Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
1998-07-20
2002-06-04
Schuberg, Darren (Department: 2872)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S002000, C385S007000, C385S008000, C385S010000
Reexamination Certificate
active
06400881
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based up on and claims priority to Japanese Patent Application Number 09-216050 filed Aug. 11, 1997, the contents being incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device in general and more particularly to an acousto-optic device that may be used in an optical transmission system to add, drop and modulate selected wavelengths.
2. Description of the Related Art
In recent years, progress to a highly sophisticated information society has generated a tremendous amounts of information and an optical communication system using an optical fiber has been introduced as a way of transmitting such information. With this optical communication system, the transmission capacity has been increased year by year with the realization of a high speed modulation rate. A modulation rate of Gb/s or higher has already been put into practical use.
However, the need for a transmission systems which can transmit a large amount of data, such as that from image information, is expected to increase in the future. Such a high capacity system may be now required to have the transmission capacity of one Tb/s or more. The current systems cannot satisfy the requirement for the above transmission capacity only by improving the modulation rate. Therefore, an optical wavelength multiplex transmission/communication system is considered indispensable, and there have been attempts in recent years to introduce such a system.
An important element for realizing optical wavelength multiplex communication is the optical wavelength filter. This filter can combine onto a single optical fiber light beams of different (perhaps many different) wavelengths respectively generated by different light sources and can branch light beams of the different wavelengths transmitted through the single optical fiber to respective different fibers and detectors. The filter is thus a key device of the optical wavelength multiplex transmission system. The filter is required to satisfy different requirements depending on the system in which it is used. For example, the filter should be able to work with different numbers of wavelengths, from several wavelengths to about 100 wavelengths. The filter should be able to work with different wavelengths interval, from 1 nm or less to several tens of nm. The filter should be extremely low cost for application to an access system.
There are several devices which utilize mutual interference between an acoustic wave and an optical beam.
FIG. 1
is a perspective view of such a device, in which Ti metal is thermally diffused into an X-Y cut LiNbO
3
substrate
2
to form a channel waveguide
1
, and a flat waveguide. On the substrate
2
, this device has a waveguide lens
3
and a transducer
4
formed from a comb-tooth type electrode for exciting a surface acoustic wave (SAW).
In this device, a light beam is converted to a parallel light beam by the waveguide lens
3
. A SAW generates a refractive index grating from the photo acoustic effect of the SAW. The light beam is diffracted by this grating into different directions depending on the frequency of SAW. When this diffracted light beam is condensed by lens
5
, the diffracted light beam is focused to different points because the device functions as an optical deflector.
FIG. 2
is a top view of another example of a related art device utilizing the refractive index grating created by a SAW. This device, a collinear AO module with an inhomogeneous SAW waveguide, was present at the Photonics in Switching conference, at Sendai, Apr. 21-25, 1996. In this device, parallel optical waveguides
7
,
8
are formed on a Y cut LiNbO
3
substrate
6
and a thin film
9
formed of Ta
2
O
5
is formed on the substrate as a SAW waveguide. In operation, even number mode light and odd number mode light are combined by the refractive index grating between the parallel optical waveguides
7
,
8
. As before, the SAW creates the refractive index grating. A selected wavelength of a light beam incident to the optical waveguide
7
is switched to the optical waveguide
8
. The selected wavelength corresponds to the refractive index grating created by the SAW. In this device, the grating is weighted through a change in width a(z) and thickness h(z) of the thin film
9
which guides the SAW. The thin film
9
reduces a siderobe in the optical waveform. Moreover, a device in which weighting is realized by forming the SAW waveguide crossing the optical waveguide is also known.
FIG. 3
is a top view of an optical waveguide device which extracts a light beam having a selected wavelength and executes modulation by rotating the main axis of the waveguide refractive index for the selected wavelength to thereby rotate the polarization of the selected wavelength. The selected wavelength corresponds to the frequency of the SAW generated in the device. Optical waveguides
11
,
12
are formed by diffusing Ti in a X cut LiNbO
3
substrate
10
and creating deeply diffused regions
14
of Ti on both sides of a region
13
for guiding a SAW generated by a SAW transducer
15
. To generate the SAW transducer (IDT)
15
is provided with a radio frequency (RF) signal.
The TE (transverse electric) and TM (transverse magnetic) mode beams of an incident light beam are isolated by a crossing type polarization beam splitter (crossing type PBS)
16
, and thereby the TE mode beam is incident to the optical waveguide
12
, while the TM mode beam is incident to the optical waveguide
11
. In optical waveguide
11
, the light beam of a selected wavelength corresponding to the SAW is converted from the TM mode to the TE mode through rotation of polarization. In optical waveguide
12
, the TE mode beam of the selected wavelength is converted to a TM mode beam through rotation of the polarization.
In this example, the TM mode beams of non-selected, non-rotated wavelength light are output from the optical waveguide
11
to the non-selected beam side via a crossing type PBS
17
, while the TE mode beam of the selected wavelength is output from waveguide
11
to the selected beam side. In optical waveguide
12
, the TE mode beams of non-selected, non-rotated light are output to the non-selected beam side and the TM mode beam of the selected wavelength is output to the selected beam side. Thereby, the selected wavelength can be extracted and modulated using this optical waveguide device. In the
FIG. 3
device, absorbing bodies
19
and
20
are SAW absorbing bodies for preventing the SAW from being reflected at end faces of the substrate.
In the
FIG. 3
device, the SAW is propagated at a higher rate in the deeply diffused region
14
of Ti than in the substrate due to the influence of Ti. The SAW is thus trapped and propagated in the region
13
where the propagation rate of the SAW between deeply diffused regions
14
of Ti is rather low. Therefore regions
14
function as a SAW waveguide.
In the filter and switch of the optical waveguide shown in
FIG. 1
or
FIG. 2
, where even and odd number modes are coupled, filtering and switching can be realized independently respectively for the TE mode beam and the TM mode beam. This can occur because the SAW is generated or formed with perfect symmetry, but the propagation constants of the TE mode beam and the TM mode beam in the optical waveguide formed on the plane are generally different.
Moreover, coupling between two adjacent optical waveguides is believed to depend on the polarization of the TE and TM mode beams. Therefore, filter and switch characteristics also depend on the polarization. This causes a problem in a device which is required to control a light beam having a desired polarization, which requirement may be present in an optical communication system.
To eliminate the polarization dependency, it has been proposed to use two pairs of the FIG.
1
and
FIG. 2
devices to isolate the input beam by polarization. These two paris would correspond to the desired polarization. However, it is not practical to
Nakazawa Tadao
Seino Minoru
Taniguchi Shinji
Assaf Fayez
Schuberg Darren
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