U.S. patent number RE37,044 [Application Number 09/257,555] was granted by the patent office on 2001-02-06 for tunable add/drop optical filter.
This patent grant is currently assigned to Chorum Technologies, Inc.. Invention is credited to Kuang-Yi Wu.
United States Patent |
RE37,044 |
Wu |
February 6, 2001 |
Tunable add/drop optical filter
Abstract
In all-optical networks, optical switching and routing become
the most important issues for interconnecting the transport network
layers. This invention describes a novel tunable optical add/drop
filter for the all-optical wavelength-division-multiplexing (WDM)
network applications. This filter can add or drop part of the high
transmission capacity signals of a WDM link. It can be used to
decentralized access point in the access network or as small core
network node to realizing branching points in the network topology.
It works in both wavelength and space domains. It has the
advantages of: 1) High throughput and low voltage operation; 2)
Wide tuning range and therefore, high channel capacity; 3) High
isolation and high directivity between input and output ports; 4)
Compact device packaging is possible as compares to the
conventional grating and mechanical switching type of add/drop
filter; 5) Multiple ports add/drop tunable filters can be realized
with this invention to interconnect multiple WDM networks. This
novel add/drop filter can be used in various WDM topologies. It
enhances the performance of the conventional tunable filter by
re-routing the rejected wavelengths back to network, which not only
save the precious optical energy, but also cut down the return loss
of the device.
Inventors: |
Wu; Kuang-Yi (Plano, TX) |
Assignee: |
Chorum Technologies, Inc.
(Richardson, TX)
|
Family
ID: |
24523362 |
Appl.
No.: |
09/257,555 |
Filed: |
February 23, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
629523 |
Apr 10, 1996 |
05606439 |
Feb 25, 1997 |
|
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Current U.S.
Class: |
349/117; 385/14;
385/31; 385/39 |
Current CPC
Class: |
G02B
6/272 (20130101); G02B 6/2746 (20130101); G02B
6/29302 (20130101); G02B 6/29358 (20130101); G02B
6/29361 (20130101); G02B 6/29383 (20130101); G02B
6/29395 (20130101); G02F 1/31 (20130101); H04J
14/021 (20130101); H04Q 11/0003 (20130101); G02B
6/2766 (20130101); G02B 6/2773 (20130101); G02F
1/09 (20130101); H04J 14/0212 (20130101) |
Current International
Class: |
G02F
1/31 (20060101); G02B 6/34 (20060101); G02F
1/29 (20060101); H04Q 11/00 (20060101); G02F
1/01 (20060101); G02F 1/09 (20060101); H04J
14/02 (20060101); G02F 001/1335 (); G02B 006/12 ();
G02B 006/26 () |
Field of
Search: |
;349/117,96
;385/24,31,39,14 ;359/114,124,117,130,122,156 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Melman et al., "Tunable birefringent wavelength-division
multiplexer/demiutiplexer", Electronic Letters, vol. 21, No. 15,
pp. 634-635, Jul. 1985. .
Glance, Tunable Add/drop Optical Filter Providing Arbitrary Channel
Arrangements, IEEE Photon. Lett., 7(11) 1303, 1995. .
Dono et al., "A Wavelength Division Multiple Access Network For
Computer Communication", IEEE J. Sol. Area Comm., 8(6), 983, 1990.
.
Title, "Tunable Birefringenet Filters", Opt. Engrg., vol. 20, pp.
815, 1981. .
Wang, "Acousto-optic Tunable Filters Spectrally Modulate Light",
Laser Focus World, May 1992. .
Cheung, "Acoustooptic Tunable Filters in Narrowband WDM Networks:
System Issues and Network Applications" IEEE J. Sele. Area Comm.
