U.S. patent number RE39,525 [Application Number 11/027,587] was granted by the patent office on 2007-03-20 for reconfigurable optical add and drop modules with servo control and dynamic spectral power management capabilities.
This patent grant is currently assigned to Capella Photonics, Inc.. Invention is credited to Joseph E. Davis, Jeffrey P. Wilde.
United States Patent |
RE39,525 |
Wilde , et al. |
March 20, 2007 |
Reconfigurable optical add and drop modules with servo control and
dynamic spectral power management capabilities
Abstract
This invention provides a novel wavelength-separating-routing
(WSR) apparatus that uses a diffraction grating to separate a
multi-wavelength optical signal by wavelength into multiple
spectral channels, which are then focused onto an array of
corresponding channel micromirrors. The channel micromirrors are
individually controllable and continuously pivotable to reflect the
spectral channels into selected output ports. As such, the
inventive WSR apparatus is capable of routing the spectral channels
on a channel-by-channel basis and coupling any spectral channel
into any one of the output ports, thereby constituting a dynamic
optical drop module (RODM). By operating an RODM in reverse, a
dynamic optical add module (ROAM) is also provided. The RODM (or
ROAM) of the present invention may be further equipped with
servo-control and power-management capabilities. Such RODMs and
ROAMs can be used as building blocks to construct dynamically
reconfigurable optical add-drop multiplexers (OADMs) and other WDM
optical networking systems.
Inventors: |
Wilde; Jeffrey P. (Morgan Hill,
CA), Davis; Joseph E. (Morgan Hill, CA) |
Assignee: |
Capella Photonics, Inc. (San
Jose, CA)
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Family
ID: |
44720436 |
Appl.
No.: |
11/027,587 |
Filed: |
December 31, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09938426 |
Aug 23, 2001 |
6625346 |
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60277217 |
Mar 19, 2001 |
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Reissue of: |
10143651 |
May 8, 2002 |
06661948 |
Dec 9, 2003 |
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Current U.S.
Class: |
385/24; 385/37;
385/34 |
Current CPC
Class: |
G02B
6/3512 (20130101); G02B 6/2931 (20130101); G02B
6/29313 (20130101); G02B 6/3588 (20130101); G02B
6/29391 (20130101); G02B 6/29385 (20130101); G02B
6/29395 (20130101); G02B 6/29383 (20130101); G02B
6/32 (20130101); G02B 6/3556 (20130101); G02B
6/3592 (20130101); G02B 6/34 (20130101); G02B
26/0833 (20130101); G02B 6/3586 (20130101); G02B
6/356 (20130101) |
Current International
Class: |
G02B
6/28 (20060101) |
Field of
Search: |
;385/24,34,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Font; Frank G.
Assistant Examiner: Mooney; Michael P.
Attorney, Agent or Firm: Young; Barry N.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-in part of U.S. patent
application Ser. No. 09/938,426, filed Aug. 23, 2001, which is
incorporated herein by reference in its entirety for all purposes,
and which claims priority from U.S. Provisional Patent Application
No. 60/277,217, filed on Mar. 19, 2001.
Claims
What is claimed is:
1. An optical apparatus, comprising: a) a plurality of input ports
for receiving input optical signals and an output port; b) a
wavelength-separator, for separating said input optical signals
from said input ports by wavelength into respective spectral
channels; c) a beam-focuser, for focusing said spectral channels
into corresponding spectral spots; and d) an array of channel
micromirrors positioned to receive said spectral channels, said
channel micromirrors being individually and continuously
controllable to reflect a subset of said spectral channels into
said output port.
2. The optical apparatus of claim 1 further comprising a
servo-control assembly, including a spectral monitor for monitoring
optical power levels of said reflected spectral channels into said
output port, and a processing unit responsive to said optical power
levels for providing control of said channel micromirrors.
3. The optical apparatus of claim 2 wherein said servo-control
assembly maintains said optical power levels at a predetermined
value.
4. The optical apparatus of claim 1 wherein said plurality of input
ports and said output port comprise fiber collimators.
5. The optical apparatus of claim 4 further comprising an array of
collimator-alignment mirrors, in optical communication with said
wavelength-separator and said fiber collimators, for adjusting
alignment of said input optical signals from said input ports
respectively and directing said reflected spectral channels into
said output port.
6. The optical apparatus of claim 5 wherein each
collimator-alignment mirror is rotatable about at least one
axis.
7. The optical apparatus of claim 5 further comprising first and
second arrays of imaging lenses, in a telecentric arrangement with
said collimator-alignment mirrors and said fiber collimators.
8. The optical apparatus of claim 1 wherein each channel
micromirror is pivotable about one axis.
9. The optical apparatus of claim 1 wherein each channel
micromirror is pivotable about two axes.
10. The optical apparatus of claim 9 wherein said plurality of
input ports and said output port are arranged in a two-dimensional
array.
11. The optical apparatus of claim 1 wherein each channel
micromirror is a silicon micromachined mirror.
12. The optical apparatus of claim 1 wherein said plurality of
input ports and said output port are arranged in a one-dimensional
array.
13. The optical apparatus of claim 1 wherein said beam-focuser
comprises at least one focusing lens having first and second focal
planes.
14. The optical apparatus of claim 13 wherein said
wavelength-separator and said channel micromirrors are placed
respectively in said first and second focal planes.
15. The optical apparatus of claim 1 wherein said
wavelength-separator comprises an element selected from the group
consisting of ruled diffraction gratings, holographic diffraction
gratings, echelle gratings, curved diffraction gratings, and
dispersing prisms.
16. The optical apparatus of claim 1 further comprising a
quarter-wave plate optically interposed between said
wavelength-separator and said channel micromirrors.
17. The optical apparatus of claim 1 wherein said input optical
signals contain no common wavelengths.
18. The optical apparatus of claim 17 wherein each channel
micromirror receives a unique one of said spectral channels.
19. The optical apparatus of claim 1 wherein said input optical
signals contain a plurality of common wavelengths.
20. The optical apparatus of claim 19 wherein at least one channel
micromirror receives a plurality of said spectral channels, said at
least one channel micromirror being controlled such to reflect a
selected one of said received spectral channels into said output
port.
