Optical head device with optically variable aperture

Abstract: The optical head device includes a laser light source (1), a beam splitter (3) for reflecting light emitted from the laser light source, a converging lens (4) for converging light reflected from the beam splitter as a minute beam spot on a recording surface of an optical disk medium, a photo-detector (5) for receiving modulated reflected light from the recording surface of the optical disk medium through the converging lens and the beam splitter, and a diffraction grating disposed between the laser light source and the beam splitter. The device is featured by the provision of a variable aperture element (7) which is disposed between the converging lens and the beam splitter and which is capable of controlling the transmission and blocking of the light depending on the polarization directions. The arrangement enables the reproduction of the data recorded in optical disk media having different thicknesses by a single objective lens.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to an optical head device, and more particularly to an optical head device for use in an optical medium recording and reproduction device. (2) Description of the Related Art A conventional optical head device of the kind to which present invention relates is shown in Fig. 1. Such a device includes, as disclosed, for example, in Japanese patent application Kokai publication No. Hei 5-205282, a semiconductor laser 51 as the light source, a diffraction grating 52, a beam splitter 53, an objective lens 54 and a photo-detector 55. It is designed to provide a minute beam spot on a recording medium (optical disk medium) 100 and to make the reflected light a reproduction signal of data. The light emitted from the semiconductor laser 51 is formed into three beams for detecting tracking error signals when transmitted through the diffraction grating 52, and the three beams are converged on the objective lens 54 after being reflected from the beam splitter 53. The reflected light from the signal surface of the recording medium 100 is incident on the light receiving surface of the photo-detector 55 through the objective lens 54 and the beam splitter 53, which is then converted to electrical signals. During the focusing error signal detection for controlling the minute beam spot by following a "surface position instability" of the recording medium 100, when the light which is reflected from the recording medium 100 is again incident on the objective lens 54 and passes through the beam splitter 53, the astigmatism is generated and the light is incident on the photo-detector 55, thus detecting the focusing error signal. This is a conventionally known astigmatism method. During the tracking error signal detection for following the eccentricity of the recording medium 100, the three beams formed by the diffraction grating 52 mentioned above are irradiated on the recording medium 100 and, by setting the rotating position of the diffraction grating 52 such that ±1-order beams are positioned respectively at the two sides of the tracking pit, the tracking error detection is carried out by taking the difference between ±1-order beams. In the optical data reproduction device as above, an increase in the data capacity is desired so that, in a device such as above, the easiest method for the realization of high density data capacity is to reduce the size of a beam spot irradiated on the recording medium 100. The size of the minute beam spot on the recording medium 100 largely dependents on the laser wave length λ and the magnitude of the NA of the objective lens. Therefore, the way to reduce the size of the irradiated spot, which is employed nowadays, is to use a design such that the laser wave length λ is made short or the NA is made large. However, although it is possible for the optical head to be designed with a short laser wave length or a large NA, when the reproduction is made of a recording medium having conventional recording density and if there is a thickness difference in the medium, the optical head suffers from a large influence of spherical aberration, thus making it impossible to form a desired minute beam spot on the surface of the medium. Figs. 2A and 2B show the relationship between the conventional objective lens 54 and thicknesses of the recording media. With the conventional objective lens 54, as shown in Fig. 2A, aberration compensation of the objective lens 54 is made in correspondence to the thickness of the optical disk medium 100, so that it is possible to converge the received beam to the diffraction limit on the recording surface of the optical disk medium 100. On the other hand, when the light which passes through the objective lens 54 that is designed and formed in the state as shown in Fig. 2A, is incident on the recording medium 101 having a different thickness (thicker), the converged beam deviates from the optimally designed value of the objective lens for the reason that, as shown in Fig. 2B, the distance which the converged and refracted beam is transmitted through the recording medium 101 becomes longer. Therefore, even if the optical disk medium 100 was one having a high density data capacity and the optical disk medium 101 was one having a conventional density data capacity, the reproduction of data from the optical disk medium 101 requires that the spot