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Patent 2486498 Summary

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(12) Patent Application: (11) CA 2486498
(54) English Title: ELECTRO-OPTIC MODULATORS
(54) French Title: MODULATEURS ELECTRO-OPTIQUES
Status: Dead
Bibliographic Data
(51) International Patent Classification (IPC):
  • G02F 1/035 (2006.01)
(72) Inventors :
  • BREUKELAAR, IAN GREGORY (Canada)
  • WORT, PHILIP MICHAEL (Canada)
  • BIDNYK, SERGE (Canada)
  • BERINI, PIERRE SIMON JOSEPH (Canada)
(73) Owners :
  • SPECTALIS CORP. (Canada)
(71) Applicants :
  • SPECTALIS CORP. (Canada)
(74) Agent: RIDOUT & MAYBEE LLP
(74) Associate agent:
(45) Issued:
(86) PCT Filing Date: 2003-06-02
(87) Open to Public Inspection: 2003-12-11
Availability of licence: N/A
(25) Language of filing: English

Patent Cooperation Treaty (PCT): Yes
(86) PCT Filing Number: PCT/CA2003/000787
(87) International Publication Number: WO2003/102676
(85) National Entry: 2004-11-12

(30) Application Priority Data:
Application No. Country/Territory Date
2,388,574 Canada 2002-05-31

Abstracts

English Abstract




A low loss, low drive voltage plasmon-polariton electro-optic modulator based
on mode cutoff comprises a waveguide structure formed by a thin metallic strip
(106) surrounded by material (102, 104) having a relatively low free charge
carrier density. The metallic strip has finite width and thickness with
dimensions such that optical radiation having a wavelength in a predetermined
range couples to the metallic strip and propagates along the length of the
metallic strip as a plasmon-polariton wave. The surrounding material comprises
two distinct portions (102, 104) with the metallic strip extending between
them. The modulator comprises means (108A, 112) for varying an electric field
applied to at least one portion so as to vary the value of the electromagnetic
property and thereby the propagation characteristics of the plasmon-polariton
wave.


French Abstract

Un modulateur électro-optique à plasmon-polariton à faible coût et basse tension d'excitation basé sur la coupure de mode comprend une structure de guide d'onde composée d'une bande métallique mince (106) entourée d'un matériau (102, 104) ayant une densité de porteuse de charge libre relativement basse. La bande métallique présente une largeur finie et une épaisseur aux dimensions telles qu'un rayonnement optique, d'une longueur d'onde dans une gamme prédéterminée, se couple à la bande métallique et se propage sur la longueur de la bande métallique sous la forme d'une onde plasmon-polariton. Le matériau enveloppant comprend deux parties distinctes (102, 104) entre lesquelles s'étend la bande métallique. Le modulateur comprend des moyens (108A, 112) permettant de faire varier un champ électrique appliqué à au moins une partie, afin de varier la valeur de la propriété électromagnétique et ainsi les caractéristiques de propagation de l'onde plasmon-polariton.

Claims

Note: Claims are shown in the official language in which they were submitted.



30

CLAIMS

1. Modulation means comprising input means (107), output means (109) and a
waveguide
structure therebetween;
the waveguide structure formed by a thin metallic strip (106) surrounded by
material
having a relatively low free charge carrier density, the surrounding material
comprising first
(102) and second (104) distinct portions with the metallic strip extending at
an interface
between respective juxtaposed surfaces (102",104") of the first and second
distinct portions,
the metallic strip having finite width and thickness, the width being greater
than the thickness,
dimensioned such that optical radiation emitted by the input means having a
wavelength in a
predetermined range couples to the metallic strip and, when said first and
second distinct
portions are substantially index-matched, propagates along the length of the
metallic strip as
a plasmon-polariton wave with its transverse electric field substantially
perpendicular to the
width of the metallic strip;
the input means being arranged to couple said optical radiation to one end of
the
metallic strip so as to excite said plasmon-polariton wave and said output
means being coupled
to the opposite end of the metallic strip so as to receive the plasmon-
polariton wave;
at least the first distinct portion (102) comprising an electro-optic material
having a
preferred axis along which its refractive index changes in response to an
applied electric field;
electrode means (100) extending adjacent, preferably substantially
longitudinally
parallel to, the metallic strip, at least part of the first distinct portion
extending between said
electrode means and said metallic strip;
and control means (112) for applying a voltage (V T) to the electrode means so
as to
establish an electric field (E) in said first distinct portion, the
orientation of said preferred axis
of the electro-optic material relative to said metallic strip, and the
positioning of the electrode
means relative to said metallic strip both being such that variation of said
refractive index is
in a direction that extends transversely of, and preferably is substantially
perpendicular to,
said width of the metallic strip;



31

said control means (112) being operable to modulate said voltage so as to vary
said
refractive index of said first distinct portion relative to that of said
second distinct portion,
such that a plasmon-polariton wave propagating along the metallic strip will
be
correspondingly modulated.

2. Modulation means according to claim 1, wherein the electrode means
comprises first
(100) and second (108) electrodes disposed at opposite sides, respectively, of
the metallic strip
(106), and spaced apart in said perpendicular direction, said first distinct
portion (102) being
between the first electrode (100) and the metallic strip (106) and the second
distinct portion
(104) being between the second electrode (108) and the metallic strip (106),
said second
distinct portion (104) also comprising electro-optic material having a
preferred axis along
which its refractive index changes in response to an applied electric field,
the respective
preferred axes of the first and second distinct portions both extending
substantially parallel to
said perpendicular direction but oriented in opposite directions, and wherein
the control means
(112) applies said voltage between the first and second electrodes so as to
establish said
electric field (E) in the same direction in both first and second distinct
portions (Figs 1 - 3).

3. Modulation means according to claim 1, wherein the electrode means
comprises first
(100) and second (108) electrodes disposed at opposite sides, respectively, of
the metallic strip
(106), and spaced apart in said perpendicular direction, said first distinct
portion (102) being
between the first electrode (100) and the metallic strip (106) and the second
distinct portion
(104) being between the second electrode (108) and the metallic strip (106),
said second
distinct portion (104) also comprising an electro-optic material having a
preferred axis along
which its refractive index changes in response to an applied electric field,
the respective
preferred axes of the first and second distinct portions both extending
substantially parallel to
said perpendicular direction and both oriented in the same direction, the
control means (112)
being connected to the first and second electrodes (100, 108) and to the
metallic strip (106)
and applying first (V A) and second (V B) voltages to the first (100) and
second (108) electrodes
respectively, relative to the metallic strip (106), so as to establish first
(E A) and second (E B)


32

electric fields in the first and second distinct portions respectively, the
first and second electric
fields being in opposite directions, and wherein the control means (112)
modulates both
applied voltages (V A,V B) (Figs 6 - 8).

4. Modulation means according to claim 3, wherein the control means comprises
a first
voltage source for applying a first potential difference between the metallic
strip and the first
electrode and a second voltage source for applying a second potential
difference between the
metallic strip and the second electrode. (Figs 6-8).

5. Modulation means according to any one of claims 1 to 4, wherein the electro-
optic
material of the first distinct portion (102) comprises a crystalline material
selected from the
symmetry groups 2mm, 3, 3m, 4, 4mm, 6, 6mm.

6. Modulation means according to any one of claims 1 to 5, wherein the electro-
optic
material of the second distinct portion (104) comprises a crystalline material
selected from the
symmetry groups 2mm, 3, 3m, 4, 4mm, 6, 6mm.

7. Modulation means according to claim 1, wherein the electrode means
comprises an
electrode (100) spaced apart from said metallic strip (106) in said
perpendicular direction and
the control means (112) applies said voltage (V T) between the metallic strip
(106) and the
electrode (100) and wherein said preferred axis extends parallel to said
perpendicular direction
(Fig 9).

8. Modulation means according to claim 7, wherein the electro-optic material
of the first
distinct portion (102) comprises a crystalline material selected from the
symmetry groups
2mm, 3, 3m, 4, 4mm, 6, 6mm.

9. Modulation means according to claim 5, 6 or 8, wherein the crystalline
material
comprises z-cut lithium niobate, its z-axis being the preferred axis.