21. An optical apparatus comprising: a) a plurality of input ports
for receiving input optical signals and an output port; b) a
wavelength-separator, for separating said input optical signals
from said input ports by wavelength into respective spectral
channels; c) a beam-focuser, for focusing said spectral channels
into corresponding spectral spots; d) an array of channel
micromirrors positioned to receive said spectral channels, said
channel micromirrors being individually and continuously
controllable to reflect a subset of said spectral channels into
said output port; and e) an array of collimator-alignment mirrors,
for adjusting alignment of said input optical signals from said
input ports respectively and directing said reflected spectral
channels into said output port.
22. The optical apparatus of claim 21 further comprising a
servo-control assembly, including a spectral monitor for monitoring
optical power levels of said spectral channels coupled into said
output port, and a processing unit responsive to said optical power
levels for providing control of said channel micromirrors.
23. The optical apparatus of claim 22 wherein said servo-control
assembly maintains said optical power levels at a predetermined
value.
24. The optical apparatus of claim 21 wherein each
collimator-alignment mirror is rotatable about at least one
axis.
25. The optical apparatus of claim 21 wherein each channel
micromirror is continuously pivotable about one axis.
26. The optical apparatus of claim 21 wherein each channel
micromirrors is pivotable about two axes.
27. The optical apparatus of claim 26 wherein said plurality of
input ports and said output port are arranged in a two-dimensional
array.
28. The optical apparatus of claim 27 wherein said
collimator-alignment mirrors are arranged in a two-dimensional
array.
29. The optical apparatus of claim 21 further comprising first and
second arrays of imaging lenses, in a telecentric arrangement with
said collimator-alignment mirrors and said plurality of input ports
along with said output port.
30. The optical apparatus of claim 21 wherein said
wavelength-separator comprises an element selected from the group
consisting of ruled diffraction gratings, holographic diffraction
gratings, echelle gratings, curved diffraction gratings, and
dispersing prisms.
31. The optical apparatus of claim 21 wherein said plurality of
input ports and said output port are in a one-dimensional
array.
32. The optical apparatus of claim 31 wherein said
collimator-alignment mirrors are in a one-dimensional array.
33. The optical apparatus of claim 21 wherein said plurality of
input ports and said output port comprise fiber collimators.
34. A method of performing dynamic wavelength multiplexing,
comprising: a) receiving input optical signals from a plurality of
input ports; b) separating said input optical signals by wavelength
into respective spectral channels; c) focusing said spectral
channels onto an array of channel micromirrors; and d) dynamically
and continuously controlling said channel micromirrors, so as to
reflect a subset of said spectral channels into an output port.
35. The method of claim 34 further comprising the steps of
monitoring optical power levels of said reflected spectral channels
into said output port and providing feedback control of said
channel micromirrors.
36. The method of claim 35 further comprising the step of
maintaining said optical power levels at predetermining values.
37. The method of claim 34 wherein step d) comprises reflecting
each of said spectral channels into said output port.
38. The method of claim 34 wherein a plurality of said spectral
channels impinge onto a particular channel micromirror, and wherein
step d) includes controlling said particular channel micromirror so
as to reflect a selected one of said received spectral channels
into said output port.
Description
FIELD OF THE INVENTION
This invention relates generally to optical communication systems.
More specifically, it relates to a novel class of dynamically
reconfigurable optical add-drop multiplexers (OADMs) for wavelength
division multiplexed optical networking applications.
BACKGROUND
As fiber-optic communication networks rapidly spread into every
walk of modern life, there is a growing demand for optical
components and subsystems that enable the fiber-optic
communications networks to be increasingly scalable, versatile,
robust, and cost-effective.
Contemporary fiber-optic communications networks commonly employ
wavelength division multiplexing (WDM), for it allows multiple
information (or data) channels to be simultaneously transmitted on
a single optical fiber by using different wavelengths and thereby
significantly enhances the information bandwidth of the fiber. The
prevalence of WDM technology has made optical add-drop multiplexers
indispensable building blocks of modern fiber-optic communications
networks. An optical add-drop multiplexer (OADM) serves to
selectively remove (or drop) one or more wavelengths from a
multiplicity of wavelengths on an optical fiber, hence taking away
one or more data channels from the traffic stream on the fiber. It
further adds one or more wavelengths back onto the fiber, thereby
inserting new data channels in the same stream of traffic. As such,
an OADM makes it possible to launch and retrieve multiple data
channels (each characterized by a distinct wavelength) onto and
from an optical fiber respectively, without disrupting the overall
traffic flow along the fiber. Indeed, careful placement of the
OADMs can dramatically improve an optical communication network's
flexibility and robustness, while providing significant cost
advantages.
Conventional OADMs in the art typically employ
multiplexers/demultiplexers (e.g, waveguide grating routers or
arrayed-waveguide gratings), tunable filters, optical switches, and
optical circulators in a parallel or serial architecture to
accomplish the add and drop functions. In the parallel
architecture, as exemplified in U.S. Pat. No. 5,974,207, a
demultiplexer (e.g., a waveguide grating router) first separates a
multi-wavelength signal into its constituent spectral components. A
wavelength switching/routing means (e.g., a combination of optical
switches and optical circulators) then serves to drop selective
wavelengths and add others. Finally, a multiplexer combines the
remaining (i.e., the pass-through) wavelengths into an output
multi-wavelength optical signal. In the serial architecture, as
exemplified in U.S. Pat. No. 6,205,269, tunable filters (e.g.,
Bragg fiber gratings) in combination with optical circulators are
used to separate the drop wavelengths from the pass-through
wavelengths and subsequently launch the add channels into the
pass-through path. And if multiple wavelengths are to be added and
dropped, additional multiplexers and demultiplexers are required to
demultiplex the drop wavelengths and multiplex the add wavelengths,
respectively. Irrespective of the underlying architecture, the
OADMs currently in the art are characteristically high in cost, and
prone to significant optical loss accumulation. Moreover, the
designs of these OADMs are such that it is inherently difficult to
reconfigure them in a dynamic fashion.
U.S. Pat. No. 6,204,946 of Askyuk et al. discloses an OADM that
makes use of free-space optics in a parallel construction. In this
case, a multi-wavelength optical signal emerging from an input port
is incident onto a ruled diffraction grating. The constituent
spectral channels thus separated are then focused by a focusing
lens onto a linear array of binary micromachined mirrors. Each
micromirror is configured to operate between two discrete states,
such that it either retroreflects its corresponding spectral
channel back into the input port as a pass-through channel, or
directs its spectral channel to an output port as a drop channel.
