U.S. patent application number 10/103469 was filed with the patent office on 2002-09-12 for tunable periodic filter.
Invention is credited to Krol, Mark F., Wu, Qi.
Application Number | 20020126935 10/103469 |
Document ID | / |
Family ID | 22263501 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020126935 |
Kind Code |
A1 |
Krol, Mark F. ; et
al. |
September 12, 2002 |
Tunable periodic filter
Abstract
A polarization-interferometry based tunable periodic filter
includes polarization defining components such as polarizing beam
splitters or polarizing beam displacers located on the input and
output sides of a phase retarder such as a birefringent crystal. A
polarization independent input consisting of multiple optical
channels having a periodic frequency spacing is converted to a
branched output of optical channels in which each branch has a
periodic frequency spacing that is different from that of the
input, and which are interleaved with each other. The output period
is tunable by adjusting the phase delay of orthogonal polarization
components. A contrast ratio of .gtoreq.20 dB can be realized. The
device allows the mux/demux of up to 200 WDM channels with a 50 GHz
frequency spacing. Applications of the device include a band
splitter, a wavelength selective cross-connect, and a wavelength
monitor.
Inventors: |
Krol, Mark F.; (Painted
Post, NY) ; Wu, Qi; (Painted Post, NY) |
Correspondence
Address: |
William S. Francos, Esq.
VOLENTINE FRANCOS, PLLC
SUITE 150
12200 SUNRISE VALLEY DRIVE
RESTON
VA
20191
US
|
Family ID: |
22263501 |
Appl. No.: |
10/103469 |
Filed: |
March 20, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10103469 |
Mar 20, 2002 |
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09356217 |
Jul 16, 1999 |
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6370286 |
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60097464 |
Aug 21, 1998 |
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Current U.S.
Class: |
385/11 ;
359/487.02; 359/487.06; 359/489.05; 359/489.07; 359/489.09;
359/489.19; 385/15 |
Current CPC
Class: |
G02B 6/2773 20130101;
G02B 6/29302 20130101; G02B 6/29395 20130101; G02B 6/272 20130101;
G02B 6/2706 20130101; G02B 6/2935 20130101; G02B 6/2766
20130101 |
Class at
Publication: |
385/11 ; 385/15;
359/487 |
International
Class: |
G02B 006/27; G02B
006/26; G02B 027/28 |
Claims
We claim:
1. A tunable optical channel routing device, comprising: an input
for a plurality of optical channels having a frequency period;
first means for separating a polarization state of the input
optical channels into orthogonal (s and p) polarization states;
means for temporally retarding one of the orthogonal polarization
states with respect to the other orthogonal polarization state for
producing an ordinary beam and an extraordinary beam for each of
the polarization states; second means for separating the
polarization states of an output from the retarding means into
orthogonal (s and p) polarization states; and an output for the
plurality of optical channels from the second means wherein each
channel of a first group of output channels has a center frequency
and the first group of output channels has a frequency period that
is different from the frequency period of the input channels, and
wherein each channel of a second group of output channels has a
center frequency and the second group of output channels has a
frequency period different from the frequency period of the input
channels and is interleaved with the first group; further wherein
the device has a contrast ratio .gtoreq.20 dB over a spectral band
from about 1520 nm to 1570 nm.
2. The device of claim 1, wherein the first and second means for
separating the polarization states each comprises at least one of a
polarizing beam splitter and a polarizing beam displacer and
further wherein the means for temporally retarding one of the
orthogonal polarizations with respect to the other orthogonal
polarization comprises at least one of a birefringent crystal and a
birefringent optical fiber.
3. The device of claim 2, further comprising means for fine-tuning
the center frequency of the output channels.
4. The device of claim 2, further comprising means for changing the
length of the birefringent fiber for tuning the frequency period of
the output channels.
5. The device of claim 1, further comprising an optical path length
compensator positioned in a propagation path of at least one of the
first group of output channels and the second group of output
channels.
6. The device of claim 5, wherein the optical path length
compensator is an achromatic half-wave plate.
