U.S. patent application number 09/874641 was filed with the patent office on 2001-11-22 for optical wavelength router based on polarization interferometer.
Invention is credited to Wu, Kuang-Yi, Zhou, Gan.
Application Number | 20010042821 09/874641 |
Document ID | / |
Family ID | 26881968 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010042821 |
Kind Code |
A1 |
Zhou, Gan ; et al. |
November 22, 2001 |
Optical wavelength router based on polarization interferometer
Abstract
A method and apparatus for optical wavelength routing separates
even and odd optical channels from an input WDM signal. The input
beam is first converted to at least one pair of
orthogonally-polarized beams. A split-mirror resonator has a front
mirror with two regions having different reflectivities, and a
reflective back mirror spaced a predetermined distance behind the
front mirror. Each of the orthogonally-polarized beams is incident
on a corresponding region of the front mirror of the split-mirror
resonator. A portion of each beam is reflected by the front mirror,
which the remainder of each beam enters the resonator cavity where
it is reflected by the back mirror back through the front mirror.
The group delay of each reflected beam is strongly dependent on
wavelength. The two reflected beams from the resonator are combined
and interfere in a birefringent element (e.g., a beam displacer or
waveplates) to produce a beam having mixed polarization as a
function of wavelength. The polarized components of this beam are
separated by a polarization-dependent routing element (e.g., a
polarized beamsplitter) to produce two output beams containing
complimentary subsets of the input optical spectrum (e.g., even
optical channels are routed to output port A and odd optical
channels are routed to output port B).
Inventors: |
Zhou, Gan; (Plano, TX)
; Wu, Kuang-Yi; (Plano, TX) |
Correspondence
Address: |
DORR CARSON SLOAN & BIRNEY, PC
3010 EAST 6TH AVENUE
DENVER
CO
80206
|
Family ID: |
26881968 |
Appl. No.: |
09/874641 |
Filed: |
June 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09874641 |
Jun 5, 2001 |
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09547813 |
Apr 11, 2000 |
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6229139 |
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60186314 |
Mar 2, 2000 |
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Current U.S.
Class: |
250/225 |
Current CPC
Class: |
H04J 14/06 20130101;
G02B 6/29302 20130101; H04J 14/02 20130101; G02B 6/29386 20130101;
Y10S 359/90 20130101 |
Class at
Publication: |
250/225 |
International
Class: |
G02F 001/01; H01J
040/14 |
Claims
We claim:
1. An optical wavelength router comprising: a beam displacer
separating an input beam into a first beam and second beam having
orthogonal polarizations; a split-mirror resonator having: (a) a
front mirror with a partially-reflective first region and a second
region having a higher reflectivity than said first region; and (b)
at least one reflective back mirror spaced a predetermined distance
from said front mirror; wherein said first beam is incident on said
first region, and said second beam is incident on said second
region of said front mirror, so that a portions of said first and
second beams are reflected by said front mirror and the remainders
of said first and second beams are transmitted through said front
mirror and are reflected by at least one of said back mirrors
through said front mirror so that the group delay of said first and
second beam are dependent on wavelength; a birefringent element
combining and interfering said first and second beams reflected
from said split-mirror resonator to produce a beam having mixed
polarization as a function of wavelength; a polarization-dependent
routing element separating the polarized components of said
mixed-polarization beam to produce two output beams containing
complimentary subsets of the spectrum of the input beam.
2. The optical wavelength router of claim 1 wherein said
birefringent element comprises a beam displacer.
3. The optical wavelength router of claim 1 wherein said
birefringent element comprises at least one waveplate.
4. The optical wavelength router of claim 1 wherein said
polarization-dependent routing element comprises a polarized
beamsplitter.
5. The optical wavelength router of claim 1 wherein said
split-mirror resonator further comprises a plurality of reflective
back mirrors arranged in a reflective ring behind said front
mirror.
