U.S. patent application number 11/693533 was filed with the patent office on 2007-07-19 for low loss electrodes for electro-optic modulators.
This patent application is currently assigned to The Government of the US, are represented by the Secretary of the Navy. Invention is credited to James H. Cole, Marta M. Howerton, Robert P. Moeller.
Application Number | 20070165977 11/693533 |
Document ID | / |
Family ID | 34994270 |
Filed Date | 2007-07-19 |
United States Patent
Application |
20070165977 |
Kind Code |
A1 |
Cole; James H. ; et
al. |
July 19, 2007 |
Low loss electrodes for electro-optic modulators
Abstract
An electro-optic modulator includes a substrate, at least two
parallel optical waveguides, at least one ground plane, at least
one active electrode with at least two lower portions of the active
electrode, and an upper portion connected to the lower portions,
the lower portions spaced apart from each other, each of the two
lower portions of the active electrode extending over one of the
optical waveguides. An electro-optic phase modulator has at least
one optical waveguide and at least one active electrode formed on a
face of the substrate, the active electrode having a wider upper
portion and a narrower lower portion, the lower portions of the
active electrode aligned with and extending over one of the optical
waveguides. A bridge electrode has at least one narrower lower
portion, and a wider upper portion, the lower portion for being
arranged over an optical waveguide formed in a substrate.
Inventors: |
Cole; James H.; (Great
Falls, VA) ; Moeller; Robert P.; (Ft. Washington,
MD) ; Howerton; Marta M.; (Fairfax Station,
VA) |
Correspondence
Address: |
NAVAL RESEARCH LABORATORY;ASSOCIATE COUNSEL (PATENTS)
CODE 1008.2
4555 OVERLOOK AVENUE, S.W.
WASHINGTON
DC
20375-5320
US
|
Assignee: |
The Government of the US, are
represented by the Secretary of the Navy
Washington
DC
|
Family ID: |
34994270 |
Appl. No.: |
11/693533 |
Filed: |
March 29, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11080974 |
Mar 14, 2005 |
7224869 |
|
|
11693533 |
Mar 29, 2007 |
|
|
|
60556012 |
Mar 12, 2004 |
|
|
|
Current U.S.
Class: |
385/8 ;
385/9 |
Current CPC
Class: |
G02F 1/2255 20130101;
G02F 1/0356 20130101; G02F 2201/122 20130101; G02F 1/0316 20130101;
G02F 2201/16 20130101 |
Class at
Publication: |
385/008 ;
385/009 |
International
Class: |
G02F 1/295 20060101
G02F001/295 |
Claims
1. An electro-optic modulator comprising: a substrate having at
least one optical waveguide formed on a first face of the
substrate; at least one active electrode formed on the first face
of the substrate aligned over the optical waveguide; the active
electrode having at least one narrower lower portion and wider
upper portion, the active electrode having a height that is at
least 5 times as large as the width of the lower portion, the
active electrode operating to induce a refractive index change in
the optical waveguide.
2. The electro-optic modulator of claim 1, the upper portion having
a width and height sufficient to match velocities of a signal in
the optical waveguide and of an RF signal in the active
electrode.
3. The electro-optic modulator of claim 1, wherein the width of the
upper portion is at least twice as great as the width of the lower
portion.
4. The electro-optic modulator of claim 1, wherein the width of the
upper portion is at least three times as great as the width of the
lower portion.
5. The electro-optic modulator of claim 1, wherein the width of the
upper portion is at least four times as great as the width of the
lower portion.
6. The electro-optic modulator of claim 1, wherein the width of the
lower portion is about eight microns.
7. The electro-optic modulator according to claim 1, further
comprising: at least one ground plane formed on the first face of
the substrate spaced apart from the active electrode.
8. The electro-optic modulator according to claim 1, further
comprising: ground planes formed on opposite sides of the active
electrode, spaced apart from the active electrode.
9. The electro-optic modulator according to claim 8, wherein the
ground plane does not overlie the optical waveguide in an active
region of the modulator.
10. The electro-optic modulator according to claim 1, further
comprising an insulator element arranged on either side of the
lower portion to support the upper portion.
11. The modulator according to claim 1, the lower narrower portion
of the active electrode formed by depositing a first layer of
metallization on the substrate, the upper wider portion of the
active electrode formed by depositing at least a second layer of
metallization over the first layer.
12. The modulator according to claim 11, the upper wider portion of
the active electrode being formed by depositing a third layer over
the second layer in active regions of the modulator, but no third
layer in bend regions of the modulator.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application under 35 U.S.C.
.sctn. 121 of application Ser. No. 11/080,974, filed on Mar. 14,
2005, which is a non provisional application under 35 U.S.C. .sctn.
119(e) of provisional application No. 60/556,012, filed in the
United States on Mar. 13, 2004, each of which are incorporated
herein by reference in their entireties.
BACKGROUND
[0002] 1. Field of the Invention
[0003] This application is related to electro-optic modulators and
electrodes for use in electro-optic modulators.
[0004] 2. Description of Related Art
[0005] Electrooptic devices, such as optical modulators, have the
ability to change a particular characteristic of an optical signal,
such as its intensity, phase, or polarization. Electro-optic
modulators, particularly lithium niobate (LiNbO.sub.3) modulators,
have application in radio frequency analog links, digital
communications and electric field sensing. Electro-optic modulators
are useful for modulating an optical signal in a waveguide with an
RF or other frequency electrical signal.
[0006] A variety of electro-optic modulators are disclosed in
Wooten, E. L, Kissa, K. M., Yi-Yan, A., Murphy, E. J., Lafaw, D.
A., Hallemeier, P. F., Maack, D., Attanasio, D. V., Fritz, D. J.,
McBrien, G. J., Bossi, D. E., "A review of Lithium Niobate
Modulators for Fiber-Optic Communications Systems," IEEE Journal of
Selected Topics in Quantum Mechanics, Vol. 6, No. 1, 2000.
[0007] Electro-optic modulators formed in x-cut and z-cut lithium
niobate are disclosed in U.S. Pat. No. 5,416,859 to Burns et al.,
U.S. Pat. No. 6,016,198 to Burns et al., U.S. Pat. No. 6,304,685 to
Burns, U.S. Patent Application Publication No. 2004/0061918 A1,
U.S. Patent Application Publication No. 2004/0095628 A1, U.S.
Patent Application Publication No. 2004/0136634 A1, U.S. Patent
Application Publication No. 2004/0151414 A1, U.S. Pat.No.
