GB2052132A - Optical System for Reproducing Information - Google Patents

Abstract

An optical system for reproducing information has a semiconductor laser 21, forming a light source, which emits a beam L of light towards an optical disk (not shown). The beam L passes through an opening 24 in a holder 25, through a beam splitter 22, and is then incident on a plano-convex lens 23, with its planar surface nearest to the laser 21 which collimates the beam L. The beam L is then focused onto the disk and reflected. The reflected beam, which varies in intensity in dependence upon the information recorded on the disk, is reflected by a joint plane in the beam splitter 22 and received by a photodetector (not shown). With this arrangement, the elliptical beam from the laser 21 is rendered circular by opening 24 and is focused as a circular spot on the disc and aberrations produced by the system are less significant. To increase the optical efficiency, the lens 23 and the beam splitter 22 may be combined in a one-piece structure (Figures 4, 5). <IMAGE>

Description

SPECIFICATION Optical System for Reproducing Information The present invention relates to an optical system for reproducing information and more particularly to an optical system for optically reproducing information stored in a recording medium.

One known apparatus for reproducing information optically is the optical disk apparatus in which a semiconductor laser, for example, is used as a light source, and the light beam emitted by the laser is projected onto an information medium (the disk) by an optical system, so that information recorded on the disk is reproduced or information is recorded onto the disk.

Figure 1 of the accompanying drawings is a diagram showing schematicaíly the construction of a known optical disk apparatus. In the figure a light beam emitted by a semiconductor laser 1 is guided to a first lens 5 through a prism assembly 20. As shown, the prism assembly 20 is composed of three prisms 2, 3 and 4, which are formed as a unitary optical component. The light beam enters the prism assembly 20 from the side nearest the semiconductor laser 1 passes through the prisms 2 and 3 and towards the lens 5. A light beam entering the prism assembly 20 from the side nearest the lens 5 is reflected at the joint plane A between the prisms 2 and 3 towards the prism 4.The light beam passing to the first lens 5 is substantially collimated by this lens, and is projected as a small light spot on a disk 8 through a second lens 6 which is supported by a voice coil 7. The light beam is then reflected by the disk 8, and the reflected beam is received at a photodetector 9 after having passed through the second lens 6, the first lens 5 and the prisms 3 and 4.

If information is recorded on the disk 8 (by way of example, information recorded in the form of depressions in the disk surface in dependence upon the information recorded), the reflected beam has its intensity modulated in dependence upon the recorded information, so that the information is obtained in the form of output signals from the photodetector 9. The coií 7 moves the second lens 6 small distances at a high speed, and is used to control a light spot, i.e. for an auto-focusing control or for a tracking control.

In such an apparatus, the laser beam emitted from the semiconductor laser 1 has a divergence (or exhibits a ratio of eccentricity) of about 3 to 1 and therefore shows an anisotropic (elliptic) farfield pattern. If the beam is focused on the disk unchanged, the beam spot does not form an isotropic (circular) distribution pattern on the disk.

Hence the frequency characteristics of the insormation read from the optical disk are degraded.

In known apparatus when a semiconductor laser is used as the light source, the size of the aperture of the lens through which the beam is transmitted is set to an appropriate value in order to make the spot produced by the beam have a circular distribution. This may be done by utilizing the numerical aperture (NA) of the lens so that an elliptic distribution pattern is converted into a circular distribution pattern.

With such known apparatus, however, there is the disadvantage that a lens having a predetermined numerical aperture must be used.

Also, when the numerical aperture NA is small, combined lenses (the number of lenses being 2 or 3) must be used, which complicates the construction of the apparatus.

In the apparatus shown in Fig. 1, the output from the photodetector must be sufficiently large so that the information signals may be detected with satisfactory signal-to-noise ratio. To do this facets of the prism and the lens, for example, one facet 3a of the prism 3 and one facet 5a of the first lens 6, may be provided with thin films which prevent the reflection of the light beam and reduce as much as possible the loss of light due to the prism and the lens. However, the elements of the optical system i.e. the prism and the lenses, are all separate, so that the optical efficiency is low and the number of surfaces of the prism, the lens, etc. onto which an anti-reflection film must be evaporated is large, resulting in high cost of manufacture.Also, since the prism and the lenses are separate, the adjustment of the optical axes of the prism and the lenses is difficult, and the whole apparatus is large due to the large number of components involved.

According to the present invention, there is provided an optical system for reproducing information onto or from a disk or other recordimg medium, having a semiconductor laser forming a light source and a plano-convex lens through which in use a beam of light passes from the laser onto a disk or other recording medium, the lens having its planar surface nearer to the laser in the optical path.

Thus, the present invention can provide a simple optical system which converts a beam to be projected on a disk surface into a circular distribution pattern and also reduces the effect on the spot of aberrations caused by the lens.

Also the optical system can have a high optical efficiency, while using a small number of components and being capable of simple adjustment.

Embodiments of the present invention will now be described in detail, by way of example. with reference to the accompanying drawings, in which: Fig. 1 is a diagram showing a prior-art optical disk apparatus and has already been referred to; Fig. 2 is a diagram showing a first optical system embodying the present invention; Figs. 3(a) and 3(b) are diagrams explaining the operation of the system shown in Fig. 2; Fig. 4 is a diagram showing a second opticai system embodying the present invention; and Figures 5(a) and 5(b) are diagrams showing alternative arrangements of a part of a third embodiment of the present invention.