6. A method for optical wavelength routing predetermined
complementary subsets of the optical spectrum in an input beam,
said method comprising: separating an input beam into a first beam
and second beam having orthogonal polarizations; providing a
split-mirror resonator having: (a) a front mirror with a
partially-reflective first region and a second region having a
higher reflectivity that said first region; and (b) at least one
reflective back mirror spaced a predetermined distance from said
front mirror; wherein said first beam is incident on said first
region and said second beam is incident on said second region of
said front mirror, so that portions of said first and second beams
are reflected by said front mirror and the remainders of said first
and second beams are transmitted through said front mirror and are
reflected by at least one of said back mirrors through said front
mirror so that the group delay of said first and second beams are
dependent on wavelength; combining and interfering said first and
second beams reflected from said split-mirror resonator to produce
a beam having mixed polarization as a function of wavelength;
separating the polarized components of said mixed-polarization beam
to produce two output beams containing complimentary subsets of the
spectrum of the input beam.
7. The method of claim 6 wherein said step of separating the input
beam into said first and second beams is performed by a
birefringent element.
8. The method of claim 6 wherein said step of separating the
polarized components of said mixed-polarization beam is performed
by a polarized beamsplitter.
9. The method of claim 6 wherein said split-mirror resonator
further comprises a plurality of reflective back mirrors arranged
in a reflective ring behind said front mirror.
10. The method of claim 6 wherein said step of combining and
interfering said first and second beams reflected from said
split-mirror resonator is performed by a birefringent element.
11. An optical wavelength router comprising: a beam displacer
separating an input beam into a first beam and second beam having
orthogonal polarizations; a split-mirror resonator having: (a) a
front mirror with a partially-reflective first region and a second
region having a higher reflectivity than said first region; and (b)
a reflective back mirror spaced a predetermined distance from said
front mirror; wherein said first beam is incident on said first
region and said second beam is incident on said second region of
said front mirror, so that a portions of said first and second
beams are reflected by said front mirror and the remainders of said
first and second beams are transmitted through said front mirror
and are reflected by at least one of said back mirrors through said
front mirror so that the group delay of said first and second beam
are dependent on wavelength; a zero-order beam displacer combining
said first and second beams reflected from said split-mirror
resonator with only a negligible amount of birefringence added to
said first and second beams; at least one waveplate generating
birefringence in said combined beam from said zero-order beam
displacer, thereby providing a predetermined difference in the
optical path lengths between different polarized components of said
combined beam to produce a beam having mixed polarization as a
function of wavelength; and a polarization-dependent routing
element separating the polarized components of said
mixed-polarization beam to produce two output beams containing
complimentary subsets of the spectrum of the input beam.
12. The optical wavelength router of claim 11 wherein said
zero-order beam displacer comprises: a first birefringent element
having its optical axis oriented in a predetermined direction; and
a second birefringent element having its optical axis oriented at
approximately 90 degrees relative to the optical axis of said first
birefringent element.
13. The optical wavelength router of claim 11 wherein said
zero-order beam displacer comprises: a first birefringent element
having its optical axis oriented in a predetermined direction; a
second birefringent element having its optical axis oriented at
approximately 90 degrees relative to the optical axis of said first
birefringent element; and a zero-order half-wave plate between said
first and second birefringent elements.
14. An optical wavelength router comprising: a first beam displacer
separating an input beam into a pair of orthogonally-polarized
beams; a first polarization rotator rotating the polarization of at
least one of said beams so that both beams have a first
polarization; a polarization-dependent routing element routing said
beam pair along a predetermined optical path; a birefringent
element separating said beam pair into two pairs of
orthogonally-polarized beams; a split-mirror resonator having: (a)
a front mirror with a partially-reflective first region and a
second region having a higher reflectivity that said first region;
and (b) a reflective back mirror spaced a predetermined distance
from said front mirror; wherein two beams of said two pairs of
orthogonally-polarized beams are incident on said first region, and
the other two beams are incident on said second region of said
front mirror, so that a portions of said beam pairs are reflected
by said front mirror and the remainders of said beam pairs are
transmitted through said front mirror and are reflected by said
back mirror through said front mirror so that the group delay of
said beam pairs are dependent on wavelength; wherein said beam
pairs reflected by said split-mirror resonator interfere and
combine within said birefringent element to produce two beams
having mixed polarization as a function of wavelength; wherein said
polarization-dependent routing element separates the polarized
components of said mixed-polarization beams so that those
components of said mixed-polarization beams having said first
polarization are routed as a pair of beams along a first optical
path toward said first polarization rotator and said first beam
displacer, and those components of said mixed-polarization beams
having a polarization orthogonal to said first polarization are
routed as a pair of beams along a second optical path; wherein said
first polarization rotator rotates the polarization of at least one
of said beam pair along said first optical path so that said beam
pair becomes orthogonally polarized; wherein said first beam
displacer combines said orthogonally-polarized beam pair exiting
said first polarization rotator to produce a first output beam
containing a subset of the optical spectrum of the input beam; a
second polarization rotator rotating the polarization of at least
one of said beam pair along said second optical path so that said
beam pair becomes orthogonally polarized; and a second beam
displacer combining said orthogonally-polarized beam pair exiting
said second polarization rotator to produce a second output beam
containing a subset of the optical spectrum of the input beam.