5,388,170, U.S. Pat. No. 5,712,935, U.S. Pat. No. 6,522,793, U.S.
Pat. No. 6,600,843, U.S. Patent Application Publication No.
2004/0114845 A1, U.S. Pat. No. 5,153,930, U.S. Pat. No. 5,189,713,
U.S. Pat. No. 5,953,466, U.S. Pat. No. 6,501,867, U.S. Patent
Application Publication No. 2004/0066549, U.S. Patent Application
Publication No. 2004/0145797, and U.S. Patent Application
Publication No. 2003/0228081. Electro-optic devices with a lithium
niobate substrate are also disclosed in U.S. Patent Application
Publication No. 2004/0202395, U.S. Patent Application Publication
No. 2004/0240036, U.S. Patent Application Publication No.
2004/0240790, U.S. Patent Application Publication No. 2004/0247220,
U.S. Patent Application Publication No. 2004/0264832, U.S. Pat. No.
5442,719, U.S. Pat. No. 5,497,233, U.S. Pat. No. 6,128,424, and
U.S. Patent Application Publication No. 2004/0067021.
[0008] Reflection traveling-wave interferometric modulators are
disclosed in W. K. Burns, M. M. Howerton, R. P. Moeller, R. W.
McElhanon, A. S. Greenblatt, "Broadband reflection traveling-wave
LiNbO3 modulator", OFC '98 Technical Digest, 1998, pp. 284-285, and
in W. K. Burns, M. M. Howerton, R. P. Moeller, R. W. McElhanon, A.
S. Greenblatt, "Reflection Traveling Wave LiNbO3 Modulator for Low
V.pi. Operation," LEOS 1997, IEEE p 60-61, and in W. K. Burns, M.
M. Howerton, R. P. Moeller, A. S. Greenblatt, R. W. McElhanon,
"Broad-Band Reflection Traveling-Wave LiNbO3 Modulator," IEEE
Photonic Technology Letters, Vol. 10, No. 6, June 1998, pp.
805-806.
[0009] Mach-Zehnder traveling-wave electro-optic modulators with
waveguides formed in a z-cut lithium niobate substrate are
disclosed in W. K. Burns, M. M. Howerton, R. P. Moeller, R.
Krahenbuhl, R. W. McElhanon, and A. S. Greenblatt, "Low-Drive
Voltage, Broad-Band LiNbO.sub.3 Modulators with and Without Etched
Ridges," Journal of Lightwave Technology, Vol. 17, No. 12, Dec
1999, pp. 2551-2555 and in M. M. Howerton, R. P. Moeller, A. S.
Greenblatt, and R. Krahenbuhl, "Fully Packaged, Broad-band
LiNbO.sub.3 Modulator with Low Drive Voltage", IEEE Photonics
Technology Letters, Vol. 12, No. 7, July 2000, pp. 792-794. A 40
Gb/s Mach-Zehnder modulator with traveling wave electrode is
disclosed in M. Sugiyama, M. Doi, S. Taniguchi, T. Nakazawa, and H.
Onaka, "Driver-less 40 Gb/s LiNbO.sub.3 Modulator with Sub-1 V
Drive Voltage", OFC 2002, FB6-2-FB6-4.
[0010] Integrated optical photonic RF phase shifters are disclosed
in E. Voges, K. Kuckelbaus, and B. Hosselbarth, "True time delay
integrated optical RF phase shifters in lithium niobate",
Electronics Letters, Vol. 33, No. 23, 1997, pp. 1950-1951.
[0011] Waveguide horns for use in electro-optic modulators are
disclosed in U.S. Pat. No. 6,356,673 to Burns et al. Electrodes
suitable for use in electro-optic modulators are disclosed in U.S.
Pat. No. 6,381,379 to Burns et al. Additional electro-optic
modulators are disclosed in U.S. Pat. No. 6,393,166 to Burns, U.S.
Pat. No. 6,526,186 to Burns, and U.S. Pat. No. 6,535,320 to
Burns.
[0012] Lithium-tantalate based electro-optic modulators are
discussed in W. K. Burns, M. M. Howerton, and R. P. Moeller,
"Performance and Modeling of Proton Exchanged LiTaO3 Branching
Modulators", Journal of Lightwave Technology, Vol. 10, No. 10, Oct
1992, pp. 1403-1408.
[0013] Multiple-pass reflective electro-optic modulators are
disclosed in commonly assigned patent application Ser No.
10/165,940, now issued as U.S. Pat. No. 6,862,387, incorporated by
reference in its entirety, and in M. M. Howerton, R. P. Moeller,
and J. H. Cole, "Subvolt Broadband Lithium Niobate Modulators" 2002
NRL Review, pp 177-178. The low-loss compact turns increase the
active length of a modulator and achieve a reduction in drive
voltage V.pi. without sacrificing a great deal of space on the
substrate material.
[0014] Electrodes for use in lithium niobate modulators are also
discussed in R. Krahenbuhl and M. M. Howerton, "Investigations on
Short-Path-Length High-Speed Optical Modulators in LiNbO.sub.3 with
Resonant-Type Electrodes", Journal of Lightwave Technology, Vol.
19, No. 9, September 2001, pp. 1287-1297.
[0015] Mach Zehnder interferometers with etched ridges between the
electrodes and waveguides are disclosed in W. K. Burns, M. M.
Howerton, R. P. Moeller, R. W. McElhanon, and A. S. Greenblatt,
"Low Drive Voltage, 40 GHz LiNbO.sub.3 Modulators", OFC '99, pp
284-286.
SUMMARY
[0016] An embodiment of the invention is directed to an
electro-optic modulator comprising: a substrate having at least two
parallel optical waveguides formed on a first face of the
substrate; at least one active electrode formed on the first face
of the substrate; at least one ground plane formed on the first
face of the substrate spaced apart from the active electrode; the
active electrode having at least two lower portions of the active
electrode proximal to the substrate, and an upper portion connected
to the lower portions, the lower portions spaced apart from each
other, each of the two lower portions of the active extending over
one of the optical waveguides.
[0017] The electro-optic modulator can be a Mach-Zehnder modulator.
One of the two optical waveguides can be reversed poled compared to
another of the two optical waveguides. The two optical waveguides
can be joined at each end by a y-branch.
[0018] In one embodiment, ground planes are arranged on opposite
sides of the active electrode. The gap between the spaced apart
lower portions of the active electrode can be filled with an
electrical insulator, such as a polymer.
