U.S. patent application number 09/922989 was filed with the patent office on 2003-02-13 for bandwidth variable wavelength router and method of operation.
This patent application is currently assigned to Chorum Technologies LP. Invention is credited to Cheng, Chi-Hao, Li, Shuxin, Liu, Jian-Yu, Wu, Kuang-Yi, Xia, Tiejun.
Application Number | 20030030881 09/922989 |
Document ID | / |
Family ID | 25447926 |
Filed Date | 2003-02-13 |
United States Patent
Application |
20030030881 |
Kind Code |
A1 |
Xia, Tiejun ; et
al. |
February 13, 2003 |
BANDWIDTH VARIABLE WAVELENGTH ROUTER AND METHOD OF OPERATION
Abstract
An optical device comprises a first birefringent crystal having
a first length, a second birefringent crystal having a second
length, and a dynamic polarization rotator. An optical signal
propagating through the first and second birefringent crystals has
an effective optical path length based, at least in part, upon the
first length of the first birefringent crystal and the second
length of the second birefringent crystal. The dynamic polarization
rotator adjusts the effective optical path length of the optical
signal in response to a control signal.
Inventors: |
Xia, Tiejun; (Richardson,
TX) ; Cheng, Chi-Hao; (Dallas, TX) ; Li,
Shuxin; (Plano, TX) ; Liu, Jian-Yu; (Garland,
TX) ; Wu, Kuang-Yi; (Plano, TX) |
Correspondence
Address: |
Baker Botts L.L.P.
2001 Ross Avenue, Suite 600
Dallas
TX
75201-2980
US
|
Assignee: |
Chorum Technologies LP
|
Family ID: |
25447926 |
Appl. No.: |
09/922989 |
Filed: |
August 3, 2001 |
Current U.S.
Class: |
359/246 ;
359/320 |
Current CPC
Class: |
G02F 1/313 20130101 |
Class at
Publication: |
359/246 ;
359/320 |
International
Class: |
G02F 001/03; G02F
001/07; G02F 001/29 |
Claims
What is claimed is:
1. An optical device, comprising: a first birefringent crystal
having a first length; a second birefringent crystal having a
second length, wherein an optical signal propagating through the
first and second birefringent crystals has an effective optical
path length based, at least in part, upon the first length of the
first birefringent crystal and the second length of the second
birefringent crystal; and a dynamic polarization rotator operable
to adjust the effective optical path length of the optical signal
in response to a control signal.
2. The device of claim 1, wherein the dynamic polarization rotator
is operable to change the polarization state of at least a portion
of the optical signal by approximately ninety degrees such that the
effective optical path length is based, at least in part, upon the
difference between the first length of the first birefringent
crystal and the second length of the second birefringent
crystal.
3. The device of claim 1, wherein the dynamic polarization rotator
is operable to change the polarization state of at least a portion
of the optical signal by either approximately zero degrees or
approximately three-hundred-sixty degrees such that the effective
optical path length is based, at least in part, upon the addition
of the first length of the first birefringent crystal and the
second length of the second birefringent crystal.
4. The device of claim 1, further comprising a third birefringent
crystal having a third length and a second dynamic polarization
rotator operable to adjust the effective optical path length of the
optical signal in response to a second control signal.
5. The device of claim 1, wherein the first length of the first
birefringent crystal is not equal to the second length of the
second birefringent crystal.
6. The device of claim 1, further comprising a plurality of
birefringent waveplates arranged such that when the optical signal
propagates through the first and second birefringent crystals, the
polarization rotator, and the plurality of birefringent waveplates,
it is processed into a first subset of wavelengths having a first
polarization and a second subset of wavelengths having a second
polarization.
7. The device of claim 6, wherein the first subset of wavelengths
and the second subset of wavelengths each comprise a plurality of
wavelength channels, each wavelength channel having a particular
bandwidth such that: if the dynamic polarization rotator changes
the polarization state of beam components associated with the
optical signal by approximately ninety degrees, then each
wavelength channel has a first bandwidth; and if the dynamic
polarization rotator changes the polarization state of beam
components associated with the optical signal by either
approximately zero degrees or approximately three-hundred-sixty
degrees, then each wavelength channel has a second bandwidth that
is narrower than the first bandwidth.
8. The device of claim 6, wherein the plurality of birefringent
waveplates are oriented at a substantially common angle about an
optical axis, the optical device further comprising a plurality of
polarization rotators arranged among the plurality of birefringent
waveplates.
9. The device of claim 8, wherein each of the birefringent
waveplates introduces a phase delay between a first polarization
component of the optical signal and a second polarization component
of the optical signal.
10. The device of claim 9, wherein each of the polarization
rotators arranged among the birefringent waveplates is operable to
change the polarization state of at least one of the first
polarization component and the second polarization component.
11. The device of claim 8, wherein: a first polarization rotator
arranged among the birefringent waveplates is oriented at a first
angle about the optical axis; and a second polarization rotator
arranged among the birefringent waveplates is oriented at a second
angle about the optical axis.
12. The device of claim 11, wherein the first and second subsets of
wavelengths are based, at least in part, upon the first angle of
the first polarization rotator and the second angle of the second
polarization rotator.
13. The device of claim 8, wherein the first and second subsets of
wavelengths are based, at least in part, upon the common angle of
the plurality of birefringent waveplates.
14. The device of claim 8, wherein the common angle of the
plurality of birefringent waveplates is approximately zero
degrees.
15. The device of claim 8, wherein the first subset of wavelengths
is approximately complementary to the second subset of
wavelengths.
16. The device of claim 11, wherein adjusting at least one of the
first angle of the first polarization rotator and the second angle
of the second polarization rotator adjusts the first and second
wavelength subsets.
17. The device of claim 1, further comprising a plurality of
birefringent waveplates arranged such that a first signal
comprising a first subset of wavelengths having a first
polarization and a second signal comprising a second subset of
wavelengths having a second polarization are processed by the first
and second birefringent crystals, the polarization rotator, and the
birefringent waveplates into a third signal comprising the first
and second subsets of wavelengths.
18. The device of claim 8, wherein at least one of the plurality of
polarization rotators arranged among the birefringent waveplates
comprises a dynamic half-wave plate operable to change the
polarization state of beam components associated with the optical
signal.
19. The device of claim 18, wherein the dynamic half-wave plate
changes the polarization state of the beam components such that the
first polarization of the first subset of wavelengths is
substantially orthogonal to the second polarization of the second
subset of wavelengths.
20. The device of claim 18, wherein the dynamic half-wave plate
changes the polarization state of the beam components such that the
first polarization of the first subset of wavelengths is
substantially equal to the second polarization of the second subset
of wavelengths.
21. The device of claim 19, further comprising a polarization based
routing waveplate operable to route the first subset of wavelengths
for receipt by a first output port and the second subset of
wavelengths for receipt by a second output port.
22. The device of claim 20, further comprising a polarization based
routing waveplate operable to route the first subset of wavelengths
and the second subset of wavelengths for receipt by an output
port.
23. A method for adjusting the effective optical path length of an
optical signal, comprising: propagating an optical signal through a
first birefringent crystal having a first length; propagating the
optical signal through a second birefringent crystal having a
second length; rotating the polarization of beam components
associated with the optical signal such that the effective optical
path length of the optical signal is based, at least in part, upon
the first length of the first birefringent crystal and the second
length of the second birefringent crystal.
24. The method of claim 23, wherein rotating comprises rotating the
polarization of the beam components by approximately ninety degrees
such that the effective optical path length of the optical signal
is based, at least in part, upon the difference between the first
length of the first birefringent crystal and the second length of
the second birefringent crystal.
25. The method of claim 23, wherein rotating comprises rotating the
polarization of the beam components by either approximately zero
degrees or approximately three-hundred-sixty degrees such that the
effective optical path length of the optical signal is based, at
least in part, upon the addition of the first length of the first
birefringent crystal and the second length of the second
birefringent crystal.
26. The method of claim 23, wherein the first length of the first
birefringent crystal is substantially equal to the second length of
the second birefringent crystal.
27. The method of claim 23, wherein the first length of the first
birefringent crystal is not substantially equal to the second
length of the second birefringent crystal.
28. The method of claim 23, further comprising propagating the
optical signal through a plurality of birefringent waveplates such
that the optical signal is processed into a first subset of
wavelengths having a first polarization and a second subset of
wavelengths having a second polarization.