15. The optical wavelength router of claim 11 wherein said
birefringent element comprises at least one beam displacer.
16. The optical wavelength router of claim 11 wherein said
birefringent element comprises at least one waveplate and a beam
displacer.
17. The optical wavelength router of claim 16 wherein said beam
displacer comprises a zero-order beam displacer.
18. The optical wavelength router of claim 11 wherein said
polarization-dependent routing element comprises a polarized
beamsplitter.
19. The optical wavelength router of claim 11 wherein said first
polarization rotator comprises a half-wave plate.
20. The optical wavelength router of claim 11 wherein said second
polarization rotator comprises a half-wave plate.
21. An optical wavelength router comprising: a first beam displacer
separating an input beam into a pair of orthogonally polarized
beams; a first polarization rotator rotating the polarization of at
least one of said beams so that both beams have a first
polarization; a polarization-dependent routing element routing said
beam pair along a predetermined optical path; at least one
waveplate producing orthogonally-polarized components in said beam
pair; a second beam displacer spatially separating the
orthogonally-polarized components in said beam pair into two pairs
of orthogonally-polarized beams; a split-mirror resonator having:
(a) a front mirror with a partially-reflective portion and a
totally-reflective portion; and (b) a reflective back mirror spaced
a predetermined distance from said front mirror, wherein two beams
of said two pairs of orthogonally-polarized beams are reflected by
said reflective portion of said front mirror, and the other two
beams are transmitted through said partially-reflective portion of
said front mirror and are reflected by said back mirror through
said partially-reflective portion of said front mirror thereby
providing a group delay that is dependent on wavelength; wherein
said second beam displacer combines said beam pairs reflected by
said split-mirror resonator to produce two beams having mixed
polarization that interfere within said waveplate to produce two
beams having mixed polarization as a function of wavelength;
wherein said polarization-dependent routing element separates the
polarized components of said mixed-polarization beams so that those
components of said mixed-polarization beams having said first
polarization are routed as a pair of beams along a first optical
path toward said first polarization rotator and said first beam
displacer, and those components of said mixed polarization beams
having a polarization orthogonal to said first polarization are
routed as a pair of beams along a second optical path; wherein said
first polarization rotator rotates the polarization of at least one
of said beam pair along said first optical path so that said beam
pair becomes orthogonally polarized; wherein said first beam
displacer combines said orthogonally-polarized beam pair exiting
said first polarization rotator to produce a first output beam
containing a subset of the optical spectrum of the input beam; a
second polarization rotator rotating the polarization of at least
one of said beam pair along said second optical path so that said
beam pair becomes orthogonally polarized; and a third beam
displacer combining said orthogonally-polarized beam pair exiting
said second polarization rotator to produce a second output beam
containing a subset of the optical spectrum of the input beam.
22. The optical wavelength router of claim 21 wherein said second
beam displacer comprises a zero-order beam displacer.
23. The optical wavelength router of claim 21 wherein said
polarization-dependent routing element comprises a polarized
beamsplitter.
Description
RELATED APPLICATION
[0001] The present application is based on the Applicants' U.S.
Provisional Patent Application Ser. No. 60/186,314, filed on Mar.
2, 2000, entitled "Optical Wavelength Router Based On Polarization
Interferometer."
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to the field of
optical communications systems. More specifically, the present
invention discloses an optical wavelength router for wavelength
division multiplex (WDM) optical communications.