[0019] An upper portion of the active electrode can have a width
about equal to the width of the optical waveguides plus the width
of the gap between the optical waveguides.
[0020] One embodiment of the modulator has zero chirp.
[0021] The substrate can be lithium niobate, and in one embodiment
is z-cut lithium niobate. A buffer layer of an electrical insulator
such as silicon dioxide can be arranged formed on the substrate
between the optical waveguides and the electrodes.
[0022] The parallel optical waveguides can be formed by titanium
diffusion into the substrate. A groove can be etched in the
substrate between the optical waveguides. The grooves can be etched
in the substrate surface between the active electrode and the
ground plane. The grooves in the substrate can be etched into the
substrate before one of the optical waveguides is reverse-poled, or
after one of the optical waveguides is reverse-poled.
[0023] In another embodiment of the invention, the optical
waveguides, the active electrode, and the ground plane extend from
an active region into a bend region, the bend region having a
reflector arranged at an end of the optical waveguides, the active
electrode and the ground plane having a thickness in the bend
region less than a thickness in an active portion of the optical
waveguides. The modulator can include 0-5 or more 180 degree bend
regions. In an embodiment, the substrate is etched in the linear
active regions between the bend regions and is not etched in the
bend region. Another embodiment of the invention is directed to a
phase modulator having a substrate with at least one optical
waveguide formed on a first face of the substrate; at least one
active electrode formed on the first face of the substrate aligned
over the optical waveguide: the active electrode having at least
one narrower lower portion and wider upper portion having a width
and height sufficient to match or nearly match velocities of a
signal in the optical waveguide and of a RF signal in the active
electrode, the active electrode having a height that is at least 5
times as large as the width of the lower portion, the active
electrode operating to induce a refractive index change in the
optical waveguide.
[0024] Another embodiment of the invention is directed to an
electrode for inducing an index change in an optical waveguide of
an electro-optic modulator, including a first elongated portion
having a length greater than its width and height; and a second
elongated portion electrically coupled to the first portion, the
second portion having a width at least twice the width of the first
portion. Another embodiment of the invention is directed to an
electrode for inducing an index change in each of two optical
waveguide arms of a Mach-Zehnder modulator (MZM) having a first and
a second elongated parallel portions separated by a gap, and a
third portion electrically coupled to the first and second
elongated parallel portions, the third portion having a width at
least equal to the sum of the first and second portions and the
gap.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] A more complete appreciation of the invention will be
readily obtained by reference to the following example embodiments
and the accompanying drawings.
[0026] FIG. 1 is a plan view of an electro-optic modulator
according to a first embodiment of the invention.
[0027] FIG. 2 is a cross-sectional view of the modulator of FIG. 1
at line A-A.
[0028] FIG. 3 is a cross sectional view of an electro-optic
modulator according to a second embodiment of the invention.
[0029] FIG. 4 is a cross-sectional view of the modulator of FIG. 3
in an active region at line B-B.
[0030] FIG. 5 illustrates a view of the electrodes in the bend
region corresponding to FIG. 3 and 4.
[0031] FIG. 6 is a graph illustrating projected drive voltage
versus frequency for bridge electrodes according to embodiments of
the invention.
[0032] FIG. 7 is of electrode transmission versus frequency for
coplanar waveguides and for bridge electrodes in accordance with an
embodiment of the invention.
[0033] FIG. 8 illustrates the electrode loss versus electrode gap
for coplanar waveguide structures.
[0034] FIG. 9 illustrates the projected drive voltage versus
frequency for coplanar waveguide structures.
[0035] FIG. 10 illustrates the loss coefficient, dc drive voltage
V.pi. (DC) and 20 GHz drive voltage V.pi. (20 GHz) for coplanar
waveguide structures.
[0036] FIG. 11 is an expanded view of the bridge electrode
according to FIG. 2 or 4.
[0037] FIG. 12 illustrates a lower level of a bridge electrode
according to an embodiment of the invention.
[0038] FIG. 13A-B are cross sectional views of an electro-optic
phase modulator according to an embodiment of the invention.
DETAILED DESCRIPTION
[0039] When configured as MZMs, the electro-optic modulators and
electrode structures described herein are useful for amplitude
modulation of optical signals. The electrode structures described
herein are also suitable for electro-optic phase modulators.
[0040] A waveguide is formed of a substrate material and a
conveying medium. The substrate can be made of any suitable
material, including ferroelectric materials such as lithium
niobate, suitable for titanium diffused or proton exchange
waveguides; lithium tantalate (LiTaO.sub.3), which is typically
used with proton exchange waveguides; barium titanate
(BaTiO.sub.3); strontium barium niobate (SrBaNbO.sub.3); various
polymers; and semiconductor materials such as indium gallium
arsenide phosphide (InGaAsP), indium phosphide (InP), gallium
arsenide (GaAs), and gallium aluminum arsenide (GaAlAs). The
conveying medium can be any suitable material which has a higher
refractive index than the substrate after formation of the optical
waveguide. Since lithium niobate has good long term stability, low
optical loss, a strong electro-optic coefficient, and the ability
to operate at high frequencies, the description that follows will
be made in the context of titanium-diffused waveguides in lithium
niobate substrates, although it should be understood that other
materials can be used.
[0041] An embodiment of the invention herein provides an
electro-optic device, and specifically, a Mach-Zehnder modulator
with an extremely low-loss electrode. The low-loss electrodes
described herein are desired for high frequency applications and
allow the incorporation of recently developed low-loss, integrated
compact turns described in U. S. Pat. No. 6,862,387, suitable for
interconnecting multiple devices or regions on the same chip,
increasing the device active length, and increasing the device
density. Compact turns in an integrated optic modulator allows an
increase in the active length of the modulator without sacrificing
a great deal of space on the substrate material. The drive voltage
at dc is inversely proportional to the device length. Accordingly,
compact turns also provide a means for achieving a desirable
reduction in drive voltage.
[0042] An embodiment of the invention includes a low-loss electrode
structure that provides extremely low electrical loss. The low-loss
electrode structure is useful in electro-optic devices with long
active lengths, since an increase in active length increases the
overall electrode loss.
