1. Field of the Invention
The present invention generally relates to an optical pickup for irradiating a light beam as an information reproducing light onto an information record medium such as an optical disk, on which information is recorded by a phase pit or is magnetically recorded, and reproducing the information by detecting the reflection light from the information record medium. More particularly, it is related to a construction of an objective lens portion for condensing or focusing the light beam to form a light spot having an optimum size on a reproduction position of the information record medium.
2. Description of the Related Art
There is a so-called CD (Compact Disk)/LD (Laser Disk) compatible player, which can reproduce both of the CD and LD by a single player. In this CD/LD compatible player, the distance from a surface of the disk to the information record surface (i.e. the thickness of the protection layer) of the CD is the same as that of the LD (commonly 1.2 mm). Thus, by use of a single optical pickup for condensing or focusing the light beam onto a focus position, it is possible to reproduce both of these two types of optical disks.
There is a DVD (Digital Video or Versatile Disk) as a high density record medium, in which the memory capacity is drastically improved as compared with the conventional CD, and to which one movie etc. can be recorded. Thus, it is desirable to develop a CD/DVD compatible player which can reproduce both of the DVD and the CD.
Here, comparing the constructions of the CD and DVD, the thickness of the protection layer of the DVD is about half of the CD (i.e. 0.6 mm) because of the requirement for the high recording density. Therefore, if both types of these disks are tried to be reproduced by use of the optical pickup of single focal type, for example, if the light beam is condensed or focused optimally for the DVD, there is generated an aberration such as a spherical aberration of the light beam with respect to the CD since the protection layer of the CD where the light beam is passing through is thicker than the DVD. This results in a problem that the light beam cannot be optimally condensed or focused with respect to the CD.
Further, the CD and the DVD are different from each other in the size of the information pit formed for recording the information (more concretely, the length of the shortest pit of the information pit of the CD is about 0.87 .mu.m, while the length of the shortest pit of the DVD is about 0.4 .mu.m). Thus, in order to precisely read the information pits of these two types of disks, it is necessary to form a light spot on the CD or the DVD, which has an optimum size with respect to the size of the information pit of either type of the disks to be reproduced.
Here, the size (diameter) of the light spot is proportional to the ratio of the wavelength of the light beam with respect to the numerical aperture of the objective lens for condensing or focusing the light beam onto the information record surface. Namely, assuming that the wavelength of the light beam is set to be constant, as the numerical aperture is increased, the size of the light spot is decreased. Therefore, in case that the CD and the DVD are reproduced by an optical pickup of single focal type, if the wavelength of the light beam is set to be constant and the numerical aperture is set to be suitable for the information pit of the DVD, for example, the light spot becomes so small for the information pit of the CD as to cause a distortion in the reproduced signal of the CD, so that it is difficult to precisely read the information pit of the CD, which is another problem.
Therefore, in order to solve the above explained problems, there is an optical pickup of bifocal type provided with a bifocal lens capable of irradiating two light beams to be focused on one straight line at different positions and forming two light spots, which have sizes suitable for the information pits of the CD and the DVD respectively. This bifocal lens is explained here in detail.
The bifocal lens is constructed by disposing a diffraction element and an objective lens on one optical axis. The bifocal lens separates the light beam, which is emitted by a laser diode, is reflected by a half mirror and is made to be a collimated light beam by the collimator lens, into three lights i.e. the 0 order light and the .+-.1 order lights, by the diffraction element, and focuses the 0 order light and the +1 order light on different positions from each other on the same optical axis by use of the optical property that the focal length of the 0 order light and the focal length of the +1 order light are different from each other. At this time, out of the light beam, a component which has passed through the diffraction element becomes the 0 order light, while a component which has diffracted by the diffraction pattern of the diffraction element becomes the +1 order light.
More concretely, as for the focal position, the bifocal lens is constructed such that the +1 order light is focused at a farther position from the objective lens than the 0 order light, and that the 0 order light is optimally condensed or focused at the information record surface of the DVD, while the +1 order light is optimally condensed or focused at the information record surface of the CD.
Here, the diffraction pattern to form the +1 order light out of the light beam is formed within a smaller area than the aperture of the objective lens (i.e. the input area of the light beam). Thus, the diffraction does not occur at the portion of the diffraction element where the diffraction pattern is not formed, so that all of the light beam incident to this portion become a transmitted light (i.e. the 0 order light). By this construction, it is possible to increase the numerical aperture with respect to the 0 order light (i.e. the light beam for the DVD) while it is possible to decrease the numerical aperture with respect to the +1 order light to be the numerical aperture suitable for the CD, since the +1 order light is generated only in the area where the diffraction pattern is formed. In other words, the diffraction pattern is formed in the area having the numerical aperture (NA) corresponding to the size of the light spot by which the information from the CD can be precisely read.
By use of this bifocal lens, it is possible to reproduce both of the CD and DVD by use of a single optical pickup.
Here, in the above described bifocal lens, the diffraction element and the objective lens are disposed on the same optical path, in order to form the bifocal lens by using the diffraction element and the objective lens as a single lens.
