Note: Descriptions are shown in the official language in which they were submitted.
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TECHNICAL FIELD
The invention relates to optical devices and is especially applicable to
waveguide
structures and integrated optics.
BACKGROUND ART
This specification refers to several published articles. For convenience, the
articles are cited in full in a numbered list at the end of the description
and cited by
number in the specification itself. The reader is directed to them for,
reference.
In the context of this patent specification, the term "optical radiation"
embraces
electromagnetic waves having wavelengths in the infrared, visible and
ultraviolet ranges.
The terms "finite" and "infinite" as used herein are used by persons skilled
in this
art to distinguish between waveguides having "finite" widths in which the
actual width is
significant to the performance of the waveguide and the physics governing its
operation
and so-called "infinite" waveguides where the width is so great that it has no
significant
effect upon the performance and physics of operation.
As explained in copending international patent application No. PCT/CA00/01525,
to which the reader is directed for reference, at optical wavelengths, the
electromagnetic
properties of some metals closely resemble those of an electron gas, or
equivalently of a
cold plasma. Metals that resemble an almost ideal plasma are commonly termed
"noble
metals" and include, among others, gold, silver and copper. Numerous
experiments as
well as classical electron theory both yield an equivalent negative dielectric
constant for
many metals when excited by an electromagnetic wave at or near optical
wavelengths
[l,2]. In a recent experimental study, the dielectric function of silver has
been accurately
measured over the . visible optical spectrum and a very close correlation
between the
measured dielectric function and that obtained via the electron gas model has
been
demonstrated [3] .
It is known that the interface between semi-infinite materials having positive
and
negative dielectric constants can guide TM (Transverse Magnetic) surface
waves. In the
case of a' metal-dielectric interface at optical wavelengths, these waves are
termed
plasmon-polariton modes and propagate as electromagnetic fields coupled to
surface
plasmons (surface plasma oscillations) comprised of conduction electrons in
the metal [4] .
It is known to use a metal film of a certain thickness bounded by dielectrics
above
and below as an optical slab (planar, infinitely wide) waveguiding structure,
with the core
of the waveguide being the metal film. When the film is thin enough, the
plasmon
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the metal, thus creating supermodes that exhibit dispersion with metal
thickness. The
modes supported by infinitely wide symmetric and asymmetric metal film
structures are
well-known, as these structures have been studied by numerous researchers;
some notable
published works include references [4] to [10].
In general, only two purely bound TM ~ modes, each having three field
components, are guided by an infinitely wide metal film waveguide. In the
plane
perpendicular to the direction of wave propagation, the electric field of the
modes is
comprised of a single component, normal to the interfaces and having either a
symmetric
or asymmetric spatial distribution across the waveguide. Consequently, these
modes are
denoted sb and ab modes, respectively. The sb mode can have a small
attenuation constant
and is often termed a long-range surface plasmon-polariton. The fields related
to the ab
mode penetrate further into the metal than in the case of the sb mode and can
be much
lossier by comparison. Interest in the modes supported by thin metal films has
recently
intensified due to their useful application in optical communications devices
and
components. Metal films are commonly employed in optical polarizing devices
[1l]
while long-range surface plasmon-polaritons can be used for signal
transmission [7]. In
addition to purely bound modes, leaky modes are also known to be supported by
these
structures.
Infinitely wide metal film structures, however, are of limited practical
interest
since they offer one-dimensional (1-D) field confinement only, with
confinement
occurring along the vertical axis perpendicular to the direction of wave
propagation,
implying that modes will spread out laterally as they propagate from a point
source used
as the excitation. Metal films of finite width have recently been proposed in
connection
with polarizing devices [12], but merely as a cladding.
In addition to the lack of lateral confinement, plasmon-polariton waves guided
by
a metal-dielectric interface are in general quite lossy. Even long-range
surface plasmons
guided by a metal film can be lossy by comparison with dielectric waveguides.
Known
devices exploit this high loss associated with surface plasmons for the
construction of
plasmon-polariton based modulators and switches. Generally,. known plasmon-
polariton
based modulator and switch devices can be classified along two distinct
architectures. The
first architecture is based on the phenomenon of attenuated total reflection
(ATR) and the
second architecture is based on mode coupling between a dielectric waveguide
and a
nearby metal. Both architectures depend on the dissipation of optical power
within an
interacting metal structure.
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upon a dielectric-metal structure placed in optical proximity, to a surface
plasmon-
polariton mode supported by the metal structure. At a specific angle of
incidence, which
depends on the materials used and the particular geometry of the device,
coupling to a
plasmon mode is maximised and a drop in the power reflected from the metal
surface is
observed. ATR based modulators make use of this attenuated reflection
phenomenon
along with means for varying electrically or otherwise at least one of the
optical
parameters of one of the dielectrics bounding the metal structure in order to
shift the angle
of incidence where maximum coupling to plasmons occurs. Electrically shifting
the angle
of maximum coupling results in a modulation of the intensity of the reflected
light.
Examples of devices that are based on this architecture are disclosed in
references [23] to
[36J. Reference [42] discusses an application of the ATR phenomenon for
realising an
optical switch or bistable device.
Mode coupling devices are based on the optical coupling of light propagating
in a
dielectric waveguide to a nearby metal film placed a certain distance away and
in parallel
with the dielectric waveguide. The coupling coefficient between the optical
mode
propagating in the waveguide and the plasmon-polariton mode supported by the
nearby
metal film is adjusted via the materials selected and the geometrical
parameters of the
device. Means are provided for varying, electrically or otherwise, at least
one of the
optical parameters of one of the dielectrics bounding the metal. Varying an
optical
parameter (the index of refraction, say) varies the coupling coefficient
between the optical
wave propagating in the dielectric waveguide and the lossy plasmon-polariton
wave
supported by the metal. This results in a modulation in the intensity of the
light exiting
the dielectric waveguide. References [37] to [40] disclose various device
implementations
based upon this phenomenon. Reference [41] further discusses the physical
phenomenon
underlying the operation of these devices.
These.known modulation and switching devices disadvantageously require
relative
high control voltages and have limited electrical/optical bandwidth.
The afore-mentioned co-pending international patent application No.
PCT/CA00/01525 disclosed a waveguide structure comprising a thiw strip having
finite
width and thickness with dimensions such that optical radiation having a
wavelength in a
predetermined range couples to the strip and propagates along the length of
the strip as a
plasmon-polariton wave. The strip may comprise a material having a relatively
high free
charge carrier density', for example a conductor or certain classes of highly-
doped
semiconductor. The surrounding material may have a relatively low free charge
carrier
density, i.e. an insulator or an undoped or lightly doped semiconductor.
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transverse plane, i.e. perpendicular to the direction of propagation, and,
since suitable
low-loss waveguides can be fabricated from such strip, it is useful for signal
transmission
and routing or to construct components such as couplers, power splitters,
interferometers,
modulators, switches and other typical components of integrated optics. In
such devices,
different sections of the strip serve different functions, in some cases in
combination with
additional electrodes. The strip sections may be discrete and concatenated or
otherwise
interrelated, or sections of one or more continuous strips.
A characteristic of such a thin, (mite-width plasmon-polariton waveguide is
polarization sensitivity. In particular, external radiation linearly polarised
along the
direction perpendicular to the plane of the strip is coupled effectively to
the waveguide in
an end-fire arrangement.
An object of the present invention is to provide a low-loss polarisation
insensitive
plasmon-polariton waveguide structure.
