Optical waveguides – Planar optical waveguide
Reexamination Certificate
2002-03-27
2004-11-16
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S002000, C385S008000, C385S040000
Reexamination Certificate
active
06819851
ABSTRACT:
This application claims the benefit of Japanese Application P2001-101,729, filed Mar. 30, 2001, and Japanese Application P2002-015,167, filed Jan. 24, 2002, the entireties of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical waveguide devices and travelling-wave optical modulators utilizing the same.
2. Related Art Statement
A traveling-wave optical modulator using lithium niobate (LiNbO
3
), lithium tantalate (LiTaO
3
) or gallium-arsenide (GaAs) for the optical waveguide has excellent properties and may realize a broadband modulation at a high efficiency. Lithium niobate and lithium tantalate are excellent ferroelectric materials having large electro-optic coefficients and can control light within a short optical path. Factors suppressing the modulation frequency of the traveling-wave optical modulator include velocity mismatch, dispersion, electrode conductor loss and dielectric loss.
The concept of velocity mismatch will now be further explained. In a traveling-wave optical modulator, the velocity of the light propagating through the optical waveguide largely differs from that of the signal microwave propagating through the electrode. Assume that the light and the modulation wave propagating through the crystal have different velocity Vo and Vm, respectively. For example, calculation was made for an LiNbO
3
optical modulator having planar type electrodes. The effective refractive index of LiNbO
3
single crystal is 2.15, and the velocity of the light propagating through the optical waveguide is inversely proportional to the refractive index. On the other hand, the effective index for modulating wave is given by a square root of the dielectric constant near a conductor propagating the wave. LiNbO
3
is a uniaxial crystal, and the dielectric constant in the Z-axis direction is
28
and those in the X-axis and Y-axis directions are
43
. Therefore, even if an influence of air having a dielectric constant of 1, the effective index microwave in the LiNbO
3
optical modulator having a conventional structure is about 4, which is about 1.9 times 2.15. Thus, the velocity of the light wave is about 1.9 times as much as that of the modulating microwave.
The upper limit of bandwidth “fm” of the optical modulator or the modulating velocity is inversely proportional to a difference in velocity between the light wave and the modulating wave. That is, fm=1/(Vo−Vm). Therefore, assuming that the power loss by electrode is zero, the upper limit of bandwidth “fm” x electrode length “
1
”=9.2 GHz.cm. Actually, it has been reported that in an optical modulator having an electrode length “
1
”=2.5 mm, fm=40 GHz. The effect due to the upper limit of modulation speed becomes more substantial as the electrodes become longer. Therefore, an optical modulator having a broadband modulation and low driving voltage has been earnestly demanded.
The inventors have considered the following idea. That is, the velocity matching between signal microwave and light wave may be realized by applying a thin plate with a thickness of, for example, 10 &mgr;m for an optical waveguide substrate. It is, however, difficult to obtain a plate with such a small and uniform thickness by grinding. The resulting plate has a low strength and may easily be warped so that it may be useless.
The assignee filed a Japanese patent laid-open number 133, 159/1998 (U.S. Pat. No. 6,219,469), and disclosed a travelling wave optical modulator for giving the solution. The modulator has an optical waveguide substrate having a thinner portion with a thickness of not more than 10 &mgr;m where the optical waveguide is formed. It is thereby possible to realize high-speed modulation without forming a buffer layer made of silicon dioxide, and to advantageously reduce the product “V&pgr;·L” of a driving voltage V&pgr; and a length of an electrode “L”.
SUMMARY OF THE INVENTION
The inventors have studied the whole process for manufacturing a travelling wave optical modulator. They have tried to form a recess on the surface of an optical waveguide substrate by machining, as described in the Japanese Patent 133, 159/1998, to form a thinner portion with a thickness of, for example, not more than 10 &mgr;m. They found the following problems.
FIG. 10
schematically shows such substrate
16
. A deep recess
17
is formed by, for example laser beam working or grinding, from the back face
16
b
of the substrate
16
. The substrate
16
has a thickness of, for example, 0.3 mm and the thinner portion
16
c
has a thickness of, for example, 10 &mgr;m. A thicker portion
16
a
remains after the working in the substrate
16
to preserve the mechanical strength.
16
d
is a worked surface.
In an actual working process, however, it may be difficult to form the recess with an ideal shape shown in FIG.
10
. For example, the recess is formed by laser beam working using a lens. As the recess
17
is deeper, the focus of the lens moves so that the worked surface
16
d
is curved or rounded. It is therefore difficult to maintain the thickness of the thinner portion
16
c
at a specific value. The thickness of the central part of the thinner portion
16
c
tends to be considerably smaller than that of the peripheral parts of the thinner portion. As a result, when the working is performed so that the thickness of the thinner portion
16
c
is maintained not larger than a specified value, for example not larger than 10 &mgr;m, over a sufficiently wide area, the thickness of the central part of the thinner portion
16
c
becomes considerably smaller than 10 &mgr;m. In other words, if the thickness of the peripheral part of the thinner portion
16
c
is adjusted to 10 &mgr;m, the central part of the thinner portion
16
c
is made considerably smaller than 10 &mgr;m. Such thin central part with a thickness of smaller than 10 &mgr;m may easily be broken as
16
e
. When the substrate
16
is worked using a grinding stone, the above problems may not be avoided.
An object of the invention is to provide an optical waveguide device having an optical waveguide substrate having a mechanical strength sufficient for handling, being effective for reducing off-specification products due to warping, cracks and fracture in the substrate, and effective for improving the propagating velocity of signal wave applied onto its electrode.
Another object of the invention is to apply the above optical waveguide device to a travelling-wave optical modulator to improve the velocity matching of signal wave applied onto its electrode and light wave propagating through the optical waveguide.
The invention provides an optical waveguide device comprising an optical waveguide substrate and a supporting substrate supporting the optical waveguide substrate wherein the optical waveguide substrate comprises a main body made of an electro-optic material and having a first main face and a second main face, an optical waveguide formed in or on the main body and an electrode formed on the side of the first main face of the main body. The supporting substrate is joined with the second main face of the main body, and a low dielectric portion with a dielectric constant lower than that of the electro-optic material is formed in the supporting substrate.
The invention further provides a travelling-wave optical modulator having the device, wherein a voltage for modulating the light propagating through the optical waveguide is applied by means of the electrode.
The inventors studied the problems described above, and have reached the conclusion that the problems might not be totally avoided in a process of forming the thicker portion
16
a
for improving the strength and an space
17
for improving the propagating velocity of microwave signal in the substrate
16
, as shown in FIG.
10
. The inventors have tried to ensure a sufficient strength of a device by providing a separate supporting substrate. That is, the supporting substrate is joined with an optical waveguide substrate to provide a mecha
Aoki Kenji
Kondo Atsuo
Kondo Jungo
Mitomi Osamu
Burr & Brown
NGK Insulators Ltd.
Ullah Akm Enayet
LandOfFree
Optical waveguide device and a travelling-wave optical... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical waveguide device and a travelling-wave optical..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical waveguide device and a travelling-wave optical... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3325975