33

10. Modulation means according to claim 1, wherein the first distinct portion
(102) is
oriented with said preferred axis transversely to said metallic strip (106),
preferably extending
substantially parallel to the width of the metallic strip (106), the electrode
means comprises
first and second electrodes (134,136) disposed at opposite sides,
respectively, of the metallic
strip (106) and spaced apart along said preferred axis with said first
distinct portion (102)
therebetween, and the control means (112) applies said voltage between the
first and second
electrodes (134,136) such that the direction of the electric field (E) in the
first distinct portion
(102) is parallel to said preferred axis (Figs. 10 - 12).

11. Modulation means according to claim 10, wherein the electro-optic material
of the first
distinct portion (102) comprises a crystalline material selected from the
symmetry group
overbar(4)2m.

12. Modulation means according to claim 11 wherein the crystalline material
comprises
DKDP with its z-axis being the preferred axis.

13. Modulation means according to claim 7 or 10, wherein the electro-optic
material of
the first distinct portion (102) comprises a cubic crystalline material having
3 axes along
which its refractive index will change to a greater extent than in other
directions, one of said
three axes being preselected as said preferred axis.

14. Modulation means according to claim 13, wherein the cubic crystalline
material
comprises an aggregate of similarly oriented single crystal domains.

15. Modulation means according to claim 13, wherein the cubic crystalline
material
comprises a single crystal.



34

16. Modulation means according to claim 7 or 10, wherein the electro-optic
material of
the first distinct portion (102) comprises PLZT x/65/35 with x variable in the
range from
about 8 to about 10, having 3 axes along which its refractive index will
change to a greater
extent than in other directions, one of said three axes being preselected as
said preferred axis.

17. Modulation means according to claim 1, wherein a channel is provided in
one of said
juxtaposed surfaces at the two distinct portions and the metallic strip is
accommodated at least
partially in the channel.

18. Modulation means according to claim 1, wherein a layer of dielectric
material index-
matched to the two distinct portions when they are index-matched is provided
between the
juxtaposed surfaces of the two distinct portions and the metallic strip
extends in a channel in
said layer.

19. Modulation means according to claim 1, wherein the metallic strip has a
width in the
range from about 8 µm to about 0.15 µm and a thickness in the range from
about 100 nm to
about 5 nm.

20. Modulation means according to claim 19, wherein the first distinct portion
comprises
electro-optic material having a refractive index of about 2 to 2.5, and the
metallic strip has
a width in the range from about 3 µm to about 0.15 µm and a thickness in
the range of about
50 nm to about 5 nm, said waveguide structure supporting propagation of a
plasmon-polariton
wave having a wavelength in the range from about 0.8 µm to 2 µm,
preferably a wavelength
used for optical communications.

21. Modulation means according to claim 20, wherein the metallic strip has a
width in the
range from about 1.2 µm to about 0.7 µm and thickness in the range from
about 25 nm to
about 15 nm.




35

22. Modulation means according to claim 21, wherein the width is about 1 µm
and the
thickness is about 20 nm, said waveguide structure supporting propagation of a
plasmon-
polariton wave having a wavelength in the range from about 1.3 µm to 1.7
µm, preferably a
wavelength used for optical communications.

23. Modulation means according to claim 19, wherein the first distinct portion
comprises
electro-optic material having a refractive index of about 1.4 to 1.8, and the
metallic strip has
a width in the range from about 8 µm to about 0.5 µm and a thickness in
the range of about 50
mn to about 5 nm, said waveguide structure supporting propagation of a plasmon-
polariton wave
having a wavelength in the range from about 0.8 µm to 2 µm, preferably a
wavelength used for
optical communications.

24. Modulation means according to claim 23, wherein the metallic strip has a
width in the
range from about 6 µm to about 0.7 µm and thickness in the range from
about 25 nm to about
15 nm.

25. Modulation means according to claim 24, wherein the width is about 4 µm
and the
thickness is about 20 nm, said waveguide structure supporting propagation of a
plasmon-
polariton wave having a wavelength in the range from about 1.3 µm to 1.7
µm, preferably a
wavelength used for optical communications.

26. Modulation means according to claim 2, 3 or 10,, wherein said second
electrode (108)
has a width in the range from about 40 µm to about 1 µm, especially from
about 30 µm to about
10 µm and preferably is about 20 µm.

27. Modulation means according to claim 26 wherein the distance of the second
electrode
(108) from the metallic strip (106) is in the range from about 20 µm to
about 5 µm.



36

28. Modulation means according to claim 1 wherein the metallic strip has width
and
thickness of the same order.
29. Modulation means according to claim 28, wherein the metallic strip is
substantially
square in cross-section.
30. Modulation means according to any one of claims 1 to 29 wherein the
control means
supplies said modulation signal sufficient to modulate the plasmon-polariton
wave substantially
to extinction.

Description

Note: Descriptions are shown in the official language in which they were submitted.




CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
ELECTRO-OPTIC MODULATORS
CROSS-REFERENCE TO RELATED APPLICATIONS
This invention claims priority from Canadian patent application number
2,388,574
filed May 31, 2003, the contents of which are incorporated herein by
reference.
TECHNICAL FIELD
The invention relates to modulation or variable attenuation of optical
radiation and is
especially applicable to modulation means employing plasmon-polariton
waveguides.
BACKGROUND ART
Known SPP modulator devices exploit the 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.
ATR based devices depend upon the coupling of an optical beam, which is
incident
upon a dielectric-metal structure placed in optical proximity, to a surface
plasmon-polariton
mode supported by a 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
maximized 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 United States patents numbers 5155617, 5157541,
5075796,



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WO 03/102676 PCT/CA03/00787
2
4971426, 4948225, 4915482, 4451123, 4432614, 4249796 and 5625729, the contents
of
which are incorporated herein by reference.
The ATR phenomenon may also be employed in an optical switch or bistable
device,
as disclosed in United States patent number 4583818, the contents of which are
incorporated
herein by reference.
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.
Examples of such
mode-coupling SPP modulators are disclosed in United States patents numbers
5067788,
6034809, the contents of which are incorporated herein by reference. The paper
' The
proximity Effect of Conductors in Optical Waveguide 1?evices: coupling to
Plasmon-Polariton
Modes' by P. Berirci, SPIE T~ol. 4111, pp. 60-68, ,July 2000', further
discusses the physical
phenomenon underlying the operation of these devices.
These known modulation devices disadvantageously have limited optical
bandwidth
and, in the case of the ATR devices, are not readily coupled to input and
output waveguides,
such as optical fibers.
Modulators are known which do not use plasmon waveguide technologies, but are
based upon voltage induced waveguiding, mode overlap changes, or mode
extinction. For
examples of these types of modulators, the reader is referred to 'Voltage-
Induced Optical
Waveguide Modulator in Lithium Niobate', by Jaeger et al., IEEE Journal of
Quantum
Electr-orcics, vol. 25, No. 4, 1989, pp. 720-728 and 'Improved Mode Extinction
Modulator
Using a Ti-hZdiffused LiNb0.3 Channel Waveguide ', by Ashley et al., Applied
Physics Letters,



CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
3
Vol. 45, No. 8, 1984 pp. 840-842, the contents of which are incorporated
herein by reference.
In these types of modulators, the waveguide core is non-existent or weakly
confining and the
applied voltage either creates a core region where the index of refraction is
raised enough to
confine a mode or reduces the effective index of the mode below cut-off to
induce radiation.
These types of modulators have been demonstrated to suffer from at least one
or all of the
following limitations: high on state insertion loss, high drive voltage, and
low off state
extinction.
International patent applications Nos. WO 01/48521 and WO 03/001258 (Berini)
disclose a modulator which can readily be coupled to a waveguide and which has
more
extended optical bandwidth than such known devices. The modulator comprises a
waveguide
structure formed by a thin strip of material having a relatively high free
charge carrier density
surrounded by material having a relatively low free charge carrier density,
the strip having
finite width arid 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 surrounding material comprises two distinct
portions with the
strip extending between them. At least one of the two distinct portions has at
least one
variable electromagnetic property, and the device further comprises adjusting
means for
varying the value of that electromagnetic property so as to vary the
characteristics of the
waveguide structure and thereby the propagation characteristics of the plasmon-
polariton
wave. The adjusting means modulates an electric field in the at least one of
the distinct
portions. While such a modulator advantageously may provide a relatively high
optical
bandwidth and be readily coupled to a waveguide, such as an optical fiber
or,integrated optics
waveguide channel, for it to be used effectively in optical communications it
would be
desirable for it to have a very low operating voltage, low insertion loss in
the on state and
deep extinction.
SUMMARY OF THE INVENTION:
The present invention seeks to eliminate, or at least mitigate, one or more of
the disadvantages or limitations of known modulators or at least provide an
alternative and,



