JPH0453008B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical information processing device, and particularly to an optical information processing device using a semiconductor laser element as a light source.

[Traditional background]

In recent years, development of optical information processing devices that use semiconductor laser elements as light sources in place of gas lasers has become active. Optical disks are one example. An optical disk uses a semiconductor laser element to reproduce information signals stored on a disk (disk) or to store information in a disk at high density. In other words, in order to record and reproduce information signals on a disk using a semiconductor laser,
The beam emitted from the semiconductor laser element must be formed into a circular light spot with a diameter of about 1 μm on the disk using a coupling lens and an objective lens that constitute an optical system.

In general, semiconductor laser devices have different vertical and horizontal ratios of their light emitting regions, so the beam divergence angle is extremely irregular. The divergence angle of this semiconductor laser beam differs depending on the structure of the semiconductor laser element. That is, as shown in FIG. 1, the angles at e ^-2 in the horizontal and vertical directions of the output light distribution in the far-field image of the beam from the semiconductor laser device are respectively θθ.
For example, in an SCP type semiconductor laser, θ=8°, θ⊥=24°, and θ⊥/θ=3 (1). In addition, in the BH type semiconductor laser, θ = 16°, θ⊥ = 32°, and θ⊥ / θ = 2 ... (2), and in the BH type laser, the beam divergence angle ratio θ⊥ / θ is 2, and the CSP For type lasers, it is 3. Note that the horizontal axis in FIG. 1 is the spread angle, and the vertical axis is the light intensity. FIG. 2 shows an example of a conventional optical information processing apparatus for forming an isotropic spot with a diameter of about 1 μmφ on a disk when the beam divergence angle of the semiconductor laser device described above is not isotropic.

In FIG. 2, a beam having a non-isotropic divergence angle emitted from one end face of the semiconductor laser element 1 is formed into a light spot 5 on a disk 4 by a coupling lens 2 and an objective lens 3. The photodetector 6 is a means for detecting the optical output of the semiconductor laser element 1. Note that A is the optical axis. In FIG. 2, the numerical aperture NA of the coupling lens 2 has the relationship NA=sinθ (3) where θ is the half angle of view formed by the semiconductor laser 1 and the lens 2. Regarding the beam divergence angle of the semiconductor laser element 1, if the magnitudes at e ^-2 in the horizontal and vertical directions are θ and θ⊥ as described above, then using such a semiconductor laser element In order to form a square spot 5, the numerical aperture NA of the coupling lens 2 must be selected so that θθ<θ⊥= (4). That is, by reducing the numerical aperture of the coupling lens 2 and blocking off-axis rays, the optical axis A is
It is necessary to make the intensity distribution of the light emitted from the coupling lens 2 isotropic by using only the beam around (θ=0). The divergence angle of the beam shown in Fig. 1 and (1),
From equations (3) and (4), for a CSP type laser, NA = 0.1 θ = 5.7° (θ < θ⊥) ...(5), then the beam after passing through the condenser lens 2 is almost isotropic. Therefore, an isotropic spot 5 is formed on the disk 4.

However, in this configuration, only a portion of the light beam emitted from the semiconductor laser element is irradiated onto the disk, so there is a drawback that the light utilization efficiency of the laser element is poor. In particular, in the case of recording, the metal thin film provided on the disk must be dissolved and holes must be formed, which requires several times the amount of light compared to the case of reproduction. Furthermore, when a semiconductor laser element emits a certain amount of light or more, its lifetime becomes short. Therefore, in an optical information processing device using a semiconductor laser element, it is absolutely necessary to increase the light utilization efficiency of the laser element and to suppress the optical output as much as possible from the viewpoint of lifespan and reliability. Further, normally, the disk rotates with a vertical deviation of about 1 mm. In order to prevent the diameter of the spot from changing regardless of the vertical movement of the rotary disk, it is necessary to optically detect a defocus signal and perform automatic focusing.

Now, in the configuration shown in FIG. 2, when the reflected light from the disk 4 returns to the semiconductor laser element 1, the output of the semiconductor laser 1 increases or decreases depending on the strength of the reflected light from the disk. It can be reproduced by the output of the detector 6. This technique is described in Japanese Patent Application Laid-open No. 49-69008.

On the other hand, in order to detect the deviation of the focus of the light beam from the disk, a technology was developed in Japanese Patent Application Laid-Open No. 53/1987 that detects the deviation of the focus by slightly vibrating the light source or lens in the direction of its optical axis and synchronously detecting the laser output. −
This is proposed in Publication No. 17706. However, this technique has the disadvantage that the automatic focus control pull-in range is narrow. Figure 3 shows the change in output from the semiconductor laser element 1 when the disk 4 is slightly moved along the optical axis direction in the configuration shown in Figure 2, when the numerical aperture NA of the coupling lens 2 is 0.1. It is something. As is clear from Fig. 3, the coupling lens 2
It can be seen that when the numerical aperture NA is very small at 0.1, the automatic focusing range is only 10 μm.
This is because the focal position of the reflected return light spot near the end face of the laser element changes greatly due to the displacement of the disk. In other words, the reflected return light spot on the end face of the laser element is significantly blurred due to the focus shift, and this causes the automatic focus pull-in range to be limited.
This results in a small number of 10 μm.
The optical information processing apparatus that returns the reflected light from the disk to the semiconductor laser element in this way has the disadvantage that the pull-in range is small, as described above. For this reason, automatic focus control was difficult and information could not be reproduced from a disk with large vertical vibrations.

