Pick-up device

Abstract: A pick-up device which includes a light source (7) and a condenser lens (2) for effecting recording and reproduction of information by projecting light beam as collected by the condenser lens onto a recording medium (1), and is characterized in that there is further provided a diffraction grating (3) between the light source (7) and the condenser lens (2) for dividing light returning from the recording medium (1) by the diffraction grating (3) so as to be led to a photo-detector (6), thereby to obtain an optical detection signal.

Description: The present invention generally relates to a pick-up arrangement and more particularly, to a pick-up device suitable for use in an optical disc apparatus such as a so-called compact disc player, video disc player or the like. Commonly, an optical disc apparatus is so arranged as to effect recording by forming fine holes or grooves in a spiral or concentric pattern on the surface of a disc-like recording medium or by subjecting the surface of the recording medium to magnetic variations, and also to effect reproduction by projecting a collected spot light onto the recorded portion of the recording medium so as to derive the reflected light or transmitted light as reproduction information. In Fig. 1, there is shown a general construction of a conventional optical information reading pick-up device as incorporated in an optical disc apparatus. In the known pick-up device as shown in Fig. 1, the optical system includes a collimate lens Lm and a beam splitter Sp provided in a barrel Br so as to collect and project laser light emitted from a semi-conductor laser W onto a recording medium M through said collimate lens Lm, said beam splitter Sp and an objective lens Lo. After being subjected to modulation at the information recorded portion on the recording medium M, the light thus projected is divided by the beam splitter Sp through the objective lens Lo, and one portion of the divided light is incident upon a photo-detector Pd via a detecting system convex lens Ld and a cylindrical lens Lc. The conventional optical information reading pick-up device having the optical system as described above, in order to correctly record the information on the recording medium or so as to accurately reproduce the recorded information therefrom, is to be controlled in such a manner that the collected light spot correctly traces the information portion (referred to as tracking) as recorded in the spiral or concentric configuration (i.e. information recorded track), with a simultaneous control for focusing of the light spot. Moreover, for the actual application, since the above pick-up device is subjected to influences due to eccentricity or fluttering of the disc, and external vibrations, etc., it is required to provide a mechanism to derive such deviations as error signals for automatic correction. For this purpose, it has been a normal practice to effect the focus control and the tracking control through employment of an electromagnetic driving unit including a magnetic circuit and a coil means. For effecting the control up to the high frequency region during the above tracking driving, it is desirable that the driving portion is compact in size and light in weight. From the above point of view, there has also been conventionally proposed an arrangement in which only the objective lens Lo is separated from the barrel for driving, but in such a construction, deviation in axes is produced between an optical axis of the pick-up device and that of the objective lens, thus resulting in deterioration in the quality of the pick-up control signal, for which some countermeasures are required. Furthermore, in order to avoid the deterioration in the recording signal and reproducing signal with respect to the disc, a driving unit having a sufficiently high accuracy is required so as to prevent inclination of the lens optical axis from taking place during driving of the objective lens. Meanwhile, in the pick-up device as shown in Fig. 1, in which the whole pick-up assembly is driven, cross-talk between the tracking control and the focus control tends to take place since the barrel Br is not formed into axial symmetry. More specifically, on the assumption that the pick-up device is driven in a direction at right angles with the recorded surface of the information carrier or recording medium in the disk-like shape, the phenomenon that the optical axis is deviated from the direction normal to the recorded surface, is difficult to be avoided in the pick-up device which is not of the axial symmetry. Additionally, in the prior art pick-up device as shown in Fig. 1, an astigmatism producing optical system including the convex lens Ld and the cylindrical lens Lc is required to obtain the focusing error signal for effecting the focus control, such an optical system becomes an obstruction in realizing the reduction of size, weight and cost for the pick-up device. From EP-A-0 147 749 an optical head is known in which between the light source and the condenser lens a polarization prism, serving as a light diffracting means, is arranged. Through this polarization prism the light reflected from the recording medium is directed to a light detecting means comprising several photodetectors. However, all the reflected light impinging on the condenser lens is directed to the photodetectors by means of the polarization prism so that cross-talk from one track of the recording medium to the other may occur. From US-A-4 358 200, DE-B-33 46 812 and "optische Fehlerdetektion" aus Neues aus der Technik Nr. 6 vom 15. Dezember 1980, Seite 3, it is known to use a diffraction grating in an optical pick-up device, in which the detector means is located at one side of the light source. Transmissive and reflective diffraction gratings with blaze characteristics are known as such. It is the object of the present invention to provide an optical pick-up device of the above described type, which is arranged to suppress the undesirable cross-talk phenomenon. According to the invention this object is solved by an optical pick-up device having the features of the attached claim. In the pick-up device according to the invention the light detecting means is a single detector placed at one side of the light source. The light diffracting means is a diffraction grating having a blaze