Abstract:
This invention relates to a method for detecting and compensating the disk tilt of an optical disk and an apparatus using it. The disk tilt causes the coma which degrades the carrier-to-signal ratio of a pickup. To reduce the coma induced by the disk tilt of an optical disk, the pickup of the present invention adapts a two-dimension (2-D) grating to produce a plurality of laser beams for detecting the disk tilt in the radial and tangential direction of the optical disk. In addition, according to the radial and tangential tilts of the disk, an actuation device adjusts a reflection angle of a reflective mean to change the incident angles of the laser beams for the compensation of the coma induced by the disk tilt of the optical disk.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and an apparatus for detecting and compensating disk tilt, more particular to a technique for pickups to solve the reading-error problem induced by coma as there are optical disk tilts in the radial and tangential directions. 
     BACKGROUND OF THE INVENTION 
     Optical disks include a transparent substrate having a recording layer where data is recorded and stored. Data can be stored on the recording layer in various forms, including pits, marks, and magneto-optic domains. In an optical disk system, a laser beam is focused by an objective lens through the transparent substrate and onto the stored data. The laser beam is then reflected back through the same objective lens for focusing. Since surface defects on the disk, such as dust particles and scratches, can have dimensions on the order of the focused spot size of the laser beam, the laser beam is typically focused onto the rear surface of the disk substrate to ensure that any surface defects will be out of focus with respect to the recording layer containing the data. Any spherical aberration caused by focusing the laser beam through the substrate will generally be corrected by the design of the objective lens. 
     Typically, the optical disk is not perfectly flat, and any local deviations from flatness appear as a slight tilt of the front surface of the disk with respect to the incident beam. In addition, when the optical disk placed on a turntable of a player is warped, the front surface of the disk is tilted relative to the optical axis of the focused laser beam, and coma aberration occurs. Additional tilt components can be caused by spindle misalignment or disk droop. The disk tilt causes a degradation of the focused spot quality of the laser beam, which results in a decrease in the carrier-to-noise ratio during readout, an increase in crosstalk and intersymbol interference, and a reduction in recording sensitivity. 
     Future generations of optical disks will most likely utilize shorter wavelengths and higher numerical aperture objective lenses, both of which increase the area data density of the disks. Unfortunately, the sensitivity to disk tilt increases if the wavelength and numerical aperture quantities are adjusted to meet a higher data density. Several systems have been proposed that attempt to dynamically correct for the effects of disk tilt, for example, by tilting the objective lens or the entire optical head. 
     As a prior art, U.S. Pat. No. 5,065,380 discloses a tilt-detection method by adapting a pickup with three laser beams in the radial direction of an optical disk. The method measures only the radial tilt but not the tangential tilt, and it can not resist the tracking motion of the pickup. 
     U.S. Pat. No. 5,523,989 discloses another pickup with a tilt-compensation method which measures the disk tilt according to the push-pull signal (the interference between a zero-order diffracted light beam and +1, −1 first-order diffracted light beams reflected by the optical disk). The pickup compensates the coma by using a servo actuator. The pickup of the invention needs an additional lens and its size is larger than that of a traditional pickup. 
     Disclosed in U.S. Pat. No. 5,805,543, a method adapts two consecutive laser pulses to measure the disk tilt. Its calculation is too laborious to be realized on line. One type of tilt sensor measures tilt with respect to the disc surface, but only after the information beam is tracking and focused in an information track. Such systems are described in U.S. Pat. No. 5,206,848 to Kusano. Because these devices require relative stability between the optical recording actuator and the disc, they are ineffective for measuring inertial tilt during rapid actuator movements. 
     An apparatus for producing a tilt error signal representative of the tilt of an optical disk is disclosed in U.S. Pat. No. 5,805,543. The apparatus includes a source of laser beam for focusing a laser beam onto the disk and reflecting such beam from the disk, a beam splitter positioned to receive the reflected light beam and to direct the light beam in a first direction, and a structure for separating the reflected light beam from the beam splitter into at least four portions and for producing detection signals for each portion. The apparatus further includes circuitry responsive to the detection signals for producing first and second tracking error signals, the first and second tracking error signals being produced by different combinations of the detection signals and both being sensitive to cross-track diffraction, and circuitry responsive to the first and second tracking error signals for producing the tilt error signal. The apparatus has no actuator to compensate the disk tilt. 