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4
to this end, provides a plasmon-polariton modulation means of the kind
disclosed in
International patent applications Nos. WO 01/48521 and WO 03/001258,
characterized inthat
the surrounding material comprises first and second distinct portions with the
strip extending
between them and at least one electrode is positioned adjacent the strip, at
least the first
distinct portion being an electro-optic material having a preferred axis along
which its
refractive index changes preferentially. The modulation means further
comprises control
means for modulating a voltage applied to the at least one electrode so as to
modulate the
electric field. The direction of the electric field, the orientation of the
material of the first
distinct portion both relative to the strip, are arranged so that modulation
of said electric field
produces a corresponding modulation of the plasmon-polariton wave.
The foregoing summary of the invention does not necessarily disclose all the
features
essential for defining the invention; the invention may reside in a sub-
combination of the
disclosed features.
According to the present invention, there is provided modulation means
comprising
input means (107), output means (109) and a waveguide structure therebetween;
the waveguide structure formed by a thin metallic strip ( 106) surrounded by
material
having a relatively low free charge carrier density, the surrounding material
comprising first
(102) and second (104) distinct portions with the metallic strip extending at
an interface
between respective juxtaposed surfaces (102",104") of the first and second
distinct portions,
the metallic strip having finite width and thickness dimensioned such that
optical radiation
emitted by the input means having a wavelength in a predetermined range
couples to the
metallic strip and, when said first and second distinct portions are
substantially index-matched,
propagates along the length of the metallic strip as a plasmon-polariton wave
with its
transverse electric field substantially perpendicular to the width of the
metallic strip;
the input means being arranged to couple said optical radiation to one end of
the
metallic strip so as to excite said plasmon-polariton wave and said output
means being coupled
to the opposite end of the metallic strip so as to receive the plasmon-
polariton wave;



CA 02486498 2004-11-15
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at least the first distinct portion (102) comprising an electro-optic material
having a
preferred axis along which its refractive index changes in response to an
applied electric field;
electrode means (100) extending longitudinally parallel to the metallic strip,
at least
part of the first distinct portion extending between said electrode means and
said metallic strip;
5 and control means (112) for applying a voltage (VT) to the electrode means
so as to
establish an electric field (E) in said first distinct portion, the
orientation of said preferred axis
of the electro-optic material relative to said metallic strip, and the
positioning of the electrode
means relative to said metallic strip both being such that variation of said
refractive index is
in a direction that extends transversely of, and preferably is substantially
perpendicular to,
said width of the metallic strip;
said control means (112) being operable to modulate said voltage so as to vary
said
refractive index of said first distinct portion relative to that of said
second distinct portion,
such that a plasmon-polariton wave propagating along the metallic strip will
be
correspondingly modulated.
Such a modulation means advantageously may operate with.deep extinction, i.e.,
the
propagating wave is substantially cut off. The term "cut off" refers to the
elimination of any
supported bound propagating plasmon-polariton waves. The change in the
electromagnetic
property, specifically the refractive index (or permittivity) due to the
asymmetry in the index
of refraction may be sufficient that the propagating plasmon-polariton wave is
no longer
supported; or is so highly attenuated as to be effectively not supported.
The electrode means may comprises first (100) and second (108) electrodes
disposed
at opposite sides, respectively, of the metallic strip (106), and spaced apart
in said
perpendicular direction, said first distinct portion (102) being between the
first electrode (100)
and the metallic strip (106) and the second distinct portion (104) being
between the second
electrode ('108) and the metallic strip (106), said second distinct portion
(104) also comprising
electro-optic material having a preferred axis along which its refractive
index changes in
response to an applied electric field, the respective preferred axes of the
first and second
distinct portions both extending substantially parallel to said perpendicular
direction but
oriented in opposite directions, and wherein the control means (112) applies
said voltage



CA 02486498 2004-11-15
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6
between the first and second electrodes so as to establish said electric field
(E) in the same
direction in both first and second distinct portions.
Alternatively, the electrode means may comprise first (100) and second (108)
electrodes disposed at opposite sides, respectively, of the metallic strip
(106), and spaced apart
in said perpendicular direction, said first distinct portion (102) being
between the first
electrode (100) and the metallic strip (106) and the second distinct portion
(104) being
between the second electrode (108) and the metallic strip (106), said second
distinct portion
(104) also comprising an electro-optic material having a preferred axis along
which its
refractive index changes in response to an applied electric field, the
respective preferred axes
of the first and second distinct portions both extending substantially
parallel to said
perpendicular direction and both oriented in the same direction, the control
means (112) being
connected to the first and second electrodes (100, 108) and to the metallic
strip (106) and
applying first (VA) and second (VB) voltages to the first (100) and second
(108) electrodes
respectively, relative to the metallic strip (106), so as to establish first
(EA) and second (EB)
electric fields in the first and second distinct,portions respectively, the
first and second electric
fields being in opposite directions, and wherein the control means (112)
modulates both
applied voltages (VA,VB).
It is also possible to use the strip as an electrode. Thus, in one embodiment
of the
invention the electrode means comprises an electrode (100) spaced apart from
said metallic strip
(106) in said perpendicular direction and the control means (112) applies said
voltage (VT)
between the metallic strip ( 106) and the electrode (100), the prefeiTed axis
of the material of the
first distinct portion extending parallel to said perpendicular direction.
In yet another preferred embodiment of the invention, the first distinct
portion ( 102)
is oriented with said preferred axis transversely to said metallic strip
(106), preferably
extending substantially parallel to the width of the metallic strip (106), the
electrode means
comprises first and second electrodes (134,136) disposed at opposite sides,
respectively, of
the metallic strip (106) and spaced apart along said preferred axis with said
first distinct
portion (102) therebetween, and the control means (112) applies said voltage
between the first



CA 02486498 2004-11-15
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7
and second electrodes (134,136) such that the direction of the electric field
(E) in the first
distinct portion (102) is parallel to said preferred axis.
The foregoing , and other objects, features, aspects and advantages of the
present
invention will become more apparent from the following detailed description,
in conjunction
with the accompanying drawings, of preferred embodiments of the invention,
which are
described by way of example only and without limitation to the combination of
features
necessary for carrying the invention into effect.
BRIEF DESCRIPTION OF DRAWINGS:
Figure 1 is a perspective schematic illustration of a modulator according to a
first
embodiment;
Figure 2 is a plan view of the modulator of Figure 1;
Figure 3 is a partial cross-sectional end view of the modulator of Figure 1;
Figure 4 shows curves representing attenuation and location of cut-off points
for
plasmon-polariton waves in the modulator;
Figure Sa illustrates variation of normalized insertion loss with electric
field for the
modulator;
Figure Sb illustrates experimental outputs of the modulator;
Figure 6 is a perspective schematic view of a modulator which is a second
embodiment
of the invention;
Figure 7 is a plan view of the modulator of Figure 6;
Figure 8 is a partial cross-sectional end view of the modulator of Figure 6;
Figure 9 is a partial cross-sectional end view of a third embodiment of the
invention;
Figure 10 is a schematic perspective view of a fourth embodiment of the
invention;
Figure 11 is a plan view of the embodiment of Figure 10;
Figure 12 is a partial cross-sectional end view of the eobodirnent of Figure
10; and
Figure 13 illustrates the relative orientations of the index changes and the
electric field
associated with the plasmon-polariton wave.



