Optical recording and reproducing apparatus for tracking wobbling guide grooves

Abstract: An optical recording and reproducing apparatus uses a laser to form one or more spots on an information recording medium preformatted with wobbling guide grooves, and detects tracking error. If the differential push-pull method of detecting tracking error is used, a center spot is flanked by satellite spots distant by an odd multiple of half the repeating period of the wobble. If the three-beam method is used, the distance is an odd multiple of one-fourth the repeating period of the wobble. If the push-pull method is used, a split photodetector generates a pair of electrical signals. The sum and difference of these signals are filtered, then synchronously detected, and the result is combined with the difference signal to generate a corrected tracking error signal.

Description: This invention relates to optical recording and reproducing apparatus employing media preformatted with wobbling guide grooves, more particularly to the detection of tracking error in such apparatus. Optical reproducing apparatus is already widely manufactured and sold in the form of compact disc players, and apparatus is now becoming available that can both reproduce and record information, using write-once-read-many media or overwritable media. In both types of apparatus information is read from (or written on) spiral or concentric tracks by means of a laser beam. The beam is kept on-track by a system that detects tracking error, and a servo system that corrects the detected tracking error. A variety of methods are used to detect tracking error. In the well-known push-pull method, the laser beam is reflected from the medium to a split photodetector. Signals from the two halves of the detector are added to obtain the reproduced information signal, and mutually subtracted to obtain a tracking error signal. In another method known as the three-beam, triple-beam, tri-beam, or twin-spot method, three laser beams are focused onto the medium, creating three spots that are aligned at an angle to the tracks. The center spot is used for reading or writing information. The other two spots, referred to as satellite spots, are reflected to two separate photodetectors, and the difference between the photodetector output signals is used as the tracking error signal. Still another method, known as the differential push-pull method, also employs three spots, but reflects each spot to a split photodetector that generates a difference signal. The tracking error signal is obtained by adding the two difference signals from the satellite spots and subtracting the difference signal from the center spot. These three methods were originally developed for use with media having regular circular or spiral tracks, but there has been a recent movement toward media on which the tracks are formed in wobbling guide grooves. For example, standards for compact disc (CD) media have been proposed in which absolute address information and information for constant linear velocity control are encoded in the wobble. A general problem arising with such media is that if the tracking error detection system detects the wobble, the tracking servo may attempt to track the wobble, thereby defeating the purpose of the wobble. In the push-pull method, for example, if the servo system tracks the wobble accurately, the information encoded in the wobble is lost. A similar problem occurs in the differential push-pull method when the spacing between the three spots is one-fourth the repeating period of the wobble. A somewhat different problem can occur in the three-beam method. If the two satellite spots are separated by a distance equal to the repeating period of the wobble, the servo system tracks the wobble at the locations of the satellite spots, thereby doubling the amplitude of the wobble at the central read/write spot. In recording, this leads to poorly, formed pits; in reproducing, it degrades the quality of the reproduced signal. The wobble-tracking problem is related to the problem of tracking offset, which can be caused by tilt of the rotating medium or track-following movement of the objective lens. This problem occurs particularly when the push-pull method is used. Previous attempts to correct offset by means of sensors that detect media tilt and objective-lens movement have been unsatisfactory, in part because they increase the complexity and cost of the apparatus. Attempts to avoid the second type of offset by moving the optical system as a whole instead of just the objective lens also increase the cost of the apparatus, because a more powerful actuator is needed. WO 91/01551 reflecting the preamble of claim 1 relates to tracking methods for use with recording media having wobbling tracks. It proposes the use of a three spot system with satellite spots either side of a centre spot, the satellite spots being an integer multiple of the track wobble wavelength apart in the longitudinal direction, but illustrates a system with the satellite spots half the wobble wavelength apart. No details of how a tracking signal is generated are given. EP-0,324,949 A1 and EP-0,426,879 discloses a three beam tracking method for use with a recording medium having a non-wobbling guide track. It is accordingly an object of the present invention to generate a tracking error signal with reduced wobble and offset components. Another object of the invention is to reduce the cost of optical recording and reproducing apparatus. According to the present invention, there is provided an optical apparatus for recording