximately 90-degrees; and recombining said reflected first and
second sets of optical beams by wavelength into reflected spectral
channels.
22. The method of claim 21 further comprising the step of
monitoring said optical power levels at a predetermined value.
23. A method of dynamic routing of a multi-wavelength optical
signal in a polarization diversity arrangement, comprising:
decomposing said multi-wavelength optical signal into first and
second polarization components; rotating a polarization of said
second polarization component to be substantially parallel to a
polarization of the first polarization component; separating said
first, and second polarization components by wavelength
respectively into first and second sets of optical beams; focusing
said first and second sets of optical beams onto an array of
micromirrors; dynamically controlling said micromirrors to reflect
said first and second sets of optical beams into selected output
ports; rotating a polarization of said reflected first set of
optical beams by approximately 90-degrees; recombining said
reflected first and second sets of optical beams by wavelength into
reflected spectral channels monitoring optical power levels of said
reflected spectral channels coupled into said output pods; and
providing Feedback control of said micromirrors.
24. The method of claim 23 further comprising the step of
maintaining said optical power levels at a predetermined value.
25. A method of dynamic routing of a multi-wavelength optical
signal in a polarization diversity arrangement, comprising:
adjusting an alignment of said multi-wavelength optical signal;
decomposing said multi-wavelength optical signal into first and
second polarization components; rotating a polarization of said
second polarization component It) be substantially parallel to a
polarization of the first polarization component; separating said
first and second polarization components by wavelength respectively
into U) first and second sets of optical beams; focusing said first
and second sets of optical beams onto an array of micromirrors
dynamically controlling said micromirrors to reflect said first and
second sets of optical beams into selected output ports; rotating a
polarization of said reflected first set of optical beams by
approximately 90-degrees; and recombining said reflected first and
second sets of optical beams by wavelength into reflected spectral
channels.
26. The method of claim 25 further comprising the step of coupling
of said reflected spectral channels into selected output ports.
27. A method of dynamic routing of a multi-wavelength optical
signal in a polarization diversity arrangement, comprising:
decomposing said multi-wavelength optical signal into first and
second polarization components; adjusting a relative alignment
between said first and second polarization components; rotating a
polarization of said second polarization component to be
substantially parallel to a polarization of the first polarization
component; separating said first and second polarization components
by wavelength respectively into first and second sets of optical
beams; focusing said first and second sets of optical beams onto an
array of micromirrors; dynamically controlling said micromirrors to
reflect said first awl second sets of optical beams into selected
output pods; rotating a polarization of said reflected first set of
optical beams by approximately 90-degrees; and recombining said
reflected first and second sets of optical beams by wavelength into
reflected spectral channels.
28. The method of claim 27 further comprising the step of adjusting
a relative alignment between said reflected first and second sets
of optical beams.
Description
BACKGROUND
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.
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 (being
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 demultiplexed 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 optical 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 optical power levels of various spectral channels
in these OADMs. 5) The inherent high cost and 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 OF THE INVENTION
The invention provides a polarization diversity
wavelength-separating-routing (WSR) apparatus and method which
minimizes insertion loss and polarization-dependent loss (PDL).
In WSR apparatus with which the invention may be used, a
multi-wavelength optical signal is provided from an input port to a
wavelength-separator which separates the multi-wavelength optical
signal by wavelength into multiple spectral channels. Each channel
may be characterized by a distinct center wavelength and associated
bandwidth. A beam-focuser may focus the spectral channels into
corresponding spots onto a plurality of channel micromirrors
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. Each output port may receive any number
of the reflected spectral channels.
In one aspect, the WSR apparatus of the invention employs a
polarization diversity arrangement to overcome
polarization-sensitive effects the constituent optical elements may
possess. A polarization-displacing unit and a polarization-rotating
unit may be disposed along the optical path between the fiber
collimators providing the input and output ports and the
wavelength-separator which separates the input multi-wavelength
optical signal into the constituent wavelengths. The
polarization-displacing unit decomposes the input multi-wavelength
optical signal into first and second polarization components. The
polarization-rotating unit may subsequently rotate the polarization
of the second polarization component so that its polarization is
substantially parallel to the first polarization component, e.g.,
by 90-degrees. The wavelength-separator separates the incident
optical signals by wavelength into first and second sets of optical
beams, respectively. The beam-focuser may focus the first and
second sets of optical beams into corresponding focused spots,
impinging onto the channel micromirrors. The first and second
optical beams associated with the same wavelength may impinge onto
(and be manipulated by) the same channel micromirror. The channel
micromirrors may be individually controlled such that the first and
second sets of optical beams are deflected, upon reflection. The
reflected first set of optical beams may subsequently undergo a
rotation in polarization by, e.g., 90 degrees, by the
polarization-rotating unit. This enables the
polarization-displacing unit to recombine the reflected first and
second sets of optical beams by wavelength respectively into
reflected spectral channels, prior to being coupled into the output
ports.