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8
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first embodiment of the invention, the modulator illustrated in Figures l, 2
and 3,
comprises a die formed by two wafer portions of electro-optic material 102 and
104 having
respective opposed flat surfaces 102" and 104" wafer-bonded together. A thin
metal
waveguide strip 106 deposited in a trench 105 formed (e.g. etched) in the
surface 102"
extends the length of the die. Opposite ends of strip 106 are coupled to input
and output
waveguides 107 and 109, e.g., optical fiber waveguides, waveguides in an
integrated optics
device, or other plasmon polariton waveguides. The input waveguide is arranged
such that
the substantially TM polarized plasmon-polariton wave is excited efficiently.
For example,
the waveguide 107 could be a polarization-maintaining fiber connecting a
polarized light
source to the strip 106 with appropriate orientation. Alternatively, the input
waveguide 107
could be the output plasmon-polariton waveguide of the external cavity laser
disclosed in
copending US Provisional patent application number 601459,717 filed December
20, 2002,
the contents of which are incorporated herein by reference. Light coupled into
the strip 106
will propagate along the strip 106 as a plasmon-polariton wave in the manner
described in
United States patent No. 6,442,231 and International patent applications Nos.
WO 01/48521
and WO 03/001258, the contents of which are incorporated herein by reference,
and leave the
modulator by way of output waveguide 109.
A ground plane electrode 100 provided on the lowermost (as shown) surface of
wafer
portion 102 extends across the entire length and width thereof. A second
electrode comprises
an elongate section 108A extending along the uppermost (as shown) surface of
wafer portion
104 so as to overlie the strip 106. Opposite ends of the section 108A are
connected by
branches 108B and 108C, respectively, to one output terminal of a modulation
control unit
112 and the ground plane electrode 100 is connected to the other output
terminal.
The ground plane electrode 100 need not extend across the entire length and
width of
the modulator. For example, it could be an elongate electrode extending the
length of the strip
106 and either side of it and be coupled by way of a conductive via to a strip
electrode running
alongside the electrode 108A which would then connect it to the modulation
control unit 112.



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9
The modulation control unit 112 applies a DC bias voltage (VB) and high
frequency
modulation (V~) voltage across the electrodes. The high frequency signal (VRF)
is a data
signal which modulates the light propagating along the strip 106.
In this preferred embodiment, both of the wafer portions 102 and 104 comprise
LiNb03,
The z-axes of the wafer portions 102 and 104, respectively, are aligned
antiparallel.
Specifically, the positive z face of one crystal wafer portion is bonded to
the positive z face
of the other crystal wafer portion (or the negative face with the negative
face) so that
refractive index changes of opposite signs are produced in the layers, as
shown in Figure 3
by variation of the electric field produced by the modulation voltage VRF~
Figure 5a shows experimental results for a modulator of length about 2rnm
configured
as shown in Figure 1 in which the strip 106 comprises a 0.95~,m wide, 22nm
thick Au metal
film in LiNb03 wafer portions 102 and 104 and the light source supplies light
at a free space
wavelength of ~,=1550nm. Figure 5a is a plot of normalized insertion loss
versus electric
field intensity E applied to the wafer portions 102 and 104. It will be
appreciated that the
voltage required to be applied to the electrodes for a particular electric
field strength will
depend upon the spacing between the electrodes (or electrode and strip in some
embodiments).
Examples of specific voltages and spacings will be given later. As shown in
Figure, the
modulator has a very linear operation over part of its transfer
characteristic. An extinction
ratio of about l2dB is seen for an applied field of 2V/p,m. This corresponds
to an index of
refraction asymmetry of approximately 5.8*10-4.
Figure 5b shows a sequence of experimental outputs for the preferred
embodiment of
Figure 1. Figure 5b shows the negative of the actual image for clarity, such
that a darker spot
represents a higher optical intensity. In the top image the mode is in the on-
state and is very
symmetric as seen by the dark circle at the center. As an electric field is
applied, the mode
becomes asymmetric as shown in the middle diagram. The bottom diagram shows
the mode
cutoff. The background light seen in the first diagram is due mainly to mode
coupling
mismatch as these modulators did not incorporate mode matching sections for
coupling to
fiber. Such mode matching sections, e.g. tapers, can be incorporated into the
modulator



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WO 03/102676 PCT/CA03/00787
design in the manner described in United Statespatent No. 6,442,231 and
International patent
applications Nos. WO 01/48521 and WO 03/0012581, supra.
The bottom image in Figure 5b shows that the waveguide 'is indeed cutoff at an
applied
field of 2 V/~,m, since at this drive level, almost no light is guided by the
center region.
5 The dimensions of the strip, spacing of the electrodes, operating voltage,
wavelength
of excitation and materials used for the strip and the wafer portions can be
varied according
to the application for the modulator. The manner in which they can be
determined for a
particular application will now be described.
Although the above-described embodiment employs lithium niobate for the wafer
10 portions 102 and 104, it could employ instead other materials selected from
the class of linear
electro-optic materials where the index of refraction change is in the
direction parallel to the
applied field. This effect is well known and well documented by Yariv and Yeh
in a book
entitled "Optical Waves in Crystals", John Wiley & Sons, New York, chap 7
(1984), so
minimal theory will be repeated here. This effect is common in linear electro-
optic materials
from crystal symmetry groups such as the 3m group which includes Lithium
Niobate
(LiNb03), and Lithium Tantalate (LiTa03), the 2mm group which includes
Potassium Titanyl
Phosphate also known as KTP (KTiOP04), the 4mm group which includes the
tetragonal
linear phases of PLZT, electro-optic materials from symmetry groups 3, 4, 6
and 6mm. Other
crystals that are z-cut such that the z-axis or extraordinary axis of the
crystal is oriented
parallel to the applied electric field are suitable. Certain electro-optic
polymers are also
suitable candidate materials. The change 0n in the index of refraction along
this same axis is
given by
0n = - 1 n3rE
2
where n is the nominal zero applied electric field index of refraction of the
electro-optic
material, r is the electro-optic coefficient and E is the applied field. The
effect is linear with
E and the sign of the change in index of refraction depends on the sign of the
applied field.
When using these particular materials in this modulator it is advisable to
align the z-



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11
axis of the crystal with the normal to the largest dimension of the cross-
section of the strip 106
and parallel to the applied electric field. In this way the highly TM
polarized plasmon-polariton
wave is maximally affected.
Figure 4 shows the theoretical modal attenuation curves for the modulator of
Figure
1 using Au as the strip 106 both of the wafer portions 102 and 104 are made
from z-cut
LiNb03, and the wavelength of excitation has a free-space wavelength of
1550nm. Mode
cutoff occurs at the points where the modal_attenuation vanishes. At such a
point the optical
field extends completely into the highest index cladding region so there is no
modal loss
associated with the metal waveguide strip106. At these points a bound mode is
no longer
supported and any input light is radiated away from the waveguide core. The on-
state
attenuation can be found at the centre of the curve at the point where the
substrate index of
refraction change is zero (ie. where the upper and lower cladding regions 102
and 104 are
index matched) .
For lithium niobate and other materials with index of refraction around n=2.2,
good
dimensions for the metallic strip are widths of w=0.7~,m to w=1.2~,m for
thicknesses of
t= l5nm to t=25nm. These dimensions will keep the on-state modal insertion
loss below
IOdB/cm.
Particularly good strip dimensions can be inferred from Figure 4. When the
strip 106
has width w=l,um and thickness t=20nm and an insertion loss of about 2.5
dB/cm, it is
considered cutoff (as. the modal attenuation decreases to zero), for a change
of index of
refraction of 0n=2.7* 10-4. Cutoff plasmon-polariton waveguides are not
limited to Au films
in lithium niobate; using appropriate dimensions for the strip 106, the
modulator will work
for a variety of metals and electro-optic material combinations. This device
is also not limited
to operating at ~,0=1550nm, but for appropriate dimensions will also work over
the entire
optical communications range, typically extending from 0.8 ~,m to 2 ~,m.
It is usually desirable for the drive voltage VRF to be low enough for the
modulation
control unit 112 to use conventional drive circuitry. The operating voltage
applied to achieve
a required electric field strength in the vicinity of the strip 106 can be
reduced if the external
electrodes are brought closer to the strip 106. However, if they are so close
to the strip that