characteristic so as to direct onto the light detector only the light reflected from that position of the recording medium from which information is to be reproduced. By the above arrangement of the present invention, an improved pick-up device has been advantageously provided, with substantial elimination of disadvantages inherent in the conventional pick-up devices of this kind. Further features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a schematic side sectional view showing a general construction of a conventional optical information reading pick-up device which may be incorporated in an optical disc apparatus (already referred to); Fig. 2 is a schematic side sectional view showing construction of an improved optical information reading pick-up device according to one preferred embodiment of the present invention; Fig. 3 is a diagram schematically showing construction of the pick-up device of Fig. 2 having a blaze characteristic; and Fig. 4 is a fragmentary perspective view showing on an enlarged scale, part of a diffraction grating of Fig. 3. Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring to the drawings, there is shown in Fig. 2 a pick-up device P3 according to a preferred embodiment of the invention while Fig. 3 shows a schematic diagram of the arrangement of Fig. 2. Generally, the pick-up device P3 in Fig. 2 includes the barrel 5 extending in a direction intersecting at right angles with the recording surface la of the recording medium 1, and the objective lens 2, diffraction grating 3', collimate lens 4, and semi-conductor laser or laser diode 7 provided at one side of the detector 6 sequentially disposed within the barrel 5 in the above order in a direction away from the recording medium 1. In the pick-up device P3 as described above, after being subjected to collimation by the collimate lens 4, the light emitted from the semi-conductor laser 7 is transmitted through the diffraction grating 3', and thereafter, advances in a direction in which zero-order light intersects at right angles with the recording surface la of the recording medium 1 so as to be subsequently collected by the condenser lens 2 for being projected onto the recording surface la. The light thus projected is modulated at the recorded information portion on the recording medium 1, and after being collected by the objective lens 2, subjected to diffraction by the diffraction grating 3'. The light thus diffracted by the diffraction grating 3' is collected onto the detector 6 by the collimate lens 4, and thus, it becomes possible to reproduce the information recorded on the recording medium 1. It should be noted here that a major aspect to the pick-up device P3 in Fig. 2 resides in the construction of the diffraction grating as described hereinbelow. More specifically, in the pick-up device P3, the diffraction grating is so constructed as to have a blaze characteristic, and Fig. 3 shows the construction of the diffraction grating 3' provided with the blaze characteristic, and applicable to the arrangement of Fig. 2. As shown on an enlarged scale in Fig. 4, the diffraction grating 3' in Fig. 3 has its one surface formed into a saw-tooth shape. Here, it is intended to cause the direction of the refracted light owing to the shape and the direction of the diffracted light to coincide with each other so as to blaze the light which has passed through the diffraction grating 3', thereby to concentrate energy in a direction of the diffraction. For the above purpose, the shape is so determined as to satisfy a following equation on the assumption that light is vertically incident upon the flat face side of the diffraction grating 3'. n sinα = sinβ d sin (β-α) = Kλ where n: refractive index of the diffraction grating 3' d: grating pitch λ: wavelength of light K: integral number By the employment of the diffraction grating 3' as described above, as shown in Fig. 3, the light emitted from a point D where the semi-conductor laser 7 is disposed, is diffracted by the diffraction grating 3' and forms light spots only at two points A and B on the recording medium 1. Of these two points, the reproducing signal by the light modulated at the point A of the recording medium 1 is the correct signal, and the reflected light is diffracted by the diffraction grating 3', and reaches a point F for the detector 6 and the point D for the semi-conductor laser 7. Meanwhile, the light based on the returning light modulated at a point B (on the other track) slightly spaced from the correct spot A and not subjected to diffraction by the diffraction grating 3' reaches a point E without the detector 6 and the point D where the semi-conductor laser 7 is disposed. Accordingly, only the returning light from the correct spot position A reaches the detector 6, and thus, an improved pick-up device free from influence of cross-talk and having favorable characteristics with a high efficiency for light utilization, may be presented. As is clear from the foregoing description, according to the present invention, it is possible to realize the high performance pick-up device having the symmetrical nature, an compact in size and light in weight can be realized.

Claims: Optical pick-up device for recording and/or reproducing information onto and from a recording medium, comprising: a light source (7), a condenser lens (2) for collecting the light of said light source (7) and projecting it onto said recording medium, a light diffraction grating (3') provided between said light source (7) and said condenser lens (2) for dividing the light reflected from said recording medium, a light detecting means for receiving the light reflected from said recording medium and diffracted by said light diffraction grating (3'), said light detecting means being a single detector (6) placed at one side of said light source (7), characterized in that, said light diffraction grating (3') is a blazed grating with grooves having a steep and shallow face so as to deflect onto said light detector (6) only the light reflected from that position (A) of said recording medium from which information is to be reproduced.

Inventors: INOUE TERUAKI, KURATA YUKIO, NAKATA YASUO, OGATA NOBUO, SATO HIDEAKI, INOUE, TERUAKI, KURATA, YUKIO, NAKATA, YASUO, OGATA, NOBUO, SATO, HIDEAKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1986
Main classification: G11B7
IPC codes: G11B7