JP2584739B2 - Focus detection device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus detection device, and more particularly to a focus detection device suitable for use in an optical information recording / reproducing device for recording and reproducing various information on a recording medium such as an optical disk. It concerns the device.

(Prior art)

As a focus detection apparatus as described above, for example,
A so-called astigmatism method as shown in Japanese Patent No. 57013 is known. This example is shown in FIG. Here, a light beam 42 emitted from a light source 41 such as a semiconductor laser is applied to a first surface 47 of a wedge-shaped plate 43.
And is condensed by the objective lens 44 on the information track 46 of the optical disc 45. The return light beam 49 reflected by the optical disk 45 passes through the objective lens 44 again, enters the wedge-shaped plate 43 from the first surface 47, is internally reflected by the second surface 48, and emerges from the first surface. And is detected by the photodetector 50. The returning light beam 49 generates astigmatism by transmitting through the wedge-shaped plate 43, and the shape of the light beam on the photodetector 50 changes according to the focusing state on the optical disk 45.
By detecting this change on the four-divided light receiving surface of the photodetector 50, focus detection is performed.

However, in the conventional apparatus as described above, the light source and the photodetector are arranged on the same side with respect to the wedge-shaped plate. In other words, a certain distance is required (that is, in a direction perpendicular to the surface of the optical disk), and this has been an obstacle to making the device thinner.

Further, in the above-described conventional apparatus, when the wavelength of the light source changes due to a temperature change or the like, the refraction angle of the return light by the wedge-shaped plate changes, and the light receiving position of the return light on the photodetector greatly shifts. There was a problem.

[Summary of the Invention] An object of the present invention is to provide a focus detection device which solves the above-mentioned problems of the prior art with a simple and thin structure by making a prism work optically equivalent to a parallel plate.

An object of the present invention is to provide a light source, a light condensing unit for condensing a light beam emitted from the light source on an object, and a first and a second surface. A prism that reflects the light and guides the light to the light condensing means, makes the return light from the object enter from a first surface, emits the light from a second surface, and generates astigmatism in the return light; Second
Receiving means for receiving return light emitted from the surface, and detecting from the astigmatism of the return light the in-focus state of the light beam on the object. After light is reflected by the inner surface of the prism, the second
And the prism is formed such that the mirror image of the second surface reflected on the inner reflecting surface is substantially parallel to the first surface.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic view showing a first reference example of the present invention. Here, a light beam 2 emitted from a light source 1 such as a semiconductor laser is partially reflected by a first surface 3a of a prism 3, and is condensed on an information recording carrier 6 via a collimator lens 4 and an objective lens 5. Next, the return light flux 10 reflected by the information recording carrier 6
Passes through the objective lens 5 and the collimator lens 4 again to become convergent light, and a part of the light enters the prism 3 from the first surface 3a. Further, this light beam 10 is reflected on the inner surface of the reflecting film formed on the second surface 3b, and is emitted from the third surface 3c of the prism which has been subjected to anti-reflection treatment for the wavelength to be used. The light is detected by a photodetector 7 disposed at a position facing 1.

The returning light beam 10 passes through the prism 3,
The photodetector 7 is located at an appropriate position between the meridional convergence point 8 and the spherical convergence point 9 of the light beam 10 due to astigmatism. Here, assuming that the vertical magnification of the optical system is γ and the change in the distance between the information recording carrier 6 and the objective lens 5 is δ (when focusing is δ = 0), the convergence points 8 and 9 are substantially equal. It moves by 2δ / γ, and the spot shape of the light beam in the photodetector 7 changes with this movement. The light receiving surface of the light detector 7 is divided into four light receiving portions 11, 12, 13, and 14 as shown in FIG.
Can be detected. For example, at the time of focusing (δ = 0), the spot has a substantially circular shape as shown in FIG. 15, and the amount of light incident on each light receiving unit is substantially equal. On the other hand, when the objective lens 5 is too far away from the information recording carrier 6 (δ is positive) and is in a so-called front focus state, the spot has a shape like 16 and the amount of light incident on the light receiving portions 11 and 13 is The light amount relatively increases as compared with the amount of light incident on the units 12 and 14. In the case where the objective lens 5 is too close to the information recording carrier 6 (δ is negative), that is, in the so-called rear focus state, the spot shape becomes like 17 and the light receiving portion is formed.
The amount of light incident on 12 and 14 is relatively increased as compared with the light receiving units 11 and 13. Therefore, a focus error signal (a so-called S-shaped curve) is obtained from the difference between the sum signal of the light receiving units 11 and 13 and the sum signal of the light receiving units 12 and 14. In the optical information recording / reproducing apparatus, the objective lens 5 is moved in the optical axis direction based on the focus error signal to perform focus control.

FIG. 3 is a principle view showing that astigmatism is generated by a prism. In FIG. 3, consider that a minute light beam emitted from point O is refracted at points P and Q on the prism surface. Let the convergence points of the meridional and spherical convergence (imaginary) convergence of the light flux refracted at point Q be Q'm and Q's, respectively.
_{If 1} , ▲ = d, the astigmatic difference ΔP ′ can be expressed by the following equation (1).

Here, n ′ is the refractive index of the prism, i ₁ and i ₁ ′ are the incident angle and refraction angle at point P, and i ₂ and i ₂ ′ are the incident angle and refraction angle at point Q. The focus detection sensitivity in the above-described device is determined by the above equation (1) and the magnification of the optical system.

FIG. 4 is a schematic diagram showing the behavior of a light beam incident on the prism 3 used in the first reference example. Here, the ray emitted from point O proceeds to points P, R, Q, and S. However, due to the design of the optical system, a plane T that is substantially parallel to the optical axis incident on the information recording carrier surface and the angle θ formed by the ray are is important. By setting this angle to approximately 0 °, the structure of the apparatus and the manufacture of mechanical parts and the like become easy. Although a detailed derivation method of the equation is omitted, θ can be expressed as follows by various quantities shown in FIG.

_{θ = i '3 + 2 ......} (2) where i' _3, the refractive index of the prism n ', the incident angle i ₁ at the point P, Bale continued at an angle _{1, 2,} etc. Snell's law to the prism And can be easily calculated. As described above, by using this design technique, the focus detection device can be manufactured to be smaller and thinner at lower cost.

Next, the brightness of an image formed on the photodetector surface by the prism in the first reference example will be described. In the above reference example, a prism divergence and a convergent light beam are incident. When the divergence and the convergence angle are large, the difference between the transmittance and the reflectance of the linearly polarized light component with respect to each surface of the prism increases (that is, the angle characteristic becomes large). Have). Therefore, by causing the polarization plane of the light beam from the light source to be incident on the meridional plane at an angle of approximately 45 degrees so that the S and P polarization components are balanced with respect to the incident surface of the light beam, the S and P polarization components are The average transmittance and the reflectance can be kept almost constant with respect to the change in the incident angle of the light beam. An example is shown below.

FIG. 5 shows the polarization characteristics of the first surface 3a of the prism (the surface that reflects the light beam from the light source and folds the light beam in the direction of the collimator lens 4) in the first reference example. In the figure, TA is the average transmittance, and three dotted lines above this line are P-polarized light,
The three lower solid lines show the transmittance of S-polarized light. RA is the average reflectance, and the dotted line shows the reflectance of P-polarized light and the solid line shows the reflectance of S-polarized light.

Two sets of curves indicated by 21a to d, 22a to d, and 23a to d in the drawing are P, S polarized light when the incident angles to the first surface of the prism are 35, 45, and 55 degrees, respectively. It shows the transmittance and reflectance of the components.
As is clear from the figure, when only the P or S polarized light component is used for the first surface, the transmittance and the reflectance differ depending on the angle characteristics. Therefore, when considering the first surface reflected light beam, the brightness differs above and below the spherical plane of the light beam depending on the magnitude of the incident angle. Since this effect occurs on each surface of the prism, unnecessary light and dark patterns are generated by the prism on the sensor surface, which adversely affects focus detection. Therefore, as described above, by setting the polarization plane of the light beam from the light source to approximately 45 degrees with respect to the meridian plane, the S-polarized light and the P-polarized light become almost equal, and the angular characteristics such as TA and RA disappear, and the A focus error signal can be obtained.

