JPH0740374B2 - Light head - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head, and more particularly to an optical recording / reproducing apparatus which corrects a beam shape of laser light output from a semiconductor laser into a predetermined shape by an optical system and uses the same. The present invention relates to an optical head suitable for use.

[Background of the Invention]

In an optical disc device for recording / reproducing information with an optical head, a metal film formed on the surface of a rotating disc disk is irradiated with a light beam, and at the time of recording information, the irradiation power of the light beam is modulated to A hole (pit) corresponding to the information "1" or "0" is formed in the metal film, and the change in the reflected light due to the presence or absence of the pit is detected by irradiating the metal film with a weak light beam during reproduction. Then, the information is recognized. As an information recording / reproducing optical head of such an optical disk apparatus, a semiconductor laser element is used as a light source in order to reduce the size and weight, and this is combined with a collimator / lens system for focusing a light beam and an objective lens. Structural optics are used. In this case, in order to supplement the output power of the semiconductor laser and obtain the high irradiation power necessary for forming pits during information recording, the collimator lens that focuses the output light of the semiconductor laser element into parallel light has a large numerical aperture. Things are desired.

By the way, the optical head using the semiconductor laser has some problems. One is that, as shown in FIG. 1 (B), the output light from the semiconductor laser usually shows an elliptical intensity distribution. For example, in the case of a laser device known as HL8300 series of Hitachi, The intensity distribution is at an ellipse where the ratio of the short axis to the long axis at half maximum is approximately 1: 2.

Therefore, even if the numerical aperture of the collimator lens is set to about 0.3 to 0.5 in order to obtain the output intensity required for recording, the output light from the collimator lens has the elliptic intensity distribution as described above.

In order to accurately record / reproduce information on / from the disc, the light spot converged by the objective lens is shown in FIG. 1 (C).
It is desired that the shape is a perfect circle as shown in. In order to obtain a spot having such a perfect circular intensity distribution, an optical element for beam shape conversion is provided in the optical path between the collimator lens and the objective lens or between the semiconductor laser and the collimator lens. It is necessary to arrange the collimator lens 12 as shown in FIG. 1 (A).
Arrange the triangular prism 13 behind the collimator lens
The elliptical laser light shown in FIG. 1 (B) focused into parallel light at 12 is magnified only in the minor axis direction by the prism 13 and converted into the perfect circular light shown in FIG. 1 (C).

Another problem with an optical head using a semiconductor laser is that the output characteristics of the laser are easily affected by temperature fluctuations, and when the environmental temperature rises or the output power is increased from the reproduction level to the recording level, The point is that the oscillation wavelength of light varies. For example, if the ambient temperature rises by 20 ° C, the oscillation wavelength will increase by about 5 nm and the output power will increase by 3
When it is increased from mW to 20 mW, the oscillation wavelength becomes longer by 3 to 4 nm.

Therefore, as shown in FIG. 1A, when the triangular prism 13 is simply arranged and the elliptical output light is converted into the circular light, the reproduction output state is switched to the recording output state, or Triangular prism 13 when the environmental temperature rises
The emission direction from changes as shown by the dotted line in Fig. 2,
The optical axis shifts when passing through the objective lens 21 and the optical disc
The light spot on 22 is displaced. Further, as a matter of course, even when the recording output state is changed to the reproduction output state, the direction of the light emitted from the triangular prism is changed, and the position of the light spot is displaced to the optical disc 22, so that the reflected light is reflected. The optical axis of the light will shift, and the light incident on the photodetector will shift. The characteristic in which the prism emission light direction changes due to the change in the oscillation wavelength is called the prism wavelength dispersion characteristic.

In the case of FIG. 2, the focal length of the objective lens 21 is 4.5 mm,
Material SF-11 for the triangular prism 13 (refractive index = 1.763066 at 83
When 0 nm and apex angle α = 30.79 °) are used, when the laser wavelength fluctuation is 4 nm, for example, the light spot after passing through the objective lens 21 has a fluctuation of about 0.6 μm. Therefore, in an optical disc having a track pitch of only about 1.6 μm, if the oscillation wavelength fluctuates, a serious error will occur in the positioning of the light spot on the optical disc 22.

[Object of the Invention]

In view of the characteristics of the semiconductor laser device described above, the object of the present invention is to convert the output light from the semiconductor laser into light of a predetermined intensity distribution shape, and to easily suppress the optical axis shift due to the change of the oscillation wavelength of the laser. It is to provide an optical head having a configuration.

[Outline of Invention]

In order to achieve the fulfilled purpose, in the present invention, attention is paid to the fact that the optical axis shift due to wavelength dispersion can be eliminated by combining prisms made of different materials.
Two triangular prisms are combined and interposed between the lens and the objective lens, and these prisms simultaneously cancel the beam shape correction and the optical axis shift compensation due to wavelength dispersion.

That is, since the optical axis shift caused by the wavelength change of the semiconductor laser is caused by the wavelength dispersion characteristic of the triangular prism as described above, the optical axis shift caused by the wavelength dispersion is almost eliminated by combining the prisms made of different materials. Can be eliminated, for example, SF-11 and LaK10 as a triangular prism.
It is possible to eliminate the deviation of the optical axis of the light emitted from the triangular prism by appropriately selecting the angle of each prism by using.

In this case, simply arranging a combination prism of different materials in the optical path for the purpose of simply eliminating the deviation of the optical axis increases the number of parts, complicates the structure of the optical head, and is expensive in terms of price. Will be.