As such, the pass-through signal (i.e., the combined pass-through
channels) shares the same input port as the input signal. An
optical circulator is therefore coupled to the input port, to
provide necessary routing of these two signals. Likewise, the drop
channels share the output port with the add channels. An additional
optical circulator is thereby coupled to the output port, from
which the drop channels exit and the add channels are introduced
into the output port. The add channels are subsequently combined
with the pass-through signal by way of the diffraction grating and
the binary micromirrors.
Although the aforementioned OADM disclosed by Askyuk et al. has the
advantage of performing wavelength separating and routing in free
space and thereby incurring less optical loss, it suffers a number
of limitations. First, it requires that the pass-through signal
share the same port/fiber as the input signal. An optical
circulator therefore has to be implemented, to provide necessary
routing of these two signals. Likewise, all the add and drop
channels enter and leave the OADM through the same output port,
hence the need for another optical circulator. Moreover, additional
means must be provided to multiplex the add channels before
entering the system and to demultiplex the drop channels after
exiting the system. This additional multiplexing/demultiplexing
requirement adds more cost and complexity that can restrict the
versatility of the OADM thus-constructed. Second, the optical
circulators implemented in this OADM for various routing purposes
introduce additional optical losses, which can accumulate to a
substantial amount. Third, the constituent optical components must
be in a precise alignment, in order for the system to achieve its
intended purpose. There are, however, no provisions provided for
maintaining the requisite alignment; and no mechanisms implemented
for overcoming degradation in the alignment owing to environmental
effects such as thermal and mechanical disturbances over the course
of operation.
U.S. Pat. No. 5,906,133 of Tomlinson discloses an OADM that makes
use of a design similar to that of Aksyuk et al. There are input,
output, drop and add ports implemented in this case. By positioning
the four ports in a specific arrangement, each micromirror, which
is switchable between two discrete positions, either reflects its
corresponding channel (coming from the input port) to the output
port, or concomitantly reflects its channel to the drop port and an
incident add channel to the output port. As such, this OADM is able
to perform both the add and drop functions without involving
additional optical components (such as optical circulators used in
the system of Aksyuk et al.). However, because a single drop port
is designated for all the drop channels and a single add port is
designated for all the add channels, the add channels would have to
be multiplexed before entering the add port and the drop channels
likewise need to be demultiplexed upon exiting from the drop port.
Moreover, as in the case of Askyuk et al., there are no provisions
provided for maintaining the requisite optical alignment in the
system, and no mechanisms are implemented for combating degradation
in the alignment due to environmental effects over the course of
operation.
As such, the prevailing drawbacks suffered by the OADMs currently
in the art are summarized as follows: 1) The wavelength routing is
intrinsically static, rendering it difficult to dynamically
reconfigure these OADMs. 2) Add and/or drop channels often need to
be multiplexed and/or demultiplexed, thereby imposing additional
complexity and cost. 3) Stringent fabrication tolerance and
painstaking optical alignment are required. Moreover, the optical
alignment is not actively maintained, rendering it susceptible to
environmental effects such as thermal and mechanical disturbances
over the course of operation. 4) In an optical communication
network, OADMs are typically in a ring or cascaded configuration.
In order to mitigate the interference amongst OADMs, which often
adversely affects the overall performance of the network, it is
essential that the optical power levels of spectral channels
entering and exiting each OADM be managed in a systematic way, for
instance, by introducing power (or gain) equalization at each
stage. Such a power equalization capability is also needed for
compensating for non-uniform gain caused by optical amplifiers
(e.g., erbium doped fiber amplifiers) in the network. There lacks,
however, a systematic and dynamic management of the optical power
levels of various spectral channels in these OADMs. 5) The inherent
high cost and heavy optical loss further impede the wide
application of these OADMs. In view of the foregoing, there is an
urgent need in the art for optical add-drop multiplexers that
overcome the aforementioned shortcomings in a simple, effective,
and economical construction.
SUMMARY OF THE INVENTION
The present invention provides a wavelength-separating-routing
(WSR) apparatus and method which employ an input port and a
plurality of output ports; a wavelength-separator; a beam-focuser;
and an array of channel micromirrors.
In operation, a multi-wavelength optical signal emerges from the
input port. The wavelength-separator separates the multi-wavelength
optical signal into multiple spectral channels, each characterized
by a distinct center wavelength and associated bandwidth. The
beam-focuser focuses the spectral channels into corresponding
spectral spots. The channel micromirrors are positioned such that
each channel micromirror receives one of the spectral channels. The
channel micromirrors are individually controllable and movable,
e.g., continuously pivotable (or rotatable), so as to reflect the
spectral channels into selected ones of the output ports. As such,
each channel micromirror is assigned to a specific spectral
channel, hence the name "channel micromirror". And each output port
may receive any number of the reflected spectral channels.
A distinct feature of the channel micromirrors in the present
invention, in contrast to those used in the prior art, is that the
motion, e.g., pivoting (or rotation), of each channel micromirror
is under analog control such that its pivoting angle can be
continuously adjusted. This enables each channel micromirror to
scan its corresponding spectral channel across all possible output
ports and thereby direct the spectral channel to any desired output
port.
In the WSR apparatus of the present invention, the
wavelength-separator may be provided by a ruled diffraction
grating, a holographic diffraction grating, an echelle grating, a
curved diffraction grating, a dispersing prism; or other
wavelength-separating means known in the art. The beam-focuser may
be a single lens, an assembly of lenses, or other beam-focusing
means known in the art. The channel micromirrors may be provided by
silicon micromachined mirrors, reflective ribbons (or membranes),
or other types of beam-deflecting means known in the art. And each
channel micromirror may be pivotable about one or two axes. The
input and output ports may be provided by fiber collimators, e.g.,
arranged in a one-dimensional or two-dimensional array. In the
latter case, the channel micromirrors must be pivotable
biaxially.
The WSR apparatus of the present invention may further comprise an
array of collimator-alignment mirrors, in optical communication
with the wavelength-separator and the fiber collimators, for
adjusting the alignment of the input multi-wavelength signal and
directing the spectral channels into the selected output ports by
way of angular control of the collimated beams. Each
collimator-alignment mirror may be rotatable about one or two axes.