DISCLOSURE OF INVENTION:
The present invention seeks to eliminate, or at least mitigate, one or more of
the
disadvantages of the prior art.
According to one aspect of the present invention there is provided a waveguide
structure comprising a strip having a substantially square cross-section with
dimensions of
the same order (less than 10) such that optical radiation having a wavelength
in a
predetermined range couples to the strip and propagates along the length of
the strip as a
plasmon-polariton wave. The strip may comprise a material having a relatively
high free
charge carrier density, for example a conductor or certain classes of highly-
doped
semiconductor. The surrounding material may have a relatively low free charge
carrier
density, i.e. an insulator or an undoped or lightly doped semiconductor.
Such a strip of finite width offers two-dimensional (2-D) conf'mement in the
transverse plane, i.e. perpendicular to the direction of propagation, and,
since suitable
low-loss waveguides can be fabricated from such strip, it is useful for signal
transmission
and routing or to construct components such as couplers, power splitters,
interferometers,
modulators, switches and other typical components of integrated optics. In
such devices,
different sections of the strip serve different functions, in some cases in
combination with
additional electrodes. The strip sections may be discrete and concatenated or
otherwise
interrelated, or sections of one or more continuous strips.
For example, where the optical radiation has a free-space wavelength of 1550
fzfn,
and the waveguide is made of a strip of a noble metal surrounded by a good
dielectric,
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of about 200 nm to 150 nm, preferably about 1 ~0 nm.
The strip could be straight, curved, bent, tapered, and so on.
The dielectric. material may be inhomogeneous, for example a combination of
5 slabs, strips, laminae, and so on. The conductive or semiconductive strip
may be
inhomogeneous, for example a gold-layer sandwiched between thin layers of
titanium.
The plasmon-polariton wave which propagates along the structure may be excited
by an appropriate optical field incident at one of the ends of the waveguide,
as in an end-
fire configuration, and/or by a different radiation coupling means.
The low free-carrier density material may comprise two distinct portions with
the
strip extending therebetween, at least one of the two distinct portions having
at least one
variable electromagnetic property, and the device then may further comprise
means for
varying the value of said electromagnetic property of said one of the portions
so as to
vary the propagation characteristics of the waveguide structure and the
propagation of the
plasmon-polariton wave.
In some embodiments of the invention, for one said value of the
electromagnetic
property, propagation of the plasmon-polariton wave is supported and, for
another value
of said electromagnetic property, propagation of the plasmornpolariton wave is
at least
inhibited. Such embodiments may comprise modulators or switches.
Different embodiments of the invention may employ different means of varying
the electromagnetic property, such as varying the size of at least one of said
portions,
especially if it comprises a fluid.
The at least one variable electromagnetic property of the material may
comprise
permittivity, permeability or conductivity.
Where the portion comprises an electro-optic material, the variable
electromagnetic property will be permittivity, which may be varied by applying
an
electric field to the portion, or changing an electric field applied thereto,
using suitable
means.
Where the portion comprises a magneto-optic material, the variable
electromagnetic properly will be permittivity which may be varied by applying
a magnetic
field to the portion or changing a magnetic field applied thereto, using
suitable means.
Where the portion comprises a thermo-optic material, the electromagnetic
property may be permittivity and be varied by changing.the temperature of the
material.
Where the portion comprises an acousto-optical (photoelastic) material, the
electromagnetic property may be permittivity and be varied by changing
mechanical strain
in the material.
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electromagnetic property will be permeability and may be varied by applying a
magnetic
field to the material or changing a magnetic field applied thereto, by
suitable means.
Where the portion comprises a semiconductor material, the electromagnetic
property will be conductivity or permittivity and may be varied by changing
free charge
carrier density in said portion, using suitable means. .
Additionally or alternatively, the propagation of the plasrizon-polariton wave
may
be varied by varying an electromagnetic property of the strip. For example,
the
permittivity of the strip may be varied by changing the free charge carrier
density or by
changing or applying a magnetic field through the strip.
Various objects, features, aspects and advantages of the present invention
will
become more apparent from the following detailed description, taken in
conjunction with
the accompanying drawings, of a preferred embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1(a) and 1(b), labelled PRIOR ART, are a cross-sectional illustration
and
a plan view, respectively, of a symmetric waveguide structure as disclosed and
claimed in
copending PCT application No. PCT/CA00/01525, in which the core is comprised
of a
lossy metal film of thickness t, width w, length l and permittivity s2
embedded in a
cladding or background comprising an "infinite" homogeneous dielectric having
a
permittivity s1;
Figures 2(a) and 2(b) are a cross-sectional illustration and a plan view,
respectively, of a symmetric waveguide structure having a substantially square
metal
cross-section of width w, thickness t, length l and permittivity s2 embedded
in a cladding
or background comprising an "infinite" homogeneous dielectric having a
permittivity $1;
Figures 3(a) and 3(b) give the dispersion characteristics of the ss~ and ss~
modes
supported by a square cross-section waveguide as a function of the waveguide
cross-
sectional dimension w = t for two metals of interest. (a) Normalised phase
constant; (b)
Attenuation;
Figures 4(a),(b),(c),(d),(e) and (f) illustrate the spatial distribution of
the six field
components related to the ss~x° mode supported by a square cross-
section metal waveguide
of dimensions w = t = 180 nm. The waveguide cross-section is located in the x -
y plane
and the metal region is outlined as the rectangular dashed contour. The field
distributions
are normalized such that r~urx ~ Re{Ex} ~ = l;
Figures 5(a),(b),(c),(d),(e) and (f) illustrate the spatial distribution of
the six field
components related to the ss~ mode supported by a square cross-section metal
waveguide
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and the metal region is outlined as the rectangular dashed contour. The field
distributions
are normalized such that max ~ Re{Ev} ~ = 1;
Figure 6 shows the spatial distribution of Re{Ex} related to the ss~x°
mode, plotted
in the x direction along the top horizontal edge of the metal region, for
waveguides having
different cross-sectional dimensions. The spatial distribution of the main
transverse
electric field component related to the fundamental mode supported by an
optical fibre is
also shown for comparison;
Figure 7 shows the spatial distribution of Re{Eve related to the ss~°
mode, plotted
in the x direction along the top horizontal edge of the metal region, for
waveguides having
different cross-sectional dimensions. The spatial distribution of the main
transverse
electric field component related to the fundamental mode supported by an
optical fibre is
also shown for comparison;
Figures 8(a) and 8(b) are a cross-sectional view and a plan view,
respectively, of a
second embodiment of the invention in the form of an asymmetric waveguide
structure
formed by a metal region having a substantially square cross-section of
thickness t, width
w and permittivity EZ supported by a homogeneous semi-inf'mite substrate of
permittivity
s~ and with a cover or superstrate comprising a homogeneous semi-infinite
dielectric of
permittivity s3;
Figures 8(c) and 8(d) give the attenuation of the ssbx° and ss~°
modes supported by
the square cross-section waveguide of Figure 8(a) as a function of dielectric
asymmetry;