8(6), 1015, 1990..
|
Primary Examiner: Parker; Kenneth
Assistant Examiner: Nguyen; Dung
Attorney, Agent or Firm: Morrison & Foerster LLP
Claims
I claim:
1. A tunable add/drop filter .[.which comprises.].
.Iadd.comprising.Iaddend.:
a nonreciprocal setup for optical channels return to WDM network, a
tunable filter for predetermined optical channel selection, and a
reciprocal optical add/drop setup for add/drop operations,
.[.where, the non-reciprocal optical setup is a combination of an
optical birefringent element which has a thickness of d, a Faraday
rotator that rotates eigen polarization of input light beam by
45.degree., a polarization beam combiner and a right angle prism
placed at an entrance side of the birefringent element to recombine
returned optical signals; the tunable filter for selectively
dropping or adding the optical channels; and the reciprocal optical
add/drop setup comprises two optical birefringent elements with
thickness of (1/2)d, and with their polarization eigen planes
oriented at 45.degree. and -45.degree. relatively to a polarization
eigen plane of the birefringent element in the non-reciprocal
setup, respectively..].
.Iadd.wherein said non-reciprocal optical setup comprises a
combination of an optical birefringent element, which has a
thickness of d, and a Faraday rotator that rotates eigen
polarization of input light beams by 45.degree., and
wherein said non-reciprocal optical setup further comprises a
polarization beam combiner and a right angle prism placed at an
entrance side of said birefringent element to recombine returned
optical signals..Iaddend.
2. The tunable add/drop filter of claim 1, wherein said tunable
filter .[.is.]. .Iadd.comprises .Iaddend.a liquid crystal based,
birefringent filter, a pair of halfwave plates are added to the
front and back facets of the filter; said two halfwave plates are
placed to intersect two orthogonally polarized incident light
paths, respectively; a first one of the halfwave plates is placed
on a first light path that rotates an ordinary light wave into
extraordinary light wave that aligns a polarization of the light
wave to an optical axis of the liquid crystal film; a second one of
the halfwave plates is placed on a second light path that rotates
the extra-ordinary light wave into ordinary light wave.
3. The tunable add/drop filter of claim 2, wherein said tunable
birefringent filter is a patterned liquid crystal based filter with
a plurality of sections; each filter section can selectively filter
through optical channel and said tunable add/drop filter forms a
tunable multiple-port add/drop filter..Iadd.
4. The tunable add/drop filter of claim 1, wherein said reciprocal
optical add/drop setup comprises two optical birefringent
elements..Iaddend..Iadd.
5. The tunable add/drop filter of claim 1, wherein said tunable
filter comprises a filter selected from the group consisting of:
piezo-tuned, Fabry-Perot tunable filters; liquid crystal based
Fabry-Perot tunable filters; tunable polarization interference
filters; and acoustooptic tunable filters..Iaddend..Iadd.
6. The tunable add/drop filter of claim 1, wherein said tunable
filter further comprises a pair of halfwave plates, each of said
pair located on opposite sides of said filter,
respectively..Iaddend..Iadd.
7. A tunable add/drop filter comprising:
a tunable filter which is selectively adjustable to pass only
desired frequencies of light waves;
means for splitting an optical input polarization into two eigen
orthogonal polarizations;
means for rotating said two orthogonal eigen polarizations by
45.degree. and passing the rotated orthogonal eigen polarizations
to a first side of said tunable filter;
means for spatially combining an optical output from a second side
of said tunable filter;
means for spatially combining undesired frequencies of said two
orthogonal eigen polarizations which are rejected by said tunable
filter; and
means for outputting a combination of said undesired
frequencies..Iaddend..Iadd.
8. The tunable add/drop filter of claim 7, further comprising means
for inputting an optical signal to said second side of said tunable
filter; and
means for splitting said optical signal into two eigen orthogonal
polarizations and passing the split optical signal to said second
side of said tunable filter; and
means for spatially combining desired frequencies of said split
optical signal which are outputted from said first side of said
tunable filter with said undesired of frequencies of said two
orthogonal eigen polarizations which are rejected by said tunable
filter and outputting a combined signal through said means for
outputting..Iaddend..Iadd.
9. A tunable add/drop filter comprising: .Iaddend.
.Iadd.selectively adjustable means for passing only desired
frequencies of light waves;
means for splitting an optical input signal into two eigen
orthogonal polarizations;
means for rotating said two orthogonal eigen polarizations by
45.degree. and passing the rotated orthogonal eigen polarizations
to a first side of said selectively adjustable means;
means for spatially combining an optical output from a second side
of said selectively adjustable means;
means for redirecting frequencies of said two orthogonal
polarizations, which are rejected by said selectively adjustable
means, through said means for splitting such that said rejected
frequencies follow different paths than the split optical input
signal; and
means for spatially combining said rejected frequencies of said two
orthogonal eigen polarizations..Iaddend..Iadd.