diameter be made smaller than a predetermined NA as, otherwise, the reproduction would not be satisfactory. In an attempt to overcome the above problem, there has been proposed a method which enables the reproduction of data from recording media having different thicknesses. For example, Japanese Patent Application Kokai Publication No. Hei 4-219627 proposes the provision of a mechanism for attaching/detaching of a parallel plate to the light emission surface of a diaphragm lens, and Japanese Patent Application Kokai Publication No. Hei 5-205282 proposes the provision of cover plates to optical disks. However, in a reproduction system for recording media having different thicknesses as described above, it is necessitated to provide special mechanisms and, as a result, the size of the overall device becomes large resulting in an increase in costs. That is, the first problem in the conventional examples explained above is that, in the optical head for making the reproduction of high density data, it is not possible to make data production from media having different thicknesses. The reason for the above is that, according to the conventional technique, the objective lens of the optical head device with which the laser spot diameter is reduced is designed with reference to the thicknesses of disks and the refractive indexes so that the aberration is optimally compensated in the designing step. Therefore, in a high density data recording medium, since the design itself is optimized, there may be no problem in the designed reproduction. However, in the case where a minute beam spot is irradiated on a surface of the medium having thicknesses with the values thereof deviating from the designed values, there is a large influence due to the spherical aberration so that a spot diameter with a smaller NA than the desired NA is irradiated, and this makes it impossible to carry out the reproduction. A second problem is that, as a means to overcome the problems existing in the prior art, although attempts have been made to increase the spot diameter by making the NA of the objective lens smaller through the control of the aperture of the objective lens, none of such aperture control methods have been of any practical use. The reason for the above is that, in an optical disk reproduction device, in order to irradiate a minute beam spot on data recorded on the recording medium, it is necessary for the spot to follow the "surface position instability" and the "eccentricity" of the recording medium and, for this purpose, it requires the supporting mechanism to move in two axes, one to follow the direction of the surface position instability and the other to follow the direction of the eccentricity. Thus, if the aperture to be controlled is provided on the objective lens, the weight is added to the movable portion thus adversely affecting the capability to follow the surface position instability and the eccentricity. If this capability to follow is to be ensured, an increase in the size of the driving mechanism has been inevitable. EP-A-0745 977 (prior art under Article 54(3) EPC) discloses an optical head device which comprises a light source for projecting laser light, a beam splitter for reflecting the laser light onto an optical disk, an objective for focusing the reflected light on an information recording surface of the optical disk to form a micro spot thereon, an optical detector for receiving a light reflected on the information recording surface and converting the light component into an electrical signal, a variable aperture device for varying a diameter of the micro spot according to an external input, the variable aperture device being provided between the beam splitter and the objective, and a variable rotation angle diffraction grating for varying a rotation angle of a subbeam for tracking of the laser light according to an external input, the variable rotation angle diffraction grating being provided between the light source and the beam splitter. EP-A-0 731 457 (prior art under Article 54(3) EPC) discloses an improved optical pick-up apparatus which is capable of reading data irrespective of disk type capable of reading various kinds of disks having different thicknesses and writing densities using only one pick-up apparatus. The optical pick-up apparatus includes a light source, a beam splitter for passing through or splitting a beam from the light source, an objective lens for condensing the beam onto a corresponding disk among disks having different thicknesses and different densities, a numerical aperture control unit for controlling an effective numerical aperture of the objective lens so as to execute a focussing operation with respect to a certain disk, and a photo-detector for receiving the beam reflected by the disk and transmitted from the beam splitter. US-A-5,281,797 discloses a short wavelength optical disk head, comprising at least one coherent light source, a focusing optical system for focusing the light beam radiated out of said light source onto an optical disk, a photodetector for detecting a light beam reflected from or having transmitted through said optical disk to convert said light to an electrical signal and a leading optical system for leading the reflected or transmitted light to the photodetector. The focusing optical system has an aperture diaphragm which can change its aperture