and reproducing information on or from an information recording medium having guide grooves that are disposed at a certain pitch and that wobble with a certain repeating period, said apparatus comprising: a laser for emitting at least one laser beam; an optical system for receiving said laser beam and forming a centre spot and two satellite spots on said information recording medium, said centre spot and said two satellite spots being aligned in a straight line at an angle to said guide grooves with said centre spot disposed midway between said two satellite spots and distant from each of said two satellite spots by an odd multiple of substantially one-half said repeating period in a longitudinal direction of said guide grooves; a first split photodetector for receiving reflected light from said centre spot and generating a first pair of electrical signals; a second split photodetector for receiving reflected light from one of said two satellite spots and generating a second pair of electrical signals; a third split photodetector for receiving reflected light from another of said two satellite spots and generating a third pair of electrical signals; and a tracking error detection circuit for generating a first difference signal from said first pair of electrical signals, a second difference signal from said second pair of electrical signals, and a third difference signal from said third pair of electrical signals, then adding said second difference signal and said third difference signal and subtracting their sum from said first difference signal to generate a tracking error signal. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Fig. 1 is a sectional view of the optical system and related parts of an optical recording and reproducing apparatus according to the invention employing the differential push-pull method. Fig. 2 is a plan view of the information recording medium in Fig. 1, showing the positions of the spots. Fig. 3 is a schematic diagram of the tracking error detection circuit of the apparatus in Fig. 1. Fig. 4 is a simplified sectional view of the optical system, illustrating offset caused by tilt of the information recording medium. Fig. 5 is another simplified sectional view of the optical system, illustrating offset caused by movement of the objective lens. Fig. 6 is another plan view of the information recording medium. Optical recording and reproducing apparatus employing the differential push-pull method of tracking error detection will be described with reference to the attached drawings. The drawings are illustrative and do not restrict the scope of the invention, which should be determined solely from the appended claims. Referring to Fig. 1, a first optical recording and reproducing apparatus according to the invention comprises a disc-shaped information recording medium 2 that rotates in a plane at right angles to the page, and a semiconductor laser 4 that emits a laser beam 5. An optical system 6 diffracts and focuses the laser beam 5 to form a center spot 8 and two satellite spots 10 and 12 on the information recording medium 2. Three split photodetectors 14, 16, and 18 receive light reflected from the information recording medium 2 and convert the light to electrical signals that are input to a tracking error detection circuit to be shown later. The optical system 6 comprises a collimator lens 20, a diffraction grating 22, a polarizing beam splitter 24, a quarter-wave plate 26, an objective lens 28, and a convergent lens 30. The collimator lens 20 converts the divergent laser beam 5 emitted by the laser 4 to a parallel beam. The diffraction grating 22 separates this parallel beam into three beams: a zeroth-order diffracted beam 32, and two first-order diffracted beams 34 and 36. These three beams pass through the polarizing beam splitter 24 and the quarter-wave plate 26 and are focused by the objective lens 28 onto the information recording medium 2, the zeroth-order diffracted beam 32 forming the center spot 8 and the first-order diffracted beams 34 and 36 forming the two satellite spots 10 and 12. After reflection from the information recording medium 2, the three beams 32, 34, and 36 re-enter the optical system 6, pass through the objective lens 28 and the quarter-wave plate 26, are reflected in the polarizing beam splitter 24, and are directed by the convergent lens 30 onto the three split photodetectors 14, 16, and 18. The zeroth-order diffracted beam 32 is received by the first split photodetector 14, while the two first-order diffracted beams 34 and 36 are received by the second and third split photodetectors 16 and 18. Fig. 2 is a plan view showing part of the information recording medium 2. When the information recording medium 2 is manufactured, it is preformatted with guide grooves 38 that wobble with a substantially constant repeating period W around center lines 40. The wobbling is substantially in phase between adjacent grooves. The center lines 40 are drawn as double lines representing where the edges of the guide grooves 38 would be if the guide grooves did not wobble. The wobble is greatly exaggerated for clarity in the drawings; the actual amplitude of the wobble is nominally one twenty-fifth of the spacing P of the guide grooves 38. This spacing P will be referred to below as the track pitch. Information is recorded in the guide grooves 38 in the form of pits 41. The areas between the guide grooves 38 are referred to as land areas 42. As shown in the diagram, the spatial frequency of the pits 41 is much higher than the spatial frequency of the wobble. The pits 