The polarization-displacing unit may comprise one or more
polarization-displacing elements, each being a birefringent beam
displacer, or a polarizing-beam-splitting element, e.g., a
polarizing beam splitter in conjunction with a suitable
beam-reflector. The polarization-rotating unit may include one or
more polarization rotating elements, each being a half-wave plate,
a Faraday rotator, or a liquid crystal rotator known in the
art.
A distinct feature of the channel micromirrors in the WSR apparatus
is that the motion 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, the wavelength-separator may be a ruled
diffraction grating, a holographic diffraction grating, an echelle
grating, a curved diffraction grating, a transmission 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 silicon micromachined mirrors, reflective
ribbons (or membranes), or other types of beam-deflecting means
known in the art. Each channel micromirror may be pivotable about
one or two axes. 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 may be
pivotable biaxially.
In another aspect, the WSR apparatus of the invention may 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
for 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 such
that the collimator-alignment mirrors are effectively "imaged" onto
the corresponding fiber collimators to ensure an optimal
alignment.
In another aspect, the WSR apparatus of the invention may include a
servo-control assembly, in communication with the channel
micromirrors and the output ports. The servo-control assembly
serves to monitor the optical 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 into 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 optical 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 SR apparatus of
the 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, in the following discussion.
The WSR apparatus of the invention affords a variety of optical
devices, including a novel class of dynamically reconfigurable
optical add-drop multiplexers (OADMs), that 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 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 optical 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, and lower in cost and optical loss. 6) The
use of a polarization diversity scheme renders the
polarization-sensitive effects inconsequential in the OADM. This
enables the OADM to minimize the insertion loss; and enhance
spectral resolution in a simple and cost-effective construction
(e.g., by making use of high-dispersion diffraction grating
commonly available in the art). The polarization diversity scheme
further allows the overall optical paths of the two polarization
components for each spectral channel to be substantially equalized,
thereby minimizing the polarization-dependent loss. Such attributes
would be particularly desirable in WDM optical networking
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1D show a first embodiment of a
wavelength-separating-routing (WSR) apparatus with which the
invention may be employed, and the modeling results demonstrating
the performance of the WSR apparatus;
FIG. 2A depicts a second embodiment of a WSR apparatus with which
the invention may be employed;
FIGS. 2B-2C show a third embodiment of a WSR apparatus with which
the invention may be employed;
FIG. 3 shows a fourth embodiment of a WSR apparatus with which the
invention may be employed;
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 invention;
FIG. 5 depicts an exemplary embodiment of an optical add-drop
multiplexer (OADM) according to the invention;
FIG. 6 shows an alternative embodiment of an OADM according to the
invention;
FIGS. 7A-7B depict a fifth embodiment of a WSR apparatus according
to the invention employing a polarization diversity
arrangement;
FIGS. 7C-7D depict two exemplary embodiments of a
polarization-displacing unit that may be used in the WSR apparatus
shown in FIGS. 7A-7B;
FIG. 8A shows a sixth embodiment of a WSR apparatus according to
the invention, employing a polarization diversity arrangement;
FIG. 8B depicts a seventh embodiment of a WSR apparatus according
to the invention employing a polarization diversity arrangement;
and
FIG. 8C shows an eighth embodiment of a WSR apparatus according to
the invention employing a polarization diversity arrangement.
DETAILED DESCRIPTION
In this specification and appended 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 with which the
invention may be employed. By way of example to illustrate the
general principles and the topological structure of a
wavelength-separating-routing (WSR) apparatus of the 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 collimator 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 select 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 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 planes (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. This 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.