Referring first to Fig. 2, a light beam L from a semiconductor laser 21 is emitted towards a polarizing beam splitter 22 and enters a planoconvex lens 23. A circular opening 24 is provided in a part of a holder 25 which supports the polarizing beam splitter 22, the opening being in the wall of the holder 25 nearest the laser. The angle 2a which, as seen in Fig. 2, the opening subtends at the semiconductor laser 21 is such as to satisfy the relation a < 0// < 0l where t.L and 8, denote half divergence angles of the beam of the semiconductor laser in a direction perpendicularto the junction of the semiconductor laser and in a direction parallel to the junction respectively.This construction makes the numerical aperture of the lens 23 approximately sin a, so that a lens of a predetermined numerical aperture NA need not be used. Also, a single plano-convex lens 23 may be used rather than the combined lenses used in the prior art.

The planar surface of the lens 23 is arranged so as to face the semiconductor laser 21, so that aberrations do not occur. This is due to three factors. Firstly, since the semiconductor laser employed as the light source has a single oscillation wavelength (for example, 8,300 A), a correction lens for correcting any chromatic aberration is unnecessary. Secondly, the numerical aperture NA is small since the half divergence angle of the semiconductor laser 21 is also small (for example, 8#8 0), Consequently, a correction lens for correcting the part of the beam away from the optical axis is unnecessary. Thirdly, the wave front of the beam of the semiconductor laser is approximately a spherical wave and need - not be corrected. Thus, a single lens may be used.

When making a lens which includes a curved surface, a jig which conforms with the curvature must be prepared. In addition, the step of polishing the lens material into a plane is necessary before the curved surface may be formed. Therefore, a plano-convex lens has the advantage that the manufacturing process is simplified.

The method of setting the plano-convex lens 23 will now be described. In the case of Fig. 3(a), where the curved surface is nearest the laser, the incident beam enters the lens at an angle ss defined relative to the normal to the entrance facet. In the case of Fig. 3(b) where the plane surface is nearest the laser beam is incident at an angle P, and the beam emerges at an angle y relative to the normal to the exit facet. It is clear that # > # and # > y.

In order to reduce the aberrations due to the lens 23, the angle of the incident beam relative to the normal to the entrance facet should be made as small as possible. This angle affects greatly the spherical aberration in particular. It is clear that the arrangement illustrated in Fig. 3(b) is better i.e. the plano-convex lens must be arranged so that its planar surface is arranged so as to face the light source.

As described above, the lens which the beam from the semiconductor laser enters is constructed from a single plano-convex lens, the planar surface of the lens facing the light source and an effective opening provided midway in the optical path, whereby an optical head can be achieved with a simple construction.

Fig. 4 is a diagram showing the construction of a second embodiment of the present invention in which the optical system has been simplified. A light beam emitted from a semiconductor laser 41 passes through prisms 42 and 43 and a first lens 45 to be substantially collimated, and the collimated light beam is projected as a small light spot onto a disk 8 by a second lens 46 which is supported by a voice coil (not shown). The reflected beam from the optical disk 8 passes through the second lens 46, the first lens 45 and the prism 43, and is reflected by the joint plane A between the prisms 42 and 43. It then passes through a lens 44 and is received by a photodetector 9. In this second embodiment, one facet 5a of the first lens 45 is planar and is attached to one facet 3a of the prism 43 so that the first lens and the prism form a unitary structure.By attaching the first lens 45 to the prism 43, it becomes unnecessary to evaporate anti-reflection films on the facet 5a of the lens 45 and the facet 3a of the prism 43, and also the light loss is reduced. Also, the optical system may be easily adjusted, it is proof against vibrations and it can be miniaturized.

Figs. 5(a) and 5(b) are diagrams each showing arrangements of a part of a third embodiment.

The component 30 shown in Fig. 5(a) is obtained by forming the first lens 45, the prism 43 and the lens 44 shown in Fig. 4, in one-piece during manufacture, whilst the component 40 shown in Fig. 5(b) is obtained by forming the first lens 45 and the prism 43 in one-piece during manufacture. The one-piece structure (monolithic) may be achieved with a known technique for example, by resin molding of plastics material. By forming the first lens 45 and the prism 43 in one-piece during manufacture, the number of components of the optical system may be reduced.

Thus, an optical system according to the present invention, may have a high optical efficiency, may be formed from a small number of components and adjusted easily.

Claims (7)

Claims

1. An optical system for reproducing information onto or from a disk or other recording medium, having a semiconductor laser forming a light source and a plano-convex lens through which in use a beam of light passes from the laser onto a disk or other recording medium, the lens having its planar surface nearer to the laser in the optical path.

2. An optical system according to claim 1 wherein the beam of light from the laser passes through an aperture arranged in the optical path between the laser and the lens, the aperture being smaller than the divergence at the location of the aperture of the beam of light from the laser.

3. An optical system according to claim 1 or claim 2, further including a prism which derives a reflected beam from the disc or other recording medium.

4. An optical system according to claim 3, wherein the lens and the prism are formed in onepiece.

5. An optical system for reproducing information substantially as herein described with reference to and as illustrated in Figs. 2, 3(a) and 3(b) or Fig. 4 or Figs. 5(a) and 5(b) of the accompanying drawings.

6. An optical system for reproducing information onto or from a disk or other recording medium, having a semiconductor laser, a planoconvex lens located in a path of the light beam from the laser to the disc or other recording medium and a prism which extracts a beam reflected from the disk or other recording medium, wherein the said lens and the said prism are formed unitarily.

7. An optical system according to claim 6 wherein said lens and said prism are formed in one-piece.