JPH056263B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical device, particularly to an optical device for beam shaping of a semiconductor laser.

[Conventional technology]

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.
Alternatively, information is recorded on a disk at high density. In other words, in order to record or reproduce information signals on a disk using a semiconductor laser, a light beam emitted from a semiconductor laser element is directed onto a disk with a diameter of about 1 μm using a coupling lens and an objective lens that constitute an optical system. It must be formed as a minute light spot.

In general, a semiconductor laser device has an anisotropic beam spread angle because its light emitting region has a different aspect ratio. 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.
For example, in a CSP 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 the beam divergence angle ratio is 2 in the BH type laser and 3 in the CSP type. There is. Note that the horizontal axis in FIG. 1 is the spread angle, and the vertical axis is the light intensity. Second
The figure shows that when the beam divergence angle of the semiconductor laser device mentioned above is not isotropic, a diameter of 1 μm is placed on the disk.
An example of a conventional optical information processing device for forming an isotropic spot of approximately φ is shown. In addition, semiconductor lasers have a characteristic that the divergence origin of the diverging beam is different in the direction perpendicular to the junction surface of the semiconductor and in the horizontal direction (astigmatism difference).For this reason, the semiconductor laser is used as a light source to form an image. Then, there is a problem that the image becomes blurred. This amount of astigmatism (distance in the optical axis direction of the divergence origin in the vertical and horizontal directions of the beam) is
Even if the semiconductor laser is of the same type, it differs from one individual to another.

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 detector for the 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 is the same as that between the semiconductor laser 1 and the lens 2.
If the half angle of view formed by Regarding the beam divergence angle of the semiconductor laser element 1, as mentioned above, if the magnitudes at e ^-2 in the horizontal and vertical directions are θ and θ⊥, then using such a semiconductor laser element, it is possible to spread the beam on the disk 4 isotropically. In order to form a typical 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 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, if we block such off-axis rays,
If only a portion of the light beam emitted from the semiconductor laser is irradiated onto the disk, the light utilization efficiency of the laser element is poor. In particular, when recording, the metal thin film provided on the disk must be melted to 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, when using a semiconductor laser device as a light source, it is absolutely necessary to increase the light utilization efficiency of the laser device and to suppress the optical output as much as possible from the viewpoint of lifespan and reliability.

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 photodetector 6. This technique is described in Japanese Patent Application Laid-open No. 49-69008.
Normally, the disk rotates with a vertical vibration of about 1 mm, and the depth of focus of the objective lens is about 2 μm, so in order to reproduce signals from the disk, the optical spot must always be kept at a diameter of about 1 μm on the disk. Focusing is necessary to achieve this.

In order to detect the deviation of the focus of the light beam from the disk, JP-A No. 53-17706 discloses a technology that detects the deviation of the focus by micro-vibrating the light source or lens in the optical axis direction and synchronously detecting the laser output. It is proposed in the official gazette. It has been previously reported that the automatic focus control pull-in range by this technique is narrow, about 10 μm (Electronic Materials, February 1979, p. 67).
Figure 3 shows when the numerical aperture of the coupling lens 2 is NA=0.1.
This figure shows the change in output from the semiconductor laser element 1 when the disk 4 is slightly moved along the optical axis direction. As is clear from FIG. 3, when the numerical aperture NA of the coupling lens 2 is as small as 0.1, the automatic focusing range is only 10 μm. An optical information processing device that returns light reflected from a disk to a semiconductor laser element has the drawback of a small pull-in range, as described above. For this reason, automatic focus control was difficult, and information could not be reproduced from a disk with large vertical shake.

Furthermore, Japanese Patent Application Laid-Open No. 53-100844 describes an optical system that uses a plurality of prisms to expand anisotropic semiconductor laser light in the short axis direction and shape it into an isotropic light beam.

In FIG. 9 of Japanese Unexamined Patent Publication No. 100844/1984, an isotropic light beam is obtained from an anisotropic light beam using a prism. In this example, the light from the light source is collimated by cylindrical lenses 25 and 26 and a rotationally symmetrical lens (coupling lens) 27. This rotationally symmetric lens is larger than the diameter of the light beam. In other words, the size of the lens that handles the laser beam is clearly stated in equation (8) of JP-A-53-100844. This means that if the beam diameter of 1/e ² or more is 89.2% of the aperture diameter of the imaging lens (that is, if the lens is made larger by a certain percentage than the beam diameter), the intensity at the center of the image can be maximized.