     A combined sensor for measuring tilt and tracking position of a lens holder with respect to an optical recording actuator base is disclosed in U.S. Pat. No. 5,732,054. A light source and two photosensitive bi-cell detectors are secured to the actuator base so that a beam from the light source strikes the detectors, and an optical slot or flag is secured to the lens holder between the light source and the detectors, for creating an image on the bi-cell detectors. Output from the bi-cell detectors is converted into information on tilt and position of the lens holder relative to the actuator base. 
     SUMMARY OF THE INVENTION 
     The main objective of the invention is to provide a method and a pickup to detect the radial and tangential tilts of an optical disk and compensate the coma induced by the disk tilt. The pickup adapts a two-dimension (2-D) grating to produce a plurality of laser beams for detection. These laser beams include a zero-order (0-order) diffracted laser beam, +1, −1 first-order radial diffracted laser beams and +− first-order (+1-order and −1-order) tangential diffracted laser beams. In addition, two detectors are adapted in the pickup to measure the interference of the reflected laser beams corresponding to the radial and tangential tilts. According to the radial and tangential tilts of the disk, a reflective mean is controlled and actuated to change the incident angle of the reading laser beam of the pickup for the compensation of the coma induced by the disk tilt. 
     The other objective of the invention is to provide a simple actuator which adjusts the reflection angle of a reflective mean to change the incident angle of the reading laser beam of the pickup for the compensation of the coma induced by the disk tilt. The actuator includes piezoelectric driving devices or electromagnetic driving devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The description is made with reference to the accompanying drawings in which: 
     FIG. 1 is the perspective view of a pickup according to the present invention; 
     FIG. 2 is the top view of a two-dimension grating used in the pickup shown in FIG. 1; 
     FIG. 3 shows the focused pattern of the laser beams diffracted by the grating shown in FIG. 2; 
     FIG. 4A illustrates the incident angle of the laser beam focused onto the optical disk according to the pickup of the present invention without disk tilt; 
     FIG. 4B illustrates the incident angle of the laser beam focused onto the optical disk according to the pickup of the present invention with disk tilt; 
     FIG. 5A illustrates the energy distribution of the diffracted laser beam reflected from the optical disk according to the pickup of the present invention without disk tilt; 
     FIG. 5B illustrates the energy distribution of the diffracted laser beam reflected from the optical disk according to the pickup of the present invention with disk tilt; 
     FIG. 6 is the diagram of the detection circuit of the pickup according to the present invention; 
     FIG. 7 illustrates the location relation of the detector and the diffracted laser spots according to the present invention; 
     FIG. 8 is an embodiment of the actuation device of the pickup according to the present invention, which is used to compensate the coma induced by the disk tilt; and 
     FIG. 9 is another embodiment of the actuation device of the pickup according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, according to the present invention, a pickup for the data access and the disk-tilt detection of an optical disk  30  includes a laser source  10 , a two-dimension (2-D) grating  20 , an optical system, a detection circuit  50  and an actuation device  60 . The laser source  10  can generate a laser beam for the data access and the disk tilt detection such as a laser diode. The 2-D grating  20  diffracts the laser beam from the laser source  10  to a plurality of laser beams in the tangential and radial directions of the optical disk  30  for the disk-tilt detection. The optical system of the pickup according to the present invention includes a beam splitter  40 , a collimator  42 , a reflective device  44  and an objective lens  46  for the data access and the disk-tilt detection. The detection circuit  50  transfers the light intensity of the diffracted laser beams reflected from the optical disk  30  to the data signal recorded on the optical disk  30  and the tilt signals in the tangential and radial directions. According to the present invention, the actuation device  60  can adjust the reflection angle of the reflective device  44  to change the incident angle of any laser beam of the pickup which is focused onto the optical disk  30  for compensating the coma induced by the disk tilt. 
     The laser beam generated by the laser source  10  first travels through the 2-D grating  20 . Then, the 2-D grating  20  diffracts the laser beam to a main laser beam and a plurality of minor laser beams. These laser beams sequentially travel through the beam splitter  40  and the collimator  42  collimating them. These collimated laser beams are then reflected to the objective lens  46  by the reflective device  44 , and the objective lens  46  focuses them onto the optical disk  30  for the data access and disk-tilt detection. Next, these focused laser beams are reflected by the recording layer of the optical disk  30  and sequentially travel through the objective lens  46 , the reflective device  44 , the collimator  42  and the beam splitter  40  reflecting them onto a photo sensing device of the detection circuit  50 . The detection circuit  50  transfers the light intensity of these backward laser beams to data signals recorded on the optical disk  30  and the tilt signals in the tangential and radial directions. According to the calculated tilt signals, the actuation device  60  tunes the reflection angle of the reflective device  44  to change the incident angles of these focused laser beams onto the optical disk  30  for compensating the coma induced by the disk tilt. 
