U.S. patent application number 16/481877 was filed with the patent office on 2020-04-30 for relaxed tolerance adiabatic coupler for optical interconnects.
The applicant listed for this patent is ARIZONA BOARD OF REGENTS ON BEHALF OF THE UNIVERSITY OFARIZONA. Invention is credited to Erfan M. FARD, Thomas L. Koch, Robert A. NORWOOD, Stanley K. Pau.
Application Number | 20200132931 16/481877 |
Document ID | / |
Family ID | 62979591 |
Filed Date | 2020-04-30 |
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United States Patent
Application |
20200132931 |
Kind Code |
A1 |
FARD; Erfan M. ; et
al. |
April 30, 2020 |
RELAXED TOLERANCE ADIABATIC COUPLER FOR OPTICAL INTERCONNECTS
Abstract
An optical arrangement includes an optical printed circuit board
(OPCB) having at least a first optical waveguide having a first end
located on the OPCB. The optical arrangement also includes at least
one photonic integrated circuit (PIC) mounted to the OPCB. The PIC
includes a second optical waveguide. The first waveguide has a
second end located on a portion of the second waveguide to
optically couple light between the PIC and the first waveguide. The
portion of the second waveguide on which the second end of the
first waveguide is located has an inverse taper. The inverse
tapered portion is defined by a plurality of segments. The segments
of the inverse tapered portion each have a length and a taper rate
that causes each segment to make an equal contribution to any
radiation losses in the mode transformation of light being coupled
between the first and second waveguides.
Inventors: |
FARD; Erfan M.; (Tucson,
AZ) ; NORWOOD; Robert A.; (Tucson, AZ) ; Koch;
Thomas L.; (Tucson, AZ) ; Pau; Stanley K.;
(Tucson, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ARIZONA BOARD OF REGENTS ON BEHALF OF THE UNIVERSITY
OFARIZONA |
Tucson |
AZ |
US |
|
|
Family ID: |
62979591 |
Appl. No.: |
16/481877 |
Filed: |
January 30, 2018 |
PCT Filed: |
January 30, 2018 |
PCT NO: |
PCT/US18/15965 |
371 Date: |
July 30, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62452284 |
Jan 30, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/1223 20130101;
G02B 2006/12061 20130101; G02B 6/02038 20130101; G02B 6/126
20130101; G02B 6/1228 20130101; G02B 6/14 20130101 |
International
Class: |
G02B 6/122 20060101
G02B006/122; G02B 6/02 20060101 G02B006/02; G02B 6/14 20060101
G02B006/14; G02B 6/126 20060101 G02B006/126 |
Goverment Interests
GOVERNMENT FUNDING
[0002] This invention was made with government support under Grant
No. FA8650-15-2-5220, awarded by Air Force Material Command. The
government has certain rights in the invention.
Claims
1. An optical coupler, comprising: a photonic integrated circuit
(PIC) having at least one tapered-waveguide output port; and a
second waveguide being sufficiently close to the tapered-waveguide
output port to enable an adiabatic transition of an optical signal
from the at least one tapered-waveguide output port to the second
waveguide.
2. The optical coupler of claim 1, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, at least one of the segments being a linear taper and
the inverse taper of at least another of the segments being an
exponential taper.
3. The optical coupler of claim 1, wherein the tapered-waveguide
output port is an Si-based or Si.sub.3N.sub.4-based waveguide.
4. The optical coupler of claim 1, wherein the second optical
waveguide is a prefabricated polymer waveguide.
5. The optical coupler of claim 1, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, wherein the segments of the inverse tapered portion
each have a length and a taper rate that causes each segment to
make an equal contribution to any radiation losses in the mode
transformation of light being coupled between the first and second
optical waveguides.
6. The optical coupler of claim 1, wherein the second waveguide has
a core thickness less than 5 microns.
7. The optical coupler of claim 1, wherein the second waveguide and
the at least one tapered waveguide define an angle of less than 10
degrees between them.
8. The optical coupler of claim 1, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, all of the segments being a linear taper.
9. The optical coupler of claim 1, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, all of the segments being an exponential taper.
