Tracking control apparatus for a recording/reproducing apparatus on or from an optical disk

Abstract: A tracking control apparatus for use with an optical recording/reproducing apparatus includes a stationary optical section (5) provided with a laser light source (11); a moveable optical section (15) provided with an objective lens (6) by means of which three laser beams emanating from the laser light source are irradiated on an optical disk (1), a plurality of photodetectors (20) having a light receiving surface divided into at least two segments adapted to receive the three reflected laser beams from the optical disk, and a tracking control signal computing device (21) for taking the difference of detection outputs from the light receiving surfaces of said photodetectors (20) to produce plural tracking error signals and dividing the tracking error signals by the total light quantities received by the photodetectors (20) for normalising the tracking error signals to derive tracking signals.

Description: This invention relates to a tracking control apparatus for a recording and/or reproducing apparatus for recording signals on and/or reproducing signals from an optical disk, and may be advantageously applied inter alia to tracking control by a differential push-pull method. One tracking servo control method in a recording and/or reproducing apparatus on or from an optical disk which has been proposed uses a tracking servo employing a differential push-pull (DPP) method. The tracking servo employing this DPP method is an improvement over the method for tracking error detection by the push-pull method employing a signal laser beam as disclosed for example in Japanese patent publication number 32735/1986. In the DPP method, the laser light emitted from a laser light source, such as a semiconductor laser diode, is separated by the grading method into three laser beams, ie, the 0'th order beam and ± first order diffracted beams. Of these three laser beams, the 0'th order beam is irradiated onto a record track TR of an optical disk 1 as a laser spot for reproduction SP PB , whereas the two ± first order diffraction beams are irradiated as two auxiliary laser spots SP S1 and SP S2 so that these two beams are offset ahead and behind the reproduction laser spot SP PB in the track pitch direction with respect to the record track TR (figure 1). In this manner, the return lights from the three laser spots SP PB , SP S1 and SP S2 , derived from the three laser beams, are obtained. The return light from the reproduction laser spot SP PB is received by a four-segment photodetector 2, while the return lights from the two auxiliary laser spots SP S1 , SP S2 are received by two two-segment photodetectors 3, 4 respectively, each having its division line parallel to the record track TR of the optical disk 1. On light reception, outputs S A , S B , S C , S D , S E , S F , S G and S H produced by light receiving segments 2A, 2B, 2C and 2D making up the four-segment photodetector 2 and light receiving segments 3E, 3F, 4G and 4H of the two-segment photodetectors 3, 4 respectively, are subject to an arithmetic operation is performed in accordance with the following formula (1) S TE = [(S A + S D ) - S B + S C )] - k₁ [(S E - S F ) + k₂(S G - S H )] (1) wherein K₁ and k₂ are constants, and S TE , the tracking error signal. Meanwhile, with the above described recording and/or reproducing apparatus on or from an optical disk, there has been proposed an optical head in which an optical system exclusive of the optical disk is stationarily arranged in the main body of the apparatus and the objective lens is placed on an actuator and driven thereby to be shifted to a desired record track on the optical disk to enable high speed accessing. This desired record track is referred to hereinafter as the access position. With the above described optical head, supposing that, if an optical axis 6L of an objective lens 6 is positioned with an offset from the 0'th order light L₀, among the three laser beams LB₀, LB+₁ and LB₋₁ of the 0'th and ± first order diffraction lights emanating from a stationary optical unit 5, the coupling efficiency of the ± first order refraction lights LB+₁ and LB₋₁ with respect to the object lens 6 is caused to fluctuate as a function of changes in the access positions P n0 , P n1 of the objective lens 6. The result is that a deviation is brought about in the equilibrium of the sum signal PP2 of second and third push-pull outputs PP21, PP22, which is produced from the two-segment photodetectors 3, 4 and which is represented by the formula PP2 = (S E - S F ) + (S B - S N ) (3) with respect to the first push-pull output PP1 which is produced by the four-segment photodetector 2 and which is represented by the formula PP2 = (S A - S D ) + (S B - S C ) (2) Thus the inconvenience is caused that the offset produced when the objective lens 6 is off-centre with respect to the record track TR cannot be cancelled. In addition, since the coupling equilibrium of the ± first order diffracted light beams LB+₁, LB+₁ with respect to the objective lens 6 is changed as a function of the access positions of the objective lens 6, an offset is produced in the second and third push-pull outputs PP21 and PP22 from the two two-segment photodetectors 3, 4 so as to be superimposed on the detected tracking error signal S TE . EP-A-0,201,603 discloses a tracking control apparatus for use with an optical recording and/or reproducing apparatus comprising: a stationary optical section provided with a laser light source outputting a single beam and a beam splitter for splitting said beam into three beams; a moveable optical section provided with an objective lens by means of which said