[0043] FIG. 1 illustrates a single pass electro-optic Mach-Zehnder
modulator 100 in accordance with an exemplary embodiment of the
invention. The modulator includes optical waveguides formed in a
substrate and electrodes arranged to induce index changes in the
optical waveguides. A single hot input electrode receives a RF
signal from a coplanar waveguide input horn. The base layer of the
hot input electrode is split into two electrodes that are
approximately the same width as the two optical waveguide arms of
the modulator. A wider upper portion of the hot electrode
electrically connects the lower electrode portions. The RF input
signal in the electrode portions overlying the optical waveguide
arms induces index changes in the optical waveguides arms. The
electrode is designed such that the velocity of the RF signal
traveling down the electrodes matches, as closely as possible, the
velocity of the optical wave traveling within the waveguide under
the electrode. One of the optical waveguide arms is reverse poled
relative to the other waveguide arm. Thus, the in-phase RF input
signal traveling through the electrode induces opposite phase
shifts in the optical waveguide arms. Further details are provided
in the following paragraphs. Note that FIG. 1-5 are not to
scale.
[0044] In the exemplary embodiment shown in FIG. 1 and 2, the
substrate 160 is z-cut lithium niobate, and the optical waveguide
channels 140 and 142 are formed by high temperature indiffusion of
a titanium that has been photolithographically defined.
[0045] The optical waveguides are formed in a pattern such that an
input waveguide 132 is split at a y-branch 136 into two parallel
waveguides 140 and 142, which are then recombined into a single
output waveguide 134 by an output y-branch 138. In an alternative
embodiment (not shown), each of the parallel waveguides 140 and 142
can be coherently combined using a directional coupler providing
two separate outputs from the device, without the output y-branch
138 and output waveguide 130.
[0046] As seen in FIG. 2, a buffer layer 170 can be arranged
between the z-cut lithium niobate substrate 160 and the electrodes
110, 120, and 150 at least in the areas that will underlie the
electrodes. The buffer layer 170 can be silicon dioxide or other
suitable material. If the substrate is x-cut lithium niobate, no
buffer layer is necessary provided that the electrodes are beside
the waveguides rather than on top of them. In another embodiment
(not shown), the buffer layer 170 can also be disposed over the
substrate 160 in the regions between the electrodes.
[0047] As illustrated in FIG. 1 and 2, the ground electrodes 110
and 120 are arranged on either side of and spaced apart from the
central active electrode 150. Details of suitable materials and
suitable techniques for diffusing titanium into the substrate are
found in commonly assigned U.S. Pat. No. 6,862,387.
[0048] A cross-sectional view of the active electrode is shown in
FIG. 2, corresponding to the central linear active portion of the
modulator between the turns. The active or hot electrode 150 is
arranged over both of the parallel waveguides 140 and 142. The hot
electrode 150 has a bridge configuration, with two lower portions
152 and 154 aligned preferably directly over the parallel
waveguides 140 and 142. The hot electrode's lower portions 152 and
154 are approximately the same width as the parallel waveguides 140
and 142. The lower portions of the electrode 150 are separated by a
distance that is approximately the same width as the distance
between the optical waveguide channels 140 and 142. The space 162
can be empty (e.g., filled with air) or can be filled with an
electrically insulating material preferably with a low dielectric
constant.
[0049] The upper portion 156 of the bridge-shaped hot electrode 150
has a width approximately the same as the optical waveguides 140
and 142 plus the distance between the optical waveguides, and is
aligned over and in physical contact with the lower portions of the
electrode.
[0050] FIG. 1 shows the electrode 150 with the upper portion 156
cut away to illustrate the alignment of the electrode legs or lower
portions 152 and 154 over the optical waveguide arms 140 and 142.
In the region 144 where the optical waveguide arms are
approximately linear and parallel, the lower parts of the electrode
150 are aligned over the optical waveguide arms.
[0051] The active electrode and ground electrodes are configured to
match as closely as possible the velocity of the RF signal in the
active electrode 150 and the velocity of the optical signal in the
optical waveguide channels 140 and 142. For example, the overall
width of the active electrode is not limited to the width of the
optical waveguide, as in some conventional MZMs, but may be equal
to the sum of the widths of the optical waveguides plus the
edge-to-edge separation distance between the two optical
waveguides. The larger size of the bridge electrode allows much
wider gap spacing between the active center electrode 150 and the
ground electrodes 110 and 120 than in conventional MZM designs. The
wider gap spacing allows the electrodes to be made much thicker (in
the z direction of FIG. 2), resulting in considerably lower
electrical losses in the electrodes.
[0052] At either end of the active region 144, the two electrode
legs 152, 154 that form the base of the electrode bridge 150 are
recombined and preferably terminated with a resistor equal to the
characteristic impedance of the transmission line. For an
unamplified input signal, this configuration provides an additional
2.sup.1/2 direct improvement over an electro-optic modulator with
one hot electrode over one waveguide and a ground plane over the
over waveguide and an equivalent active modulation length.
[0053] In the exemplary embodiment of FIG. 1-2, an optical source
such as a laser (not shown) provides optical energy into the
optical waveguide input 132. The device of FIG. 1-2 is configured
to receive optical energy at the 1.5 micron wavelength, however,
optical energy at other wavelengths is also suitable. For example,
optical energy at the 1.3 micron wavelength can be carried by the
optical waveguide 132. The dimensions of the optical waveguide can
also be adjusted to carry optical energy at longer or shorter
wavelengths.
[0054] Another exemplary embodiment of an electro-optic modulator
is illustrated in FIG. 3-5.
[0055] FIG. 3 illustrates a multiple-pass electro-optic
Mach-Zehnder modulator 300 in accordance with another exemplary
embodiment of the invention. The modulator 300 includes optical
waveguides formed in a substrate and electrodes arranged to induce
index changes in the optical waveguides. A single hot input
electrode 350 receives a RF signal from a coplanar waveguide input
horn.
[0056] The base layer of the hot input electrode 350 is split into
two electrodes that pass over two optical waveguide arms of the
modulator. The RF input signal induces index changes in the optical
waveguides modulating the velocity of the propagating optical
signals. One of the optical waveguide arms is reverse poled
relative to the other waveguide arm. Thus, the in-phase RF input
signal traveling through the electrodes induces opposite phase
shifts in the optical waveguides. The electrode is designed such
that the velocity of the RF signal traveling down the electrodes
matches, as closely as possible, the velocity of the optical wave
traveling within the waveguide under the electrode. Further details
are provided in the following paragraphs.
[0057] As illustrated in FIG. 3, the active region is folded back
and forth in order to increase the interaction length, resulting in
a reduced drive voltage. The modulator 300 includes a long active
modulation region 380 with five approximately linear portions in
which the active electrode overlies the waveguide arms. Compact
reflective waveguide turns 382 are provided at the ends of the
linear portions, as disclosed in U. S. Pat. No. 6,862,387 and
discussed in the following paragraphs.