41 need not follow the wobble of the guide grooves 38; normally they follow the center lines 40 as shown in the drawing. Due to the exaggerated wobble in the drawings, some pits are shown as departing from the guide grooves, but in practice, such departure would be either nonexistent or extremely slight. The center spot 8 and the two satellite spots 10 and 12 are aligned in a straight line at an angle to the guide grooves 38 (more precisely, at an angle to their center lines 40). The center spot 8 is disposed midway between the two satellite spots 10 and 12, at a distance L from each satellite spot in the horizontal direction in the drawing, i.e., the longitudinal direction of the guide grooves. L is equal to an odd multiple of half the repeating period W. In the drawing L is equal to one-half W, but the invention can be practiced with other odd multiples such as three-halves W, five-halves W, and so on. L is the distance between the spots in the direction of the guide grooves 38 (the horizontal direction in the drawing). The two satellite spots 10 and 12 are also displaced from the center spot 8 in the vertical direction in the drawing, i.e., in the lateral direction of the guide grooves, by a distance Q equal to one-half the track pitch P. Q is much smaller than L, so the true distance of the center spot 8 from the satellite spots 10 and 12 is substantially equal to L. Fig. 3 is a schematic diagram illustrating the three photodetectors 14, 16, and 18 and the tracking error detection circuit 43 to which they are connected. As shown, each split photodetector comprises two separate light-receiving surfaces that generate two separate electrical signals. The tracking error detection circuit 43 comprises four differential amplifiers 44, 46, 48, and 50, a summing amplifier 52, and two variable-gain amplifiers 54 and 56. The differential amplifier 44 receives the two electrical signals D 1 and D 2 output by the first split photodetector 14 and generates a first difference signal E 0 . The differential amplifier 46 receives the two electrical signals D 3 and D 4 output by the second split photodetector 16 and generates a second difference signal E 1 . The differential amplifier 48 receives the two electrical signals D 5 and D 6 output by the third split photodetector 18 and generates a third difference signal E 2 , which is amplified by the variable-gain amplifier 54. The summing amplifier 52 adds the amplified third difference signal E 2 to the second difference signal E 1 . The variable-gain amplifier 56 amplifies the resulting sum E 3 . The differential amplifier 50 subtracts the amplified sum E 3 from the first difference signal E 0 to generate a tracking error signal TE. The tracking error signal TE is supplied to a tracking servo system, not shown in the drawings, that positions the three spots 8, 10, and 12 by moving the objective lens 28 in Fig. 1, or by moving the entire optical system 6 together with the laser 4 and the split photodetectors 14, 16, and 18. The tracking servo system is adapted to adjust the positions of the spots 8, 10, and 12 so as to reduce the value of the tracking error signal TE to zero. The tracking error detection circuit 43 also supplies a constant linear velocity control signal CLV to a spindle servo system, not shown in the drawings, that controls the rotation of the information recording medium 2. The first difference signal E 0 can be used as this constant linear velocity control signal CLV, as shown in the drawing. Another summing amplifier 57 receives the two electrical signals D 1 and D 2 output from the first split photodetector 32 and generates their sum as a reproduced information signal RF. This signal RF is supplied to signal-processing circuits, not shown in the drawings, that reproduce the information encoded in the pits 41. Next the difference signals E 0 , E 1 , and E 2 will be described in more detail. The three difference signals E 0 , E 1 , and E 2 have four components: a high-frequency information component due to the pits 41; a lower-frequency wobble component due to the wobble of guide grooves 38; a still lower-frequency tracking error component due to displacement of the center spot 8 from the center line 40 of the guide groove 38 being followed; and an offset component generated by such factors as tilt of the information recording medium 2 and movement of the objective lens 28. Fig. 4 illustrates how tilt of the information recording medium 2 can generate offset. For clarity, the diffraction grating 22 and the quarter-wave plate 26 are omitted and only the zeroth-order diffracted beam 32 is shown. The center spot 8 is exactly centered in a guide groove 38. If the information recording medium 2 were not tilted, reflected light from the center spot 8 would follow the dashed arrows and illuminate the two halves of the photodetector 14 equally. Because of the tilt, however, the reflected light follows the solid lines and illuminates the two halves of the photodetector 14 unequally, causing a dc offset in the difference signal. Fig. 5 illustrates how displacement of the objective lens 28 can generate offset. This problem occurs when track following is performed by moving the objective lens 28 without moving the photodetectors. If the objective lens 28 is centered over the guide groove 38 being tracked, as indicated by dashed lines, reflected light follows the dashed arrows and illuminates the two halves of the photodetector 14 equally. If the objective lens 28 has been moved downward in the drawing, however, as indicated by solid lines, reflected light illuminates