7. The device of claim 5, wherein the optical path length
compensator is a transparent material having an index of refraction
and a physical thickness sufficient to adjust an optical path
length difference between the first group of output channels and
the second group of output channels.
8. The device of claim 1, wherein the input comprises multiple
groups of input channels.
9. The device of claim 1, wherein the first and second groups of
output channels each have a frequency spacing .ltoreq.200 GHz.
10. The device of claim 1, wherein the first and second groups of
output channels each have a frequency spacing .ltoreq.100 GHz.
11. The device of claim 1, wherein the first and second groups of
output channels each have a frequency spacing .ltoreq.50 GHz.
12. An optical channel routing device, comprising a concatenated
plurality of devices according to claim 8, wherein at least one of
a set of inputs and outputs of one device is connected to a
respective one of a set of outputs and inputs of another
device.
13. The device of claim 2, comprising two birefringent crystals
such that each one of the orthogonal polarization components passes
through a respective birefringent crystal.
14. The device of claim 2, wherein the birefringent crystal is one
of YVO.sub.4, calcite and rutile.
15. The device of claim 1, wherein the frequency period of the
output channels is substantially constant over the wavelength range
from about 1520 nm to 1570 nm.
16. The device of claim 2, wherein the means for retarding one of
the orthogonal polarizations with respect to the other orthogonal
polarization comprises a polarization maintaining fiber pigtailed
to an end of the birefringent fiber.
17. A tunable optical channel routing device, comprising: an input
for a plurality of optical channels having a frequency period; a
first polarization beamsplitter in an optical path of the input
channels for separating and transmitting an output comprising one
of an orthogonal (s or p) polarization state of the input and for
reflecting the other orthogonal (p or s) polarization state of the
input; means for steering the reflected polarization state of the
input without changing the polarization state; a birefringent
crystal in the first polarization beamsplitter output optical paths
having a thickness L and a c-axis oriented at 45.degree. with
respect to the s and p polarization states such that the crystal
propagates an output comprising an ordinary beam and an
extraordinary beam; means for steering an output from the
birefringent crystal without changing the polarization state; a
second polarization beamsplitter in the optical path of the output
from the birefringent crystal for separating and transmitting a
first group of output channels comprising a portion of the
orthogonal (s and p) polarization states of the output and for
reflecting a second group of output channels comprising another
portion of the orthogonal (p and s) polarization states of the
output; wherein each channel of a first group of output channels
has a center frequency and the first group of output channels has a
frequency period that is different from the frequency period of the
input channels, and wherein each channel of a second group of
output channels has a center frequency and the second group of
output channels has a frequency period different from the frequency
period of the input channels and is interleaved with the first
group; further wherein the device has a contrast ratio .gtoreq.20
dB over a spectral band from about 1520 nm to 1570 nm.
18. The device of claim 17 wherein the crystal is rotatable about
the c-axis for fine tuning the center frequency of the output
channels.
19. The device of claim 17 further comprising a phase compensator
located in an optical path after the first polarization beam
splitter for fine tuning the center frequency of the output
channels.
20. The device of claim 19 wherein the phase compensator is a
liquid crystal.
21. The device of claim 17 wherein said device is a microoptic
assembly.
22. A tunable optical channel routing device, comprising: an input
for a plurality of optical channels having a frequency period; a
first polarization beam displacer having a thickness, d, in an
optical path of the input channels for separating and transmitting
an output comprising orthogonal (s and p) polarization states of
the input; a birefringent crystal in the first polarization beam
displacer output optical path having a thickness L and a c-axis
oriented at 45.degree. with respect to the s and p polarization
states such that the crystal propagates an output comprising at
least one of an ordinary beam and an extraordinary beam; a second
polarization beam displacer having a thickness, d, in the crystal
output optical path for separating and transmitting an output
comprising orthogonal (s and p) polarization states of the second
polarization beam displacer output, one of which is a second group
of output channels; an optical path length compensator located in
an optical path of one of the s and p polarization states output
from the second polarization beam displacer; a third polarization
beam displacer having a thickness, 2d, in the second polarization
beam displacer output optical path of one of the s and p
polarizations for combining and transmitting a first group of
output channels, wherein each channel of a first group of output
channels has a center frequency and the first group of output
channels has a frequency period that is different from the
frequency period of the input channels, and wherein each channel of
a second group of output channels has a center frequency and the
second group of output channels has a frequency period different
from the frequency period of the input channels and is interleaved
with the first group; further wherein the device has a contrast
ratio .gtoreq.20 dB over a spectral band from about 1520 nm to 1570
nm.