[0058] The optical waveguides are formed in a pattern such that an
input waveguide 332 is split at a y-branch 336 into two parallel
waveguide channels 340 and 342. At the end of the optical waveguide
opposite the input portion 332, the optical waveguide channels 340
and 342 each include a compact 180 degree waveguide s-bend turn. As
illustrated in FIG. 3, the modulator 300 includes reflective
surfaces 381 and 383 at the edges of the substrate 360 adjacent the
optical waveguide turns 382. The reflective surfaces can be gold or
dielectric mirrors, or any other suitable material with a high
reflectivity at the wavelength of the optical energy carried by the
optical waveguides.
[0059] As discussed in U.S. Pat. No. 6,862,387, reflective surfaces
381 and 383 can be formed on an edge of the substrate 360 which
should be polished to a smooth surface before application of the
reflective surface. Smoothness of the substrate edge in the
vicinity of the optical waveguides should be about or better than
1/5 of the typical wavelength of 1.5 microns or about 0.3 microns.
Waveguide portions 340a and 340b, for example, meet at an apex 343.
As discussed in U.S. Pat. No. 6,862,387, the distance of the apex
of the optical waveguide to the reflecting surface 381 in the y
direction can be between 0 and plus or minus 14 microns. The
optical waveguides 340 and 342 can be between about 50 to 500
microns apart, measured from the centerline of the optical
waveguides, and should be sufficient to limit coupling via the
evanescent wave effect between waveguide arms. The reflective
surface 381 is not exactly at 90 degrees to the incoming light in
the optical waveguide arms. The offset from 90 degrees can be below
about 10 degrees, providing a compact modulator with more than one
pass and therefore a longer active region. Reflective surfaces can
also be formed by etching a groove in the substrate, and depositing
a material that is highly reflective at the optical frequency, as
discussed in U.S. Pat. No. 6,862,387.
[0060] Reflective surfaces can also be formed by etching a groove
in the substrate, and making use of total internal reflection if
the incident angle is sufficiently large
[0061] Light in the optical waveguide arm 340a will propagate along
the lower s-bend half and will be reflected by reflective surface
381, proceeding along the upper s-bend half to the optical
waveguide arm 340b. Similarly, light in the optical waveguide arm
342a will propagate along the lower s-bend half and will be
reflected by reflective surface 381, proceeding along the upper
s-bend half to the optical waveguide arm 342b. After being
reflected by the reflecting surfaces and modulated in the active
regions 380 between the compact s-bend turns, the signals in the
optical waveguide arms will be recombined at the output y-branch
338 and will exit the modulator through output waveguide 334. In an
alternative embodiment (not shown), each of the parallel waveguides
340 and 342 can be coherently combined using a directional coupler
providing two separate outputs from the device, without the output
y-branch 338 and output waveguide 334.
[0062] In the active regions 380 of the modulator, the optical
waveguide arms 340 and 342 follow the path of the electrodes 310,
320, and 350. Compact reflective waveguide turns in the bending
regions allow compact spacing of the adjacent active regions in the
x-direction. In alternative embodiment (not shown), the optical
waveguides can follow can follow a semicircular path in the bending
regions, however, this would require several times more space to
contain the same number of active regions or transition to a high
index gradient waveguide.
[0063] The substrate 360 is preferably formed of a crystalline
material having a high electro-optic coefficient such as lithium
niobate or lithium tantalate. Other suitable materials include
barium titanate, strontium barium niobate, a polymer, indium
gallium arsenide phosphide, indium phosphide, gallium arsenide, and
gallium aluminum arsenide. In the exemplary embodiment shown in
FIG. 3 and 4, the substrate 360 is z-cut lithium niobate, and the
optical waveguide channels 340 and 342 are formed by high
temperature indiffusion of a titanium that has been
photolithographically defined.
[0064] As illustrated in FIG. 3 and 4, the ground electrodes 310
and 320 are arranged on either side of and spaced apart from the
centrally arranged hot or active electrode 350. A cross-sectional
view of the active electrode 350 is shown in FIG. 4, corresponding
to the central linear active portion of the modulator between the
turns. The active or hot electrode 350 is arranged over both of the
parallel waveguides 340 and 342. The hot electrode 350 has a bridge
configuration, with two lower portions 352 and 354 aligned
preferably directly over the parallel waveguides 340 and 342 in the
linear portions of the active region. As illustrated in FIG. 4, the
hot electrode's lower portions 352 and 354 are approximately the
same width as the parallel waveguides 340 and 342. The lower
portions of the electrode 350 are separated by a distance that is
approximately the same width as the distance between the optical
waveguide channels 340 and 342. The space 362 can be empty (e.g.,
filled with air) or can be filled with an electrically insulating
material preferably with a low electro-optic coefficient.
[0065] As illustrated in FIG. 4, the upper portion 356 of the
bridge-shaped hot electrode 350 has a width approximately the same
as the optical waveguides 340 and 342 plus the distance between the
optical waveguides, and is aligned over and in physical contact
with the legs or lower portions of the electrode. The electrode can
be formed in two or more steps, with a base layer being deposited
first to form the lower electrode portions, and the upper portion
of the electrode being formed of subsequently applied layer or
layers of metallization. The upper portion 356 of the hot electrode
350 can have a width approximately equal to the widths of the
electrode legs plus the gap between the legs, as seen in FIG. 4.
The upper portion must be in electrical contact with the electrode
base portions, although it is not necessary that the upper portion
of the electrode be precisely aligned over the electrode base
portions. The width of the upper portion can be slightly less than
the widths of the electrode legs plus the gap between the legs. The
width of the upper portion can also be greater than the widths of
the electrode legs plus the gap between the legs, as seen in the
electrode embodiments illustrated in FIG. 13A-B.
[0066] FIG. 3 shows the electrode 350 with the upper portion 356
cut away to illustrate the alignment of the electrode legs or lower
portions 352 and 354 extending over and aligned with the optical
waveguide arms 340 and 342 in the parts of the active region 380
where the optical waveguides are approximately linear and
parallel.
[0067] A buffer layer 370 can be arranged between the z-cut lithium
niobate substrate 360 and the electrodes 310,320, and 350 at least
in the areas that will underlie the electrodes. The buffer layer
370 can be silicon dioxide or other suitable material. If the
substrate is x-cut lithium niobate, no buffer layer is necessary.
The buffer layer 370 can also be disposed over the substrate 360 in
the regions between the electrodes.