[0004] 2. Statement of the Problem
[0005] Wavelength division multiplexing is a commonly used
technique that allows the transport of multiple optical signals,
each at a slightly different wavelength, on an optical fiber. The
ability to carry multiple signals on a single fiber allows that
fiber to carry a tremendous amount of traffic, including data,
voice, and even digital video signals. As an example, the use of
wavelength division multiplexing permits a long distance telephone
company to carry thousands or even millions of phone conversations
on one fiber. By using wavelength division multiplexing, it is
possible to effectively use the fiber at multiple wavelengths, as
opposed to the costly process of installing additional fibers.
[0006] In wavelength division multiplexing techniques, multiple
wavelengths can be carried within a specified bandwidth. It is
advantageous to carry as many wavelengths as possible in that
bandwidth. International Telecommunications Union (ITU) Draft
Recommendation G.mcs, incorporated herein by reference, proposes a
frequency grid which specifies various channel spacings including
100 GHz and 200 GHz. It would be advantageous to obtain 50 GHz
spacing. Separating and combining wavelengths with these close
spacings requires optical components which have high peak
transmission at the specified wavelengths and which can provide
good isolation between separated wavelengths.
[0007] One technique which has been developed to accomplish the
demultiplexing of closely spaced wavelengths is to cascade a series
of wavelength division demultiplexing devices, each device having
different wavelength separating characteristics. At typical
application involves cascading an interferometric device such as an
arrayed waveguide device having a narrow spacing of transmission
peaks (e.g., 50 GHz) with a second interferometric device which has
a coarser spacing and correspondingly broader transmission peaks
(e.g., 100 GHz spacing). The cascade of devices provides the
separation of wavelengths by subdividing the wavelengths once in
the first device, typically into a set of odd and even channels,
and then separating wavelengths in the subsets in following devices
in the cascade.
[0008] Arrayed waveguide, fused biconical taper, fiber Bragg
grating, diffraction grating, and other interferometric wavelength
demultiplexing devices can be constructed to have the appropriate
characteristics for the first or second stage devices in the
cascade. However, traditional interferometric devices have the
characteristic that as the spacing of the channels is decreased,
the transmission peaks become narrower, and are less flat over the
wavelength region in the immediate vicinity of each peak than a
device with wider channel spacings. As a result, when using a
traditional device in the first stage of a cascade, the
transmission peaks may not have a high degree of flatness, and any
drift or offset of a wavelength from its specified value may result
in significant attenuation of that wavelength. In addition, the
isolation between wavelengths is frequently unsuitable with
conventional interferometric devices and can result in unacceptable
cross-talk between channels.
[0009] With increasing numbers of wavelengths and the close
wavelength spacing which is utilized in dense wavelength division
multiplexing systems, attenuation and cross-talk must be closely
controlled to meet the system requirements and maintain reliable
operations. As an example, 40 or 80 wavelengths can be generated
using controllable wavelength lasers, with transmission signals
modulated onto each laser. It is desirable to be able to
demultiplex these channels. Although the lasers can be controlled
and the wavelengths stabilized to prevent one channel from drifting
into another, there is always some wavelength drift which will
occur.
[0010] For the foregoing reasons, there is a need for a wavelength
division demultiplexing device which tolerates wavelength drift,
maintains a high degree of isolation between channels, and is able
to separate large numbers of wavelengths.
[0011] 3. Prior Art
[0012] FIG. 1 illustrates a prior art interferometer that shares
some of the basic principles employed in the present invention. An
input laser beam is split into two beams by a beamsplitter 10. One
beam propagates toward a mirror 14 and is reflected back by this
mirror. The other beam propagates toward a resonator 12 and is also
reflected back. The resonator 12 is a Fabry-Perot cavity with a
partially-reflective front mirror and a totally-reflective back
mirror. The resonator 12 reflects substantially all of the incident
optical power back regardless of wavelength, but the group delay of
the reflected light is strongly dependent on wavelength. The two
reflected beams from the mirror 14 and from the resonator 12
interfere at the beamsplitter 10 and the resulting output is split
into two beams, one at output A, and the other in a different
direction at output B. The two output beams contain complimentary
subsets of the input optical spectrum, as shown for example in FIG.
2. Such a wavelength router concept has been proposed by B. B.
Dingle and M. Izutsu, "Multifunction Optical Filter With A
Michelson-Gires-Tournois Interferometer For
Wavelength-Division-Multiplex- ed Network System Applications,"
Optics Letters, vol. 23, p.1099 (1998) and the references
therein.
[0013] The two output ports A and B divide the spectral space
evenly with alternating optical channels being directed to each
output port (i.e., optical channels 1, 3, 5, 7, etc. are directed
to output port A, while channels 2, 4, 6, etc. are directed to
output port B). This function has sometimes been called an optical
interleaver.
[0014] 4. Solution to the Problem
[0015] The present invention address the problems associated with
the prior art using a polarization-based interferometer to
implement an optical interleaver capable of separating closely
spaced optical channels with minimal cross-talk.
SUMMARY OF THE INVENTION
[0016] This invention provides a method and apparatus for optical
wavelength routing in which an input beam is converted to at least
one pair of orthogonally-polarized beams. A split-mirror resonator
has a front mirror with two regions having different
reflectivities, and a reflective back mirror spaced a predetermined
distance behind the front mirror. Each of the
orthogonally-polarized beams is incident on a corresponding region
of the front mirror of the resonator. A portion of each beam is
reflected by the front mirror, which the remainder of each beam
enters the resonator cavity where it is reflected by the back
mirror back through the front mirror. The group delay of each
reflected beam is strongly dependent on wavelength. The two
reflected beams from the resonator are combined and interfere in a
birefringent element (e.g., a beam displacer or waveplates) to
produce a beam having mixed polarization as a function of
wavelength. The polarized components of this beam are separated by
a polarization-dependent routing element (e.g., a polarized
beamsplitter) to produce two output beams containing complimentary
subsets of the input optical spectrum (e.g., even optical channels
are routed to output port A and odd optical channels are routed to
output port B).
[0017] These and other advantages, features, and objects of the
present invention will be more readily understood in view of the
following detailed description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The present invention can be more readily understood in
conjunction with the accompanying drawings, in which:
[0019] FIG. 1 is a simplified diagram of a prior art interferometer
using a beamsplitter and a Fabry-Perot cavity resonator.
[0020] FIG. 2 is a graph showing an example of the spectral
response of the interferometer in FIG. 1.
[0021] FIG. 3 is a diagram of an optical wavelength router
embodying the present invention.
[0022] FIG. 4(a) is a detail perspective view of the split-mirror
resonator in FIG. 3.
[0023] FIG. 4(b) is an exploded view of its components of the
split-mirror resonator corresponding to FIG. 4(a).
[0024] FIG. 5(a) is a diagram of an alternative embodiment of a
split-mirror resonator using three mirrors in a ring
configuration.
[0025] FIG. 5(b) is a diagram of another alterative embodiment of a
split-mirror resonator using four mirrors in a ring
configuration.
[0026] FIG. 6 is a diagram of an alternative embodiment of the
optical wavelength router using the ring resonator from FIG.
5(a).
[0027] FIG. 7 is a diagram of another alternative embodiment of the
optical wavelength router.
[0028] FIG. 8 is an isometric view of one embodiment of the
zero-order beam displacer shown in FIG. 7
[0029] FIG. 9 is a side view of an alternative embodiment of the
zero-order beam displacer shown in FIG. 7.
[0030] FIG. 10 is an isometric view of another alternative
embodiment of the zero-order beam displacer shown in FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
[0031] FIG. 3 is a diagram showing a one possible implementation of
the present optical wavelength router based on a polarization
interferometer. A collimated beam from an optical fiber propagates
along the Z axis and is incident into the first beam displacer 31.
For example, a birefringent element consisting of a material such
as calcite, rutile, lithium niobate, YVO.sub.4-based crystals, and
the like could be used as the beam displacers in the present
invention. The first beam displacer 31 splits the input beam into
two beams having orthogonal polarizations (e.g., along the X and Y
directions, respectively). A half-wave plate ({fraction
(.lambda./2)}) 32 rotates the polarization of one of these beams by
90 degrees, so that both beams have the same polarization. For
example, both beams exiting the half-wave plate 32 in FIG. 3 are
polarized along the Y axis.
[0032] Both beams then pass through a polarized beamsplitter (PBS)
33 without significant attenuation. A second beam displacer 34
splits the Y-polarized beam pair into two pairs of beams that are
orthogonally polarized in the XY plane. One pair of these beams is
polarized at 45 degrees relative to the X axis, while the other
pair is polarized at 135 degrees relative to the X axis. The two
pairs of beams are incident onto and reflected by a split-mirror
resonator (SMR) 35.
[0033] FIGS. 4(a) and 4(b) show the structure of the split mirror
resonator 35 in FIG. 3. The resonator 35 is formed by a front
mirror 41 and a back mirror 43 separated a predetermined distance
by a center spacer 42. The front mirror 41 is a split mirror in
which part of the surface is coated with a high-reflectivity
coating and part of the surface is only partially reflective (e.g.,
18% reflectivity). The degree of reflective of both regions is a
matter of design. For example, the high-reflectivity region can be
100% reflective, or only partially reflective so long as it is more
reflective than the other region of the front mirror 41. For
example, this can be accomplished by applying a split coating to
the front mirror 41. The second mirror 43 has a high
reflectivity.
[0034] Returning to FIG. 3, the second beam displacer 34 produces
two pairs of orthogonally-polarized beams. The first beam pair
strikes the highly-reflective region of the front mirror 41 and is
largely reflected back along the Z axis to the second beam
displacer 34 without propagating through the resonator 35. In
contrast, the second beam pair strikes the partially-reflective
region of the front mirror 41 and is partially transmitted through
the front mirror 41 into the resonator cavity between the front and
back mirrors 41 and 43. A portion of the second beam pair is also
reflected back along the Z axis to the second beam displacer 34
without propagating through the resonator 35. The transmitted
portions of the first and second beam pairs are reflected by the
back mirror 43 through the front mirror 41 of the resonator 35
toward the second beam displacer 34. The split-mirror resonator 35
reflects substantially all of the incident optical power back
regardless of wavelength, but the group delay of the reflected
beams is strongly dependent on wavelength.
[0035] Thus, both pairs of reflected beams from the split mirror
resonator 35 back-propagate along the negative Z axis (moving
toward the left in FIG. 3) and are recombined into one pair of
beams by the second beam displacer 34. Due to the birefringence of
the second beam displacer 34, a difference in the optical path
lengths between the two beam pairs is generated. As a result, the
polarization state of the back-propagating beam pair exiting the
second beam displacer 34 is a function of optical wavelength. In
other words, this back-propagating beam pair has mixed polarization
as a function of the optical wavelengths carried by the beams.
[0036] The back-propagating beam pair enters the polarized
beamsplitter 33. The components of the beam pair that are polarized
along the Y axis are transmitted through the polarized beamsplitter
33 toward the first beam displacer 31, while those components that
are polarized along the X axis are reflected by the polarized
beamsplitter 33 toward a third beam displacer 37, as illustrated in
FIG. 3. It should be expressly understood that other types of
polarization-dependent routing elements could be employed to
separate the components of the back-propagating beam pair. For
example, an angled beamsplitter, beam displacer, or other
birefringent element could substituted for this purpose.
[0037] One of the beams in the transmitted beam pair passes through
the half-wave plate 32 which rotates its polarization by 90
degrees, so that the transmitted beams have orthogonal
polarizations. These beams are then recombined by the first beam
displacer 31 into a single beam at output port A. Similarly, one of
the beams in the reflected beam pair passes through a half-wave
plate 36 which rotates its polarization by 90 degrees, so that the
reflected beams become orthogonally polarized. These beams are
recombined by the third beam displacer 37 into a single beam at
output port B.
[0038] Thus, this device functions as an optical interleaver. The
outputs beams at output ports A and B contain two complimentary
subset of the input optical spectrum, similar to those shown in
FIG. 2, with alternating optical channels in the input spectrum
being routed to each output port. If desired, this device can be
extended in a cascade architecture with multiple stages of optical
interleavers to progressively separate individual channels or
groups of channels.
[0039] The embodiment of the split-mirror resonator shown in FIGS.
