Coplanar integrated optical waveguide electro-optical modulator

Details Coplanar integrated optical waveguide electro-optical modulator Coplanar integrated optical waveguide electro-optical modulator

2002-06-28

2004-07-06

Lee, John R. (Department: 2881)

Optical waveguides

Directional optical modulation within an optical waveguide

Electro-optic

C385S004000, C385S002000, C385S003000, C385S009000, C359S315000, C359S316000, C359S319000, C359S320000, C359S322000

Reexamination Certificate

active

06760493

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of integrated optics. More specifically, the invention relates to integrated optical waveguide electro-optical modulators, that is devices based on the electro-optic effect in which optical beams propagate through optical waveguides integrated in an electro-optic substrate material, particularly optical intensity, i.e. amplitude, interferometric modulators of the Mach-Zehnder type. Still more particularly, the invention relates to a coplanar integrated optical waveguide electro-optical modulator, in which the electrodes necessary for applying a modulating electric field are arranged on a same substrate surface.
2. Technical Background
Integrated electro-optical devices, such as modulators and switches, are fabricated on substrates of electro-optic material. Among all the known substrate materials, lithium niobate (LiNbO
3
) is probably the most widely used because of the enhanced electro-optic properties thereof and the possibility of making low loss optical waveguides. Another known substrate material is for example lithium tantalate (LiTaO
3
).
Electro-optic materials show an electro-optic response, a second-order non-linear property which is characterized by a tensor. This tensor relates the polarization changes at optical frequencies (i.e., refractive index changes) of the material to low-frequency modulating electric fields, that is modulating electric fields at frequencies much lower than those of the optical fields. Phase and amplitude modulation of optical fields can be obtained by applying external electric fields, which modify the material refractive index via the electro-optic effect.
Overlooking, for simplicity, the tensorial nature of the electro-optic effect, the refractive index change &Dgr;n(&ohgr;) at the optical frequency &ohgr; is proportional to the product of an electro-optic coefficient r and the modulating electric field Eo: &Dgr;n(&ohgr;)∝r·Eo.
In the case of a LiNbO
3
crystal the electro-optic coefficient having the highest value is r
33
≈30 pm/V. The electro-optic coefficient r
33
relates the refractive index change experienced by electromagnetic waves polarised along the c (also called z) crystal axis to the component of the modulating electric field along the same axis.
For this reason LiNbO
3
crystal substrates are generally made available in z-cut slices, with the z crystal axis normal to the substrate surfaces of largest area, since this configuration is the one ensuring superior modulation performances even at relatively high modulation frequencies.
A Mach-Zehnder interferometric electro-optical modulator is a device capable of providing an electrically-induced amplitude modulation of an optical signal. In a Mach-Zehnder interferometric electro-optical modulator the voltage required to drive the modulator is reduced when the two optical modes propagating along the two interferometer arms experience changes of the refractive index having opposite sign. This is achieved by properly designing the electrode geometry, so that the component of the modulating electric field along the z axis has opposite signs (i.e. opposite orientations with respect to the z axis orientation) in the two interferometer arms. The resulting device is said to have a push-pull configuration.
FIGS. 1
to
4
show typical examples of push-pull Mach-Zehnder interferometric electro-optical modulators. Specifically,
FIGS. 1 and 2
schematically show, respectively in top-plan and in cross-sectional views, a so-called coplanar waveguide (“CPW”) configuration.
FIGS. 3 and 4
schematically show, again in top plan and in cross-section, a so-called double coplanar strip (“CPS”) configuration.
The electro-optic performances of devices based on the above cited configurations are extensively described in literature. For example, the performances of the CPW configuration is discussed in K. Noguchi et al., ‘40-Gbit/s Ti:LiNbO3 optical modulator with a two-stage electrode’, IEICE Trans. Electron., vol. E81-C, p. 316 (1998) and in K. Noguchi et al., ‘Millimiter-wave Ti:LiNbO3 optical modulators’, J. of Lightwave Tech., vol.16, p.615 (1998). The double CPS configuration is for instance described in U.S. Pat. No. 5,388,170.
Referring to
FIGS. 1 and 2
, in a z-cut LiNbO
3
substrate
1
a Mach-Zehnder interferometer is integrated comprising an input optical waveguide
2
or input channel, a first Y-junction
3
for splitting an input optical signal propagating along the input waveguide
2
into two optical signals propagating along two generally parallel optical waveguides
41
,
42
forming the interferometer arms, a second Y-junction
5
, spaced apart from the first Y-junction, for combining the two optical signals into an output optical signal propagating along an output optical waveguide
6
or output channel. The waveguides
2
,
41
,
42
and
6
are formed by conventional techniques in correspondence of a surface
7
of the substrate
1
perpendicular to the z crystal axis. The substrate forms a single ferroelectric domain so that throughout the substrate the z crystal axis keeps a same orientation, for example the orientation shown by the arrow in FIG.
2
.
In the region of the interferometer arms, a first metal electrode
8
is superimposed over the surface
7
above the waveguide
42
and extends for a section
421
thereof, a second metal electrode
9
is superimposed over the surface
7
above the waveguide
41
and extends for a section
411
thereof substantially corresponding to the section
421
of the waveguide
42
, and a third metal electrode
10
is superimposed over the surface
7
and extends, laterally to the second electrode
9
and on the opposite side of the first electrode
8
, for a segment substantially corresponding to the section
411
of waveguide
41
. Conventionally, a buffer layer
11
, typically of silicon dioxide (SiO
2
), is formed over the surface
7
for separating the metal electrodes
8
,
9
and
10
from the optical fields in the waveguides
41
,
42
so to avoid attenuation of said optical fields.
The electrodes
8
,
9
and
10
are used for applying a modulating electric field useful for varying, by electro-optic effect, the refractive index in the two waveguides
41
,
42
. The electrodes
8
and
10
are electrically connected to a reference potential (ground), and are therefore called ground electrodes. The electrode
9
is electrically connected to a modulating potential V, and is called hot electrode. The shape and layout of the electrodes are properly designed so as to allow the operation of the device up to the microwave region of the spectrum of the modulating electric field. By applying a modulating electric field, the refractive index of the two waveguides
41
,
42
undergoes opposite variations and the optical signals propagating along such waveguides correspondingly undergo opposite phase shifts (push-pull effect). An amplitude modulated output optical signal is thus obtained in waveguide
6
, the amplitude depending on the overall phase shift.
The main disadvantage of the CPW configuration is the asymmetry of the structure, which gives rise to an asymmetry in the interaction between each optical mode propagating along the interferometer arms and the modulating electric field. Such an asymmetry causes different phase shifts in the two interferometer arms, thus inducing chirps in the phase of the amplitude modulated output optical field. This asymmetry is inherent to the device, since in order to have opposite phase shifts in the two interferometer arms the two waveguides must be placed one under the hot electrode and the other under the ground electrode. The efficiency of the phase shift induced on the optical mode propagating through a waveguide by the modulating electric field depends on the overlap between the modulating electric field and the optical mode, and is expressed by an overlap factor &Ggr;. The ratio of the overlap factors &Ggr;h in the waveguide
41
under the hot electrode
9
and &Ggr;g in the waveg

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