JPH0630166B2 - Optical head device - Google Patents

Description

The present invention relates to an optical head device used for recording / reproducing of so-called optical disc, digital audio disc, video disc and the like.

[Conventional technology]

As shown in FIG. 2, a conventional optical head device used for video discs, digital audio discs, and optical discs (hereinafter collectively referred to as optical discs) has a semiconductor laser 1 as a light source and a radiated light 2 from the semiconductor laser 1. To the disk surface 4, an image forming lens 3, a beam splitter prism 8, a spherical concave lens 5, a cylindrical lens 6,
A focus error detecting unit and a tracking error detecting unit each including the four-division photodetector 7 are provided.

Such a conventional optical head device has a size of about 40 × 40 × 30 mm ³ even though it has been put to practical use, and therefore it is heavy, and the optical disc device can be made smaller and lighter or the stack type can be made larger. It was an obstacle to the realization of a capacity optical disk.

In order to solve this problem, the present applicant has achieved a reduction in the number of optical elements by performing three functions of a beam splitter, a focus error detection optical system, and a tracking error detection optical system with one grating lens. Applied for a lightweight optical head device. Examples of these applications are Japanese Patent Application No. 61-121575, Japanese Patent Application No. 61-121577, and Japanese Patent Application No. 61-156406. As an example, an optical head device described in Japanese Patent Application No. 61-156406, which has a configuration similar to that of the optical head device of FIG. 2, will be described.

FIG. 3 is a perspective view showing the basic structure of the optical head device described in Japanese Patent Application No. 61-156406. The emitted light 2 of the semiconductor laser 1 passes through the grating lens 17 as the 0th-order diffracted light, and is converged on the disk surface 4 by the imaging lens 3. Disc surface 4
The reflected light from is converged by the imaging lens 3, diffracted by the grating lens 17, and reaches the four-division photodetector 20 beside the semiconductor laser 1 as diffracted lights 18 and 19. The four-division photodetector 20 is composed of four photodetection elements 11, 12, 13, and 14 divided by division lines 24 and 25.

The grating lens 17 forms a circular beam on the photodetector 20, and a grating corresponding to an interference fringe of a converging wavefront including astigmatism having focal lines orthogonal to the front and back thereof and a spherical wave converging the semiconductor laser 1. Have a pattern. In FIG. 3, the pitch of the grid is drawn larger than it actually is in order to make the arrangement easy to understand. The grating lens 17 is composed of an upper grating lens 22 and a lower grating lens 23 having different diffraction angles with a boundary line 21 parallel to the direction connecting the semiconductor laser 1 and the four-division photodetector 20 as a boundary. The light diffracted by the upper grating lens 22 becomes a semicircular light beam rotated by 90 ° due to astigmatism and reaches the dividing line 25 of the photodetection elements 13 and 14. On the other hand, the light diffracted by the lower grating lens 23 becomes a semicircular light beam rotated by 90 ° due to astigmatism and reaches the dividing line 25 of the photodetection elements 11 and 12. Therefore, when the converging light is focused on the disk surface 4, the four-division photodetector is arranged so that the intensity of both diffracted lights of the four-division photodetector 20 incident on the photodetection elements 11, 12, 13, and 14 are equal. Second by placing 20
Similar to the conventional astigmatism method shown in the figure, V (11) + V (13)-
The focus error signal can be detected by V (12) -V (14). Here, V (11) to V (14) are output voltages of the respective elements 11 to 14 of the photodetector.

On the other hand, the tracking error signal utilizes the fact that the intensity distribution of reflected light becomes unbalanced when the narrowed spot on the disk surface 4 deviates from the track position. In the configuration of FIG. 3, the intensity ratio of the diffracted lights 18 and 19 changes due to the track shift, so V (11) + V (12) -V (13) of the four-division photodetector 20.
The tracking signal can be detected by taking -V (14).

The reproduction signal from the disc can be detected by taking the sum V (11) + V (12) + V (13) + V (14) of the light quantity of the four-division photodetector 20.

[Problems to be solved by the invention]

In the optical head device using the above-mentioned grating lens, the light beam is the 0th-order diffracted light on the outward path and the 1st-order diffracted light on the return path and is transmitted through the grating lens. Therefore, the light utilization rate is (0th-order diffraction efficiency) ×
(First-order diffraction efficiency). This value is only 10% at most in the case of the concave-convex grid having a rectangular cross section, and there is a problem that the light utilization rate is low.

An object of the present invention is to solve the above-mentioned drawbacks and to provide a small-sized optical head device having high light utilization efficiency.

[Means for solving problems]

The present invention provides a light source, an imaging lens that narrows down an image of the light source onto a recording medium, a photodetector having a light-receiving surface arranged beside the light source, and the light source and the imaging. An optical head which is provided between lenses and is composed of a plurality of divided grating lenses that divide reflected light from the recording medium that has passed through the imaging lens and guide it to the light receiving surface of the photodetector. In the apparatus, the grating lens is a grating lens in which at least the concave portion of the surface unevenness grating formed on the birefringent medium is filled with a substance having a refractive index substantially equal to the ordinary or extraordinary refractive index of the birefringent medium, and the grating A quarter wave plate is arranged between the lens and the imaging lens.