10. The tunable add/drop filter of claim 9, further comprising
means for outputting the combination of said rejected
frequencies..Iaddend..Iadd.
11. The tunable add/drop filter of claim 9, further comprising:
means for splitting a second optical input signal into two eigen
orthogonal polarizations and passing the split second optical
signal to said second side of said selectively adjustable
means;
means for directing frequencies of said split optical signal, which
pass through said selectively adjustable means, along said
different paths and with the same polarizations as said rejected
frequencies of said first optical signal; and
means for spatially combining said rejected frequencies of said
first optical signal and the passed frequencies of said second
optical signal..Iaddend..Iadd.
12. The tunable add/drop filter of claim 11, further comprising
means for outputting the combination of said rejected frequencies
of said first optical signal and the passed frequencies of said
second optical signal..Iaddend..Iadd.
13. A tunable add/drop filter comprising:
at least two input ports for inputting optical signals;
selectively adjustable means for passing only desired frequencies
of light waves;
means for splitting an optical input signal from each said input
port into two eigen orthogonal polarizations;
means for rotating each pair of said two orthogonal eigen
polarizations by 45.degree. and passing the rotated orthogonal
eigen polarizations to a first side of said selectively adjustable
means;
means for spatially combining outputs received from a second side
of said selectively adjustable means to form optical outputs,
wherein each optical output corresponds to a respective pair of
said two orthogonal eigen polarization inputted to said first side
of said selectively adjustable means;
means for redirecting frequencies of each of said pairs of said two
orthogonal polarizations, which are rejected by said selectively
adjustable means, through said means for splitting such that said
rejected frequencies follow different paths than the split optical
input signals; and
means for spatially combining said rejected frequencies, wherein
each spatial combination of rejected frequencies corresponds to a
respective pair of said two orthogonal eigen polarizations rejected
from said first side of said selectively adjustable
means..Iaddend..Iadd.
14. The tunable add/drop filter of claim 13, further comprising at
least two output ports for outputting said spatial combinations of
rejected frequencies..Iaddend..Iadd.
15. The tunable add/drop filter of claim 14, wherein a number of
said input ports equals a number of said output
ports..Iaddend..Iadd.
16. The tunable add/drop filter of claim 13, further
comprising:
at least two input ports for inputting optical add signals;
means for splitting said optical add input signal into two eigen
orthogonal polarizations each, and passing the split optical add
signals to said second side of said selectively adjustable
means;
means for directing frequencies of said split optical add signals,
which pass through said selectively adjustable means, along said
different paths and with the same polarizations as said rejected
frequencies of said optical input signals; and
means for spatially combining said rejected frequencies of said
optical input signal and the passed frequencies of said optical add
signals, wherein each spatial combination of rejected frequencies
and passed frequencies corresponds to a respective pair of said
input ports for inputting optical signals and said input ports for
inputting optical add signals..Iaddend..Iadd.
17. The tunable add/drop filter of claim 16, further comprising
means for outputting said spatial combinations of said rejected
frequencies and said passed frequencies..Iaddend..Iadd.
18. The tunable add/drop filter of claim 17, wherein said means for
outputting comprises at least two output ports..Iaddend..Iadd.
19. The tunable add/drop filter of claim 18, wherein a number of
said input ports for inputting optical signals, said input ports
for inputting optical add signals, and said output ports are all
the same..Iaddend..Iadd.
20. A tunable add/drop filter comprising
a nonreciprocal setup for optical channels return to WDM network, a
tunable filter for predetermined optical channel selection, and a
reciprocal optical add/drop setup for add/drop operations,
wherein said reciprocal optical add/drop setup comprises two
optical birefringent elements, and wherein said two optical
birefringent elements of said reciprocal optical add/drop setup
have polarization eigen planes oriented at 45.degree. and
-45.degree. relative to a polarization eigen plane of said
birefringent element in said non-reciprocal setup,
respectively..Iaddend..Iadd.