29. The method of claim 28, wherein the first subset of wavelengths
and the second subset of wavelengths each comprise a plurality of
wavelength channels, each wavelength channel having a particular
bandwidth, such that: if the step of rotating comprises rotating
the polarization of beam components by approximately ninety
degrees, then each wavelength channel has a first bandwidth; and if
the step of rotating comprises rotating the polarization of beam
components by either approximately zero degrees or approximately
three-hundred-sixty degrees, then each wavelength channel has a
second bandwidth that is narrower than the first bandwidth.
30. The method of claim 28, wherein the plurality of birefringent
waveplates are oriented at a common angle about an optical
axis.
31. The method of claim 30, further comprising imparting a phase
delay between a first polarization component of the optical signal
and a second polarization component of the optical signal using at
least one of the plurality of birefringent waveplates.
32. The method of claim 31, further comprising rotating the
polarization of at least one of the first polarization component
and the second polarization component using one of a plurality of
polarization rotators arranged among the plurality of birefringent
waveplates.
33. The method of claim 32, wherein: a first polarization rotator
arranged among the birefringent waveplates is oriented at a first
angle about the optical axis; and a second polarization rotator
arranged among the birefringent waveplates is oriented at a second
angle about the optical axis.
34. The method of claim 33, wherein the first and second subsets of
wavelengths are based, at least in part, upon the first angle of
the first polarization rotator and the second angle of the second
polarization rotator.
35. The method of claim 30, wherein the first and second subsets of
wavelengths are based, at least in part, upon the common angle of
the plurality of birefringent waveplates.
36. The method of claim 30, wherein the common angle of the
plurality of birefringent waveplates is approximately zero
degrees.
37. The method of claim 28, wherein the first subset of wavelengths
is approximately complementary to the second subset of
wavelengths.
38. The method of claim 33, wherein adjusting at least one of the
first angle of the first polarization rotator and the second angle
of the second polarization rotator adjusts the first and second
wavelength subsets.
39. The method of claim 30, further comprising rotating the
polarization of beam components associated with the optical signal
using one of a plurality of polarization rotators arranged among
the plurality of birefringent waveplates such that the first
polarization of the first subset of wavelengths is substantially
orthogonal to the second polarization of the second subset of
wavelengths.
40. The method of claim 30, further comprising rotating the
polarization of beam components associated with the optical signal
using one of a plurality of polarization rotators arranged among
the plurality of birefringent waveplates such that the first
polarization of the first subset of wavelengths is substantially
equal to the second polarization of the second subset of
wavelengths.
41. The method of claim 39, further comprising routing the first
subset of wavelengths for receipt by a first output port and the
second subset of wavelengths for receipt by a second output
port.
42. The method of claim 40, further comprising routing the first
subset of wavelengths and the second subset of wavelengths for
receipt by an output port.
43. An optical wavelength router, comprising: a first input port
operable to receive a first input signal having a first bit-rate; a
second input port operable to receive a second input signal having
a second bit-rate; and an optical device comprising: a first
birefringent crystal having a first length; a second birefringent
crystal having a second length, wherein an optical signal
propagating through the first and second birefringent crystals has
an effective optical path length based, at least in part, upon the
first length of the first birefringent crystal and the second
length of the second birefringent crystal; and a dynamic
polarization rotator operable to adjust the effective optical path
length of the optical signal in response to a control signal such
that the optical device operates in the first state to process the
first input signal and the optical device operates in the second
state to process the second input signal.
44. The router of claim 43, wherein the first bit-rate is different
from the second bit-rate.
45. The router of claim 43, wherein: the first input signal
comprises a plurality of wavelength channels, each wavelength
channel having a first spectral bandwidth; the second input signal
comprises a plurality of wavelength channels, each wavelength
channel having a second spectral bandwidth different from the first
spectral bandwidth.
46. The router of claim 43, further comprising: a demultiplexer
operable to demultiplex the first input signal into a plurality of
wavelength channels; and a switch operable to process the plurality
of wavelength channels associated with the first input signal.
47. The router of claim 46, wherein the optical device further
comprises a plurality of birefringent waveplates operable to
multiplex the plurality of wavelength channels associated with the
first input signal to form an output signal.
48. The router of claim 47, wherein the birefringent waveplates are
oriented at a substantially common angle about an optical axis and
further comprising a plurality of polarization rotators arranged
among the plurality of birefringent waveplates.
49. The router of claim 43, wherein the dynamic polarization
rotator is operable to change the polarization state of at least a
portion of the first input signal by approximately ninety degrees
such that the effective optical path length of the first input
signal is based, at least in part, upon the difference between the
first length of the first birefringent crystal and the second
length of the second birefringent crystal.
50. The router of claim 43, wherein the dynamic polarization
rotator is operable to change the polarization state of at least a
portion of the first input signal by either approximately zero
degrees or approximately three-hundred-sixty degrees such that the
effective optical path length of the first input signal is based,
at least in part, upon the addition of the first length of the
first birefringent crystal and the second length of the second
birefringent crystal.
Description
RELATED APPLICATIONS
[0001] This application and copending application serial number
______ , entitled "Switchable Wavelength Router and Method of
Operation", filed on ______ ; and copending application serial
number ______ , entitled "Wavelength Router and Method of
Operation", filed on ______share portions of a common
specification. These applications have been commonly assigned to
Chorum Technologies LP.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to optical communication
systems, and more particular, to a bandwidth variable wavelength
router.
BACKGROUND OF THE INVENTION
[0003] A wavelength router has applications in wavelength division
multiplexed (WDM) optical networking environments. Prior designs
for wavelength routers based upon polarization based techniques are
often difficult to manufacture, provide static spectral processing,
and are not effective in multi-bit-rate networking
environments.
SUMMARY OF THE INVENTION
[0004] In one embodiment of the present invention, an optical
device comprises a plurality of birefringent waveplates and a
plurality of polarization rotators. The birefringent waveplates are
oriented at a substantially common angle about an optical axis. The
polarization rotators are arranged among the plurality of
birefringent waveplates such that a wavelength division multiplexed
optical signal propagating through the polarization rotators and
the birefringent waveplates is processed into a first subset of
wavelengths comprising substantially a first polarization and a
second subset of wavelengths comprising substantially a second
polarization.
[0005] In another embodiment of the present invention, an optical
device comprises a first birefringent crystal having a first
length, a second birefringent crystal having a second length, and a
dynamic polarization rotator. An optical signal propagating through
the first and second birefringent crystals has an effective optical
path length based, at least in part, upon the first length of the
first birefringent crystal and the second length of the second
birefringent crystal. The dynamic polarization rotator adjusts the
effective optical path length of the optical signal in response to
a control signal.
[0006] In yet another embodiment of the present invention, an
optical device comprises a plurality of birefringent waveplates and
a plurality of polarization rotators. The polarization rotators are
arranged among the plurality of birefringent waveplates such that
an optical signal propagating through the polarization rotators and
the birefringent waveplates is processed into a first subset of
wavelengths comprising substantially a first polarization and a
second subset of wavelengths comprising substantially a second
polarization. At least one of the plurality of polarization
rotators is operable to change the polarization state of beam
components associated with the optical signal.
[0007] Some, none, or all of the embodiments described herein may
embody some, none, or all of the advantages described herein.
Manufacturing birefringent crystals or waveplates having unique
angles, as with prior art wavelength routers, is a delicate
process. An advantage provided by at least one embodiment of the
present invention is that the birefringent waveplates are all
arranged at a substantially common angle, (e.g., approximately zero
degrees) with respect to a reference optical axis. In this respect,
the cost and complexity associated with manufacturing and arranging
the birefringent waveplates is reduced. For example, a designer is
free to choose any common angle for all of the birefringent
waveplates. As a result, angles near vulnerable cleavage planes,
which induce chipping or cracking, can be readily avoided. Damage
and waste are further reduced through efficient raw material
utilization. For example, because all birefringent crystals are cut
at a substantially common angle, an angle can be selected which
results in the best yield.
[0008] Not only do the principles of the present invention
advantageously reduce complexity and enhance flexibility of design
and fabrication as described above, they facilitate a compact
single piece waveplate implementation of a wavelength router. For
example, because the birefringent waveplates may be oriented at a
substantially common angle, it becomes possible to replace multiple
longitudinally aligned individual birefringent waveplates with
fewer waveplates arranged with the polarization rotators in a
compact assembly that uses an optical beam path that is folded. In
one embodiment, the multiple birefringent waveplates may be
replaced by a single birefringent waveplate oriented at an angle.
The compact size of the wavelength router results in higher optical
device densities and a robust operation.