Figure 9 is a plan view of a waveguide with opposite sides stepped to provide
different widths;
Figure 10 is a plan view of a waveguide which is tapered and slanted;
Figure 11 is a plan view of a trapezoidal waveguide;
Figure 12 is a plan view of a waveguide having curved side edges and suitable
for
use as a transition piece;
Figure 13 is a plan view of a curved waveguide section suitable for
interconnecting waveguides at a corner;
Figure 14 is a plan view of a two-way splitter/combiner formed by a
combination
of three straight waveguide sections and one tapered waveguide section;
Figure 15 is a plan view of an angled junction using a slanted section;
Figure 16 is a plan view of a power divider formed by a trapezoidal section
and
pairs of concatenated bends;
Figure 17 is a plan view of a Mach-Zehnder interferometer formed using a
combination of the waveguide sections;
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waveguide structure of Figure 17;
Figures 18(b) and 18(c) are inset diagrams illustrating alternative ways of
applying
a modulation control voltage;
Figure 19 is a plan view of a modulator using the Mach-Zehnder waveguide
structure of Figure 17 and illustrating magnetic field control;
Figure 20(a) is a plan view of an edge coupler formed by two parallel strips
of
straight waveguide with various other waveguides for coupling signals to and
from them;
Figure 20(b) is an inset diagram illustrating a way of applying a modulation
control voltage;
Figure 21(a) is a perspective view of a coupler in which the parallel strips
are not
co-planar;
Figure 21(b) is an inset diagram illustrating a way of applying a modulation
control voltage;
Figure 22 is a plan view of an intersection formed by four sections of
waveguide;
Figures 23(a) and 23(b) are a schematic front view and corresponding top plan
view of an electro-optic modulator employing the waveguide structure of Figure
8(a);
Figures 24(a) and 24(b) are a schematic front view and corresponding top view
of
an alternative ~electro-optic modulator also using the waveguide structure of
Figure 8(a);
Figure 24(c) illustrates an alternative connection arrangement of the
modulator of
Figure 24(a);
Figure 25 is a schematic front view of a third embodiment of electro-optic
modulator also using the waveguide structure of Figure 8(a);
Figure 26 is a schematic front view of a magneto-optic modulator also using
the
waveguide structure of Figure 8(a);
Figure 27 is a schematic front view of a thermo-optic modulator also using the
waveguide structure of Figure 8(a);
Figure 28 is a schematic perspective view of an electro-optic switch also
using the
waveguide structure of Figure 8(a);
Figure 29 is a schematic perspective view of a magneto-optic switch also using
the
waveguide structure of Figure 8(a);
Figure 30 is a schematic perspective view of a thermo-optic switch also using
the
waveguide structure of Figure 8(a);
DESCRIPTION OF THE PREFERRED EMBODIMENTS
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the invention, their theoretical basis will first be explained with reference
to Figures 1 to
8.
The waveguide structure shown in Figure 2(a) was analysed using the Method of
Lines (MoL) applied in a manner similar to that disclosed in the afore-
mentioned co-
pending PCT/CA00/01525 application. The MoL is a numerical technique that can
be
used to solve suitably defined boundary value problems based on Maxwell's
equations.
The method can be used to generate the modes supported by a waveguide
structure of
interest. A mode is described by its mode fields and its propagation constant
y = a+j(3,
where a is the attenuation constant and (3 is the phase constant.
The square cross-section waveguide as shown in Figure 2(a) exhibits 90°
rotational
symmetry about its centre longitudinal axis. A consequence of this property is
that many
of the modes supported by the structure are degenerate (two or more modes are
said to be
degenerate with respect to each other if they have identical propagation
constants). In the
square cross-section waveguide, two of the fundamental modes are degenerate
and can be
made long-ranging. One of these modes has its main transverse electric field
component
directed along the x axis and is denoted the ss~ mode. The other has its main
transverse
electric field component directed along the y axis and is denoted the ss~
mode. When
excited in an end-fire arrangement, a square cross-section waveguide properly
dimensioned to support only these two fundamental long-ranging plasmon-
polariton
modes, will appear polarisation insensitive.
The physical quarter symmetry of the square cross-section waveguide structure
is
exploited in the MoL when solving for the modes. This is achieved by placing
electric
and magnetic walls along the y and x axes, respectively, of Figure 2(a), to
generate the
ss~x° mode, and by placing magnetic and electric walls along the y and
x axes,
respectively, to generate the ss~ mode.
It is assumed that the metal region shown in Figure 2(a) can be modelled as an
electron gas over the wavelengths of interest. According to classical or Drude
electron
theory, the complex relative permittivity of the metal region is given by the
well-known
plasma frequency dispersion relation [4]:
2 2
~z.' 1 ~~pvz J ~(~z+ ~) (1)
where e~ is the excitation frequency, ~p is the electron plasma frequency and
v is the
effective electron collision frequency, often expressed as v = 1/i with i
defined as the
relaxation time of electrons in the metal. Materials that are characterized by
a high
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doped semiconductors. Such materials are often said to support a free electron
gas.
Figures 3(a) and 3(b) show the geometrical dispersion curves computed using
the
MoL, of the ssUx° and ss~° modes supported by the square cross-
section waveguide
5 structure illustrated in Figure 2(a), as a function of the cross-sectional
dimension w = t,
over the range 100 nm to 1000 nm. The optical free-space wavelength of
analysis is set to.
~,o = 1550 nm and the background dielectric is SiOz (Er.l = 2.085136). Two
cases for
the metal are shown: Au (E~.2 = -131.9475 - j 12.65) and A1 (s~.2 = -253.9264
j46.08). In Figure 3(a), the phase constant of the modes is normalised to ~3o
where Rio =
10 w/co, with c° being the velocity of light in free space. The mode
power attenuation in
dB/cm plotted in Figure 3 (b) is related to the attenuation constant a via:
_20
attenuation = 100 a loge)
As is seen from Figure 3(b), the modes exhibit a vanishing attenuation as the
size
of the square cross-section is reduced, and both the ss~x° and ssw
modes remain guided
down to very small values of w = t. It is noted that mode power attenuation
values in the
range of about 10 dB/cm down to 0.1 dB/cm are achievable using structure
dimensions w
= t in the range from about 280 nm down to about 150 nm for Au and Al. These
are
good dimensions for the waveguide.
From Figure 3(a), it is noted that the phase constant of both the ss~ and ss~
modes tends towards the phase constant of a Transverse ElectroMagnetic (TEM)
wave
propagating in the inf'nute homogeneous background as the size of the square
cross-
section is reduced. The curves plotted in Figure 3(a) for both of these
degenerate modes
should be identical but due to small residual numerical errors in the MoL, the
curves are
slightly different for each mode.
Figures 4 and 5 give the spatial distribution of the six field components
related to
the ss~° and ssw modes, respectively, for the Au core at w = t = 180
nm. The
waveguide cross-section is located in the x - y plane and the metal region is
outlined as the
rectangular dashed contour. The field distributions are normalized such that
nurx ~ Re{Ex} ~ = 1 and y»crx ~ Re{Ey} ~ = 1, respectively. From Figure 4 it
is noted that
the mode power associated with the ssUx° mode is carried mainly by the
Ex and Hv field
components. Conversely,.from Figure 5 it is clear that the mode power
associated with
the ss~ mode is carried mainly by the Ey and Hx field components. It is also
observed
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metal portion, and that they decay in an exponential-like manner away from the
metal.
Observing the evolution of the fields associated with these modes as w = t is
reduced, reveals that the ssUx° and ss~ modes indeed evolve,
respectively, into the
horizontally and vertically polarised TEM waves supported by the background,
as the
metal vanishes.
