Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2001-11-13
2004-07-13
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Reexamination Certificate
active
06762874
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of forming a polarization inversion portion by applying electric field to a ferroelectric crystal, more particularly to a polarization inversion method of ferroelectrics which is capable of forming a uniform polarization inversion pattern by shortening an electric field application time. Furthermore, the present invention relates to a method of fabricating an optical wavelength conversion device, which applies the polarization inversion method of the ferroelectric.
2. Description of the Related Art
A method has been already proposed by Bleombergen et al., which changes a wavelength of a fundamental wave to that of a second harmonic by use of an optical wavelength conversion device providing an area in which a self-polarization (domain) of ferroelectric showing a nonlinear optical effect is periodically inverted (see Physical Review, Vol. 127, No. 6, pp. 1918 (1962)). In this method, a cycle &Lgr; of a polarization inversion portion is set to an integer multiple of a coherence length&Lgr;c which is given by the following equation (1)
&Lgr;
c
=2&pgr;/{&bgr;(2&ohgr;)−2&bgr;(&ohgr;)} (1)
where &bgr; (2&ohgr;) is a propagation constant of a second harmonic and &bgr; (&ohgr;) is a propagation constant of a fundamental wave. Thus, a phase matching of the second harmonic can be achieved. When the wavelength conversion is performed by use of a bulk crystal of a nonlinear optical material, a wavelength that is subjected to the phase matching is limited to a specific wavelength which is inherent to the crystal. However, according to the above described method, the phase matching that is a so-called pseudo phase matching can be effectively achieved by using the cycle &Lgr; satisfying the equation (1) for an desired wavelength.
As a method forming the above described periodic polarization inversion structure, one has been known as described in Japanese Unexamined Patent Publication No.7(1995)-72521. In this method, after periodic electrodes having a predetermined pattern are formed on one surface of a ferroelectric crystal showing a nonlinear optical effect, which has been subjected to a single polarization, a ferroelectric crystal is corona-charged by the periodic electrodes and a corona wire disposed on a surface opposite to that one surface, and electric field is applied to the ferroelectric crystal, thus converting a portion opposite to one of the periodic electrodes of the ferroelectric crystal to a local polarization inversion portion.
In addition to the method which applies the corona charging, a method has been known as is described in Japanese Patent No. 3005225, in which an entire surface electrode is formed on the entire surface of ferroelectric opposite to a surface where periodic electrodes having a predetermined pattern are formed, and electric field is directly applied to the ferroelectric by use of the entire surface electrode and the periodic electrodes, thus forming a local polarization inversion portion.
When the polarization of the ferroelectric crystal is inverted by any of the above described conventional methods, one polarization inversion portion is formed in the portion opposite to a corresponding one of the periodic electrodes of the ferroelectric crystal.
Incidentally, in the case where a periodic polarization inversion structure having a particularly long cycle or a large area is formed in forming the periodic polarization inversion portions in the ferroelectric crystal by use of the above described electrodes, a long electric field application time is required. If the electric field application time is long, a problem is recognized, in which an inversion width is large in an earlier polarization inversion portion where a polarization inversion first starts, and the inversion width is small in a latter polarization inversion portion where the polarization inversion is started later, thus creating unevenness of the width of the polarization inversion portion.
When the foregoing problem occurs in the optical wavelength conversion device in which the periodic polarization inversion structure is formed in a ferroelectric crystal showing then on linear optical effect, a cycle or width ratio of the polarization inversion portions is uneven and a decrease in wavelength conversion efficiency is brought about.
SUMMARY OF THE INVENTION
In consideration of the foregoing circumstances, an object of the present invention is to provide a polarization inversion method of ferroelectrics, which is capable of accurately forming polarization inversion portions having a desired pattern in a short electric field application time.
Another object of the present invention is to provide a method of fabricating an optical wavelength conversion device, which is capable of forming a periodic polarization inversion structure having an excellent periodicity in a nonlinear optical crystal that is ferroelectric.
A first polarization inversion method of ferroelectric of the present invention is one in which one electrode having a shape corresponding to the area is not disposed in an area of a ferroelectric crystal where a polarization is desired to be inverted, but one in which a plurality of electrodes smaller than the area are disposed in this area, and electric field is applied to the ferroelectric crystal through the plurality of electrodes, the method comprising the steps of: forming electrodes having a predetermined pattern on one surface of the ferroelectric crystal that has been subjected to a single polarization; and forming a local polarization inversion portion in the ferroelectric crystal by applying electric fields to front and back surfaces of the ferroelectric crystal via the electrodes, wherein one polarization inversion portion having a desired pattern is formed by allowing portions of the ferroelectric crystal respectively corresponding to the plurality of electrodes and portions between these portions to be polarization-inverted.
A second polarization inversion method of ferroelectrics of the present invention is a method in which the first method is applied especially to a case where a periodic polarization inversion structure is formed. In this second method, periodic electrodes are used as the foregoing electrodes, which include a plurality of electrode groups formed periodically, each being composed of a plurality of electrodes; and each polarization inversion portion is formed for a corresponding one of the electrode groups, thus forming a periodic polarization inversion structure in which the polarization inversion portion is periodically and repetitively formed.
In the polarization inversion method of ferroelectric according to the present invention, a corona wire is disposed on one surface of a ferroelectric crystal opposite to one surface thereof, and electric field should be applied by corona charging by use of the corona wire and the electrodes.
The first and second polarization inversion methods of ferroelectric of the present invention are particularly effective when the ferroelectric crystal is a LiNb
x
Ta
1-x
O
3
(0<x<1) crystal or a crystal doped with one of MgO, ZnO and Sc.
On the other hand, a method of fabricating an optical wavelength conversion device of the present invention is a method to which the foregoing second polarization inversion method of ferroelectrics of the present invention is applied. In this method of fabricating an optical wavelength conversion device of the present invention, a nonlinear optical crystal is used as a ferroelectric crystal that has been subjected to a single polarization, and a periodic polarization inversion structure corresponding to a periodic pattern of the foregoing one group of the electrodes is formed in the nonlinear optical crystal.
In general, when a polarization of the ferroelectric crystal is inverted, it is proved by experiments that an inversion nucleus first occurs, and then a polarization inversion grows around the inversion nucleus. When electric field is applied to th
Fuji Photo Film Co. , Ltd.
Lee John D.
Sughrue & Mion, PLLC
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