JPH0533466B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a polarizing beam splitter used in an optical head in an optical information recording and reading device. In this case, the light intensity distribution in a plane perpendicular to the optical axis of the generated light beam tends to be elliptical, so the polarizing beam splitter can also serve as an optical means for shaping the light beam into a perfect circle. It is something.

[Prior art]

Gas lasers such as helium-neon lasers and argon lasers have traditionally been used as light sources in optical heads for optically recording or reproducing information on optical information carriers (hereinafter referred to as disks), but in recent years small-sized lasers have been used. Semiconductor lasers that can be directly modulated have come into use. As is well known, semiconductor lasers generally have a light output of
At 15 to 25 mW, it is small compared to conventional gas lasers, and its light beam is anisotropic, with a far-field pattern that is elliptical (in other words, the cross-sectional pattern of the emitted beam from the semiconductor laser is not a perfect circle). It is customary to have an elliptical shape.

Therefore, in an optical head using a semiconductor laser as a laser light source, it is necessary to improve the light utilization efficiency of the optical system from the laser light source to the objective lens. Furthermore, in order to record information at high density on a disk, the light beam incident on the objective lens is made into an isotropic light beam (the beam is cut in the direction perpendicular to its emission direction) with equal aspect ratios. It is also necessary to shape the light beam so that its cross-sectional shape is a perfect circle.

Conventionally, in optical heads using semiconductor lasers, in order to improve light utilization efficiency, the light beam emitted from the semiconductor laser is converted into a parallel light beam by a collimating lens with a large numerical aperture, and the collimating lens and objective lens are It is customary to provide a correction means in the optical path for converting an anisotropic light beam into an isotropic light beam.
For this reason, optical heads as shown in FIGS. 1 and 2 have conventionally been used.

FIG. 1 is a perspective view of an optical head using a shaping prism as a light beam correction means. In FIG. 1, an anisotropic light beam 11 emitted radially from a semiconductor laser 1 is converted into a parallel light beam 12 by a collimating lens 2, and an incident light beam 13 that is isotropic in the vertical and horizontal directions is converted into a parallel light beam 12 by a shaping prism 3. formatted into.

The incident light 13 enters the incident surface 4a of the polarizing beam splitter 4, passes through the polarizing surface 4b, and then exits as the first output light 14 from the first output surface 4c. The first emitted light 14 passes through the quarter wavelength plate 5, is focused by the objective lens 6, and is irradiated onto the disk 7. The reflected light from the disk 7 passes through the objective lens 6 and the quarter-wave plate 5 again, enters the first output surface 4c of the polarizing beam splitter 4, and enters the polarizing surface 4b.
The second output light 1 is reflected from the second output surface 4d.
It emits as 5. After the second emitted light 15 is focused by the condensing lens 8 and passes through the cylindrical lens 9,
The detection element 10 is irradiated with light.

In the above configuration, although the first output light 14 and the second output light 15 are perpendicular to each other, the light beam 12 incident on the shaping prism 3 is
In order to intersect obliquely with the second emitted light 15, it is necessary to arrange the semiconductor laser 1 and the collimating lens 2 obliquely with respect to the other optical components shown in FIG. Therefore, there is a drawback that it is difficult to assemble each optical component with high precision.

Furthermore, although most of the incident light 13 that enters the incident surface 4a of the polarizing beam splitter 4 passes through the polarizing surface 4b and exits from the first exit surface 4c,
Since the light beam 11 emitted from the semiconductor laser 1 is not completely linearly polarized, part of the incident light 13 is reflected by the polarization plane 4b, further reflected by the plane 4e, and then reflected again by the polarization plane 4b. According to
The light is emitted from the incident surface 4a, traces the same optical axis as the incident light 13 in the opposite direction, and returns to the semiconductor laser 1. The fact that the emitted light from the laser returns to the laser again has a disadvantage, especially in semiconductor lasers, in that noise is generated in the laser light.

FIG. 2 is a plan view of a conventional optical head in which two cylindrical lenses are used as light beam shaping means instead of the shaping prism in FIG. 1. Note that optical components not shown are the same as those shown in FIG. 1.