the two halves of the photodetector 14 unequally. The resulting offset in the difference signal changes with movement of the objective lens 28, but the change becomes appreciable only after the objective lens 28 has moved much farther than the track pitch P. Over reasonable periods of time, such as during the following of a single track in Fig. 2, this offset can also be treated as a dc component of the difference signal. Next the use of the constant linear velocity control signal CLV will be described. The spindle servo system has a bandpass filter that rejects the dc offset component, the low-frequency tracking error component, and the high-frequency information component of the constant linear velocity control signal CLV. The spindle servo thus responds only to the wobble component. The spindle servo is adapted to control the rotation of the information recording medium 2 so that the wobble component has a constant center frequency of, for example, 22.05 kilohertz as in the proposed compact-disc standard. Although the repeating period W of the wobble is substantially constant, it is not exactly constant; it is modulated slightly to encode absolute address information. This modulation is detected as a frequency modulation of the constant linear velocity control signal CLV with a maximum deviation of ±1 kilohertz. Thus during normal operation the wobble has a frequency of 22.05 ± 1 kilohertz. Next the tracking error signal TE will be described. The tracking servo system has a low-pass filter that rejects the high-frequency information component of the tracking error signal TE, so the high-frequency information components of the difference signals E 0 , E 1 , and E 2 will be ignored in this description. Taken together, the tracking error and wobble components can be approximated by a sine function of the deviation of the spot from the guide groove. Referring to Fig. 6 (in which the pits 41 are omitted for clarity), let U be the deviation of the guide grooves 38 from the center line 40 at the location of the center spot 8. Since the satellite spots 10 and 12 are separated from the center spot 8 by a distance L equal to half the repeating period W of the wobble, at the satellite spots 10 and 12 the guide grooves 38 deviate by an equal amount -U in the opposite direction. Let X be the tracking error, i.e. the deviation of the center spot 8 from the center line 40 (in the drawings X is zero). Then the three difference signals can be represented as follows: E 0 = A 0 sin[2π (X - U)/P] + B 0 E 1 = A 1 sin[2π (X + Q + U)/P] + B 1 E 2 = A 2 sin[2π (X - Q + U)/P] + B 2 The constants A 0 , A 1 , and A 2 are the amplitudes of the tracking error plus wobble components of the three difference signals and B 0 , B 1 , and B 2 are their offset components. Since Q = P/2 and hence 2π Q/P = π , the formulas for E 1 and E 2 can be simplified as follows. E 1 = -A 1 sin[2π (X + U)/P] + B 1 E 2 = -A 2 sin[2π (X + U)/P] + B 2 From Figs. 4 and 5 it can be understood that offset affects all three difference signals in the same way; that is, the amplitudes and offsets are related as follows: A 0 /B 0 = A 1 /B 1 = A 2 /B 2 The gain G 1 of the variable-gain amplifier 54 is set to A 1 /A 2 . The signal E 3 can therefore be expressed as follows: E 3 = E 1 + G 1 E 2 = -2A 1 sin[2π (X + U)/P] + 2B 1 The gain G 2 of the variable-gain amplifier 56 is set to A 0 /(2A 1 ). The tracking error signal TE is therefore: TE = E 0 - G 2 E 3 = A 0 sin[2π (X - U)/P] + A 0 sin[2π (X + U)/P] = 2A 0 sin(2π X/P)cos(2π U/P) Since the wobble U is much smaller than the track pitch P, cos(2π U/P) is substantially equal to unity. To a close approximation, TE is therefore expressible as follows: TE = 2A 0 sin(2π X/P) As a sine function of X, this tracking error signal TE accurately represents the tracking error, without offset or wobble. The tracking servo therefore tracks the center line 40 accurately, resulting in a high-quality constant linear velocity control signal CLV, a high-quality reproduced information signal RF during reproduction, and well-formed pits during recording. In the case of a compact disc with a constant linear velocity between 1.2 and 1.4 meters per second in which the wobble frequency is 22.05 ± 1 kilohertz, to be equal to half the repeating period of the wobble, the separation L between the center spot 8 and the satellite spots 10 and 12 should be 24 to 35 microns. The invention is not restricted to this particular range of values however, odd multiples of half the repeating period of the wobble also being usable. The apparatus in Fig. 1 uses a diffraction grating to produce three laser beams, but the invention is not restricted to this means of beam production. Instead of a diffraction grating, it is possible to employ a monolithic semiconductor laser array that emits three laser beams from a single lasing region, or a hybrid semiconductor laser array having three lasing regions, each of which emits a single laser beam. The apparatus shown in Figs. 1 to 6 has the advantage that by eliminating or reducing the offset component of the tracking error signal, it eliminates or reduces the need for sensors to detect tilt of the information recording medium 2 or movement of the objective lens 28, and increase the range over which track following can be carried out by moving the objective lens 28 without moving the entire optical system. Other modifications that will be obvious to those skilled in the art can be made in the apparatus shown in Figs. 1 and 3 without departing from the scope of the present invention as set forth in the following claims.

