Liquid crystal cells – elements and systems – Liquid crystal optical element – Liquid crystal diffraction element
Reexamination Certificate
1998-07-22
2001-10-16
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal optical element
Liquid crystal diffraction element
C349S129000, C349S123000
Reexamination Certificate
active
06304312
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical head for reading and/or writing information on an optically recording medium such as a CD (compact disc), a CD-ROM, another optical disc, and a magneto-optical disc, and a diffraction element suitable for such an optical head.
BACKGROUND ART
As an optical head which writes optical information or reads optical information on an optical disc, a magneto-optical disc and so on, there have been known one wherein a prism type beam splitter is used as an optical part for directing (beam-splitting) a signal ray reflected from a recording surface of a disc to a detector, and one wherein a diffracting grating or a hologram element is used as such an optical part.
In the past, the diffraction grating or the hologram element for the optical head has had a rectangular grating with a rectangular section (an isotropic diffracting grating in a relief form) formed on a glass substrate or a plastic substrate by dry etching or injecting molding, thereby reflecting a ray to obtain a beam splitting function.
In order to increase the utilization efficiency of a ray in comparison with an isotropic diffraction grating having about 10% of light ray utilization efficiency, the utilization of polarization can be thought out When polarized light is utilized, there has been a method wherein a prism type beam splitter is combined with a quarter-wave plate to improve efficiency in a forward path (a direction from a light source toward an optical recording medium) and in a backward path (a direction from the optical recording medium toward the light source and a detector) so as to raise reciprocation efficiency.
However, the prism type polarization splitter is expensive, and another system has been groped for. As a system, there has been known a method wherein a flat plate of birefringent crystal such as LiNbO
3
is used and the flat plate has an anisotropic diffraction grating formed thereon to obtain polarization selectivity. However, such a birefringent crystal per se is expensive, and application of the birefringent crystal to commercial utilization is difficult. When the grating is formed by proton exchanging, the proton in the proton exchange liquid is usually liable to diffuse in a LiNbO
3
substrate, creating a problem in that it is difficult to provide the grating with a fine pitch.
The isotopic diffraction grating has about 10% of reciprocation efficiency at the maximum because the utilization efficiency in the forward path is about 50% and the utilization efficiency in the backward path is about 20% as stated earlier.
The present invention has been made to solve these problems, and has an object to provide an optical head having high utilization efficiency of light ray and high reliability.
DISCLOSURE OF INVENTION
The present invention provides an optical head which irradiates a ray from a light source to an optically recording medium through a diffraction element so as to read information and/or write information, characterized in that the diffraction element includes an optically anisotropic diffraction grating which comprises an optically anisotropic polymer liquid crystal sandwiched between two transparent substrates, and that the polymer liquid crystal has an alignment direction thereof periodically changed, and a diffraction element suitable for such an optical head.
In a preferred mode of the present invention, both of the two transparent substrates have electrodes mounted thereon, and at least one of the electrodes comprises periodically formed electrode pieces. By such arrangement, portions with the periodically formed split electrodes and portions without the split electrodes can have different alignment states in a liquid crystal material on application of an electric field, facilitating formation of an optically anisotropic diffraction grating by the electric field.
In another preferred mode of the present invention, both of the electrodes comprise periodically formed electrode pieces, and the two electrodes are unsymmetrical arranged between the two transparent substrates. It means that both electrodes are unsymmetrical with respect to a central plane located at the center between the two transparent substrates and parallel to the two transparent substrates in such a state that a polymer liquid crystal cell has been formed. By such arrangement, the split electrodes of both of the two electrodes confront one another, having different locations and/or different sizes, and aligned portions of the polymer liquid crystal formed by the split electrodes can be unsymmetrical as a pair of upper and lower split electrodes are viewed. The optically anisotropic diffraction grating wherein either one of ±1st order diffraction rays has a higher diffraction efficiency can be easily formed by the electric field.
In these modes, it is preferable that the periodically formed electrode pieces of the at least one of the electrodes have a width of 30-45% of a periodical pitch thereof.
Both of the electrodes may comprise periodically formed electrode pieces, and the periodically formed electrode pieces of both electrodes may have a width of 30-45% of a periodical pitch thereof. In this case, the upper and lower electrode pieces may have the same width.
In another preferred embodiment of the present invention, at least one of the transparent substrates has a side thereof contacting the liquid crystal formed with an alignment layer, and at least one of the alignment layers includes alignment layer pieces which periodically have different alignment forces which periodically have different alignment forces. By such arrangement, the split alignment layers which are formed by the alignment layer pieces having periodically different alignment forces can form a certain distribution in the alignment state of the polymer liquid crystal. When one period includes several split alignment layers, the polymer liquid crystal can have an unsymmetrical alignment state in the periodical direction. In this manner, the optically anisotropic diffraction grating wherein either one of ±1st order diffraction rays partially has a high diffraction efficiency can be formed easily.
In another preferred mode of the present invention, the alignment layers of both of the two transparent substrates respectively include alignment layer pieces which periodically have different alignment forces, and the alignment layer pieces which periodically have different alignment forces are unsymmetrical arranged between the two transparent substrates. It is mean that the alignment layer pieces are unsymmetrical with respect to a central plane locating at the center between the two transparent substrates and parallel to the two transparent substrates in such a state that the polymer liquid crystal cell has been formed. By such arrangement, the respective split alignment layers on both substrates which are formed by the alignment layer pieces periodically having different alignment forces can make aligned portions of the polymer liquid crystal by the split alignment layers unsymmetrical as viewed a pair of upper and lower split alignment layers are viewed. In this manner, the optically anisotropic diffracting grating wherein either one of ±1st order diffraction rays has a higher diffraction efficiency can be formed easily by the alignment layers.
In another preferred mode of the present invention, an alignment layer is formed on an electrode, the alignment layer is one which is subjected to rubbing so as to extend perpendicularly to a longitudinal direction of the electrode.
In addition, the present invention provides a method for manufacturing an optical head which irradiates a ray from a light source to an optically recording medium through a diffraction element so as to read information and/or write information, characterized in that the method comprises providing electrodes on two transparent substrates; forming at least one of the electrodes so as to comprise periodically formed electrode pieces; providing an alignment layer on a side of at least one of the two transp
Gunjima Tomoki
Hotaka Hiroki
Sato Hiromasa
Tanabe Yuzuru
Asahi Glass Company Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Parker Kenneth
Qi Mike
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