diameter depending on the pit size on an optical disk in order to allow recording on and reproducing from not only a short wavelength disk, but also a long wavelength optical disk. The aperture diameter is made larger for the smaller pit size and vice versa, and the light source output is adjusted depending on the aperture diameter. EP-A-0 470 807 reflecting the nearest prior art discloses an optical head comprising two converging optical systems. Each system comprises a semiconductor laser as a light source, a collimating lens for converting a laser beam from the semiconductor laser into a parallel beam, a beam splitter to divide the beam into two beams, a mirror to change the direction of the beam, an objective lens to converge the beam onto an optical disk, a detecting lens to converge the reflected lights which have been divided by the beam splitter and a photo detector to obtain an information reproduction signal, a focussing error signal and a tracking error signal from the converged reflected lights. Each objective lens is designed to have a numerical aperture so that the aberration by the disk substrate of a corresponding given thickness is corrected. According to the determined thickness of the disk substrate one of the converging optical systems is used. JP-A-6020298 discloses a single optical head for optical disks having different recording densities or skew characteristics. The size of a light beam projected from a light source projected from a light source along an optical path to the disk surface is adjusted by adjusting the size of an aperture diaphragm disposed in the optical path between a beam splitter and the optical disk. A aperture diaphragm adjusting section comprises a variable aperture diaphragm section with an iris diaphragm and an actuator adjusting the iris diaphragm. JP-A-5 120 720 discloses a system for use in an optical medium recording and reproducing device. A light beam of a light source is focused by an objective lens with an aperture onto an optical recording medium. The diameter of the aperture of the objective lens can be changed by a means for changing the aperture. A laser power control part inputs a laser power control signal which is corrected in accordance with the transmission efficiency of a light beam determined by the aperture to a laser driving circuit. The optical power of the light source is changed in accordance with transmission efficiency of the light beam such that the optical power irradiating the optical recording medium is made constant when various kinds of optical recording medium with different recording density are recorded and reproduced by changing the diameter of the aperture of the objective lens. SUMMARY OF THE INVENTION An object of the present invention, therefore, is to overcome the problems existing in the prior art, and to provide an optical head device which, when reproducing predetermined data from an optical disk medium having high density data by irradiating a laser spot on the optical disk medium, enables the reproduction of data by one optical head device from optical disk media having different thicknesses. According to the invention an optical head device according to claim 1 is provided. Preferred embodiments are given in dependent claims 2 and 3. The arrangement according to the invention enables the reproduction of the data recorded in a plurality of optical disk media having different thicknesses by a single objective lens without requiring a change of the lens. Since the single objective lens can satisfy the needs for carrying out the reproduction without requiring a plurality of objective lens or a plurality of optical heads in correspondence to the thicknesses of the media, it is possible to provide an optical head device which is low cost and is compatible with the conventional disk, and which permits more general or flexible use as compared with the conventional optical head devices. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be apparent from the following description explained with reference to the accompanying drawings, in which: Fig. 1 is a structural diagram of a conventional optical head device; Figs. 2A and 2B are diagrams for use in explaining the relationship between the conventional objective lens and thicknesses of the recording media, Fig. 2A showing the recording medium as having a normal thickness and Fig. 2B showing the recording medium as being a high density recording medium and as being thicker; Fig. 3 is a structural diagram of an optical head device ; Fig. 4 is a plan view of an example of a variable aperture element included in the device shown in Fig. 3; Figs. 5A and 5B are diagrams for use in explaining the relationship between the converging lens (objective lens) and thicknesses of the recording media in the device according to Fig. 3, Fig. 5A showing the recording medium as having a normal thickness and Fig. 5B showing the recording medium as being a high density recording medium and as being thicker; and Fig. 6 is a structural diagram of an optical head device of a second embodiment according to the invention. Figs. 3 to 5 show an optical head device of an embodiment not falling under the claims. Numeral 1 depicts a semiconductor laser as a laser light source, numeral 3 depicts a beam splitter which reflects the light emitted from the semiconductor laser 1, and numeral 4 depicts a converging