JPH02306440A - Objective lens for optical head and the head - Google Patents

Abstract

PURPOSE:To reduce the diameter of a converged beam by forming a circular mask at the center part of an aspherical mold lens to shield the center part of the lens within a certain radius range. CONSTITUTION:An objective lens 1 is equal to an aspherical mold lens like an aspherical plastic lens, an aspherical press glass lens, etc. A circular mask 2 is formed at the center part of the lens 1 with a coating of an opaque material, etc., to shield the center part of the lens 1 within a certain radius range. When a light beam is made incident on the lens 1, the aberration is corrected by a diffracting function of the mask 2. As a result, the diameter of a converged beam can be reduced without using a means which increases the NA of a lens.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to an optical tisf device (optical information recording device) that records information on a recording medium using optical means or reproduces information that has already been recorded. The present invention relates to an optical head used in a recording/reproducing apparatus) and an objective lens used in this optical head.

[Prior Art] Examples of the above-mentioned optical disc devices include a read-only optical disc device, a write-once optical disc device that can write once, and a rewritable tiff device that can record, play, and erase. Conventional objective lenses used in the optical head of this type of optical disk device include spherical glass combination lenses, aspherical plastic lenses, aspherical glass press lenses, and others.

[Problem to be solved by the invention] By the way, since write-once type and rewritable type optical heads in particular have a recording operation, it is necessary to irradiate a smaller spot on the optical head than that of a reproduction-only type optical head. Since a small spot can be obtained by reducing the converging beam diameter (beam waist diameter) do shown in Fig. 4, it is necessary to make the converging beam diameter (I, 1) sufficiently small. The diameter d is the wavelength λ of the laser beam
, is determined by the numerical aperture NA (NA=a/L) of the objective lens, and d,=K・λ/NA (however, K is a constant), Therefore, in order to reduce the convergent beam diameter d, For this purpose, it is necessary to reduce the wavelength λ of the laser beam or use a lens with a high NA.

On the other hand, if the distance between the objective lens and the disk surface during operation, that is, the working distance, is short, collisions between the objective lens and the disk are likely to occur, damaging the objective lens or the disk. As a condition, the above-mentioned working distance is required to be long.

However, there are conflicting demands between increasing the NA of the lens to narrow down the spot diameter and increasing the working distance to prevent damage to the disk or lens due to collision between the disk and the lens.

That is, increasing the NA of the lens means shortening the focal length f when the lens radius a is constant, which shortens the working distance of the objective lens.

Conventionally, a method of placing a band-shaped light-shielding plate in the optical path from the laser light source to the objective lens to reduce the diameter of the converged beam by the objective lens is known as the super-i method. Since it is a band-shaped light shielding plate, it is possible to focus the convergent beam in the width direction of the light shielding plate, but there is no effect of converging the convergent beam in the length direction of the light shielding plate.

The present invention has been made in view of the above circumstances, and its main purpose is to obtain an objective lens that can simultaneously satisfy the requirements of using a lens with a low NA and reducing the diameter of a convergent beam.

[Means for Solving the Problems] The invention of claim 1 which solves the above problems includes providing a circular mask in the center of the aspherical molded lens to shield this center within a certain radius range. This is a characteristic objective lens for an optical head.

The invention of claim 2 provides an optical head in which a light beam generated by a laser light source is focused by an objective lens to form a light spot on an optical disk signal surface, and a signal detection section detects the reflected light reflected from the optical disk signal surface. An optical head characterized in that the objective lens according to claim 1 is used as the objective lens, and a circular slit for shielding a diffraction ring is provided in front of the signal detection section.

[Function]' In the objective lens having the above configuration, aberrations are corrected by the diffraction effect due to the presence of the circular mask, and the convergent beam diameter is reduced two-dimensionally, that is, in two orthogonal directions. In this case, a ring-shaped diffraction ring (side lobe) is generated around the reduced convergent beam (main lobe).

In the optical head of the second aspect, the useless diffraction ring in the return light reflected by the disk and incident on the information signal photodetector is blocked by the circular slit, and only the main lobe is incident on the photodetector. do.

[Example] Hereinafter, an example of the present invention will be described with reference to FIGS. 1 to 3.

FIGS. 1A and 1B show an objective lens according to an embodiment of the present invention. This objective lens 1 is an aspherical molded lens such as an aspherical plastic lens or an aspherical pressed glass lens, and a circular mask 2 that shields the center of the lens within a certain radius range is coated with an opaque material, for example. This is the configuration provided. The material for the aspherical pressed glass lens is heat-resistant PMMA (
Polymethyl methacrylate) is preferred. Further, the material of the aspherical pressed glass lens may be general optical glass such as soda lime glass.

When a light beam is incident on this objective lens 1, a convergent beam with an intensity distribution shown by a solid line in FIG. 2 is obtained. In other words, there is a main lobe (convergent beam that forms the necessary spot on the disk surface) M that is focused two-dimensionally (in two perpendicular directions) at the center, which is the original convergent beam, and a ring-shaped main lobe that is generated around it. Weak diffraction rings (side lobes) are obtained.

