JPS631652B2 - - 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.

[Background of the invention]

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 spread angle is anisotropic. 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 θ, respectively.
Assuming θ⊥, for example, in a CSP type semiconductor laser, θ=8°, θ⊥=24°, and θ⊥/θ=3 (1). In addition, for the BH type semiconductor laser, θ = 16°, θ⊥ = 32°, and θ⊥/θ = 2 ... (2), and the beam divergence angle ratio θ⊥/θ is 2 for the BH type laser, and θ⊥/θ = 2 for the CSP type laser. For 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 following relationship, where θ is the half angle of view formed by the semiconductor laser 1 and the lens 2: NA=sinθ (3). Regarding the beam divergence angle of the semiconductor laser device 1, as mentioned above, if the magnitudes at e ^-2 in the horizontal and vertical directions are θ and θ⊥,
Using such a semiconductor laser element, the disk 4
In order to form an isotropic spot 5 on the top, 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 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, resulting in poor light utilization efficiency of the laser element. 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.

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 photodetector 6. This technique is described in Japanese Patent Application Laid-open No. 49-69008.

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 achieve precision in machining, being expensive, and complicating the arrangement of the optical system.In addition, converging a light spot into a circular spot of 1 μm is difficult to achieve with a cylindrical lens. This is difficult due to the large influence of astigmatism due to the use of

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical information processing device that can form an isotropic spot on a disk with a simple optical system and with high light utilization efficiency.

[Summary of the invention]

In order to achieve this object, the present invention is characterized in that a prism for beam conversion 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. Furthermore, in the present invention, this prism is integrated with an optical element for extracting the reflected light from the disk, thereby reducing the reflection loss of light due to the insertion of the prism, thereby further improving the light utilization efficiency.

[Embodiments of the invention]

First, the principle of the present invention will be explained using the reference example shown in FIG.

In the present invention, the condition (4) of the coupling lens that the light spot 5 on the disk 4 has an isotropic shape is satisfied.
Ignoring the equation, the numerical aperture NA of the coupling lens 2 is increased (NA=0.5). Numerical aperture of the above coupling lens 2
NA (=sinθ) is set so that θ<θ⊥≦θ (6).

Equation (6) is completely opposite to Equation (4) above. However, in reality, the far-field image shown in Figure 1 is
If the vertical spread angle θ⊥ at e ^-2 approximately satisfies the numerical aperture NA (=sin θ) of the coupling lens, most of the beam from the semiconductor laser element will actually be incident on the coupling lens. . Therefore, effectively, it is sufficient to satisfy θ<θ≦θ⊥ ...(7).

In this way, when 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, since the light beam passing through the coupling lens 2 that satisfies equation (7) is not isotropic, the light spot 5 is also not isotropic. Therefore, in the present invention, in order to solve this problem, as shown in the reference example of FIG. 3, a prism 7 is inserted after the coupling lens 2 that satisfies equation (7). In Fig. 7, the prism is a right-angled prism with an apex angle of θ _d and a refractive index of N, the incident angle is θ _i , and the ratio of the incident beam diameter I to the refracted beam diameter O is m = O/I. , these are given by the following equations, respectively.

However, m is set depending on the structure of the semiconductor laser device used. That is, the prism 7 expands the horizontal direction of the spread of the beam from the semiconductor laser element so that it coincides with the vertical direction, thereby converting it into an isotropic beam. Therefore, when most of the beam from the semiconductor laser device is incident on the coupling lens, m needs to match the beam divergence angle ratio θ⊥/θ in order to obtain an isotropic beam. be. For example, when using a BH type semiconductor laser element, m=2 according to equation (2), and when using a CSP type semiconductor laser element, m=3 according to equation (1). Therefore, BK7 (N=
1.510), from equation (8), the prism is For CSP type semiconductor laser devices becomes. In addition, SF-11 (N
= 1.764), for BH type semiconductor laser device, For CSP type semiconductor laser devices becomes.

Therefore, for a BH type semiconductor laser device with a beam divergence angle expressed by equation (2), equation (9)
For a CSP semiconductor laser device having a beam divergence angle expressed by equation (1), the prism 7 expressed by equation (11) may be replaced by a prism expressed by equation (10) or equation (12). 7 as the coupling lens 2 in Fig. 3.
By inserting the beam immediately after the beam, it is possible to convert it into an isotropic beam. This isotropic light beam is irradiated onto the disk 4 by the objective lens 3 as an isotropic spot. However,
It becomes possible to irradiate the disk as an isotropic spot without blocking part of the beam from the semiconductor laser element having light emitting regions with different vertical and horizontal ratios. Moreover, since a prism is used, no aberration occurs.

In FIG. 3, 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 shown by the arrow in the figure.

FIG. 4 is a diagram showing the configuration of another reference example of the present invention, and the same reference numerals as in FIG. 3 indicate the same or equivalent parts. In the example of FIG. 4, unlike the example of FIG. 3, the vertical direction of the beam spread is reduced so that it coincides with the horizontal direction, and the prism 7
The plane of incidence is arranged opposite to the example of FIG. That is, the beam from the semiconductor laser element 1 is
As shown by the black circle in the figure, the light is set to S-polarized light (the plane of polarization vibrates perpendicular to the plane of the paper), which is converted to P-polarized light by the 1/2 wavelength plate 11 and then incident on the prism 7. . By doing so, the objective lens 3 can be made smaller.