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.) The reflective surface of each channel
micromirror lies in the x-y plane as defined in the figure and is
movable, e.g., pivotable or deflectable about an axis along the
x-direction in an analog, i.e., continuous, manner. Each spectral
channel, upon reflection, is deflected in the y-direction, e.g.,
downward, relative to its incident direction, so as 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 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, e.g., 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
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. The exemplary modeling results
thus described demonstrate the unique capabilities of the WSR
apparatus of the invention.
FIG. 1A is but one of many embodiments of a WSR apparatus with
which the invention may be used. 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. Each
micromirror may be pivoted about one or two axes. It is important
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 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 invention, to best
suit a given application.
A WSR apparatus embodying the 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.
FIG. 2A depicts a second embodiment of a WSR with which the
invention may be used. 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 elements 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 22-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 elements 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. 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
4-f relay (or 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 with which the
invention may be used. 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 elements 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 imaging 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 relay (or imaging) arrangement with respect to the
two-dimensional collimator-alignment mirror array 320 and the
two-dimensional fiber collimator array 350. Each of 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
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 within this specification.
FIG. 4A depicts a schematic illustration of a first embodiment of a
WSR-S apparatus according to the invention. The WSR-S apparatus 400
comprises a WSR apparatus 410 and a servo-control assembly 440. The
WSR 410 may be substantially identical to the apparatus 100 of FIG.
1A, or any other embodiment in accordance with the invention. The
servo-control assembly 440 includes a spectral monitor 460, for
monitoring the optical 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 optical 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 optical power
levels of the spectral channels coupled into the output ports. The
optical 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
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 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
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
optical power measurements from the spectral monitor 460 to provide
dynamic control of the channel micromirrors 430 along with the
collimator-alignment mirrors 485, so as 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
any one of the spectral power monitoring devices known in the art
that is capable of detecting the optical 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 optical 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 optical
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 as 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 a processing unit in a servo-control system
are known in the art. A skilled artisan would 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 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 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 invention may be used to construct a variety of
optical devices and utilized in many applications. Moreover, a
novel class of optical add-drop multiplexers (OADMs) may be built
upon the WSR-S (or WSR) apparatus of the invention, as exemplified
in the following embodiments.
FIG. 5 depicts an exemplary embodiment of an optical add-drop
multiplexer (OADM) according to the 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 receives
a multi-wavelength optical signal. The constituent spectral
channels of this optical signal 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 optical 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 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
substantially identical to 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 670 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.
As discussed above, the diffraction efficiency of a diffraction
grating is polarization-sensitive, and such polarization-sensitive
effects may give rise to significant insertion loss and
polarization-dependent loss (PDL) in an optical system. The
situation is further exacerbated in WDM optical networking
applications, where the polarization state of WDM signals is
typically indeterminate and may vary with time. This can produce an
undesirable time-varying insertion loss that may cause the optical
signals to fall below acceptable levels or render them unusable.
Thus, it is desirable to avoid such polarization-sensitive effects,
and the invention affords a polarization diversity scheme that
addresses this, as will now be described.
FIG. 7A depicts a schematic top view and FIG. 7B depicts a
schematic side view of a fifth embodiment of a WSR apparatus of the
invention that employs a polarization diversity arrangement that
minimizes polarization-sensitive effects. (The schematic top and
side views in FIGS. 7A-7B and the following figures are presented
with respect to the perspective view of FIG. 1A.) WSR apparatus 700
may make use of the general architecture and a number of the
elements used in the embodiment of FIG. 1A, as indicated by those
elements labeled with the same numerals. The input port 110-1
provides a multi-wavelength optical signal, which may be of
indeterminate time varying polarization and which may contain
wavelengths .lamda..sub.1 through .lamda..sub.M, for instance, to a
polarization-displacing unit 720. The polarization-displacing unit
may be disposed along the optical path between the array of fiber
collimators 110 (including the input port 110-1 and the output
ports 110-2 through 110-N, as shown in FIG. 7B below) and the
diffraction grating 101. The polarization-displacing unit 720
serves to separate or decompose the input multi-wavelength optical
signal into a first p-polarization component and a second
orthogonal s-polarization component. Assuming that p-polarization
is the "preferred" polarization direction of the diffraction
grating 101, i.e., the diffraction efficiency is higher for the
p-polarization component than for the s-polarization component, the
p-polarization component of the input optical signal maybe output
as a first optical signal 722 from the polarization displacing
unit. The second s-polarization component of the input optical
signal may be rotated by 90-degrees, by a polarization-rotating
unit 730 to produce a second optical signal 732 also having
p-polarization. Thus, the two optical signals 722, 732 incident
onto the diffraction grating 101 both possess p-polarization.