Therefore, in the present invention, a prism having an isosceles right triangle functioning as a beam splitter and a triangular prism for beam shape correction are attached to each other, and a dielectric film is deposited on a boundary surface of both prisms, thereby achieving an initial film-like effect. And it is possible to reduce the number of parts and downsize at the same time. Hereinafter, details of the present invention will be described with reference to examples.

Example of Invention

FIG. 3 is a diagram showing an example of the configuration of the optical head according to the present invention, in which 11 is a semiconductor laser, 12 is a collimator lens,
Reference numeral 13 is a first triangular prism for beam shape correction, 14 is an optical path change, that is, a second triangular prism which acts as a polarization beam splitter for directing reflected light from the recording medium 22 toward a photodetector 25, 15 Is a quarter wavelength plate, 21 is an objective lens, 23 is a focusing lens, 24 is a cylindrical lens, and 25 is a photodetector. The lenses 23 and 24 and the photodetector 25 form an optical system for obtaining a focus shift signal, a track shift signal, and a data signal from the optical disc 22.

In the above configuration, for example, the material SF-
11 right-angled isosceles triangles and triangular prism 13
A material of LaSF-16 is used for the above, and a vapor deposition layer having a function as a polarization beam splitter is formed on the surface 30 corresponding to the hypotenuse of the prism 14. An achromatic prism can be obtained by bonding the two prisms 13 and 14 of different materials on the surface 30. Also, a laser light source (semiconductor laser) having an elliptic intensity distribution
The output light from 11 is converted into parallel light by the collimator lens 12 and then incident on the end face 13A of the prism 13 at a predetermined angle to enlarge the minor axis direction of the ellipse and to provide a light with a perfect circular intensity distribution. Can be converted into a beam. The light beam is a surface
After passing through 30, the recording medium 22 is irradiated with light from one end face 14A of the prism 14 through the quarter-wave plate 15 and the objective lens 21. Further, the return light reflected from the recording medium passes through the objective lens 21 and the quarter-wave plate 15 and is incident on the end surface 14A of the prism 14, and is reflected by the superposing surface 30 of the two prisms 14 and 13. Then, it goes from the other end surface 14B of the prism 14 to the detection optical system.

When the magnification m of the luminous flux of incoming and outgoing light is set to, for example, 2 by the combination of the prisms 13 and 14, one apex angle (β angle) of the prism 13 on the light source side is set to 76.167 °, and the incidence on this prism 13 is made. The incident surface 13A having the light angle (θ angle) may be set to 65.315 ° with respect to the normal line. At this time, for example, when the oscillation wavelength of the semiconductor laser 11 is 830 nm, the refractive index of the prism 13 is ne = 1.760298, and the refractive index of the prism 14 is nd = 1.763066. Even if the wavelength changes ± 30 nm, The deviation of the emitted light is suppressed to about 0.001 °, and the deviation of the light spot focused on the recording medium by the objective lens 21 is about 0.08 μm. The track pitch of the optical disk 22 is 1.6 μm
Therefore, the deviation of the light spot is only about 5% in terms of the deviation rate from the track, and this value is a negligible value that hardly affects the recording and reproducing of information.

In the above structure, a part of the incident light of the prism 13 is reflected by the surface 30 as shown by the destruction 40, and the bottom surface of the prism 13 is reflected.
There is a possibility that the light will be reflected by 13C and input to the detection optical system from the end surface 14B of the prism 14. Conventionally, such a prism 13
To prevent the incident beam from reaching the detector 25, for example, the beam splitter is placed at a slight tilt,
The detector is arranged at a position where only the return light that enters the prism 14 from the end face 14A and goes out from the end face 14B can be received.
However, the problem of reflected light is that the prism on the light source side
This can also be prevented by roughening the bottom surface 13C of 13 to suppress reflection on this surface, and by adopting this method, an optical head with a simple configuration and easy to assemble can be obtained.

〔The invention's effect〕

As described above, according to the present invention, even if there is a change in the oscillation wavelength due to a change in the temperature of the semiconductor laser or the like, it is possible to easily suppress the deviation of the optical axis of the output light from the optical head, and it is possible to realize a perfect circular light An optical head capable of obtaining an output light beam having an intensity distribution can be realized with a simple structure.

[Brief description of drawings]

FIGS. 1 (A), (B), and (C) are views for explaining an optical system of a conventional optical head that corrects the output intensity distribution of a semiconductor laser into a circle, and FIG. 2 shows light by wavelength dispersion of a triangular prism. FIG. 3 is a diagram for explaining the axis deviation, and FIG. 3 is a diagram showing an optical system of an optical head showing an embodiment of the present invention. 11: semiconductor laser, 12: collimator lens, 21: objective lens, 22: information recording medium (optical disc), 13, 14: triangular prism, 25: detector.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeru Nakamura 1-280 Higashi Koigakubo, Kokubunji City, Tokyo Central Research Laboratory, Hitachi, Ltd. (72) Yoshito Tsunoda 1-280 Higashi Koigakubo, Kokubunji, Tokyo Hitachi Ltd. Central Research Center

Claims (1)

[Claims] 1. A semiconductor laser which emits light with an emission intensity distribution that is substantially elliptical, a collimator lens that corrects light from the laser into parallel light, and an elliptical intensity distribution of parallel light that is corrected by the collimator lens. And a first prism for correcting the intensity distribution into a circular intensity distribution, in which light from the semiconductor laser is wavelength-dispersed on the parallel light emission surface of the first prism, the intensity distribution of which is corrected. A second prism that corrects the axis deviation is attached, and
An optical head, wherein a thin film layer having a beam splitting function is formed on the bonding surface.