10. An optical coupler, comprising: a photonic integrated circuit
(PIC) having at least one tapered-waveguide output port; and a
second waveguide located within 1000 nm of, and substantially in
parallel with, the at least one tapered-waveguide output port to
enable an adiabatic transition of an optical signal from the at
least one tapered-waveguide output port to the second waveguide,
wherein the second waveguide has a core thickness less than 5
microns.
11. The optical coupler of claim 10, wherein the second waveguide
is located within 500 nm of the at least one tapered-waveguide
output port.
12. The optical coupler of claim 10, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, at least one of the segments being a linear taper and
the inverse taper of at least another of the segments being an
exponential taper.
13. The optical coupler of claim 10, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, all of the segments being a linear taper.
14. The optical coupler of claim 10, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, all of the segments being an exponential taper.
15. The optical coupler of claim 10, wherein the tapered waveguide
output port is an Si-based or Si.sub.3N.sub.4-based waveguide.
16. The optical coupler of claim 10, wherein the second optical
waveguide is a prefabricated polymer waveguide.
17. The optical coupler of claim 10, wherein the tapered-waveguide
output port includes an inverse tapered portion having a plurality
of segments, wherein the segments of the inverse tapered portion
each have a length and a taper rate that causes each segment to
make an equal contribution to any radiation losses in the mode
transformation of light being coupled between the first and second
optical waveguides.
18. The optical coupler of claim 10, wherein the second waveguide
has a core and cladding, a refractive index difference between the
core and the cladding being no more than 0.1.
19. The optical coupler of claim 7, wherein the second waveguide
has a core and cladding, a refractive index difference between the
core and the cladding being no more than 0.3.
20.-32. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/452,284, filed Jan. 30, 2017, the contents of
which are incorporated herein by reference.
BACKGROUND
[0003] An important problem in optical packaging involves the
optical interconnection of planar-integrated photonic integrated
circuits (chip-to-chip connections) and the connection of such
circuits to the external world. Photonic integrated circuits (PICs)
refer to waveguide-based photonic components, including optical
integrated devices such as lasers, optical amplifiers, switches,
filters, modulators, isolators, splitters, phase shifters, variable
attenuators, detectors, and the like. PICs can also include
integration with semiconductor devices such as CMOS devices. PICs
allow systems with high complexity and multiple functions to be
integrated on a single substrate to thereby allow the generation,
detection, propagation and modulation of both optical and
electrical signals. PICs may employ a variety of different material
systems, including silicon (Si), silicon nitride (Si.sub.3N.sub.4),
polymer, silicon dioxide, lithium niobate, InP, GaAs, and graphene,
and optical interconnection processes should be compatible with
these material systems. Existing wire bonding techniques that have
been successfully applied to electrical connections in electronic
integrated circuits cannot be easily extended to optical connection
in a PIC. Therefore, interfacing on-chip guiding media with their
on or off-chip counterparts, i.e. intra-chip, inter-chip, and
chip-to-board communications is a focus of one aspect of the
present disclosure.
[0004] Current interfacing technologies typically attempt to couple
waveguides located on photonic chips to optical fibers or to
optical modes that are similar in size to those in optical fibers.
These technologies require costly high-precision and low-throughput
placement tools due to the demanding tolerances in positioning
accuracy to achieve efficient and low-variance coupling in
manufacture. When designed to afford more relaxed tolerances in
placement accuracy, these current techniques typically forfeit very
high coupling efficiency and also often have a large footprint on
the PIC chips and surrounding coupling apparatus, leading to higher
manufacturing costs. Additionally, even with precision placement
tools, these solutions often have significant
polarization-dependent loss which can have an undesirable impact on
system performance.
SUMMARY
[0005] In accordance with one aspect of the subject matter
described herein, an optical coupler includes a photonic integrated
circuit (PIC) having at least one tapered-waveguide output port.
The optical coupler also includes a second waveguide that is
sufficiently close to the tapered-waveguide output port to enable
an adiabatic transition of an optical signal from the at least one
tapered-waveguide output port to the second waveguide.
