JPH0664750B2 - Optical head - Google Patents

Description

[Technical Field of the Invention] The present invention relates to recording / recording of an optical disc, etc. by a semiconductor laser.
The present invention relates to an optical head that performs reproduction and performs focus servo by an astigmatism method.

[Technical background of the invention and its problems]

2. Description of the Related Art Conventionally, there is an optical head shown in FIG. 2 as an optical head for recording / reproducing an optical disc by using a semiconductor laser.

In the figure, 11 is a semiconductor laser that outputs a laser beam, and 12
Is a collimator lens that collimates the divergent light emitted from the semiconductor laser 11, and 13 is a prism that shapes the beam of the collimated light beam, and uses a polarization beam splitter at the bonding surface between the beam shaping unit 13a and the right-angle prism 13b. Printer (PBS) surface P '
Are formed.

14 is a quarter-wave plate, 15 is an objective lens that focuses the laser light on the disk D and collects the reflected light from the disk D, and 16 is the detection light for performing focus servo by the astigmatism method. A condenser lens that focuses the reflected light,
Reference numeral 17 is a cylindrical lens for producing astigmatism in the detection light, and 18 is a light receiving element.

The elliptical cone-shaped divergent light emitted from the semiconductor laser 11 is collimated by the collimator lens 12 into parallel light having an elliptical cross section, and the elliptical light flux is incident on the beam shaping unit 13a of the prism 13. When the light beam incident on the beam shaping unit 13a is refracted, the minor axis direction of the ellipse is expanded to be shaped into a light surface having a circular cross section, reflected by the reflection surface P ₂ ′, and emitted toward the disk D. At this time, the beam-shaped parallel light is PS
The light passes through the surface P ′ and the quarter-wave plate 14, is circularly polarized by the quarter-wave plate 14, and is focused on the disk D by the objective lens 15.

The reflected light from the disk D is condensed by the objective lens 15 and is transmitted as a parallel light flux through the quarter-wave plate 14 to form a PBS surface P '.
The light beam that has been circularly polarized at the time of passing through the quarter-wave plate 4 is linearly polarized and reflected by the BPS surface P ′ toward the condenser lens 16 side.

Then, the light flux incident on the condenser lens 16 is converged in the vicinity of the light receiving surface of the light receiving element 18 in a state where astigmatism is generated by the condenser lens 16 and the cylindrical lens 17, and the light receiving element 18 is focused. An output according to the spot shape is performed, and a focus error signal is obtained based on this output.

However, in the conventional optical head as described above, since a cylindrical lens is used to generate astigmatism, the number of parts is large, the number of assembling steps is large, and it is difficult to adjust the optical system and the size is small. It was not suitable for commercialization.

[Object of the Invention]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above conventional problems, reduce the number of parts, reduce the number of assembling steps, and provide an optical head suitable for miniaturization.

[Outline of Invention]

According to the present invention, the output light of the semiconductor laser, which is collimated by the collimator lens, is refracted by the first prism to shape the cross-sectional shape of the light flux, and the light flux is passed through the quarter-wave plate on the optical disc by the objective lens. The reflected light from the optical disc which is focused by the objective lens and condensed by the objective lens and transmitted through the quarter-wave plate is reflected by the polarization beam splitter formed by attaching the second prism to the first prism. An optical head configured to extract, a first prism that refracts a light beam to shape the cross-sectional shape of the light beam, and a polarization beam splitter that is attached to the prism to form a polarization beam splitter Different second prisms, and a second quarter-wave plate provided at a position for transmitting the light flux reflected by the polarization beam splitter, And a reflection lens for reflecting and converging the light beam transmitted through the second quarter-wave plate to the quarter-wavelength side, and reflecting the reflected light from the disk reflected and extracted by the polarization beam splitter. It is reflected / focused by a lens, and the above-mentioned second is placed before and after this reflection / focusing.
Of the 1/4 wavelength plate and the polarizing beam splitter to be refracted, and this refraction causes astigmatism in the light beam focused by the reflection lens to perform focus servo by the astigmatism method. I was allowed to.

〔Example〕

FIG. 1 is a diagram showing an optical system of an optical head according to an embodiment of the present invention.

In the figure, 1 is a semiconductor laser that outputs a laser beam, 2
Is a collimator lens for converting scattered light emitted from the semiconductor laser 1 into parallel light.

Reference numeral 3 denotes a shaping prism that also serves as a beam splitter for performing beam shaping on the collimated light beam and extracting the detection light. Two types of prisms made of materials having different refractive indexes, that is, a first prism 3a and A right-angled prism 3b as a second prism is bonded and formed, and a polarization beam splitter (PBS) surface P is formed on this bonded surface.

Reference numeral 4 is a first quarter-wave plate for allowing the detection light to be reflected on the PBS surface P, and 5 is for focusing the light flux on the disk D and for converging the reflected light from the disk D. It is an objective lens that collimates light into parallel light.