21. The tunable add/drop filter of claim 20, wherein said
non-reciprocal optical setup comprises a combination of an optical
birefringent element, which has a thickness of d, and a Faraday
rotator that rotates eigen polarization of input light beams by
45.degree...Iaddend..Iadd.
22. The tunable add/drop filter of claim 21, wherein said
non-reciprocal optical setup further comprises a polarization beam
combiner and a right angle prism placed at an entrance side of said
birefringent element to recombine returned optical
signals..Iaddend..Iadd.
23. The tunable add/drop filter of claim 21, wherein said
reciprocal optical add/drop setup comprises two optical
birefringent elements, each having a thickness of (1/2)d, and with
their polarization eigen planes oriented at 45.degree. and
-45.degree. relative to a polarization eigen plane of said
birefringent element in said non-reciprocal setup,
respectively..Iaddend..Iadd.
24. The tunable add/drop filter of claim 23, wherein said tunable
filter further comprises a pair of halfwave plates, each of said
pair located on opposite sides of said liquid crystal based,
birefringent filter, respectively..Iaddend..Iadd.
25. The tunable add/drop filter of claim 20, wherein said tunable
filter comprises a liquid crystal based, birefringent
filter..Iaddend..Iadd.
26. The tunable add/drop filter of claim 25, wherein said tunable
filter comprises a patterned liquid crystal based, birefringent
filter with a plurality of sections..Iaddend..Iadd.
27. The tunable add/drop filter of claim 20, wherein said tunable
filter comprises a filter selected from the group consisting of:
piezo-tuned, Fabry-Perot tunable filters; liquid crystal based
Fabry-Perot tunable filters; tunable polarization interference
filters; and acoustooptic tunable filters..Iaddend..Iadd.
28. A tunable add/drop filter comprising
a nonreciprocal setup for optical channels return to WDM
network,
a tunable filter for predetermined optical channel selection, and a
reciprocal optical add/drop setup for add/drop operations,
wherein said non-reciprocal optical setup comprises a combination
of an optical birefringent element, which has a thickness of d, and
a Faraday rotator that rotates eigen polarization of input light
beams by 45.degree., and
wherein said reciprocal optical add/drop setup comprises two
optical birefringent elements, each having a thickness of (1/2)d,
and with their polarization eigen planes oriented at 45.degree. and
-45.degree. relative to a polarization eigen plane of said
birefringent element in said non-reciprocal setup,
respectively..Iaddend..Iadd.
29. The tunable add/drop filter of claim 28, wherein said tunable
filter further comprises a pair of halfwave plates, each of said
pair located on opposite sides of said liquid crystal based,
birefringent filter, respectively..Iaddend..Iadd.
30. The tunable add/drop filter of claim 28, wherein said tunable
filter comprises a filter selected from the group consisting of:
piezo-tuned, Fabry-Perot tunable filters; liquid crystal based
Fabry-Perot tunable filters; tunable polarization interference
filters; and acoustooptic tunable filters..Iaddend.
Description
FIELD OF THE INVENTION
In this invention, a tunable add/drop filter for the
wavelength-division-multiplexing (WDM) network applications is
described. This filter can add or drop part of the high
transmission capacity signals of a WDM link.
BACKGROUND OF THE INVENTION
The communication environment is evolving towards increasingly
heterogeneous but interconnected networks. The growth of demand for
existing services and the introduction of new advanced services is
expected to create a large increase of traffic flow in the near
future. The current evolution of telecommunication network is led
by asynchronous and synchronous transfer modes (Asynchronous
Transfer Mode(ATM), Synchronous Optical Network (SONET),
Synchronous Digital Hierarchy (SDH)), which require primarily
electronic technologies for processing and switching. Although the
necessary hardware building blocks are available to design wide
area networks, complex .[.issue arises with.]. .Iadd.issues arise
with .Iaddend.the management of network resources. In order to
simplify the transfer task, the layer structure of the transport
network and the use of optical means are preferred.
In all-optical networks, optical switching and routing become the
most important issues for interconnecting the transport network
layers. This invention describes a tunable optical add/drop filter
for the optical WDM network application. This filter can add or
drop part of the high transmission capacity signals of a WDM link.
It can be used to .[.decentralized access point.].
.Iadd.decentralize access points .Iaddend.in the access network or
as a small core network node .[.to.]. .Iadd.in .Iaddend.realizing
branching points in the network topology. It works in both
wavelength and space domains.