That is, in the case of a full aperture lens without a circular mask 2, the intensity distribution will be as shown by the broken line, but in the objective lens 1 of the present invention with the circular mask 2 in the center, the original intensity distribution will be as shown by the solid line. The convergent beam diameter (main lobe diameter) do is smaller than that of a full aperture lens. The reason why a convergent beam with such an intensity distribution is obtained is that aberrations are corrected by the diffraction effect caused by the presence of the circular mask, and due to this aberration correction effect, the convergent beam diameter is two-dimensionally Two directions perpendicular to each other (smaller in the x and y directions. For example, numerical aperture NA = 0.45, shielding by a circular mask
When the objective lens of the present invention has a hardness ratio of -0.2 to 05, and the laser beam wavelength is λ-〇, 78 μm, the working distance W is
With D=1 and 9 rom, a spot diameter smaller than that of a full aperture lens can be achieved.

An example of an optical head using the objective lens 1 described above is shown in FIG.

In the figure, reference numeral 3 denotes an optical disk, and an imaging optical system that focuses the laser beam from the semiconductor laser 4 onto the surface of the disk 3 includes a collimator lens 5, a shaping prism 6, a polarizing beam splitter 7, and a quarter-wave plate. 8. The objective lens 1 of the present invention described above is arranged in order toward the disk 3. The information signal detection optical system that detects the information signal obtained by diffraction and reflection from above the recording pit on the disk 3 includes a beam splitter 9, a condensing lens 10, a photodetector 11 for detecting the information signal, It is composed of circular sleeves 1 to 12 having circular small holes arranged in front of the detector 11. Further, an error signal detection system for obtaining an error signal for driving the objective lens actuator and controlling the positioning of the focused beam together with the objective lens with high precision includes a critical angle prism 13 and a photodetector 1 for error signal detection.
It consists of 4.

The laser beam emitted from the semiconductor laser 4 passes through a collimator lens 5, a shaping prism 6, and a polarizing beam splitter 7.1/
After passing through the four-wavelength plate 8, the light is focused by the objective lens 1 to form a spot on the disk surface. This spot has the intensity distribution shown in FIG. 2 described above, and a two-dimensionally narrowed and sufficiently efficient spot can be obtained.

The return light reflected by the disk 3 passes through the objective lens 1 and 1/4 wavelength plate 8 along the same path, and then passes through the polarizing beam splitter 7.
The light beam is reflected in the right angle direction, passes through the beam splitter 9 and the condenser lens 10, and passes through the circular slit 12. As mentioned above, the returned light contains side lobes as well as the main lobe, which is the original spot, but these side lobes are blocked by the circular slit 12, and only the main lobe component is transmitted, and the light is detected by the photodetector 11. Detected by

In this way, unnecessary side lobes do not enter the photodetector 11, so crosstalk due to side lobes can be prevented.

Note that the convergent beam diameter that can be reduced by the circular mask 2 and the intensity of the generated sight lobe are related to each other, so the light shielding rate of the lens by the circular mask and the wavelength λ of the laser beam are
The convergent beam diameter and sidelobe intensity are controlled by appropriately selecting a combination of

[Effects of the Invention] 8. Since the present invention is configured as described above, it has the following effects.

Since a circular mask was provided at the center of the aspherical lens, aberrations were corrected by this circular mask, and the diameter of the convergent beam could be reduced without using means to increase the NA of the lens.

Since the diameter of the converging beam can be made small, high-density recording is possible, and application to write-once and rewritable systems that require a small spot due to recording operations is facilitated.

Since the diameter of the converging beam can be made smaller by using a circular mask, the amount of aberration correction can be reduced when designing an aspherical molded lens (in other words, when designing the lens surface shape for aberration correction), and the aberration correction lens design becomes easier.

Since a low NA lens can be used (that is, a lens with a long focal length can be used), a long working distance can be obtained as an objective lens, and damage to the disk or lens due to collision between the disk and lens can be avoided. It can be prevented.

Furthermore, since a low NA lens can be used, the cost is reduced.

[Brief explanation of drawings]

FIG. 1 (A) is a front view of an objective lens showing an embodiment of the present invention, FIG. 1 (B) is a cross-sectional view of the same, and FIG. , FIG. 3 is a diagram of the optical system configuration of an optical head using the same objective lens, and FIG. 4 is an explanatory diagram of the convergence state of a light beam transmitted through a general objective lens. 1... Aspherical molded lens (objective lens), 2...
・Circular mask, 12...Circular slit.

Claims (2)

[Claims] (1) An objective lens for an optical head, characterized in that a circular mask is provided at the center of an aspherical molded lens to shield the center within a certain radius range. (2) A light beam generated by a laser light source is focused by an objective lens to form a light spot on the optical disc signal surface,
An optical head in which a signal detection section detects reflected light reflected on an optical disk signal surface, as the objective lens according to claim 1.
An optical head characterized in that the objective lens described above is used and a circular slit for shielding a diffraction ring is provided in front of the signal detection section.