The first and second polarization components (optical signals 722,
732) emerging from the polarization-displacing unit 720 and the
polarization-rotating unit 730, respectively, may undergo an
unamorphic beam magnification by a beam-modifying unit 740 and
emerge as spatially separated and magnified beams 742, 744 which
impinge upon the diffraction grating 101. The configuration may be
such that the beam-modifying unit 740 preferentially enlarges the
beam size in the direction perpendicular to the groove lines on the
diffraction grating 101. This magnifies the optical beams in a
direction perpendicular to the groove lines of the grating so that
the focused beams produced by the focusing lens 102 are narrower in
this direction, i.e., perpendicular to the groove lines. This
enables use, for example, of rectangular shaped micromirrors. The
diffraction grating 101 subsequently separates the magnified first
and second polarization components 742, 744 by wavelength into
first and second sets of diffracted optical beams. Each set of
optical beams comprises multiple wavelengths .lamda..sub.1 through
.lamda..sub.M, which are diffracted by the diffraction grating 101
at different angles. The focusing lens 102 in turn focuses the
diffracted optical beams into corresponding focused spots which
impinge onto the channel micromirrors 103. Each focused spot may be
elliptical in cross-section. Further, the first and second
diffracted optical beams having the same wavelength, e.g.,
.lamda..sub.i, are arranged to impinge onto the same channel
micromirror, e.g., the channel micromirror 103-i, see FIG. 7B. In
this way, each channel micromirror handles concurrently two optical
beams having the same polarization and wavelength.
FIG. 7B depicts a schematic side view of the WSR apparatus 700,
where only the second multiple wavelength polarization component
732, 744 (on the forward path), along with the reflected first set
of optical beams (on the return path), are shown. For purposes of
illustration and clarity, several channel micromirrors are
explicitly identified in this figure, while the array of channel
micromirrors as a whole is also indicated by the numeral 103. As
described above with respect to FIGS. 1A-1B, the channel
micromirrors 103 are individually controllable and movable, e.g.,
pivotable about an axis 750 (which may be parallel to the x-axis
shown in FIG. 1B and perpendicular to the plane of FIG. 7B). Hence,
each channel micromirror is capable of directing its corresponding
optical beams into any one of the output ports 110-2 through 110-N
byway of its pivoting motion. By way of example, the channel
micromirror 103-k may be controlled to direct the first and second
optical beams with wavelength .lamda..sub.k into the first output
port 110-2; the channel micromirror 103-j may be controlled to
direct the first and second optical beams with wavelength
.lamda..sub.j into the second output port 110-3; the channel
micromirror 103-i may be controlled to direct the first and second
optical beams with wavelength .lamda..sub.i into the third output
port 110-4, and so on. Note that a plurality of the channel
micromirrors may be individually controlled to direct their
corresponding reflected optical beams into the same output
port.
Referring to FIG. 7A, the first and second sets of optical beams
reflected from the respective channel micromirrors 103 are
deflected out of the plane of the figure (as indicated by the side
view of FIG. 7B); hence they are not explicitly shown in the top
view of FIG. 7A. With reference to FIG. 7B, it will be apparent to
those skilled in the art that the reflected first and second sets
of optical beams each undergo an anamorphic beam demagnification by
way of the beam-modifying unit 740, thereby resuming the beam size
of the input optical signal. The reflected first set of optical
beams subsequently undergoes a 90-degree polarization rotation by
the polarization-rotating unit 730, whereby the reflected first and
second sets of optical beams are polarized in two orthogonal
directions upon entering the polarization-displacing unit 720. This
enables the polarization-displacing unit 720 to recombine the
reflected first and second sets of optical beams by wavelength
respectively into reflected spectral channels, prior to being
coupled into selected ones of the output ports 110-2 through
110-N.