Claims: An optical apparatus for recording and reproducing information on or from an information recording medium having guide grooves that are disposed at a certain pitch and that wobble with a certain repeating period said apparatus comprising: a laser (4) for emitting at least one laser beam, an optical system (6) for receiving said laser beam and forming a centre spot (8) and two satellite spots (10 12) on said information recording medium said centre spot and said two satellite spots being aligned in a straight line at an angle to said guide grooves with said centre spot disposed midway between said two satellite spots the apparatus being characterized in that the centre spot is distant from each of said two satellite spots by an odd multiple of substantially one-half said repeating period in a longitudinal direction of said guide grooves and further comprises: a first split photodetector (14) for receiving reflected light from said centre spot and generating a first pair of electrical signals (D1, D2); a second split photodetector (16) for receiving reflected light from one of said two satellite spots and generating a second pair of electrical signals (D3, D4); a third split photodetector (18) for receiving reflected light from another of said two satellite spots and generating a third pair of electrical signals (D5, D6) and a tracking error detection circuit (43) for generating a first difference signal (E0) from said first pair of electrical signals (D1, D2) a second difference signal (E1) from said second pair of electrical signals (D3, D4), and a third difference signal (E2) from said third pair of electrical signals (D5, D6), then adding (52) said second difference signal and said third difference signal and subtracting (50) their sum from said first difference signal to generate a tracking error signal (TE). The apparatus of claim 1, wherein said centre spot is distant from each of said two satellite spots by substantially half said pitch of said grooves in a lateral direction of the grooves. A. The apparatus of claim 1 or 2, wherein said laser (4) emits a single laser beam and said optical system has a diffraction grating (22) for separating said single laser beam into three beams. The apparatus of claim 1 or 2, wherein said laser (4) is a monolithic semiconductor laser array that emits three laser beams from a single lasing region. The apparatus of claim 1 or 2, wherein said laser (4) is a hybrid semiconductor laser array having three lasing regions each of which emits a single laser beam. The apparatus of any one of claims 1 to 5 further comprising means to rotate said information recording medium with a constant linear velocity between 1.2 and 1.4 meters per second such that said tracks wobble at a frequency of 22.05± 1 kilohertz. The apparatus of claim 6, wherein said centre spot is separated from each of said satellite spots by a distance not less than 24 microns and not greater than 35 microns.

Inventors: ITO TAKESHI, KIME KENJIRO, MATOZAKI TOSHIYA, NAKAMURA KEIJI, SHINODA MASAHISA, YOSHIHARA TORU, ITO, TAKESHI, KIME, KENJIRO, MATOZAKI, TOSHIYA, NAKAMURA, KEIJI, SHINODA, MASAHISA, YOSHIHARA, TORU, Ito, Takeshi, c/o Mitsubishi Denki K.K., KIME, KENJIRO, C/O MITSUBISHI DENKI K.K., Matozaki, Toshiya, c/o Mitsubishi Denki K.K., Nakamura, Keiji, c/o Mitsubishi Denki K.K., Shinoda, Masahisa, c/o Mitsubishi Denki K.K., Yoshihara, Toru, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: G11B7
IPC codes: G11B7