three laser beams emanating from said stationary optical section are, in use, irradiated on an optical disk, and at least three photodetectors each having a light receiving surface divided into at least two segments adapted to receive a respective one of the three reflected laser beams from said optical disk, and tracking control signal computing means for taking the difference of the detection outputs from said two segments of said light receiving surfaces of each of said photodetectors to produce plural tracking error signals. US-A-4,571,712 discloses the principle of normalization of the detection outputs using the total beam power. The invention is characterised in that said computing means is further adapted for normalising said tracking error signals on the basis of the total light quantities received by the respective photodetector to derive tracking control signals. Thus it is possible with the tracking control apparatus according to the present invention to effect stable detection of the tracking control signal by dividing the differential push-pull detection outputs of the 0'th order light and the ± first order diffracted light beams by the total light quantities of the 0'th order light beam and the ± first order diffracted light beams. The invention will be further described by way of nonlimitative example with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram for illustrating the tracking control by the differential push-pull method. Figure 2 is a schematic diagram showing the relation between the laser light source and the objective lens. Figure 3 is a diagrammatic view showing the recording and/or reproducing apparatus on or from an optical disk to which the tracking control apparatus according to the present invention to applied. In the following description, the same numerals are used to depict the parts or components which are the same as those used in figures 1 and 2 in the illustration of the tracking control method employing the aforementioned DPP method and the description thereof will be omitted. Referring to figure 3, the recording and/or reproducing apparatus according to the present invention includes an optical head device 10 formed by a stationary optical section 5 and a moveable optical section 15. The stationary optical section 5 of the optical head device 5 is made up of a laser light source 11, a collimator lens 12, a grating 13 for separating the laser light L₁ emitted by the laser light source 11 into three laser beams, that is a 0'th order light beam and ± first order diffracted light beams, a deflected light beam splitter 14 and a photodetector 20 for receiving and detecting a return light beam LB B from the optical disk 1. The moveable optical section 15, placed on an optical head actuator, not shown, is moved in a controlled manner in a direction shown by the arrow in figure 3, that is, in a direction normal to the record track TR of the optical disk 1, and is provided with an objective lens 6 and a biaxial actuator 7 for displacing the objective lens 6 in two mutually perpendicular directions, ie, in the focusing direction parallel to an optical axis of the objective lens 6 and the tracking direction included in a horizontal plane normal to the optical axis of the objective lens 6. By thus separating the optical head device into the stationary optical section 5 and the moveable optical section 15, the weight of the moveable section of the optical head device may be reduced. Thus it becomes possible with this separate type optical head device 10 to shorten the time involved in accessing a record track on the optical disk as compared with the optical head device for the conventional recording and/or reproducing apparatus with a built-in optical system extending from the laser light source to the objective lens. With the above described optical head device 10, the laser light L1 emitted from the laser light source 11 is separated by the collimator lens 12 and the grating 13 into three laser beams LB consisting of the 0'th order light beam and the ± first order diffracted light beams before being transmitted through the deflected light beam splitter 14 so as to be incident on the moveable optical part 15. The laser beam LB incident on this moveable optical section 15 is bent through 90° at a mirror 16 and converged by the objective lens 6 supported on the biaxial actuator 17 so as to project three light spots SP PB , SP S1 and SP S2 on the record track TR of the optical disk 1. The optical disk 1 is supported rotatably about a spindle shaft 18 as the centre of notation, by chucking means, not shown, and is driven by a spindle motor 19 so as to rotate at a predetermined speed. The reflected light from the three light spots SP PB , SP S1 and SP S2 , thus converged on the record track TR of the optical disk 1, is bent through 90° at the mirror 16 via the objective lens 6 so as to be incident as the return light beam LB B from the moveable optical section 15 into the stationary optical section 5. The return light beam LB B incident on the stationary optical part 5 is bent through 90° by the deflected beam splitter 14 before being received by the photodetector 20 consisting of a four-segment photodetector 2 and two-segment photodetectors 3, 4 having a division line parallel to the record track TR of the optical disk 1. Light reception outputs S A , S B , S C , S D ; S E , S F , S G , S H , produced from light receiving elements 2A, 2B, 2C, 2D of the four-segment photodetector 2 and light receiving elements 3E, 3F, 4G and 4H, are input into a tracking control signal computing circuit 21, where an arithmetic operation is performed in accordance with the following equation (4); The arithmetic operation based on