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12
they couple to the plasmon-polariton wave, this interaction would cause the
modal attenuation
to increase. Such an increase can be offset by a decrease in the width or
thickness of the SPP
waveguide. It can also be mitigated by using a layer of a transparent
conductive dielectric
material such as ITO in place of the external electrodes, or as an
intermediate layer with a
thickness of about 1 ~,m between the external electrodes 108A or 100 and the
electro-optic
regions 104 and 102 respectively. Such a material behaves as a conductor at
low frequencies
and a dielectric at optical wavelengths.
Typical index of refraction differences in the wafer portions 102 and 104
sufficient for
cutoff are 10'4 to 10-3, although lower and higher values are required for
weaker and stronger
confining waveguides, respectively. For the purpose of consistency between the
description of
the above-described embodiment and the descriptions of other embodiments to be
described
later, an index of refraction asymmetry of 2* 10-4 will be used for cutoff
throughout.
From Figure 4 this corresponds to a waveguide width of about 0.7 to 0.8~,m in
the case
of an Au strip 106 in lithium niobate wafer portions 102 and 104. Similar
strip dimensions are
needed for most metals in cladding materials of similar index of refraction.
Assuming a simple parallel plate field model, the applied electric field
strength between
electrodes 100 and 108A shown in Figure 3 is approximately found by:
E=-(2*d)~'*(VB+V~.*sin(c~at))
where c.~ is the angular frequency of the modulating signal. The changes in
index of refraction
in regions 104 and 102 respectively are found from:
On,=-O.Sn3rE
~n2 -Vin,
The lower the ratio V~:(V~+VB) is the better since this represents more of the
RF
power being replaced by a constant DC source, which lowers the power
dissipated by the
modulator. To lower the high-frequency drive voltage, the modulator must be
biased away from
the D = 0 point. The amount of bias voltage required will be determined by the
optical
performance at the bias point.
The mode cutoff effect is a strong function of the strip geometry. For
appropriate
strip dimensions as mentioned above, an asymmetry in the index of refraction
of On,2 2 ~ 10-4 is



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13
sufficient for cutoff to occur, where ~n~2=~Onl-OnZ~=2*~On~~ represents the
total difference in
index between the portions 104 and 102.
LiNb03 has a high frequency electro-optic coefficient of approximately 30 pm/V
and an
extraordinary index ofrefraction of approximately 2.1377 at a free space
wavelength of 1550nm.
. 5 It is possible to calculate the necessary electric field for ~n~z 2 X10-4
as E~"IOf~0.68 V/pm.
From this the total required applied voltage is VT V~+VB is found:
VT(d)=2*d*Ecutoff
For a few representative cases, VT values are:
VT (S~,m) = 6.8 V
VT (10~m) =14 V
VT (SOO~m) = 680 V
The bias voltage VB will be at least half of this value and the high frequency
voltage V~.
will thus oscillate the total voltage between 0 volts and VT. A modulator
constructed from an
Au strip in LiNb03 should then be operational with VB 3.4V and V~ 3.4V.
In this embodiment there will be very little chirp as the phase increase
caused by an
index of refraction increase in one portion (say, 102) is compensated by a
phase decrease
caused by an index of refraction decrease in the other portion (104).
Because the strip 106 is a conductor, it too can be used as an electrode.
Thus, Figures
6, 7 and 8 illustrate a second embodiment in which the operating voltage is
applied between strip
106 and external electrodes 100a and 108 on the outermost surface of the
electro-optic wafer
portions 102 and 104, respectively. A plurality of optically non-invasive
branch connector
elements 114, similar in width and thickness to waveguide strip 106, spaced at
intervals along
the length of the waveguide strip 106 extend laterally from one side of the
waveguide strip 106
to a parallel via 116 which is filled and capped with an electrode pad 118 so
that electrical
contact can be made to the waveguide 106 from the top of the modulator die. It
should be noted
that electrode pad 118 does not overlie the strip 106, and is positioned
sufficiently far away from
the strip 106 so as to not interact with the plasmon-polariton wave
propagating along the strip
106.
Figure 7 shows connecting electrodes 122 and 124 whereby the modulation
control



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14
unit112 can be connected to the modulator from one side. The electrodes 100a
and 108 are
connected in common to one terminal of the modulation control unit 112 and the
electrode pad
118, and hence strip 106, are connected to the other terminal. As a result,
the electric fields
applied to the wafer portions 102 and 104 are in~opposite directions, as shown
in Figure 8.
The electro-optic wafer portions 102 and 104 use the linear electro-optic
effect
described with respect to the embodiment of Figures 1 to 3. Using z-cut LiNb03
again, the
z-axis of the crystal above and below the waveguide strip 106 are aligned
parallel.
Specifically, the positive z face of one crystal wafer portion 104 is bonded
to the negative z
face of the other crystal wafer portion 102 (or the negative z face of 104
with the positive z
face of 102) so that refractive index changes of opposite sign are produced in
the layers, as
shown in Figure 8, by variation of the electric field produced by the
modulation voltage VHF.
Since the applied electric fields are oppositely directed with respect to the
center as shown
in Figure 8, they produce index of refraction changes of opposite signs in the
two wafer
portions 102 and 104.
The on-state optical performance is the same as, or very similar to, that of
the
embodiment of Figures 1 to 3.
The applied electric field strength and index of refraction changes are
approximately
found by:
E = ~ d ~~e + ~RF sin( w t ) )
On, =-~n3~~E
oh2 = -v~,
Again requiring On,z= 2 x 10~' to cutoff the mode and converting this to E now
gives:
E~"~off = 0.68 (V/~cm), which is the same field strength as in the first
embodiment. The total
voltage VT=VRF+VB is now halved since the field is applied over half the
distance:
V'r(d) = d'kE~~coff
For a few representative cases, VT values are:
VT (S~.m) = 3.4 V



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VT (1 Op,m) = 6.8 V .
VT (SOO~,m) = 340 V
Because this second embodiment uses the strip 106 as an electrode, and applies
the
operating voltage to the strip and to electrodes above and below the optical
region, the required
5 voltage is reduced by 50% as compared with the embodiment of Figures l, 2
and 3. Again the
bias voltage will be at least half of this value and the high frequency
voltage will thus oscillate
the total voltage between 0 volts and VT. A modulator constructed from an Au
strip in
LiNb03 should then be operational with VB=1.7V and V~=1.7V.
In this embodiment there will be very little chirp as the phase increase
caused by an
10 index of refraction increase in one portion (say,102) is compensated by a
phase decrease caused
by an index of refraction decrease in the other portion (104).
When the strip 106 is used as an electrode, it is possible to omit one of the
outermost
electrodes. Figure 9 illustrates a third embodiment in which the uppermost
electrode 108 and
the associated drive circuitry have been omitted; otherwise the modulator is
similar to that
15 described with reference to Figure 8. The strip 106 again acts as an
electrode (as in the
second embodiment) and the modulation control unit 112 applies the operating
voltage
between the strip 106 and the ground plane electrode 100a. As before, the
strip 106 may be
connected by a series of branch electrode elements 114 and a via 116 to an
electrode pad 118
on the outmost surface of wafer portion 104. It should be noted that electrode
pad 118 does
not overlie the strip 106, and is positioned sufficiently far away from the
strip 106 so as to not
interact with the plasmon-polariton wave propagating along the strip 106. In
this embodiment,
the lower electro-optic wafer portion 102 uses the linear electro-optic effect
as described in
the previous embodiments and requires the z-axis of the material to be
parallel to the applied
field as shown. The z-axis can be oriented either positive or negative with
respect to the
electric field, but must be aligned with the polarization of the plasmon-
polariton wave. The
applied electric field strength and index of refraction changes are
approximately found by:



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16
E = - ~ (hB + h~ sin( c~ t ) )
On2 = - 1 ~c3y. ,E
2
Ohl = 0
In this case the total change in index of refraction is contained within the
lower electro-
optic wafer portion 102. The wafer portions 102 and 104 can be made of the
same material.
Alternatively, the lower "active" wafer portion 102 could be of an electro-
optic material and
the upper "non-active" wafer portion 104 could be a passive dielectric, so
long as Onla= ~ On1-
On2 ~ =0 for zero applied electric field.
Again using an index of refraction asymmetry of L~n~z 2 X 10-4 for cutoff to
occur, the
necessary electric field applied is now E~u~of~=1.36 Vl~m for LiNb03. From
this the required
voltage is VT V~+VB, so VT(d)=d*E~utoee~
For a few representative cases, VT values are:
VT(S~m) - 6.8 V
VT(10~m) - 14 V
VT(SOO~m) - 680 V
It should be noted that the operating voltage for the embodiment of Figure 9
is the
same as in the first embodiment, since, although the electric field is applied
across a smaller
separation, only one portion is affected. Again the bias voltage will be at
least half of this
value and the high frequency voltage will thus oscillate the total voltage
between 0 volts and
VT. A modulator constructed with a Au strip 106 in LiNb03 wafer portions 102
and 104
should then be operational with VB=3.4V and VRF=3.4V.
It should be recognized that the electric field may be applied between either
wafer
portion 102 or 104 although it is only shown for the lower wafer portion 104
in Figure 9.
A version of the embodiment of Figure 9 could be constructed using a I~err
material
where the change in index of refraction is a function of the square of the
applied electric field.
Since the effect is independent of the sign of the applied field, however, the
same modification
cannot be made to the embodiments of Figures 1 and 8.



