JPS6153775B2 - - Google Patents

Description

[Detailed description 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. 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 recorded on a disk (disc) or to record information on the 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 aspect 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 θ.
, θ⊥, for example, in a CSP type semiconductor laser, θ=8°, θ⊥=24°, and θ⊥/θ=3
…(1) becomes. In addition, for 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 3 for CSP type lasers. 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 θ<θ<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 coupling 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, when recording, it is necessary to melt the thin metal film provided on the disk and form holes, which requires several times the amount of light compared to when reproducing. 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, the light source or lens is slightly vibrated in the direction of its optical axis, and the laser output is synchronously detected. As a result, the technology for detecting defocus was developed in Japanese Patent Application Laid-Open No. 53
- 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 of 0.1 is very small, 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 as a result, the automatic focus pull-in range is 10 μm.
This results in a small number called m. The optical information processing apparatus that returns the reflected light from the disk to the semiconductor laser element as described above 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 shake. 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 with precision in machining, being expensive, and complicating the arrangement of the optical system; This is difficult because astigmatism has a large effect on the lens used. Furthermore, in order to increase the efficiency of the optical system, it is necessary to increase the transmittance of the cylindrical lens, and anti-reflection coating is required. The present invention is an optical information processing device that uses a semiconductor laser whose output light beam is anisotropic, has excellent light utilization efficiency, and is capable of forming an isotropic spot on a recording medium with a simple optical system. The purpose is to provide In order to achieve this object, the present invention is characterized in that a prism is provided in the optical system that guides the beam from the semiconductor laser element to the information recording medium. That is, 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 increases the numerical aperture of the coupling lens to increase the light output from the semiconductor laser element. By making almost all of the non-isotropic beam incident on the coupling lens and placing a prism after the coupling lens, the non-isotropic beam that has passed through the coupling lens is converted into an isotropic beam. It is something. In addition, in the present invention, focusing on the fact that the polarized light of the semiconductor laser is horizontal to the junction surface, the prism is designed so that the semiconductor laser light enters as P-polarized light and expands the horizontal distribution of the semiconductor laser light. and arranged so that the beam is output perpendicularly from the exit end face of this prism,
It is characterized by almost no reflection loss when entering and exiting the prism, and the efficiency of using the light beam is greatly increased. The present invention will be explained below with reference to the drawings. 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 approximately satisfies the vertical divergence angle θ⊥ at e ^- ₂ of the far-field pattern shown in Figure 1, most of the beam from the semiconductor laser element can be effectively absorbed by the coupling lens. It will be made to be incident on. 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-angle prism with an apex angle of θα and a refractive index of N, the incident angle is θi, and the ratio of the incident beam diameter I to the refracted beam diameter O is m=O/I, then these are as follows. Each is given by the following formula. 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 beam divergence angle ratio θ.
It is necessary to match ⊥/θ. For example, BH
When using a type semiconductor laser element, m=2 according to equation (2). Assuming that the refractive index of the prism N=1.7636, if θ _i and θ〓 are selected so that the reflectance R ₁₁ is as small as possible, then from equation (7),

[Table] Therefore, for a BH type semiconductor laser having a beam divergence angle expressed by equation (2), the prism 7 expressed by equation (8) is inserted immediately after the coupling lens 2 in FIG. This makes it possible to convert the beam 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. As described above, in this embodiment, since the semiconductor laser light is incident on the prism as P-polarized light, there is almost no reflection loss when the semiconductor laser light is incident on the prism. For example, when S-polarized light is incident, its reflectance R _s is R _s = sin ² (θ _i - θ〓) / sin ² (θ _i + θ〓), so in the case of the above-mentioned BH type semiconductor laser, R _s = 0.32, and the reflection loss at the time of incidence is large, and the efficiency of light utilization is significantly reduced. FIG. 6 is a diagram showing the configuration of a reference example of the present invention, and the same reference numerals as in FIG. 5 indicate the same or equivalent parts. In FIG. 6, unlike the embodiment shown in FIG. 5, the vertical direction of the beam spread is reduced so that it coincides with the horizontal direction, and the incident surface of the prism 7 is It is placed opposite to the example. That is, the beam from the semiconductor laser element 1 is
As shown by the black circle in the figure, the S-polarized light (the plane of polarization vibrates perpendicular to the plane of the paper) is set, and this is converted to P-polarized light by the 1/2 wavelength plate 11, and then enters the prism 7. be. 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 quarter-wave 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. As explained above, according to the present invention, it is possible to efficiently form an isotropic (circular) spot on a disk even if a semiconductor laser element having no isotropic divergence angle is used as a light source. Since the prism is arranged so that the light enters as P-polarized light and expands its horizontal distribution, there is almost no reflection loss when it enters the prism, and the output end face of the prism is perpendicular to the refracted light. Because of this arrangement, there is an advantage that there is little reflection loss when the light beam is emitted from the prism, and that the light beam is used efficiently. In the above explanation, a case was described in which a change in the amount of light reflected from the disk is detected as a change in laser light from the other direction of the semiconductor laser. Of course, the present invention is also applicable to detecting changes.

[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, FIG. 5 is a diagram showing the configuration of an embodiment of the present invention, and FIG. 6 is a diagram showing the configuration of a reference example of the present invention. FIG. 7 is a diagram for explaining the operation of the present invention, FIG. 8 is a diagram for explaining 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 diverging beam whose divergence angle is different in the horizontal direction and perpendicular direction to the joint surface and whose intensity distribution is anisotropic with respect to the optical axis, and which is polarized in the horizontal direction. The output semiconductor laser element and substantially θ
<θ≦θ⊥ (where θ is the half angle of view formed by the above laser element, and θ and θ⊥ are the horizontal and vertical spread angles of the light beam from the above laser element), and the light beam from the above laser element is A prism having a collimating lens, an entrance end surface into which the collimated light beam is incident, and an exit end surface through which the light beam is output, the entrance end surface being parallel to the perpendicular direction, and the prism comprising: a prism disposed such that its output end face is perpendicular to the light beam refracted by the prism; an objective lens that focuses the light beam output from the prism onto an information recording medium; 1. An optical information processing device comprising: means for detecting a change in the amount of light of a light beam. 2. An optical means for extracting the reflected beam from the medium is provided in the optical path between the prism and the objective lens, and the detecting means includes a photodetector that receives the reflected beam from the optical means. An optical information processing device according to claim 1.