The collimator-alignment mirrors may be arranged in a
one-dimensional or two-dimensional array. First and second arrays
of imaging lenses may additionally be optically interposed between
the collimator-alignment mirrors and the fiber collimators in a
telecentric arrangement, thereby "imaging" the collimator-alignment
mirrors onto the corresponding fiber collimators to ensure an
optimal alignment.
The WSR apparatus of the present invention may further include a
servo-control assembly, in communication with the channel
micromirrors and the output ports. The servo-control assembly
serves to monitor the optical power levels of the spectral channels
coupled into the output ports and further provide control of the
channel micromirrors on an individual basis, so as to maintain a
predetermined coupling efficiency of each spectral channel into one
of the output ports. As such, the servo-control assembly provides
dynamic control of the coupling of the spectral channels into the
respective output ports and actively manages the optical power
levels of the spectral channels coupled into the output ports. (If
the WSR apparatus includes an array of collimator-alignment mirrors
as described above, the servo-control assembly may additionally
provide dynamic control of the collimator-alignment mirrors.)
Moreover, the utilization of such a servo-control assembly
effectively relaxes the requisite fabrication tolerances and the
precision of optical alignment during assembly of a WSR apparatus
of the present invention, and further enables the system to correct
for shift in optical alignment over the course of operation. A WSR
apparatus incorporating a servo-control assembly thus described is
termed a WSR-S apparatus, thereinafter in the present
invention.
As such, the aforementioned WSR (or WSR-S) apparatus of the present
invention may be used as a "reconfigurable optical drop module"
(RODM) that is capable of dynamically routing any wavelength in the
input multi-wavelength optical signal to any one of the output
ports. Further, by operating an RODM in reverse (e.g., the output
ports serving as multiple input ports and the input port providing
for an output port), the RODM can also combine (or "add") multiple
input optical signals from the input ports and direct the combined
optical signal to the output port, hence providing for a
"reconfigurable optical add module" (ROAM). The input optical
signals to an ROAM may each contain one or more wavelengths (or
spectral channels).
The RODMs and ROAMs of the present invention may be utilized as
building blocks for constructing a variety of optical networking
systems, such as reconfigurable optical add-drop multiplexers
(OADMs).
In an exemplary embodiment of an OADM of the present invention, a
first WSR-S (or WSR) apparatus is cascaded with a second WSR-S (or
WSR) apparatus. The output ports of the first WSR-S (or WSR)
apparatus include a pass-through port and one or more drop ports.
The second WSR-S (or WSR) apparatus includes a plurality of input
ports and an exiting port. The configuration is such that the
pass-through channels from the first WSR-S apparatus and one or
more add channels are directed into the input ports of the second
WSR-S apparatus, and consequently multiplexed into an output
multi-wavelength optical signal directed into the exiting port of
the second WSR-S apparatus. That is, in this embodiment, one WSR-S
apparatus (e.g., the first one) serves as an RODM, effective to
perform dynamic drop function. The other WSR-S apparatus (e.g., the
second one) functions as an ROAM, providing dynamic add function.
There are essentially no fundamental restrictions on the
wavelengths that can be added or dropped, other than those imposed
by the overall communication system.
Those skilled in the art will recognize that the aforementioned
embodiments provide only two of many embodiments of a dynamically
reconfigurable OADM according to the present invention. Various
changes, substitutions, and alternations can be made herein,
without departing from the principles and the scope of the
invention. Accordingly, a skilled artisan can design an OADM in
accordance with the present invention, to best suit a given
application.
The OADMs of the present invention provide many advantages over the
prior art devices, notably: 1) By advantageously employing an array
of channel micromirrors that are individually and continuously
controllable, an OADM of the present invention is capable of
routing the spectral channels on a channel-by-channel basis and
directing any spectral channel into any one of the output ports. As
such, its underlying operation is dynamically reconfigurable, and
its underlying architecture is intrinsically scalable to a large
number of channel and port counts. 2) The add and drop spectral
channels need not be multiplexed and demultiplexed before entering
and after leaving the OADM respectively. And there are not
fundamental restrictions on the wavelengths to be added or dropped.
3) The coupling of the spectral channels into the output ports is
dynamically controlled by a servo-control assembly, rendering the
OADM less susceptible to environmental effects (such as thermal and
mechanical disturbances) and therefore more robust in performance.
By maintaining an optimal optical alignment, the optical losses
incurred by the spectral channels are also significantly reduced.
4) The optical power levels of the spectral channels coupled into
the output ports can be dynamically managed according to demand, or
maintained at desired values (e.g., equalized at a predetermined
value) by way of the servo-control assembly. This spectral
power-management capability as an integral part of the OADM will be
particularly desirable in WDM optical networking applications. 5)
The use of free-space optics provides a simple, low loss, and
cost-effective construction. Moreover, the utilization of the
servo-control assembly effectively relaxes the requisite
fabrication tolerances and the precision of optical alignment
during initial assembly, enabling the OADM to be simpler and more
adaptable in structure, and lower in cost and optical loss. 6) The
underlying OADM architecture allows a multiplicity of the OADMs
according to the present invention to be readily assembled (e.g.,
cascaded) for WDM optical networking applications. The novel
features of this invention, as well as the invention itself, will
be best understood from the following drawings and detailed
description.
BRIEF DESCRIPTION OF THE FIGURES
FIGS. 1A-1D show a first embodiment of a
wavelength-separating-routing (WSR) apparatus according to the
present invention, and the modeling results demonstrating the
performance of the WSR apparatus;
FIGS. 2A-2C depict second and third embodiments of a WSR apparatus
according to the present invention;
FIG. 3 shows a fourth embodiment of a WSR apparatus according to
the present invention;
FIGS. 4A-4B show schematic illustrations of two embodiments of a
WSR-S apparatus comprising a WSR apparatus and a servo-control
assembly, according to the present invention;
FIG. 5 depicts an exemplary embodiment of a reconfigurable optical
add module (ROAM) employing a servo-control assembly, according to
the present invention; and
FIG. 6 shows an alternative embodiment of an OADM according to the
present invention.
DETAILED DESCRIPTION
In this specification and appending claims, a "spectral channel" is
characterized by a distinct center wavelength and associated
bandwidth. Each spectral channel may carry a unique information
signal, as in WDM optical networking applications.
