Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2001-03-30
2003-06-24
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S003000, C385S008000, C385S009000
Reexamination Certificate
active
06584240
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 the data transmission rate increases in recent years, optical modulators for modulating a data signal from an electric signal into an optical signal are developed energetically in the field of long distance communication systems for communication such as submarine optical communication.
One of such optical modulators as just described is, for example, such a single drive optical modulator
20
as shown in FIG.
8
. Referring to
FIG. 8
, the single drive optical modulator
20
shown includes a substrate
21
on which a Mach-Zehnder optical waveguide
22
is formed, and an electrode
23
formed integrally on the substrate
21
and including a single signal electrode
23
A and a grounding electrode
23
B.
FIG. 9
is a sectional view taken along line A-A′ of the single electrode optical modulator
20
shown in FIG.
8
. As seen in
FIG. 9
, the single electrode optical modulator
20
is configured such that the electrode
23
is formed on the substrate
21
, which is made of, for example, lithium niobate (LiNbO
3
) and cut (Z-axis cut) in the Z-axis direction of the crystal orientation, together with the Mach-Zehnder optical waveguide
22
.
The Mach-Zehnder optical waveguide
22
is formed by thermal diffusion of titanium (Ti) or a like substance on the substrate
21
and includes a Y branching waveguide
22
A and two straight arm waveguides
22
B-
1
and
22
B-
2
on the light incoming side and a Y branching waveguide
22
C on the light outgoing side. The electrode
23
includes the single signal electrode
23
A and the grounding electrode
23
B and converts, when a voltage signal (microwave) of, for example, NRZ (Non Return to Zero) data or the like is applied to the signal electrode
23
A, the voltage signal into an NRZ optical signal.
As shown in
FIG. 8
, the single signal electrode
23
A is formed so as to establish electric connection between two connector contacts on a one-side edge portion of the substrate
21
in its widthwise direction, and is formed such that part of it extends along and above the portion at which the straight arm waveguide
22
B-
1
is formed. Further, the grounding electrode
23
B is formed such that it is disposed on the opposite sides of the single signal electrode in a spaced relationship by a predetermined distance thereby to form a coplanar line on the substrate
21
.
When light from a light source not shown is introduced into the single electrode optical modulator
20
having the configuration described above with reference to
FIGS. 8 and 9
, while the light propagates in the Mach-Zehnder optical waveguide
22
, it is modulated into an NRZ optical signal by the signal electrode
23
A to which a voltage signal (microwave) of NRZ data or the like is applied. The modulated NRZ optical signal goes out of the single electrode optical modulator
20
.
Where such a single electrode optical modulator
20
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 suppress the loss of a microwave which advances through the electrode and suppress the deterioration of the extinction ratio.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical modulator which suppresses the loss of a microwave which advances through an electrode and loss of light which propagates in a waveguide and makes the losses in individual arm waveguides substantially equal to each other to suppress the deterioration of the extinction ratio to improve the transmission quality.
In order to attain the object described above, according to an aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect and having formed thereon a ridge, first and second grooves which are positioned on the opposite sides of the ridge, 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 and including a first Y branching waveguide, first and second arm waveguides which are branched from the first Y branching waveguide and one of which is included in the ridge, and a second Y branching waveguide at which the first and second arm waveguides join together, an electrode formed on the substrate and including a signal electrode formed on the ridge and a grounding electrode formed on the first and second banks for controlling light propagating in the optical waveguide, and first and second recesses formed at symmetrical positions with respect to the ridge on the first and second banks, respectively.
With the optical modulator, since the first and second recesses are formed at the symmetrical positions with respect to the ridge
14
a
on the first and second banks, respectively, also the electric field distribution in the substrate by an electric signal provided to the signal electrode can be made symmetrical with respect to the ridge. Consequently, the optical modulator is advantageous in that the loss of a microwave which advances through the signal electrode can be suppressed.
Preferably, the substrate is made of LiNbO
3
, and more preferably, the substrate made of LiNbO
3
is a Z-axis cut substrate.
The optical modulator may be configured such that the grounding electrode is provided on the first and second recesses and an air gap is formed in each of the first and second recesses or part of the grounding electrode is filled in the first and second recesses.
Preferably, the ridge and the first and second banks have top faces which are set in a substantially same level with one another, and more preferably, the first and second recesses have a depth set substantially equal to the depth of the first and second grooves.
Preferably, the signal electrode contacts with the ridge with a width smaller than the width of the ridge.
Preferably, a buffer layer is formed between the substrate and the electrode, and more preferably, the buffer layer is provided also in the first and second recesses.
Preferably, a silicon layer is placed on the substrate, and more preferably, the buffer layer is provided also in the first and second recesses.
The optical modulator is advantageous in that the absorption loss of light which propagates in the optical waveguide can be suppressed by the buffer layer and electric charge generated by a pyroelectric effect can be made uniform by the silicon layer to suppress the variation of the operating point by a temperature variation.
Further, since the buffer layer or the silicon layer is formed also in the first and second recesses
13
-
1
,
13
-
2
, the optical modulator is advantageous also in that adjustment of the characteristic impedance, which should be kept to a fixed value set in advance, and the speed matching between a microwave and light can be performed readily by setting of the thickness of the buffer layer or the silicon layer.
Preferably, one of the first and second arm waveguides is provided at a location of the other one of the first and second banks nearer to the ridge than a corresponding one of the first and second recesses.
With the optical modulator, since the one arm waveguide is provided nearer to the ridge than the other recess, the structure of the substrate portion at which the arm waveguide which is not included in the ridge is formed can be formed substantially same as the structure of the ridge. Therefore, the optical modulator is advantageous in that the losses of the arm waveguides can be made substantially equal to each other to suppress the deterioration of the extinction ratio.
According to another aspect of the present invention, there is provided an optical modulator, comprising a Z-axis cut substrate made of LiNbO
3
and having for
Doi Masaharu
Ohyama Masaaki
Sugiyama Masaki
Taniguchi Shinji
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