[0006] In accordance with another aspect of the subject matter
described herein, an optical arrangement includes an optical
printed circuit board (OPCB) having at least a first optical
waveguide having a first end located on the OPCB. The optical
arrangement also includes at least one photonic integrated circuit
(PIC) mounted to the OPCB. The PIC includes at least a second
optical waveguide. The first optical waveguide has a second end
located on a portion of the second optical waveguide to optically
couple light between the PIC and the first optical waveguide. The
portion of the second optical waveguide on which the second end of
the first optical waveguide is located has an inverse taper. The
inverse tapered portion is defined by a plurality of segments. The
segments of the inverse tapered portion each have a length and a
taper rate that causes each segment to make an equal contribution
to any radiation losses in the mode transformation of light being
coupled between the first and second optical waveguides.
[0007] This Summary is provided to introduce a selection of
concepts in a simplified form. The concepts are further described
in the Detailed Description section. Elements or steps other than
those described in this Summary are possible, and no element or
step is necessarily required. This Summary is not intended to
identify key features or essential features of the claimed subject
matter, nor is it intended for use as an aid in determining the
scope of the claimed subject matter. The claimed subject matter is
not limited to implementations that solve any or all disadvantages
noted in any part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIGS. 1 and 2 show a cross-sectional and top view,
respectively, of one example of an arrangement that includes a
photonic integrated circuit (PIC) (a photonic chip) mounted on an
optical printed circuit board (OPCB) (not shown).
[0009] FIG. 3 shows an expanded top view of a conventional tapered
waveguide (it should be noted that the aspect ratio of the tapered
waveguide is not to scale).
[0010] FIG. 4 shows one example of a segmented tapered waveguide in
accordance with the techniques described herein.
[0011] FIG. 5 shows a graph of the coupling efficiency as a
function of the lateral misalignment between the polymer and the
tapered silicon waveguide for the TE and TM polarization modes.
[0012] FIG. 6 shows a cross-sectional view of an arrangement
similar to that shown in FIG. 1, except with a misalignment between
the optical waveguide of the PIC and the polymer waveguide.
DETAILS OF THE INVENTION
[0013] Many of the design constraints imposed on current
interfacing technologies arise because they are generally concerned
with coupling from semiconductor photonic chip waveguides, which
typically have mode sizes that are sub-micron, to optical fibers or
optical modes compatible with optical fibers, which typically have
dimensions on the order of 5-10 microns. This mode size mismatch
creates severe design constraints that can be reduced or even
eliminated using the techniques described herein because optical
fibers are not employed. Instead, a waveguide design is introduced
that may incorporate substantially thinner and higher-index cores.
This design can provide, in some embodiments, previously
unattainable performance, including high efficiency coupling with
substantially reduced footprint, relaxed alignment tolerances, and
lower polarization dependence. These features are important for
enabling high-throughput manufacturing with lower-cost,
lower-precision assembly tools. Furthermore, some embodiments can
accommodate short-distance inter-chip and optical printed circuit
board connections.
[0014] In one aspect, the subject matter described herein
encompasses a wide variety of designs that become possible without
the constraint to match optical fiber modes. As illustrated below,
these designs may include thin waveguide cores and taper structures
that are enabled by new regimes of dimensions and/or material index
contrast. In addition, these designs can offer substantial
improvements in performance.
[0015] In some embodiments the performance parameters that are to
be optimized when designing an optical coupler between the tapered
waveguide of a photonic integrated circuit (PIC) and an
interconnect waveguide may be chosen so that the coupler possesses
one or more (or all) of the following attributes: high misalignment
tolerance (so that it is compatible with low-cost, high-throughput
placement and assembly tools); high performance) to save valuable
on-chip signal power); small footprint) so that it is cost
effective to produce with silicon photonics manufacturing
techniques); and polarization-independence.