lens (objective lens) which converges the reflected light from the beam splitter as a minute beam spot on a recording surface of an optical disk medium 100. Also, numeral 5 depicts a photo-detector which receives the modulated reflected light from the recording surface of the optical disk medium 100 through the converging lens 4 and the beam splitter 3. Further, there is a diffraction grating 2 between the semiconductor laser 1 and the beam splitter 3. Between the converging lens 4 and the beam splitter 3, there is provided a variable aperture element 7 which is, as explained later, formed by an acousto-optic element as a base. In the optical system of the optical head device shown in Fig. 3, the light emitted from the semiconductor laser 1 is transmitted through the diffraction grating 2, thus being formed into three beams for detecting tracking errors. The direction of these beams is changed towards the objective lens 4 by the beam splitter 3. The light reflected by the beam splitter 3 passes through the variable aperture element 7 before entering into the objective lens 4. The variable aperture element 7 is set so as to allow the light at the aperture of the objective lens 4 to be transmitted. The laser light having passed through the variable aperture element 7 and the objective lens 4 is converged to a minute light spot whose diameter is smaller than the numerical aperture of the objective lens 4. The minute light spot is incident on the data recording surface of the optical disk medium 100. The reflected light from the data recording surface of the optical disk medium 100 is again incident on the objective lens 4 and on the beam splitter 3 after passing through the variable aperture element 7. As shown in Fig. 3, the light incident on the beam splitter 3 is transmitted therethrough while being refracted in accordance with Snell laws of refraction. Of the light transmitted through the beam splitter 3, the light that is parallel to the plane of the drawings is expanded while the light that is vertical to the plane of the drawings passes through as it is (which is the function of the beam splitter 3) so that there occurs astigmatism. By this astigmatism, a focusing error detection signal is detected. Next, the light having passed through the beam splitter 3 is converged at the photo-detector 5. Thus, the photo-detector 5 detects a reproduction signal, a focus error signal and a tracking error signal. Fig. 4 is a structural diagram of the variable aperture element 7. On an ordinary glass plate or a crystal optical element 8, there is formed a ring 9 whose aperture diameter is matched to that of the objective lens 4 by an optical element that is capable of controlling the transmission and blocking of the light depending on the polarization directions. An example of the optical element capable of controlling transmission and blocking of the light is an acousto-optic element. By causing the current to flow from the electrode 10 to the ring 9 formed by the acousto-optic element, it is possible to increase or decrease the extent to which the transmission is permitted depending on the polarization directions, thus making it possible to vary the aperture. Here, as shown in Fig. 5A, the objective lens 4 that is designed and formed to have thicknesses corresponding to the conventional medium is capable of irradiating a minute beam spot on the recording surface of the optical disk medium 100. However, as shown in Fig. 5B, in the optical disk medium 101 which has a different thickness, the aperture is controlled such that the minute beam spot is formed on the optical disk medium 101 with the aperture ratio being smaller than the NA of the actual objective lens 4. That is, since the aperture ratio is reduced to smaller than the NA of the actual objective lens 4, the spot diameter formed on the optical disk medium itself becomes large but, on the other hand, the spherical aberration becomes small, thus enabling the reproduction of the optical disk medium 100 having data density as in the conventional example. In the device shown in Fig. 3, when use is made of the objective lens formed with its thickness being matched to a high density medium and the reproduction is made of an optical disk media having different thicknesses with the data densities being as in the conventional example, although the converged spot is enlarged by the control of the aperture, the diameter of the spot is sufficient to reproduce the media with data densities as in the conventional example. Also, as to the amount of aberration, as compared with that generated when the aperture control is not made and the optical disk media having different thicknesses are reproduced, the aberration suppression can be made correspondingly to the enlarged portion of the converged spot diameter. For the above reason, in the embodiment as shown in Fig. 3, it is possible to reproduce the data recorded on optical disk media having different thicknesses using one objective lens, that is, without the need of providing a plurality of objective lenses or a plurality of overall devices, thus enabling the provision of an optical head device which is low cost and is compatible with the conventional disks. Now, an optical head device of a preferred embodiment according to the invention is explained with reference to Fig. 6. The optical head device of the preferred embodiment