JPH0550528U - Optical pickup device - Google Patents

Abstract

(57) [Abstract] [Purpose] To reduce the size of the focus error signal optical system of an optical pickup device and increase the degree of freedom in design. [Configuration] incident angle inclined plane-parallel plate 2 at an angle of theta ₁ with respect to the optical axis of the light beam focused in such a way that the aperture angle u with the lens 1 is arranged such that the theta _1, parallel to its rear Rights angle of incidence inclined at theta ₂ of angle faceplate 3 to theta ₁ of the slope and the reverse direction are arranged so that theta _2. The thickness t ₁ of the plane-parallel plate ₂ is larger than the thickness t ₂ of the plane-parallel plate 3, and the incident angle θ ₁ is smaller than the incident angle θ ₂ .

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to an optical pickup device for an optical disc player, and more particularly to an optical system for obtaining a focus error signal.

[0002]

[Prior Art]

 In an optical pickup device of an optical disk player, a focus error signal for performing focusing control is obtained by receiving a light beam having astigmatism by a four-division light receiving element.

FIG. 9 shows an example of such a conventional optical system. Input elevation angle tilted plane-parallel plate 2 at an angle of theta ₅ with respect to the optical axis of the light beam focused in such a way that the aperture angle u is disposed so that theta ₅ by the lens 1 as shown in FIG, behind Further, the plane-parallel plate 3 is arranged at an angle of θ _{6 in} the direction opposite to the inclination of θ ₅ so that the incident angle becomes θ ₆ .

The astigmatisms generated by the plane-parallel plates 2 and 3 have the same direction and are added, and the coma aberrations generated by the plane-parallel plates 2 and 3 have opposite directions and cancel each other out. Predetermined astigmatism is required to detect the focus error signal, and coma difference becomes an obstacle. In order to correct coma, the thicknesses t ₅ and t _{6 of the} plane parallel plates 2 and 3 are made equal to the refractive indices n ₃ and n ₄ , respectively, and the absolute values of the inclination angles θ ₅ and θ ₆ are made equal.

[0005]

[Problems to be solved by the device]

 In the above-mentioned conventional optical system, in order to correct the coma aberration and obtain the required amount of astigmatism, parallel plane plates of the same thickness are symmetrically arranged, that is, the absolute value is obtained by reversing the inclination angle direction. It was difficult to achieve miniaturization to satisfy the condition of equalization.

The present invention has been made in view of the above points, and an object thereof is to provide an optical pickup device that can be downsized by increasing the degree of freedom of the inclination angle of the plane parallel plate. To provide.

[0007]

[Means for Solving the Problems]

 In the optical pickup device of the present invention, two parallel plane plates are arranged in the focus detection optical path so as to be tilted in directions opposite to each other with respect to the optical axis, so that coma aberration is corrected and desired astigmatism is obtained. In the pickup device, two parallel plane plates having different thicknesses are used as the parallel plane plates, and the inclination angle of the thin parallel plane plate is made larger than that of the thick parallel plane plate.

Further, in the optical pickup device, the refractive index of the optical material of the thin parallel plane plate is made smaller than the refractive index of the optical material of the thick parallel plane plate.

Further, the thicker parallel plane plate in the optical pickup device is replaced with a right-angled isosceles prism.

Further, the parallel plane plate in the optical pickup device is replaced with a right-angled isosceles prism.

[0011]

[Action]

Coma W _c obtained by the following equation occurs convergent light obliquely incident on the plane-parallel plate

[0012]

[Formula 1] Here, t and n are the thickness and refractive index of the plane-parallel plate, respectively, and θ and u are the incident angle and the aperture angle of the convergent light incident on the plane-parallel plate, respectively.

A graph shown in FIG. 1 is obtained when t and u in the equation 1 are fixed and n is a parameter and comatic aberration with respect to the incident angle θ is obtained. On the other hand, the amount of astigmatism monotonically increases as the incident angle θ 1 increases.

Therefore, the plane-parallel plates having the same thickness and the same refractive index are tilted as two incidence angles before and after the coma maximum value point in the graph of FIG. The astigmatism is larger than that when the parallel plane plates are symmetrically arranged (the absolute values of the incident angles are made equal) after correcting the coma aberration by making the oblique directions opposite to each other.

As shown in the graph of FIG. 1, in the range of the refractive index of the optical material that can be normally used, the incident angle at the maximum point of coma difference exceeds 50 °. It is not practical to arrange the face plates asymmetrically to correct coma.

Therefore, the parallel plane plates are arranged asymmetrically in the region where the coma aberration increases, but the coma aberration per unit thickness of the parallel plane plate having a small incident angle is the coma per unit thickness of the parallel plane plate having a large incident angle. Since it is smaller than the aberration, comatic aberration cannot be canceled out if the thickness of both plane-parallel plates is the same.

According to the optical pickup device of the first aspect, by increasing the thickness of the plane-parallel plate having a small incident angle, the coma aberration caused thereby causes the coma aberration produced by the plane-parallel plate having a large incident angle. It can be erased. And the size of the optical system can be made smaller than the conventional one.

A graph shown in FIG. 2 is obtained when t and u in the equation 1 are fixed and the coma aberration with respect to the refractive index n is obtained using the incident angle θ as a parameter. FIG. 3 shows an enlarged range of the actual refractive index in FIG. As shown in this graph, the coma aberration decreases as the refractive index increases in the range of the refractive index of ordinary optical materials.

According to the optical pickup device of the second aspect, by selecting the material of the thin plane-parallel plate, that is, the plane-parallel plate having a large incident angle as the optical material having a small refractive index, the coma aberration can be made the same. It is possible to make the incident angle smaller while keeping it.