Further, it is needless to say that the change of the transmittance and the reflectance with respect to the change of the incident angle of the light beam can be reduced similarly to the above-mentioned example even when the optical thin film is formed on the reflection surface and the transmission surface in the above-mentioned embodiment.

Next, the return light amount ratio to the light source will be described. Let R be the reflectivity of the first surface of the prism described in the above example, and consider the light amount ratio returning to the light source via the collimator lens, the objective lens, and the information carrier. Assuming that the amount of light emitted from the light source is 100 and the transmittance from the light source of the optical element other than the first surface of the prism to the light source via the information carrier is T, the return light amount ratio to the light source is 100 · T · R ² (%). When a semiconductor laser (LD) is used as a light source, the return light amount and LD
There is a correlation between the noises. Therefore, the return light amount ratio of 10
By appropriately adjusting the reflectance R of the prism first surface 3a of 0 · T · R ² (%), the return light amount ratio to the LD changes, and the LD
Noise can be reduced.

FIG. 6 is a schematic diagram showing a second reference example of the present invention,
The same members as those in FIG. 1 are denoted by the same reference numerals, and detailed description will be omitted. In the present embodiment, a quarter-wave plate (hereinafter referred to as a λ / 4 plate) 30 is arranged between the collimator lens 4 and the objective lens 5, and an optical element that produces a polarization characteristic on the first surface 3 a of the prism 3. A thin film is provided. The first surface 3a shows a polarization characteristic as shown in FIG. 7, and shows almost 100% transmittance for P-polarized light like TP, and almost 100% for S-polarized light like TS and almost 100% transmittance. Nearly% is reflected. TA indicates the average transmittance.

In FIG. 6, when the light beam 2 from the light source 1 is set to S-polarized light, the light beam 2 is almost reflected by the surface 3a,
The light becomes clockwise circularly polarized light by the λ / 4 plate 30 and enters the information recording carrier 6. Also, the return light reflected by the information recording carrier 6 is left-handed circularly polarized light, passes through the λ / 4 plate 30 again, becomes P-polarized light,
Most of the light passes through the first surface 3a of the prism and is guided to the photodetector 7. In this reference example, the light source 1
As a result, the amount of light incident on the photodetector 7 can be increased and energy efficiency can be improved.

If the noise level can be reduced by guiding a certain amount of return light to the light source as described above, the λ / 4 plate 30
By appropriately rotating the direction of the crystal axis, the amount of returned light can be adjusted.

FIG. 8 is a schematic diagram showing a first embodiment of the present invention.
This embodiment is the same as the first embodiment except that the prism 3 shown in FIG. 1 is replaced with a prism 33 having a different shape. The same members as those in FIG. Is omitted.

Here, the return light flux 10 incident from the first surface 33a of the prism 33 is reflected by the reflection film formed on the second surface 33b,
The light is totally reflected by the first surface 33a, emitted from the third surface 33c, and guided to the photodetector 7. In the present embodiment, the return light flux is internally reflected twice in the prism as described above, so that the optical path length in the prism is extended, and the distance between the prism and the photodetector is increased as compared with the first reference example. It is possible to reduce the size and further reduce the size. In addition, the optical axes of the light beam from the light source and the light beam emitted from the prism can be set substantially parallel, which facilitates optical adjustment and is advantageous in making the device thinner.