It should be appreciated that the rotation in polarization produced
by a polarization-rotating element, e.g., the polarization-rotating
unit 730, may have slight variations about a prescribed angle,
e.g., 90-degrees, due to imperfections that may exist in a
practical system. Such variations, however, will not significantly
affect the overall performance of the invention.
In the embodiment of FIGS. 7A-7B, the polarization-displacing unit
720 may be in the form of a single polarization-displacing element,
corresponding to the array of fiber collimators 110. FIG. 7C shows
two schematic views of an exemplary embodiment of a
polarization-displacing element 720A which may be a birefringent
beam displacer well known in the art. The first schematic
represented by dashed box 761 of FIG. 7C illustrates a top view of
the polarization-displacing element 720A, where an incident optical
beam 770, e.g., the multi-wavelength optical signal in the
embodiment of FIGS. 7A-7B, is decomposed into first and second
polarization components 772, 774 polarized in two orthogonal
directions, as illustrated in the figure. Notice that the two
polarization components are spatially displaced and propagate in
parallel, upon emerging from the polarization-displacing element
720A. The second schematic represented by dashed box 762 of FIG. 7C
depicts an exemplary cross-sectional top view of the
polarization-displacing element 720A, where two parallel optical
beams 776, 778 polarized in two orthogonal directions, e.g., the
first and second optical beams associated with wavelength
.lamda..sub.i in the embodiment of FIGS. 7A-7B, are recombined by
way of traversing the polarization-displacing element 720A into a
single optical beam 780, e.g., the reflected spectral channel with
wavelength .lamda..sub.i in the embodiment of FIGS. 7A-7B. As such,
the polarization-displacing element 720A acts as a
polarization-separating element for optical beams propagating in
one direction; and serves as a polarization-combining element for
optical beams traversing in the opposite direction.
Those skilled in the art will appreciate that rather than using a
birefringent beam displacer, the polarization-displacing element
720A may alternatively be provided by a suitable
polarizing-beam-splitting element, e.g., a polarizing beam splitter
commonly used in the art along with an appropriate beam-deflector
or prism (such that the two emerging polarization components
propagate in parallel). Such a polarizing-beam-splitting element
provides a substantially similar function to the aforementioned
birefringent beam displacer. In general, a polarization-displacing
element in the invention may be embodied by any optical element
that provides a dual function of polarization separating and
combining, as depicted in FIG. 7C.
Likewise, the polarization-rotating unit 730 may comprise a single
polarization-rotating element, e.g., a half-wave plate, a liquid
crystal rotator, a Faraday rotator, or any other means known in the
art that is capable of rotating the polarization of an optical beam
by a prescribed angle, e.g., 90 degrees.
Alternatively, the polarization-displacing unit 720 may comprise a
plurality of polarization-displacing elements, each corresponding
to one or more fiber collimators 110 in the embodiment of FIGS.
7A-7B. By way of example, FIG. 7D depicts a schematic side view of
a polarization-displacing unit 720B which may be an array of
polarization-displacing elements 720-1 through 720-N. Each
polarization-displacing element may be a birefringent beam
displacer, a polarizing-beam-splitting element, or any other
suitable means known in the art, as described above with respect to
FIG. 7C. In this case, the polarization-rotating unit 730 may
include one or more polarization-rotating elements, each as
described above. As a way of example, FIG. 7D also shows a
schematic side view of a polarization-rotating unit 730B as an
array of polarization-rotating elements 730-1 through 730-N, which
may be in a one-to-one correspondence with the
polarization-displacing elements 720-1 through 720-N. As such, the
polarization-displacing unit 720B, along with the
polarization-rotating unit 730B, may be implemented in the
embodiment of FIGS. 7A-7B so that the polarization-displacing
elements 720-1 through 720-N are in a one-to-one correspondence
with the fiber collimators 100 that provide the input port 110-1
and the output ports 110-2 through 110-N.