[Action]

For example, when the concave portion of the surface concavo-convex grating formed on the birefringent medium is filled with a substance having a refractive index almost equal to its ordinary light refractive index,
For ordinary light, the grating is refractive index matched and no longer acts as a grating, but merely serves as a medium having an ordinary refractive index. On the other hand, for extraordinary light, the extraordinary light acts as a phase grating composed of the refractive index of the extraordinary light and the refractive index of the filling material, diffracts the extraordinary light, and effectively changes the optical path. Such a “polarizing beam splitter” was designed by the present applicant on July 18, 1986.
Apply by date. Therefore, an isolator can be constructed by combining such a polarization beam splitter and a quarter-wave plate. That is, the light beam is transmitted through the grating lens without being diffracted in the forward direction and is transmitted almost 100%, and is transmitted through the quarter-wave plate to be circularly polarized and converged on the disk. Since the rotation direction of the circularly polarized light of the light beam reflected by the disc is the reverse of that of the incident light, it becomes linearly polarized light that is orthogonal to the polarization direction of the outgoing light beam by the 1/4 wavelength plate, and almost 100% diffracted by the grating lens. To be done. As a result, a high light utilization rate can be obtained.

〔Example〕

 Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view showing the basic structure of an embodiment of the present invention. The structure and operation of the optical head device is a grating lens.
It is the same as the optical head device of FIG. 3 except that 26 is a polarization type grating lens and that a quarter wavelength plate 27 is added. Therefore, description of the same components will be omitted and only polarization will be described. explain. The grating lens 26 is formed by forming a grating lens composed of an upper grating lens 29 and a lower grating lens 30 as a concave-convex grating on a calcite substrate in which the boundary line 28 and the vibration direction of the extraordinary light are parallel to each other. Refractive index n _e = 1.
An acrylic resin having a refractive index n = 1.49 which is almost equal to 485 is applied as an embedding material to form a flat surface.

In the optical head device configured as described above, the emitted light of the semiconductor laser 1 reaches the grating lens 26 as linearly polarized light parallel to the boundary line 28 of the grating lens 26. Since the grating lens 26 has the above-described configuration, the emitted light of the semiconductor laser 2
Passes through the grating lens 26 without being diffracted, and
Reach the four-wave plate 27. Since the optical axis of the quarter-wave plate 27 is set to 45 ° with respect to the boundary line 28 of the grating lens 26,
The transmitted light becomes right-handed circularly polarized light and converges on the disk surface 4. The reflected light from the disk surface 4 becomes circularly polarized light by reflection and reaches the quarter-wave plate 27, and the boundary line 28 of the grating lens 26.
Is converted into linearly polarized light perpendicular to. Since the polarization direction of the ordinary light oscillation direction of the grating lens 26, the grating lens 26 acts as a phase grating consisting of a refractive index of the ordinary refractive index n _{o =} 1.654 and n = 1.49. Since the refractive index is as large as 0.1644, it is possible to obtain a diffraction efficiency of almost 100% by appropriately setting the groove depth when the original unevenness is obtained. Diffracted light 18 and 19 from the grating lens 26 is 4
Reach the split photodetector 20.

In this embodiment, the astigmatism type lens is used as the grating lens, but the dual focus method, the four-division grating lens, or the Foucault method (double knife edge method) is used.
It goes without saying that any of the above grating lenses may be used.

〔The invention's effect〕

According to the optical head device of the present invention, it is possible to realize an optical head device having a small number of parts, a small size, and high light utilization efficiency.

[Brief description of drawings]

FIG. 1 is a perspective view showing a basic configuration of an embodiment of the present invention, FIG. 2 is a sectional view showing an example of a conventional optical head device, and FIG. 3 is an example of a conventional optical head device using a grating lens. FIG. 1 ... Semiconductor laser 2 ... Synthetic light 3 ... Imaging lens 4 ... Disk surface 5 ... Concave spherical lens 6 ... Cylindrical lens 7,20 4 photodetector 8 ... Beam splitter prism 11, 12,13,14 …… Photodetector 17 …… Grating lens 18,19 …… Diffracted light 21,28 …… Boundary line 22,29 …… Upper grating lens 23,30 …… Lower grating lens 24,25… … Dividing line 26 …… Lattice lens 27 …… Quarter wave plate

Claims (1)

[Claims] 1. A light source, an imaging lens for narrowing down an image of the light source onto a recording medium, a photodetector disposed beside the light source and having a light-receiving surface divided into a plurality of portions, and the light source and the connection. Light composed of a plurality of divided grating lenses, which are provided between the image lenses and divide the reflected light from the recording medium that has passed through the imaging lens and guide the divided light to the light receiving surface of the photodetector. In the head device, the grating lens is a grating lens in which at least the concave portion of the surface concavo-convex grating formed on the birefringent medium is filled with a substance having a refractive index substantially equal to the ordinary or extraordinary refractive index of the birefringent medium, An optical head device characterized in that a quarter-wave plate is arranged between a grating lens and the imaging lens.