JP2005174417A - Optical head and adjustment method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problems that a side spot is deviated from the center position of a side spot light receiving element caused by the manufacturing error of an optical element such as a diffraction grating and space errors among a laser diode, the diffraction grating and a collimator, and consequently the distortion of the tracking error signal by a DPP method occurs. <P>SOLUTION: In addition to the rotation adjustment of a diffraction grating 2 around an optical axis, the position of the diffraction grating 2 in the direction of the optical axis is adjusted so as to position a side spot by a side beam on the side spot light receiving element 12 of a photodetector 11. Additionally, the diffraction grating 2 is arranged in a nonparallel luminous flux. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical head for recording or reproducing information used in an optical information recording / reproducing apparatus, and more particularly to an optical head using a differential push-pull method (hereinafter referred to as DPP method) and an adjustment method thereof.

  Conventionally, a DPP method is known as one of tracking servo systems in a drive device for an optical recording disk such as a CD-R or a DVD-R. In the DPP method, a tracking error signal is generated by calculating an output signal of each light receiving element obtained from a main beam and two side beams. This DPP method is described in, for example, Japanese Patent Application Laid-Open No. 2003-16666 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2002-279659 (Patent Document 2).

  Specifically, as shown in FIG. 1, a diffraction grating 2 is arranged in the forward path of the beam emitted from the laser diode 1, and the polarization beam splitter 4, collimator 5, quarter wavelength plate 6, and objective lens 7 are interposed. Then, each spot is formed on the optical disc 8 by three beam lights of zero-order diffracted light (main beam) and two diffracted lights (side beam). Further, the reflected light from the optical disk 8 is received by the light receiving element of the photodetector 11 through the cylindrical lens 10 for detecting the focusing error by the astigmatism method. In FIG. 1, reference numeral 3 denotes an APC sensor used for light amount control of the laser diode 1, and 9 denotes a sensor lens.

  FIG. 2 shows the light receiving surface of the photodetector 11. As shown in FIG. 2, the photodetector 11 includes a light receiving element 12 that receives a main spot by a main beam, and light receiving elements 13 and 14 that receive a side spot by a side beam. The light receiving element 12 of the main spot is divided into four vertical and horizontal, and the light receiving elements 13 and 14 of the side spot are divided into two vertical.

  Here, as shown in FIG. 2, when the divided light receiving element outputs of the light receiving elements 12 to 14 are denoted by A, B, C, D, E, F, G, and H, calculation is performed using these signals. As a result, a tracking error signal is generated. That is, a push-pull signal (PPm) based on the main beam is generated from the output signal of the main spot light-receiving element 12, and a push-pull signal (PPs) based on the side beam is generated from the output signals of the side spot light-receiving elements 13 and 14.

  Specifically, for example, the DPP signal is obtained by the following arithmetic expression using the arithmetic circuit shown in FIG. Inputs A to H of the arithmetic circuit in FIG. 3 correspond to outputs A to H of the light receiving elements 12 to 14.

PPm = (A + D)-(B + C)
PPs = (E−F) + (G−H)
DPP = PPm−K × PPs
= (A + D)-(B + C) -K {(E-F) + (GH)}
Here, K is a constant determined so as to correct and calibrate the difference in light intensity between the 0th order diffracted light, the + 1st order diffracted light, and the −1st order diffracted light.

In this case, the arrangement of the spots on the optical disk 8 is such that the main spot 17 by the main beam is on the groove 15 and the sub-spots 18 and 19 by the sub-beam are as shown in FIG. 4 by adjusting the rotation of the diffraction grating 2 around the optical axis. It is located on the land 16 at a symmetrical position with the main spot 17 in between. (When the main spot 17 is on the land 16, the sub-spots 18 and 19 are on the groove 15.)
When the main spot 17 is traced on the boundary between the land 15 and the groove 16 as shown in FIG. 5 by adjusting the rotation of the diffraction grating 2 around the optical axis, a tracking error signal is generated by calculation between the sub beams. .

