 |
Description  |
|
|
FIELD OF THE INVENTION
In this invention, a tunable optical 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 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 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.
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.
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
wave 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 a pair of birefringent elements with their
polarization eigen plane 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 the two birefringent elements
in the reciprocal setup is 1/2 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 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 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 through 53 and go back to the network
with a spectrum 723.
The light wave propagates within the add/drop filter can be further
explained as follow. In the dropping operation, the incoming network
signals carry 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 at port 52. The rejected channels by the tunable filter, on
the other hand, reflects back to the non-reciprocal setup 54. Because of
the non-reciprocal property of 54, light propagates backward in a
different path as in the forward propagating direction. 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 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 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 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 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 22 and 23 oriented at .+-.45.degree. and collected by
the output lens 12. Since the thickness of 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 combined birefringent elements (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. 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 acoustooptic tunable
filters (X. Wang, Acousto-optic tunable filters spectrally modulate light,
Laser Focus World, May 1993.). When fixed filters, for example the
interference filters, are used in this 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 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, multiple WDM networks interconnections where
simultaneously 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 in the access network or as small core network
node to 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 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 823 of the add/drop filter 811 is linked to the
each other. 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 821
and 822, optical channels can still be loaded up and down from the its WDM
network 801 and 802, respectively. This greatly increase 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
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 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.
* * * * *
|
|
 |
|
 |
Description |
|