In FIG. 2, an anisotropic light beam 11 emitted radially from a semiconductor laser 1 is converted into a parallel light beam 12 by a collimating lens 2, and is isotropic in the vertical and horizontal directions by cylindrical lenses 20 and 21. incident light 13. The incident light 13 enters the incident surface 4a of the polarizing beam splitter 4, passes through the polarizing surface 4b, and then exits from the first output surface 4c. The reflected light from the disk that returns along the same optical axis as the first output light 14 enters the first output surface 4c of the polarization beam splitter 4 again, and the polarization surface 4b
The light is reflected by the light beam and exits as second outgoing light 15 from the second outgoing surface 4d. In the configuration shown in Figure 2,
Since the optical axis of the light beam 12 and the optical axis of the incident light 13 coincide, there is no need to arrange the semiconductor laser 1 and the collimating lens 2 obliquely with respect to other optical components constituting the optical head. However, as the number of parts increases, the amount of light decreases due to the increase in the number of boundary surfaces, and the generation of stray light increases. Furthermore, in order to accurately match the optical axes of the collimating lens 2 and the cylindrical lenses 20 and 21 and to accurately arrange their mutual positions, there is a drawback that assembly and adjustment become complicated.

[Purpose of the invention]

An object of the present invention is to eliminate the drawbacks of the prior art described above, to correct the anisotropy of semiconductor laser light, and to adjust the optical axis of incidence to a polarizing beam splitter.
An optical system in which one of the two output optical axes orthogonal to each other is parallel to each other, stray light caused by the reflection surface from the polarization plane of the polarization beam splitter does not return to the semiconductor laser, and the number of parts is small. This provides a polarizing beam splitter that can realize a polarizing head.

[Summary of the invention]

In order to achieve the above object, the present invention includes an entrance surface, a first and a second exit surface, a polarization plane therein, and a reflection plane that guides the light beam incident on the incidence surface to the polarization plane. and a first and second light beam whose output optical axes are perpendicular to each other from the first and second output surfaces in response to the light beam being incident on the input surface. Emits,
The incident optical axis to the incident surface and each of the first and second output optical axes are located in the same plane, and the incident optical axis and the first output optical axis are perpendicular to each other, and the incident light In a polarizing beam splitter in which the oblique angle of the axis with respect to the incident plane is sufficient to shape the incident oblique beam from an anisotropic beam to an isotropic beam, the above-mentioned incident plane, reflective surface, polarizing plane, In addition to the first output surface and the second output surface, the first output surface, the first output optical axis, and the second output surface are provided with a surface parallel to the incident optical axis and a perpendicular surface to the incident optical axis. and the second output optical axis are arranged to be oblique to each other.

[Embodiments of the invention]

Hereinafter, details of the present invention will be explained with reference to embodiments shown in the drawings. FIG. 3 is a perspective view of an optical head using a polarizing beam splitter as an embodiment of the present invention.

In FIG. 3, an anisotropic light beam 11 emitted radially from a semiconductor laser 1 is converted into an incident light beam 12 which is a parallel light beam by a collimating lens 2, and is converted into an incident light beam 12 which is a parallel light beam.
By obliquely entering the beam 0a, it is shaped into a light beam 13 that is isotropic in the vertical and horizontal directions, and is reflected by the reflective surface 30b on the inner surface of the polarizing beam splitter 30, so that it is perpendicular to the optical axis of the incident light beam 12. It becomes a light beam 16, passes through the polarization plane 30c, and is emitted from the first emission surface 30d as the first emission light 14 from the emission surface 30.
It emits obliquely to d.

The first emitted light 14 passes through a quarter wavelength plate 5,
The light is focused by the objective lens 6 and irradiated onto the disk 7 . The reflected light from the disk 7 passes through the objective lens 6 and the quarter-wave plate 5 again, obliquely enters the first output surface 30d of the polarizing beam splitter 30, is reflected by the polarizing surface 30c, and is reflected by the second polarizing surface 30c. From the output surface 30e, the second output light 15 that is orthogonal to the first output light 14 is output obliquely to the second output surface 30e. The second emitted light 15 becomes a convergent light beam 16 by the condensing lens 8, passes through the cylindrical lens 9, and then is irradiated onto the detection element 10.

4 is a plan view of only the polarizing beam splitter 30 in FIG. 3 viewed from the direction of arrow A in FIG. 3. FIG.

In the configuration shown in FIG. 4, the optical refractive index of the polarizing beam splitter 30 at the wavelength of the semiconductor laser is n.
The magnification factor of the light beam 12 in the plane parallel to the paper surface for correcting the anisotropy of the light beam is m
Then, the illustrated angle θ ₁ in the splitter 30 is expressed by the following equation (1), and the angle θ ₂ is also expressed by the equation (2).

θ ₁ = ₁ + 45° (1) θ ₂ = ( ₂ − ₁ ) / 2 + 90° (2) ₂ = sin ^-1 (sin ( ₁ )/n) (4) In the above configuration, the angle θ ₃ ≠ θ ₄ , the incident angle of the incident light 12 with respect to the incident surface 30a is ₁ , and the incident light beam 12 is assumed to have a cross-sectional shape that is longer in the direction perpendicular to the paper than in the direction parallel to the paper.