FIG. 1A depicts a first embodiment of a
wavelength-separating-routing (WSR) apparatus according to the
present invention. By way of example to illustrate the general
principles and the topological structure of a
wavelength-separating-routing (WSR) apparatus of the present
invention, the WSR apparatus 100 comprises multiple input/output
ports which may be in the form of an array of fiber collimators
110, providing an input port 110-1 and a plurality of output ports
110-2 through 110-N (N.gtoreq.3); a wavelength-separator which in
one form may be a diffraction grating 101; a beam-focuser in the
form of a focusing lens 102; and an array of channel micromirrors
103.
In operation, a multi-wavelength optical signal emerges from the
input port 110-1. The diffraction grating 101 angularly separates
the multi-wavelength optical signal into multiple spectral
channels, which are in turn focused by the focusing lens 102 into a
spatial array of distinct spectral spots (not shown in FIG. 1A) in
a one-to-one correspondence. The channel micromirrors 103 are
positioned in accordance with the spatial array formed by the
spectral spots, such that each channel micromirror receives one of
the spectral channels. The channel micromirrors 103 are
individually controllable and movable, e.g., pivotable (or
rotatable) under analog (or continuous) control, such that, upon
reflection, the spectral channels are directed into selected ones
of the output ports 110-2 through 110-N by way of the focusing lens
102 and the diffraction grating 101. As such, each channel
micromirror is assigned to a specific spectral channel, hence the
name "channel micromirror". Each output port may receive any number
of the reflected spectral channels.
For purposes of illustration and clarity, only a selective few
(e.g., three) of the spectral channels, along with the input
multi-wavelength optical signal, are graphically illustrated in
FIG. 1A and the following figures. It should be noted, however,
that there can be any number of the spectral channels in a WSR
apparatus of the present invention (so long as the number of
spectral channels does not exceed the number of channel mirrors
employed in the system). It should also be noted that the optical
beams representing the spectral channels shown in FIG. 1A and the
following figures are provided for illustrative purpose only. That
is, their sizes and shapes may not be drawn according to scale. For
instance, the input beam and the corresponding diffracted beams
generally have different cross-sectional shapes, so long as the
angle of incidence upon the diffraction grating is not equal to the
angle of diffraction, as is known to those skilled in the art.
In the embodiment of FIG. 1A, it is preferable that the diffraction
grating 101 and the channel micromirrors 103 are placed
respectively at the first and second (i.e., the front and back)
focal points (on the opposing sides) of the focusing lens 102. Such
a telecentric arrangement allows the chief rays of the focused
beams to be parallel to each other and generally parallel to the
optical axis. In this application, the telecentric configuration
further allows the reflected spectral channels to be efficiently
coupled into the respective output ports, thereby minimizing
various translational walk-off effects that may otherwise arise.
Moreover, the input multi-wavelength optical signal is preferably
collimated and circular in cross-section. The corresponding
spectral channels diffracted from the diffraction grating 101 are
generally elliptical in cross-section; they may be of the same size
as the input beam in one dimension and elongated in the other
dimension.
It is known that the diffraction efficiency of a diffraction
grating is generally polarization-dependent. That is, the
diffraction efficiency of a grating in a standard mounting
configuration may be considerably higher for P-polarization that is
perpendicular to the groove lines on the grating than for
S-polarization that is orthogonal to P-polarization, especially as
the number of groove lines (per unit length) increases. To mitigate
such polarization-sensitive effects, a quarter-wave plate 104 may
be optically interposed between the diffraction grating 101 and the
channel micromirrors 103, and preferably placed between the
diffraction grating 101 and the focusing lens 102 as is shown in
FIG. 1A. In this way, each spectral channel experiences a total of
approximately 90-degree rotation in polarization upon traversing
the quarter-wave plate 104 twice. (That is, if a beam of light has
P-polarization when first encountering the diffraction grating, it
would have predominantly (if not all) S-polarization upon the
second encountering, and vice versa.) This ensures that all the
spectral channels incur nearly the same amount of round-trip
polarization dependent loss.
In the WSR apparatus 100 of FIG. 1A, the diffraction grating 101,
by way of example, is oriented such that the focused spots of the
spectral channels fall onto the channel micromirrors 103 in a
horizontal array, as illustrated in FIG. 1B.
Depicted in FIG. 1B is a close-up view of the channel micromirrors
103 shown in the embodiment of FIG. 1A. By way of example, the
channel micromirrors 103 are arranged in a one-dimensional array
along the x-axis (i.e., the horizontal direction in the figure), so
as to receive the focused spots of the spatially separated spectral
channels in a one-to-one correspondence. (As in the case of FIG.
1A, only three spectral channels are illustrated, each represented
by a converging beam.) Let the reflective surface of each channel
micromirror lie in the x-y plane as defined in the figure and be
movable, e.g., pivotable (or deflectable) about the x-axis in an
analog (or continuous) manner. Each spectral channel, upon
reflection, is deflected in the y-direction (e.g., downward)
relative to its incident direction, so as to be directed into one
of the output ports 110-2 through 110-N shown in FIG. 1A.
As described above, a unique feature of the present invention is
that the motion of each channel micromirror is individually and
continuously controllable, such that its position, e.g., pivoting
angle, can be continuously adjusted. This enables each channel
micromirror to scan its corresponding spectral channel across all
possible output ports and thereby direct the spectral channel to
any desired output port. To illustrate this capability, FIG. 1C
shows a plot of coupling efficiency as a function of a channel
micromirror's pivoting angle .theta., provided by a ray-tracing
model of a WSR apparatus in the embodiment of FIG. 1A. As used
herein, the coupling efficiency for a spectral channel is defined
as the ratio of the amount of optical power coupled into the fiber
core in an output port to the total amount of optical power
incident upon the entrance surface of the fiber (associated with
the fiber collimator serving as the output port). In the
ray-tracing model, the input optical signal is incident upon a
diffraction grating with 700 lines per millimeter at a grazing
angle of 85 degrees, where the grating is blazed to optimize the
diffraction efficiency for the "-1" order. The focusing lens has a
focal length of 100 mm. Each output port is provided by a
quarter-pitch GRIN lens (2 mm in diameter) coupled to an optical
fiber (see FIG. 1D). As displayed in FIG. 1C, the coupling
efficiency varies with the pivoting angle .theta., and it requires
about a 0.2-degree change in .theta. for the coupling efficiency to
become practically negligible in this exemplary case. As such, each
spectral channel may practically acquire any coupling efficiency
value by way of controlling the pivoting angle of its corresponding
channel micromirror. This is also to say that variable optical
attenuation at the granularity of a single wavelength can be
obtained in a WSR apparatus of the present invention. FIG. 1D
provides ray-tracing illustrations of two extreme points on the
coupling efficiency vs. .theta. curve of FIG. 1C: on-axis coupling
corresponding to .theta.=0, where the coupling efficiency is
maximum; and off-axis coupling corresponding to .theta.=0.2
degrees, where the representative collimated beam (representing an
exemplary spectral channel) undergoes a significant translational
walk-off and renders the coupling efficiency practically
negligible. The exemplary modeling results thus described
demonstrate the unique capabilities of the WSR apparatus of the
present invention.