On the other hand, as shown in FIG. 8 showing a partial cross-section view of a diffraction element, the diffraction element used for the above described bifocal lens is provided with: a diffraction area (the diffraction pattern), where a plurality of diffraction surfaces are coaxially formed; and a transmission area, which is formed at an outer circumference side of the diffraction area and where the incident light beam L is transmitted as it is. The diffraction surfaces HR are connected to each other by stepped surfaces HC which form the stepped portions. The cross-sectional shape of the diffraction surfaces HR and the stepped surfaces HC is such a saw-tooth shape that the stepped surfaces HC are substantially perpendicular to an incident surface HI on the side to which the light beam L is incident. The production of this diffraction element H is performed by forming the diffraction surfaces HR directly onto the material of the diffraction element H by use of a diamond cutter, or by forming a die, which has a shape corresponding to the diffraction surfaces HR, and molding the material of the diffraction element H such as resin, polymer etc. into the die.
However, considering the case where the information recorded on the DVD is reproduced by use of the above explained bifocal lens, the +1 order light, the -1 order light and the higher order lights, which are not necessary for the reproduction of the information form the DVD, are generated other than the 0 order light, which is necessary for the reproduction of the information form the DVD, at the diffraction area of the diffraction element H. Since the center of the diffraction element H and the center of the objective lens R are disposed on the same optical path to each other, the reflection lights of these .+-.1 order lights and the higher order lights from the optical disk are condensed at a position same as the reflection light of the 0 order light, on the photo-detector within the optical pickup of bifocal type.
This point is explained in more detail with referring to FIGS. 9A to 9C. In FIGS. 9A to 9C, the central axis of C.sub.R of an objective lens R and the central axis C.sub.H, of the diffraction element H are made coincident to each other, and each of the lens surface of the objective lens R and the diffraction element H is indicated by a single line respectively, for the simplicity of the explanation.
As shown in FIG. 9A, if the 0 order light L.sub.0 arrives at an information record surface DD of the DVD, it is reflected by the information record surface DD, passes through the same optical path as before arriving at the information record surface DD, and is further reflected to the photo-detector. However, in this case, as shown in FIG. 9B, after the -1 order light L.sub.-1 is reflected by the information record surface DD, it passes through the same optical path as the incident light of the +1 order light, arrives at the light diffraction element H, is diffracted by the diffraction element H, and arrives at the photo-detector through the same optical path as the 0 order light L.sub.0.
In the same manner, as shown in FIG. 9C, after the +1 order light L.sub.+1 is reflected by the information record surface DD, it passes through the same optical path as the incident light of the -1 order light, arrives at the light diffraction element H, is diffracted by the diffraction element H, and arrives at the photo-detector through the same optical path as the 0 order light L.sub.0. Consequently, the component based on the reflection lights of the .+-.1 order lights L.sub.-1 and L.sub.+1 and the higher order lights are mixed as the noise component into the detection signal based on the reflection light of the 0 order light L.sub.0 to be detected as the genuine signal.
Here, assuming that the diffraction efficiencies for the 0 order light L.sub.0 and the +1 order light L.sub.+1 are 40%, and that the diffraction efficiency of the -1 order light L.sub.-1 is 4%, the light intensity of the refection light of the 0 order light L.sub.0 to be detected as the genuine signal is about 16% with respect to the original light beam L, while that of the other lights which should not be detected is about 3.2%. Namely, the light intensity ratio of the reflection light to be detected as the genuine signal with respect to the other reflection lights is 5:1. Thus, the detection signal component based on the reflection lights other than the 0 order light L.sub.0, which should not be detected, becomes the noise in the RF (Radio Frequency) signal based on the detection signal during the generation of the RF siganl, and becomes a cause of making the focus servo control (pulling-in of the focus servo etc.) unstable.
On the other hand, since the above explained diffraction element H is constituted of the diffraction area (the diffraction pattern) and the transmission area, the light intensity of the transmitted light (i.e. the 0 order light L.sub.0) as the light beam corresponding to the thickness of the protection layer of the DVD is drastically decreased at the time of transmitting through the diffraction area as shown in FIG. 10A, at a time of reproducing the information from the DVD. As a result, as shown in FIG. 10B, each side lobe of the light beam for the DVD which is condensed on the DVD through the objective lens R becomes large, so that it gives undesirable influence onto the focusing condition of the main lobe on the DVD, and the reproduced signal from the DVD is certainly degraded, which is another problem.
Further, since, on the cross-sectional shape of the above explained diffraction element H, the stepped portion HC is substantially vertical shape with respect to the incident surface HI as shown in FIG. 8, the use of a sharp edged blade (e.g. a bite) B' as shown in FIG. 11 is necessary for forming the diffraction element H. Thus, there is another problem that the edge of the blade B' can be easily broken, the operation efficiency is considerably low and it becomes the safety hazard. Furthermore, since a protruding portion P' of the saw-tooth shaped portion is sharpened, the diffraction element H formed in this manner can be easily broken at this protruding portion P', which is another problem.