Optical waveguides – Planar optical waveguide
Reexamination Certificate
1999-08-31
2001-08-28
Ben, Loha (Department: 2873)
Optical waveguides
Planar optical waveguide
C385S130000, C385S002000, C359S305000, C359S245000, C438S492000, C257S134000
Reexamination Certificate
active
06282357
ABSTRACT:
This application is based on application No. 10-248580 filed in Japan, the contents of which are hereby incorporated by reference.
BACKGROUD OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide and an acousto-optic deflector, and more particularly to an acousto-optic deflector which deflects light traveling in an optical waveguide by surface acoustic waves excited on the optical waveguide.
2. Description of the Prior Art
In an image forming apparatus which uses a laser beam as a light source, generally, the laser beam is deflected by a polygon mirror to scan a photosensitive member. In recent years, however, speed-up of scanning has been demanded, and in order to comply with this demand, various kinds of waveguide type acousto-optic deflectors have been developed.
Well-known structures for such acousto-optic deflectors are a structure wherein a piezoelectric thin film waveguide layer of ZnO or the like is deposited on a substrate of glass, sapphire or silicon with a thermally oxidized surface and a structure wherein a waveguide layer of Ti is formed on a substrate of LiNbO
3
by dispersion or by protone exchange. Surface acoustic waves are excited on such a waveguide layer, and the surface acoustic waves and light (a laser beam) traveling in the waveguide layer are made interact. When the light and the surface acoustic waves intersect at an angle satisfying a Bragg condition, Bragg diffraction of the light occurs, thereby resulting in deflection of the light.
When the substrate is sapphire or silicon, Sezawa waves with a high electromechanical coupling coefficient can be excited as the surface acoustic waves, and this is expected to be practical. Also, a silicon substrate is inexpensive and stable. Sezawa waves mean surface acoustic waves which are Rayleigh waves in a high-order mode.
In an optical deflector using a silicon substrate, however, while surface acoustic waves are propagated in the waveguide layer, the energy of the waves is taken by the substrate, and the surface acoustic waves are attenuated. In the meantime, while light is traveling in the waveguide layer, the energy of the light is taken by the substrate, and the light is attenuated. In using silicon for a substrate, there are advantages that the cost is inexpensive and that Sezawa waves, which are efficient, can be excited, whereas there is a problem that surface acoustic waves and light traveling in the waveguide layer are attenuated greatly, thereby lowering the efficiency of use of light. In order to compensate for the lowered efficiency of use of light, it is possible to apply larger electric power to the interdigital transducer or to strengthen the intensity of light emitted from the light source; it, however, results in cost-up of the optical deflector.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical waveguide and an acousto-optic deflector which use a silicon substrate, have a high efficiency of use of light and are inexpensive.
From experiments which were made by the inventors so as to attain the object, it became apparent that the attenuation of surface acoustic waves depends on the resistivity of a silicon substrate and the thickness of a buffer layer and that the attenuation of light depends on the thickness of a buffer layer.
Based on these facts, an acousto-optic deflector according to the present invention has a buffer layer and a piezoelectric thin film waveguide layer on a silicon substrate with a resistivity of not more than 20 &OHgr;cm and an interdigital transducer formed on the waveguide layer. Sezawa waves are excited on the waveguide layer by the interdigital transducer, and light traveling in the waveguide layer is deflected by the Sezawa waves.
According to the present invention, since a silicon substrate with a resistivity of not more than 20 &OHgr;cm is used, the propagation loss of surface acoustic waves (Sezawa waves) can be reduced, and the oscillation efficiency can be improved. Sezawa waves are high-order mode waves of Rayleigh waves, and the electromechanical coupling coefficient is large. Also, a silicon substrate is inexpensive and has stable characteristics. The present invention uses a combination of a silicon substrate and Sezawa waves which have such advantages, and the oscillation efficiency of Sezawa waves is high, which results in an improvement in efficiency of use of light. Further, because a buffer layer which has a refractive index lower than that of the substrate is provided, it is possible that a piezoelectric thin film with a refractive index lower than that of the substrate serves as an optical waveguide layer.
Preferably, the waveguide layer is a ZnO film, and the thickness h of the ZnO film and the wavelength A of surface acoustic waves have a mutual relationship of 0.2<h/&lgr;<0.5. Experimentally, under this condition, the propagation loss of Sezawa waves is reduced, which results in an improvement in efficiency of use of light.
Moreover, the buffer layer is preferably an SiO
2
film. An SiO
2
film can be formed easily by oxidizing a surface of a silicon substrate, and with the SiO
2
film, the propagation loss of light traveling in the waveguide layer can be reduced. Especially if the thickness of the SiO
2
layer is not less than 4000 Å, the propagation loss of light is reduced, and as a synergetic effect with the arrangement of using a silicon substrate with a resistivity of not more than 20 &OHgr;cm, the efficiency of use of light can be more improved.
REFERENCES:
patent: 5138407 (1992-08-01), Hirtz et al.
patent: 5298457 (1994-03-01), Einthoven et al.
patent: 5689362 (1997-11-01), Kadota
patent: 5790719 (1998-08-01), Mitomi et al.
patent: 5825524 (1998-10-01), Faderl et al.
patent: 5852702 (1998-12-01), Nishida et al.
patent: 5878175 (1999-03-01), Sonoda et al.
Iwamoto Tsuyoshi
Kadota Michio
Ben Loha
Burns Doane , Swecker, Mathis LLP
Murata Manufacturing Co. Ltd.
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