[0016] In some embodiments the optical coupler may employ adiabatic
coupling with waveguide and taper designs that are able to optimize
the aforementioned performance parameters. Adiabatic coupling
introduces variations in a composite waveguide structure along the
propagation length of the coupler. For an output coupler, the
adiabatic coupling occurs between an initial launch waveguide and a
second output waveguide, and the composite structure comprises the
two waveguides positioned in close proximity and nearly in parallel
along the propagation length of the waveguides. The concept of
adiabatic coupling relies on making gradual changes along the
length of one, or both, of these waveguides in such a manner that
the fundamental optical mode is able to transition along the length
of the composite structure from being primarily centered on the
core of one of these waveguides to primarily being centered on the
core of the second waveguide in such a way as to limit radiation
loss, i.e. light escaping from the waveguide structure to free
space. The aforementioned variations may be changes in the core
thicknesses, widths, or index of refraction, or other physical
attributes of the waveguides. To be considered adiabatic, the
variations in the waveguides should be accomplished in such a
gradual manner along the length so as to avoid the optical signal
radiating energy out of the local fundamental mode of the composite
waveguide system. Adiabatic coupling is to be distinguished from
directional coupling in that the waveguide physical parameters are
modified along the length of the coupler such that the optical
signal remains in the fundamental mode of the composite waveguide
system; in a directional coupler, light in one waveguide transfers
to an adjacent waveguide, with each of the corresponding modes
being distinct.
[0017] In some embodiments the adiabatic coupling is accomplished
through the placement of an output waveguide approximately in
parallel along the length, and in close proximity to the surface of
a waveguide on a PIC, which serves as an output port for the PIC.
For instance, in some cases the output waveguide preferably may be
approximately parallel with the waveguide on the PIC so that an
angle of less than 10 degrees is formed between them, more
preferably with an angle of less than 5 degrees between them, and
more preferably still with an angle of zero between them (i.e., the
two waveguides are perfectly in parallel). Likewise, in some cases,
depending in part on the refractive index difference between the
output waveguide and the waveguide on the PIC, the distance between
the two waveguides may be, for purposes of illustration, preferably
less than 1000 nm, more preferably less 500 nm and more preferably
still between 50-500 nm.
[0018] In some cases the variations along the length of the
composite waveguide system are accomplished by tapering the width
of the waveguide core on the PIC. That is, significant aspects of
the optical coupler described herein are directed to the output
waveguide design and to the taper designs of the waveguide on the
PIC that are enabled by the output waveguide design, which together
may lead to substantial improvements in adiabatic coupling
performance relative to conventional optical couplers.
[0019] FIGS. 1 and 2 show a cross-sectional and top view,
respectively, of one example of an arrangement that includes a
photonic integrated circuit (PIC) (a photonic chip) mounted on an
optical printed circuit board (OPCB) (not shown). In some cases the
PIC may be serving as an optically enabled interposer in a stacked
chip subassembly. The cross-sectional view of FIG. 1 is taken along
line A-A in FIG. 2. The PIC 110 has an optical waveguide 112 with
an inverse adiabatic taper for coupling light between the PIC 110
and another optical waveguide. In this example the optical
waveguide that couples light to and from the tapered waveguide 112
of the PIC 110 includes a prefabricated polymer waveguide core 114
that may be formed, for example, in a substrate 116 such as a film
or tape that can serve as the cladding. The film or tape with the
polymer waveguide core 114 embedded therein is mounted to the OPCB
such that one end overlaps with the tapered waveguide 112 of the
PIC 110. The polymer waveguide core 114 is aligned with the
underlying tapered waveguide 112 to within some lateral placement
precision. As FIG. 2 indicates, there may be some lateral offset
between the polymer waveguide core 114 and the underlying tapered
waveguide 112 due to the finite lateral placement precision that
can be achieved. Despite this lateral offset, mode coupling between
the two waveguides is still achievable.
[0020] It should be noted that in various embodiments substrate 116
is a free-standing, self-supporting structure and is not to be
construed as a thin film layer that is formed on a free-standing,
self-supporting structure and which does not exist apart from the
free-standing, self-supporting structure. However, as with those
embodiments in which the substrate 116 is a film or tape, the
substrate 116 is generally flexible, at least to some limited
degree.