23. The device of claim 22 wherein the optical path length
compensator is an achromatic half-wave plate.
24. The device of claim 17 wherein the means for steering the
reflected polarization states are right angle prisms.
25. A tunable optical channel routing device, comprising: an input
for a plurality of optical channels having a frequency period; a
first polarization beamsplitter in an optical path of the input
channels for separating and transmitting an output comprising one
of an orthogonal (s or p) polarization state of the input and for
reflecting the other orthogonal (p or s) polarization state of the
input; a first birefringent fiber coupled to the first polarization
beamsplitter output for propagating a portion of the orthogonal (s
and p) polarizations such that the fiber propagates an output
comprising an ordinary beam and an extraordinary beam; a second
birefringent fiber coupled to the first polarization beamsplitter
output for propagating another portion of the orthogonal (s and p)
polarizations such that the fiber propagates an output comprising
an ordinary beam and an extraordinary beam; a second polarization
beamsplitter coupled to both the first birefringent fiber output
and the second birefringent fiber output for separating and
transmitting a first group of output channels comprising orthogonal
(s and p) polarization states of the output and for reflecting a
second group of output channels comprising orthogonal (s and p)
polarization states of the output; wherein each channel of a first
group of output channels has a center frequency and the first group
of output channels has a frequency period that is different from
the frequency period of the input channels, and wherein each
channel of a second group of output channels has a center frequency
and the second group of output channels has a frequency period
different from the frequency period of the input channels and is
interleaved with the first group; further wherein the device has a
contrast ratio .gtoreq.20 dB over a spectral band from about 1520
nm to 1570 nm.
26. The device of claim 25 wherein the first and second
birefringent fibers are coupled to the first and second
polarization beamsplitters by polarization maintaining fiber
pigtails.
27. The device of claim 26 wherein the first and second
birefringent fibers have polarization axes and the polarization
maintaining fiber pigtails have polarization axes wherein the
birefringent fiber axes are oriented at 45 degrees to the
polarization maintaining fiber axes.
28. The device of claim 25 further comprising means for stretching
at least one of the birefringent fibers for fine-tuning the center
frequency of the output channels.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention generally relates to a device and method for
routing (e.g., dividing and subdividing) bands of optical signals
and, more particularly, to a multi-port, tunable periodic filter
based on polarization interferometry, and a method for tuning the
transmission peaks and frequency spacing of the optical
signals.
[0003] 2. Description of Related Art
[0004] The demand for increased data transmission capability
continues to grow. Users of DWDM systems are pressing for greater
bandwidth utilization with 50 GHz and tighter channel spacing,
adding further challenge to upgrade existing DWDM's struggling with
100 GHz channel spacing.
[0005] Several approaches for providing building blocks for
all-optical networks capable of meeting the challenging demands of
service providers and users involve tunable filters. These include
(but are not limited to), e.g., cascaded Fabry-Perot (resonant
cavity) and Mach-Zehnder (interferometry) components for squeezing
more and more channels into a free spectral range or limited
bandwidth. Such devices and examples of their utilization are
described in Green, Fiber Optic NETWORKS, Prentice Hall, cha. 4,
(1993). Disadvantages associated with these devices include, e.g.,
long response time or slow tuning speed, poor crosstalk performance
and device complexity and fabrication tolerance. Other lattice and
Mach-Zehnder component filter designs used as band splitters are
also described in EP 0 724 173A1, U.S. Pat. No. 5,680,490 and OFC
'98 Technical Digest, paper ThQ7 by Nolan et al. These devices lack
optimum contrast ratio and desired tuning capability.