[0009] In a particular embodiment of the present invention, the
spectral bandwidth of the wavelength channels associated with
output signals is made variable in response to control signals
applied to portions of the birefringent waveplates. By implementing
the birefringent waveplates using a dynamic polarization rotator
positioned between birefringent crystals, the effective optical
path length propagating through the birefringent waveplate can be
increased or decreased. By increasing the optical path length of an
optical signal, the bandwidth of each wavelength channel associated
with the output signals is narrowed. By decreasing the optical path
length of an optical signal, the bandwidth of each wavelength
channel associated with the output signals is widened. As a result,
the use of a dynamic polarization rotator to control the effective
path length of an optical signal facilitates variable bandwidth
wavelength routing.
[0010] In another embodiment of the present invention, dynamic
polarization rotators may be operated by the application of control
signals to produce a switchable wavelength router. A technical
advantage of a switchable wavelength router is that it provides a
switchable beam path control in optical network applications. This
allows the switchable wavelength router to function as an optical
wavelength router in an optical network and to perform, for
example, protection switching and restoration of optical data
paths. Additionally, it can recognize new wavelength bands and
switch subsets of wavelength channels among outputs.
[0011] Other technical advantages of the present invention will be
readily apparent to one skilled in the art from the following
figures, description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a more complete understanding of the present invention,
and for further features and advantages thereof, reference is now
made to the following description taken in conjunction with the
accompanying drawings, in which:
[0013] FIG. 1 illustrates one embodiment of a wavelength
router;
[0014] FIG. 2A is a perspective diagram illustrating an angle of
cutting or rotation of an exemplary birefringent crystal;
[0015] FIG. 2B is a perspective diagram illustrating an angle of
cutting or rotation of an exemplary polarization rotator having an
optical axis;
[0016] FIGS. 3A-3B illustrates another embodiment of a wavelength
router;
[0017] FIG. 4 illustrates one embodiment of a bandwidth variable
wavelength router;
[0018] FIG. 5 is a graphical illustration showing relative
amplitude varying by frequency in various stages of operation;
[0019] FIG. 6 is a schematic representation illustrating various
principles of a bandwidth variable wavelength router;
[0020] FIG. 7 is a graphic representation of the relative spectral
responses of wavelength channels processed by the exemplary
birefringent crystal configurations of FIG. 6 depicting bandwidths
in arbitrary wavelength units horizontally and relative amplitudes
vertically;
[0021] FIGS. 8A-8B illustrate another embodiment of a bandwidth
variable wavelength router;
[0022] FIG. 9 is a simplified block diagram illustrating one
example of an application of a bandwidth variable wavelength
router;
[0023] FIG. 10 illustrates one embodiment of a cascaded
architecture that includes a wavelength router communicatively
coupled to bandwidth variable wavelength routers;
[0024] FIG. 11 illustrates one embodiment of a switchable
wavelength router;
[0025] FIG. 12 illustrates one example of the operation of
switchable wavelength router to yield four different output
states;
[0026] FIGS. 13A-13B illustrate still another embodiment of a
wavelength router; and
[0027] FIGS. 14A-14B illustrate one example application of
switchable wavelength routers to facilitate first and second
optical communication traffic patterns.
DETAILED DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 illustrates one embodiment of a wavelength router 10
that includes a first birefringent element 12, a first polarization
rotator 14, wavelength filter 16, polarization dependent routing
element 18, second and third polarization rotators 20 and 24, and
second and third birefringent elements 22 and 26. In general,
router 10 receives an input signal 28 and, based at least in part
upon the orientation of elements within filter 16, generates output
signals 30 and 32 comprising particular wavelength channels of
input signal 28.
[0029] Birefringent elements 12, 22, and 26 comprise birefringent
materials that allow a particularly polarized portion of an optical
signal (e.g., vertically polarized portion) to pass through without
changing course because they are ordinary waves in the element. In
contrast, oppositely polarized waves (e.g., horizontally polarized
portion) are redirected at an angle because of the birefringent
walk-off effect. The angle of redirection is a well-known function
of the particular material chosen. Examples of materials suitable
for construction of the elements 12, 22, and 26 include calcite,
rutile, lithium niobate, YVO4 based crystals, and the like.
[0030] Polarization rotators 14, 20, and 24 comprise twisted
nematic liquid crystal rotators, ferroelectric liquid crystal
rotators, pi-cell based liquid crystal rotators, magneto-optic
based Faraday rotators, acousto-optic or electro-optic based
polarization rotators, or any other suitable device that shifts,
rotates, or otherwise changes the polarization state of an optical
signal by a desired degree. In a particular embodiment, rotators
14, 20, and 24 change the polarization state of an optical signal
or beam component by either zero degrees (i.e., no rotation) or by
ninety degrees.
[0031] Polarization dependent routing element 18 comprises
polarization beam splitters (PBS), birefringent elements, or any
other suitable device that routes beam components of an optical
signal along particular optical paths based upon the polarization
of the particular beam component. Although element 18 is
illustrated in FIG. 1 as a pair of polarization beam splitters, it
should be understood that other types of polarization dependent
routing elements may be substituted without departing from the
scope of the invention.
[0032] Input signal 28 comprises an optical signal having a
plurality of wavelength channels (e.g., a wavelength division
multiplexed (WDM) or a dense wavelength division multiplexed (DWDM)
optical signal). Each channel has a particular range of wavelengths
(or frequencies). As used herein, the terms "channel" or "spectral
band" refer to a particular wavelength or range of wavelengths (or
frequencies) that define a unique information signal or group of
signals. Each channel is usually evenly spaced from adjacent
channels, although this is not necessary. Output signals 30 and 32
each comprise an optical signal that includes a subset of the
wavelength channels associated with input signal 28.
[0033] Wavelength filter 16 comprises birefringent waveplates
50a-c, generally referred to as a birefringent waveplate 50,
arranged among polarization rotators 52a-d, generally referred to
as a polarization rotator 52. Although FIG. 1 illustrates three
birefringent waveplates 50 and four polarization rotators 52, it
should be understood that any suitable number of birefringent
waveplates 50 and polarization rotators 52 may be used to perform
the wavelength filtering process described herein. In a particular
embodiment, waveplates 50 and rotators 52 are arranged in an
alternating sequence starting and ending with rotators 52 as
illustrated in FIG. 1. Polarization rotators 52 generally comprise
static half-wave plates. In one embodiment described in greater
detail with respect to FIG. 11, polarization rotators 52 may
comprise dynamic half-wave plates that shift, rotate, or otherwise
change the polarization state of a signal by a desired degree in
response to a control signal.
[0034] Each birefringent waveplate 50 comprises any suitable number
and combination of birefringent crystals that impart a particular
amount of phase delay between beam components of an optical signal
based, at least in part, upon the indices of refraction associated
with the birefringent material used to construct the birefringent
crystals. In one embodiment, birefringent waveplate 50 comprises a
single birefringent crystal. In other embodiments, described in
greater detail below with regard to FIGS. 4-6, birefringent
waveplate 50 comprises an arrangement of birefringent crystals and
one or more polarization rotators. In prior wavelength filters,
birefringent waveplates 50 are oriented at different angles with
respect to a particular reference optical axis. This cutting or
rotation angle .theta. is described in greater detail below with
respect to FIG. 2A. By arranging each waveplate 50 at a unique
angle .theta. with respect to the reference optical axis, each
waveplate 50 imparts a particular degree of polarization rotation
upon beam components of an optical signal. Polarization rotators 52
are therefore not used in prior wavelength filters. Waveplates 50
used in prior wavelength filters therefore impart phase delay and
perform polarization rotation to achieve a desired transmission
spectrum.
[0035] Manufacturing birefringent crystals having unique angles
.theta. for prior art wavelength routers is a delicate process. An
advantage provided by the present invention is that birefringent
waveplates 50 are all arranged at a common angle, .theta., (e.g.,
zero degrees) with respect to the reference optical axis. In this
respect, the cost and complexity associated with manufacturing and
arranging birefringent waveplates 50 is reduced. For example, a
designer is free to choose any common angle .theta. for all of the
birefringent waveplates 50. As a result, angles .theta. near
vulnerable cleavage planes, which induce chipping or cracking, can
be readily avoided. Damage and waste are further reduced through
efficient raw material utilization. For example, because all
birefringent crystals are cut at an approximately common angle
.theta., an angle .theta. can be selected which results in the best
yield.
[0036] Polarization rotators 52 are then arranged at particular
angles, .phi., with respect to the reference optical axis. For
example, polarization rotator 52a may be oriented at a first angle
with respect to the reference optical axis while polarization
rotator 52b may be oriented at a second angle with respect to the
reference optical axis. The combination of birefringent waveplates
50 and polarization rotators 52 therefore provides the appropriate
amount of phase delay and polarization rotation upon the beam
components of an optical signal to provide the desired transmission
spectrum. Another advantage provided by the present invention is
that polarization rotators 52 may be manufactured from materials
(e.g., quartz) that are easier to obtain and fabricate at
particular angles .phi. without chipping or cracking.
[0037] In operation, birefringent element 12 spatially separates
input signal 28 into a first beam component 56 having a first
polarization (e.g., vertical polarization, indicated using a dot)
and a second beam component 58 having a second polarization (e.g.,
horizontal polarization, indicated using a line). Beam components
56 and 58 may also be referred to as polarization components 56 and
58. Polarization rotator 14 changes the polarization state of beam
component 56 such that it has the same polarization (e.g.,
horizontal polarization) as beam component 58. Alternatively,
polarization rotator 14 may be positioned such that it changes the
polarization state of beam component 58 to match that of beam
component 56.
[0038] In the embodiment illustrated in FIG. 1, wavelength filter
16 receives beam components 56 and 58 having at least substantially
horizontal polarizations. The combination of birefringent
waveplates 50 and polarization rotators 52 associated with filter
16 imparts a phase delay and a polarization state change upon beam
components 56 and 58 to generate two eigen states for each beam
component 56 and 58. The first eigen state carries a first subset
of wavelength channels associated with signal 28 with the same
polarization as the beam component 56 and 58 received by filter 16
(e.g., horizontal polarization, as depicted in FIG. 1). The second
eigen state carries a second, complementary, subset of wavelength
channels with an approximately orthogonal polarization (e.g.,
vertical polarization, as depicted in FIG. 1). In this respect, the
polarization of the incoming beam component 56 and 58 and the two
output polarizations for each beam component 56 and 58 form a pair
of spectral responses. By manipulating the orientation angle .phi.
of one or more polarization rotators 52 with respect to the
reference optical axis, the amount of phase delay introduced by
particular birefringent waveplates 50 may be increased or
decreased. In this respect, the first and second subsets of
wavelength channels may be controlled.
[0039] In a particular embodiment, birefringent waveplates 50 and
polarization rotators 52 are arranged such that alternating
wavelength channels are coded with one of horizontal or vertical
polarization and the complementary wavelength channels are coded
with the other of horizontal or vertical polarization (e.g., even
channels coded with horizontal polarization and odd channels coded
with vertical polarization, or vice-versa). A wavelength router 10
that achieves such a symmetric output spectra may be followed by
additional stages of wavelength routers 10 in a cascaded assembly
to form a demultiplexer. Each cascaded wavelength router 10 has a
narrower spectral response to further slice the wavelength spectra
and produce even narrower spectral bandwidths. A particular type of
cascaded assembly is described in greater detail with reference to
FIG. 10.
[0040] In another embodiment, a particular asymmetric output
spectra may be achieved so that the wavelength router 10 may be
used as an add/drop filter in a WDM network node. In this
embodiment, a specific wavelength channel or subset of wavelength
channels may be added or dropped through the narrower band of
asymmetric spectra of the wavelength router 10, while the remaining
wavelength channels continue past the wavelength router 10 through
the wider complementary spectrum. This allows WDM signals to enter
or leave a WDM network at a particular node.
[0041] Polarization dependent routing element 18 routes the first
and second subsets of wavelength channels based upon their
polarizations. For example, element 18 directs the first subset of
wavelength channels having a horizontal polarization along a first
optical path toward birefringent element 22. Element 18 directs the
second subset of wavelength channels having a vertical polarization
along a second optical path toward birefringent element 26. To
recombine the spectra of the first subset of wavelength channels,
polarization rotator 20 and birefringent element 22 are used. To
recombine the spectra of the second subset of wavelength channels,
polarization rotator 24 and birefringent element 26 are used.
Output signal 30 therefore comprises the first subset of wavelength
channels associated with input signal 28 while output signal 32
comprises the second, complementary, subset of wavelength
channels.
[0042] FIG. 2A is a perspective diagram illustrating the angle of
cutting or rotation of an exemplary birefringent crystal, such as,
in one embodiment, birefringent waveplate 50. A birefringent
crystal has a first refractive index n1 for light polarized along
an optical axis 70 and a second refractive index n2 for light
polarized along an optical axis 72 perpendicular to optical axis
70. For convenience, laboratory axes are labeled x, y, and z, with
an input optical beam 74 propagating parallel to the z axis and
having an input polarization 76 oriented parallel to the y
direction perpendicular to z. The birefringent crystal is cut or
rotated so that optical axis 70 is oriented at an angle .theta. in
the x-y plane relative to the y-oriented direction of input
polarization 76. The angle .theta. is defined as the cutting or
rotation angle of the birefringent crystal. The propagation
direction of an output optical beam 78 is oriented parallel to the
z axis. Depending on the length, d, and angle .theta. of the
birefringent crystal, the possible output beam polarizations 80 can
be oriented in the x-y plane over the full three-hundred-sixty
degree range of directions radially relative to the z-axis along
which output optical beam 78 propagates.
[0043] FIG. 2B is a perspective diagram illustrating the angle of
cutting or rotation of an exemplary polarization rotator 52 having
an optical axis 90. For convenience, laboratory axes are labeled x,
y, and z, with an input optical beam 74 propagating parallel to the
z axis and having an input polarization 76 oriented parallel to the
y direction perpendicular to z. Polarization rotator 52 is cut or
rotated so that optical axis 90 is oriented at an angle .phi. in
the x-y plane relative to the y-oriented direction of input
polarization 76. The angle .phi. is defined as the cutting or
rotation angle of polarization rotator 52. Depending on the length,
1, and cutting or rotation angle .phi. of polarization rotator 52,
the possible output beam polarizations 94 can be oriented radially
about output optical beam 92 over the full three-hundred-sixty
degree range of directions in the x-y plane.
[0044] FIGS. 3A-3B illustrate one embodiment of a wavelength router
100 that includes a birefringent waveplate 50 and a plurality of
polarization rotators 52. Because the birefringent waveplates 50
described above with regard to FIG. 1 may be oriented at a common
angle .theta., it becomes possible to replace the multiple
longitudinally aligned individual birefringent waveplates 50 of
FIG. 1 with fewer waveplates 50 arranged with the polarization
rotators 52 in a compact assembly that uses an optical beam path
that is folded. In one embodiment, the multiple birefringent
waveplates 50 of FIG. 1 may be replaced by a single birefringent
waveplate 50 oriented at angle .theta.. Wavelength router 100
further includes birefringent elements 12, 22, and 26; polarization
rotators 14, 20, and 24; polarization dependent routing element 18;
and reflective elements 102.
[0045] In operation, birefringent element 12 spatially separates
input signal 28 into a first beam component 56 having a first
polarization and a second beam component 58 having a second
polarization. Polarization rotator 14 changes the polarization
state of beam component 56 such that it has the same polarization
as beam component 58. Alternatively, polarization rotator 14 may be
positioned such that it changes the polarization state of beam
component 58 to match that of beam component 56. These operations
and the resulting beam components 56 and 58 are illustrated in FIG.
3B. Portions of FIG. 3A depict a single beam path for beam
components 56 and 58 for illustrative purposes only.
[0046] Beam components 56 and 58 propagate through polarization
rotators 52 and birefringent waveplate 50 in multiple passes along
a folded optical path that is created using reflective elements
102a and 102b. The combination of birefringent waveplate 50,
encountered by beam components 56 and 58 in multiple passes, and
polarization rotators 52a-d imparts a phase delay and a
polarization state change upon beam components 56 and 58 to
generate two eigen states for each beam component 56 and 58. The
first eigen state carries a first subset of wavelength channels
associated with signal 28 with the same polarization as the beam
component 56 and 58 received by polarization rotator 52a. The
second eigen state carries a second, complementary, subset of
wavelength channels with the orthogonal polarization. In this
respect, the polarization of the incoming beam component 56 and 58
and the two output polarizations for each beam component 56 and 58
form a pair of spectral responses.
[0047] Polarization dependent routing element 18 routes the first
and second subsets of wavelength channels based upon their
polarizations. For example, element 18 together with reflective
element 102c directs the first subset of wavelength channels having
a horizontal polarization along a first optical path toward
birefringent element 22. Element 18 directs the second subset of
wavelength channels having a vertical polarization along a second
optical path that is reflected using reflective element 102d toward
birefringent element 26. To recombine the spectra of the first
subset of wavelength channels, polarization rotator 20 and
birefringent element 22 are used. To recombine the spectra of the
second subset of wavelength channels, polarization rotator 24 and
birefringent element 26 are used. Output signal 30 therefore
comprises the first subset of wavelength channels associated with
input signal 28 while output signal 32 comprises the second,
complementary, subset of wavelength channels. A technical advantage
of wavelength router 100 is its compact size resulting in higher
optical device densities and a robust operation.
[0048] FIG. 4 illustrates one embodiment of a bandwidth variable
wavelength router 110 that includes a first birefringent element
12, a first polarization rotator 14, wavelength filter 112,
polarization dependent routing element 18, second and third
polarization rotators 20 and 24, and second and third birefringent
elements 22 and 26. In general, router 110 receives an input signal
28 and, based at least in part upon the orientation of elements
within filter 112, generates output signals 30 and 32 comprising
particular wavelength channels of input signal 28. The bandwidth of
the wavelength channels associated with output signals 30 and 32 is
variable in response to control signals 114a-c applied to elements
within filter 112. In a particular embodiment, wavelength router
110 further comprises a controller 116 and a network management
module 118.
[0049] Wavelength filter 112 comprises birefringent waveplates
50a-c. In one embodiment of router 110, filter 112 also comprises
polarization rotators 52a-d. In another embodiment of router 110,
filter 112 does not include polarization rotators 52a-d. Therefore,
rotators 52a-d are illustrated using dashed lines. A birefringent
waveplate 50 comprises birefringent crystals 120 separated by one
or more polarization rotators 122. A first birefringent crystal 120
of a birefringent waveplate 50 has a first crystal length, d1, and
a second birefringent crystal 120 of a birefringent waveplate 50
has a second crystal length, d2. In a particular embodiment, length
d2 is different from length d1. As a result, an optical signal
propagating in series through the birefringent crystals 120 of a
birefringent waveplate 50 has an optical path length based, at
least in part, upon the crystal lengths d1 and d2 of birefringent
crystals 120.
[0050] The polarization rotator 122 of a birefringent waveplate 50
comprises a dynamic half-wave plate that changes the polarization
state of an optical signal by a desired degree (e.g., ninety
degrees) in response to a control signal 114. As will be described
in greater detail below with respect to FIG. 5, by changing or not
changing the polarization state of an optical signal propagating in
series through birefringent crystals 120 of a birefringent
waveplate 50, the effective crystal length of birefringent crystals
120 and, therefore, the optical path length of the optical signal,
may be controlled. For example, if the polarization rotator 122
changes by ninety degrees the polarization state of an optical
signal propagating in series through birefringent crystals 120 of
waveplate 50, then the optical path length of the optical signal is
based, at least in part, upon the difference between lengths d1 and
d2 of birefringent crystals 120. The optical path length of the
optical signal is therefore decreased. If the polarization rotator
122 changes by zero or three-hundred sixty degrees the polarization
state of an optical signal propagating in series through
birefringent crystals 120 of waveplate 50, then the optical path
length of the optical signal is based, at least in part, upon the
addition of lengths d1 and d2 of birefringent crystals 120. The
optical path length of the optical signal is therefore increased.
Although birefringent waveplates 50 illustrated in FIG. 4 include
two birefringent crystals 120 separated by one polarization rotator
122, it should be understood that a birefringent waveplate 50 may
comprise any number and combination of birefringent crystals 120
separated by an appropriate number of polarization rotators 122.
For example, a birefringent waveplate 50 may comprise birefringent
crystals 120 having lengths d1, d2, and d3, and polarization
rotators 122 arranged among the birefringent crystals 120 in an
alternating sequence. In this respect, the optical path length of
an optical signal propagating in series through the birefringent
crystals 120 and the polarization rotators 122 may be controlled
(e.g., increased or decreased) with a higher degree of precision
and granularity.
[0051] Referring back to FIG. 2A, a birefringent crystal, such as
birefringent crystals 120 of a birefringent waveplate 50, has a
length, d, and a cutting or rotation angle, .theta.. The
birefringence, .DELTA.n, of a birefringent crystal 120 is defined
as the difference between first and second refractive indices n1
and n2 such that .DELTA.n=(n2-n1). The spectral bandwidth of each
wavelength channel associated with output signals 30 and 32 is a
function of the product of crystal length, d, and birefringence,
An, (i.e., d.times..DELTA.n), such that as this product becomes
larger, the bandwidth of each wavelength channel becomes narrower
and as this product becomes smaller, the bandwidth of each
wavelength channel becomes wider. As described above, the effective
lengths of birefringent crystals 120 of a birefringent waveplate 50
may be modified using a polarization rotator 122 to increase or
decrease the optical path length of an optical signal propagating
in series through the crystals 120. By increasing the optical path
length of an optical signal, such as portions of input signal 28,
as it propagates through filter 112, the bandwidth of each
wavelength channel associated with output signals 30 and 32 is
narrowed. By decreasing the optical path length of an optical
signal, such as portions of input signal 28, as it propagates
through filter 112, the bandwidth of each wavelength channel
associated with output signals 30 and 32 is widened.
[0052] In operation, birefringent element 12 spatially separates
input signal 28 into a first beam component 56 having a first
polarization (e.g., vertical polarization, indicated using a dot)
and a second beam component 58 having a second polarization (e.g.,
horizontal polarization, indicated using a line). Polarization
rotator 14 changes the polarization state of beam component 56 such
that it has the same polarization (e.g., horizontal polarization)
as beam component 58. Alternatively, polarization rotator 14 may be
positioned such that it changes the polarization state of beam
component 58 to match that of beam component 56.
[0053] As illustrated in FIG. 4, wavelength filter 112 receives
beam components 56 and 58 having horizontal polarizations. In the
embodiment of wavelength filter 112 that does not include
polarization rotators 52, birefringent waveplates 50 are oriented
at selected angles, .theta., such that they impart a phase delay
and a polarization state change upon beam components 56 and 58 to
generate two eigen states for each beam component 56 and 58. In the
embodiment of wavelength filter 112 that does include polarization
rotators 52, birefringent waveplates 50 are each oriented at a
common angle, .theta., and the combination of birefringent
waveplates 50 and polarization rotators 52 imparts a phase delay
and a polarization state change upon beam components 56 and 58 to
generate two eigen states for each beam component 56 and 58.
Irrespective of the embodiment of wavelength filter 112 used in
wavelength router 110, the first eigen state carries a first subset
of wavelength channels associated with signal 28 with the same
polarization as the beam component 56 and 58 received by filter 16
(e.g., horizontal polarization, as depicted in FIG. 1). Moreover,
the second eigen state carries a second, complementary, subset of
wavelength channels with the orthogonal polarization (e.g.,
vertical polarization, as depicted in FIG. 1). In this respect, the
polarization of the incoming beam component 56 and 58 and the two
output polarizations for each beam component 56 and 58 form a pair
of spectral responses.
[0054] Polarization dependent routing element 18 routes the first
and second subsets of wavelength channels based upon their
polarizations. For example, element 18 directs the first subset of
wavelength channels having a horizontal polarization along a first
optical path toward birefringent element 22. Element 18 directs the
second subset of wavelength channels having a vertical polarization
along a second optical path toward birefringent element 26. To
recombine the spectra of the first subset of wavelength channels,
polarization rotator 20 and birefringent element 22 are used. To
recombine the spectra of the second subset of wavelength channels,
polarization rotator 24 and birefringent element 26 are used.
Output signal 30 therefore comprises the first subset of wavelength
channels associated with input signal 28 while output signal 32
comprises the second, complementary, subset of wavelength
channels.
[0055] The spectral bandwidth of the wavelength channels associated
with output signals 30 and 32 are variable in response to control
signals 114. Referring now to FIG. 5, if control signals 114a-c
cause polarization rotators 122a-c to change by ninety degrees the
polarization of beam components 56 and 58 within each particular
birefringent waveplate 50, then the optical path lengths of beam
components 56 and 58 are based, at least in part, upon the
difference between lengths d1 and d2 of birefringent crystals 120.
Therefore, the optical path lengths of beam components 56 and 58
are decreased such that the bandwidth of each wavelength channel
associated with output signals 30 and 32 is widened, as illustrated
in State I of FIG. 5. If control signals 114a-c cause polarization
rotators 122a-c to change by zero or three-hundred-sixty degrees
the polarization state of beam components 56 and 58 within each
particular birefringent waveplate 50, then the optical path lengths
of beam components 56 and 58 are based, at least in part, upon the
addition of lengths d1 and d2 of birefringent crystals 120.