The following describes various device structures that have been
used for the add/drop filter design. The first structure [Cheung,
"Acoustoopic Tunable Filters in Narrowband WDM networks: System
Issues and Network Applications," IEEE J. Sele. Area Comm. 8(6),
1015, 1990.] uses four 1.times.N demultiplexers and N's 2.times.2
optical switches. The structure is complicated and the
interconnections are difficult.
The second tunable add/drop filter, similar to the first geometry,
has recently been proposed and demonstrated by Glance at AT&T.
[Glance, "Tunable add/drop optical filter providing arbitrary
channel arrangement", IEEE Photon. Lett., 7(11), 1303, 1995 and
U.S. Pat. No. 5,488,500.] This filter provides the advantage of
arbitrary channel arrangement, but still suffers a costly 6 dB
optical coupling loss, because of the two array waveguide grating
demultiplexers used in the structure.
The third type of wavelength-space switch [Dono et al, "A
wavelength division multiple access network for computer
communication", IEEE J. Sol. Area Comm., 8(6), 983, 1990.] has been
widely used in various WDM networks, for example the IBM Rainbow
Network. This structure uses a passive star-coupler that combines
and splits the incoming light signals into N receivers. The
receivers built with a tunable filter then select the desired
channels. It has .[.the.]. broadcast capability and the control
structure of this implementation is very simple. However, .[.the
undesirable feature of the broadcast star, the splitting loss can
be very high when the users number is large..]. .Iadd.an
undesirable feature of the broadcast star is that splitting losses
can be very high when the number of users is large..Iaddend.
The add/drop filter presented in this invention can re-route the
unused channels, which are rejected by the tunable filter, back to
the network and save the precious optical energy in the network. It
includes the design of an optical isolator, that also dramatically
cuts down the return loss of the device, another important
performance requirement for high-speed WDM devices. It operates in
both wavelength and space domains that provides hybrid
functionality with a relatively simple structure. It is ideal for
the WDM applications.
DESCRIPTION OF THE DRAWINGS
FIGS. 1a, 1b are the building blocks of a tunable add/drop optical
filter of this invention. It consists of three primary parts of
non-reciprocal optical setup, tunable filter, and reciprocal
optical setup.
FIG. 2 is a schematic representation of an exemplary tunable
add/drop filter of this invention.
FIGS. 3a, 3b are schemes illustrating the operation of a tunable
add/drop filter of this invention. The polarization states
progressing through the optical elements is indicated and the
definition of the signs are shown in the insert.
FIG. 4 is a structure representation of an exemplary tunable
add/drop filter of this invention which incorporates a liquid
crystal Fabry-Perot tunable filter. A pair of half waveplates are
added into the two light paths, respectively, to rotate the
polarizations match to the optical axis to the liquid crystal
Fabry-Perot tunable filter.
FIG. 5 is a schematic representation of a multi-port add/drop
tunable filter. The input/output ports have multiple fibers that
carry the optical signals from multiple WDM networks. Each add/drop
layer can independently drop or add a desired optical frequency
through the sectioned tunable filter.
FIGS. 6a, 6b, 6c show optical switches combined with tunable
add/drop filters to form tunable multiple-port add/drop
filters.
SUMMARY DESCRIPTION OF THE INVENTION
The present invention includes a tunable add/drop filter that
utilizes the unique operational characteristics of a non-reciprocal
optical setup and a reciprocal optical setup for wavelength
re-routing, and a tunable filter for wavelength selection. The
non-reciprocal optical setup provides the functionality for optical
channels to go in and out of the filter with high isolation. The
reciprocal optical setup is used to keep the light were paths
.[.stay.]. the same during the add/drop operations. In between the
two optical setups, a filter is inserted to select the desired
channels that pass through the add/drop filter.