this equation (4) consists in computing the tracking control signal S TE10 using the light reception outputs S A to S H . That is, with the present tracking control signal computing circuit 21, in consideration that the first push-pull output PP1 obtained from the aforementioned four-segment photodetector 2 on the basis of the formula (2) and the second and third push-pull outputs PP21 and PP22 obtained from the two-segment photodetectors 3, 4 on the basis of the formula (3) are approximately proportionate to the total light quantities TC₂, TL₃ and TL₄ of the photodetectors 2, 3 and 4 represented by the formulas TL₂ = S A + S D + S E + S C (5) TL₃ = S E + S F (6) TL₄ = S G + S H (7) the push-pull outputs PP1, PP21, PP22 obtained from the photodetectors 2 to 4 are divided by the total light quantities TL₂, TL₃ and TL₄, respectively, for normalisation. In this manner, even supposing that the 0'th order light LB₀ and the optical axis LL of the objective lens 6 are arranged with an offset from each other, the tracking control signal computing circuit 21 functions to eliminate this offset to enable detection of stable tracking control signal S TE10 at all times. With the above described arrangement, the differential push-pull detection outputs PP1, PP21 and PP22 of the 0'th order light beam and ± first order diffracted light beams are divided by the total light quantities TL₂, TL₃ and TL₄, respectively, for normalisation, to produce stable tracking control signals S TE10 , whereby a tracking control device may be realised which is able to perform stable tracking control of the optical head which is adapted to move only the objective lens 6 to effect a higher speed accessing. With the above illustrative embodiment, the tracking error control signal computing circuit 21 itself functions as an automatic gain control (AGC) circuit, so that, even if there should be laser power fluctuations, as in the case of the overwritable recording and/or reproducing apparatus, there is no necessity for providing an AGC circuit and hence the overall construction may be simplified despite the provision of a division circuit as the tracking control signal computing circuit 21. Although the present invention is applied in the above embodiment to the tracking control device for the recording and/or reproducing apparatus on or from an optical disk, the present invention may also be applied extensively to other optical recording and/or reproducing apparatus such as a recording and/or reproducing apparatus for use with a magneto-optical disk. As explained hereinabove, the differential push-pull detection outputs of the 0'th order light beam and ± first order diffracted light beams, obtained in accordance with the differential push-pull method, are divided by the total light quantities of the reflected light beams derived from the 0'th order light beam and the ± first order diffracted light beams, respectively, for normalisation, so that a tracking control apparatus may be realised which enables detection of stable tracking control signals at all times. In this manner, a recording and/or reproducing apparatus may be realised which enables stable tracking control of the optical head which may be accessed at a higher speed to moving only the objective lens.

Claims: A tracking control apparatus for use with an optical recording and/or reproducing apparatus comprising: a stationary optical section (5) provided with a laser light source (11) outputting a single beam and a beam splitter (13) for splitting said beam into three beams; a moveable optical section (15) provided with an objective lens (6) by means of which said three laser beams emanating from said stationary optical section are, in use, irradiated on an optical disk, and at least three photodetectors (2, 3, 4) each having a light receiving surface divided into at least two segments (A-M) adapted to receive a respective one of the three reflected laser beams from said optical disk, and tracking control signal computing means (21) for taking the difference of the detection outputs from said two segments of said light receiving surfaces of each of said photodetectors to produce plural tracking error signals and characterised in that: said computing means is further adapted for normalising said tracking error signals on the basis of the total light quantities received by the respective photodetector to derive tracking control signals. The tracking control apparatus according to claim 1 wherein said tracking signal computing means are adapted for dividing said tracking error signals by the total light quantities received by the light receiving surfaces of the respective photodetector. The tracking control apparatus according to claim 2 wherein, when a light spot on an optical disk of a first one of the three laser beams emanating from said laser light source is on a track of said optical disk, light spots on said optical disk of second and third ones of the three laser beams are irradiated at offset positions from said track on said optical disk at the upstream and downstream sides of said first laser beam, and wherein the tracking control signal S TE is given by wherein S a , S b denote detection outputs from the segments of the light receiving surface of that one of said photodetectors which receives the reflected light of said first laser beam S c , S d , S e and S f denote detection outputs from the segments of the respective ones of the light receiving surfaces of those of the photodetectors which receive the reflected light beams of said second and third laser beams, and K denotes a coefficient.

Inventors: KOBAYASHI YUHEI C O SONY CORPO, KOBAYASHI, YUHEI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G11B7, G11B11