     It is noted that according to the present invention the method for detecting and compensating the disk tilt at least includes the following steps: 
     a. detecting the tilts of the optical disk  30  in the tangential and radial directions; and 
     b. tuning the reflection angle of the reflective device  44  to change the incident angle of the focused laser beams onto the optical disk  30  according to the detected tilt signals. 
     The 2-D grating  20  is defined by a plurality of straight lines where some straight lines are parallel to the radial direction and the residual straight lines are parallel to the tangential direction (i.e. the track direction of the optical disk  30 ) as shown in FIG.  2 . The main laser beam diffracted by the 2-D grating is a zero-order diffracted laser beam, and the minor laser beams includes +1, −1 first-order diffracted laser beams in the radial direction and +1, −1 first-order diffracted laser beams in the tangential direction. These minor laser beams are symmetrically distributed corresponding to the main laser beam. The focused pattern of the main and minor laser beams on the optical disk  30  is shown in FIG. 3, and they are denoted by S 1  to S 5  (the numbers inside the quotations present the order of two dimensional diffraction). 
     Please refer to FIGS. 4A and 5A. As there exists no disk tilt induced by the optical disk  30 , the incident angles of every +1-order diffracted laser beam and −1-order diffracted laser beam are equal (denoted by φ). The light-intensity distribution on the photo sensing device of the laser beams reflected from the optical disk is schematically presented by the dark areas A 1  and A 2 , and the A 1 &#39;s and A 2 &#39;s areas are equal (A 1 =A 2 ). Referring to FIGS. 4B and 5B, as there exists a tilt angle θ on the optical disk  30 , the incident angles of the +1-order diffracted laser beam and −1-order diffracted laser beam are φ−θ and φ+θ. In addition, the light-intensity distribution on the photo sensing device of the laser beams reflected from the optical disk is schematically presented by the dark areas A 1  and A 2 , and the A 1 &#39;s and A 2 &#39;s areas are different (A 1 ≠A 2 ). Consequently, according to the difference of the areas A 1  and A 2 , the photo sensing device of the detection circuit  50  can measure the tilt angles of the optical disk  30  in the radial and tangential directions. 
     Referring to FIGS. 6 and 7, the detection circuit  50  includes a photo sensing device having a first pair of photo sensors ( 51   a  and  51   b ) and a second pair of photo sensors ( 52   a  and  52   b ), a radial-tilt detection circuit  70  and a tangential-tilt detection circuit  80 . The photo sensors  51   a  and  51   b  respectively transfer the light intensities of the reflected +1-order and −1-order laser beams S 2  and S 3  in the radial direction of the optical disk  30  to a first and a second electrical signals which are used to calculate the radial tilt. Similarly, the photo sensors  52   a  and  52   b  respectively transfer the light intensities of the reflected +1-order and −1-order laser beams S 4  and S 5  in the tangential direction of the optical disk  30  to a third and a forth electrical signals which are used to calculate the tangential tilt. The radial-tilt detection circuit  70  calculates the difference of the first and second electrical signals and exports a radial-tilt signal. Similarly, the tangential-tilt detection circuit  80  calculates the difference of the third and forth electrical signals and exports a tangential-tilt signal. 