Figures 6 and 7 show the spatial distribution of the real part of the Ex and
Ev field
components associated with the ssbx° and ss~ modes, respectively, for
the Au core. The
distributions are plotted in the x direction along the top horizontal edge of
the metal
portion and they are shown for various cross-sectional dimensions. The
distribution of the
main transverse electric field component of the fundamental mode supported by
a Single
Mode Fibre (SMF) having a numerical aperture of 0.14 and a diameter of 8.2
l,un is also
shown on both figures for comparison. All fields are normalised to a maximum
value of
unity. It is noted that as the metal portion vanishes, the field distribution
flattens out
extending further away from the metal into the dielectric region, resembling
more and
more the uniform distribution of the associated TEM wave supported by the
background.
Metal dimensions w = t in the range of 200 to 150 nm provide reasonable
confinement
though it is clear that a trade-off between confinement and attenuation exists
in these
structures.
Table I below lists the attenuation and coupling losses to SMF of the ss~ and
ss~°
modes supported by the square cross-section Au waveguide structure. The
coupling loss L
in dB is defined as:
L = -201og~C~
The overlap factor C in the above is defined as:
C = f f EjEg * ds
s
Where the field Ef is taken as the main transverse electric field component of
the
fundamental fibre mode supported by a SMF (NA=0.14, dia = 8.2) and the field
Es is
taken as being either the EX or Ey field component of the ss~x° and
ss~° modes,
respectively. Before evaluation of the overlap factor, the fields are
normalised such that
the following hold:
f f EfEfds=1 f f EgEgds=1
s s
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domain over which the waveguide mode fields are computed.
TABLE I: Attenuation and coupling losses to SMF of the ssUx° and
ss~° modes supported
by square cross-section Au/Si02 waveguides at a free-space operating
wavelength of
1550 nm.
ssbx sS~Y
'
w = Att. L w = Att. L
t dB/cm dB t dB/cm dB
nm nm
150 0.11 5.81 150 0.10 5.86
160 0.29 4.51 160 0.27 4.56
170 0.68 3.50 170 0.64 3.54
180 1.39 2.71 180 1.32 2.76
190 2.58 2.11 190 2.46 2.15
200 '4.41 1.64 200 4.24 1.68
250 27.54 0.69 250 27.81 0.68
From Table I, it is observed that the coupling losses to fibre increase as the
mode
fields spread out further into the surrounding dielectric. For the materials
considered in
this example, an Au core having dimensions of about w = t = 180 nm provides a
reasonable compromise between attenuation, confinement and coupling losses.
Its losses decrease with decreasing cross-section and the main degenerate long-
ranging modes, the ss~ ° and ss~ modes, do not appear to have cut=off
dimensions. The
losses related to these modes could be made arbitrarily small though it must
be
remembered that a trade-off against confinement is necessary. The fields must
remain
reasonably well confined in order for the light to round bends of small radii.
Due to the nature of their field distributions, the ssbx° and ss~ modes
are excitable
using a simple end-fire technique similar to the one employed to excite
surface plasmon
polariton modes [19,6]. This technique is based on maximising the overlap
between the
incident field and that of the mode to be excited. In reference [22], the
present inventor et
al. disclosed that plasmon-polariton waves supported by thin metal films of
finite width
had been observed experimentally at optical communications wavelengths using
this
method of excitation. Advantageously in the case of the square cross-section
waveguides,
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direction.
In Figure 8(a) the square cross-section metal region is supported by a semi-
infinite
homogeneous substrate of permittivity s~= n12 and covered by a different semi-
infinite
homogeneous dielectric of permittivity s3 = n32, where m and n2 are the
refractive indices
of the dielectrics. Introducing such an asymmetry into the structure can break
the
degeneracy of the ss~x° and ss~ modes. If the asymmetry is small, about
~ s1- s3 ~ ~ 10-3,
then the ss~X° and ss~ modes will remain long-ranging as w = t is
reduced but cut-off
dimensions below which purely bound propagation will not occur, are
introduced. As the
asymmetry ~ El - s3 ~ increases, the cut-off dimensions also increase.
Figures 8(c) and 8(d) show the attenuation of the ss~° and ss~ modes
as the
asymmetry in the structure depicted in Figure 8(a) is increased. The metal
selected is Au,
and the wavelength of analysis is set to ~,o = 1550 nm. In Figure 8(c), a
square cross-
section of w = t = 180 nm is used while in Figure 8(d) a square cross-section
of cu = t
= 200 nm is used. Figures 8 (c) and (d) were generated for the case 81 =
(1.444)2 and s3
_ (1.444 ~ ~)2. From these Figures, it is observed that the attenuation
vanishes as the
degree of asymmetry increases. This is due to the decreasing conf'mement of
the mode.
As the asymmetry increases, the mode fields spread out more and more from the
metal
core, and eventually all light is lost to radiation in the background
dielectrics. It is noted
for the geometry analysed, that the mode can be "cut-off" by inducing (electro-
optically
or otherwise) a modest asymmetry (~ ~ O. S x 10~') to 1. S x 104 in the
dielectrics
surrounding the metal.
The long-ranging modes supported by the square cross-section waveguide are
quite sensitive to the asymmetry in the structure. This high sensitivity is
useful as a
small induced asymmetry (created via an electro-optic effect present in the
dielectrics
say) can effect a large change in the propagation characteristics of the long-
ranging
modes. This physical phenomenon forms the basis of some of the device
architectures
described herein.
The existence of the long-ranging ss~x° and ss~ modes in a symmetric
structure
makes the square cross-section metal waveguide attractive for applications
requiring short
propagation distances. The waveguide is polarisation independent and offers
two-
dimensional field conf'mement in the transverse plane, rendering it useful as
the basis of
an integrated optics technology. Interconnects, power splitters, power
couplers,
interFerometers and other integrated optics components could be built using
the guides.
Finally, the structures are quite simple and are inexpensive to fabricate.
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14
Figures 3 to 8 assuming an operating free-space wavelength of ~,o = 1550 nm.
This
wavelength was selected as an example due to its widespread use for optical
fibre
communications and it is not intended to limit the application of the
invention to this
wavelength. Indeed, the invention can be used over a very broad wavelength
range from
the visible ~,o --- 500 nm to the infra-red ~,o -- 10 p.m.
For operation at a wavelength of about 500 nm, width and thickness of about 40
nm to about 70 nm give good performance. For operation at a wavelength of
about
10,000 nm, width and thickness of about 500 nm to about 2000 nm give good
performance.
Examples of practical waveguide structures, and integrated optics devices
which
can be implemented using the invention, will now be described with reference
also to
Figures 9 to 30. Unless otherwise stated, where a waveguide structure is
shown, it will
have a general construction similar to that shown in Figures 2(a) and 2(b) or
that shown in
Figures 8(a) and 8(b), with w = t or w ~ t.
The waveguide structure 100 shown in Figures 2(a) and 2(b) comprises a strip
of
finite thickness t and width w of a first material having a high free (or
almost free) charge
carrier density, surrounded by a second material which has a very low free
carrier
density. The strip material can be a metal or a highly doped semiconductor and
the
background material can be a dielectric. The strip has a substantially square
cross-section
W = t.
Suitable materials for the strip include (but are not limited to) gold,
silver, copper,
aluminium and highly n- or p-doped GaAs~ InP or Si, while suitable materials
for the
surrounding material include (but are not limited to) glass, quartz, polymer
and undoped
or very lightly doped GaAs, InP or Si. Particularly suitable combinations of
materials
include Au for the strip and SiOz for the surrounding material.
The thickness t and the width W of the strip are selected equal and small
enough
such that the waveguide supports two degenerate orthogonally polarised long-
ranging
plasmon-polariton modes at the free-space operating wavelength of interest.
Suitable
dimensions for AulSiOz waveguides at an operating free-space wavelength of
1550 nm
are about w = t = 150 nm to w = t = 200 nm.
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strips of gold (Au) or Aluminium (Al) embedded in silicon dioxide (SiOz) for
various
widths and thicknesses w = t of the metal strip: Analyses were carried out
with an
optical free space wavelength of 1550 nm. The curves show that very low
attenuation
5 values can be obtained with metal strips of practical dimensions. The case w
= t =180
nm are good dimensions for the Au/Si02 waveguides, providing a reasonable
trade-off
between confinement and attenuation. The case w = t = 240 nm are good
dimensions for
the Al/SiOz waveguides, providing a reasonable trade-off between confinement
and
attenuation.
10 Unless otherwise stated when structure dimensions are mentioned from this
point
onward, they refer to the square cross-section Au waveguides embedded in SiOz
at an
operating optical free-space wavelength of 1550 nm. Similar dimensions are
needed for
most material combinations.
The plasmon-polariton field may be excited by optical radiation coupled to the
15 strip in an end-fire manner from a fiber butt-coupled to the input of the
waveguide. The
output of the waveguide can also be butt-coupled to a fibre. Alternatively,
the waveguide
could be excited at an intermediate position by an alternative means, for
example using
the so-called attenuated total reflection method (ATR).
The length l shown in Figure 2(b) is arbitrary and will be selected to
implement a
~ desired interconnection. It has been demonstrated that a straight waveguide
100 with the
square cross-sectional dimensions set out above is low-loss and polarisation
insensitive.
Figure 9 shows a transition waveguide section 102 having stepped sides which
can
be used to interconnect two sections of waveguide having different widths. The
larger
width WZ can be set to about 250 nm to more effectively couple the waveguide
to the
input/output fibres. The reduced width Wl helps to reduce the insertion loss
of the
. waveguide. A corresponding step in thickness can also be introduced such
that each
waveguide section has a square cross-section, thus keeping the waveguides
polarisation
insensitive.
Figure 10 shows an angled section 104 which can be used as an interconnect or
transition. Its dimensions, W~, W2 and 1 and the angles ~1 and ~2, are
adjusted for a
particular application as needed. Usually the angles are kept small, in the
range of 1 to
15 degrees and the input and output widths are usually similar. Although the
sides of the
angled section 104 shown in Figure 10 are tapered, they could be parallel. It
should also
be appreciated that the angle of the inclination could be reversed, i.e. the
device could be
symmetrical about the bottom right hand corner shown in Figure 10 or
transposed about
that axis if not symmetrical about it. The widths Wl and Wz are maintained
approximately
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16
waveguide.
Figure 11 shows a tapered transition waveguide section 106, which can be
used to interconnect two waveguides of different widths. The length of the
taper is
usually adjusted such that the angles are small, usually in the range of 1 to
15
degrees. The taper angles at the two sides are not necessarily the same. Such
a
configuration might be used as an input port, perhaps as an alternative to the
layout
shown in Figure 9, or as part of another device, such as a power splitter.
Taper
profiles other than linear (as shown) could be used, such as exponential,
parabolic or
sinusoidal. The widths Wl and W2 are maintained approximately equal to t. Any
symmetry of the structure shown can be used.
Figure 12 illustrates an alternative transition waveguide section 130 which
has
curved sides, rather than straight as in the trapezoidal transition section
disclosed in
Figure 11. In Figure 12, the curved sides are shown as sections of circles of
radius Rl
and R2, subtending angles y and ~z respectively, but it should be appreciated
that various
functions can be implemented, such as sinusoidal, exponential or parabolic.
The widths
Wl and WZ are maintained approximately equal to t.
Any of the transition waveguide structures shown in Figures 9, 10, 11 and 12
could be used in a splitter/combiner, as will be described later.
~ Figure 13 shows a curved substantially square cross-section waveguide
section
108 which can be used to redirect the plasmon-polariton wave. The angle ~ of
the bend
can be in the range of 0 to 360 degrees and the bending radius R can be in the
range of a
few microns to a few centimetres. For a 45-degree bend, a radius of 0.5 to 2
cm is
appropriate. The critical dimensions are the radius R and the positions of the
input and
output straight sections 100 of substantially square cross-section waveguide.
Although the
device will work, and the structure 108 will convey the plasmon-polariton wave
around
the bend, there is leakage out of the bend (from the exterior curve) and also
reflection
back in the direction from which the wave came. Reduced radiation and
reflection is
obtained, when the input and output waveguides 100 are offset outwards
relative to the
ends of the bend. The reason for this is that the straight waveguide sections
100 have an
optical field extremum that peaks along the longitudinal centre line, and then
decays
towards the edges. In the bend, the extremum of the optical field distribution
shifts
towards the exterior of the curve. This results in increased radiation from
the external
edge of the curve and increased reflection back to the input waveguide 100 due
to a
mismatch in the field distributions. Offsetting the input and output
waveguides 100
towards the outside of the curve aligns the extrema of their optical fields
more closely
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minimise both the radiation and the reflection. The tighter the radius R, the
greater the
radiation from the exterior of the curve, so the offset Ol is related to the
radius R and the
optimum values would have to be determined according to the specific
application.
It should also be noted that it is not necessary to connect the input and
output
waveguides 100 directly to the curve. As shown in Figure 13, it is possible to
have a
short spacing dl between the end of the input waveguide 100 and the adjacent
end of the
curved section 108. Generally speaking, that spacing d~ should be minimised,
even zero,
and probably no more than a few optical wavelengths. A similar offset Oz and
spacing d2
could be provided between the bend 108 and the output straight waveguide 100.
Although Figure 13 shows no gradual transition between the straight waveguides
100 at the input and output and the ends of the curved section 108, it is
envisaged that, in
practice, a more gradual offset could be provided so as to reduce edge effects
at the
corners.
Figure 14 shows a two-way power splitter 110 formed from a trapezoidal section
106 with a straight substantially square cross-section waveguide section 100
coupled to its
narrower end 112 and two angled substantially square cross-section waveguide
sections
104 coupled side-by-side to its wider end 114. The distances between the input
waveguide 100 and the narrower end 112 of the tapered section 106, and between
the
output waveguides 104 and the wider end 114 of the tapered section 106, dl, d2
and d3,
respectively, should be minimised. The angle between the output waveguides 104
is
usually in the range of 0.5 to 10 degrees and their widths are usually
similar. The offsets
Sl and Sa between the output waveguides and the longitudinal centre line of
the
trapezoidal section 106 preferably are set to zero, but could be non-zero, if
desired, and
vary in size. Ideally, however, the output sections 104 should together be
equal in width
to the wider end 114.
Offset Sl need not be equal to offset SZ but it is preferable that both are
set to zero.
The widths of the output sections 104 can be adjusted to vary the ratio of the
output
powers. The dimensions of the centre tapered section 106 are usually adjusted
to
minimise input and output reflections and radiation losses in the region
between the output
sections 104.
It should also be noted that the centre tapered section 106 could have angles
that
vary according to application and need not be symmetrical.
Moreover, any of the alternative transition waveguide structures shown in
Figures
9, 10 and 12 could be substituted.
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18
transition section having a width broader than the width of the input
waveguide 100 so
that the transition section favoured multimode propagation causing
constructive/destructive interference patterns throughout its length. The
length could be
selected so that, at the output end of the rectangular transition section, the
constructive
portions of the interference pattern would be coupled into the different
waveguides
establishing, in effect, a 1-to-N power split. Such a splitter then would be
termed a
multimode interferometer-based power divider.
It should be appreciated that the device shown in Figure 14 could also be used
as a
combiner. In this usage, the light would be injected into the waveguide
sections 104 and
combined by the tapered centre section 106 to form the output wave which would
emerge
from the straight waveguide section 100.
In either the Y splitter or the interferometer-based power divider, the number
of
arms or limbs 104 at the output could be far more than the two that are shown
in Figure
14.
It is also feasible to have a plurality of input waveguides. This would enable
an
NxN divider to be constructed. The dimensions of the transition section 106
then would
be controlled according to the type of splitting/combining required.
As shown in Figure 15, an angled substantially square cross-section waveguide
section 104 may be used to form an intersection between two straight
substantially square
cross-section waveguide sections 100, with the dimensions adjusted for the
particular
application. It should be noted that, as shown in Figure 15, the two straight
sections 100
are offset laterally away from each other by the distances Ol and O2,
respectively, which
would be selected to optimise the couplings by reducing radiation and
reflection losses, in
the manner discussed with reference to Figure 13. The angle of the trapezoidal
section
104 will be a factor in determining the best values for the offsets O~ and O2.
The sections
100 and 104 need not be connected directly together, but could be spaced by
the distances
d~ and d2 and/or coupled by a suitable transition piece that would make the
junction more
gradual (/.e., the change of direction would be more gradual).
The embodiments of Figures 13 and 15 illustrate a general principle of
aligning
optical fields, conveniently by offsets, wherever there is a transition 'or
change of
direction of the optical wave and an inclination relative to the original
path, which can
cause radiation and reflection if field extreme are misaligned. Such offsets
would be
applied whether the direction-changing sections were straight ox curved.
As illustrated in Figure 16, a power divider 116 can also be implemented using
a
pair of concatenated curved sections 108 instead of each of the angled
sections 104 in the
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nearest to the wider end 114 of the tapered section 106 curves outwards from
the
longitudinal centre line of the tapered section 106 while the other curved
section curves
oppositely so that they form an "S" bend. Also, the curved sections in each
pair are
offset by distance O~ or OZ one relative to the other for the reasons
discussed with respect
to the bend 108 shown in Figure 13. Other observations made regarding the
power
divider and the curved section disclosed in Figures 13 and 14 respectively,
also hold in
this case.
Figure 17 illustrates a Mach-Zehnder interferometer 118 created by
interconnecting two power splitters 110 as disclosed in Figure 14. Of course,
either or
both of them could be replaced by the power sputter 116 shown in Figure 16.
Light
injected into one of the ports, i.e. the straight section 100 of one power
splitter 110/116,
is split into equal amplitude and phase components that travel along the
angled arms 104
of the sputter, are coupled by straight sections 100 into the corresponding
arms of the
other splitter, and then are recombined to form the output wave.
If the insertion phase along one or both arms of the device is modified, then
destructive interference between the re-combined waves can be induced. This
induced
destructive interference is the basis of a device that can be used to modulate
the intensity
of an input optical wave. The lengths of the arms 100 are usually adjusted
such that the
phase difference in the re-combined waves is 180 degrees for a particular
relative change
in insertion phase per unit length along the arms. The structure will thus be
long if the
mechanism used to modify the per unit length insertion phase is weak (or short
if the
mechanism is strong).
Figure 18(a) illustrates a modulator 120 based on the Mach-Zehnder 118
disclosed
in Figure 17. As illustrated also in Figure 18(b), parallel plate electrodes
122 and 124 are
disposed above and below, respectively, each of the strips 100 which
interconnects two
angled sections 104, and spaced from it, by the dielectric material, at a
distance large
enough that optical coupling to the electrodes is negligible. The electrodes
are connected
in common to one terminal of a voltage source 126, and the intervening strip
100 is
connected using a minimally invasive contact to the other terminal. Variation
of the
voltage V applied by source 126 effects the modulating action. According to
the plasma
model for the strip 100, a change in the carrier density of the latter (due to
charging +2Q
or -2Q) causes a change in its permittivity, which in turn causes a change in
the insertion
phase of the arm. (The change induced in the permittivity is described by the
plasma
model representing the guiding strip 100 at the operating wavelength of
interest. Such
model is well known to those of ordinary skill in the art and so will not be
described
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example.) This change is sufficient to induce destructive interference when
the waves in
both arms re-combine at the output combiner.
Figure 18(c) illustrates an alternative connection arrangement in which the
two
5 plate electrodes 122 and 124 are connected to respective ones of the
terminals of the
voltage source 126. In this case, the dielectric material used as the
background of the
waveguide is electro-optic (LiNb03, an electro-optic polymer,...). In this
instance, the
applied voltage V effects a change in the permittivity of the background
dielectric, thus
changing the insertion phase along the arm. This change is sufficient to
induce
10 destructive interference when the waves in both arms re-combine at the
output combiner.
It will be noted that, in Figure 18(a), one voltage source supplies the
voltage Vl
while the other supplies the voltage Vi. V1 and Vz may or may not be equal.
For both cases described above, it is possible to apply voltages in opposite
polarity
to both arms of the structure. This effects an increase in the insertion phase
of one arm
15 and a decrease in the insertion phase of the other arm of the Mach-Zehnder
(or vice
versa), thus reducing the magnitude of the voltage or the length of the
structure required
to achieve a desirable degree of destructive interference at the output. For
this connection,
it is understood that the design described in Figure 18(c) comprises a
material that exhibits
a linear electro-optic effect.
20 Also, it is possible to provide electrodes 122 and 124 and a source 126 for
only
one of the intervening strips 100 in order to provide the required
interference.
It should be appreciated that other electrode structures could be used to
apply the
necessary voltages. For example, the electrodes 122 and 124 could be coplanar
with the
intervening strip 100,. one on each side of it. By carefully laying out the
electrodes as a
microwave waveguide, a high frequency modulator (capable of modulation rates
in excess
of 10 Gbit/s) can be achieved.
Figure 19 illustrates an alternative implementation of a Mach-Zehnder 128
which
has the same set of waveguides as that shown in Figure 17 but which makes use
of
magnetic fields B applied to either or both of the middle straight section
arms to induce a
change in the permittivity tensor describing the strips. (The change induced
in the tensor
is described by the plasma model representing the guiding strip at the
operating
wavelength of interest. Such model is well known to those of ordinary skill in
the art
and so will not be described further herein. For more information the reader
is directed
to reference [21], for example.) The change induced in the permittivity tensor
will induce
a change in the insertion phase of either or both arms thus inducing a
relative phase
difference between the light passing in the arms and generating destructive
interference
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modulates the intensity of the light transmitted through the device. The
magnetic field B
can be made to originate from current-carrying wires or coils disposed around
the arms
100 in such a manner as to create the magnetic field in the desired
orientation and
intensity in the optical waveguides. The magnetic field may have one or all of
the
orientations shown, BX, BY or B~ or their opposites. The wires or coils could
be fabricated
using plated via holes and printed lines or other conductors in known manner.
Alternatively, the field could be provided by an external source, such as a
solenoid or
toroid having poles on one or both sides of the strip.
Figure 20(a) illustrates a coupler 139 created by placing two substantially
square
cross-section waveguide strips 100" parallel to each other and in close
proximity over a
certain length. The separation S~ between the strips 100" could be from 1 l,un
(or less) to
40 Nrn and the coupling length I~ could be in the range of a few microns to a
few dozen
millimeters depending on the separation S~, the dimensions of the strips 100",
the
materials used, the operating wavelength, and the level of coupling desired.
The gaps between the input and output of the waveguide sections shown would
ideally be set to zero and a lateral offset provided between sections where a
change of
direction is involved. C~irved sections could be used instead of the sections
104, 100 and
100" shown in Figure 20(a).
Although only two strips 100" are shown in the coupled section, it should be
understood that more than two strips can be coupled together to create an NxN
coupler.
As illustrated in Figure 20(b) a voltage can be applied to the two coupled
sections
100" via minimally invasive electrical contacts. Figure 20(b) shows a voltage
source 126
connected directly to the sections 100" but, if the sections 100, 104 and 100"
in each arm
are connected together electrically, the source 126 could be connected to one
of the other
sections in the same arm. Applying a voltage in such a manner charges the arms
of the
coupler, which, according to the plasma model for the waveguide, changes its
permittivity. If, in addition, the dielectric material placed between the two
waveguides
100" is electro-optic, then a change in the background permittivity will also
be effected as
a result of the applied voltage. The first effect is sufficient to change the
coupling
characteristics of the structure but, if an electro-optic dielectric is also
used, as suggested,
then both effects will be present, allowing the coupling characteristics to be
modified by
applying a lower voltage.
Figures 21(a) and 21(b) illustrate coupled waveguides similar to those shown
in
Figure 20(a) but placed on separate layers in a substrate having several
layers 140/1,
140!2 and 140/3. The substantially square cross-section waveguide strips could
be placed
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them. The coupled guides can also be offset from each other a distance S~, as
shown in
Figures 21(a) and 21(b). The strips could be separated by d = 1 l.un (or less)
to 20 p,rn,
the coupling length could be in the range of a few microns to a few dozens
millimeters
and the separation S~ could be in the range of -40 to +40 p,rn, depending on
the width
and thiclrness of the strips, the materials used and the level of coupling
desired.
As before, curved sections could be used instead of the straight and angled
sections shown in Figure 21(a).
Gaps can be introduced longitudinally between the segments of strip if desired
and
a lateral offset between the straight and angled (or curved) sections could be
introduced.
Though only two strips are shown in the coupled section, it should be
understood
that a plurality of strips can be coupled together on a layer and/or over many
layers to
create an NxN coupler.
As shown in Figure 21(b), a voltage source 126 could be connected directly or
indirectly to the middle (coupled) sections 100" in a similar manner to that
shown in
Figure 20(b).
As illustrated in Figure 22, an intersection 142 can be created by connecting
together respective ends of four of the angled and substantially square cross-
section
waveguide sections 104, their distal ends providing input and output ports for
the device.
When light is applied to one of the ports, a prescribed ratio of optical power
emerges
from the output ports at the opposite side of the intersection. The angles
~1...~4 can be set
such that optical power input into one of the ports emerges from the port
directly
opposite, with negligible power transmitted out of the other ,ports. Any
symmetry of the
structure shown is.appropriate.
Various other modifications and substitutions are possible without departing
from
the scope of the present invention. For example, although the waveguide
structure shown
in Figures 2(a) and 2(b), and implicitly those shown in other Figures, have a
single
. homogeneous dielectric surrounding a metal strip, it would be possible to
sandwich the
metal strip between two slabs of different dielectric material; or at the
junction between
four slabs of different dielectric material. Moreover, the multilayer
dielectric materials)
illustrated in Figure 21(a) could be used for other devices too. Also, the
metal strip
could be replaced by some other conductive material or a highly n- or p-doped
semiconductor. It is also envisaged that the conductive strip, whether metal
or other
material, could be multi-layered.
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Modulation and switching devices will now be described which make use of the
fact that an asymmetry induced in optical waveguiding structures having as a
guiding
element a square cross-section metal strip may inhibit propagation of the main
long
s ranging degenerate purely bound plasmon-polariton modes supported.
The asymmetry in the structure can be in the dielectrics surrounding the metal
strip. In this case the permittivity, permeability or the dimensions of the
dielectrics
surrounding the strip can be different. A noteworthy case is where the
dielectrics above
and below the substantially square cross-section (w = t or w ~ t) metal strip
100 have
different permittivities, in a manner similar to that shown in Figure 8(a).
A dielectric material exhibiting an electro-optic, magneto-optic, thermo-
optic, or
piezo-optic effect can be used as the surrounding dielectric medium. The
modulation and
switching devices make use of an external stimulus to induce or enhance the
asymmetry.in
the dielectrics of the structure. As shown ins Figures 8(c) and (d), a modest
asymmetry of
about 2x 10~ induced in the dielectrics has a significant deleterious effect
on the
propagation of the plasmon-polariton wave, thus causing cut-off of the modes.
Figures 23(a) and 23(b) depict an electro-optic modulator comprising two metal
strips 110 and 112 surrounded by a dielectric 114 exhibiting an electro-optic
effect. Such
a dielectric has a permittivity that varies with the strength of an applied
electric field. The
effect can be first order in the electric field, in which case it is termed
the Pockels effect,
or second order in the electric field (Kerr effect), or higher order. Figure
23(a) shows the
structure in cross-sectional view and Figure 23(b) shows the structure in top
view. The
lower metal strip 110 and the surrounding dielectric 114 form the optical
waveguide. The
lower metal strip 110 is dimensioned such that a purely bound long-ranging
plasmon-
polariton wave is guided by the structure at the optical wavelength of
interest. The strip
110 may have a substantially square cross-section, dimensioned such that the
two main
orthogonally polarised long-ranging plasmon-polariton modes are propagated.
Since the
guiding lower metal strip 110 comprises a metal, it is also used as an
electrode and is
connected to a voltage source 116 via a minimally invasive electrical contact
118 as
shown. The second metal strip 112 is positioned above the lower metal strip
110 at a
distance large enough that optical coupling between the strips is negligible.
It is noted that
the second strip can also be placed below the waveguiding strip instead of
above. The
second strip acts as a second electrode.
The intensity of the optical signal output from the waveguide can be varied or
modulated by varying the intensity of the voltage V applied by the source 116.
When no
voltage is applied, , the waveguiding structure is symmetrical and supports
plasmon
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24
induced via the electro-optic effect present in the dielectric 114, and the
propagation of
the plasmon-polariton waves is irfhibited. The degree of asymmetry induced may
be large
enough to completely cut-off the main purely bound long-ranging modes, thus
enabling' a
very high modulation depth to be achieved. By carefully laying out the
electrodes, a high
frequency modulator (capable of modulation rates in excess of 10 Gbit/s) can
be achieved.
Figures 24(a) and 24(b) show an alternative design for an electro-optic
modulator
which is similar to that shown in Figure 23(a) but comprises electrodes 112A
and 112B
placed above and below, respectively, of the substantially square cross-
section metal
optical waveguide strip 110 at such a distance that optical coupling between
the strips is
negligible. Figure 24(a) shows the structure in cross-sectional view and
Figure 24(b)
shows the structure in top view. A first voltage source 116A connected to the
metal strip
110 and the upper electrode 112A applies a first voltage V~ between them. A
second
voltage source 116B connected to metal strip 110 and lower electrode 112B
applies a
voltage VZ between them. The voltages Vl and V2, which are variable, produce
electric
fields El and Ez in portions 114A and 114B of the dielectric material. The
dielectric
material used exhibits a linear electro-optic effect. The waveguide structure
shown in
Figure 24(c) is similar in construction to that shown iri Figure 24(a) but
only one voltage
source 116C is used. The positive terminal (+) of the voltage source 116C is
shown
connected to metal strip 110 while its negative terminal (-) is shown
connected to both the
upper electrode 112A and the lower electrode 112B. With this configuration,
the electric
fields El and Ez produced in the dielectric portions 114A and 114B,
respectively, are in
opposite directions. Thus, whereas, in the waveguide structure of Figure
24(a), selecting
appropriate values for the voltages Ul and UZ induces the desired asymmetry in
the
waveguide structure of Figure 24(c), the asymmetry is induced by the relative
direction of
the electric field above and below the waveguiding strip 110, since the
voltage V applied
to the electrodes 112A and 112B produces electric fields acting in opposite
directions in
the portions 114A. and 114B of the dielectric material.
By carefully designing the electrodes, the structures shown in Figures
24(a),(b)
and (c) can operate to very high frequencies.
It should be appreciated that either or both of the first and second portions
may be
electro-optic according to the desired application and the arrangement of the
electrodes
and voltage sources) selected such that a required asymmetry in the
permittivity is
induced in the structure.
Figure 25 shows in cross-sectional view yet another design for an electro-
optic
modulator. In this case, the substantially square cross-section metal
waveguide strip 110 is
CA 02450836 2003-12-16
WO 03/001258 PCT/CA02/00971
second portion 114E below it. Electrodes 112D and 112E are placed opposite
lateral
along opposite lateral edges, respectively, of the upper portion 114D of the
dielectric 114
as shown and connected to voltage source 116E which applies a voltage between
them to
5 induce the desired asymmetry in the structure. Alternatively, the electrodes
112D, 112E
could be placed laterally along the bottom portion 114E of the dielectric 114,
the distinct
portions of the dielectric material still providing the asymmetry being above
and below
the strip.
Figure 26 shows an example of a magneto-optic modulator wherein the
10 . substantially square cross-section metal waveguide strip 110 and
overlying electrode 112F
are used to carry a current 1 in the opposite directions shown. The dielectric
material
surrounding the metal waveguide strip 110 exhibits a magneto-optic effect or
is a ferrite.
The magnetic fields generated by the current I add in the dielectric portion
between the
electrodes 110 and 112F and essentially cancel in the portions above the top
electrode
15 112F and below the waveguide 110. The applied magnetic field thus induces
the desired
asymmetry in the waveguiding structure. The electrode 112F is placed far
enough from
the guiding strip 110 that optical coupling between the strips is negligible.
Figure 27 depicts a thermo-optic modulator wherein the substantially square
cross
section metal waveguide strip 110 and the overlying electrode 1126 are
maintained at
20 temperatures Ti and T~ respectively. The dielectric material 114
surrounding the metal
waveguide exhibits a thermo-optic effect. The temperature difference creates a
thermal
gradient in the dielectric portion 1146 between the electrode 1126 and the
strip 110. The
variation in the applied temperature thus induces the desired asymmetry in the
waveguiding structure. The electrode 1126 is placed far enough from the
guiding strip
25 110 that optical coupling between the strips is negligible.
It should be appreciated that the modulator devices described above with
reference
to Figures 23(a) to 27 may also serve as variable optical attenuators with the
attenuation
being controlled via the external stimulus, i.e. voltage current, temperature,
which varies
the electromagnetic property.
Figures 28, 39 and 30 depict optical switches that operate on the principle of
"split and attenuate". In each case, the input optical signal is first split
into N outputs
using a power divider; a one-to-two power split being shown in Figures 28, 29
and 30.
The undesired outputs are then "switched off ' or highly attenuated by
inducing a large
asymmetry in the corresponding output waveguides. The asymmetry must be large
enough to completely cut-off the main purely bound long-ranging mode supported
by the
CA 02450836 2003-12-16
WO 03/001258 PCT/CA02/00971
26
waveguide structures of Figures 23, 26 or 27, respectively.
In the switches shown in Figures 28, 29 and 30, the basic waveguide
configuration is the same and comprises a substantially square cross-section
metal input
waveguide section 120 coupled to two parallel substantially square cross-
section metal
branch sections 122A and 122B by a wedge-shaped sputter 124. All four sections
120,
122A, 122B and 124 are co-planar and embedded in dielectric material 126. The
thickness of the metal strips is d3. Two rectangular electrodes 128A and 128B,
each of
thickness dl, are disposed above branch sections 122A and 122B, respectively,
and spaced
from them by a thickness d2 of the dielectric material 126 at a distance large
enough that
optical coupling between the strips is negligible. Each of the electrodes 128A
and 128B is
wider and shorter than the underlying metal strip 122A or 122B, respectively.
In the
switch shown in Figure 28, the asymmetry is induced electro-optically by means
of a first
voltage source 130A connected between metal strip 122A and electrode 128A for
applying voltage Vl therebetween, and a second voltage source 130B connected
between
metal strip 122B and electrode 128B, for applying a second voltage VZ
therebetween. In
the switch shown in Figure 29, the asymmetry is induced magneto-optically by a
first
current source 132A connected between metal strip 122A and electrode 128A,
which are
connected together by connector 134A to complete the circuit, and a second
current
source 132B connected between metal strip 122B and electrode 128B, which are
connected together by connector 134B to complete that circuit.
In the switch shown in Figure 30, the asymmetry is induced thermo-optically by
maintaining the metal strips 122A and 122B at temperature To and the overlying
electrodes 128A and 128B at temperatures Tl and Tz, respectively.
It will be appreciated that, in the structures shown in Figures 28, 29 and 30,
the
dielectric surrounding the metal strip will be electro-optic, magneto-optic,
or thermo-
optic, or a magnetic material such as a ferrite, as appropriate.
In general, any of the sources, whether of voltage, current or temperature,
may be
variable.
Moreover, either of the sputter configurations shown in Figures 14 and 16
could
be substituted for that shown in Figures 28, 29 and 30.
Although the switches shown in Figures 28, 29 and 30 are 1 x 2 switches, the
invention embraces 1 x N switches which can be created by adding more branch
sections
and associated electrodes, etc.
It will be appreciated that, where the surrounding material is acousto-optic,
the
external stimulus used to induce or. enhance the asymmetry could be determined
by
CA 02450836 2003-12-16
WO 03/001258 PCT/CA02/00971
27
the electro-optic material replaced by acousto-optic material and the
electrodes 112D and
112E used to apply compression or tension to the upper portion 114D.
To facilitate description, the various devices embodying the invention have
been
shown and described as comprising several separate sections of the novel
waveguide
structure. While it would be feasible to construct devices in this way, in
practice, the
devices are likely to comprise continuous strips of metal or other high charge
carrier
density material, i.e. integral strip sections, fabricated on the same
substrate.
The foregoing examples are not meant to be an exhaustive listing of all that
is
possible but rather to demonstrate the breadth of application of the
invention. The
.inventive concept can be applied to various other elements suitable for
integrated optics
devices. It is also envisaged that waveguide structures embodying the
invention could be
applied to multiplexers and demultiplexers.
INDiJSTRIAL APPLICABILITY
Embodiments of the invention may be useful for signal transmission and routing
or to construct components such as couplers, power splitters/combiners,
interferometers,
modulators, switches, periodic structures and other typical components of
integrated
optics.
Although embodiments of the invention have been described and illustrated in
detail, it is to be clearly understood that the same is by way of illustration
and example
only and not to be taken by way of the limitation, the spirit and scope of the
present
invention being limited only by the appended claims.
CA 02450836 2003-12-16
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28
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