[0006] The inventors have therefore recognized a need for a DWDM
filter device that caters to the immediate and future requirements
for high speed network systems without the disadvantages associated
with current components and approaches. Accordingly, the invention
describes a periodic filter device that has the attributes of
accurate and easy tuning, polarization independence, high contrast
ratio over the whole 1.5 .mu.m telecommunications band,
environmental stability, conformity with the ITU grid, and others,
that will be apparent from the description, drawings and claims
which follow. Applications of the invention include, but are not
limited to, band splitters, wavelength monitors and wavelength
selective cross-connect components.
SUMMARY OF THE INVENTION
[0007] Accordingly, the invention is broadly directed to a device
for routing optical signals. Additional features and advantages of
the invention will be set forth in the description which follows,
and in part will be apparent from the description, or may be
learned by practice of the invention. The objectives and other
advantages of the invention will be realized and attained by the
apparatus and method particularly pointed out in the written
description and claims hereof as well as the appended drawings.
[0008] An embodiment of the invention is directed to a tunable
optical channel routing device including an input for a plurality
of optical channels having a frequency period; first means for
separating a polarization state of the input optical channels into
orthogonal (s and p) polarization states; means for temporally
retarding one of the orthogonal polarization states with respect to
the other orthogonal polarization state for producing an ordinary
beam and an extraordinary beam for each polarization state; second
means for separating the polarization states of an output from the
retarding means again into orthogonal (s and p) polarization
states; and, an output for the plurality of optical channels from
the second means wherein each channel of a first group of output
channels has a center frequency and the first group of output
channels has a frequency period that is different from the
frequency period of the input channels, and wherein each channel of
a second group of output channels has a center frequency and the
second group of output channels has a frequency period different
from the frequency period of the input channels and is interleaved
with the first group. The device provides a contrast ratio
.gtoreq.20 dB over the 1.5 .mu.m spectral band, which is
essentially limited by the dispersion of the retarding component
(e.g., birefringent material).
[0009] In different aspects of this embodiment, the optical
components for separating the orthogonal polarizations can be
polarizing beam splitters (PBS's) or polarizing beam displacers
(PBD's), and the components for providing the ordinary and
extraordinary beams can be a birefringent crystal or a birefringent
optical fiber. While the period of the groups of output channels is
tailored by the thickness or amount of birefringent material
traversed by the light, fine tuning is achieved by rotating the
birefringent crystal about its c-axis, changing the length of the
birefringent fiber, or through the use of a phase compensator such
as a liquid crystal. An optical path length compensator such as,
e.g., a half-wave plate or a selective index transparent material
is used as necessary to maintain equal optical path lengths to
minimize polarization mode dispersion (PMD).
[0010] 1.times.2 and cascaded 2.times.2 devices according to
embodiments of the invention provide further application
flexibility as, e.g., optical cross-connect components.
[0011] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and are intended to provide further explanation of the invention as
claimed.
[0012] The accompanying drawings are included to provide a further
understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention.
DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 schematically shows an embodiment of a polarization
independent, three-port, tunable periodic filter according to an
embodiment of the invention in which polarization means for
separating light input into orthogonal polarization states includes
two polarizing beam splitters, and phase retardation means is a
birefringent crystal that is rotatable about its c-axis;
[0014] FIG. 2 schematically shows another embodiment of a
polarization independent, three-port, tunable periodic filter
according to the invention in which polarization means for
separating light input into s and p polarization states includes
two polarizing beam displacers, and further including an optical
path length compensating half-wave plate;
[0015] FIG. 3 schematically shows another embodiment of a
polarization independent, three-port, tunable periodic filter
according to the invention in which polarization means for
separating input light into orthogonal polarization states includes
two polarizing beam splitters, and phase compensation means are two
birefringent fibers having polarization maintaining fiber pigtails;
and
[0016] FIG. 4 is a schematic illustration of the principle of
polarization interferometry using a birefringent material.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0017] Reference will now be made in detail to the present
preferred embodiments of the invention, examples of which are
illustrated in the accompanying drawings and tables presented
herein.