In exemplary embodiments of the present invention, Fabry-Perot type
filters and polarization interference filters are used. The
non-reciprocal optical setup may comprise .[.of.]. a Faraday
Rotator, a birefringent element, a polarizing beam combiner, and a
right angle prism. The reciprocal optical setup may .[.comprises of
.]. .Iadd.comprise .Iaddend.a pair of birefringent elements with
their polarization eigen .[.plane.]. .Iadd.planes
.Iaddend.orthogonal to each other and .[.are.]. .+-.45.degree. to
that of the birefringent element in the non-reciprocal setup,
respectively. To properly recombine the two orthogonally polarized
light waves at the add/drop port the thickness of .Iadd.each of
.Iaddend.the two birefringent elements in the reciprocal setup is
1/2 .Iadd.times the thickness .Iaddend.of the birefringent element
in the non-reciprocal setup.
Other features and embodiments of the present invention will become
clear to those of ordinary skill in the art by reference to the
drawings and accompanying detailed description.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The core of this tunable add/drop filter .[.composes of a.].
.Iadd.comprises a .Iaddend.tunable filter 55, a non-reciprocal
optical return setup 54, and a reciprocal optical setup 56, as
shown in FIG. 1. The spectra changed in the filter can be
understood from 700 and 701 for the adding and dropping operations,
respectively. In 700 adding operation (FIG. 1a), the channel
(wavelength) to be added into the WDM network is in 711 and enters
from 52. It combines with the spectrum 712, which already exists in
the network, and .[.exit.]. .Iadd.exits .Iaddend.at 53 with a
combined spectrum 713. In dropping operation 701 (FIG. 1b), the
network spectrum is 722. It drops part of the spectrum 721 to 52.
The rest of the returned channels then .[.re-routes.].
.Iadd.re-route .Iaddend.through 53 and go back to the network with
a spectrum 723.
The light wave .[.propagates.]. .Iadd.propagation .Iaddend.within
the add/drop filter can be further explained as .[.follow.].
.Iadd.follows.Iaddend.. In the dropping operation, the incoming
network signals .[.carry.]. .Iadd.carrying .Iaddend.multiple
wavelengths enter from port 51. The non-reciprocal optical setup 54
passes spectrum 722 to the tunable filter 55. The selected channel
721 passes through the filter and the reciprocal setup 56, and
.[.exit.]. .Iadd.exits .Iaddend.at port 52. The rejected channels
by the tunable filter, on the other hand, .[.reflects.].
.Iadd.reflect .Iaddend.back to the non-reciprocal setup 54. Because
of the non-reciprocal property of 54, light propagates backward in
a different path .[.as in.]. .Iadd.from .Iaddend.the forward
propagating .[.direction.]. .Iadd.path.Iaddend.. Therefore, it
exits at port 53 and completes the dropping operation.
For the added operation, optical signal 711 to be added into the
network enters from port 52. Because .Iadd.of .Iaddend.the
reciprocal setup of 56, light traveling in the reverse direction
follows exactly the same path as it did in the forward direction.
Therefore, spectrum 711 passes through the filter 55 that has been
tuned to the channel and enters the non-reciprocal setup 54.
Because of the non-reciprocal optical path arrangement, this added
channel joins the rest of the .[.rejected.]. channels
.Iadd.rejected .Iaddend.by the filter in the backward propagating
direction and exits at port 53. This completes the adding
operations.
A preferred structure of this invention is shown in FIG. 2. The
non-reciprocal setup is built by a combination of a Faraday rotator
25, a birefringent element 21, a polarization beam combiner 15, and
a right angle prism 14. The optical reciprocal setup is comprised
of a pair of birefringent elements 22/23 with their polarization
eigen planes 212/213 perpendicular to each other, and .[.are.].
.+-.45.degree. relative to the polarization eigen plane 211 of the
birefringent element in the non-reciprocal setup. The polarization
eigen plane is defined by the plane that contains the optical axis
of the birefringent element and also is the plane .Iadd.that
.Iaddend.contains the two orthogonal polarization states, when an
unpolarized light is incident onto the element. The add/drop
channel is selected by the tunable filter 26. The add/drop port is
designated by 12. The input and output ports to the WDM network are
11 and 13.