  That is, for example, the DPP signal can be obtained by the following arithmetic expression by the arithmetic circuit shown in FIG. Inputs E to H of the arithmetic circuit in FIG. 6 correspond to outputs E to H of the light receiving elements 13 and 14.

DPP = (E−F) −α (G−H)
Here, α is a constant determined so as to correct and calibrate the difference in light intensity between the + 1st order diffracted light and the −1st order diffracted light, and is generally about 1.
Japanese Patent Laid-Open No. 2003-16666 JP 2002-279659 A

  As a problem of these DPP methods, the spot position on the optical disk 8 can be adjusted by adjusting the rotation of the diffraction grating 2 around the optical axis, but the manufacturing error of the optical element such as the diffraction grating 2 or the laser diode 1 and the diffraction grating 2 can be adjusted. Due to assembly errors such as a gap error between the collimators 5, the side spot often does not lie on the center of the side spot light receiving elements 13 and 14 for receiving it as shown in FIG.

  Therefore, when the side spot is not on the center of the light receiving element, offset occurs in the two signals of the PPs signal, but the amplitudes of the two PPs signals are substantially the same during reproduction, and the generated offsets have opposite polarities. The offset is canceled by adding. However, at the time of recording, the preceding sub-beam is not affected by pits, and the subsequent sub-beam is affected by pits. Therefore, the offset amount of the amplitude of the two signals of the PPs signal is different, and the offset is not canceled. There was a problem that the servo control circuit was complicated.

  Further, when the main spot is traced on the boundary between the land and the groove, there is a problem that an offset is added and a large offset application is required, increasing a load on the servo control circuit.

  Furthermore, when the deviation of the side spot from the center of the side spot light receiving elements 13 and 14 is large, the PPs signal itself is distorted, resulting in a problem that the tracking error signal by the DPP method is distorted.

  As described above, when the side spot is not on the center of the side spot light receiving element due to manufacturing errors or assembly errors of the optical element, the servo control circuit becomes complicated or the tracking error signal itself by the DPP method is distorted. There was a problem.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide an optical head and a method of adjusting the same that can eliminate the distortion of the tracking error signal without complicating the configuration of the servo control circuit. There is to do.

  In order to solve the above problems, the present invention is arranged between a light source, an objective lens for condensing a light beam emitted from the light source on a recording medium, and between the light source and the objective lens, and at least a light beam from the light source is main. An optical head having a beam and a wavefront dividing element that divides the beam into two side beams on both sides of the beam, and a photodetector that receives reflected light of each beam from the recording medium. The position in the optical axis direction is adjusted so that the spot is positioned on the center of the side spot light receiving element of the photodetector.

  Further, the present invention provides a light source, an objective lens for condensing a light beam emitted from the light source on a recording medium, and disposed between the light source and the objective lens. In a method of adjusting an optical head, comprising: a wavefront splitting element that splits two side beams into three beams; and a photodetector that receives the reflected light of each beam from the recording medium. A step of adjusting the position of the photodetector so that the tracking error signal obtained at the center of the light receiving element of the main spot of the detector has a maximum amplitude, the push-pull signal by the main spot and the side beam; The rotation of the wavefront splitting element around the optical axis is adjusted so that the phase difference from the push-pull signal due to the side spot is approximately 180 degrees. Step, wherein the side spots is characterized in that it comprises a step of adjusting the position of the optical axis of the wavefront splitting element to be in the center of the side spot light receiving elements of the photodetector.

  Furthermore, the present invention provides a light source, an objective lens for condensing a light beam emitted from the light source on a recording medium, and disposed between the light source and the objective lens. In a method of adjusting an optical head, comprising: a wavefront splitting element that splits two side beams into three beams; and a photodetector that receives the reflected light of each beam from the recording medium. A step of adjusting the position of the photodetector so that the obtained tracking error signal has a maximum amplitude and is located at the center of the main spot light-receiving element of the detector, and pushing the two side beams by the two side spots Adjusting the rotation of the wavefront splitting element around the optical axis so that the phase difference between the pull signals is approximately 180 degrees; Characterized in that it comprises a step of adjusting the position of the optical axis of the wavefront splitting element to be in the center of the side spot light receiving element of the optical detector.

  According to the present invention, the wavefront splitting element is arranged in the non-parallel light beam, and the wavefront splitting element is adjusted in the optical axis direction so that the side spot is located on the center of the side spot light receiving element of the photodetector. Thus, the tracking error signal by the DPP method can be maintained at a high quality, and an optical head with easy servo control can be obtained.

  Next, the best mode for carrying out the invention will be described in detail with reference to the drawings. The basic configuration of the optical head of the present invention is the same as that of the optical system shown in FIG. 1, and the spot arrangement on the optical disk 8 is also the same. The feature of the present invention resides in the method of adjusting the diffraction grating 2 in FIG. 1 and adjusts the position in the optical axis direction in addition to the adjustment of the diffraction grating 2 around the optical axis.

  Conventionally, as shown in FIG. 4 or FIG. 5, the diffraction grating 2 is rotationally adjusted around the optical axis in order to arrange the spots. In the present embodiment, the movement of the diffraction grating 2 is further adjusted along the optical axis direction. Thereby, as shown in FIG. 8, the space | interval of a main spot and a side spot can be adjusted so that a side spot may be located on the center of the side spot light receiving elements 13 and 14. FIG. A spot indicated by a broken line in FIG. 8 indicates the position of the side spot in FIG. In this case, it is essential that the diffraction grating 2 is in a non-parallel light beam.

  Next, the adjustment principle of the present invention will be described. 9 and 10 are conceptual diagrams for explaining the principle of the present invention. First, as shown in FIG. 9, if the grating surface of the diffraction grating 2 is at the position 21, the light from the light emission point a of the light source is 0th order diffracted light L0, + 1st order diffracted light L1, and −1st order diffracted light (not shown). ). At this time, the first-order diffracted light L1 looks like light having an apparent light emission point at a point b away from the light emission point a in the plane including the light emission point a. Therefore, when the grating surface of the diffraction grating 2 is moved to the position 22, the apparent light emission point becomes a point c that is d2 away from the light emission point a. L2 represents the + 1st order diffracted light at this time.

  The light having the two points b and c as light emission points forms an image at points d and e on the information surface 25 of the optical disk 8 as shown in FIG. The light from the imaging points d and e is condensed at points f and g on the light receiving surface 27 of the photodetector 11 as shown in FIG. Here, in FIGS. 10A and 10B, the optical system from the collimator 5 to the objective lens 7 in FIG. 1 is represented by the composite optical system 24, and the optical system from the objective lens 7 to the cylindrical lens 10 is a composite. The optical system 26 is representatively shown. Thus, the movement of the diffraction grating 2 along the optical axis is the movement of the position of the side spot with respect to the main spot on the photodetector 11.

  Next, the method for adjusting the optical head according to the present embodiment will be described. First, FIG. 11 shows an adjustment method for generating the DPP signal described with reference to FIGS. 3 and 4, that is, an adjustment method for performing tracking control so that the main spot traces on the groove 15 or land 16. The description will be given with reference. The optical head adjustment described below is performed automatically while monitoring the output of each light receiving element of the photodetector 11 with an automatic adjustment device.

  In FIG. 11, first, in step 1, the optical disk 8 is set in the optical information recording / reproducing apparatus (this may be manual). Next, in step 2, the position of the photodetector 11 is adjusted so that focus servo can be applied in the X, Y, and Z directions. Here, rotation of the photodetector 11 is fixed with mechanical accuracy, and adjustment of X, Y, and Z means translation in the optical axis direction and translation in a plane perpendicular to the optical axis. The photodetector 11 can be adjusted in the X, Y, and Z directions by an automatic adjustment jig (not shown), and the position is adjusted using this jig.

  Thereafter, in step 3, the position of the photodetector 11 in the X, Y, and Z directions is set so that the main spot is positioned at the center of the main spot light receiving element 12 and the obtained tracking error signal (PPm) has the maximum amplitude. Make adjustments.