[0018] The invention deals with the filtering of optical signals
and, in particular, to the splitting and combining of bands or
groups of optical channels, as in a WDM; thus although the
embodiments described herein will discuss the demultiplexing
aspects of the invention where a band of input channels with a
known frequency period or spacing is divided into, e.g., two output
channel groups each of the channels of which have a center
frequency and a frequency spacing equal to twice that of the input,
those skilled in the art will readily appreciate that the same
considerations apply to the multiplexing aspects of the invention,
and that these do not need to, and will not, be described in detail
for an understanding of the invention.
[0019] The invention is based upon the well-known principles of
polarization interferometry using birefringent materials. Although
the invention is operated and described for use with reference to
two polarization states (s and p), the optical phenomena is
described below for a single polarization state, for clarity and
ease of understanding. As shown in FIG. 4, a birefringent material
100 is placed in between two orthogonal polarizers P1 and P2. If
the polarization of the input light is parallel to that of P1, the
system has an optical transmission coefficient, T, described by the
relation: 1 T = 1 2 [ 1 + cos ( 0 + 2 ) ] , ( 1 )
[0020] where .upsilon. is the optical frequency, and .tau. is given
by 2 = L c ( n eg - n og ) L c ( n g ) , ( 2 )
[0021] where L is the thickness of the birefringent crystal, c is
the speed of light in free-space, and .DELTA.n.sub.g is the
difference in group refractive indices between ordinary and
extraordinary beams at a certain center wavelength, e.g., 1550 nm).
The frequency period of the sinusoidal transmission function is
thus (1/.tau.), which can be precisely set by tailoring L. The
transmission peaks (i.e., center frequency of each output channel)
can be fine-tuned to align them with the ITU frequency grid,
through the phase constant .phi..sub.0 in eq.(1). In practice, this
can be achieved by slightly rotating the birefringent crystal with
respect to its c-axis as shown in FIG. 1. Rotating .delta..about.10
mrad, for example, will slightly change L and fine tune the
transmission peaks without significant beam walk-off.
Alternatively, a phase compensator such as, e.g., a liquid crystal,
may be used after the first PBS 17 to dynamically control
.phi..sub.0 and fine tune the peak positions.
[0022] In a preferred embodiment of the invention, with reference
to FIG. 1, a polarization independent, optical signal multi-channel
subdivider/combiner 10 based upon a three-port periodic tunable
filter having an output whose frequency period is equal to twice
that of the input, includes a group of elliptically polarized
(i.e., any polarization state) input signal channels 11 having a
frequency period, 1/2.tau., which are input to PBS 17. PBS 17
separates the input channels into orthogonal s and p polarizations
23, 25, transmitting one of them into birefringent crystal 21, and
reflecting the other preferably from a right angle prism (not
shown) or an equivalent device that does not affect the
polarization of the light, into crystal 21. Birefringent crystal 21
has a physical thickness, L, and is preferably a material that
exhibits a large birefringence; e.g., YVO.sub.4
(.DELTA.n.sub.g=0.21101). Other materials such as, e.g., calcite
and rutile, are also suitable, however, as large a birefringence as
possible is preferred. As shown, birefringent crystal 21 has a
c-axis orientation at 45 degrees with respect to the axes of PBS's
17 and 19 (and thus with respect to the s and p polarization
states). Each of the orthogonal polarizations 23, 25 traversing
birefringent crystal 21 is decomposed into an ordinary beam and an
extraordinary beam having a relative time delay, .tau., resulting
in an output that is elliptically polarized at PBS 19. PBS 19,
similar to PBS 17, separates and transmits the orthogonally
polarized light as a first output channel group 15, and reflects a
second output channel group 13. Output channel groups 13, 15 each
have a center frequency and a frequency period equal to 1/.tau.,
the first group 13 being out of phase with the second group 15 by
.pi..