The detailed operations of the add/drop tunable filter are shown in
FIG. 3, which is the top view of the device. The polarization
progression within the filter is also indicated. In FIG. 3a, the
forward dropping operation is realized by splitting the input
polarization into two eigen orthogonal polarizations using the
birefringent element 21. These two light beams with polarization at
(0.degree., 90.degree.) are then rotated another 45.degree. by the
Faraday Rotator 25 .Iadd.which .Iaddend.sits inside a magnet 29 and
incident onto the filter 26. The dropping channel passes through
tunable filter 26 where it has been tuned to the desired resonant
condition. The two spatially separated signals are recombined by
the second and third birefringent .[.element.]. .Iadd.elements
.Iaddend.22 and 23 oriented at .+-.45.degree. and collected by the
output lens 12. Since the thickness of .Iadd.each of .Iaddend.22
and 23 is chosen to be only 1/2 of the first birefringent element
21, the two polarizations can be combined into a single beam by
orientating 23 at 90.degree. with respect to 22. This arrangement
of beam displacement allows any incoming state of polarization to
be efficiently transmitted through the add/drop filter in the
forward direction.
For the channels (wavelengths) that are rejected by the tunable
filter 26, they backward propagate to 25 and are rotated another
45.degree.. Because this is a non-reciprocal effect, the returned
polarizations are in (90.degree., 0.degree.) states and are
orthogonal to their original input states. Hence, they travel at
different paths when passing through 21, as shown in FIG. 3b. These
two light beams are recombined by the right angle prism 14 and the
polarization beam combiner 15 and send back to the WDM network.
Similarly, the added operation can be traced as shown in FIG. 3b.
The light signal to be added into the WDM network first splits its
polarization by 22 and 23 combination with polarization angles of
(+45.degree., -45.degree.). This is based on the fact that the
input and output of the combined elements (22 and 23) are
reciprocal. This means that light traveling in the reverse
direction (i.e. the adding operation) must follow exactly the same
path as it does in the forward direction. Therefore, at the exit of
.[.this.]. .Iadd.the .Iaddend.combined birefringent element
(22/23), the spatial walk-off and the polarizations are identical
for both forward and backward traveling light waves. With filter 26
tuned to the added wavelength, light signal passes the filter and
enters 25. By adding another 45.degree. polarization to its
original state, the output polarizations become (90.degree.,
0.degree.), which are the same as the rejected wavelengths. They
are then collected by the prism 14 and polarization combiner 15 and
go into the WDM network as was explained above. This completes the
add/drop operations.
The elements used in this invention are listed below for
illustration. These shall not limit .[.to.]. the application.Iadd.,
but are for illustration purposes only.Iaddend.. The Faraday
rotator can be those based on magneto-optic materials, for
examples, yttrium iron garnet (YIG), bismuth-substituted rare earth
iron garnet (RBilG), and holmium and terbium doped garnet crystals
(HoTbBi)lG. The filter in this invention can be, piezo-tuned
Fabry-Perot optical filters, liquid-crystal based Fabry-Perot
tunable filters (U.S. Pat. No. 5,111,321, by Patel), tunable
polarization interference filters (A. Title, Tunable birefringent
filters, Optical Engineering, Vol. 20, pp. 815, 1981.), and
acoustooptical tunable filters (X. Wang, Acousto-optic tunable
filters spectrally modulate light, Laser Focus World, May
.[.1993.]. .Iadd.1992.Iaddend..). When fixed filters, for example
the interference filters, are used in the invention, they result in
fixed add/drop filters. The polarizing materials used for the
reciprocal operation can be materials with optical anisotropy, for
examples calcite, rutile, lithium niobate (LiNbO.sub.3), and
yttrium orthovanadate YVO.sub.4. All these Faraday rotators,
filters, and polarizing crystals are commercially available.
EXAMPLE 1
An example of the tunable add/drop filter can be realized by using
a liquid-crystal Fabry-Perot tunable filter as shown in FIG. 4. A
pair of halfwave plates are inserted in front of and behind of the
liquid crystal filter. A halfwave plate satisfies the equation
.DELTA.nd=.lambda./2, where .DELTA.n and d are the birefringence
and thickness of the wave plate, and .lambda. is the light
wavelength. The first wave plate 27 is added into the light path
800 to change the polarization of the decomposed input light to
match the 45.degree. optic axis of the filter 99. The second
halfwave plate 28, which is placed on the opposite side of the
filter, rotates the extra-ordinary light wave into ordinary in
light paths 801. The two then .[.recombines.]. .Iadd.recombine
.Iaddend.by the birefringent elements 22 and 23. The rest of the
operations are explained in the previous embodiment.
