Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-07-01
2004-08-24
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S002000, C385S008000, C385S131000
Reexamination Certificate
active
06782166
ABSTRACT:
BACKGROUND OF THE INVENTION
As optical data processing circuits approach multigigahertz operation rates, the need arises for high-speed optical interconnects for signal transmission and routing. This requires high-speed electro-optic modulators and switches that convert the electronic signal to optical. The electronic circuits that drive the electro-optic modulators and switches provide low voltage levels at high speeds. This dictates the performance goals required for the electro-optic modulators and switches. Currently employed electro-optic modulators and switches have drive voltages much too large for high-speed operation.
Electro-optic modulation or switching is represented generally in the top view drawing of
FIGS. 1
a
and
1
b
. As the basic form of an electro-optic switch, the directional coupler, shown in
FIGS. 1
a
and
1
b
is known, with
FIG. 1
a
showing a switch without applied drive or modulation or switching voltage and
FIG. 1
b
showing a switch with applied drive or modulation or switching voltage. A directional coupling type of electro-optic switch is one that controls the transfer of the optical signal by causing the, index of refraction of the switch's coupling portion to change by an electro-optic effect. The
FIG. 1
a
top view illustrates a switch having wave guides etched from a wafer including a layer of non-linear electro-optic polymer material.
Parallel channel wave guides separated by a finite distance for receiving one or more optical signals are represented in both
FIGS. 1
a
and
1
b
at
101
,
102
and
112
and
113
, respectively. A single optical input signal is considered for purposes of the present discussion and is represented by the bold arc at
103
and
114
in
FIGS. 1
a
and
1
b
, respectively. A symmetric mode of the optical signal, as represented at
105
, and an antisymmetric mode of the optical signal as represented at
104
in
FIG. 1
a
and at
116
and
115
in
FIG. 1
b
, respectively, are generated upon entering the switch and these modes travel along the length of the channel or switch, over such lengths as are represented at
106
and
121
in
FIGS. 1
a
and
1
b
respectively. The phase of the two modes shift as the respective signals travel the length of the wave guides, as is represented in the dotted, curved lines, shown at
108
In
FIG. 1
a
and at
119
in
FIG. 1
b
and the solid, curving lines shown at
107
and
120
in
FIGS. 1
a
and
1
b
, respectively. The symmetric mode is the mode of propagation within the other wave guide region. With no voltage applied to the
FIG. 1
switches, complete transfer of light from one channel to the next occurs at a distance that introduces a voltage independent &pgr;/2 phase shift to the modes so that the one mode couples completely to the other. Complete mode coupling and light transfer occurs at the output wave guides at
126
in
FIG. 1
a
and thereafter the complete optical signal at
111
exits the wave guide at
128
in
FIG. 1
a.
Applying an electric field to one of the channels of the directional coupler of
FIG. 1
b
over the distance L represented at
121
from the voltage source shown at
122
in
FIG. 1
b
will alter the dielectric properties of the coupler's non-linear polymer material subjected to the electric field, hence changing the index of refraction of the material and introducing a voltage dependent &pgr;/2 phase shift in the signal modes
115
and
116
and thereby modulation or switching the wave guide from
129
to
130
through which the signal exits as represented at
125
in
FIG. 1
b.
Past research has focused on exploiting the electro-optic properties of non-linear electro-optic polymers with optimized optical, structural and mechanical properties to achieve high performance electro-optic devices, such as modulators and switches. Non-linear electro-optic polymers have several attractive potential characteristics that many researchers have tried to capitalize on over the past decade. These include a high non-linearity or electro-optic coefficient enabling potential low voltage operation. a low dielectric constant for high speed modulation, low temperature processing enabling integration of optics with electronics, excellent refractive index match with optical fiber materials and simplified fabrication for lower cost.
Several technical barriers have heretofore prevented the use of non-linear electro-optic polymers from progressing toward commercialization thus far. Breakthroughs in the development of non-linear electro-optic polymers over the last couple of years have demonstrated 100+pm/V electro-optic coefficients for potential low voltage electro-optic device operation. This has led to a recently reported milestone of less than 1 Volt operation voltage. However, even though device modulation and modulation or switching voltages have been dramatically reduced by utilizing these new materials, the resulting modulation or switching voltages are still much higher than required for high speed operation.
In considering modulation and modulation or switching voltages, one must first determine those parameters that affect modulation voltage for electro-optic devices. The voltage necessary to realize the desired &pgr; phase retardation for a conventional transverse electro-optic modulator is defined as the half wave voltage V&pgr; and is given by
V
π
=
λ
d
n
3
r
33
l
,
(
1
)
where &lgr; is the wavelength, d is the thickness of the electro-optic material, n is the index of the electro-optic material, r
33
is the electro-optic coefficient of the electro-optic material and 1 is the length of the interaction region. For a given geometry, V&pgr; will be inversely proportional to the electro-optic coefficient r
33
. Thus, it is desired to maximize r
33
in order to minimize V
&pgr;
Now, the value for r
33
is determined by previous application of a large poling field across the active polymer film when heated to near its transition temperature T
g
and then allowed to cool to room temperature while keeping the electric field applied. This poling field is chosen to be as large as possible, yet just less than that which would result in dielectric breakdown of the material.
However, practical non-linear electro-optic polymer based electro-optic modulators and switches require polymer cl adding layers in addition to the non-linear electro-optic polymer core in order to confine the optical signal within the core region. The cladding layers control how much poling voltage is dropped across the core region and thus controls the non-linearity or electro-optic coefficient r
33
. The present invention overcomes the barriers to commercial use of non-linear electro-optic polymers by maximizing the poling efficiency of and in-turn maximizing the electro-optic coefficient of non-linear electro-optic polymer materials making up the core layer within an electro-optic wave guide device structure that includes cladding layers and conductive charge sheet layers. The present invention will render lower operating voltages, shorter device lengths and also reduce optical propagation loss.
SUMMARY OF THE INVENTION
The present invention provides a non-linear electro-optic polymer based, integrated optic, electro-optic device utilizing a non-linear electro-optic polymer for the optical wave guide core layer sandwiched between two very thin optically transparent electrically conductive charge sheet poling electrode layers which are, in turn, sandwiched between two optical wave guide cladding layers.
It is an object of the present invention to provide a non-linear electro-optic polymer based, integrated optic, electro-optic device having a maximized electro-optic coefficient.
It is another object of the present invention to provide a non-linear electro-optic polymer based, integrated optic, electro-optic device having minimized device operating voltages.
It is another object of the invention to provide a non-linear electro-optic polymer based, Integrated optic, electro-optic device having maximized realizable device speed.
Grote James G.
Nelson Robert L.
Hollins Gerald B.
Kundert Thomas L.
Palmer Phan T. H.
Tollefson Gina S.
United States of America as represented by the Secretary of the
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