From the above, the beam deflection 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 the optical information processing device for recording and reproducing predetermined information has been described, the present invention provides a prism in the optical system that guides the beam from the semiconductor laser element to the disk, extracts the reflected light from the disk with this prism, and processes the reflected light. The present invention relates to an optical information processing device that records and reproduces predetermined information by detecting changes in light with a photodetector.

FIG. 6 is a diagram showing an example of the configuration of such an optical information processing device, and is a diagram showing a reference example to facilitate understanding of the present invention. In this example, a prism 9 and a quarter-wave plate 8 are arranged between the prism 7 and the objective lens 3 in the configuration shown in FIG. With this configuration, the reflected light from the disk 4 is directed to the prism 9.
The photodetector 10 can detect changes in the reflected light. In the example shown in FIG. 6, 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.

In the above description, the reaction loss due to the insertion of the prism 7 was not explained at all, but it is desirable to select the refractive index N of the prism so that the reflection loss due to the prism 7 is as small as possible. Here, the reflectances R _I and R _O at the entrance surface P _I and exit surface P _O of the prism 7 will be explained using FIG. 7. These are given by the following equations, respectively.

Figures 8 and 9 show the relationship between the reflectances R _I and R _O and the refractive index N of the prism obtained from equations (8) and (13). If = 2,
FIG. 9 shows the case where m=3. Note that R _O ′ in the figure will be described later. The reflection loss of the prism is R _I and R _O
The least is when the sum of is the smallest. Therefore, from the figure, when m = 2, a material with a refractive index N of about 1.4,
When m=3, it can be seen that a material with a refractive index N of about 1.7 is most preferable as the prism material.

Therefore, as the material for prism 7, BK7 (N=1.510) is used for BH type semiconductor laser device, and CSP
SF-11 (N=1.764) for type semiconductor laser devices
It is preferable to use

Furthermore, in order to reduce reflection loss due to the prism 7, it is also effective to coat the entrance/exit surface of the prism 7 with a single-layer or multi-layer anti-reflection film. In this case, by adjusting the refractive index N of the prism, it is possible to sufficiently reduce the reflectance R _O at the incident surface P _I and coat only the exit surface P _O with an antireflection film. That is, when using a BH semiconductor laser element, as is clear from FIG.
By setting RI in the range of 1.65 to 2.45, it is possible to reduce the reflectance R _I to 1% or less. For example, if SF-11 is used as the prism material, the reflectance R _I at the incident surface P _I can be set to 0.004.
On the other hand, when using a CSP type semiconductor laser element, as is clear from FIG.
By setting within the range of 3.55, the reflectance R _I can be reduced to 1%.
It is possible to do the following. For example, rutile (TiO ₂ ) and tellurium oxide (TeO ₂ ) are used as prism materials.
Crystals such as the following can be used.

Now, in the present invention, FIGS.
As shown in the figure, the beam conversion prism 7 and the reflected light extraction prism 9 are integrally constructed to reduce reflection loss.

FIG. 10 shows an example in which the prism 7 and the prism 9 are integrally constructed of the same material. In the figure, 79 is an integrated prism, and 1/
A four-wavelength plate 8 is also bonded to the prism 79. By integrating, reflection loss at the exit surface of the prism 7 and the entrance surface of the prism 9 can be eliminated.
FIG. 11 shows an example in which the prism 7 and the prism 9 are integrally constructed from different materials. For example, when the material of the prism 9 is BK7 (N=1.510), the reflectance R _O ′ at the interface 20 between the prisms 7 and 9 is calculated as R _O ′=(N− 1.510/N+1.510) ² ...(14) is given. In Figures 8 and 9, m=
2. The relationship between the reflectance R _O ' and the refractive index N when m=3 is shown by a dotted line. As is clear from the figure, it is possible to significantly improve the reflection loss caused by the prism 7. For example, the material for Prism 7 is SF-11.
(N=1.764), R _I =0.004 R _O ′=0.006 for a BH type semiconductor laser device, R _I =0.067 R _O ′=0.006 for a CSP type semiconductor laser device.

〔Effect of the invention〕

As explained above, according to the present invention, an isotropic (circular) spot can be efficiently formed on a disk even if a semiconductor laser element without an isotropic divergence angle is used as a light source, and the prism There is also little reflection loss of light due to searching, and the efficiency of light use can be significantly improved.

[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 FIGS. 3 and 4 are configurations of reference examples of the present invention. FIG. 5 is a diagram for explaining the operation of the present invention. FIG. 6 is a diagram for explaining the configuration of another reference example of the present invention. FIG. 7 is a diagram for explaining the present invention.
8 and 9 are diagrams showing the relationship between the reflectance and the refractive index at the entrance and exit surfaces of the prism, and FIGS. 10 and 11 are diagrams showing the configuration of essential parts of an embodiment of the present invention, respectively. FIG.

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 first 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 a first prism disposed such that the output end face is perpendicular to the light beam refracted by the prism, and an objective that focuses the light beam output from the first prism onto an information recording medium. a second prism provided in the optical path between the first prism and the objective lens to take out the reflected beam from the medium; and a second prism for receiving the reflected beam taken out by the second prism. An optical information processing device comprising a photodetector, wherein the second prism is integrated with the first prism. 2. Claim 1, wherein the first prism is made of a transparent material with a refractive index of 1.764.
The optical information processing device described in Section 1. 3. Claim 1, wherein the first prism is made of a transparent material with a refractive index of 1.510.
The optical information processing device described in Section 1.