[0021] The cross-sectional view in FIG. 1 shows one particular
embodiment that employs a silicon-based (e.g., silicon on insulator
(SOI)) tapered waveguide 112 located on a PIC 110 and a polymer
waveguide 116 that is fabricated, for example, from SU-8. Of
course, more generally any suitable material system may be employed
for the tapered waveguide such as silicon nitride, polymer, lithium
niobate, InP and GaAs, for example. In addition, examples of
alternative polymers 114 that may be employed for the prefabricated
waveguide include, without limitation, ZPU12/ZPU13, Lightlink,
Ormocer, EpoCore/EpoClad, SEO 250, MAPTMS/ZPO,
polymethylmethacrylate, polycarbonate, Cytop, and RHTi1. In some
embodiments a polymer material is chosen so that the refractive
index difference between the core and cladding of the prefabricated
waveguide is below some specified value (e.g., no more than 0.3, no
more than 0.1).
[0022] FIG. 3 shows an expanded top view of a conventional tapered
waveguide 300 (it should be noted that the aspect ratio of the
tapered waveguide is not to scale). As shown, the conventional
tapered waveguide has a continuous linear taper extending along its
length. However, it has been found that such a linear taper may not
be efficient in terms of the footprint it occupies.
[0023] Inspection of the mode profile as light travels down the
tapered waveguide and couples into the polymer waveguide or vice
versa (from the polymer waveguide into the tapered waveguide)
reveals that there are sections of the taper where the mode
transformation is relatively small. Accordingly, in some
embodiments these sections can be reduced in length so that their
contribution to the undesirable radiation losses that may occur
during the mode transformation process is approximately equal to
that of the other segments. In addition, the taper rate and taper
type (e.g., linear, exponential, etc.) may be independently
determined for each segment so that this criterion is satisfied. In
some embodiments this may give rise to tapers that have various
combinations of segments that are linearly and exponentially
tapered, tapers that have all linear segments, or tapers in which
all the segments are exponentially tapered.
[0024] FIG. 4 shows one example of a segmented tapered waveguide
400 that may be employed to satisfy one or both of the
aforementioned criteria. In this example, for purposes of
illustration only and not a limitation on the subject matter
disclosed herein, 8 segments 4101, 4102 . . . 4208 are employed. In
this example the first segment 4101 (the leftmost segment in FIG.
4) contributes relatively minimally to the mode transformation and
accordingly is relatively short in length. On the other hand, the
last segment 4108 (the rightmost segment in FIG. 4) makes a much
larger contribution to the mode transformation and therefore is
larger in length. Moreover, as shown, in this example selected ones
of the segments have an exponential taper, whereas the remaining
segments have a linear taper. As noted above, the length and width
change of each segment in such an example is adjusted so that each
section contributes an approximately equal amount to the total
radiation loss of the tapered waveguide adiabatic transition.
[0025] The configuration of each segment of any given taper, such
as their individual lengths, thicknesses, widths and tapering
algorithms (e.g., linear, exponential with various values of the
argument), may be chosen using any suitable means. For instance,
commercially available software may be employed to simulate the
propagation of light through the various waveguide materials.
Simulations may be executed for different configurations and the
configuration that best optimizes any desired performance
parameters may be selected.
[0026] Table 1 shows one example of a segmented tapered waveguide
that may be employed in the particular arrangement shown in FIGS. 1
and 2. In this example the taper is 200 .mu.m in length and
includes 8 segments. The performance values shown in Table 1 and
the tables that follow are for light with a wavelength of 1550 nm.
Assuming no misalignment between the two waveguides, the coupling
efficiency for this design was found to be 98.465% (that
corresponds to 0.067 dB of loss) for the TE polarization mode and
99.135% (that corresponds to 0.038 dB of loss) for the TM
polarization mode.
TABLE-US-00001 TABLE 1 SEGMENT 1 2 3 4 5 6 7 8 TAPER 450 to 400 to
350 to 300 to 250 to 200 to 150 to 100 to 400 350 300 250 200 150
100 50 nm nm nm nm nm nm nm nm TAPER Linear Linear Linear Linear
Expo- Linear Linear Lin TYPE nential SEGMENT 0.5 0.5 0.5 1.5 30 80
62 25 LENGTH (.mu.M)
[0027] Table 2 shows another example of a segmented tapered
waveguide that may be employed in the particular arrangement shown
in FIGS. 1 and 2. This example assumes that there is no
misalignment between the tapered waveguide and the overlying
polymer waveguide. In this example the taper is only 96 .mu.m in
length and includes 4 segments.