6 is a second quarter-wave plate, and 7 is a reflecting lens. The second quarter-wave plate 6 and the reflecting lens 7 are the prism 3
The detection light from the disk D, which is adhered to the right-angled prism 3b, is reflected by the PBS surface P and is incident, is converged by the reflection lens 7 and is returned to the PBS surface P. At this time, the converged detection light is the PBS surface. Make P transparent.

Reference numeral 8 denotes a light receiving element that outputs according to the spot shape of the detection light having astigmatism, and a focus error signal is obtained based on the output of this light receiving element 8.

The semiconductor laser 1 is an elliptical cone having a minimum emission angle in a direction parallel to the pn junction surface of the semiconductor element and a maximum emission angle in a direction perpendicular to the pn junction surface, and is mainly composed of pn. The laser light polarized in the direction perpendicular to the bonding surface is output.

The laser light emitted from the semiconductor laser 1 is collimated by the collimator lens 2, and the cross section of the light flux has an elliptical shape whose major axis is in the direction perpendicular to the plane of the drawing. When incident on the surface P ₁ of the shaping prism 3 and refracted, the minor axis direction of the ellipse is expanded and shaped into a light beam having a circular cross section. Then, the beam-shaped light beam is reflected by the reflecting surface P ₂ of the first prism 3a and is incident on the PBS surface P.

The beam-shaped parallel light passes through the PBS surface P and the quarter-wave plate 4, and the linearly polarized light polarized in the direction parallel to the plane of the drawing is circularly polarized by the quarter-wave plate and the objective lens 5 Is converged on the disk D.

The reflected light from the disc D is condensed by the objective lens 5 and is transmitted as a parallel light flux through the quarter wavelength plate 4 and is incident on the PBS surface P. The circularly polarized light flux is linearly polarized in the direction perpendicular to the plane of the paper.

Therefore, the above-mentioned luminous flux is the second 1/4 on the PBS surface P.
The light is reflected to the side of the wave plate 6, passes through the quarter wave plate 6, and enters the reflecting lens 7. Then, the light flux is converged by the reflecting lens 7 and further transmitted through the quarter-wave plate 6 to be linearly polarized light which is polarized in a direction parallel to the plane of the drawing, and transmitted from the right-angle prism 3b through the PBS surface P. Then first
Is incident on the prism 3a.

At this time, the optical axis of the focused light beam is incident on the PBS surface P in an inclined state, and the light flux of the right angle prism 3b is smaller than that of the first prism 3a. When passing through P, the light is refracted at a refraction angle smaller than the incident angle and astigmatism occurs.

In this way, the light flux having astigmatism is focused near the light receiving surface of the light receiving element 88, and the focus servo is performed according to the output of the light receiving element 8.

Therefore, by using the beam shaping prism as described above, it is not necessary to use a conventional cylindrical lens or the like, and the number of parts can be reduced.

The light beam refracted / reflected by the first prism is also refracted when passing through the PBS surface, but the beam shaping is performed by these first and second refractions.

Further, the angle at which the astigmatic light beam is emitted from the first prism is 90 ° in this embodiment,
You may make it select this angle suitably.

〔The invention's effect〕

As described above, according to the present invention, 2 having different refractive indexes
A prism for shaping in which two prisms are bonded together to form a polarization beam splitter is used, and the reflected light from the disk reflected by the polarization beam splitter is 1 /
The four-wave plate is transmitted and is reflected / focused by a reflection lens, and the focused light beam is refracted by the polarization beam splitter surface to generate astigmatism for performing focus servo. The number of assembling steps can be reduced, and an optical head suitable for downsizing can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing an optical system of an embodiment of the present invention, and FIG. 2 is a diagram showing a conventional optical head. 3 ... Shaping prism, 6 ... Quarter wave plate, 7 ... Reflective lens.

Claims (1)

[Claims] 1. A first prism prism refracts the output light of a semiconductor laser collimated by a collimator lens to shape the cross-sectional shape of a light beam, and the light beam is passed through a quarter-wave plate to an optical disk by an objective lens. The reflected light from the optical disc which is converged upward, condensed by the objective lens and transmitted through the quarter wavelength plate is formed by a polarization beam splitter formed by bonding a second prism to the first prism. An optical head for reflecting and extracting light, which refracts a light beam in order to shape the cross-sectional shape of the light beam.
And a second prism having a refractive index different from that of the first prism and a second prism having a refractive index different from that of the first prism, and a prism provided at a position where the light beam reflected by the polarization beam splitter is transmitted. Two
Of the 1/4 wavelength plate and a reflection lens that reflects and focuses the light flux transmitted through the second 1/4 wavelength plate to the 1/4 wavelength plate side, and is reflected / extracted by the polarization beam splitter. The reflected light from the disk is reflected / focused by the reflective lens, and before and after the reflection / focus, the second quarter-wave plate is transmitted and the polarizing beam splitter is transmitted to be refracted. The optical head is characterized in that the astigmatism is generated in the light beam focused by the reflecting lens in order to perform the focus servo by the astigmatism method.