Optical deflection element

Details Optical deflection element Optical deflection element

2000-03-03

2002-05-07

Bovernick, Rodney (Department: 2874)

Optical waveguides

Directional optical modulation within an optical waveguide

Electro-optic

C385S004000, C385S009000, C385S129000, C385S130000, C385S131000

Reexamination Certificate

active

06385355

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical deflection element, and more particularly to an optical waveguide element capable of two-dimensionally deflecting a light beam incident to an optical waveguide by electro-optic effect. The optical deflection element is applicable to the entire spectrum of optoelectronics including laser printer, digital copier, facsimile, display, optical interconnection, optical crossconnect, bar-code reader, glyph code reader, optical disk pickup, optical scanner for surface inspection, optical scanner for surface shape presumption, and the like.
2. Description of the Prior Art
A typical laser beam optical deflector used in a laser beam printer, digital copier, facsimile, and the like has a rotating polygonal mirror called a polygon mirror for deflecting a beam from a gas laser and a semiconductor laser and an f&thgr; lens for converging the laser beam reflected by the rotating polygonal mirror to the state of line motion of a uniform rate on an imaging surface such as a photoreceptor. Such an optical deflector employing a polygon mirror is large in size, and has problems in that it is lacking in durability and generates noise because the polygon mirror is mechanically, fast rotated by a motor, and a light scanning speed is limited by the number of rotations of the motor. A galvanomirror and a cantilever mirror are problematic in terms of deflection precision.
On the other hand, optical deflection elements taking advantage of acoustooptical effect are available as solid, electrical laser beam optical deflectors. Of them, optical waveguide elements smaller than bulk acoustooptical elements are especially expected. The optical waveguide elements are under study of application to a printer or the like as laser beam optical deflection elements to solve the drawbacks of laser beam optical deflectors employing a polygon mirror. The optical deflection elements of optical waveguide type have: an optical waveguide constructed from LiNbO
3
and ZnO; a unit for making a laser light beam incident to the optical waveguide; comb electrodes for pumping surface acoustic waves for deflecting the light beam within the optical waveguide by acoustooptical effect; and a unit for emitting the deflected light beam from the optical waveguide. Additionally, as required, thin-film lenses and the like are added to the elements. However, optical deflection elements taking advantage of acoustooptical effect generally have the following problems: the upper limit of laser deflection speed due to sonic limitation; reduced light utilization efficiency and processing of zero-order light because of diffraction efficiency as low as several tens of percent; and an expensive and large power supply unit for controlling frequencies of several hundred MHz. For this reason, it has been difficult to apply such optical deflection elements to laser printer, digital copier, facsimile, display, optical interconnection, bar-code reader, glyph code reader, optical disk pickup, and the like.
On the other hand, there are known prismatic optical deflection elements, as described in the literature A. Yariv, Optical Electronics, 4th ed. (New York, Rinehart and Winston, 1991) pp. 336 to 339, that employ oxide ferroelectric materials having electro-optic effect higher in modulation speed than acoustooptical effect. Although bulk elements employing ceramics or monocrystals are available as the prismatic optical deflection elements, they have been incapable of providing a practical deflection angle because of its large size and a substantially high driving voltage. Prismatic domain inversion optical deflection elements or prismatic electrode optical deflection elements are described in the literature Q. Chen, et al., J. Lightwave Tech. vol. 12 (1994) 1401. and Japanese Published Unexamined Patent Application No. Hei 1-248141. The prismatic domain inversion optical deflection elements or prismatic electrode optical deflection elements have prisms disposed in cascaded form by using a LiNbO
3
monocrystalline wafer on which a Ti diffused optical waveguide or proton exchanged optical waveguide is formed. An electrode separation of approximately 0.5 mm, which is equal to the thickness of a LiNbO
3
monocrystalline wafer, is required. Therefore, there exist problems in that a driving voltage is still high and a deflection angle as small as approximately 0.2 degrees obtained with a driving voltage of ±600 V, as described in the above-described literature, is far from a practical level. There is disclosed in Japanese Published Unexamined Patent Application No. Hei 2-311827 a method which changes the effective refractive index of an optical waveguide by acoustooptical effect and changes emission angles from a prism coupler. However, with the disclosed configuration, electrodes are not disposed so as to change the refractive index of an optical waveguide portion in which a prism coupler is disposed. Accordingly, there exists a problem in that emission angles cannot actually be changed, and even if emission angles could be changed, because of the construction that electrodes are disposed on the optical waveguide surface, the electrode interval would increase and a practical deflection angle could not be obtained as in the example of the above-described literature.
On the other hand, the inventor proposed (Japanese Published Unexamined Patent Application No. Hei 9-5797) a prismatic deflection element to solve the above-described driving voltage problem wherein the prismatic deflection element has an oxide optical waveguide having epitaxial or single orientation with electro-optic effect and a light source to make a light beam incident to the optical waveguide, and employs a thin-film optical waveguide provided with electrodes for deflecting the light beam within the optical waveguide by electro-optic effect. However, a distribution of electric field of a laser beam propagating through an optical waveguide s penetrates into a substrate. The absorption coefficient of a substrate having a practical resistivity is often large, penetrationg components are strongly absorbed by free carriers in a conductive substrate, and propagation loss in a thin-film optical waveguide reaches tens of dB/cm due to the absorption in addition to loss due to scattering of the optical waveguide itself, raising a problem that light utilization efficiency is practically insufficient.
Generally, in elements having a coplanar electrode placement, an SiO
2
clad layer is inserted between a metal electrode on an optical waveguide and the optical waveguide to prevent the penetration of electric fields into the metal electrode and avoid absorption of propagating light. However, there is a problem in that, if SiO
2
were provided between a conductive substrate and an oxide optical waveguide, an oxide optical waveguide having epitaxial or single orientation with electro-optic effect could not be fabricated because SiO
2
is amorphous. Moreover, there is a problem in that the relative dielectric constant of an oxide optical waveguide material having electro-optic effect ranges from several tens to several thousands, which is very large compared with the relative dielectric constant 3.9 of SiO
2
, and since series capacitors are formed as an equivalent circuit in the described-above construction of a thin-film waveguide on a conductive substrate, an effective voltage applied to the thin-film optical waveguide is no more than several percent of an applied voltage, ultimately causing a significant increase in a driving voltage. In an i-GaAs waveguide of a compound semiconductor, an i-AlGaAs clad layer is inserted between the i-GaAs waveguide and n-AlGaAs lower clad layer to prevent the penetration of electric fields into the n-AlGaAs lower clad layer, thereby avoiding absorption by free carriers of the n-AlGaAs lower clad layer. However, a method for providing the same construction for an oxide optical waveguide has not been known wherein the oxide optical waveguide is made of a material entirel

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