FIG. 9 is a schematic diagram showing the behavior of light rays incident on the prism 33 used in the first embodiment. Here, the plane including the incident point P and the plane including the image Q ′ of the emission point Q can be made substantially parallel. If the two planes are determined in this way, not only can the same effect as a parallel plate be achieved by a simple component called a prism, but also the number of components of the optical system can be reduced. Under the above conditions, FIG.
Assuming that i ₁ = i ₂ ′ and i ₁ ′ = i ₂ , and the right side of the above equation (1) is zero and the parallel plane interval is d ₀ , the astigmatic difference is given by the following equation (3) Is represented by

This is an expression representing astigmatic difference by a parallel plate.

In the first embodiment, focus detection can be performed on the same principle as in the first embodiment. The same applies to the setting of the polarization direction as described with reference to FIG.

FIG. 10 is a schematic view showing a second embodiment of the present invention in which a λ / 4 plate 30 is added to the apparatus shown in FIG. 8, and the same members as those in FIG. Detailed description is omitted.

On the first surface 33a of the prism 33, as in the second reference example,
An optical thin film that produces polarization characteristics as shown in FIG. 7 is provided. The isolation function of the first surface 33a and the λ / 4 plate 30 is the same as that of the second embodiment and will not be described in detail. However, this embodiment can also perform focus detection similarly to the above embodiments. is there.

The present invention is not limited to the embodiments described above, and various applications are possible. For example, each of the collimator lens and the objective lens can be constituted by a molded lens, a hologram lens, a gradient index lens, a flat microlens, or the like. It is also possible to incorporate the collimator lens and the objective lens in a single lens barrel, or to configure an optical system with the optical element. In this way, not only the number of parts can be reduced, but also a low-cost, small-sized optical system can be configured, as compared with a lens of a type in which a lens is incorporated in a commonly used lens barrel.

Further, when the present invention is applied to an optical information recording / reproducing apparatus, in order to make a light beam follow a track on an information carrier,
Tracking may be performed by inserting a grating between the light source and the prism and using a known method (a so-called three-beam method). Furthermore, the present invention can be combined with other tracking methods (for example, pupil plane push-pull method, heterodyne method, etc.).

〔The invention's effect〕

As described above, the present invention relates to a focus detection device using astigmatism caused by a prism, in which return light incident from a first surface of a prism is reflected by an inner surface and then emitted from a second surface. Since the mirror image of the second surface reflected on the reflecting surface was substantially parallel to the first surface, the device was configured to be thin, and the same effect as that of the parallel plate could be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a first reference example of the present invention, FIG. 2 is a diagram showing a change in spot shape of return light on a light receiving surface of a photodetector, and FIG. FIG. 4 is a diagram for explaining the behavior of light rays incident on the prism, FIG. 5 is a diagram showing the transmittance and reflectance characteristics of the first surface of the prism, and FIG. 6 is a second reference of the present invention. FIG. 7 is a schematic diagram showing an example, FIG. 7 is a diagram showing the polarization characteristics of the first surface of the prism in the second reference example, FIG. 8 is a schematic diagram showing the first embodiment of the present invention, and FIG. FIG. 10 is a diagram for explaining the behavior of a light beam incident on a prism, FIG. 10 is a diagram for explaining a second embodiment of the present invention, and FIG. 11 is a schematic diagram showing a configuration of a conventional focus detection device. 1 ... light source, 3 ... prism, 4 ... collimator lens, 5 ...
... Objective lens, 6 ... Information record carrier, 7 ... Photodetector.

Claims (1)

(57) [Claims] 1. A light source, a condensing means for condensing a light beam emitted from the light source on an object, and a first and a second surface, wherein the light beam from the light source is reflected by the first surface. A prism that guides the return light from the object from a first surface and emits the return light from a second surface to generate astigmatism in the return light; A return means for receiving return light emitted from the surface, and detecting means for detecting a focus state of a light beam on the object from astigmatism of the return light. The light is emitted from the second surface after being reflected by the inner surface of the prism, and the mirror image of the second surface reflected on the surface that reflects the inner surface is substantially parallel to the first surface. A focus detection device formed to work equivalently to a parallel plate.