TABLE-US-00002 TABLE 2 SEGMENT 1 2 3 4 TAPER 450 to 350 to 250 nm
250 to 150 nm 150 to 50 nm 350 nm TAPER TYPE Linear Linear Linear
Linear SEGMENT 1 10 60 25 LENGTH (.mu.M)
[0028] The polymer waveguide that in part overlies the tapered
waveguide can be tailored to the particular taper design that is
employed. For instance, it has been found that for some embodiments
a reduction in the thickness of the polymer waveguide core can
significantly increase the coupling efficiency. This is illustrated
in Table 3 for a taper with four segments having a thickness of 2
.mu.m and 1.4 .mu.m. As shown, when the thickness of the polymer is
reduced to 1.4 .mu.m, the coupling efficiency increases to 99.253%
(that corresponds to 0.032 dB of loss) for the TE polarization mode
and 98.074% (that corresponds to 0.084 dB of loss) for the TM
polarization mode.
TABLE-US-00003 TABLE 3 # of segments 1 4 4 Polymer thickness 2
.mu.m 2 .mu.m 1.4 .mu.m Taper length 200 .mu.m 95 .mu.m 96 .mu.m TE
coupling 92.335% 93.305% 99.253% efficiency TM coupling 98.992%
94.012% 98.074% efficiency
[0029] In some embodiments the thickness of the polymer waveguide
core is preferably less than about 3 .mu.m, and more preferably
less than 2 .mu.m.
[0030] FIG. 5 shows a graph of the coupling efficiency as a
function of the lateral misalignment between the polymer and the
tapered silicon waveguide for the TE and TM polarization modes (see
the cross-sectional view of FIG. 6, which illustrates the
misalignment). As the graph indicates, the coupler exhibits less
than 0.1 dB excess loss with a lateral misalignment of +1-1.8 .mu.m
between the polymer and the tapered silicon waveguide for the TE
polarization mode, and 0.1 dB excess loss with +/-1.2 .mu.m offset
for the TM polarization mode. Even a misalignment of +/-2.75 .mu.m
and +/-2.9 .mu.m exhibits less than 1 dB of additional loss for the
TE and TM polarization modes, respectively. The coupler exhibits
almost no loss when the polymer and silicon waveguides are aligned.
Table 4 shows the coupling efficiency when there is no offset and a
2.5 .mu.m offset.
TABLE-US-00004 TABLE 4 TE coupling TM coupling efficiency
efficiency No offset 98.4% 97.1% 2.5 .mu.m offset 84.6% OR 0.65 dB
80.5% OR 0.81 dB additional loss additional loss
[0031] In one example, the V-number for the segmented waveguide may
be in the range of 0.2-20 for the vertical direction and 0.1-20 for
the horizontal direction. The vertical direction refers to the
direction normal to the surface of the PIC and the horizontal
direction refers to the orthogonal direction. A segmented waveguide
configured in this manner will give rise to a significant amount of
optical coupling. The V-number in one direction is defined to
be
V=2.pi.d/.lamda.*sqrt(n.sub.1.sup.2-n.sub.2.sup.2)
where d is the thickness of the slab, .lamda. is the wavelength of
light, n.sub.1 and n.sub.2 are the core and cladding refractive
indices.
[0032] Reference in the specification to "an embodiment," "one
embodiment," "some embodiments," or "other embodiments" means that
a particular feature, structure, or characteristic described in
connection with the embodiments is included in at least some
embodiments, but not necessarily all embodiments. The various
appearances of "an embodiment," "one embodiment," or "some
embodiments" are not necessarily all referring to the same
embodiments. If the specification states a component, feature,
structure, or characteristic "may", "might", or "could" be
included, that particular component, feature, structure, or
characteristic is not required to be included. If the specification
or claim refers to "a" or "an" element, that does not mean there is
only one of the element. If the specification or claims refer to
"an additional" element, that does not preclude there being more
than one of the additional element.
[0033] While certain exemplary embodiments have been described and
shown in the accompanying drawings, it is to be understood that
such embodiments are merely illustrative of and not restrictive on
the broad invention, and that this invention not be limited to the
specific constructions and arrangements shown and described, since
various other modifications may occur to those ordinarily skilled
in the art.
* * * * *