In the above-mentioned known example, the size of the imaging lens (the lens on the image point side, also called the objective lens) is larger than the diameter of the light beam (for example, the above-mentioned known example No. 14), in accordance with the above idea.
figure). Furthermore, when designing an optical system, it should be considered as common sense for those skilled in the art that the relationship between the diameter of the lens and the diameter of the light beam is unified under similar conditions throughout the entire optical system. This is because even if a large lens is used in the rear stage of the optical system, if a small lens is used in the front stage, the photometer that enters the rear stage lens will be limited by the front stage lens, and vice versa. There is no rational reason to use a large lens in the first stage and a small lens in the second stage.

Therefore, JP-A-53-100844 has no choice but to use a coupling lens (lens on the light source side) that satisfies the condition of equation (8).

Here, when calculating assuming that the wavelength λ of the laser is 0.8 μm, in the above-mentioned known example, based on equation (8), the conditions for the coupling lens are r/R = 0.892 ∴tanθ⊥/tanθ = 0.892, and the beam Assuming that the vertical spread angle θ⊥ is 24°, θ becomes 26.5°. When the astigmatism difference tolerance is calculated by substituting the value into the formula NA ² δz≦0.69λ, which will be explained later, the astigmatism difference tolerance becomes: δz≦2.77. Therefore, when the astigmatism difference of the laser is about 2.8 μm or more, it is affected and the astigmatism difference must be corrected using an optical system such as a cylindrical lens, as described in Japanese Patent Application Laid-open No. 53-100841. I don't get it. However, a cylindrical lens is a lens that intentionally creates and cancels an astigmatic difference that is completely opposite to the astigmatic difference of a light source, and requires extremely high precision and is difficult to manufacture. Furthermore, due to variations in the characteristics of individual semiconductor lasers, it is almost impossible to adapt and manufacture cylindrical lenses one by one.

[Problem to be solved by the invention]

The present invention solves the above-mentioned problems, efficiently shapes the light beam from a semiconductor laser device that does not have an isotropic divergence angle, and has an astigmatism difference, and also uses a cylindrical lens to reduce the astigmatism difference. It is an object of the present invention to provide a semiconductor beam shaping optical device capable of forming a sharp image without having to be corrected by the process, with a configuration suitable for mass production.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a semiconductor laser light source in which the intensity distribution of the output light beam around the optical axis is anisotropic with respect to the optical axis, and an entrance end surface on which the light beam output from the light source is incident. and a plurality of prisms having an output end face from which the light beam is emitted, the optical device shapes the intensity distribution of the output light beam from the light source, A rotationally symmetrical coupling lens that collimates and guides the light beam to the prism has the following conditions: θ<θ≦θ⊥ NA=sinθ (However, the horizontal and vertical directions of the output light distribution in the far-field image of the light beam from the light source
Let the angles at e ^-2 be θ and θ⊥, respectively, and the numerical aperture of the above coupling lens be NA. ) is characterized by satisfying the following.

By having the above configuration, the present invention can form an isotropic spot on a medium with a simple optical system with high light utilization efficiency even if a semiconductor laser element having no isotropic divergence angle is used as a light source.

Further, the prism can be formed by arranging the entrance end face of the prism so as to form a Brillister angle with respect to the output light beam of the light source, and arranging the exit end face of the prism so as to be perpendicular to the light beam refracted by the prism. Light loss at the interface can be minimized.

The effects of the prescription of the coupling lens of the present invention will be described below.

Semiconductor lasers have the problem that the emitted light beam has an elliptical cross section, and that the focal point of the diverging light differs in the direction perpendicular to and horizontal to the junction surface of the semiconductor, which is the problem of astigmatism. Yes. That is, light in a direction perpendicular to the junction surface of the semiconductor laser has a focal point on the end face of the laser element, and light in a horizontal direction has a focal point at a point recessed into the back.

The present invention exhibits an excellent effect in that all the disadvantages caused by the characteristics of the semiconductor laser described above can be eliminated by using the coupling lens and prism defined as described above.

That is, first, by increasing the size of the coupling lens so that θ<θ⊥, the light from the semiconductor laser is effectively used and the elliptical beam is made circular by the action of the prism. Further, by defining the upper limit of the coupling lens as θ≦θ⊥, the influence of the astigmatism difference can be reduced. Astigmatism is the difference in the light emitting point vertically and horizontally, which causes the image to become blurred when imaged with a normal optical system.
If the numerical aperture of the coupling lens is appropriately selected, images can be formed without any problem even with a normal optical system. The details are explained below.

The center intensity I(0,0) of the formed image is I(0,
It is expressed as 0)=1-(2π/λ) ² (Δψ) ² .

Here, the wavelength of the λ laser and Δψ indicate the standard deviation of the wavefront. The cause of Δψ is the astigmatism difference mentioned above.

