JPS62124634A - Focus detector - Google Patents

Abstract

PURPOSE:To minimize the length of an optical path and to scale down a detector by arranging a beam splitter between a light source and a collimator lens and disposing a critical angle prism on a reciprocating optical path between the collimator lens and an objective one. CONSTITUTION:The critical angle prism 5 and the polarization beam splitter 2 are arranged on the reciprocating optical paths between the collimator lens 4 and the objective lens 7 and the space between the light source 1 and the collimator lens 4, respectively. Simultaneously a photodetector 9 is arranged away from the conjugate position of the light source 1 with respect to the collimator lens 4. Then the flux from the light source 1 is irradiated on an object through the beam splitter 2, the collimate lens 4, the critical angle prism 5 and the objective lens 7, and the photodetector 9 is so constituted to receive the reflected light through the objective lens 7, the critical angle lens 5, the collimator lens 4 and the beam splitter 2. Thus the length of the optical path can be minimized to reduce the number of parts, whereby the detector can be scaled down.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention focuses a recording beam or a reproduction beam onto an optical recording medium such as a video disc, a compact disc, an optical data disc, etc. so that it is focused correctly. The present invention relates to a focus detection device for projecting images.

(Prior art) As a conventional focus detection device, for example, Japanese Patent Application Laid-Open No. 59-201
There is a critical angle method as described in Japanese Patent No. 238. In this focus detection device, as shown in FIG. 5, linearly polarized light from a semiconductor laser 21 is made incident on a polarizing beam splitter 23 as a parallel beam by a collimator lens 22, and transmitted through the polarizing film 24. After the light is totally reflected by the reflecting prism 25 at right angles, it is guided to the objective lens 27 via the wavelength plate 26 and projected onto the optical disk in the form of a spot. The reflected light from the optical disk follows the same optical path as the outgoing path until it reaches the polarizing beam splitter 23 and enters the polarizing film 24, but on the returning path, the polarization direction is perpendicular to the polarizing direction of the outgoing path due to the action of the grass wave plate 26. Therefore, all the returning light is reflected by the polarizing film 24. The return light from the optical disk reflected by the polarizing film 24 passes through the critical angle prism 28 and is received by the photodetector 29.

The critical angle prism 28 has a reflective surface 28a that is set so that the incident angle with respect to a returning light ray from the optical disk, for example, the central ray, is approximately a critical angle.
The returned light is reflected at point a and made incident on the photodetector 29. Further, the photodetector 29 has four light receiving areas 1i1i 29a to 29d divided into two in the radial direction and tangential direction of the optical disc, and these light receiving areas td
A data signal, a focus error signal, and a radial error signal are obtained based on the outputs of 29a to 29d.

That is, in FIG. 5, when the objective lens 27 is in focus on the optical disc, the critical angle prism 28
The return light from the optical disk that enters becomes a parallel beam of light,
Since the angle of incidence on the reflecting surface 28a is approximately the critical angle for all the light rays, all the light rays are totally reflected as shown in FIG.
29d is uniformly incident on J. On the other hand, if the returned light that deviates from the focused state and enters the critical angle prism 28 becomes convergent light as shown in FIG. 6B or divergent light as shown in FIG. 6C, the returned light toward the reflective surface 28a becomes The incident angle of is greater than the critical angle on one side of the plane orthogonal to the plane of incidence passing through the optical axis, and smaller on the other side. Light and darkness occur. The brightness and darkness in the tangential direction on the photodetector 29 is determined by the relationship between the incident angles of the light rays on both sides of the plane perpendicular to the plane of incidence passing through the optical axis of the returned light on the reflective surface 28a. Since it is reversed in the case of divergent light and divergent light, the sixth
In Figures B and C, the brightness is reversed. Therefore, if the respective outputs of the light receiving areas 29a to 29d are a, b, c, and d, a focus error signal can be obtained by (a + b) - (c + d). Note that the radial error signal can be obtained by (a + c) - (b + d >), and the data signal can be obtained by (a + b + c + d).

(Problems to be Solved by the Invention) However, in the conventional focus detection device described above, since the polarizing beam splitter 23 is disposed between the collimator lens 22 and the reflecting prism 25, the distance between them is If the optical disc is required for a long time, the reflected light from the optical disc is separated from the optical path from the semiconductor laser 21 to the optical disc by the polarizing beam splitter 23, and then the critical angle prism 28
Since the light is made incident on the photodetector 29 through the polarization beam splitter 23 and the photodetector 29, the distance between the polarization beam splitter 23 and the photodetector 29 becomes long. For this reason, there is a problem that the entire device becomes large.