[0023] In another aspect of this embodiment (not shown), two
independently adjustable birefringent crystals can be used in place
of sole birefringent crystal 21, to alleviate the parallelism
requirement of the surfaces of a single crystal, which should be
better than 0.1 mrad in order to maintain a constant phase for the
two beams.
[0024] Low insertion loss devices of the type described above can
be fabricated as micro-optic assemblies similar to polarization
independent optical isolators which exhibit a typical insertion
loss of about 0.6 dB. Before pigtailing, the devices can be further
cascaded and integrated in one package to form 1.times.N, e.g.,
1.times.4 or 1.times.8, channel subdividers. In this way the
insertion loss can be kept low by reducing the amount of fiber
pigtailing.
[0025] In an alternative embodiment according to the invention, as
shown in FIG. 2, the polarizing beam splitters 17, 19 of FIG. 1 are
replaced by polarizing beam displacers 37, 39, each having a
thickness, d. The resulting smaller beam separation (.about.1.5 mm)
in the YVO.sub.4 crystal relaxes the parallelism requirement on the
crystal faces. An additional advantage of the beam displacers is
their typically higher extinction ratio over PBS's. As shown in
FIG. 2, an elliptically polarized input channel group 11 is input
to PBD 37 which separates the beam into mutually orthogonal s and p
polarizations. These enter and propagate through birefringent
crystal 41 wherein each is decomposed into an ordinary beam and an
extraordinary beam, and emerge as elliptically polarized light.
This output then enters the second PBD 39 which separates the light
signals passing through it, again into orthogonal polarizations
represented by (15a, 15b) and 13, respectively. Beams 15a and 15b
pass through half-wave plate 43 which rotates the polarization of
each beam as a way to minimize or eliminate PMD. Orthogonally
polarized beams 15a and 15b then pass through PBD 45, having a
thickness equal to 2d, for recombination into output channel group
15. Output channel group 13 is deflected by polarization
maintaining means 66. Similar to that described above, output
channel groups 13, 15, each have a frequency spacing that is twice
as large as that of input group 11. The channel center frequencies
can be fine tuned by positioning a phase retarder, such as a liquid
crystal, after PBD 37. In an alternative aspect of this embodiment,
the .lambda./2 plate could be replaced by a piece of material
providing optical path length equalization (e.g., having a
selective index of refraction and thickness to equalize the path
lengths).
[0026] In another embodiment, illustrated in FIG. 3, a periodic
group of input channels 11 propagate to a first PBS 17 that
separates and transmits either s or p-polarized light 23, and which
reflects p or s-polarized light 25. The output from PBS 17 is
coupled into birefringent fibers (BRF's) 71a, 71b through
polarization maintaining fiber pigtails 73, and is then coupled
into second PBS 19 again through polarization maintaining fiber
pigtails. Second PBS 19 separates and reflects a first group of
output signals 13 and transmits a second group of output signals
15; each group having a frequency spacing equal to twice that of
input group 11. Typically, a BRF length of 6 m generates
approximately a 10 ps time delay, T, (i.e., a 100 GHz frequency
period). The filter can be fine-tuned by stretching the BRF to
change L. Although the total insertion loss may be higher (i.e.,
due to pigtailing to a micro-optic component), the fiber based
device can be easily assembled. Ideally an all fiber PBS would
reduce excess insertion loss.
[0027] In all of the preceding embodiments, the devices are
inherently polarization insensitive. When the extinction ratio of
the PBS's or PBD's are in the range of 30-40 dB, the contrast ratio
of the embodied devices is 20 dB or more.
[0028] It will be apparent to those skilled in the art that various
modifications and variations can be made in the apparatus and
method of the present invention without departing from the spirit
or scope of the invention. Thus, it is intended that the present
invention cover the modifications and variations of this invention
provided they come within the scope of the appended claims and
their equivalents.
* * * * *