In optical theory, it can be said that if I(0,0) is 0.8 or more, the optical system can form an image without any trouble without any particular correction. That is, it is sufficient to satisfy the following conditions: 1−(2π/λ) ² (Δψ) ² ≧0.8 ∴Δψ≦0.07λ.

By the way, assuming that the semiconductor laser has an astigmatic difference of δz, Δψ is given by the following equation.

Δψ=6 ⁻² ·W ₂₂ W ₂₂ =NA ² δz/4 NA=sinθ NA is the numerical aperture of the lens, and θ is the half angle of view.

Combining the above equations, NA ² δz≦0.69λ is the condition under which imaging can be performed without any trouble.

Here, it is assumed that λ is approximately 0.8 μm. And NA ² is the formula
Assuming the CSP laser in (1), taking the distance between θ and θ⊥, assuming θ=16°, it is approximately 0.075. Then, the permissible amount of astigmatism difference according to the present invention is δz≦7.36 μm, which means that an astigmatism difference of up to 7.36 μm can be tolerated. In general, semiconductor lasers have individual characteristics that vary, and the astigmatism difference is not constant, but it is possible to image most semiconductor lasers without any problems.

[Embodiments of the invention]

As mentioned above, in order to form an isotropic spot 5 on the disk 4 using the semiconductor laser element 1 having an anisotropic divergence angle, the numerical aperture of the coupling lens 2 is set to (4) and (5). ) must be made small as shown in the formula. However, in this case, only a portion of the light emitted from the semiconductor laser element is irradiated onto the disk, resulting in poor utilization efficiency of the light from the laser element. Moreover, due to the displacement of the disk,
The focal position of the reflected return light spot near the end face of the laser element changes significantly. That is, the reflected return light spot on the end face of the laser element is significantly blurred due to the focal shift, and as a result, the automatic focusing range becomes as small as 10 μm.

The present inventors disregarded equation (4), which is a condition for the coupling lens that the light spot 5 on the disk 4 is isotropic, and calculated the focusing range when the numerical aperture NA of the coupling lens 2 was increased. I tried to find it experimentally. FIG. 4 shows changes in the output from the semiconductor laser element when the disk 4 is slightly moved along the optical axis direction for several types of coupling lenses with different numerical apertures NA (sin θ). As is clear from Fig. 4, the larger the numerical aperture of the coupled lens, the larger the automatic focusing range.
This means that complete automatic focusing can be achieved for the top and bottom of the disk. Moreover, as the numerical aperture of the coupling lens increases, more beams from the semiconductor laser element are incident on the coupling lens, increasing the light utilization efficiency. However, the numerical aperture of the coupling lens
If the NA approximately satisfies the vertical spread angle θ⊥ at e ^-2 of the far-field image shown in Figure 1, most of the beam from the semiconductor laser element will actually be incident on the coupling lens. . 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. In the present invention,
In order to solve this problem, as shown in Figure 5,
Prisms 7 and 8 are placed after the coupling lens 2 that satisfies the formula. FIG. 5 is a diagram showing an example of an optical information processing device using the optical device of the present invention. Fifth
In the figure, the prisms 7 and 8 have their apex angles θα,
Assuming a right-angle prism with a refractive index of N, an incident angle of θ ₁ , and a ratio of the incident beam diameter I to the refracted beam diameter O (beam magnification) as m=O/I, these are given by the following equations. .

Further, the reflectance of P-polarized light (polarized light vibrating parallel to the plane of the paper) at the incident surface of the prism is given by the following equation.

R _P = tan ² (θ _i - θα) / tan ² (θ _i - θα) ...(8) Here, if θ _i + θα = 90°, R _P = 0, and the loss of light on the prism's incident surface is zero. becomes.

In the case of θ _i =θα, there is no effect of the prism because the incidence is perpendicular. ) Here, R _P = 0 because
tanθ _i = N, which is called the Brieucster angle
It's time. At this time, equation (7) has the following relationship.

However, the beam magnification m is set depending on the structure of the semiconductor laser device used. That is, the prism extends the horizontal direction of the spread of the beam from the semiconductor laser element so that it coincides with the vertical direction, thereby making the intensity distribution of the light emitted from the prism isotropic. Therefore, when most of the beam from the semiconductor laser device is incident on the coupling lens, in order to obtain an isotropic beam, the beam magnification m must be made to match the beam divergence angle ratio θ⊥/θ. There is a need. For example, when using a CSP type semiconductor laser element, the beam magnification m is set to 3 from equation (1). At this time, the shapes of the prisms 7 and 8 are as follows from equation (9): m=3 N=1.732 θ _i =60° θα=30° R _P =0 (10).