[0068] The electrode structure may be terminated with a resistor
with the characteristic impedance of the transmission line.
[0069] The active electrode and ground electrodes are configured to
match as closely as possible the velocity of the RF signal in the
active electrode 350 and the velocity of the optical signal in the
optical waveguide channels 340 and 342 in the linear region between
the turns. For example, the overall width of the active electrode
is not limited to the width of the optical waveguide, as in some
conventional MZMs, but can be equal to about twice the width of the
a waveguide plus the edge-to-edge separation between the two
waveguides, or greater. The larger size of the bridge electrode
allows much wider gap spacing between the active center electrode
350 and the ground electrodes 310 and 320. The wider gap spacing
allows the electrodes to be made much thicker (in the z direction),
resulting in lower electrical losses in the electrodes.
[0070] The surface of the substrate 360 can be removed by etching
or other suitable removal technique in the gaps between each ground
electrode 310, 320 and the hot electrode 350. The substrate surface
can also be removed in the space 362 between the two lower portions
of the active electrode 350. FIG. 4 illustrates that the in the
linear part of the active region of the modulator, the upper
surface of the lithium niobate substrate 360 has been removed by
etching in the space 362 between the active electrode legs 352, 354
and in the region 364, 366 between the ground electrodes 310, 320
and the active electrode 350. Removing a portion of the substrate
appears to improve the velocity matching between the RF and optical
signals by increasing the velocity of the RF signal in the active
electrode. Removal of a portion of the substrate also can affect
the impedance of the electrode. The bridge electrode, when
optimized to match the RF and optical velocities, has a
characteristic impedance lower than 50 ohms without etching.
[0071] Etching the substrate surface in the bend regions is
difficult to accomplish without damaging the optical waveguide
crossings near the reflective s-bends. To impedance match the
bridge electrode in the turning region, the height of the electrode
is reduced to a height less than the height in the linear active
region to obtain 50 ohms without etching the lithium niobate
substrate. For example, when the bridge electrode height is 90
microns in the linear part of the modulator, the bridge electrode
height in the turning region is 30 microns thick. The reduced
height of the electrodes in the bending region, however, can cause
a mismatch between the velocities of the RF and optical signals.
Therefore, the physical lengths of the optical waveguides and the
electrodes are selected to match the total transit time of the
optical and electrical signals through the bend region, ensuring
they are in phase as they transition through the turn into the next
linear active region. A three dimensional electromagnetic model can
be used to determine the appropriate lengths of the optical
waveguides and electrodes in the bend region and linear active
regions. FIG. 5 illustrates the bridge electrode 350 and the ground
electrodes 310 and 320 in the turning region of the modulator.
[0072] The side walls of the electrodes can be perpendicular to the
substrate, or can be slightly flaring outward so the upper portion
of the electrodes are wider than the lower portions, as illustrated
in FIG. 2 and 4. The upper portion of the electrode side walls
could also be narrower than the lower portion (not shown).
[0073] In an alternative embodiment (not shown), the optical
waveguides underlie the hot electrode even in the bend region, and
reflective s-bends are not provided at the edges of the substrate.
This embodiment, while having the advantage that etching can be
performed between the electrodes without harming the optical
waveguides, does not include compact turns, so requires more space
on the substrate.
[0074] Modulators based on x-cut lithium niobate have typically
been limited to a lower frequency range than modulators based on
z-cut lithium niobate because for x-cut modulators the optical
waveguides must go between the electrodes to utilize the larger
electro-optic coefficient, r33. This limits the electrode gap
width. Narrow gap widths require a shorter electrode height for
velocity matching which in turn results in higher losses than
experienced for the tall electrodes allowed for z-cut devices. As a
result for x-cut devices there is a trade-off between higher drive
voltage V.pi. and frequency response. However, for applications in
which the frequency range and drive voltage are not critical, the
bridge electrode described herein can be included in x-cut lithium
niobate based modulators. For such modulators, the lower portions
of the bridge electrode can be arranged on either side of a
waveguide, rather than directly aligned over the optical waveguide
arms.
[0075] The following is a description of a suitable method for
forming the electro-optic modulators of FIG. 1-5. A substrate is
suitably selected as a z-cut optical-grade commercial lithium
niobate wafer. The z plane is the plane perpendicular to the
crystal axis (z) and is the largest face of the substrate. The
wafer can be approximately 3-4 inches in diameter with a 1 mm
thickness, although larger or smaller size wafers can be used. The
wafer is cleaned in trichloroethylene, acetone, methanol,
detergent, and deionized water. Titanium is sputtered at room
temperature over the z face to a thickness of 600 angstroms.
Optical waveguides can be formed in the substrate
photolithographically by spin-coating photoresist on the substrate,
prebaking the photoresist at 90 degrees C. for 25 minutes, exposing
the photoresist to UV light through the optical waveguide
photomask, with the optical waveguides aligned along the y-axis of
the substrate. The photoresist is then developed to eliminate it,
and postbaked at 110 degrees C. for 45 minutes to fully harden it
in the optical waveguide regions. Finally, the titanium, is etched
away by the use of ethylene diamine tetraacetic acid (EDTA). The
final titanium strip width after etching is 8 um and produces
single-mode waveguides after indiffusion of titanium. The substrate
is then placed in a furnace and heated to an elevated temperature
of 1000 degrees C. for 10 hours in wet oxygen. This technique
produces high quality optical waveguides with very low propagation
losses in straight channels, for example, losses of approximately
0.1 dB/cm.
[0076] A poling mask is used to define a poling electrode over one
of the two waveguide regions. A voltage is applied to the poling
electrode to reverse the ferroelectric domains in only one of the
two waveguides. The net effect of this poling is a reduction in
modulator drive voltage. After reverse poling is completed, the
poling electrode is chemically etched away. An etch mask is then
used to define the areas on the substrate that are to be etched by
ion milling or another suitable technique. The lithium niobate
ridge (unetched area) can be slightly wider than the electrode
footprint, for example, to minimize optical loss in the active
region associated with roughness of the etched surface.
[0077] A buffer layer of silicon dioxide is deposited over the
wafer after etching of the substrate. Alternatively, the buffer
layer can be deposited prior to etching.
[0078] The layering process for the electrodes is illustrated by
FIG. 4. A first modulator electrode mask is used to define the
foundation (lower portion or legs) of the bridge electrode, and a
first layer of the ground planes. The electrodes are plated
everywhere to a height h1 of 20 microns. In a preferred embodiment
the electrodes are formed of gold. For structural support, the
volume between the legs of the bridge electrode can be filled with
a polymer or other suitable material before the upper layer is
applied.
[0079] A second modulator mask is used to define the center hot
electrode and all other electrodes, and the electrodes are plated
to an additional height h2 of 10 microns. This completes the
formation of the 30 micron electrode height in the turning
region.
[0080] A third modulator mask is used to form the mold for the
linear part of the modulator between the turns, where the hot
electrode and the ground planes are plated with another 60 microns
of electrode material (h3), for a total of 90 microns in height (in
the z direction).
[0081] The input and output horns couple RF input energy to the
first active region and from the last active region of the
modulator. The input and output horns can have a thickness of
approximately 20 microns. In this embodiment, the horns are not
impedance matched. Their length is less than the wavelength of the
RF energy, so impedance matching is not necessary. The impedance,
effective refractive index, and gold thickness in microns of the
FIG. 3-5 embodiment is as follows: TABLE-US-00001 Z gold (ohms) n
eff thickness characteristics linear active region 48 2.14 90
microns velocity and impedance matched turns 48 3.41 30 microns
impedance matched horns (RF to active) 38 2.6 20 microns not
impedance matched
[0082] A significant advantage of the modulators described herein
is that they are inherently chirp-free. For a Mach-Zehnder
modulator, chirp is the ratio of the phase modulation to the
amplitude modulation where the phase modulation is the time
averaged phase modulation for both waveguides in the Mach-Zehnder
modulator. Previous Mach-Zehnder modulators that apply different
electrical fields to the two waveguide arms are susceptible to
chirp due to the differential in the electric field applied to each
electrode over the two waveguides. In MZMs with one electrode at
ground potential (zero electrical field) and the other electrode at
the maximum applied voltage, the optical waveguide under the
grounded electrode has only a small contribution to the average
phase while the other optical waveguide has most of the modulation
phase shift.
[0083] In contrast, the electrode bridges configurations disclosed
herein and illustrated in FIG. 1-5 allows the same electrical field
to be applied to both waveguides. Therefore, the magnitude of the
optical signal's phase shift is the same in each waveguide. Since
one of the optical waveguides is reverse poled, the sign of the
phase modulation in one of the optical waveguides is reversed
relative to the other waveguide. This results in a time averaged
phase modulation of zero for equal length waveguides. Thus, the
bridge structure electrodes are inherently chirp-free.
[0084] Another advantage of the electrode configuration disclosed
herein is that the fabrication tolerances for the bridge structure
are reduced compared to coplanar waveguide structures. Although
more masks are used to define the bridge structure, only the bottom
electrode layer of 20 .mu.m requires precise alignment during the
photolithography stage. In contrast, other low-loss coplanar
waveguides not having a bridge structure require thicker electrodes
in a single step (e.g., greater than 40 microns), making the
alignment during the photolithography process much more
difficult.
[0085] For a number of modulator applications a modulator drive
voltage of 0.5 V or less is desirable. Further, operation without a
low noise amplifier between the electrical source (such as an
antenna) and the modulator can be desirable due to a lack of
electrical power locally, to minimize local power consumption, or
to eliminate distortion products created by the nonlinearities of
the amplifier. For example, with drive voltages of 0.5 V or less,
microwave transmission from antennas can be accomplished without
any amplifier at the antenna and with RF gain in the fiber optic
link and noise figure of the same order as a low noise
amplifier.
[0086] FIG. 6 and the following table illustrate the projected
drive voltage for single pass and multi-pass modulators using the
bridge electrode configuration described herein. The modulator
illustrated in FIG. 3-5 having a low-loss bridge electrode
structure, five active regions, and compact waveguide turns
provides a Lithium niobate Mach-Zehnder modulator design with a
projected drive voltage of 0.6 V or less through 20 GHz. The
modulator illustrated in FIG. 1-2 and 3-5 are projected to have a
drive voltage of 1.4 V or less through 20 GHz. TABLE-US-00002 Model
at 1.55 micron optical signal wavelength: V.pi.(V) Passes DC 10 GHz
20 GHz Interaction Length (cm) 1 1.2 1.33 1.4 7.4 3 0.4 0.55 0.67
23.4 5 0.2 0.4 0.57 39.4
[0087] Achieving the low drive voltage over the 0-20 GHz frequency
range is very useful for applications where amplifiers cannot be
used between the RF source and the modulator, particularly where
weight, size, power dissipation and power consumption are issues.
Further, by eliminating amplifiers in RF systems, the lack of
amplifier noise and distortion generated by the inter-modulation
products of the amplifier improves the sensitivity of the RF
system. System applications include wing mounted antenna array
telemetry, space based systems and commercial and military
telecommunication systems in which significant cost savings can be
achieved while increasing reliability. With drive voltages of less
than 0.5 V, microwave signals from antennas can be received without
any amplifier at the antenna and transmitted over optical fiber
with RF gain in the link and noise figure of the same order as a
low noise amplifier.
[0088] The modulators and electrodes of FIG. 1-5 are also suitable
for modulation at frequencies greater than 20 GHz.
[0089] The following discussion is provided to clarify the
advantages of providing a single electrode with reverse poled
waveguides compared to other modulator types. Four possible
configurations of a single pass modulator are as follows: [0090] 1)
a hot electrode over one waveguide with no electrode over the other
waveguide; [0091] 2) a hot electrode over one waveguide with a
ground plane over the other waveguide; [0092] 3) a hot electrode
over one waveguide with a second hot electrode over the other
waveguide, operating with two electrical driving signals which are
180 degrees out of phase; and [0093] 4) a hot electrode over one
waveguide with a second hot electrode over the other waveguide
operating with either a single electrical signal or two electrical
driving signals in phase. The optical waveguide under one electrode
is reverse poled to change the sign of the modulation.
[0094] Push-pull is a method of combining two signals that are out
of phase to get more modulation effect between the two waveguides
forming the Mach-Zehnder interferometer. The push pull method can
be implemented through electrode or optical design. The electrode
configurations of (3) and (4) are referred to as push-pull.
Electrode designs consistent with configuration (2) can also
provide a small increase in modulation efficiency compared with
configuration (1) due to non-negligible field intensity under the
ground electrode. It should also be noted that the push-pull
configurations (3) and (4) are sometimes erroneously considered to
provide twice the modulation due to the push-pull configuration. In
the case of a RF source without amplification, the power must be
divided between the two electrodes. Since these modulators respond
to the voltage developed across the electrodes, the maximum
improvement over configuration (1) modulators is 2.sup.1/2 for
typical 50 ohm systems rather than a full factor of 2.