is featured in that it is provided with a single converging lens (objective lens) 4, a photo-detector 5, two kinds of laser light sources which correspond to thicknesses of the optical disk medium, collimating lens respectively corresponding to the two kinds of laser light sources, and diffraction gratings respectively corresponding to the two kinds of laser light sources. As shown in Fig. 6, the optical system of the optical head device of the second embodiment includes the first laser light source 11, the first beam splitter 13 which receives and reflects the reflected light from the first laser light source 11 through the first collimating lens 11A and the first diffraction grating 12, the converging lens 4 which converges the reflected light from the first beam splitter 13 as a minute beam spot on a recording surface of the optical disk 100, and the photo-detector 5 which receives the modulated reflected light from the recording surface of the optical disk medium 100 through the converging lens 4 and the first beam splitter 13. The optical head device includes the second beam splitter 23 provided between the first beam splitter 13 and the photo-detector 5, and the second semiconductor laser 21 as a second laser light source which sends a predetermined laser light to the second beam splitter 23. In Fig. 6, there are provided a convex lens 15 and a cylindrical lens 16 between the second beam splitter 23 and the photo-detector 5. Between the second semiconductor laser 21 and the second beam splitter 23, there are provided a second collimating lens 21A and a second diffraction grating 22. The parallel light flux outputted from the second collimating lens 21A is set such that the width thereof is narrower than that of the parallel light flux outputted from the first collimating lens 11A. As a means to set the width of the parallel light flux outputted from the second collimating lens 21A narrower than that of the parallel light flux outputted from the first collimating lens 11A, the second collimating lens 21A provided is formed to be the same as the first collimating lens 11A and the second laser light source provided is one in which the beam emitting angle thereof is smaller than the beam emitting angle of the first laser light source. Alternatively, the second semiconductor laser 21 may be formed to be the same as the first semiconductor laser 11 and the second collimating lens 21A may be formed to be the same as the first collimating lens 11A, and an aperture diaphragm element may be provided between the second collimating lens 21A and the second diffraction grating 22. In the preferred embodiment, the laser light sources may be appropriately used depending on the thicknesses of the optical disk medium so that, in addition to achieving the same advantageous effects as those in the embodiment shown in Fig. 3, the preferred embodiment enables the achievement of a higher precision reproduction signal of data recorded in the optical disk medium. While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made without departing from the true scope of the invention, as defined in the appended claims.

Claims: An optical head device comprising: a first laser light source (11) and a second laser light source (21); a first collimating lens (11A) and a second collimating lens (21A); a first diffraction grating (12) and a second diffraction grating (22); a first beam splitter (13) for reflecting the light from said first laser light source received through said first collimating lens and said first diffraction grating; a second beam splitter (23) for reflecting the light emitted from said second laser light source, the second diffraction grating being disposed between said second laser light source and said second beam splitter; a converging lens (4) for converging light reflected from both said beam splitters as a minute beam spot on a recording surface of an optical disk medium (100; 101); and a photo-detector (5) for receiving, through said converging lens and said beam splitters, modulated reflected light from said recording surface of said optical disk medium; said optical head device being arranged such that the width of the parallel light flux outputted from said second collimating lens (21A) is narrower than a width of a parallel light flux outputted from said first collimating lens (11A); and wherein said second beam splitter (23) is provided between said first beam splitter (13) and said photo-detector (5). An optical head device according to claim 1, in which said second collimating lens (21A) is one identically formed as said first collimating lens (11A), said second laser light source (21) being set such that an emitted light beam angle therefrom is smaller than an emitted light beam angle from said first laser light source (11). An optical head device according to claim 1, in which said second laser light source (21) is one identically formed as said first laser light source (11), and said second collimating lens (21A) is one identically formed as said first collimating lens (11A), and which further comprises a variable aperture diaphragm element disposed between said second collimating lens (21A) and said second diffraction grating (22).

Inventors: TAKAHASHI JUN-ICHI, TAKAHASHI, JUN-ICHI, Takahashi, Jun-ichi, c/o NEC Corp.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: G11B7
IPC codes: G11B7