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17
The Kerr effect is described by Yariv and Yeh in their book entitled "Optical
Waves
in Crystals", John Wiley & Sons, New York, p256 (1984). It dominates in a
quadratic
electro-optic material such as certain phases of BaTi03, PZT, the chalcogenide
glasses, in
particular the As-Se-S based glasses As2S3 and AsZSe3, and some polymers,
among other
materials. Of note is PLZT, Which is a composition dependant ferroelectric
material usually
consisting of the following ratios of atoms: (Pbt_XLaX)(ZryTi,_Y)03 and is
commonly referred to
by composition as (x/y/1-y). The relaxor phases with La atomic percentages in
the range from
x=8 to x=10, with y=35 have high Kerr electro-optic coefficients, in
particular the compositions
8/65/35, 8.5/65/35 and 9/65/35. Certain electro-optic polymers are also
suitable candidate
materials.
An external electric field will induce a change in the index of refraction in
either the
direction aligned with the applied field or perpendicular to it or both.
Typically the index
change is smaller in the direction perpendicular to the applied field thus it
is advantageous to
exploit the index change parallel to the applied field making it suitable for
this embodiment.
Kerr materials occur typically in amorphous or cubic form. In the case of a
cubic
crystal, the applied field should be directed parallel to one of the crystal
axes so long as the
normal to the surface to the waveguide is similarly aligned. The change in
index of refraction
is
Zo ~n = -1 ~3~2
2
where n is the nominal zero field index of refraction of the electro-optic
material, R is the
Kerr electro-optic coefficient and E is the applied electric field. The effect
is quadratic with
E and the sign does not depend on the sign of the applied electric field. Thus
for a particular
material with R > 0 the index of refraction can only be decreased along this
direction. The
applied electric field and change in index of refraction is approximately
found from:



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18
E = - ~ ~VB + TlRF sin( wt)~
drat = - ~ n3R ~ EZ
On,=0
Again using an index of refraction asymmetry of Onl2 = 2 X 10-4 for cutoff to
occur, and
using the K.err material PLZT in the 9/65/35 relaxor composition for region
102 with a Kerr
electro-optic coefficient R =10* 10-'6 mz/Vz and an index of refraction of n =
2.3, the necessary
applied electric field is E~utoff= 0.18 V/~m. From this the required voltage
is VT= V~+ V~, so
VT(d) = d*E~"~off .
For a few representative cases, V.,. values are:
VT(S~,m) - 0.9 V
VT(10~m) - 1.8 V
VT(SOO~.m) - 90 V
Using a 9/65/35 PLZT based modulator is seen to greatly reduce the necessary
drive
voltage for modulation. A bias voltage VB as discussed above may not be
necessary due to
the low drive voltage of this construction.
In this embodiment the chirp can be designed either positive or negative
depending
on whether the index of refraction in the active region is increased or
decreased.
It should be recognized that the electric field may be applied between either
wafer
portion 102 or 104 although it is only shown for the lower wafer portion 104
in Figure 9.
A fourth embodiment, which makes use of another class of linear electro-optic
materials, will now be described with reference to Figures 10 , 11 and 12. In
this class of
linear electro-optic materials, especially those from the symmetry group 42m
namely KDP
(KHZPO4), DKDP or KD*P (KDZP04), ADP ((NH4)HZP04), AD*P ((NH4)DZPO4) and also
including others, where the crystal wafer is cut so that the normal to its
largest face is at a 45
degree angle to the x and y axes of the crystal and the z axis is in the plane
of this surface, an
applied electric field parallel to the z-axis will cause a change in the index
of refraction 0n



CA 02486498 2004-11-15
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19
normal to the surface given by
~n = 1 ~c3~E
2
where n is the nominal zero field index of refraction of the electro-optic
material, r is the
electro-optic coefficient and E is the applied electric field. The effect is
linear with E and the
sign of the change in index of refraction depends on the sign of the applied
field.
When using these particular materials in this modulator it is advisable to
align the x
and y axes of the crystal at a 45 degree angle to the normal to the largest
dimension of the
strip cross-section and to have the applied electric field acting along the z-
axis of the crystal
as shown in Figure 12. In this way, the highly TM polarized plasmon-polariton
wave is
maximally affected. This specific orientation of KDP is well documented by
Yariv and Yeh
in their book entitled "Optical Waves in Crystals", John Wiley & Sons, New
York, p226
(1984) and will not be exhaustively described here.
The modulator shown in Figures 10 - 12 uses KDP or like materials oriented as
described above, with external electrodes 134 and 136 extending within the
upper wafer
portion 104 one on each side of the waveguide strip 106. The electrodes 134
and 135 are
connected to electrode pads 118a and 118b, respectively. The modulation
control unit 112
applies the operating voltage between the electrodes 134 and 136 in a manner
similar to that
shown in Figure 1.
It should be appreciated that the external electrodes 134 and 136 could be
placed in
either wafer portion 102 or 104 and that the second wafer portion not located
between the
electrodes could be made up of a passive dielectric or another electro-optic
material, so long
as Onl2= ~ Ont-Onz ~ = 0 when no field is applied. If a similar electro-optic
material is used
for the second wafer portion, it should be oppositely oriented to the first
wafer portiozz so that
the electric fields fringing into the second wafer portion do not lower the
asymmetry, but
work to increase it. With wafers so aligned the electrodes 134 and 136 could
also extend
below the plane of the waveguide strip into region 102 for a larger effect.
The applied electric field and changes in index of refraction are
approximately found



CA 02486498 2004-11-15
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by:
E = - d (IrB + V aF sin( w t ) )
5 On1 = _ 2 h3~ . E
On2=0
In this case (Figure 10) the total asymmetry in index of refraction is
contained within
the electro-optic material in wafer portion 104.
Again using an index of refraction asymmetry of ~n12= 2 x 10-4 for cutoff to
occur,
10 the necessary electric field applied for the case of, say, DKDP with
n.=1.4834, r=25.Spm/V
is E~~~off=4.8 V/,um. From this the required voltage is VT=V~+VB, so
VT(d)=d*E~"toff
For a few representative cases, VT values are:
VT(lOp,m) = 48 V
VT(15~m) = 72 V
15 The voltage required in this case is higher than previous cases since the
external
electrodes 134 and 136 must still be placed a similar distance away from the
waveguide strip
106, but only one half of the device is actively contributing to the total
asymmetry seen by the
plasmon-polariton mode. As well, the nominal index of refraction for DKDP is
lower than
that of LiNb03. Again, the bias voltage will be at least half of this value
and the high
20 frequency voltage will thus vary the total voltage between 0 volts and VT.
A modulator
constructed from an Au waveguide in DKDP should then be operational with
VB=24V and
VRF=24V.
In this embodiment the chirp can be designed either positive or negative
depending on
whether the index of refraction in the active region is increased or
decreased.
Modulators constructed using DKDP and similar materials having an index of
refraction of approximately n=1.5 will operate with metallic strip widths
between w=O.S~m
and 8~,m and thickness between t=Snrn and t=SOnm. Modulators will perform
better though
with narrower thinner strips with width in the range w=0.7~,m to w=6~,m and
thickness between



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21
t=l5nm and t=25nm. In particular a metallic strip in said material with width
w=4p,m and
thickness t=20nm will have a modal attenuation of about 1dB/cm.
This embodiment would also work for other linear and quadratic electro-optic
materials
such as LiNb03 and PLZT, but is not the optimal geometry for these particular
materials.
General
The modulation devices described herein are predicated upon the fact that an
asymmetry induced in optical waveguiding structures comprising a thin narrow
metallic strip
as a guiding element may inhibit propagation of the main long-ranging purely
bound plasmon-
polariton wave supported. In the embodiments described herein, the asymmetry
in the
waveguide structure is induced in distinct dielectric portions above and below
the metallic strip.
The distinct dielectric portions comprise electro-optic material and the
asymmetry in the
waveguide structure is induced by electro-optically changing the refractive
index of one distinct
portion relative to the other.
The plasmon-polariton wave is akin to a surface wave and thus is very
sensitive to the
refractive index of the material in the immediate vicinity of the metallic
strip. For thinner and
narrower metallic strips, this sensitivity is increased.
The modulation devices make use of an external electrical stimulus to induce
or enhance
the asymmetry in the dielectrics of the structure, via electro-optically
induced changes in
refractive index. It should be appreciated that, under no applied electric
field, the substantially
matched refractive indices of the cladding materials allow the plasmon-
polariton wave to
propagate unimpeded. As the refractive indices vary one relative to the other,
radiation begins
to occur and the insertion loss of the modulator begins to increase. This
trend continues until
the waveguide is cut-off, at which point the purely bound long-ranging plasmon-
polariton
wave is not supported.
Generally, a plasmon-polariton waveguide having a metallic strip of large
aspect ratio
supports substantially TM polarized light, ie.: the transverse electric field
component of the
optical mode is aligned substantially along the normal to the largest
dimension in he
waveguide cross-section, and as such requires an asymmetry in the index of
refraction along
this direction. This can be achieved using one or both of the two common
electro-optic effects,



CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
22
the Pockels effect and the Kerr effect, depending on the particular embodiment
and electro-
optic materials selected.
Guidelines for the selection of the materials, the orientation of the
materials relative to
the strip, the orientation of the applied electric field relative to the
strip, and the location of the
electrode means relative to the strip are given as follows, with reference to
Figure 13:
~ for a metallic strip having a width greater than its thickness,
~ the strongest electric field component of the plasmon-polariton wave, EPP,
is
directed along the perpendicular to the strip width,
~ thus the largest available index change in portions 102 andlor 104 should
occur
substantially parallel with this direction,
~ the electrode means are disposed such that the applied electric field is
oriented
in the direction that requires the least electric field strength to effect the
refractive index change,
~ the distance between the electrode means and the metallic strip should be
small,
~ the distance between the electrode means should be small,
~ the applied electric field should overlap well with a large portion of the
plasmon-polariton wave (the mode size of the plasmon-polariton wave can be
in the range between w = 5 ~,m and w = 40 p,m).
Applying such guidelines ensures that the drive voltage is low. Electrode
means are
not shown in Figure 13, but should be placed within or outside of this
structure, as described
in the foregoing description of the preferred embodiments.
It should be understood that, in each of the embodiments, other material
orientations
may still allow the modulator means to function, but may require an increase
in the applied
voltage. This increase can be three to four times greater, or more, depending
on the particular
material and~its associated electro-optic coefficients.
In the embodiments described herein, the low insertion loss or "on" state is
achieved
when no. drive voltage is applied to the modulator, and the high insertion
loss or "off" state
is achieved when the drive voltage is applied to the modulator. It should be
appreciated that
any embodiment could operate in the converse manner by applying a DC bias
voltage to either



CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
23
or both of the distinct portions; i.e: the low insertion loss or "on" state is
achieved when drive
voltage is applied to the modulator, and the high insertion loss or "off'
state is achieved when
no drive voltage is applied to the modulator.
Wavelengths and Interface Means
The modulator will operate with radiation:
having a wavelength such that a plasmon-polariton wave is supported,;
at optical wavelengths;
at optical communications wavelengths;
at wavelengths in the range of 800 nm to 2 ~,m;
at wavelengths near 1550 nm;
at wavelengths near 1310 nm;
at wavelengths near 850 nm,;
at wavelengths near 980 nm.
It should be appreciated that references to wavelength should be interpreted
as meaning
the centre wavelength of the spectrum associated with the input radiation.
Unlike a Mach-Zehnder modulator, modulation means embodying the present
invention
are not based on interferometry and so do not require coherent input radiation
for operation. The
plasmon-polariton wave will be cutoff whether the radiation is coherent or
incoherent; hence the
modulator operates with laser or LED input radiation.
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, andlor by different radiation coupling means. This incident
optical field can be
in the optical communications wavelength range such that a plasmon-polariton
wave is excited
for the particular modulator geometry. The modulator is broadband and can
operate over the
optical C and L bands with little or no bias voltage tuning required.



CA 02486498 2004-11-15
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24
Dimensions
The modulators will have good "on" state performance if the metallic strip has
a width
in the range from about 8 p,m to about 0.15 ~,m and a thickness in the range
from about 100 nm
to about 5 nm, particular dimensions depending on the index of refraction of
the surrounding
material and the wavelengths
For metallic strips bounded by distinct portions comprising materials such as
lithium
niobate and PLZT, having a refractive index in the range from about 2 to about
2.5, as discussed
in three of the embodiments, the range of dimensions for the metallic strip is
the width in the
range from about 3 ~,m to about 0.15 ~,m and the thickness in the range from
about 50 nm to
about 5 nm. Such wavegitide structures support propagation of a plasmon-
polariton wave having
a wavelength in the range from about 0.8 ~,m to 2 ~.m. Good dimensions for the
metallic strip
are a width in the range from about 1.2 ~,m to about 0.7 ~m and thickness in
the range from
about 25 nm to about 15 nrn. A good choice for the dimensions of the metallic
strip are a width
of about 1 ~,m and a thickness of about 20 nm for operation in the wavelength
range from about
1.3 ~,m to 1.7 Vim. The wavelength selected is preferably a wavelength used
for optical
communications. Other materials having an index of refraction approximately in
the same range
require strip dimensions in approximately the same ranges.
For metallic strips bounded by distinct portions comprising materials such as
DI~DP,
having a refractive index in the range from about 1.4 to about 1.8, as
discussed in one of the
embodiments, the range of dimensions for the metallic strip is the width in
the range from about
8 ~,m to about 0.5 ~,m and the thickness in the range from about 50 nm to
about 5 nm. Such
waveguide structures suppart propagation of a plasmon-polariton wave having a
wavelength in
the range from about 0.8 ~m to 2 ~,m. Good dimensions for the metallic strip
are a width in the
range from about 6 ~,m to about 0.7 ~,m and thickness in the range from about
25 nm to about
15 nm. A good choice for the dimensions of the metallic strip are a width of
about 4 ~,m and the
thickness of about 20 nm for operation in the wavelength range from about 1.3
~,m to 1.7 p,m.
The wavelength selected is preferably a wavelength used for optical
communications. Other
materials having an index of refraction approximately in the same range
require strip dimensions
in approximately the same ranges.



CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
The width of the external electrodes (or in the case of the fourth embodiment,
the height
or thickness) should be such that it spans the mode size of the plasmon-
polariton wave. The
mode size can be in the range between w = 5 ~m and w = 40 p,m. For typical
modes described
in this invention an electrode width in the range from about 40 ~,m to about 1
~,m, especially
5 from about 30 ~,m to about 10 p,m, and preferably about 20 ~.m are good
widths.
The low voltage operation of the embodiments is made possible by the close
proximity
of the electrode means to each other and/or to the metallic strip. Electrodes
can be brought into
close proximity to the metallic strip by as much as 2 ~m to 3~,m by using ITO
layers between
the electrode and the distinct portions. A good distance of the electrode
means from the metallic
10 strip is in the range from about 20 ~,m to about 5 ~,m, depending on the
materials selected and
wavelength of operation.
For, example, where the optical radiation has a free-space wavelength of 1550
nm, and
the metallic strip is constructed from gold and is surrounded by an
appropriate electro-optic
'material such as lithium niobate, with suitably placed electrodes, and the
metallic strip has
15 dimensions of about 1 pm wide and 20 nm thick and a length of the order of
a couple of
millimeters a suitable difference between the refractive index of the two
distinct portions to
induce mode cutoff and radiation is about 2.7x10ø.
Construction
20 The distinct electro-optic portions of the device, above or below the
metallic strip, may
comprise single crystal or partially crystalline material, and may consist of
different types of
electro-optic material which are not necessarily homogeneous. The electro-
optic material may
be only on the top or only on the bottom of the metallic strip with an
appropriately matched
passive dielectric constituting the opposite cladding region. These variations
lead to alternate
25 embodiments. All embodiments will have the same or similar on-state optical
characteristics
and vary mainly in ease of fabrication and magnitude of applied voltage
necessary for
modulation.
Devices are fabricated using wafer bonding and polishing or known deposition
techniques for the cladding materials, and known lithographic and metal
deposition techniques



CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
26
for the metallic strip. The metallic strip may be embedded in a shallow trench
etched within one
of both of the portions 102 or 104, or surrounded by a planarizing dielectric
layer having a
refractive index that matches that of the claddings when no field is applied.
The metallic strip may consist of, but is not limited to consisting of, a
single metal or
a combination of metals from the group Au, Ag, Cu, Al, Pt, Pd, Ti, Ni, Mo, and
Cr,
preferred metals being Au, Ag, Cu, and Al. A single material or combination of
materials
which behave like metals, such as Indium Tin Oxide can also be used. The
metallic strip is
not necessarily homogeneous.
The external drive electrodes can be constructed from any good conductor, but
should
consist of one of the less lossy metals Au, Ag, Cu, and Al if the proximity of
the electrodes
to the plasmon-polariton wave guided by the metallic strip is such that slight
optical coupling
occurs between the strip and the electrodes.
The modulator could be straight, curved, bent, tapered, and so on. Tapered
input and
output metallic strip sections may be necessary depending on the mode size so
that coupling
loss to the input and output means is minimized.
The modulation means can comprise: a metallic strip that is homogeneous and
either
or both distinct portions that are homogeneous, a metallic strip that is
homogeneous and either
or both distinct portions that are inhomogeneous, a metallic strip that is
inhomogeneous and
either or both distinct portions that are homogeneous, a metallic strip that
is inhomogeneous and
either or both distinct portions that are inhomogeneous. An inhomogeneous
metallic strip can
be formed from a continuously variable material composition, or strips or
laminae. An
inhomogeneous distinct portion can be formed from a continuously variable
material
composition, or strips or laminae. ,
Miscellaneous
The short length of the modulator allows it to be driven as a lumped element
by an
appropriate high-frequency voltage circuit. For example, a 2 mm long modulator
in LiNbO3
(e~=28) with top and bottom electrodes (as described in the first embodiment)
of width 10 ~,m
separated from each other by 10. ~,m has a capacitance of:. C - Eo*e~*A/d -



CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
27
(8.854x10-'2)*28*(1Ox10-6)*(2x10-3)/lOxlOv = 0.5 pF. A capacitance of 0.5 pF
can be driven
to frequencies beyond 10 GHz within a lumped element circuit.
The metallic strip is made of conductive material and as such may be used as
an
electrode to bias or drive the modulator. The term "external electrode" refers
to other
optically non-invasive electrode elements outside of the waveguide region. The
metallic strip
used to form the waveguide may also be used as part of a control circuit to
monitor
temperature and voltage within the modulator in locations where it is not
being used as a
biasing or drive electrode. Short gaps of up to a few microns in the metallic
strip can be
introduced to electrically isolate parts of the strip without deleteriously
affecting the
propagation of the plasmon-polariton wave.
. The present invention embraces not only linear amplitude optical modulators,
but also
variable optical attenuators and on/off switches; i.e., in this specification,
the term
"modulation means" embraces variable attenuators, linear modulators, digital
modulators,
on/off modulators and on/off switches. The latter cases simply mean that the
insertion loss of
the modulator varies from very low in the on state, to very high in the off
state.
It is envisaged that the metallic strip 106 in any of the above-described
embodiments
could be replaced by a strip having width and thickness of substantially the
same order as
described in United States published patent application No.20030059147 A1.
INDUSTRIAL APPLICABILITY
An advantage of modulators embodying the present invention is that they can be
readily connected to ,a polarized light source, for example by way of a
polarization-
maintaining fiber, or directly to another plasmon-polariton device, such as
the external cavity
laser disclosed in the above-mentioned United States Provisional patent
application. Such a
modulator-ECL combination could be readily provided on the same substrate and
interconnected by a common waveguide metallic strip.
In contrast to modulators which are based upon voltage induced waveguiding,
mode
overlap changes, or mode extinction, as discussed above, embodiments of the
present
invention depend upon sensitivity to the electromagnetic properties of the
material above and



CA 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
2~
below the waveguiding strip. This effect is unique to the surface wave nature
of the plasmon-
polariton waveguide. The metallic strip in embodiments of the present
invention remains
unchanged during the modulation cycle and, beyond cut-off, light is
preferentially radiated
into the high index region above or below the waveguide strip.
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 02486498 2004-11-15
WO 03/102676 PCT/CA03/00787
29
Acronyms and chemical formulae used in this specification
KTP KTiOP04 Potassium Titanyl Phosphate


DKDP KDZPO4 Deuterated Potassium Dihydrogen
Phosphate


KDP KHZP04 Potassium Dihydrogen Phosphate


ADP (NH4)HZPO
Ammonium Dihydrogen Phosphate


AD*P (NH4)DzP04 Deuterated Ammonium Dihydrogen
Phosphate


PLZT (Pb1_XLaX),(ZrYTiI_Y)03Lead Lanthanum Zirconium Titanate


PZT (Pb),(ZrYTiI_Y)03 Lead Zirconium Titanate


ITO In203:Sn02 Indium Tin Oxide


BaTiO3 Barium Titanate


As2S3 Arsenic Sulphide


AsZSe3 Arsenic (III) Selenide


SPP Surface Plasmon Polariton


ATR Attenuated Total Reflection


DC Direct Current


RF Radio Frequency or High
Frequency


TM Transverse Magnetic


TE Transverse Electric


~,m micron
mm millimeter (except in crystal symmetry groups)
nrn nanometer

Representative Drawing
A single figure which represents the drawing illustrating the invention.
Administrative Status

For a clearer understanding of the status of the application/patent presented on this page, the site Disclaimer , as well as the definitions for Patent , Administrative Status , Maintenance Fee  and Payment History  should be consulted.

Administrative Status

Title Date
Forecasted Issue Date Unavailable
(86) PCT Filing Date 2003-06-02
(87) PCT Publication Date 2003-12-11
(85) National Entry 2004-11-12
Dead Application 2009-06-02

Abandonment History

Abandonment Date Reason Reinstatement Date
2008-06-02 FAILURE TO PAY APPLICATION MAINTENANCE FEE
2008-06-02 FAILURE TO REQUEST EXAMINATION

Payment History

Fee Type Anniversary Year Due Date Amount Paid Paid Date
Registration of a document - section 124 $100.00 2004-11-15
Application Fee $200.00 2004-11-15
Maintenance Fee - Application - New Act 2 2005-06-02 $50.00 2004-11-15
Maintenance Fee - Application - New Act 3 2006-06-02 $100.00 2006-05-12
Maintenance Fee - Application - New Act 4 2007-06-04 $100.00 2007-06-01
Owners on Record

Note: Records showing the ownership history in alphabetical order.

Current Owners on Record
SPECTALIS CORP.
Past Owners on Record
BERINI, PIERRE SIMON JOSEPH
BIDNYK, SERGE
BREUKELAAR, IAN GREGORY
WORT, PHILIP MICHAEL
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.
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Document
Description 
Date
(yyyy-mm-dd) 
Number of pages   Size of Image (KB) 
Cover Page 2005-02-03 1 46
Abstract 2004-11-12 1 67
Claims 2004-11-12 7 299
Drawings 2004-11-12 10 444
Description 2004-11-12 29 1,492
Representative Drawing 2004-11-12 1 12
Fees 2006-05-12 2 57
Fees 2007-06-01 1 28
PCT 2004-11-12 3 118
Assignment 2004-11-12 8 344