JPH057769B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photodetection device for an optical reader,
In particular, the present invention relates to shortening the effective length of a photodetector that is suitable for a three-spot photodetector system.

FIG. 1 shows a photodetection device of a conventional optical reader. A laser beam oscillated by a laser diode 1 is turned into a parallel beam by a collimator lens 2, and is incident on a diffraction grating 3. The diffraction grating diffracts the incident light beam into an angular interval θ determined by its grating constant. Among these, this optical system is responsible for the autofocus signal to the disk information surface and the read information signal.
Only the second-order diffracted light (transmitted light) and the ±1st-order diffracted light that carry the tracking signal to the disk information track are used. Next, this diffracted light passes through a polarizing prism 4 and a crystal wavelength plate 5, is focused by an objective lens 6 onto a disk information surface 7, and is reflected after reading out information. This reflected light returns along the same path again and is reflected by the polarizing prism toward the lens group of the photodetection system, passes through the detection lens 8 and the cylindrical lens 9, and enters the photoelectric converter 10, which converts the optical information into electricity. Convert to signal. As described above, this optical detection system is an optical system that utilizes an astigmatism method for detecting a focus signal and a three-spot method for detecting a tracking signal. Currently, tracking signal detection methods are roughly divided into two types: the diffracted light method (2D method) and the sub-spot method (3-spot method.The 3-spot method of this conventional example is a method that utilizes the brightness and darkness of the light intensity of the sub-spots. This is a very effective method, as it provides a stable tracking signal that is not affected by the quality of the pit shape recorded on the disk information surface. However, due to the following requirements, it is necessary to downsize the photodetection optical system. One of the disadvantages is that the diameter of the light spot on the photoelectric conversion element must have a certain finite size P, and each of the three spots must be completely separated. A distance d (d>2・P) is required. However, as for the spot diameter P, it is advantageous to have a larger spot diameter from the viewpoint of spot position adjustment accuracy on the photoelectric conversion element. In order to obtain a good tracking signal at the position with the maximum radius of curvature, the distance between the three spots on the disk should be made as short as possible.Therefore, the diffraction angle θ by the diffraction grating should be set as small as possible. By the way, the effective length l of the photodetection system
is expressed by the following equation.

l=d/tanθ...(1) Therefore, the above two necessary conditions are this effective length l
This means that the sensor must be set for a long time, which hinders miniaturization of the photodetection system. Here, as an example of setting the constants of a photodetection system aiming at miniaturization, d=
When 600 (μm) and θ=0.28°, the effective length is l≒
120 (mm), but in current digital audio disks, it is preferable to make the effective length of this photodetection system at least half that length or less. As mentioned above, the three-spot method of this conventional example has a major drawback in shortening the effective length of the photodetection system.
This in turn hinders the miniaturization of optical reading devices.

An object of the present invention is to reduce the size of an optical reader by shortening the effective distance between each optical element in the photodetection device of the optical reader without changing the optical specifications.

A method of shortening the effective length from the optical system to the focal point without changing the combined focal length of the optical system by adding a concave lens to a part of the condensing optical system has long been known, but this method The invention applied this to the photodetection system of an optical reader and succeeded in significantly shortening the effective distance of the photodetection system by adding one type of concave lens to the conventional optical system.

Embodiments of the present invention will be described below with reference to FIGS. 2, 3, and 4.
This will be explained using figures. First, FIG. 2 shows an embodiment of the present invention. The configuration and function of the optical components are the same as in the conventional example except for the provision of a concave lens 11 between the detection lens 8 and the photoelectric converter 10, but the provision of this concave lens improves the effectiveness of the photodetection optical system. This made it possible to significantly shorten the length. Figure 3 below,
The principle of this shortening will be explained with reference to FIG.

The optical system shown in FIG. 3 is an equivalent representation of the conventional optical system, and has a convex lens with a focal length f, and the effective length from the lens to the focal position X ₁ 12 is L ₁ . However, L ₁ =f. The optical system in FIG. 4 is an equivalent representation of the photodetection optical system of this example.
If we have a convex lens with a focal length of f ₁ and a concave lens with a focal length of f ₂ , and the distance between the convex lens and the concave lens is m, then the combined focal length f′ and the effective length L ₂ from the convex lens to the focal position X ₁ 12 are given by the following formula. Become.

f'=f ₁・f ₂ / (f ₁ + f ₂ − m) L ₂ = m + (f ₁・f ₂ − mf ₂ ) / (f ₁ + f ₂ − m) Now, in Fig. 4, f ₁ is replaced by f ₁ <<f, and then set f ₂ , m so that f′ at that time becomes f′=f′=f=L ₁
When set, the incident parallel light beam takes a path like the solid line and focuses at X ₁ . At this time, the optical performance of the lens group shown in FIG. 4 becomes equivalent to that of the lens shown in FIG. 3 by setting f'=f. In other words, the principal plane of the lens group in Fig. 4 is at position 13 at X ₀ , which is equivalent to having a convex lens with focal length f at the position X ₀ .
The horizontal magnification and vertical magnification are also the same. This state is shown by a dashed line. Therefore, α with respect to the optical axis of the lens
For the incident light flux that enters with an inclination of , the lateral movement amounts δ ₁ and δ ₂ of the focal position are equal in both FIGS. 3 and 4.
δ ₁ = δ ₂ This state is shown by the dotted line. Therefore f′=f=L ₁
It is clear that by setting f ₁ and _m small under the conditions of It is. By applying this principle to the photodetection system as described above, it has become easy to significantly shorten the effective length of the photodetection system.

In this example, the effective length of the photodetection system is f=
Compared to the conventional example of L ₁ = 120 (mm), this was achieved by setting the optical system constants to f ₁ = 36 (mm), f ₂ = 14 (mm), and m = 26.2 (mm). The effective length of the photodetection system is approximately 52 (mm), which is less than half the size of the conventional example, and the optical reader uses a 3-spot method for the photodetection system, making it significantly more compact. It is possible to aim for

Next, the embodiment shown in FIG. 5 has the effect that the optical axes of the optical components can be easily aligned with high precision by fixing and integrating the optical elements of the photodetection system to the supporting member 14.

The embodiment shown in FIG. 6 uses a meniscus lens 14, which is a combination of a convex lens and a concave lens, and is an integral part of the optical elements of the photodetection system. The embodiment shown in FIG. 7 uses an optical lens 15 in which a convex lens and a cylindrical lens are combined and integrated, and one side is convex and the other side is shaped into a cylindrical surface. The embodiment shown in FIG.
An optical lens 16 in which a cylindrical lens and a concave lens are synthesized and integrated, and one side is cylindrical and the other side is concave.
is used. The photodetection optical system shown in Figures 6, 7, and 8 has the above configuration, which has the effect of eliminating one optical component and improving the accuracy of optical axis alignment, improving performance and ease of use. Improves sex.

As described above, according to the present invention, by providing the concave lens between the convex lens and the photoelectric conversion element, the effective distance between the convex lens and the photoelectric conversion element can be significantly reduced without changing the composite focal length. Because you can
This has a great effect on downsizing the optical system of the optical reading device. Furthermore, if the present invention is applied to a three-spot photodetection system, the effective length of the photodetection system can be significantly shortened without changing the spot spacing on the photoelectric conversion element, which is particularly effective.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a conventional system; FIG. 2 is a schematic diagram of a three-spot photodetection system according to an embodiment of the present invention; FIGS. 3 and 4 are explanatory diagrams of the operation of the present invention;
6, 7, and 8 are schematic diagrams of other embodiments of the present invention. DESCRIPTION OF SYMBOLS 1...Laser diode light source, 2...Collimating lens, 3...Diffraction grating, 4...Polarizing prism, 5...Crystal wave plate, 6...Objective lens, 7...
... Disc information surface, 8 ... Detection lens, 9 ... Cylindrical lens, 10 ... Photoelectric converter, 11
. . . Concave lens, 12 . . . Focal position, 13 .

Claims (1)

[Scope of Claims] 1. An optical reading device that reads information on an optical recording medium using reflected light of a laser beam irradiated to the optical recording medium, comprising: an optical device that converges the reflected light from the optical recording medium; an astigmatism element that causes astigmatism in the converged reflected light from the optical element; a concave lens into which the convergent light that has been astigmatized by the astigmatism element is incident; and from the concave lens. What is claimed is: 1. An optical reader comprising: a photoelectric conversion element that detects convergent light; 2. The optical reading device according to claim 1, wherein an optical element for converging the reflected light, and at least two lenses among the astigmatism element and the concave lens are incorporated into one support body. . 3. The optical reading device according to claim 1 or 2, wherein the optical element for converging the reflected light and the concave lens are combined to form a meniscus lens. 4 The astigmatism element and the optical element that converges the reflected light are combined into one lens to form one composite lens in which one side is an astigmatism element and the back side is an optical element that converges the reflected light. An optical reading device according to claim 1 or 2, characterized in that: 5. The astigmatism element and the concave lens are combined into one lens to form a composite lens in which one side is an astigmatism element and the back side is a concave lens. Or the optical reading device according to item 2.