Due to the spatial-light-modulation capability (2-Dimensional) of a
liquid-crystal Fabry-Perot filter, a multiple-port add/drop tunable
filter can be realized based on the current structure. As shown in
FIG. 5, this multi-port add/drop tunable filter can be easily
fabricated by patterning a liquid-crystal Fabry-Perot filter into
sections, and spatially aligning a series of inputs and outputs
ports together. Remember, this multi-port tunable add/drop filter
has exactly the same number of birefringent elements, Faraday
rotator, and filter as in the single-port design. The patterning of
the liquid-crystal Fabry-Perot can also be achieved
photolithographically on the controlling transparent
indium-tin-oxide (ITO) electrodes. Therefore, costs saving on
materials and a compact packaging are possible for this multi-port
filter. Potential applications include, but .[.not limit to,.].
.Iadd.are not limited to .Iaddend.multiple WDM networks
interconnections where .[.simultaneously.]. .Iadd.simultaneous
performances of .Iaddend.add/drop channels at this filter node can
be achieved.
It can also combine with a N.times.N optical switch at the add/drop
ports. In this case, multiple WDM networks are interconnected to
each other and exchange information on this optical node. It
operates in wavelength-space domain and is transparent to users and
operators. This versatile filter will release the complex design of
the high-capacity WDM network and .[.decentralized access point.].
.Iadd.decentralize access points .Iaddend.in the access
network.[.or as.]. .Iadd.. It may also be used as a .Iaddend.small
core network node .[.to.]. .Iadd.in .Iaddend.realizing branching
points in the network topology.
EXAMPLE 2
When a fixed filter, for example the interference filter, is used
in this invention a high throughput passive add/drop filter is
realized. Here, the add/drop channel is pre-defined by the
interference filter. However, only such a wavelength can go in and
out of the ports.
EXAMPLE 3
When an 1.times.2 optical switch is added onto the add/drop port,
as shown in FIG. 6a, the three-port add/drop filter becomes a
four-port add/drop filter with it's input- and output-port
separated.(See FIG. 6b) Two of .[.this.]. .Iadd.these
.Iaddend.add/drop filters 811 can be further interconnected to form
a wavelength-space switching node for multi-layered WDM systems. In
FIG. 6c, one of the add/drop .[.port.]. .Iadd.ports .Iaddend.823 of
.[.the.]. .Iadd.each of the .Iaddend.add/drop .[.filter 811 is
linked to the each other.]. .Iadd.filters 811 are linked
together.Iaddend.. The channels between the two WDM systems 801 and
802 can then be shared through this interconnected optical node.
Furthermore, because of the reciprocal nature of this add/drop
filter at the add/drop .[.port.]. .Iadd.ports .Iaddend.821 and 822,
optical channels can still be loaded up and down from the its WDM
network 801 and 802, respectively. This greatly .[.increase.].
.Iadd.increases .Iaddend.the flexibility design from the system's
perspective.
THE ADVANTAGES OF THIS INVENTION
This tunable add/drop filter can be regarded as a combination of a
tunable filter and an optical circulator. It has the merits
of.Iadd.:.Iaddend.
1. High throughput because all of the optical energies are
preserved by the re-routing characteristics of the add/drop
operations.
2. Wide tuning range when liquid-crystal Fabry-Perot, piezoelectric
Fabry-Perot, or acoustooptic tunable filter are used. Therefore,
high channel capacity is obtainable.
3. High isolation and high directivity between input and output
ports because of the use of Faraday rotator and birefringent
materials.
4. Compact device packaging is possible, as .[.compares.].
.Iadd.compared .Iaddend.to the conventional grating and mechanical
switching type of add/drop filter.
5. When the tunable filter is a liquid-crystal Fabry-Perot type,
multiple-port add/drop tunable filters can be realized by
patterning the liquid-crystal Fabry-Perot filter into sections and
spatially aligning an array of input and output fibers together.
With the output ports connected to an N.times.N switch, a
space-separated, wavelength-division demultiplexer can be realized.
This multiple-port add/drop tunable filter can potentially be used
to link multi-WDM networks without complicated electrooptic
conversion at each networking node.
* * * * *