Buffer layer structures for stabilization of a lithium...

Details Buffer layer structures for stabilization of a lithium... Buffer layer structures for stabilization of a lithium...

2002-01-04

2003-11-25

Kim, Robert H. (Department: 2882)

Optical waveguides

Temporal optical modulation within an optical waveguide

Electro-optic

C385S131000, C385S040000

Reexamination Certificate

active

06654512

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide modulator, and more particularly, to the provision of improved thermal and temporal bias stability in optical waveguide devices.
2. Discussion of the Related Art
Mach Zehnder interferometers (MZI's) used as optical modulators are of great interest for high data rate fiber optical communications systems. A great deal of research has been carried out to develop this type of device since its introduction in the mid-70's. The practicality of Ti-diffused LiNbO
3
waveguide systems has allowed wide introduction of these devices in current optical communication systems.
FIG. 1
illustrates a plan view of a related art Z-cut lithium niobate Mach-Zehnder interferometer used for modulation of an optical signal. An optical waveguide path
4
is formed inside a surface of a lithium niobate (LiNbO
3
) substrate
1
that splits into a first path
4
a
and a second path
4
b
and then recombines back into a single path
4
′. The optical waveguide paths
4
a
and
4
b
may be formed by diffusion of a metal, for example titanium, or with other dopants that will form an optical path in the lithium niobate substrate
1
. An electric field is applied to the first optical waveguide path
4
a
and the second optical waveguide path
4
b
via electrodes
2
a
and
3
, respectively, that are positioned over the first and second optical waveguide paths. Specifically, the electrode
2
a
over the first optical waveguide path
4
a
is a ground electrode and the electrode
3
over the second optical waveguide path
4
b
is an input electrode. In addition, another ground electrode
2
b
is positioned on the substrate so that ground electrodes
2
a
and
2
b
are on each side of the input electrode
3
for further control of the electric fields applied to the first and second optical waveguide paths
4
a
and
4
b
. The electrodes
2
a
,
2
b
and
3
are separated from the substrate
1
by a buffer layer
5
. The application of the electric field changes the refractive index of an optical waveguide path in proportion to the amount of electric field applied. By controlling the amount of electric field applied via the electrodes
2
a
,
2
b
and
3
, an optical signal passing through the optical waveguide paths can be modulated.
FIG. 2
is cross-sectional view of the related art Z-cut lithium niobate Mach-Zehnder interferometer along line B-B′ in FIG.
1
. The buffer layer
5
is comprised of a transparent dielectric film and is positioned between the electro-optical crystal substrate
1
and the electrodes
2
a
,
2
b
and
3
. The buffer layer
5
prevents optical absorption of the optical mode by the metal electrodes
2
a
and
3
. However, the buffer layer
5
allows electric fields that emanate from the electrode
3
to affect a refractive index change in either or both the first optical waveguide path
4
a
or the second optical waveguide path
4
b
. Typically, silicon dioxide (SiO
2
) is used as the buffer layer due to its optical transparency at 1.55 microns and its low dielectric constant.
FIG. 2
also illustrates that electro-optical crystal substrate
1
of the related art Z-cut lithium niobate Mach-Zehnder interferometer is formed so that a Y axis of the crystal orientation extends in a longitudinal direction of the lithium niobate substrate
1
along the waveguide paths
4
a
and
4
b
. The Z axis of the crystal orientation extends in the direction of the thickness of the electro-optical crystal substrate
1
such that the top and bottom surfaces of the lithium niobate substrate
1
are respectively −Z and +Z faces in terms of the crystal lattice structure of the substrate. The optical waveguide paths are commonly denoted as being within the −Z face of the lithium niobate substrate.
One of the practical difficulties in the early introduction of Z-cut LiNbO
3
devices was the pyroelectric sensitivity of LiNbO
3
, which resulted in the development of large internal fields within the devices when subjected to temperature changes or gradients across the device. This is because a change in temperature causes a change in the spontaneous polarization due to the ferroelectric properties of LiNbO
3
. As illustrated in
FIG. 2
, this results in an imbalance of charge between the Z faces of the electro-optical crystal substrate
1
, so that an electric field is generated in the Z direction perpendicularly along the waveguide paths
4
a
and
4
b
of the device. Due to the very high resistivity of LiNbO
3
, these charges take a long time to travel through the electro-optical crystal substrate
1
and neutralize themselves. This imbalance of charge impedes or lessens the effect of the electrical fields from the electrodes
2
a
,
2
b
and
3
on the waveguide paths
4
a
or
4
b
, thus decreasing the effectiveness or control in modulating optical signals. Early modulators were highly susceptible to thermal changes and strict environmental controls were necessary for thermal stabilization of the devices.
An early approach to maintain or prevent loss of modulation control due to thermal effects was to bleed off or counteract the imbalance of charge between the Z faces of a LiNbO
3
substrate. C. H. Bulmer et al. (one of the authors is an inventor in this application), “Pyroelectric Effects in LiNbO
3
Channel Waveguide Devices,” Applied Physics Letters 48, p. 1036, 1986 disclosed that metallizing the Z faces, and electrically connecting them with a high conductivity path to allow the unbalanced charge to neutralize rapidly, resulted in improved thermal stability of an X-cut device. Nonetheless, in Z-cut devices, this approach is difficult since the waveguide paths are on the Z face, and a metalized layer on this face would short out the electrodes of the device, making the device ineffective or inoperable.
Instead of a metallization layer, P. Skeath et al. (one of the authors is an inventor in this application), “Novel Electrostatic Mechanism in the Thermal Stability of Z-Cut LiNbO
3
Interferometers,” Applied Physics Letters 49, p. 1221, 1986 and I. Sawaki et al., Conference on Lasers and Electro-Optics, MF2, PP. 46-47, San Francisco, 1986 suggested a semiconducting or semi-insulating layer on the Z face under the electrodes of a Z-cut device. The semiconducting or semi-insulating layer would transfer the unbalanced charge between the Z faces of the LiNbO
3
substrate but not short out the electrodes. Although X-cut devices are commonly treated by providing metal layers or other conductive layers on the Z faces and interconnecting the conductive layers, research continues as to what semiconductor or semi-insulating layer can be best or appropriately specified for use with Z-cut devices.
Approaches attempted in the past have included Indium Tin Oxide (ITO), Silicon (Si), and Silicon Titanium Nitride (Si
x
Ti
y
N
z
) layers, which are applied in place of or above the usual SiO
2
buffer layer on a Z-cut optical waveguide device. Minford et al., “Apparatus and Method for Dissipating Charge from Lithium Niobate Devices, U.S. Pat. No. 5,949,944, Sep. 7, 1999, which is hereby incorporated by reference, proposes a silicon titanium nitride layer that has the advantage of adjustable resistivity by adjustment of the silicon/titanium ratio. However, control of the resistivity is unsatisfactory due to oxygen contamination in the silicon titanium nitride buffer layer, which results from residual background gases in the deposition system. This results in unacceptable run-to-run variation in the resistivity of a silicon titanium nitride buffer layer. Furthermore, the deposition system for a silicon titanium nitride buffer layer includes a sputtering process that requires a variety of targets with varying compositions to vary the composition of the buffer layer over a desired range and thus, is not a practical process with suitable control of the resistivity.
The effect of the electric field and the consistency of the effect of the electric field over time (i.e. temporal stabilit

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