The present invention has been made in view of these conventional problems, and it is an object of the present invention to provide a focus detection device suitably configured so that the entire device can be made compact.

[Means and effects for solving the problem] In order to achieve the above object, the present invention includes a critical angle prism in the reciprocating optical path between the collimator lens and the objective lens, and a beam splitter between the light source and the collimator lens. The photodetector is placed at a position that is conjugate with the light source with respect to the collimator lens, and the light beam from the light source passes through the beam splitter, the collimator lens, the critical angle prism, and the objective lens and irradiates the object to be irradiated. The reflected light is received by a photodetector after passing through an objective lens, a critical angle prism, a collimator lens, and a beam splitter.

〔Example〕

FIG. 1 shows an embodiment of the present invention.

In this embodiment, a scattered light beam from a light source 1 such as a semiconductor laser is incident on a polarizing beam splitter 2 and its polarizing film 3
The transmitted light is made into a parallel light beam by the collimator lens 4 and is incident on the critical angle prism 5 as 88 lights.

The light incident on this critical angle prism 5 is reflected by its reflecting surface 5a.
The light is reflected by the three-wavelength plate 6 and projected onto the optical disk 8 by the objective lens 7 in the form of a spot. The reflected light from the optical disk 8 follows the same optical path as the outgoing path up to the polarizing beam splitter 2 and is incident on the polarizing film 3, but on the returning path, the direction of polarization is perpendicular to the polarizing direction of the outgoing path due to the action of the wavelength plate 6. Since it becomes P-polarized light, it is reflected by the polarizing film 3. A photodetector 9 receives the return light from the optical disk 8 that is reflected by the polarizing film 3 .

The optical beam splitter 2 used is one in which the dependence of the transmittance and reflectance of the polarizing film 3 on the angle of incidence is kept small.

Further, the critical angle prism 5 has its apex angle set so that the incident light beam and the outgoing light beam form an angle of approximately 90°, and the reflective surface 5a has an incident angle approximately at a critical angle with respect to the central ray of the light beam. Set it so that In this embodiment, an optical thin film 5b is further provided to increase the reflection of S-polarized light that is incident on the reflective surface 5a at an approximately critical angle, and to prevent reflection of P-polarized light. As shown in Figure 2, S
Polarized light is almost totally reflected, and the reflectance of P(li light is rapidly reduced at an incident angle below the critical angle).

Such an optical thin film 5b is made by using two types of materials, one with a lower refractive index and the other with a higher refractive index than the critical angle prism 5, on the reflective surface 5a, so that the refractive index is high, low, high...
It can be constructed by stacking layers in the order of -low and high to a thickness that does not satisfy r+dcosθ-λ/4. Here, n is the refractive index of the coating material, d is the thickness of the coating material, θ is the refraction angle in the coating material, and λ is the wavelength of light.

Further, as shown in FIG. 3, the photodetector 9 is configured with four light receiving areas 98 to 9d divided into two in the radial direction and tangential direction of the optical disc 8, and is conjugate with the light source 1 with respect to the collimator lens 4. It is placed closer to the polarizing beam splitter 2 than the other position. In this way, the returned light from the optical disk 8 is made to enter the photodetector 9 as a beam having an appropriate diameter.

In the above configuration, the S-polarized light beam entering the critical angle prism 5 via the collimator lens 4 is
The optical thin film 5b coated on the reflective surface 5a acts as a reflection increasing film for S-polarized light.
It is almost totally reflected at a. Further, the return light from the optical disk 8 that enters the critical angle prism 5 as P-polarized light is transmitted through the optical thin film 5.
Since b acts as an antireflection film for P-polarized light,
Light rays incident on the reflective surface 5a at an incident angle equal to or greater than the critical angle are totally reflected, but light rays incident at an incident angle slightly smaller than the critical angle have their reflectance rapidly reduced as shown in FIG. Therefore, the return light received by the photodetector 9 is adjusted to the sixth position depending on the focal state of the objective lens 7 with respect to the optical disk 8.
Since the changes occur in the same manner as explained in FIGS. A to C, a focus error signal, as well as a data signal and a radial error signal, can be obtained in the same manner from the outputs of the light receiving areas 98 to 9d.

FIG. 4 shows another embodiment of the invention. In this embodiment, in the configuration shown in FIG. By exchanging the arrangement of I11 and photodetector 9, the light beam from the light source 1 is reflected by the polarizing film 3 of the polarization splitter 2 and made into a parallel light beam by the collimator lens 4, and the returned light from the optical disc 8 is reflected by the polarizing film 3 of the polarization splitter 2. The difference is that the light beam 3 is transmitted and incident on the photodetector 9, and the other configurations are the same as in FIG. Note that the photodetector 9 is arranged with respect to the collimator lens 4 away from a position conjugate with the light source 1 so that the returned light is incident as a suitable spot. In this embodiment as well, a focus error signal can be obtained based on the output of the photodetector 9 in the same manner as in the above-described embodiment.