On the other hand, it has been considered to use a cylindrical lens to make the narrowed spot on the disk closer to a circular shape. However, cylindrical lenses have the drawbacks of being difficult to produce accurately, being expensive, and complicating the arrangement of the optical system.
It is difficult to converge to a circular spot of 1 μm because a cylindrical lens is used and astigmatism is greatly affected.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical information processing device capable of forming a semiconductor laser beam having no isotropic divergence angle as an isotropic spot on a disk with efficient use of light. .

[Summary of the invention]

In order to achieve this objective, the present invention ignores equation (4), which is the condition of the coupling lens that the light spot on the disk has an isotropic shape, and
By increasing the numerical aperture of the coupling lens, almost all of the anisotropic beam from the semiconductor laser element enters the coupling lens, and by placing a prism after the coupling lens, the coupling lens can be This converts the non-isotropic beam that has passed through the beam into an isotropic beam. Also,
In the present invention, focusing on the fact that semiconductor laser light is polarized in a direction parallel to the cemented surface, by arranging a prism so that this laser light is polarized within the incident plane, the input and output from the prism is controlled. It almost eliminates reflection loss and increases the efficiency of light use.

[Embodiment of the Invention] FIG. 4 is a diagram showing the relationship between the position of the disk and the optical output. That is, in the configuration shown in FIG. 2, the numerical aperture NA of the coupling lens 2 is increased by ignoring equation (4), which is the condition for the coupling lens that the optical spot 5 on the disk 4 has an isotropic shape. (NA=
0.5) and shows the change in the optical output from the semiconductor laser element when the disk 4 is slightly moved along the optical axis direction. As is clear from FIG. 4, the pull-in range is 100 μm.

That is, by increasing the numerical aperture of the coupling lens, the change in the focal position of the reflected return light spot due to the displacement of the disk is reduced, and the automatic focusing range is expanded.

Thus, the larger the numerical aperture of the coupling lens, the wider the automatic focus pull-in range, and complete automatic focus can be achieved against vertical movement of the disk. Furthermore, as the numerical aperture of the coupling lens increases, more beams from the semiconductor laser element are incident on the coupling lens, and the efficiency of using light from the laser element increases. However, if the numerical aperture NA of the coupling lens satisfies the vertical spread angle θ⊥ at approximately e ^-2 of the far-field pattern shown in Figure 1, most of the beam from the semiconductor laser element can be effectively transferred to the coupling lens. It will be made incident. Therefore, in reality, it is sufficient to satisfy θ<θ≦θ⊥ (6).

However, since the light beam passing through the coupling lens 2 that satisfies equation (6) is not isotropic, the light spot 5 is also not isotropic. Therefore, in the present invention, in order to solve this problem, a prism 7 is arranged after the coupling lens 2, as shown in FIG. FIG. 9 shows the shape of the prism 7 mentioned above. 9th
In the figure, the prism is a right-angled prism with an apex angle of θα and a refractive index of N, and if the incident angle θi and the ratio of the incident beam diameter I to the refracted beam diameter O are m=O/I, these are respectively It is given by the following formula.

cosθ _i =N ² −1 N ² m ² −1 cosθα=mN ² −1 N ² m ² −1 m=cosθα cosθ _i R ₁₁ = tan ² (θ _i −θα) tan ² (θ _i +θα) ... (7) Here, R ₁₁ is the reflectance at the entrance surface of the prism. The prism 7 extends the horizontal direction of the spread of the beam from the semiconductor laser element so that it coincides with the vertical direction, thereby converting the beam into an isotropic beam.
Therefore, in order to obtain an isotropic beam when most of the beam from the semiconductor laser device is incident on the coupling lens, m must be set as the ratio of the beam divergence angle θ⊥
It is necessary to match θ. For example, when using a BH type semiconductor laser device, m=
It is 2. Assuming that the refractive index of the prism N=1.7636, θ _i , so that the reflectance R ₁₁ is as small as possible
If θα is selected, from equation (7) m = 2.03 N = 1.7636 θα = 30.935° θ _i = 65° R ₁₁ = 0.05 ... (8) Therefore, the beam divergence angle expressed by equation (2) is By inserting a prism 7 expressed by equation (8) immediately after the coupling lens 2 in FIG. 5, it is possible to convert a BH type semiconductor laser into an isotropic beam. This isotropic light beam is irradiated onto the disk 4 by the objective lens 3 as an isotropic spot. Therefore, it becomes possible to irradiate a beam as an isotropic spot onto the disk without blocking a portion of the beam from the semiconductor laser device having light emitting regions with different vertical and horizontal ratios. Moreover, since a prism is used, no aberration occurs. In a configuration in which the beam emitted from one end face of the semiconductor laser device 1 is reflected by a disk and the reflected light is returned to the end face as in this embodiment, the numerical aperture of the coupling lens 2 is determined by equation (6). Therefore, in order to make it larger, the fourth
As shown in the figure, the autofocus pull-in range has been expanded.

