Optical waveguides – Integrated optical circuit
Reexamination Certificate
2001-04-05
2003-06-17
Feild, Lynn D. (Department: 2839)
Optical waveguides
Integrated optical circuit
Reexamination Certificate
active
06580843
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
This invention relates to an optical modulator suitable for use in the field of long distance optical communication systems.
2) Description of the Related Art
As data transmission rates have increased in recent years, optical modulators for modulating a data signal from an electric signal into an optical signal are being developed energetically in the field of long distance communication systems such as submarine optical communication.
An example of an optical modulators as just described is dual drive optical modulator
30
as shown in FIG.
22
. Referring to
FIG. 22
, the dual drive optical modulator
30
shown includes a substrate
31
on which a Mach-Zehnder optical waveguide
32
is formed, and an electrode
33
formed integrally on the substrate
31
and including two signal electrodes
33
A-
1
and
33
A-
2
and a grounding electrode
33
B. The dual drive optical modulator
30
modulates incoming light from a light source not shown with an NRZ data signal.
FIG. 23
is a sectional view taken along line A-A′ of the dual drive optical modulator
30
shown in FIG.
22
. As seen in
FIG. 23
, the dual drive optical modulator
30
is configured such that the electrode
33
is integrally formed on the substrate
31
, which is made of, for example, lithium niobate (LiNbO
3
) and cut in the Z-axis direction of the crystal orientation (Z-axis cut), together with the Mach-Zehnder optical waveguide
32
.
The Mach-Zehnder optical waveguide
32
is formed by thermal diffusion of titanium (Ti) or a like substance on the substrate
31
and includes a Y branching waveguide
32
A and two straight arm waveguides
32
B-
1
and
32
B-
2
on the light incoming side and a Y branching waveguide
32
C on the light outgoing side.
The electrode
33
is formed partially on the substrate
31
with a buffer layer
35
(refer to
FIG. 23
) interposed therebetween and includes the two signal electrodes
33
A-
1
and
33
A-
2
and the grounding electrode
33
B.
The electrode
33
can modulate incoming light into an NRZ optical signal by applying NRZ data signals from NRZ data signal generators
34
A and
34
B which are hereinafter described as electric signals to the signal electrodes
33
A-
1
and
33
A-
2
.
As shown in
FIG. 22
, the signal electrodes
33
A-
1
and
33
A-
2
are each formed so as to establish an electric connection between two connector contacts on a one-side edge portion of the substrate
31
in its widthwise direction. Further, the signal electrode
33
A-
1
is formed such that part of it extends along and above the portion at which the straight arm waveguide
32
B-
1
is formed. Further, the grounding electrode
33
B is formed such that it is disposed on the opposite sides of the signal electrodes
33
A-
1
and
33
A-
2
in a spaced relationship by a predetermined distance thereby to form a coplanar line on the substrate
31
.
The NRZ data signal generator
34
A applies a voltage signal (microwave) as an NRZ data signal to the signal electrodes
33
A (
33
A-
1
and
33
A-
2
). The NRZ data signal generator
34
B applies a voltage signal (microwave) as an NRZ data signal to the signal electrode
33
B.
When light from a light source (not shown) is introduced into the dual drive optical modulator
30
having the configuration described above with reference to
FIG. 22
, while the light propagates in the Mach-Zehnder optical waveguide
32
, it is modulated into an NRZ optical signal by the signal electrodes
33
A-
1
and
33
A-
2
to which a voltage signal (microwave) of NRZ data or the like is applied.
In order to design an optical device for which high speed operation is required such as an optical modulator described above, it is necessary as a basic design item to take several parameters into consideration including (1) the drive voltage, (2) the velocity match between the optical signal and the electric signal, (3) the attenuation constant of the electric signal, (4) the characteristic impedance (normally 50 &OHgr;), (5) the wavelength chirp amount, and (6) the loss. Particularly, it is important for improvement in power consumption and transmission quality of the apparatus to lower the drive voltage of the optical device.
Where such a dual drive optical modulator as described above is used to modulate a voltage signal into a data optical signal of a transmission rate particularly of 10 Gb/s or more, preferably of approximately 40 Gb/s, it is a significant subject for improvement of the transmission quality to lower the drive voltage while arbitrating with the values of the other evaluation parameters such as the velocity match between the optical signal and the electric signal as described above.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical device which can lower the drive voltage which is used as one of the parameters for performance evaluation of the optical device.
In order to attain the objective described above, according to an aspect of the present invention, there is provided an optical device, comprising a substrate having an electro-optical effect and having formed thereon first and second ridges which extend in parallel to each other, first and second grooves which are positioned on the outer sides of the first and second ridges, respectively, a third groove which is positioned between the first and second ridges, and first and second banks which are positioned on the outer sides of the first and second grooves, respectively, a Mach-Zehnder optical waveguide formed on the substrate such that the Mach-Zehnder optical waveguide branches at a first Y branching waveguide into a first arm waveguide included in the first ridge and a second arm waveguide included in the second ridge and then joins together at a second Y branching waveguide, and electrode means formed on the substrate and including a first signal electrode formed on the first ridge, a second signal electrode formed on the second ridge and a grounding electrode formed on the first and second banks and the third groove for controlling light which propagates in the optical waveguide, the grounding electrode extending to the first groove adjacent to the first bank and the second groove adjacent to the second bank.
In the optical device, while incoming light propagates in the optical waveguide, an electric signal is applied to the electrode means to control the light which propagates in the optical waveguide. Since the grounding electrode which is a component of the electrode means extends to the first groove adjacent to the first bank and the second groove adjacent to the second bank, the drive voltage of the electric signal to be applied to the electrode means is lowered.
Accordingly, with the optical device, since the grounding electrode extends to the first groove adjacent to the first bank and the second groove adjacent to the second bank, the amplitude values of the voltages (drive voltages) to be applied to the first and second signal electrodes can be reduced while maintaining a favorable modulation performance. Consequently, the optical device is advantageous in that the transmission quality can be improved, power can be saved, and the performance of the optical modulator can be improved.
Preferably, in the optical device, a third bank is formed at a middle position between the first and second ridges in the third groove. In this instance, the grounding electrode may extend to the third groove adjacent to the opposite sides of the third bank, and the ridges may have a top face level substantially equal to that of the banks.
In the optical device, preferably the substrate is made of LiNbO
3
. The substrate made of LiNbO
3
may be Z-axis cut. The ridges may have a top face level substantially equal to that of the banks. The grooves may have depths set substantially equal to each other.
In the optical device, preferably the signal electrodes contact with the respective corresponding ridges with a contact width smaller than the width of the ridges. Preferably, a buffer layer is formed between the substrate
Doi Masaharu
Norimatsu Masaaki
Ohyama Masaaki
Taniguchi Shinji
Feild Lynn D.
Fujitsu Limited
Le Thanh-Tam
Staas & Halsey , LLP
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