Therefore, for a CSP type semiconductor laser having a beam divergence angle expressed by equation (1), the prisms 7 and 8 expressed by equation (10) are inserted immediately after the coupling lens 2 in FIG. Therefore, it is possible to convert the beam into an isotropic beam using the prisms 7 and 8. This isotropic light beam is directed to an isotropic spot 5 on the disk 4 by the objective lens 3.
This makes it possible to obtain almost all of the beam from the laser element onto the disk. Moreover, since the numerical aperture of the coupling lens 2 is increased, the retracting range of the self-splitting focus is expanded. Furthermore, since there is no loss of light due to incidence on the prism, the light utilization efficiency is excellent.

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 embodiment shown in FIG. 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 9. The light is then incident on the prisms 7 and 8. By doing so, the objective lens 3 can be made smaller.

FIG. 7 is a diagram showing the configuration of another embodiment of the present invention. In the configuration of the embodiment shown in FIG. 5, a prism 11 is disposed between the prism 8 and the objective lens 3. This embodiment makes it possible to detect changes in the reflected light from the disk 4 from the prism 11. In addition, 10 is a 1/4 wavelength plate,
12 is a photodetector.

Furthermore, in the above explanation, a case has been 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.

Further, if near the point N=√ where R _P =0, R _P is almost 0. As a practical matter, even if the refractive index of prisms 7 and 8 cannot be exactly √, the refractive index N that makes R _P = 0
If it is around (=√), R _P is almost zero, so a refractive index in that vicinity can be used satisfactorily.
FIG. 8 shows the relationship between the beam magnification m and the reflectance R _P at the entrance surface of the prism. If the prism is composed of one prism, and the light reflectance is set to be low and the beam magnification m is set to m=2 to 3, a prism with a considerably large refractive index N is required. BK in the diagram
-7 (N=1.51) and SF-11 (N=1.764), respectively, are shown by dashed lines and dashed lines. Since the ratio of θ⊥ to θ of a semiconductor laser beam is usually about 2 to 4 times, it is quite difficult to configure it with one laser. When configured with two pieces as in the present invention, it becomes possible to eliminate the above-mentioned drawbacks when configured with one piece.
In the figure, the solid line shows the relationship when two prisms made of BK-7 are used. widely known to the general public
In the case of BK-7, the refractive index N= around the wavelength of 8000 Å
It is 1.51. In this case, when m=2.28 times, R _P =
It becomes 0. Moreover, the transmittance is 99.9% or more in the region of m≈2.1 to 2.5. If you allow up to 99%, m
Can be used from ≒1.8 to 3.

As described above, according to the present invention, it is possible to increase the beam magnification with a small reflection loss using a prism having a small refractive index N, and the range of use thereof can also be expanded. Therefore, it has the advantage of being able to perform beam shaping with excellent light utilization efficiency using ordinary optical glass.

〔Effect of the invention〕

As explained above, according to the present invention, by using a plurality of prisms and defining the conditions of the coupling lens, a light beam from a semiconductor laser device that does not have an isotropic divergence angle is shaped with light utilization efficiency. The present invention has the advantage that it is possible to provide an optical system that is extremely easy to design and manufacture, and which solves the problem of image blurring due to the astigmatism difference of a nonconductor laser.

[Brief explanation of drawings]

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 focus pull-in range according to the present invention, FIG. 5, and FIG.
7 and 7 are diagrams showing the configuration of an optical information processing device using the optical device of the present invention, and FIG. 8 is a diagram showing the relationship between the reflectance at the entrance surface of the prism and the beam magnification. 1... Semiconductor laser, 2... Coupling lens, 3... Objective lens, 7, 8... Prism.

Claims (1)

[Scope of Claims] 1. A semiconductor laser light source in which the intensity distribution around the optical axis of the output light beam is anisotropic with respect to the optical axis, an incident end surface onto which the light beam output from the light source is incident, and the light beam. an optical device that shapes the intensity distribution of the output light beam from the light source; The rotationally symmetrical coupling lens that leads to the prism has the following conditions: θ<θ≦θ⊥ NA=sinθ (However, the horizontal and vertical directions of the output light distribution in the far-field image of the light beam from the above light source
Let the angles at e ^-2 be θ and θ⊥, respectively, and the numerical aperture of the above coupling lens be NA. ) An optical device characterized by satisfying the following. 2. The input end face of the prism is arranged so as to form a Brillister angle with respect to the output light beam of the light source, and the output end face of the prism is arranged so as to be perpendicular to the light beam refracted by the prism. 2. An optical device according to claim 1, characterized in that: