JP4011529B2 - Optical pickup device - Google Patents

Description

  The present invention relates to an optical pickup device used in an optical disc apparatus that optically records information on an information recording medium such as an optical disc or reproduces recorded information.

  An optical pickup device using a hologram has been proposed as a means for reducing the size, the thickness, and the reliability. The hologram described in Patent Document 1 is divided into two in the radial direction of the optical disc, and each is further divided into two in the track direction. The focus error signal is detected by half of the reflected beam from the optical disk, and the information signal is detected by the entire reflected beam. The track error signal is calculated by comparing the phase difference between the sum signal of the signals received by the two detectors at the diagonal position and the sum signal of the signals received by the two detectors at the other diagonal position. By doing so, a position signal with respect to the track, that is, a so-called phase difference signal (DPD signal) can be detected.

  However, since the DPD signal uses a diffraction pattern from an already recorded pit, a push-pull method (PP method) or a differential push-pull method (DPP method) is used as a track servo for an unrecorded optical disk.

This optical pickup device includes an integrated unit using this hologram and an objective lens for condensing the light emitted from the integrated unit on an optical disk.
JP-A-10-269588

  Here, the pickup device using the integrated unit has the following problems.

  Generally, as the track servo, a push-pull method (PP method), a three-beam method, and a differential push-pull method (DPP method) can be used. In either method, the amount of detrack is detected by detecting the light amount difference between the plurality of light receiving units, and the case where there is no light amount difference is determined to be just-on-track.

  For example, in the case of the PP method, the tracking signal is generated by detecting the difference between the right and left light quantity distributions of the reflected light of the optical disk with a two-divided detector. When the objective lens moves in the radial direction, the optical axis of the reflected light of the optical disc is shifted and the beam center is deviated from the center of the two-divided detector. Also, when the disc is tilted, the optical axis of the reflected light is similarly shifted. In any case, an offset is generated in the differential signal of the two-divided detector in spite of tracking, and it is determined that the track is detrack.

  On the other hand, the 3-beam method and the DPP method are widely used as a tracking method because an offset generated in the case of the PP method can be suppressed by dividing the beam into three. However, when information is recorded on the optical disk, since three beams are generated from one light source, the amount of light of the main beam involved in recording is reduced. As a result, the recording speed is slow, which is an obstacle to increasing the recording speed.

  The present invention has been made to solve the above-described problems, and is an optical pickup that does not cause a decrease in the amount of light of the main beam and that does not generate an offset due to the movement of the objective lens or the tilt of the disk and can provide stable track servo performance. An object is to provide an apparatus.

  According to the optical pickup device based on the present invention, the light emitting unit that emits the laser light, the hologram that diffracts the reflected light of the optical disk and guides the light to the light receiving unit, and the light receiving unit that receives the light diffracted by the hologram And an integrated unit. Further, an objective lens for condensing the laser light emitted from the light emitting unit of the integrated unit on the optical disc is provided. A first optical element is disposed between the objective lens and the integrated unit. The first optical element has a diffractive portion that diffracts part of the reflected light from the optical disk. The diffraction unit has polarization anisotropy that transmits light in the first linear polarization state and diffracts light in the second linear polarization state whose polarization direction is orthogonal to the first linear polarization state. Yes. A second optical element is disposed between the first optical element and the objective lens. The second optical element converts the polarization state of the reflected light from the optical disk into the second linear polarization state. The first optical element and the second optical element are provided integrally with the objective lens.

  Preferably, in the optical pickup device, the second optical element converts the first linear polarization state into a circular polarization state and converts the circular polarization state into a second linear polarization state. It is a board.

  More preferably, in the optical pickup device, the diffractive portion of the first optical element is a polarizing diffraction grating provided only in a portion corresponding to a part of the reflected light of the optical disc.

  More preferably, in the optical pickup device, the polarizing diffraction grating constituting the diffraction portion of the first optical element is disposed in the groove, and the lithium niobate substrate having grooves periodically provided on the surface. And a proton exchange region.

  More preferably, in the optical pickup device, a part of the reflected light of the optical disk diffracted by the diffraction part of the first optical element, and the other part of the reflected light of the optical disk transmitted through the first optical element are: The light enters the hologram of the integrated unit.

  More preferably, in the optical pickup device, the diffractive portion of the first optical element is provided on one of the first optical element divided into two by a dividing line in the radial direction of the optical disc.

  More preferably, in the optical pickup device, the light emitting unit emits laser light in a first linearly polarized state.

  In the optical pickup device, more preferably, the hologram of the integrated unit is divided into two by a dividing line in the radial direction of the optical disc, and further divided into two by a dividing line in the track direction of the optical disc. is there.

  More preferably, in the optical pickup device, the light receiving unit of the integrated unit is divided into four or more parts corresponding to the diffracted light divided into four by the hologram.

  More preferably, in the optical pickup device, the reflected light of the optical disc transmitted without being diffracted by the first optical element is further divided into two by a dividing line of the hologram in the radial direction of the optical disc.

  More preferably, in the optical pickup device, the light receiving section is divided into a plurality of parts, and is generated from a signal of a portion of the light receiving section where the reflected light of the optical disc transmitted without being diffracted by the first optical element is incident. When tracking is performed by calculating a first track error signal and a second track error signal generated from a signal of a portion of the light receiving portion on which the reflected light of the optical disk diffracted by the first optical element is incident. A third track error signal used in the above is detected.

  According to the optical pickup device of the present invention, in the one-beam tracking method, it is possible to correct an offset generated by the movement of the objective lens and the tilt of the disk, and it is possible to obtain a stable track servo performance.

  Hereinafter, the optical pickup device according to the present embodiment will be described with reference to FIGS. FIG. 1 is a front view showing an outline of the configuration of the optical pickup device in the present embodiment.

  The integrated unit 1 includes an LD (laser diode) chip 2 as a light emitting unit, a light receiving unit 3, and a hologram element 4. The objective lens 5, the first optical element 7 and the second optical element 8 are fixed to and integrated with the holder 20. The objective lens 5, the first optical element 7, and the second optical element 8 fixed to the holder 20 are driven as a unit during focusing and tracking.

  The light emitted from the LD chip 2 of the integrated unit 1 passes through the hologram element 4, and then the zero-order light of the hologram passes through the first optical element 7, the second optical element 8, and the objective lens 5. The light is condensed on the optical disk 6. The reflected light of the optical disk 6 is transmitted through the objective lens 5 and the second optical element 8, and only a part of the reflected light is diffracted by the first optical element 7. The diffracted reflected light and the reflected light transmitted through the first optical element enter the hologram element 4, and the + 1st order diffracted light of the hologram reaches the light receiving unit 3. The hologram element 4 is divided into two by a radial dividing line M1 of the optical disc 6, and is further divided into two by a dividing line M2 in the track direction of the optical disc (see FIG. 6).

  FIG. 2 is a diagram showing the structure of the first optical element. In FIG. 2, an outer shape B forming a circular shape of the reflected light beam is indicated by a dotted line. A polarizing diffraction grating 9 is formed in a portion corresponding to the semicircle of the beam of reflected light obtained by dividing the outer shape B into two by the dividing line L. The polarizing diffraction grating 9 is not provided in a portion corresponding to the remaining semicircle of the reflected light beam. The dividing line L is a line extending in the radial direction of the optical disc and passing through the center of the outer shape B forming the circular shape of the beam.

  Further, the polarizing diffraction grating 9 is composed of polarizing diffraction gratings 9a and 9b having different grating directions. The dividing line that divides the polarizing diffraction gratings 9a and 9b is a line that passes through the center of the outer shape B forming the circular shape of the beam and extends in the track direction of the optical disk. Accordingly, among the reflected light of the optical disk that has reached the first optical element 7, the reflected light of the optical disk that passes through the polarizing diffraction gratings 9a and 9b is diffracted in different directions by the polarizing diffraction gratings 9a and 9b. The The reflected light of the optical disk that passes through the part other than the part where the polarizing diffraction grating 9 is provided is not diffracted but only transmitted through the first optical element 7. At this time, the shape of the reflected light of the optical disk on the hologram element 4 is as shown in FIG.

  Further, the optical element 8 is a quarter wavelength plate, and after the linearly polarized light in the X direction in the drawing emitted from the LD chip 2 is converted into circularly polarized light, the circularly polarized light of the reflected light from the optical disk is converted into linearly polarized light in the Y direction Has a function to convert.

  FIG. 3 is a diagram showing the structure of the polarizing diffraction grating. FIG. 4 is a diagram illustrating the function of the polarizing diffraction grating. For example, as shown in FIG. 3, the polarizing diffraction grating 9 having polarization characteristics includes a lithium niobate substrate 10. Grooves 10a are periodically formed on the surface to form a periodic uneven grating. A proton exchange region 11 is disposed inside the groove 10a.

  By controlling the depth of the groove 10a and the thickness of the proton exchange region 11, the difference between the optical path length of the convex portion and the optical path length of the concave portion is an integral multiple of the wavelength for polarized light in the X direction, For polarized light, the difference between the optical path length of the convex portion and the optical path length of the concave portion can be an integral multiple of the wavelength plus a half wavelength. That is, as shown in FIG. 4, the light travels straight without being diffracted with respect to the polarization in the X direction, and diffracts according to the period of the grating with respect to the polarization in the Y direction. The structure of the polarizing diffraction grating 9 is not limited to this structure.

  FIG. 5 is a side view showing the operation of the optical pickup device of the present embodiment. As shown in FIG. 5, the light beam 12 transmitted through the optical element 7 other than the polarizing diffraction grating 9 and the light beam 13 diffracted by the polarizing diffraction grating 9 are incident on the hologram element 4.

  FIG. 6 is a block diagram illustrating processing of signals from the light receiving unit. As shown in FIG. 6, the light receiving unit 3 is divided into photodetectors R <b> 1 to R <b> 6 constituting the light receiving unit 3. Light incident on the C region of the hologram element 4 is received by the photodetectors R3 and R4, and light incident on the D region is received by the photodetectors R5 and R6. When the output signals from the respective photodetectors R3, R4, R5, and R6 are S3, S4, S5, and S6, the tracking error signal TES1 is represented by (S3 + S4) − (S5 + S6).

  In addition, light incident on the A region and the B region of the hologram element 4 is received by the photodetectors R1 and R2 in the light receiving unit 3, respectively. If the output signals from the photodetectors R1 and R2 are S1 and S2, respectively, the tracking error signal TES2 is represented by (S1-S2). Further, by performing a differential operation on TES1 and TES2, a tracking error signal TES3 = (S1−S2) −K {(S3 + S4) − (S5 + S6)} is output. Here, K is a correction coefficient.

  The focus error signal FES is represented by {(S3 + S5) − (S4 + S6)}, and the RF signal (reproduction signal) is obtained by performing the calculation of (S1 + S2 + S3 + S4 + S5 + S6) on the entire beam.

  When reproducing a read-only disc in which pits have already been recorded, the tracking servo can also use a DPD signal. In that case, a DPD signal can also be obtained by performing a comparison operation on the phase difference between (S1 + S3 + S4) and (S2 + S5 + S6) or the phase difference between S1 and S2.

  Next, the principle that no offset occurs in the TES 3 even if the objective lens 5 moves during tracking will be described. FIG. 7 is a diagram showing a relationship between a conventional hologram and reflected light from an optical disk. As shown in FIG. 7, the tracking error signal detects the difference in the amount of light incident on the A area and B area of the hologram. That is, it corresponds to the area ratio of the reflected light area a and area b of the optical disk. Here, when the objective lens 5 is shifted in the Y-axis negative direction in response to the eccentricity of the optical disc 6, the reflected light on the hologram moves in the Y-axis negative direction as shown by the dotted line.

  Therefore, the area a increases and the area b decreases. As a result, actually, even in the just track state, there is a difference between the area a and the area b, so that an offset occurs in the tracking error signal.

  FIG. 8 is a diagram illustrating the relationship between the position of the reflected light on the hologram and the intensity distribution on the first optical element before and after the objective lens is shifted. As shown in FIG. 8, the light diffracted and divided by the first optical element 7 enters the areas A and B of the hologram element 4. The incident light is represented by region a and region b. FIG. 8 shows the intensity at each position of the area a and the area b in relation to the intensity distribution on the first optical element 7. That is, before the objective lens shift, the intensity of the area a and the area b is the same. Even if the objective lens 5 is shifted, the areas a and b have the same area, but their intensity distributions are different. This will be described in detail next.

  FIG. 9 shows (a) the position of the reflected light on the hologram before and after the objective lens is shifted, (b) the intensity distribution of the emitted light on the first optical element, and (c) the first. It is a figure which shows intensity distribution of the reflected light on the optical element. As is apparent from the comparison between FIG. 9A1 and FIG. 9A2, when the objective lens 5 is shifted in the negative Y-axis direction, the reflected light of the optical disk moves on the hologram element 4 in the negative Y-axis direction. The area of the region a and the area of the region b do not change. Regardless of the amount of movement of the reflected light of the optical disk, TES2 = (S1-S2) = 0, so that no offset occurs. On the other hand, the area of the region c and the area of the region d increase or decrease according to the amount of movement of the reflected light. However, this is limited to the case where the amount of movement of the disk reflected light is smaller than half of the distance between the region a and the region b on the hologram element 4. Therefore, what is necessary is just to determine the space | interval of the area | region a and the area | region b according to the shift amount of the objective lens 5. FIG.

  However, even if the objective lens 5 is shifted in the Y-axis negative direction (radial direction of the optical disk) in the differential operation TES2 = (S1-S2), the area difference between the area a and the area b does not occur, but the LD chip As shown in FIG. 9 (b1), when the emitted light from 2 has a steep intensity distribution in the radial direction of the optical disk, the LD that enters the objective lens 5 when the objective lens 5 is shifted in the negative Y-axis direction. Radial intensity distribution center line J of the radiated light from the chip 2 (a portion where the intensity of the radiated light from the LD chip 2 is maximum in the radial direction) and the objective lens center line P1 on the objective lens 5 or the objective lens The center line P2 of the reflected light on the hologram element 4 corresponding to the objective lens center line P1 on 5 is shifted in the radial direction of the optical disk.

  Further, when reflected by the optical disk, the intensity distribution of the emitted light from the LD chip 2 incident on the objective lens 5 is reversed with respect to the objective lens center line P1 when the objective lens is shifted. Therefore, in the intensity distribution of the reflected light, the intensity distribution center J moves in the negative Y-axis direction as shown in FIG. 9C2.

  Note that P3 in FIGS. 9B1 and 9B2 is the position on the emitted light from the LD chip 2 corresponding to the objective lens center line P1 when the objective lens is shifted, and FIGS. 9C1 and 9C2 P4 in () is a position on the reflected light corresponding to the objective lens center line P1 when the objective lens is shifted. Therefore, as can be seen from FIG. 9 (c2), the amount of deviation in the center of the intensity distribution of the reflected light of the optical disk is twice that of the shift amount of the objective lens 5 in the same direction.

Therefore, a difference occurs in the intensity distribution between the region a and the region b, and the offset cannot be completely canceled. Therefore, the arithmetic expression TES3 = (S1−S2) −K × {(S3 + S4) − (S5 + S6)}
In, by optimizing the K value, in any case, it is possible to completely cancel an offset caused by an objective lens shift. The K value depends on the radial radiation angle of the LD chip 2 and the effective NA of the collimator lens.

  FIG. 10 is a block diagram illustrating processing of signals from the light receiving unit. In the FES generation method shown in FIG. 6, it has been described that the dividing line L of the optical element 7 shown in FIG. 2 and the dividing line M of the hologram element 4 shown in FIG. 6 coincide with the X-axis direction. When the dividing line on the hologram element 4 with respect to the dividing line L of the optical element 7 shown in FIG. 2 is (L), the dividing line M1 of the hologram element 4 is changed to the dividing line (L ), The positional relationship shown in FIG.

  In such a case, the dividing line used in the knife edge method is always the dividing line M1 of the hologram element 4 shown in FIG. 10, and the FES is formed by using the dividing line of the hologram element in the fixed portion. This integrated unit is less susceptible to changes and changes over time, and is more reliable.

FIG. 11 shows TES1 and TES2 when the radial radiation angle of the LD chip 2 is 9.5 ° and the effective NA of the collimator lens is 0.125. A push-pull signal generated by crossing the track is A (short cycle), and an offset signal component is B (long cycle). (2) in FIG. 11 is an offset amount of the tracking error signal TES2 generated when the LD intensity distribution with respect to the region a and the region b is different when the objective lens is shifted.

FIG. 12 shows TES1, TES2, and TES3 when the K value is optimized. In this embodiment, as shown in FIG. 12, TES1 is multiplied by 0.34 and subtracted from TES2, thereby obtaining a tracking error signal TES3 in which almost no offset due to LD intensity distribution shift at the time of objective lens shift occurs. be able to.

  Therefore, by calculating two or more types of track error signals having different offset generation amounts of the tracking error signal due to the objective lens shift, it is possible to obtain a track error signal in which no offset occurs even when the objective lens 5 is shifted.

  In addition, the said embodiment disclosed this time is an illustration in all the points, Comprising: It does not become the basis of limited interpretation. Therefore, the technical scope of the present invention is not interpreted only by the above-described embodiments, but is defined based on the description of the claims. Moreover, all the changes within the meaning and range equivalent to a claim are included.

It is a front view which shows the outline of a structure of the optical pick-up apparatus in embodiment based on this invention. It is a figure which shows the structure of a 1st optical element. It is a figure which shows the structure of a polarizing diffraction grating. It is a figure which shows the function of a polarizing diffraction grating. It is a side view which shows operation | movement of the optical pick-up apparatus of embodiment based on this invention. It is a block diagram which shows the process of the signal from a light-receiving part. It is a figure which shows the relationship between the conventional hologram and the reflected light from an optical disk. It is a figure which shows the relationship between the position of the reflected light on a hologram, and the intensity distribution on a 1st optical element before and after shifting an objective lens. Before and after the objective lens shifts, (a) the position of the reflected light on the hologram, (b) the intensity distribution of the emitted light on the first optical element, (c) on the first optical element It is a figure which shows intensity distribution of the reflected light in. It is a block diagram which shows the process of the signal from a light-receiving part. It is a figure which shows TES1 and TES2 in a predetermined condition. It is a figure which shows TES1, TES2, and TES3 at the time of optimizing K value.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Integrated unit, 2 LD chip | tip (light emission part) 3, Light receiving part, 4 Hologram element, 5 Objective lens, 6 Optical disk, 7 1st optical element, 8 2nd optical element, 9 Polarizing diffraction grating (diffractive part) ) 10 Lithium niobate substrate, 10a groove, 11 proton exchange region.

Claims (8)

An integrated unit having a light emitting unit that emits laser light, a hologram that diffracts reflected light of the optical disc and guides the light to the light receiving unit, and a light receiving unit that receives the light diffracted by the hologram;
An optical pickup device including an objective lens for condensing the laser light emitted from the light emitting unit of the integrated unit on an optical disc,
A first optical element is disposed between the objective lens and the integrated unit, and the first optical element has a diffractive portion that diffracts part of the reflected light from the optical disc, A polarization anisotropy that transmits light in the first linear polarization state and diffracts light in the second linear polarization state whose polarization direction is orthogonal to the first linear polarization state;
A second optical element is disposed between the first optical element and the objective lens, and the second optical element converts the polarization state of the light reflected from the optical disk into the second linear polarization state. Is what
The first optical element and the second optical element are provided integrally with the objective lens ,
The diffractive portion of the first optical element is provided on one side of the first optical element divided into two by a dividing line in the radial direction of the optical disc,
The hologram of the integrated unit is a four-divided hologram that is divided into two by a dividing line in the radial direction of the optical disc and further divided into two by a dividing line in the track direction of the optical disc,
The light receiving unit of the integrated unit is an optical pickup device that is divided into four or more divisions corresponding to the diffracted light divided into four by the hologram .   The said 2nd optical element is a quarter wavelength plate which converts a 1st linear polarization state into a circular polarization state, and converts a circular polarization state into a 2nd linear polarization state. The optical pickup device described.   3. The optical pickup device according to claim 1, wherein the diffractive portion of the first optical element is a polarizing diffraction grating provided only in a portion corresponding to a part of reflected light of the optical disc.   The polarizing diffraction grating that constitutes the diffraction section of the first optical element includes a lithium niobate substrate having a groove periodically provided on the surface, and a proton exchange region disposed in the groove. The optical pickup device according to claim 3.   Part of the reflected light of the optical disk diffracted by the diffraction part of the first optical element and the other part of the reflected light of the optical disk transmitted through the first optical element are incident on the hologram of the integrated unit. The optical pickup device according to claim 1. The light emitting unit emits a laser beam of a first linear polarization state, the optical pickup apparatus according to any one of claims 1 to 5. 7. The optical pickup device according to claim 1, wherein the reflected light of the optical disc transmitted without being diffracted by the first optical element is further divided into two by a dividing line of the hologram in the radial direction of the optical disc. . The light receiving unit is divided into a plurality of parts,
A first track error signal generated from a signal of a portion of the light receiving portion where the reflected light of the optical disc transmitted without being diffracted by the first optical element is incident, and the first optical element of the light receiving portion 8. The third track error signal used for tracking is detected by calculating a second track error signal generated from the signal of the portion where the reflected light of the optical disk diffracted at is incident. 9 . An optical pickup device according to claim 1.

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