FIG. 1A provides one of many embodiments of a WSR apparatus
according to the present invention. In general, the
wavelength-separator is a wavelength-separating means that may be a
ruled diffraction grating, a holographic diffraction grating, an
echelle grating, a dispersing prism, or other types of
spectral-separating means known in the art. The beam-focuser may be
a focusing lens, an assembly of lenses, or other beam-focusing
means known in the art. The focusing function may also be
accomplished by using a curved diffraction grating as the
wavelength-separator. The channel micromirrors may be provided by
silicon micromachined mirrors, reflective ribbons (or membranes),
or other types of beam-deflecting elements known in the art. Each
micromirror may be pivoted about one or two axes. It is important
that the pivoting (or rotational) motion of each channel
micromirror be individually controllable in an analog manner,
whereby the pivoting angle can be continuously adjusted so as to
enable the channel micromirror to scan a spectral channel across
all possible output ports. The underlying fabrication techniques
for micromachined mirrors and associated actuation mechanisms are
well documented in the art, see U.S. Pat. No. 5,629,790 for
example. Moreover, a fiber collimator is typically in the form of a
collimating lens (such as a GRIN lens) and a ferrule-mounted fiber
packaged together in a mechanically rigid stainless steel (or
glass) tube. The fiber collimators serving as the input and output
ports may be arranged in a one-dimensional array, a two-dimensional
array, or other desired spatial pattern. For instance, they may be
conveniently mounted in a linear array along a V-groove fabricated
on a substrate made of silicon, plastic, or ceramic, as commonly
practiced in the art. It should be noted, however, that the input
port and the output ports need not necessarily be in close spatial
proximity with each other, such as in an array configuration
(although a close packing would reduce the rotational range
required for each channel micromirror). Those skilled in the art
will know how to design a WSR apparatus according to the present
invention, to best suit a given application.
A WSR apparatus of the present invention may further comprise an
array of collimator-alignment mirrors, for adjusting the alignment
of the input multi-wavelength optical signal and facilitating the
coupling of the spectral channels into the respective output ports,
as shown in FIGS. 2A-2B and 3.
FIG. 2A depicts a second embodiment of a WSR apparatus according to
the present invention. By way of example, WSR apparatus 200 is
built upon and hence shares a number of the elements used in the
embodiment of FIG. 1A, as identified by those labeled with
identical numerals. Moreover, a one-dimensional array 220 of
collimator-alignment mirrors 220-1 through 220-N is optically
interposed between the diffraction grating 101 and the fiber
collimator array 110. The collimator-alignment mirror 220-1 is
designated to correspond with the input port 110-1, for adjusting
the alignment of the input multi-wavelength optical signal and
therefore ensuring that the spectral channels impinge onto the
corresponding channel micromirrors. The collimator-alignment
mirrors 220-2 through 220-N are designated to the output ports
110-2 through 110-N in a one-to-one correspondence, serving to
provide angular control of the collimated beams of the reflected
spectral channels and thereby facilitating the coupling of the
spectral channels into the respective output ports according to
desired coupling efficiencies. Each collimator-alignment mirror may
be rotatable about one axis, or two axes.
The embodiment of FIG. 2A is attractive in applications where the
fiber collimators (serving as the input and output ports) are
desired to be placed in close proximity to the collimator-alignment
mirror array 220. To best facilitate the coupling of the spectral
channels into the output ports, arrays of imaging lenses may be
implemented between the collimator-alignment mirror array 220 and
the fiber collimator array 110, as depicted in FIG. 2B. By way of
example, WSR apparatus 250 of FIG. 2B is built upon and hence
shares many of the elements used in the embodiment of FIG. 2A, as
identified by those elements labeled with identical numerals.
Additionally, first and second arrays 260, 270 of imaging lenses
are placed in a 4-f telecentric arrangement with respect to the
collimator-alignment mirror array 220 and the fiber collimator
array 110. The dashed box 280 shown in FIG. 2C provides a top view
of such a telecentric arrangement. In this case, the imaging lenses
in the first and second arrays 260, 270 all have the same focal
length f. The collimator-alignment mirrors 220-1 through 220-N are
placed at the respective first (or front) focal points of the
imaging lenses in the first array 260. Likewise, the fiber
collimators 110-1 through 110-N are placed at the respective second
(or back) focal points of the imaging lenses in the second array
270. And the separation between the first and second arrays 260,
270 of imaging lenses is 2f. In this way, the collimator-alignment
mirrors 220-1 through 220-N are effectively imaged onto the
respective entrance surfaces (i.e., the front focal planes) of the
GRIN lenses in the corresponding fiber collimators 110-1 through
110-N. Such a telecentric imaging system substantially eliminates
translational walk-off of the collimated beams at the output ports
that may otherwise occur as the mirror angles change.