A ray is incident obliquely from the long side of the right-angled isosceles prism, is reflected by the other two surfaces, and is emitted from the long side again. Is equivalent to that. In the optical pickup device of the third aspect, the thick parallel plane plate is replaced by the right-angled isosceles prism, and by doing so, the optical path can be U-turned and the linear distance of the optical path can be shortened. In addition, it is possible to expand the degree of freedom in design by changing the arrangement of optical system elements.

According to another aspect of the optical pickup device of the present invention, both of the plane parallel plates are replaced by right-angled isosceles prisms. With such a configuration, the optical path is made a U-turn by 2 degrees, and the optical element It is possible to change the layout and further expand the degree of freedom in design.

[0022]

【Example】

An optical system for a focus error signal of an optical pickup device according to an embodiment of the present invention will be described with reference to the drawings. FIG. 4 shows a first embodiment of this invention. As shown in the figure, the plane-parallel plate 2 is tilted at an angle of θ ₁ with respect to the optical axis of the light flux focused by the lens 1 so that the aperture angle is u, and the incident angle is θ _1. Further, the plane parallel plate 3 is arranged at an angle of θ _{2 in} the direction opposite to the inclination of θ ₁ so that the incident angle becomes θ ₂ . The plane-parallel plate 2 and the plane-parallel plate 3 have the same refractive index.

The thickness t ₁ of the plane-parallel plate ₂ is larger than the thickness t ₂ of the plane-parallel plate 3, and the incident angle θ ₁ is smaller than the incident angle θ ₂ . With such a configuration, the optical system can be downsized.

FIG. 5 shows a second embodiment of the present invention. As shown in the figure, the plane parallel plate 2 is set to θ with respect to the optical axis of the light beam focused by the lens 1 so that the aperture angle u is obtained.₃The angle of incidence is θ₃And the parallel plane plate 3 at the rear of the₃Θ in the direction opposite to the inclination of _Four The angle of incidence is θ_FourIt is arranged so that The refractive indexes of the plane-parallel plate 2 and the plane-parallel plate 3 are n.₁And n₂And n₁> N₂Is.

The thickness t of the plane-parallel plate 2₃Is the thickness t of the plane-parallel plate 3._FourLarger, angle of incidence θ₃Is the incident angle θ_FourSmaller than With such a configuration, the incident angle θ_FourIs shown in FIG. ₂ The size can be further reduced, and the constituent optical system can be further downsized.

FIG. 6 shows a third embodiment of the present invention. In this embodiment, the parallel plane plate 2 of the first embodiment shown in FIG. 4 is replaced by an isosceles right-angled prism 4 equivalent to it, and by adopting such a configuration, the converged light is directed toward the light source. It is possible to increase the degree of freedom in designing the optical system.

FIG. 7 shows a fourth embodiment of the present invention. In this embodiment, the plane parallel plate 2 of the second embodiment shown in FIG. 5 is replaced by an isosceles right-angled prism 4 equivalent thereto, and in this case, the parallel plane plate 2 is more parallel than that of the third embodiment shown in FIG. It is possible to make the incident angle of the plane plate 3 smaller.

FIG. 8 shows a fifth embodiment of the present invention. In this embodiment, the parallel plane plate 2 of the first embodiment shown in FIG. 4 is replaced by an equivalent isosceles right prism 4, and the parallel plane plate 3 is replaced by an equivalent isosceles right prism 5. With such a configuration, it is possible to make the incident direction and the outgoing direction of the convergent light the same and to downsize the optical system.

[0029]

[Effect of the device]

 According to the optical pickup device of the present invention, the focus error signal optical system can be downsized, and the degree of freedom in design can be increased.

[Brief description of drawings]

FIG. 1 is a graph showing coma aberration generated when transmitting through a plane-parallel plate.

FIG. 2 is a graph showing coma aberration that occurs when transmitting through a plane-parallel plate.

FIG. 3 is a graph showing coma aberration that occurs when transmitting through a plane-parallel plate.

FIG. 4 is an optical path diagram showing a focus error signal optical system of the optical pickup device according to the first embodiment of the present invention.

FIG. 5 is an optical path diagram showing a focus error signal optical system of an optical pickup device according to a second embodiment of the present invention.

FIG. 6 is an optical path diagram showing a focus error signal optical system of an optical pickup device according to a third embodiment of the present invention.

FIG. 7 is an optical path diagram showing a focus error signal optical system of an optical pickup device according to a fourth embodiment of the present invention.

FIG. 8 is an optical path diagram showing a focus error signal optical system of an optical pickup device according to a fifth embodiment of the present invention.

FIG. 9 is an optical path diagram showing an example of a focus error signal optical system of a conventional optical pickup device.

[Explanation of symbols]

 1 Lens 2 Parallel plane plate 3 Parallel plane plate 4 Right angle isosceles prism 5 Right angles isosceles prism

Claims (4)

[Scope of utility model registration request] 1. An optical pickup device which obtains a desired astigmatism while correcting coma aberration by arranging two parallel plane plates in the focus detection optical path so as to be inclined in directions opposite to each other with respect to the optical axis, An optical pickup device characterized in that two parallel plane plates having different thicknesses are used as the parallel plane plates, and an inclination angle of the thin parallel plane plate is larger than an inclination angle of the thick parallel plane plate. 2. The optical pickup device according to claim 1, wherein the refractive index of the optical material of the thin parallel plane plate is smaller than the refractive index of the optical material of the thick parallel plane plate. 3. An optical pickup device according to claim 1, wherein the thicker parallel plane plate in the optical pickup device is replaced by a right-angled isosceles prism. 4. An optical pickup device in which the plane parallel plate in the optical pickup device according to claim 3 is replaced with a right-angled isosceles prism.