4(a) and 4(b) has advantages in certain applications. This
embodiment can decrease the device size. More importantly, it
allows the two beam pairs to share a common path, thereby
minimizing the effects of vibration, air turbulence, and
temperature change.
[0040] Ring-shaped Resonator Structures.
[0041] Alternatively, the split-mirror resonator can be implemented
as a ring structure with more than two mirrors. For example, FIG.
5(a) shows a resonator with three mirrors 51, 52, and 53. Here, the
first mirror 51 is a split mirror, similar to the example shown in
FIG. 4(b). The other mirrors 52 and 53 are coated with a high
reflectance coating. FIG. 5(b) extends this concept to a resonator
with four mirrors 51-54 in a ring structure.
[0042] FIG. 6 shows an alternative embodiment of an optical
wavelength router using the ring resonator structure from FIG.
5(a). The input optical signal passes through a polarizer 61 that
converts the random polarization of the input beam to a known
linear polarization. For example, the polarizer 61 can be
implemented as a birefringent element 31 and half-wave plate 32 as
shown in FIG. 3 that converts the input beam into a pair of beams
having the same polarization. Alternatively a simple polarization
filter can be employed to produce a single polarized beam as shown
in FIG. 6.
[0043] The polarized beam is then separated into two
orthogonally-polarized beams by a first beam displacer 62. As
before, one of these beams strikes the highly reflective region of
the first mirror 51 and is reflected to the second beam displacer
63. The other beam passes through the partially reflective region
of the first mirror 51 and is reflected in turn by the second and
third mirrors 52 and 53 before back through the first mirror 51
toward the second beam displacer 63. The beams exiting the ring
resonator 51-53 are combined by the second beam displacer 63. Here,
again, the difference in the optical path lengths between the beams
due to the birefringence of the first beam displacer 62 and the
second beam displacer 63 produces interference between the beams
and results in an output beam having a polarization state that is a
function of optical wavelength. A polarized beamsplitter 64 (or
other polarization-dependent routing element) separates the
polarized components of the output beam from the second beam
displacer 63 to output ports A and B, respectively, to produce a
two complementary subsets of the input optical spectrum, similar to
those shown in FIG. 2.
[0044] Wavelength Router Using Waveplates and a Zero-order Beam
Displacer.
[0045] FIG. 7 shows another alternative embodiment of the present
optical wavelength router. In this device, one or more waveplates
71 are used to generate birefringence and thereby produce a
predetermined difference in the optical path lengths between
different optical polarizations. The waveplates 71 are oriented
such that the optical axis for each one is at 45 degrees relative
to the polarizing axis of the beamsplitter 33. However, the
waveplates 71 do not disturb the net beam propagation direction.
The waveplates 71 can be one piece of birefringent material
oriented at 45 degrees, or a plurality of birefringent elements
that are all oriented at 45 degrees.
[0046] The first beam displacer 31 splits the input beam into two
orthogonally-polarized beams. A half-wave plate 32 rotates the
polarization of one of these beams by 90 degrees, so that both
beams have the same polarization. Both beams then pass through a
polarized beamsplitter 33 without significant attenuation. The
waveplates 71 cause a 50/50 split of the incident optical power of
both beams into two orthogonal polarizations as a result of the 45
degree orientation of the waveplates' axis. After the waveplates
71, a second beam displacer 72 spatially separates the two
orthogonal polarizations in the beam pair to create two pairs of
beams as illustrated in FIG. 7.
[0047] A split-mirror resonator 35, as describe above and shown in
FIGS. 4(a) and 4(b), reflects both beams pairs beams back along the
negative Z axis so that they are recombined into one pair of beams
by the second beam displacer 72. Due to the birefringence of the
waveplates 71, a difference in the optical path lengths between the
orthogonally polarized beams is generated. As a result, the
polarization state of the back-propagating beam pair exiting the
waveplate 71 is a function of optical wavelength.
[0048] The back-propagating beam pair enters the polarized
beamsplitter 33 (or other polarization-dependent routing element).
