Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
1998-06-29
2002-04-09
Nguyen, Thong (Department: 2872)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S007000, C385S040000
Reexamination Certificate
active
06370308
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to acousto-optical devices, and more particularly to an acousto-optical device having a light waveguide path formed on an acousto-optical substrate and a transducer which crosses the acousto-optical waveguide path and propagates a surface acoustic device along the acousto-optical waveguide path, in which various mutual actions are caused in a light wave propagated through the light waveguide path under a surface acoustic wave controllable by an electric signal applied to the transducer.
An optical filter device is used in terminal equipment or a repeater or relay device in an optical communication system in order to separate signal lights which are transmitted in a wavelength-multiplexed formation. The mutual action of a surface-acoustic wave and light can realize a tunable wavelength filter, and the optical system can flexibly be constructed.
Examples of typical filter structures utilizing the surface-acoustic wave are as follows. A structure uses a TE-TM mode transducer which transduces a TE/TM wave of a wavelength input light to a TM/TE wave in combination with a polarizer which extracts a particular polarized wave. Another structure uses the Bragg diffraction. Yet another structure uses a coupling of the even and odd modes in a directional coupler. The above structures can realize a light-intensity modulator and an optical switch in such a way that the structures do not function as a filter.
2. Description of the Related Art
FIG. 1A
is a perspective view of a conventional TE-TM mode transducer. As shown in
FIG. 1A
, the transducer is made up of an acousto-optical substrate
1
, high-density Ti diffused areas
2
a
and
2
b
, a diffused light waveguide path (channel)
3
, an interdigital transducer
4
, and acoustic-wave absorbers
5
a
and
5
b
. The acousto-optical substrate
1
is made of, for example, an X-cut plate (Y-axis propagation) of LiNbO
3
. The transducer
4
excites a surface acoustic wave (SAW) in an area including the waveguide path
3
, and has finger electrodes
4
a
and
4
b
formed of a metal such as aluminum. The absorbers
5
a
and
5
b
are made of an acoustically soft material such as wax or rubber.
The high-density Ti diffused areas
2
a
and
2
b
are located on both sides of the substrate
1
and function to increase the acoustic velocity therein. Hence, SAW power is contained within the surface area of the substrate
1
sandwiched between the areas
2
a
and
2
b.
The Ti waveguide path
3
provided in the longitudinal direction of the substrate
1
and located in the center thereof is formed by thermally diffusing Ti. The thermal diffusion method can change the refractive indexes n
o
and n
e
of the LiNbO
3
substrate with respect to ordinary light (ray) and extraordinary light by an almost identical degree.
The SAW is generated by utilizing the piezoelectricity of LiNbO
3
in such a way that an RF (high frequency) signal is applied across the finger electrodes
4
a
and
4
b
of the transducer
4
which is directly mounted on an end surface portion (light input side) of the substrate
1
. The distance
1
between the finger electrodes
4
a
and
4
b
and the wavelength &Lgr; of the SAW has a relationship such that
1
=&Lgr;/
2
. In this case, the SAW power generated in the substrate
1
is defined by multiplying the RF signal input power by an efficiency. The SAW oriented to the light input side is acoustically absorbed by the absorber
5
a
and thus disappears immediately. The SAW directed to the light output side is propagated on the substrate portion between the areas
2
a
and
2
b
at an acoustic velocity v.
In a case where a polarized wave of TE-mode (or TM-mode) light is applied to the input end of the waveguide path
3
in the above state, the plane of polarization of the polarized wave is turned by 90° due to the acoustic-optical effect of the SAW propagated on the substrate
1
when the wave has traveled a given action length L. Hence, the polarized wave of TE-mode (or TM-mode) light is transduced into that of TM-mode (or TE-mode) light. The above rotation can be controlled by the power of the SAW. The absorber
5
b
is located in the above position. Hence, the mutual action to the SAW does not occur in the waveguide path
3
, and thus the polarized wave of the TM (or TE) mode can be obtained via the output end of the waveguide path
3
.
The following phase matching condition is satisfied in the above case:
&LeftBracketingBar;
β
TE
-
β
TM
&RightBracketingBar;
=
(
2
π
/
λ
)
&LeftBracketingBar;
N
TE
-
N
TM
&RightBracketingBar;
=
2
π
/
Λ
=
2
π
f
/
v
(
1
)
where &bgr;
TE
and &bgr;
TM
respectively denote the propagation constants of the waveguide modes TE and TM, N
TE
and N
TM
respectively denote the effective refractive indexes of the modes TE and TM, &Lgr; denotes the wavelength of the SAW, f denotes the frequency, and v denotes the phase velocity. The mode transduction is caused due to the SAW of the frequency f which satisfies equation (1), and the transduction efficiency can be controlled by the SAW power.
The following equation (2) can be obtained from equation (1):
&lgr;=&Lgr;|N
TE
−N
TM
| (2)
A numerical example will be described below. The index of double refraction |N
TE
−N
TM
| obtained used when LiNbO
3
is approximately equal to 0.072. In order to realize the above mode transduction with light having a wavelength &lgr; of 1.55 nm (frequently used in optical communications), the wavelength &Lgr; of the SAW is equal to 21.5 &mgr;m. Since the acoustic velocity (phase velocity) v on the substrate
1
is approximately equal to 3700 m/s, the RF signal is required to have a frequency f (=v/&Lgr;) of 172 MHz. The power of the RF input signal depends on the mutual action length L to the SAW. Assuming that L=30 mm, the RF power is approximately equal to 10 mW.
With the above arrangement, the TE/TM wave of the input light can efficiently be transduced into the TM/TE wave by a reduced RF signal level and reduced RF power.
A wideband acousto-optical tunable wave filter can be configured by providing, at the following state, a polarizer for extracting the TM (or TE) wave.
In the structure shown in
FIG. 1A
, the finger electrodes
4
a
and
4
b
of the transducer
4
are directly mounted on the surface of the waveguide path
3
. Hence, the light propagated through the waveguide path
3
is absorbed by the finger electrodes
4
a
and
4
b
due to the presence of the metal forming them, and thus the light has a considerable propagation loss. This is because metal has a negative dielectric constant and serves as a dielectric having a large loss due to the inertia effect of charges in metal in the light wavelength range. Particularly, the electromagnetic field distribution in the TM mode enters deeply in metal, and is greatly affected by metal (the degree of influence in the TM mode is approximately ten times that in the TE mode).
In order to reduce the propagation loss of light caused by the transducer
4
, an improved arrangement has been proposed as shown in
FIG. 1B. A
buffer layer 6 is provided between the entire area between the transducer
4
and the substrate
1
and is formed of a dielectric film such as SiO
2
. The buffer layer
6
reduces the influence resulting from the presence of metal (transducer
4
). As the thickness of the buffer layer
6
is increased, the propagation loss of the light propagated through the waveguide path
3
is drastically reduced. In a case where the TM
o
mode light can be propagated through a single-mode waveguide path, if the SiO
2
film is 0.16 &mgr;m or more in thickness, the propagation loss can be reduced to 0.1 dB or less.
However, the structure shown in
FIG. 1B
has a disadvantage in that the presence of the buffer layer
6
provided continuously between the transducer
4
and the substrate
1
greatly reduces the effi
Nakazawa Tadao
Seino Minoru
Taniguchi Shinji
Fujitsu Limited
Nguyen Thong
Staas & Halsey , LLP
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