For example, if the refractive index of the prism is n = 1.51 and the magnification m = 2.40, then if θ ₁ = 116.05° and θ ₂ = 73.87°, the light beam 12 will have an anisotropy of vertical:horizontal = 1:2.40. an input optical axis to the polarized beam splitter 30, and two output optical axes 14, 15 that are orthogonal to each other.
The output optical axes 15, which are one of the two, are parallel to each other, and an isotropic light beam 13 can be obtained.

With the configuration described above, in the polarizing beam splitter 30, a part of the light beam 16 is
Although reflected at point c, the reflected light does not return to the semiconductor laser 1 because it enters the incident surface 30a at a different angle from the light beam 13.

Further, since the parallel light beam 12 and the output light beam 15 are parallel to the disk 7, it is possible to construct a thin optical head. Moreover, the number of interfaces through which the light beam passes or is reflected is one less than the equivalent functional parts of the conventional optical head shown in FIG. 3
Since the number of lights is reduced, the problem of stray light is also alleviated, and by applying a reflective coating to the reflective surface 30b, the light utilization efficiency is also improved.

Note that in FIG. 4, the first output light 14 and the first output surface 30d do not intersect at right angles, and similarly the second output light 14 and the first output surface 30d do not intersect at right angles.
It will be recognized that the values of the angles θ ₁ , θ ₂ , θ ₃ , and θ ₄ are determined so that the angles 5 and the second exit surface 30e are not perpendicular to each other.

With the above configuration, even if there is light reflected by the first output surface 30d of the light beam 16, the reflected light will not trace the same optical axis as the light beam 16 in the opposite direction. Moreover, the above matters also apply to the second exit surface 30e.

〔Effect of the invention〕

As explained above, in an optical head using a conventional polarizing beam splitter, the reflected light from the polarizing plane returns to the semiconductor laser and generates noise, and the prism is used to correct the anisotropic light beam. When using a cylindrical lens, it is difficult to assemble optical components with high precision because the incident optical axis and the output optical axis intersect obliquely, and when the light beam is corrected with two cylindrical lenses, the incident light and the output optical axis are Although the incident light is perpendicular or parallel to the incident light, it has the disadvantage that the number of optical components increases and higher precision is required for assembling each optical component. In the head,
A prism for correcting an anisotropic light beam and a polarizing beam splitter are integrated to form a new polarizing beam splitter, and a reflecting surface ( 30b) in Fig. 4, and by appropriately making the incident optical axis and the incident surface obliquely intersect, the incident optical axis and one of the two mutually orthogonal output optical axes are perpendicular, and the other is parallel. Since the reflected light from the polarization plane does not return to the semiconductor laser, the above-mentioned drawbacks of the prior art can be solved. Furthermore, by integrating the correction prism and the splitter, the number of boundary surfaces through which the light beam passes is reduced, so that the light utilization efficiency is improved and the optical head portion can also be made thinner.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing a conventional example of an optical head, FIG. 2 is an explanatory view showing another conventional example, FIG. 3 is a perspective view showing an optical head including an embodiment of the present invention, and FIG. is a plan view of the polarizing beam splitter 30 (an embodiment of the present invention) in FIG. 3, viewed from the direction of arrow A. Description of symbols, 1...Semiconductor laser, 2...Collimating lens, 5...1/4 wavelength plate, 6...Objective lens, 7...Disc, 8...Condensing lens, 9...
Cylindrical lens, 10...detection element, 30...polarized beam splitter.

Claims (1)

[Scope of Claims] 1. An incident surface having an entrance surface, a first and a second exit surface, a polarization surface therein, and a reflection surface that guides the light beam incident on the entrance surface to the polarization surface, In response to the light beam being incident on the light beam, the first and second light beams are emitted from the first and second light emitting surfaces so that their light emitting axes are orthogonal to each other, and the light beams are directed to the light incident surface. The incident optical axis and each of the first and second output optical axes are located in the same plane, and the incident optical axis and the first output optical axis are perpendicular to each other, and the incident plane of the input optical axis is In the polarizing beam splitter, the incident light beam has an oblique angle sufficient to shape the incident light beam from an anisotropic beam to an isotropic beam, the above-mentioned incident surface, reflective surface, polarizing surface, first exit surface, In addition to the respective surfaces of the second output surface, the first output surface and the first output optical axis, and the second output surface and the second output surface are provided. A polarizing beam splitter whose emission axes are oblique to each other.