Those skilled in the art will appreciate that the exemplary
embodiments of FIGS. 7C-7D are provided as an example to illustrate
how a polarization-displacing unit, along with a
polarization-rotating unit, may be configured and operated in the
invention. Various changes and modifications may be made in this
embodiment to perform the designated functions in a substantially
equivalent manner. For example, the polarization-displacing unit
720, along with the polarization-rotating unit 730, may
alternatively be configured such that the first and second
polarization components are spatially separated along a vertical
direction that is substantially perpendicular to the plane of the
paper in the schematic top view of FIG. 7A, as opposed to being
separated horizontally in a manner as illustrated in FIG. 7A. As
will be appreciated from the teachings of the invention, one
skilled in the art would know how to implement an appropriate
polarization-displacing unit, along with a suitable
polarization-displacing unit, in a WSR apparatus, for a given
application.
Moreover, the beam-modifying unit 740 may comprise an assembly of
cylindrical lenses or prisms, in optical communication with the
polarization-displacing unit 720 along with the
polarization-rotating unit 730 and the diffraction grating 101. In
general, a beam-modifying unit may be embodied by any optical
structure that is capable of magnifying the input optical signal
and de-magnifying the reflected optical beams according to a
predetermined ratio. Such a beam-modifying unit may be particularly
useful in applications that call for a refined spectral resolution,
such as DWDM optical networking applications.
The WSR apparatus 700 of FIGS. 7A-7B is substantially similar to
the WSR apparatus 100 of FIG. 1A in operation and function and
hence achieves the advantages thereof. Furthermore, the described
polarization diversity approach renders the polarization
sensitivity of the diffraction grating 101 inconsequential in the
WSR apparatus 700. This enables the WSR apparatus 700 to minimize
the insertion loss. It also allows the WSR apparatus 700 to enhance
the spectral resolution in a simple and cost-effective
construction, e.g., by making use of high-dispersion holographic
diffraction gratings commonly available in the art. Another notable
feature of the polarization diversity scheme is that the first and
second optical beams associated with each wavelength (corresponding
to the two polarization components of each spectral channel)
effectively "exchange" their respective optical paths, upon
reflection from the micromirror, i.e., the return path of the
reflected second optical beam is substantially similar to the
forward path of the first optical beam, and vice versa. This has
the important consequence of substantially equalizing the overall
optical paths of the two polarization components for each spectral
channel, thereby minimizing the polarization-dependent loss (PDL)
and polarization-mode dispersion (PMD). These attributes are
desirable in many applications.
Those skilled in the art will appreciate that the WSR apparatus 700
of FIGS. 7A-7B may be further modified in various ways according to
the teachings of the invention. For example, the apparatus may
include an array of collimator-alignment mirrors such as described
with respect to the embodiment of FIG. 2A, 2B, or 3. FIG. 8A shows
a schematic top view of a sixth embodiment of a WSR apparatus 800A
of the invention which employs an array of collimator-alignment
mirrors 220 in a polarization diversity arrangement such as shown
in FIGS. 7A-7B. For example, WSR apparatus 800A may be built upon
the embodiments of FIGS. 2A and 7A, hence similar elements are
labeled with the same numerals. In FIG. 8A, the array of
collimator-alignment mirrors 220 (which may include the
collimator-alignment mirrors 220-1 through 220-N, as shown in FIG.
2A) may be disposed along the optical path between the fiber
collimators 110 and the polarization-displacing unit 720, such that
there is a one-to-one correspondence between the
collimator-alignment mirrors 220 and the fiber collimators 110
providing the input and output ports. As described with respect to
FIG. 2A, the collimator-alignment mirrors 220 may be controlled to
adjust the alignment of the input multi-wavelength optical signal
and to further provide angular control of the collimated beams of
the reflected spectral channels. This facilitates the coupling of
the reflected spectral channels into the respective output ports
according to desired coupling efficiencies.
In the embodiment of FIG. 8A, the collimator-alignment mirrors 220
are disposed between the fiber collimators 110 and the
polarization-displacing unit 720, and control the angular position
of the (un-split) multi-wavelength input optical signal as well as
the (combined) reflected spectral channels. There may be
applications where it is desired to provide separate control to the
first and second polarization components (on the forward path), as
well as to the reflected first and second sets of optical beams (on
the return path). FIG. 8B depicts a schematic top view of a seventh
embodiment of a WSR apparatus 800B of the invention, which achieves
this. WSR apparatus 800B of FIG. 8B may be built upon the
embodiment of FIG. 8A, hence similar elements are labeled with the
same numerals. In this case, a polarizing-beam-splitter unit 820
may be employed instead of the polarization-displacing unit 720 of
FIG. 8A, to decompose the multi-wavelength input optical signal
into first and second polarization components 822, 824 that are
propagating in two orthogonal directions. The second polarization
component 824 may be subsequently incident onto and reflected by a
first beam-deflecting unit 222, whereby it propagates parallel to
the first polarization component 822. The operation thereafter is
substantially similar to that of FIG. 8A. On the return path, the
reflected first set of optical beams is incident onto and reflected
by the beam-deflecting unit 222, so as to enable the
polarizing-beam-splitter unit 820 to recombine the reflected first
and second sets of optical beams by wavelength respectively into
reflected spectral channels.
In the embodiment of FIG. 8B, the polarizing-beam-splitter unit 820
may be a single polarizing beam splitter known in the art, in
optical communication with the array of fiber collimators 110 via
the collimator-alignment mirrors 220. It may also comprise an array
of polarizing beam splitters, e.g., in a one-to-one correspondence
with the collimator-alignment mirrors 220. The first
beam-deflecting unit 222 may comprise an array of first mirrors,
e.g., in a one-to-one correspondence with the collimator-alignment
mirrors 220. The first mirrors 222 may be individually adjustable,
so as to control the relative alignment and thereby ensure the
requisite beam parallelism between the first and second
polarization components on the forward path, which in turn warrants
the first and second optical beams associated with each wavelength
substantially coincide on the same channel micromirror. On the
return path, the first mirrors 222 may likewise adjust the relative
alignment between the reflected first and second sets of optical
beams respectively, thereby ensuring that the reflected first and
second sets of optical beams are properly recombined into the
respective spectral channels by way of the polarizing-beam-splitter
unit 820. The first mirrors 222 may be controlled on a dynamic
basis. Alternatively, the first mirrors 222 may be adjusted to
predetermined positions to enable the polarizing-beam-splitter unit
820 to achieve the requisite beam parallelism. The first mirrors
222 may be subsequently fixed in respective positions over the
course of operation. (In this way, the tolerances required for the
polarizing-beam-splitter unit 820 may be relaxed.) It should be
further appreciated that the first beam-deflecting unit 222 may
also be a static mirror, or any other beam-deflecting means known
in the art, configured such that the combination of the
polarizing-beam-splitter unit 820 and the first beam-deflecting
unit 222 effectively constitutes a polarization-displacing unit as
described above.
FIG. 8C depicts a schematic top view of an eighth embodiment of a
WSR apparatus 800C of the invention. WSR apparatus 800C may include
the elements employed in the embodiment of FIG. 8B, along with
second and third beam-deflecting units 224, 226. The second
beam-deflecting unit 224 may comprise an array of second mirrors
that are individually adjustable, e.g., in a one-to-one
correspondence with the first mirrors the first beam-deflecting
unit 222 may contain. The third beam-deflecting unit 226 may simply
be a static mirror, or other known beam-deflecting device. In this
way, the first and second polarization components 822, 824 (on the
forward path) may be independently controlled by the first and
second beam-deflecting units 222, 224, which may also control the
reflected first and second sets of optical beams (on the return
path), respectively. The collimator-alignment mirrors 220 may
further facilitate the coupling of the (combined) reflected
spectral channels into the desired output ports.
The WSR apparatus 700 (or any one of the embodiments of FIGS.
8A-8C) of the invention may further incorporate a servo-control
assembly, e.g., in a manner as described with respect to FIG. 4A
(or 4B) above. The servo-control assembly may dynamically manage
the optical power levels of the reflected spectral channels coupled
into the output ports. The servo-control assembly may also be
configured such to minimize PDL associated with the spectral
channels.
Furthermore, a dynamically reconfigurable OADM may be built upon
the WSR apparatus 700, 800A, 800B or 800C (along with an associated
servo-control assembly), e.g., in a manner similar to that
described with respect to FIG. 5 or 6. The thus-constructed OADMs
will have important advantages of low insertion loss, low PDL, and
enhanced spectral resolution, which would be particularly suitable
for WDM optical networking applications.
Those skilled in the art will recognize that the aforementioned
embodiments are provided by way of example to illustrate the
general principles of the invention. Various changes,
substitutions, and alternations can be made without departing from
the principles and the scope of the invention as defined in the
appended claims.
* * * * *