Therefore, the optical path lengths of beam components 56 and 58
are increased such that the bandwidth of each wavelength channel
associated with output signals 30 and 32 is narrowed, as
illustrated in State II of FIG. 5.
[0056] In some embodiments, controller 116 communicates particular
control signals 114 to polarization rotators 122 in response to a
control packet received from network management module 118. For
example, a control packet from network management module 118 may
contain a control message requesting a change in the spectral
bandwidth of wavelength channels associated with output signals 30
and 32 from wider wavelength channels to narrower wavelength
channels. In this respect, a higher channel density is achieved
over a particular range of wavelengths. Controller 116 receives and
interprets the control message and, in response, communicates
control signals 114a-c to polarization rotators 122a-c,
respectively, that change the operation of wavelength router 110
from State I (i.e., low channel density) to State II (i.e., high
channel density). Of course, the control packet may also request a
change in operation of wavelength router 110 from State II to State
I.
[0057] FIG. 6 is a schematic representation illustrating underlying
principles of bandwidth variable wavelength router 110. For
purposes of clarity, each of the birefringent crystals depicted in
FIG. 6 have identical birefringence, .DELTA.n. FIG. 7 is a graphic
representation of the relative spectral responses of wavelength
channels processed by the exemplary birefringent crystal
configurations of FIG. 6 depicting bandwidths in arbitrary
wavelength units horizontally and relative amplitudes
vertically.
[0058] Referring now to FIG. 6, birefringent crystals 152 and 154
have equal crystal lengths L-152 and L-154. If an optical beam
passes longitudinally in series through birefringent crystals 152
and 154 oriented at the same angle .theta., then the effective
crystal length L-156 comprises the addition of individual lengths
L-152 and L-154. As described above with regard to FIG. 4, the
spectral bandwidth of associated wavelength channels is accordingly
narrowed. If birefringent crystals 152 and 154 are rotated by
ninety degrees relative to one another about the input beam axis,
then their corresponding refractive indices are reversed so that
their birefringence is canceled. The effective crystal length is
consequently zero, and the spectral bandwidth of associated
wavelength channels is substantially unrestricted.
[0059] Birefringent crystals 162 and 164 have unequal lengths L-162
and L-164 associated with unequal spectral bandwidths B-162 and
B-164. If an optical beam passes longitudinally in series through
birefringent crystals 162 and 164 oriented at the same angle
.theta., then the effective crystal length L-166 comprises the
addition of individual lengths L-162 and L-164. Again, the spectral
bandwidth of associated wavelength channels is accordingly
narrowed, as illustrated by B-166 in FIG. 7. However, if
birefringent crystals 162 and 164 are rotated by ninety degrees
relative to one another about the input beam axis, then their
corresponding refractive indices are reversed. The effective
crystal length L-168 comprises the difference between individual
crystal lengths L-162 and L-164. The spectral bandwidth of
associated wavelength channels is accordingly widened, as
illustrated by B-168 in FIG. 7.
[0060] If a half-wave plate 172 is inserted between birefringent
crystals 152 and 154 as illustrated in optical configuration 182,
then the polarization state of an optical beam propagating through
configuration 182 will be changed by ninety degrees. For beam
propagation through birefringent crystal 154, changing the beam
polarization by ninety degrees about the beam axis is equivalent to
rotating the birefringent crystal 154 by ninety degrees about the
beam axis. Therefore, crystal lengths L-152 and L-154 of
configuration 182 will cancel, providing substantially unrestricted
wavelength channel bandwidth, as described above. Similarly,
inserting a half-wave plate 172 between birefringent crystals 162
and 164 as illustrated in optical configuration 188 causes the
polarization state of an optical beam propagating through
configuration 188 to change by ninety degrees about the beam axis.
For beam propagation through birefringent crystal 164, changing the
beam polarization by ninety degrees about the beam axis is
equivalent to rotating the birefringent crystal 164 by ninety
degrees about the beam axis. Therefore, crystal lengths L-162 and
L-164 of configuration 188 will yield an effective crystal length
of L-168, providing a wavelength channel bandwidth B-168. As a
result, changing the polarization state of a beam by ninety degrees
prior to propagation through a birefringent crystal, such as
crystals 154 and 164 of configurations 182 and 188, is equivalent
to rotating crystals 154 and 164 by ninety degrees with respect to
crystals 152 and 162, respectively. Combining rotation of the
birefringent crystals relative to one another with a half-wave
plate inserted between crystals will result in behavior equivalent
to that of combining no half-wave plate with no rotation of
birefringent crystals.
[0061] Optical configuration 192 is similar to optical
configuration 188, except that a dynamic half-wave plate 194 is
inserted in place of static half-wave plate 172 between
birefringent crystals 162 and 164. Dynamic half-wave plate 194 has
two operating states controllable by applying an external control
signal 196. In State I, dynamic half-wave plate 194 exhibits normal
half-wave plate behavior, whereas in State II, dynamic half-wave
plate 194 exhibits no half-wave plate behavior and acts
substantially as a passive transparent optical window. Accordingly,
in State I, optical configuration 192 has an effective crystal
length L-168 equal to the difference between individual lengths
L-162 and L-164, resulting in wavelength channel bandwidth B-168.
In State II, optical configuration 192 has an effective length
L-166 equal to the addition of individual lengths L-162 and L-164,
resulting in wavelength channel bandwidth B-166. As a result, the
use of a dynamic half-wave plate to control the effective crystal
length of a combination of birefringent crystals facilitates
dynamically variable bandwidth wavelength channels. Moreover, the
use of these polarization control techniques with the birefringent
crystals 120 and polarization rotators 122 illustrated in FIG. 4
facilitates variable bandwidth wavelength routing.
[0062] FIGS. 8A-8B illustrate one embodiment of a bandwidth
variable wavelength router 200 that includes a birefringent
waveplate 50 and a plurality of polarization rotators 52. Because,
in one embodiment, the birefringent waveplates 50 described above
with regard to FIG. 4 may be oriented at a common angle .theta., it
becomes possible to replace the multiple longitudinally aligned
individual birefringent waveplates 50 of FIG. 4 with fewer
waveplates 50 arranged with the polarization rotators 52 in a
compact assembly that uses an optical beam path that is folded. In
one embodiment, the multiple birefringent waveplates 50 of FIG. 4
may be replaced by a single birefringent waveplate 50 that
comprises birefringent crystals 120 and polarization rotator 122.
Wavelength router 200 further includes birefringent elements 12,
22, and 26; polarization rotators 14, 20, and 24; polarization
dependent routing element 18; and reflective elements 102.
[0063] In operation, birefringent element 12 spatially separates
input signal 28 into a first beam component 56 having a first
polarization and a second beam component 58 having a second
polarization. Polarization rotator 14 changes the polarization
state of beam component 56 such that it has the same polarization
as beam component 58. Alternatively, polarization rotator 14 may be
positioned such that it changes the polarization state of beam
component 58 to match that of beam component 56. These operations
and the resulting beam components 56 and 58 are illustrated in FIG.
8B. Portions of FIG. 8A depict a single beam path for beam
components 56 and 58 for illustrative purposes only.
[0064] Beam components 56 and 58 propagate through polarization
rotators 52 and birefringent waveplate 50 in multiple passes along
a folded optical path that is created using reflective elements
102a and 102b. The combination of birefringent waveplate 50,
encountered by beam components 56 and 58 in multiple passes, and
polarization rotators 52a-d imparts a phase delay and a
polarization state change upon beam components 56 and 58 to
generate two eigen states for each beam component 56 and 58. The
first eigen state carries a first subset of wavelength channels
associated with signal 28 with the same polarization as the beam
component 56 and 58 received by polarization rotator 52a. The
second eigen state carries a second, complementary, subset of
wavelength channels with the orthogonal polarization. In this
respect, the polarization of the incoming beam component 56 and 58
and the two output polarizations for each beam component 56 and 58
form a pair of spectral responses.
[0065] Polarization dependent routing element 18 routes the first
and second subsets of wavelength channels based upon their
polarizations. For example, element 18 together with reflective
element 102c directs the first subset of wavelength channels having
a horizontal polarization along a first optical path toward
birefringent element 22. Element 18 directs the second subset of
wavelength channels having a vertical polarization along a second
optical path that is reflected using reflective element 102d toward
birefringent element 26. To recombine the spectra of the first
subset of wavelength channels, polarization rotator 20 and
birefringent element 22 are used. To recombine the spectra of the
second subset of wavelength channels, polarization rotator 24 and
birefringent element 26 are used. Output signal 30 therefore
comprises the first subset of wavelength channels associated with
input signal 28 while output signal 32 comprises the second,
complementary, subset of wavelength channels.