[0095] For applications where there is no amplifier between the RF
driving source and the modulator, configurations (2) and (4) are
the most promising. Configuration (1) has the poorest effective
response of all configurations and configuration (3) requires a
wideband power divider and a low-loss 180 degree RF phase
shifter.
[0096] In order to maximize the modulator bandwidth and response,
traveling wave modulator designs are employed or device length is
shortened at the expense of increased drive voltage requirements.
The modulators are designed such that the velocity of propagation
of the optical wave is matched to the velocity of the microwave by
adjusting the geometry of the electrode. For high frequency
traveling wave modulators, as the active region length is increased
to reduce the dc drive voltage, the impact of increased electrode
losses becomes more significant, causing the response to
deteriorate rapidly at high frequency. Therefore, in order to take
advantage of increased active region length enabled by compact
reflective turns, extremely low loss electrode structures are
desired. The electrode designs described in this disclosure can
provide losses which are lower than conventional structures,
ultimately enabling less than 0.5 V drive voltages.
[0097] FIG. 7, a graph of electrode transmission versus frequency,
illustrates that the bridge electrodes as illustrated in FIG. 2 and
4 provide substantially reduced loss over other coplanar waveguide
designs. Curves A show measured results for a single pass modulator
with a 4.5 cm interaction length coplanar waveguide electrodes, a
25 micron gap, and a 32 micron height. Further details are provided
in M. M. Howerton, R. P. Moeller, A. S. Greenblatt, and R.
Krahenbuhl, "Fully packaged Broad-band LiNbO3 Modulator with Low
Drive Voltage,", IEEE Photon. Technol. Lett. 12, 792-794 (2000),
and fitted loss coefficient of 0.043 (GHz.sup.1/2-cm).sup.-1 for
this modulator. Curves B represent a modulator with improved
coplanar waveguide electrode morphology, resulting in a loss
coefficient of 0.025 (GHz.sup.1/2-cm).sup.-1. Curves C represent
measured and curve fitted results for a modulator with a coplanar
waveguide gap width of 45 microns and electrode height of
approximately 45 microns. The loss coefficient for this modulator
is .alpha..sub.0=0.015 (GHz.sup.1/2-cm).sup.-1. The height for this
electrode was slightly short of that appropriate for velocity
matching for the curve C modulator.
[0098] Curves D of FIG. 7 shows the electrical transmission for the
bridge structure disclosed herein with a 90 micron electrode height
in the active region, without etching between the lower portions of
the active electrode, and without etching between the active
electrode and the ground planes. The fitted curve D corresponds to
a loss coefficient of 0.008 (GHz.sup.1/2-cm).sup.-1.
[0099] Further reductions in electrode losses of CPW structures are
possible by increasing the electrode gap between the center
electrode and the ground plane of coplanar waveguide electrodes and
increasing the thickness of the electrode. Curve E represents the
projected loss of an optimized bridge design with etching,
calculated using a three dimensional finite element model. The
projected loss coefficient is 0.065 (GHz.sup.1/2-cm).sup.-1, which
includes both resistive and dielectric contributions.
[0100] Thus, the bridge electrode design disclosed herein provides
a substantial reduction in electrode loss compared to other
coplanar waveguide designs shown in FIG. 7.
[0101] FIG. 8 illustrates the electrode loss versus electrode gap
for coplanar waveguide structures. As the gap is increased, the
electrode height should also be increased to ensure that the
optical velocity and RF velocity are matched. FIG. 9 illustrates
the electrode transmission versus frequency for coplanar waveguide
structures. FIG. 10 illustrates the loss coefficient, dc drive
voltage V.pi. (DC) and 20 GHz drive voltage V.pi. (20 GHz). As
illustrated in FIG. 9 and 10, drive voltages of less than 0.5 V at
20 GHz are not feasible for the configuration (2) coplanar
waveguide.
[0102] As the modulators illustrated in FIG. 1-4 each have
reverse-poled waveguide arms and a single hot electrode, they
correspond best to the configuration (4) structure discussed in
previous paragraphs. Thus, the FIG. 1-4 modulators provide
significant advantages over coplanar waveguide structures.
[0103] FIG. 11 is an expanded view of the substrate and bridge
structure of FIG. 2 or 4. FIG. 12 illustrates the first layer of
metallization forming the base level of an active electrode 150 or
350 shown in FIG. 1-2 or 3-5. The active electrode 150 is spaced
apart from each ground plane by 100 microns, and each of the
electrode legs is 8 microns in width. The two optical waveguides
should be separated enough to limit optical crosstalk. Deep ion
etching as illustrated in FIG. 11 can limit crosstalk between the
optical waveguides while minimizing the gap between the optical
waveguides.
[0104] The upper portion of the active bridge electrode can extend
in the width direction beyond the outer edges of the electrode base
layer. A large surface area is preferred for decreasing the loss in
the electrode.
[0105] FIG. 13A-B illustrate cross sectional views of an
electro-optic phase modulator according to another embodiment of
the invention. An optical waveguide 640 is formed in a substrate
660 by the titanium indiffusion method described above, or by
another suitable method. An electrode 650 and ground plane 610 are
formed on the substrate. The hot electrode is spaced apart from a
ground plane 610. As seen in FIG. 13A, the electrode 650 has a
lower portion 652 aligned with the optical waveguide 630 in a
manner such that applying a RF signal to the electrode induces a
change in the refractive index of the optical waveguide, in turn
producing a velocity matched optical signal in the optical
waveguide. The lower portion 652 preferably has a width about the
same as the width of the optical waveguide. The upper portion 656
of the electrode is wider than the lower portion, and preferably is
at least twice, and more preferably, at least three times as wide
as the lower portion. For example, a presently preferred width for
the upper portion of the electrode is 32 microns when the lower
portion is 8 microns wide, with the electrode having a total height
of 90 microns. A polymer or other suitable electrically insulative
material can be deposited on the substrate in the areas 612 and 614
on either side of the lower portion of the electrode to provide a
stable platform on which to deposit the subsequent layer or layers
of the electrode.
[0106] As seen in FIG. 13B, the electrode can also have more than
one lower portion to provide a stable base for the subsequently
deposited layers of the electrode. In this embodiment, only one of
the lower portions 652 will be aligned over the waveguide.
[0107] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that the claimed invention may be
practiced otherwise than as specifically described.
* * * * *