The components of the beam pair that are polarized along the Y axis
are transmitted through the polarized beamsplitter 33 toward the
first beam displacer 31, while those components that are polarized
along the X axis are reflected by the polarized beamsplitter 33
toward a third beam displacer 37. One of the beams in the
transmitted beam pair passes through a half-wave plate 32 that
rotates its polarization by 90 degrees, so that the transmitted
beams have orthogonal polarizations. These beams are then
recombined by the first beam displacer 31 into a single beam at
output port A. Similarly, one of the beams in the reflected beam
pair passes through a half-wave plate 36 that rotates its
polarization by 90 degrees, so that the reflected beams become
orthogonally polarized. These beams are recombined by the third
beam displacer 37 into a single beam at output port B.
[0049] The second beam displacer 72 in FIG. 7 is preferably
constructed as shown in greater detail in FIG. 8. Two beam
displacers 81 and 82, made of similar materials and having similar
thicknesses, are aligned so that their optical axes are 90 degrees
relative to one another as shown in FIG. 8. The two beam displacers
81, 82 are then bonded together to form one piece. When an optical
beam passes through this assembly, the two input polarizations are
spatially separated, but there is no net difference in the optical
path lengths through the beam displacers 81 and 82 between the two
polarizations. In other words, FIG. 8 demonstrates a "pure" beam
displacer (i.e., a zero-order beam displacer), in which the
orthogonal input polarizations are spatially separated but at most
only a negligible amount of birefringence is added to the
beams.
[0050] A zero-order beam displacer can also be implemented as
depicted in FIG. 9. Here, a zero-order half-wave plate 92 is placed
between beam displacers 91 and 93. The two displacers 91, 93 can be
identical pieces but have their respective optical axes rotated 90
degrees from one another as shown in FIG. 9. FIG. 10 shows another
arrangement to construct a zero-order displacer with two identical
pieces of conventional displacer using a different crystal
orientation.
[0051] The embodiment illustrated in FIG. 7 is of practical
importance because of the reduced difficulty of optical alignment.
In general, either a waveplate or beam displacer can be used to
generate birefringence in an optical beam. However, the
birefringence of a conventional beam displacer is very sensitive to
its orientation. To achieve a given amount of path delay between
two polarizations, the position of a conventional displacer must be
controlled to within very tight tolerances, making it difficult to
initially align and to maintain proper alignment over a range of
operating conditions, including temperature changes and mechanical
vibration.
[0052] In contrast to a beam displacer, the amount of birefringence
from a waveplate is much less sensitive to its orientation. There
are two reasons for this difference in sensitivity. In a
conventional beam displacer as used in FIG. 3, the optical beam
usually propagates at about 45 degrees from the optical axis of the
crystal. In this configuration, the index of refraction of the
extraordinary beam is very sensitive to the exact angle between
propagation direction and the optical axis. In a waveplate, the
optical beam propagates at 90 degrees from the optical axis. In
this configuration, the index of refraction of the extraordinary
beam is relatively insensitive to the angle between the propagation
direction and the optical axis. The second reason is that in a beam
displacer, the ordinary and extraordinary rays exit the crystal
with a spatial separation. When the crystal is tilted, the physical
distance between the ordinary ray and the extraordinary ray travel
become different. In contrast, the physical distances that the
ordinary ray and extraordinary ray travel in a waveplates remain
almost unchanged.
[0053] These two effects combine to make the embodiment of the
present invention shown in FIG. 3 much more sensitive to the
perturbations to the position of the beam displacer 34. In
contrast, the implementation shown in FIG. 7 using waveplates 71 as
the interferometer is very robust.
[0054] In addition to the advantages associated with waveplates 71,
the zero-order displacer 72 introduces at most a negligible amount
of birefringence and is therefore easy to initially align and to
maintain alignment. In the device shown in FIG. 7, the waveplates
71 can be easily tuned to achieve a desire amount of birefringence
and optical path length difference. Such a design makes it possible
to produce a compact, reliable, and low-cost wavelength router for
WDM communications. The zero-order displacer can further be used to
implement a beam displacer with at most negligible inherent
differential group delay (DGD). Such zero-DGD displacers also have
zero polarization mode dispersion (PMD) and is a very important
feature for a polarization-based wavelength router.
[0055] The above disclosure sets forth a number of embodiments of
the present invention. Other arrangements or embodiments, not
precisely set forth, could be practiced under the teachings of the
present invention and as set forth in the following claims.
* * * * *