FIG. 3 shows a fourth embodiment of a WSR apparatus according to
the present invention. By way of example, WSR apparatus 300 is
built upon and hence shares a number of the elements used in the
embodiment of FIG. 2B, as identified by those labeled with
identical numerals. In this case, the one-dimensional fiber
collimator array 110 of FIG. 2B is replaced by a two-dimensional
array 350 of fiber collimators, providing for an input-port and a
plurality of output ports. Accordingly, the one-dimensional
collimator-alignment mirror array 220 of FIG. 2B is replaced by a
two-dimensional array 320 of collimator-alignment mirrors, and
first and second one-dimensional arrays 260, 270 of imaging lenses
of FIG. 2B are likewise replaced by first and second
two-dimensional arrays 360, 370 of imaging lenses respectively. As
in the case of the embodiment of FIG. 2B, the first and second
two-dimensional arrays 360, 370 of imaging lenses are placed in a
4-f telecentric arrangement with respect to the two-dimensional
collimator-alignment mirror array 320 and the two-dimensional fiber
collimator array 350. Each of the channel micromirrors 103 must be
pivotable biaxially in this case (in order to direct its
corresponding spectral channel to any one of the output ports). As
such, the WSR apparatus 300 is equipped to support a greater number
of the output ports.
In addition to facilitating the coupling of the spectral channels
into the respective output ports as described above, the
collimator-alignment mirrors in the above embodiments also serve to
compensate for misalignment (e.g., due to fabrication and assembly
errors) in the fiber collimators that provide for the input and
output ports. For instance, relative misalignment between the fiber
cores and their respective collimating lenses in the fiber
collimators can lead to pointing errors in the collimated beams,
which may be corrected for by the collimator-alignment mirrors. For
these reasons, the collimator-alignment mirrors are preferably
rotatable about two axes. They may be silicon micromachined
mirrors, for fast rotational speeds. They may also be other types
of mirrors or beam-deflecting elements known in the art.
To optimize the coupling of the spectral channels into the output
ports and further maintain the optimal optical alignment against
environmental effects such as temperature variations and mechanical
instabilities over the course of operation, a WSR apparatus of the
present invention may incorporate a servo-control assembly, for
providing dynamic control of the coupling of the spectral channels
into the respective output ports on a channel-by-channel basis. A
WSR apparatus incorporating a servo-control assembly is termed a
WSR-S apparatus, thereinafter in this specification.
FIG. 4A depicts a schematic illustration of a first embodiment of a
WSR-S apparatus according to the present invention. The WSR-S
apparatus 400 comprises a WSR apparatus 410 and a servo-control
assembly 440. The WSR 410 may make use of the embodiment of FIG.
1A, or any other embodiment in accordance with the present
invention. The servo-control assembly 440 may include a spectral
monitor 460, for monitoring the optical power levels of the
spectral channels coupled into the output ports 420-1 through 420-N
of the WSR apparatus 410. By way of example, the spectral monitor
460 is coupled to the output ports 420-1 through 420-N by way of
fiber-optic couplers 420-1-C through 420-N-C, wherein each
fiber-optic coupler serves to "tap off" a predetermined fraction of
the optical signal in the corresponding output port. The
servo-control assembly 440 further includes a processing unit 470,
in communication with the spectral monitor 460 and the channel
micromirrors 430 of the WSR apparatus 410. The processing unit 470
uses the optical power measurements from the spectral monitor 460
to provide feedback control of the channel micromirrors 430 on an
individual basis, so as to maintain a desired coupling efficiency
for each spectral channel into a selected output port. As such, the
servo-control assembly 440 provides dynamic control of the coupling
of the spectral channels into the respective output ports on a
channel-by-channel basis and thereby manages the optical power
levels of the spectral channels coupled into the output ports. The
optical power levels of the spectral channels in the output ports
may be dynamically managed according to demand, or maintained at
desired values (e.g., equalized at a predetermined value) in the
present invention. Such a spectral power-management capability is
essential in WDM optical networking applications, as discussed
above.
FIG. 4B depicts a schematic illustration of a second embodiment of
a WSR-S apparatus according to the present invention. The WSR-S
apparatus 450 comprises a WSR apparatus 480 and a servo-control
assembly 490. In addition to the channel micromirrors 430 (and
other elements identified by the same numerals as those used in
FIG. 4A), the WSR apparatus 480 may further include a plurality of
collimator-alignment mirrors 485, and may be configured according
to the embodiment of FIGS. 2A, 2B, 3, or any other embodiment in
accordance with the present invention. By way of example, the
servo-control assembly 490 may include the spectral monitor 460 as
described in the embodiment of FIG. 4A, and a processing unit 495.
In this case, the processing unit 495 is in communication with the
channel micromirrors 430 and the collimator-alignment mirrors 485
of the WSR apparatus 480, as well as the spectral monitor 460. The
processing unit 495 uses the optical power measurements from the
spectral monitor 460 to provide dynamic control of the channel
micromirrors 430 along with the collimator-alignment mirrors 485,
so as to maintain the coupling efficiencies of the spectral
channels into the output ports at desired values.
In the embodiment of FIG. 4A or 4B, the spectral monitor 460 may be
one of spectral power monitoring devices known in the art that is
capable of detecting the optical power levels of spectral
components in a multi-wavelength optical signal. Such devices are
typically in the form of a wavelength-separating means (e.g., a
diffraction grating) that spatially separates a multi-wavelength
optical signal by wavelength into constituent spectral components,
and one or more optical sensors (e.g., an array of photodiodes)
that are configured such to detect the optical power levels of
these spectral components. The processing unit 470 in FIG. 4A (or
the processing unit 495 in FIG. 4B) typically includes electrical
circuits and signal processing programs for processing the optical
power measurements provided by the spectral monitor 460 and
generating appropriate control signals to be applied to the channel
micromirrors 430 (and the collimator-alignment mirrors 485 in the
case of FIG. 4B), so as to maintain the coupling efficiencies of
the spectral channels into the output ports at desired values. The
electronic circuitry and the associated signal processing
algorithm/software for such a processing unit in a servo-control
system are known in the art. From the teachings of the present
invention, a skilled artisan will know how to implement a suitable
spectral monitor along with an appropriate processing unit to
provide a servo-control assembly in a WSR-S apparatus of the
present invention, for a given application.
The incorporation of a servo-control assembly provides additional
advantages of effectively relaxing the requisite fabrication
tolerances and the precision of optical alignment during initial
assembly of a WSR apparatus of the present invention, and further
enabling the system to correct for shift in the alignment (e.g.,
due to environmental effects) over the course of operation. By
maintaining an optimal optical alignment, the optical losses
incurred by the spectral channels are also significantly reduced.
