Optical waveguides – Polarization without modulation
Reexamination Certificate
2000-12-27
2002-10-15
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Polarization without modulation
C385S007000, C359S285000, C359S308000
Reexamination Certificate
active
06466705
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acousto-optic variable-wavelength TE/TM mode converter, and a variable-wavelength optical filter using this converter.
A TE/TM mode converter of this kind can convert TE mode light into TM mode light or vice versa by using an acousto-optic interaction. Accordingly, this converter is available for various types of optical devices or optical systems. For example, a variable-wavelength optical filter can be configured by combining this TE/TM mode converter and polarizers. Such a variable-wavelength optical filter is available, for example, as a variable-wavelength optical filter for use in a variable-wavelength coupler/splitter, etc. in a wavelength division multiplexing (WDM) optical communications system.
2. Description of the Related Art
An acousto-optic TE/TM mode converter is conventionally known. This converter is normally composed of an area where a surface acoustic wave (SAW) is excited and propagated, and an optical waveguide that is arranged to acousto-optically interact with the surface acoustic wave, which are arranged in a piezoelectric substrate.
The surface acoustic wave is excited by applying a high-frequency (RF) electric signal to a comb electrode arranged in a substrate, and propagated while being guided by a coupled acoustic waveguide arranged on the substrate. By arranging an optical waveguide in the propagation area of the surface acoustic wave, light and the surface acoustic wave are made to interact with each other. Consequently, only the light having the wavelength that phase-matches the wavelength of the surface acoustic wave is selectively TE/TM-mode-converted. The wavelength of the surface acoustic wave is changed by varying the frequency of the high-frequency signal applied to the comb electrode, so that the wavelength of TE/TM-mode-converted light can be selected. As references for such a TE/TM mode converter, for example, “Optical Integrated Circuit”(written by H. Nishihara (et al.), published by Ohmsha), “Fundamentals of Optical Electronics” written by A. Yariv, published by Maruzen Co, Ltd., etc. can be cited. For further details, please see these references.
The above described TE/TM mode converter is applicable to various optical devices with its mode conversion capability. For example, a variable-wavelength optical filter can be formed by respectively arranging polarizers in an input portion and an output portion of the optical waveguide in the above described TE/TM mode converter. In this case, the variable-wavelength optical filter can be configured as a bandpass filter or a bandstop filter (rejection filter) by changing the arrangement, etc. of the two polarizers. Thus configured variable-wavelength optical filter can simultaneously filter multiple wavelengths by applying a plurality of high-frequency electric signals to comb electrodes in a TE/TM mode converter. Namely, the lights having the wavelengths that correspond to the frequencies of the applied high-frequency electric signals can be filtered at the same time.
However, a side lobe, which is undesirable as an optical filter characteristic, occurs in such type of a variable-wavelength optical filter. As a result, a wavelength other than a selected wavelength is filtered, or the flatness of the filtering characteristic is deteriorated.
To overcome the above described problems, for example, the technique with which the intensity distribution of a surface acoustic wave is given in the longitudinal direction of an optical waveguide, which is disclosed by U.S. Pat. No. 5,400,171, is adopted. With this technique, however, a selected wavelength gets out of position or the depth of rejection is deteriorated if high-frequency electric signals having adjacent frequencies are simultaneously applied to the same acousto-optic interaction area. Accordingly, lights having multiple adjacent wavelengths cannot selectively be filtered, leading to a difficulty in use of such an optical filter for a variable-length coupler/splitter in a WDM optical communications system.
In the meantime, for example, the technique with which an interaction area in a TE/TM mode converter is separated into two areas, and high-frequency electric signals having adjacent frequencies are distributed into the respective areas may be also considered as disclosed by U.S. Pat. No. 5,455,877. The top view of the configuration of the TE/TM mode converter adopting such a technique is shown in FIG.
1
A.
The TE/TM mode converter shown in this figure comprises: an optical waveguide
1
; a coupled acoustic waveguide
2
composed of adjacent first and second acoustic waveguides
2
a
and
2
b
; first and second comb electrodes
3
and
4
respectively arranged at both ends of the coupled acoustic waveguide
2
; and a surface acoustic wave absorber
5
arranged to go across the center of the coupled acoustic waveguide
2
. The coupled acoustic waveguide
2
is partitioned off by three areas
7
a
,
7
b
, and
7
c
, which are titanium(Ti)-diffused in a piezoelectric substrate. The central Ti-diffused area
7
b
is a gap (a gap between acoustic waveguides) that separates the two acoustic waveguides
2
a
and
2
b
. The comb electrodes
3
and
4
are arranged within the first acoustic waveguide
2
a
, whereas the optical waveguide
1
is arranged within the second acoustic waveguide
2
b
.
In the TE/TM mode converter having such a configuration, if high-frequency electric signals having adjacent frequencies are respectively applied to the comb electrodes
3
and
4
, surface acoustic waves W
1
and W
2
, which correspond to the respective frequencies, occur on the surface of the piezoelectric substrate. These surface acoustic waves W
1
and W
2
propagate on the surface of the board while being guided by the coupled acoustic waveguide
2
, and is finally absorbed by the absorber
5
. Each of the surface acoustic waves W
1
and W
2
acousto-optically interacts with the light propagating through the optical waveguide
1
during this propagation. Only the lights having the wavelengths that respectively correspond to the frequencies of the above described high-frequency electric signals are selectively TE/TM-mode-converted and output from the output portion of the optical waveguide
1
. In this case, the intensity distribution of the surface acoustic waves W
1
and W
2
in the longitudinal direction of the acoustic waveguide
2
are those shown in FIG.
1
B. Substantially, the area where the surface acoustic waves W
1
and W
2
interact with light are separated into two.
Accordingly, if an optical filter is configured by using the TE/TM mode converter having such a configuration, the above described problems that the selected wavelength gets out of position, and the depth of rejection is deteriorated are overcome, whereby lights having adjacent wavelengths can selectively be filtered at the same time.
However, with the TE/TM mode converter having the configuration shown in
FIG. 1A
, the area where the surface acoustic waves W
1
and W
2
interact with light are completely separated into two in the longitudinal direction of the acoustic waveguide
2
. Therefore, the interaction length for each interaction area is shortened, and a sufficient interaction is difficult to be obtained as it is. Therefore, the power of a high-frequency signal must inevitably be intensified to secure a sufficient interaction, and at the same time, a side lobe characteristic is deteriorated.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an acousto-optic variable-wavelength TE/TM mode converter which can selectively mode-convert lights having multiple adjacent waves at the same time while preventing the power of a high-frequency signal from increasing so as to overcome the conventional problems described earlier.
Another object of the present invention is to provide a variable-wavelength optical filter which can selectively filter lights having multiple adjacent wavelengths at the same time while preventing the power of a high-frequency electric signal fr
Kubota Yoshinobu
Tanaka Takehito
Fujitsu Limited
Rojas, Jr. Omar
Sanghavi Hemang
Staas & Halsey
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