     As the tracking motion of the pickup is operated, the objective lens  46  is moved along the radial direction of the optical disk  30 . The radial motion of the objective lens  46  greatly affects the radial-tilt signal of the detection circuit  50 , and insignificantly influences the tangential-tilt signal. In order to reduce the influence of the tracking motion of the pickup, signal process is required in the radial-tilt detection circuit  70 . Since the radial-tilt signal is modulated by the tracking error signal caused by the disk run-out and the frequency is higher than the moving frequency of the objective lens  46  and tilt signal, the influence of tracking motion of the pickup can be reduced with filters. To reduce the influence of the tracking motion of the pickup, the radial-tilt detection circuit  70  includes two amplifiers ( 71   a  and  71   b ), two first processing circuits ( 75   a  and  75   b ) including a first low-pass filter and a demodulator circuit with cut-off frequency F 1 , two second low-pass filters ( 72   a  and  72   b ) with cut-off frequency F 2 , two differentiators ( 73   a  and  73   b ) and a differentiator  74 , where the cut-off frequency F 1  is higher than the cut-off frequency F 2 . The amplifiers  71   a  and  71   b  are used to amplify the first and second electrical signals from the first pair of photo sensors, respectively. The amplified signal of the first and second electrical signals is then respectively transported to the first processing circuits  75   a  and  75   b . The first processing circuit  75   a  processes amplified signal of the first electrical signal and filters out the signal induced by the run-out motion of the rotated optical disk  30  in the radial direction and infested in the first electrical signal. The first processing circuit  75   a  then exports the processed signal to the second low-pass filter  72   a . The low-pass filters  72   a  and  72   b  filter out the signal induced by the movement of the objective lens  46  and infested in the amplified and filtered signals. The filtered signal of the first electrical signal is differentiated by its amplified signal in the differentiator  73   a , and similarly, the filtered signal of the second electrical signal is differentiated by its amplified signal in the differentiator  73   b . The differentiator  74  calculates the difference of both differentiated signals imported from the differentiator  73   a  and  73   b , and exports the radial-tilt signal. In addition, the circuitry of the tangential-tilt circuit  80  is similar to that of the radial-tilt circuit  70  as shown in FIG.  6 . 
     Hence, in order to reduce the coma induced by the disk tilt, the laser beam of the pickup must be orthogonally focused on to the optical disk  30 , in other word, the incident angle of the focused laser beam is zero. It is the function of the actuation device  60 , which adjusts the reflection angle of the reflective device  44  to make the incident angle of the focused laser beam zero according to the tilt angles of the optical disk  30 . 
     In FIG. 8, an embodiment of the actuation device  60  is shown. The coma induced by the disk tilt can be reduced by rotating the reflective device  44  along the radial or tangential directions. The reflective device  44  of the invention can be a mirror or a 45° prism, and it is supported by the free ends of a plurality of rods  61   a ,  61   b ,  62   a  and  62   b  arranged along the radial and tangential directions. The other ends of these rods  61   a ,  61   b ,  62   a  and  62   b  are fixed. The actuation device  60  includes a first pair of piezoelectric actuators ( 63   a  and  63   b ) and a second pair of piezoelectric actuators ( 64   a  and  64   b ). The first pair of piezoelectric actuators  63   a  and  63   b  are respectively disposed at the fixed ends of the rods  61   a  and  61   b  on one surface, and the other surface of each piezoelectric actuator is rigidly mounted on a base  67   a . Similarly, the second pair of piezoelectric actuators  64   a  and  64   b  are respectively disposed at the fixed ends of the rods  62   a  and  62   b  on one surface, and the other surface of each piezoelectric actuator is rigidly mounted on a base  67   b . According to the radial-tilt signal exported by the detection circuit  50 , the first pair of piezoelectric actuators  63   a  and  63   b  are excited to bend the rods  61   a  and  61   b  to rotate the reflective device  44  with its rotation axis parallel to the tangential direction. Similarly, according to the tangential-tilt signal exported by the detection circuit  50 , the second pair of piezoelectric actuators  64   a  and  64   b  are excited to bend the rods  62   a  and  62   b  to rotate the reflective device  44  with its rotation axis parallel to the radial direction. Since the tilt angles are generally very small about 0.5 to 0.6 degrees, the piezoelectric actuators are suitable for this objective. 
     FIG. 9 shows another embodiment of an actuation device  60   a  of the invention. In this embodiment, a pair of electromagnetic actuators  65  and  66  are used to replace the second pair of piezoelectric actuators  64   a  and  64   b  of the first embodiment. These electromagnetic actuators  65  and  66  can be constructed by voice coils ( 442   a  and  442   b ) and permanent magnets  441   a  and  441   b , respectively. The electromagnetic actuators  65  and  66  are disposed on two opposite side walls of the reflective device  44 . By exciting the voice coils  442   a  and  442   b  to generate magnetic forces, the reflective device  44  can be rotated with its rotation axis parallel to the radial direction. 
     It is noted that method and apparatus for detecting and compensating disk tilt described above are the preferred embodiments of the present invention for the purposes of illustration only, and are not intended as a definition of the limits and scope of the invention disclosed. Any modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the present invention.