In FIG. 5, the beam from the semiconductor laser element 1 is set to be P-polarized light (the plane of polarization vibrates parallel to the plane of the paper) as indicated by the arrow in the figure.

FIG. 6 is a diagram showing the configuration of another embodiment of the present invention, in which the same reference numerals as in FIG. 5 indicate the same or equivalent parts. In the embodiment shown in FIG. 6, unlike the embodiment shown in FIG. The arrangement is opposite to that of the illustrated embodiment. That is, the beam from the semiconductor laser element 1 is set to S-polarized light (the plane of polarization vibrates perpendicular to the plane of the paper) as shown by the black circle in the figure, and this is converted to P-polarized light by the half-wave plate 11. The light is then incident on the prism 7. By doing so, the objective lens 3 can be made smaller.

From the above, the polarization of the beam is S-polarized in the vertical direction and P-polarized in the horizontal direction, as shown in FIG.

In the above explanation, by reflecting the beam emitted from one end face of the semiconductor laser element on the disk and returning the reflected light to the end face,
Although only an optical information processing device for recording and reproducing predetermined information has been described, the present invention is not limited to such an optical information processing device. The present invention can also be applied to optical information processing devices that record and reproduce predetermined information by extracting reflected light from a disk using this prism and detecting changes in the reflected light using a photodetector.

FIG. 8 is a diagram showing the configuration of an embodiment in which the present invention is applied to such an optical information processing device. In this embodiment, a prism 9 and a single wavelength plate 8 are arranged between the prism 7 and the objective lens 3 in the configuration of the embodiment shown in FIG. With this configuration, it becomes possible to extract the reflected light from the disk 4 using the prism 9 and detect changes in the reflected light using the photodetector 10. In the embodiment shown in FIG. 8, the photodetector 6 is used as a laser light output monitor for so-called automatic light output stabilization control to keep the laser light output constant.

〔Effect of the invention〕

As explained above, according to the present invention, an isotropic (circular) spot can be formed on a disk with a simple optical system even if a semiconductor laser element that does not have an equal divergence angle is used as a light source. Moreover, since the semiconductor laser beam enters the prism as P-polarized light, there is almost no reflection loss when entering and exiting the prism, and the efficiency of light utilization can be increased.

In the above explanation, the case where the amount of reflected light from the disk is detected as a change in the laser light from the other direction of the semiconductor laser is described, but the change in the reflected light is detected as a change in the driving current of the semiconductor laser 1. Of course, the present invention can also be applied to detection.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a far-field image of a semiconductor laser beam, FIG. 2 is a diagram for explaining a conventional optical information processing device, and FIG. 3 is a diagram for explaining a conventional autofocus pull-in range. FIG. 4 is a diagram explaining the automatic focusing range according to the present invention, FIGS. 5 and 6 are diagrams showing the configuration of an embodiment of the present invention, and FIG. 7 is a diagram explaining the operation of the present invention. FIG. 8 is a diagram showing the configuration of another embodiment of the present invention, and FIG. 9 is a diagram for explaining the present invention.

Claims (1)

[Claims] 1. A semiconductor laser that emits light whose intensity distribution around an optical axis is anisotropic with respect to the optical axis, and an information recording medium;
a rotationally symmetric coupling lens that collimates the light beam from the semiconductor laser; a prism that expands or contracts the collimated light beam; an optical system that converges the light from the prism onto the medium; and a means for detecting a change in the amount of light of a light beam from the optical information processing apparatus, wherein the rotationally symmetrical coupling lens satisfies the following conditions. Conditions: θ‖<θ≦θ⊥ NA=sinθ (However, the angles at e ^-2 in the horizontal and vertical directions of the output light distribution in the far-field image of the light beam from the semiconductor laser above are θ‖ and θ⊥, respectively. and the numerical aperture of the coupling lens is NA.) 2 The semiconductor laser is a semiconductor laser element that outputs a light beam polarized in the minor axis direction of an anisotropic distribution, and the optical system 2. The optical information processing device according to claim 1, further comprising a prism disposed so that the light beam of the prism is polarized within an incident plane. 3. A 1/2 wavelength plate is provided in the optical path between the prism and the laser element, and the prism is arranged so that the light beam from the wavelength plate enters perpendicularly. Optical information processing device according to scope 2.