[0066] As described above with regard to FIG. 4, the spectral
bandwidth of the wavelength channels associated with output signals
30 and 32 are variable in response to control signal 114. If
control signal 114 causes polarization rotator 122 to change by
ninety degrees the polarization of beam components 56 and 58 along
any given pass of components 56 and 58 through birefringent
waveplate 50, then the optical path lengths of beam components 56
and 58 are based, at least in part, upon the difference between
lengths d1 and d2 of birefringent crystals 120. Therefore, the
optical path lengths of beam components 56 and 58 are decreased
such that the bandwidth of each wavelength channel associated with
output signals 30 and 32 is widened, as illustrated in State I of
FIG. 5. If control signal 114 causes polarization rotator 122 to
change by zero or three-hundred-sixty degrees the polarization of
beam components 56 and 58 along any given pass of components 56 and
58 through birefringent waveplate 50, then the optical path lengths
of beam components 56 and 58 are based, at least in part, upon the
addition of lengths d1 and d2 of birefringent crystals 120.
Therefore, the optical path lengths of beam components 56 and 58
are increased such that the bandwidth of each wavelength channel
associated with output signals 30 and 32 is narrowed, as
illustrated in State II of FIG. 5. In a particular embodiment,
controller 116 communicates a particular control signal 114 to
polarization rotator 122 in response to a control packet from
network management module 118.
[0067] FIG. 9 is a simplified block diagram illustrating an
application of a bandwidth variable wavelength router, such as
routers 110 and/or 200 within a bit-rate transparent optical router
210. Optical router 210 includes optical switch 212 that
selectively interconnects input and output ports, for example,
ports 214, 216, and 218, which communicate across an optical
communications network. An optical input data stream having, for
example, 10 Gb/s bit rate wavelength channels, enters at input port
214 and is demultiplexed by a demultiplexer 220 into, for example,
odd and even wavelength channels 222 and 224, which are then
switched through optical switch 212 and propagate as channels 226
and 228, which are then multiplexed by a multiplexer 230 and
delivered to output port 218. Another optical input data stream
having 40 Gb/s bit rate wavelength channels enters router 210
through input port 216 from a different source in the network, and
is also routed to output port 218. Demultiplexer 232 demultiplexes
the 40 Gb/s data stream into, for example, odd and even wavelength
channels 234 and 236, which enter optical switch 212. Optical
switch 212 processes the two different bit-rate data streams
without difficulty, since optical switch 212 is essentially
bit-rate transparent. However, prior art wavelength routers are not
bit-rate transparent, and if multiplexer 230 is a conventional
wavelength router, it cannot adapt to varying bit rates from
different sources within the network. The result is that prior art
optical routers cannot optimally handle variable bit rate data
streams.
[0068] If multiplexer 230 is a bandwidth variable wavelength router
110 and/or 200 as described above in connection with FIGS. 4 and
8A-B, then it can adapt to accommodate both the 10 Gb/s and the 40
Gb/s input data streams. Bandwidth variable wavelength router 230
is communicatively coupled to a controller 240, which provides
control signals 242 that change the properties of dynamic half-wave
plates internal to bandwidth variable wavelength router 230. If,
for example, network management module 244 detects in the network
that bandwidth variable wavelength router 230 is initially
processing a 10 Gb/s data stream in a narrow bandwidth State II
(high wavelength channel density within a particular wavelength
range), as defined above, but is subsequently required to process a
new data stream at 40 Gb/s, then network management module 244
communicates a control packet to controller 240 indicating that
variable bandwidth wavelength routing is desired. In response,
controller 240 communicates a control signal 242 to wavelength
router 230 to change the state of operation of wavelength router
230 from narrow bandwidth State II (high wavelength channel
density) to wide bandwidth State I (low wavelength channel
density). Operating in State I, as described above, wavelength
router 230 can readily process the new 40 Gb/s data stream. In this
respect, wavelength router 230 can maximize the bandwidth
utilization for a 10Gb/s input data stream and still adapt to meet
the bandwidth requirements of a 40Gb/s input data stream.
[0069] In accordance with International Telecommunications Union
(ITU) standards, only discrete bit rate values are generally
allowed for data streams in an optical network. For example, OC48
with 2.5 Gb/s bit rate, OC192 with 10 Gb/s bit rate, and OC768 with
40 Gb/s bit rate are typical configurations. Other bit rates may be
supported as optical networks evolve. A technical advantage of
wavelength router 230 is that it may be dynamically configured to
process data streams having these and other bit rates.
[0070] FIG. 10 illustrates one embodiment of a cascaded
architecture 246 that includes a wavelength router 248
communicatively coupled to bandwidth variable wavelength routers
110a and 110b. Although the description of FIG. 10 is detailed with
respect to routers 110a-b, it should be understood that routers
200a-b may be used without departing from the scope of the present
invention. Wavelength router 248 comprises any suitable optical
device that receives an optical signal 28 and generates signals
249a and 249b. Signals 249a and 249b each comprise subsets of
wavelength channels associated with signal 28. For example, signal
249a comprises even wavelength channels associated with signal 28
and signal 249b comprises odd wavelength channels associated with
signal 28. In this respect, the channel spacing of each wavelength
channel in signals 249a and 249b is twice that of the channel
spacing of wavelength channels in signal 28. For example, if signal
28 is a 50 Ghz signal, then signals 249a and 249b are each 100 Ghz
signals. In a particular embodiment, router 248 comprises any of
the wavelength routers described herein.
[0071] Bandwidth variable wavelength routers 100a-b receive signals
249a-b and, depending upon the operation of routers 110a and 110b,
generate two or more of output signals 249c-d and 249e-f,
respectively. For example, if routers 110a and 110b are operating
in a first state, then routers 110a-b receive signals 249a and 249b
and generate signals 249c-d and 249e-f, respectively. Operating in
this state, for example, routers 110a and 110b receive 100 Ghz
signals (e.g., 249a-b) and generate 200 Ghz signals (e.g., 249c-d
and 249e-f). In this respect, architecture 246 operates as a
1.times.4 device. If routers 110aand 110b are operating in a second
state, then routers 110a-b receive signals 249a and 249b and
generate signals 249c and 249e, respectively. Operating in this
state, for example, routers 110a and 110b receive 100 Ghz signals
(e.g., 249a-b) and generate 100 Ghz signals (e.g., 249c and 249e)
having better isolation characteristics. In this respect,
architecture 246 operates as a 1.times.2 device. A technical
advantage of architecture 246 is that routers 110a and 110b provide
bandwidth variable spectral processing to meet the demands of a
growing and changing optical network.
[0072] FIG. 11 illustrates one embodiment of a switchable
wavelength router 250 that includes a first birefringent element
12, a first polarization rotator 14, wavelength filter 16,
polarization dependent routing element 18, second and third
polarization rotators 20 and 24, and second and third birefringent
elements 22 and 26. In general, router 250 receives an input signal
28 and, based at least in part upon the orientation of elements
within filter 16 and the application of control signals 260,
generates output signals 30 and 32 comprising particular wavelength
channels of input signal 28.
[0073] Each birefringent waveplate 50a-c of switchable wavelength
router 250 is oriented at a common angle .theta.. Polarization
rotators 52 of switchable wavelength router 250 comprise dynamic
half-wave plates made from, for example, liquid crystal material.
Each polarization rotator 52a-d is oriented at a particular angle
.phi. and operates in response to a corresponding control signal
260a-d, generally referred to as control signal 260.
[0074] FIG. 12 illustrates the operation of switchable wavelength
router 250 to yield four different output states 270, 272, 274, and
276. In each output state 270-276, switchable wavelength router 250
receives an input signal 28 comprising first and second subsets of
wavelength channels 280 and 282. By applying particular control
signals 260 to particular polarization rotators 52a-d, particular
subsets of wavelength channels 280 and 282 may be switched among
and between output signals 30 and 32.
[0075] In output state 270, control signals 260a-d applied to
polarization rotators 52a-d cause polarization rotators 52a-d to
operate as ordinary half-wave plates. For example, control signals
260a-d may apply zero volts to each of the polarization rotators
52a-d. As a result, the polarization of first wavelength subset 280
is orthogonal to the polarization of second wavelength subset 282.
Output signal 30 comprises first wavelength subset 280 while output
signal 32 comprises second wavelength subset 282.