It should be noted that the present invention is not limited to the embodiments just described, and numerous modifications and changes are possible. For example, the optical thin film 5b can be omitted, or a half mirror can be used in place of the polarizing beam splitter 2. Furthermore, although linearly polarized light is used in the above-described embodiments, it is also possible to use light that does not have polarization characteristics. In this case, the optical thin film 5) and the wavelength plate 6 can be omitted, and a half mirror can be used in place of the polarizing beam splitter 2. Further, the photodetector 9 can be configured to have a two-divided light receiving area exclusively for detecting focus error signals, and it can also be placed farther away than the position conjugate with the light source with respect to the collimator lens.

(Effects of the Invention) As described above, in this invention, a beam splitter is disposed between the light source and the collimator lens, and a critical angle prism is placed in the optical return path between the collimator lens and the objective lens. Because of this arrangement, the optical path length can be minimized, and the entire device can therefore be made smaller. Furthermore, the critical angle prism also functions as a reflecting prism, which was required in the conventional example, so the number of parts can be reduced, which makes it simple and inexpensive. Furthermore, since the photodetector is disposed on the convergence side of the collimator lens, its light-receiving area can be reduced, and the device can therefore be made even more compact. Also, above)
In the embodiment described above, an optical thin film 5b is provided on the reflective surface 5a of the critical angle prism 5, which increases the reflection of the outgoing light beam and prevents the incoming light beam from reflecting. Can be detected with high sensitivity.

[Brief explanation of drawings]

1, 2, and 3 are views showing one embodiment of the present invention, FIG. 4 is a view showing another embodiment of the present invention, and FIGS. 5 and 6 A to C are views showing the conventional It is a figure showing a technique. 1... Light source 2... Polarizing beam splitter 3... Polarizing film 4... Collimator lens 5... Critical angle prism 5a... Reflecting surface 5b...
・Optical thin film 6...K wavelength plate 7...Objective lens 8...Optical disk 9...Photodetector
98-9d... Light-receiving area patent applicant Olympus Optical T Gyo Co., Ltd. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Procedures Amendment Written by the Commissioner of the Japan Patent Office January 22, 1986 Michibe Uga 1, Display of the case Patent Application No. 263664 of 1985 2, Name of the invention Focus detection device 3, Person making the amendment Relationship to the case Patent applicant (087) Olympus Optical Industry Co., Ltd. 4, Agent Person 5, subject of amendment: “Detailed description of the invention” column 1 of the specification
, "P-polarized light" on page 7, line 18 of the specification is corrected as "S-polarized light." 2. The following is added between page 7, line 20 and page 8, line 1 of the same specification. , In order to simplify the drawing, the reflection direction of the polarizing beam splitter 2 is rotated 90 degrees into the paper from the original direction perpendicular to the paper in order to simplify the drawing.'' 8, page 11, line 7 Next to "Can also be used.", "When using a half mirror, the direction of reflection is the first one, like when using a polarizing beam splitter."
In the drawing, there is no longer a restriction that the direction is perpendicular to the plane of the paper. ” to join.

Claims (1)

[Claims] 1. A light source, a collimator lens that converts the light beam from the light source into a parallel light beam, an objective lens that irradiates the light beam from the collimator lens as a spot onto an irradiated object, and the collimator lens and the objective lens. is arranged between the collimator lenses and guides the light flux from the collimator lens to the objective lens, and guides the light flux reflected by the irradiated object that is focused by the objective lens to the collimator lens. a critical angle prism having a reflecting surface set such that the incident angle is approximately the critical angle; and a critical angle prism disposed between the light source and the collimator lens, which guides the light flux from the light source to the collimator lens and a beam splitter that separates a reflected light beam from the irradiated object that is reflected by a reflective surface and exits through the collimator lens into a direction different from a light beam from the light source; a photodetector disposed at a position shifted from a position conjugate with the light source with respect to the collimator lens so as to receive a reflected light beam of the light source. 2. The reflective surface of the critical angle prism is coated with an optical thin film that increases reflection for S-polarized light and prevents reflection for P-polarized light, and the light beam from the light source is incident on the reflective surface as S-polarized light. 2. The focus detection device according to claim 1, wherein the focus detection device is configured to cause the reflected light beam from the irradiated object to be incident as P-polarized light.