As such, the WSR-S apparatus thus constructed is simpler and more
adaptable in structure, more robust in performance, and lower in
cost and optical loss. Accordingly, the WSR-S (or WSR) apparatus of
the present invention may be used to construct a variety of optical
devices and utilized in many applications.
For instance, the aforementioned WSR (or WSR-S) apparatus of the
present invention may be used as a "reconfigurable optical drop
module" (RODM) that is capable of dynamically routing any
wavelength in the input multi-wavelength optical signal to any one
of the output ports. Those skilled in the art will further
appreciate that by operating an RODM in reverse (e.g., the output
ports 110-2 through 110-N serving as multiple input ports and the
input port 110-1 as an output port in the embodiment of FIG. 1A, 2A
or 2B), the RODM can also combine (or "add") multiple input optical
signals from the input ports 110-2 through 110-N and direct the
combined optical signal to the output port 110-1, hence providing
for a "reconfigurable optical add module" (ROAM), as FIG. 5 further
describes. The input optical signals to an ROAM may each contain
one or more wavelengths (or spectral channels). (An ROAM may
likewise be provided by operating the embodiment of FIG. 3 in
reverse.)
A notable advantage of an ROAM of the present invention is that a
spectral channel from any one of the input ports can be directed to
the output port by way of the pivoting motion of its corresponding
channel micromirror. This is in sharp contrast with a conventional
wavelength multiplexer known in the art where there is a one-to-one
static mapping between the incoming wavelength channels and input
ports/fibers. It may generally be desirable that the input optical
signals to an ROAM not contain common wavelengths, whereby each
channel micromirror receives a single spectral channel (originating
from one of the input ports). In the event that there are common
wavelengths in the input optical signals, a channel micromirror may
receive a plurality of spectral channels (originating from
different input ports) and selectively direct the impinging
spectral channels into the output port by way of its pivoting
position. (One skilled in the art will appreciate that care may be
taken in this case to avoid inadvertently coupling the "unwanted"
spectral channels into the input ports.) A further inherent
advantage of an ROAM (or RODM) of the present invention is that it
effectively "filters out" broadband noise (e.g., amplified
spontaneous emission (ASE) noise characteristic of semi-conductor
diode lasers) that is anywhere but in the passbands of the optical
signals coupled into the output port.
An ROAM of the present invention may further incorporate a
servo-control assembly, e.g., in a manner as depicted above with
respect to the embodiment of FIG. 4A (or 4B). FIG. 5 depicts an
exemplary embodiment of how an ROAM may be configured with a
servo-control assembly, according to the present invention. By way
of example, the embodiment of FIG. 5 may be built upon the
embodiment of FIG. 4A, hence the elements labeled with identical
numerals. In this case, the ROAM 510 may merely be the WSR
apparatus 410 of FIG. 4A operated in reverse, whereby the output
ports 420-1 through 420-N of FIG. 4A function as the input ports
520-1 through 520-N of FIG. 5; and the input port 420 of FIG. 4A
serves as the output port 520 of FIG. 5. The spectral monitor 460
of the servo-control assembly 440 may be coupled to the output port
520 by a fiber-optic couplers 520-C, whereby a predetermined
fraction of the optical signal in the output port 520 is "tapped
off" and diverted to the spectral monitor 460. The processing unit
470, in communication with the spectral monitor 460 and the channel
micromirrors 530 of the ROAM 510, uses the optical power
measurements from the spectral monitor 460 to provide feedback
control of the channel micromirrors 530 on an individual basis, so
as to maintain the coupling efficiencies of the spectral channels
in the output port 520 at desired values. (In the event that the
ROAM also employs an array of collimator-alignment mirrors, the
servo-control assembly may also provide control of the
collimator-alignment mirrors, e.g., in a manner as illustrated in
FIG. 4B.)
The RODMs and ROAMs of the present invention may be utilized as
building blocks for constructing a variety of optical networking
systems, such as reconfigurable optical add-drop multiplexers
(OADMs), as exemplified in the following.
FIG. 6 depicts an alternative embodiment of an optical add-drop
multiplexer (OADM) according to the present invention. By way of
example, OADM 600 comprises a first WSR-S apparatus 610 optically
coupled to a second WSR-S apparatus 650. By way of example, the
first WSR-S apparatus 610 may be embodied according to FIG. 4A (or
4B), and the second WSR-S apparatus 650 may be in the embodiment of
FIG. 5. The first WSR-S apparatus 610 includes an input port 620, a
pass-through port 630, and one or more drop ports 640-1 through
640-N (N.gtoreq.1). The pass-through spectral channels from the
pass-through port 630 are further coupled to the second WSR-S
apparatus 650, along with one or more add spectral channels
emerging from add ports 660-1 through 660-M (M.gtoreq.1). In this
exemplary case, the pass-through port 630 and the add ports 660-1
through 660-M constitute the input ports for the second WSR-S
apparatus 650. The second WSR-S apparatus (or ROAM) 650 serves to
multiplex the pass-through spectral channels and the add spectral
channels, and route the multiplexed optical signal into an exiting
port 670 to provide an output signal for the system.
Those skilled in the art will recognize that in the embodiment of
FIG. 6, one WSR-S apparatus (e.g., the first WSR-S apparatus 610)
serves as an RODM, effective to perform dynamic drop function. The
other WSR-S apparatus (e.g., the second WSR-S apparatus 650)
functions as an ROAM, providing dynamic add function. And there are
essentially no fundamental restrictions on the wavelengths that can
be added or dropped (other than those imposed by the overall
communication system). Moreover, the underlying OADM architecture
thus presented is intrinsically scalable and can be readily
extended to provide several cascaded WSR systems (or RODMs and
ROAMs). Additionally, the OADM of FIG. 6 may be operated in
reverse, e.g., by using the input ports as the output ports, the
drop ports as the add ports, and vice versa.
Those skilled in the art will recognize that the aforementioned
embodiment provides one of many embodiments of a dynamically
reconfigurable OADM according to the present invention. Those
skilled in the art will also appreciate that various changes,
substitutions, and alternations can be made herein without
departing from the principles and the scope of the invention as
defined in the appended claims. Accordingly, a skilled artisan can
design an OADM in accordance with the principles of the present
invention, to best suit a given application.
Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions, and alternations can be made herein without
departing from the principles and the scope of the invention.
Accordingly, the scope of the present invention should be
determined by the following claims and their legal equivalents.
* * * * *