[0076] In output state 272, a control signal 260 is applied to a
selected one of polarization rotators 52a or 52d such that it
causes the selected polarization rotator 52a or 52d to change the
polarization state of the beam components 56 and 58 by ninety
degrees with respect to the polarization state of the beam
components 56 and 58 during the operation of router 250 in output
state 270. For example, a control signal 260a may be applied to
polarization rotator 52a to cause it to change the polarization
state of beam components 56 and 58 by ninety degrees with respect
to the polarization state of the beam components 56 and 58 during
the operation of router 250 in output state 270. The remaining
control signals 260b-d cause polarization rotators 52b-d to operate
as ordinary half-wave plates. In another example, a control signal
260d may be applied to polarization rotator 52d to cause it to
change the polarization state of beam components 56 and 58 by
ninety degrees with respect to the polarization state of the beam
components 56 and 58 during the operation of router 250 in output
state 270. The remaining control signals 260a-c cause polarization
rotators 52a -c to operate as ordinary half-wave plates. As a
result, the polarization of first wavelength subset 280 is
orthogonal to the polarization of second wavelength subset 282.
Output signal 30 comprises second wavelength subset 282 while
output signal 32 comprises first wavelength subset 280.
[0077] In output state 274, a sufficient voltage is applied by
control signals 260a-d to polarization rotators 52a-d to eliminate
the birefringence properties of the liquid crystal material, for
example, from which polarization rotators 52a-d are made. In this
respect, the polarization state of beam components 56 and 58
communicated by filter 16 is the same as the polarization state of
beam components 56 and 58 received by filter 16. As a result, the
polarization of first wavelength subset 280 is substantially equal
to the polarization of second wavelength subset 282. Output signal
30 comprises first and second wavelength subsets 280 and 282.
[0078] In output state 276, a control signal 260 is applied to a
selected one of polarization rotators 52a or 52d such that it
causes the selected polarization rotator 52a or 52d to change the
polarization state of the beam components 56 and 58 by ninety
degrees. For example, a control signal 260a may be applied to
polarization rotator 52a to cause it to change the polarization
state of beam components 56 and 58 by ninety degrees. The remaining
control signals 260b-d apply a sufficient voltage to polarization
rotators 52b-d to eliminate the birefringence properties of the
liquid crystal material, for example, from which rotators 52b-d are
made. In another example, a control signal 260d may be applied to
polarization rotator 52d to cause it to change the polarization
state of beam components 56 and 58 by ninety degrees. The remaining
control signals 260a-c apply a sufficient voltage to polarization
rotators 52a -c to eliminate the birefringence properties of the
liquid crystal material, for example, from which rotators 52a -c
are made. In this respect, the polarization state of beam
components 56 and 58 communicated by filter 16 is orthogonal to the
polarization state of beam components 56 and 58 received by filter
16. As a result, the polarization of first wavelength subset 280 is
substantially equal to the polarization of second wavelength subset
282. Output signal 32 comprises first and second wavelength subsets
280 and 282.
[0079] FIGS. 13A-13B illustrate one embodiment of a wavelength
router 300 that includes a birefringent waveplate 50 and a
plurality of polarization rotators 52. Because the birefringent
waveplates 50 described above with regard to FIG. 11 may be
oriented at a common angle .theta., it becomes possible to replace
the multiple longitudinally aligned individual birefringent
waveplates 50 of FIG. 11 with fewer waveplates 50 arranged with the
polarization rotators 52 in a compact assembly that uses an optical
beam path that is folded. In one embodiment, the multiple
birefringent waveplates 50 of FIG. 11 may be replaced by a single
birefringent waveplate 50 oriented at angle .theta.. Wavelength
router 300 further includes birefringent elements 12, 22, and 26;
polarization rotators 14, 20, and 24; polarization dependent
routing element 18; reflective elements 102; and controller 116.
Polarization rotators 52 of switchable wavelength router 300
comprise dynamic half-wave plates made from, for example, liquid
crystal material. Each polarization rotator 52a-d is oriented at a
particular angle .phi. and operates in response to a corresponding
control signal 260a-d , generally referred to as control signal
260. Switchable wavelength router 300 may implement output states
270-276 using the same application of control signals 260 as
described above with regard to FIG. 12.
[0080] A technical advantage of switchable wavelength routers 250
and 300 is that they provide switchable beam path control in
optical network applications. This allows the switchable wavelength
routers 250 and 300 to function as an optical wavelength router in
an optical network and to perform, for example, protection
switching and restoration of optical data paths. Additionally, it
can recognize new wavelength bands and switch subsets of wavelength
channels among outputs. These advantages result at least in part
from using dynamic half-wave plates whose properties are controlled
by the application of control signals, which can be accomplished
adaptively or programmably.
[0081] FIGS. 14A-14B illustrate an application of switchable
wavelength routers 250 or 300 to facilitate first and second
optical communication traffic patterns 310 and 330. For example, in
a first traffic pattern 310 illustrated in FIG. 14A, traffic in a
path 312 from a network node 314 is demultiplexed for propagation
along two paths 316 and 318 at a first switchable wavelength router
250a (or 300). Traffic along path 316 enters an intermediate
network node 320, which communicates the traffic along a path 322.
Traffic along paths 318 and 322 are then multiplexed by a second
switchable wavelength router 250b (or 300), and then propagated
along path 324 to another network node 326. It should be noted that
wavelength routers 250 (or 300) are reciprocal devices that may
perform both multiplexing and demultiplexing operations. This
permits second switchable wavelength router 250b (or 300) to
perform a traffic add operation complementary to the traffic drop
operation performed by first switchable wavelength router 250a (or
300). This reciprocity further allows the entire flow of traffic in
pattern 310 to be reversed, i.e., propagation of traffic from node
326 to nodes 320 and 314. Both first and second switchable
wavelength routers 250a-b in this example operate in output state
270 described above with regard to FIG. 12. Alternatively, first
and second switchable wavelength routers 250 may both operate in
output state 272.
[0082] In a second traffic pattern 330, illustrated in FIG. 14B,
intermediate node 318 is entirely bypassed. In this case, first
switchable wavelength router 250a (or 300) operates in output state
274, so that all traffic is routed from node 314 along paths 312
and 318. Second switchable wavelength router 250b (or 300) also
operates in output state 274, thereby capturing all traffic routed
by first switchable wavelength router 250a (or 300). Second
switchable wavelength router 250b (or 300) then routes the traffic
along path 324 to node 326. Because switchable wavelength routers
250a-b (or 300) are reciprocal devices that may perform both
multiplexing and demultiplexing operations, the flow of traffic in
second traffic pattern 330 can also be reversed. Moreover, first
and second switchable wavelength routers 250a-b (or 300) can both
operate in output state 276.
[0083] By deploying switchable wavelength routers 250 (or 300) in
an optical communication network as described in FIGS. 14A-B, it is
possible to route traffic based on the data carrying capacity of
particular optical paths. For example, during the daytime, path 318
may not have enough data carrying capacity to support all of the
traffic to be communicated from network node 314 to node 326.
Switchable wavelength routers 250 (or 300) may be used to route a
portion of the traffic from node 314 to node 326 via intermediate
node 320. In this respect, paths 316 and 322 remove some of the
traffic burden from path 318. During the nighttime, however, when
the traffic flow subsides and path 318 does have enough data
carrying capacity to support all of the traffic to be communicated
from network node 314 to node 326, switchable wavelength routers
250 (or 300) may bypass intermediate node 320. In this respect,
switchable wavelength routers 250 (or 300) facilitate traffic
shaping.
[0084] Although wavelength routers 10, 100, 110, 200, 250, and 300
and their advantages have been described in detail, it should be
understood that various changes, substitutions and alterations can
be made herein without departing from the spirit and scope of the
invention as defined by the appended claims. For example,
wavelength routers 10, 100, 110, 200, 250, and 300 are reciprocal
devices, such that optical signals having the properties of output
signals 30 and 32 can be propagated in the reverse direction and
combined within wavelength routers 10, 100, 110, 200, 250, and 300
to produce an output signal having the properties of input signal
28. In this respect, wavelength routers 10, 100, 110, 200, 250, and
300 may perform both multiplexing and demultiplexing
operations.
[0085] Although the present invention has been described in several
embodiments, a myriad of changes, variations, alterations,
transformations, and modifications may be suggested to one skilled
in the art, and it is intended that the present invention encompass
such changes, variations, alterations, transformations, and
modifications as falling within the spirit and scope of the
appended claims.
* * * * *