  Subsequently, in step 4, rotation adjustment around the optical axis of the diffraction grating 2 is performed so that the phase difference between the push-pull signal (PPm) by the main beam and the push-pull signal (PPs) by the side spot becomes 180 °. Similarly, the diffraction grating 2 can be rotated around the optical axis and adjusted in position in the optical axis direction by an automatic adjustment jig (not shown), and adjustment around the optical axis is performed using this jig. Next, in step 5, the position adjustment along the optical axis of the diffraction grating 2 is performed so that the two side spots are positioned at the centers of the side spot light receiving elements 13 and 14, respectively.

  Next, in step 6, (1) the tracking error signal (PPm) has the maximum amplitude, (2) the main beam is located at the center of the main spot light receiving element 12, (3) push-pull signal (PPm) by the main spot And the phase difference between the push-pull signals (PPs) by the side spot is 180 °, and (4) the two side spots are respectively located at the centers of the side spot light receiving elements 13 and 14 within the predetermined range. It is determined whether or not each item is satisfied, and the adjustments in steps 2 to 5 are repeated until the items are satisfied.

  Finally, when all the items are satisfied, the diffraction grating 2 and the photodetector 11 are fixed in Step 7 and the adjustment is finished (this fixing may be performed manually).

  In this embodiment, when detecting the maximum amplitude of the tracking error signal (PPm), the amplitude value is monitored while applying an offset to the focus servo, and the focus servo offset is fixed to a value indicating the maximum value. doing. The phase difference of the push-pull signal between the main spot and the side spot is set to 180 ± 15 ° as a guide.

Further, whether or not each spot is at the center of the light receiving element is based on the following. That is, when the DC outputs of the divided outputs A to H of each light receiving element shown in FIG.
| Ad + Bd−Cd−Dd | / (Ad + Bd + Cd + Dd) ≦ 0.1
| Ad + Dd−Bd−Cd | / (Ad + Bd + Cd + Dd) ≦ 0.1
| Ed−Fd | / (Ed + Fd) ≦ 0.1
| Gd−Hd | / (Gd + Hd) ≦ 0.1
Meet.

  Next, an adjustment method for generating the DPP signal described with reference to FIGS. 5 and 6, that is, an adjustment method for performing tracking control so that the main spot traces on the boundary between the groove 15 and the land 16. This will be described with reference to FIG. In this case as well, the output of each light receiving element of the photodetector 11 is automatically performed while being monitored by the automatic adjustment device.

  In FIG. 12, first, the optical disk 8 is set in the apparatus in step 1 (this may be manual). Next, in step 2, the position of the photodetector 11 is adjusted so that focus servo can be applied in the X, Y, and X directions. Thereafter, in step 3, the position of the photodetector 11 in the X, Y, and Z directions is set so that the main spot is positioned at the center of the main spot light receiving element 12 and the obtained tracking error signal (PPm) has the maximum amplitude. Make adjustments. The position adjustment of the photodetector 11 is performed using an automatic adjustment jig (not shown) as in the case of FIG.

  Next, in step 4, the rotation adjustment around the optical axis of the diffraction grating 2 is performed so that the phase difference between the push-pull signals (EF) and (GH) due to the side spot is 180 °. In step 5, the position along the optical axis of the diffraction grating 2 is adjusted so that the two side spots are positioned at the centers of the side spot light receiving elements 13 and 14, respectively. Adjustment of the diffraction grating 2 around the optical axis and position adjustment in the optical axis direction are performed using an automatic adjustment jig as in the case of FIG.

  Next, in step 6, (1) the tracking error signal (PPm) has the maximum amplitude, (2) the main spot is located at the center of the main spot light receiving element 12, and (3) the push-pull signal (E− by the side spot). Whether the phase difference between F) and (GH) is 180 °, or (4) the side spot is located at the center of the side spot light receiving elements 13 and 14 is satisfied within the predetermined range. The adjustments in steps 2 to 5 are repeated until each item is satisfied.

  Finally, when all the items are satisfied, the diffraction grating 2 and the photodetector 11 are fixed in Step 7 and the adjustment is finished (this fixing may be performed manually).

  Here, in this embodiment, when detecting the maximum amplitude of the tracking error signal (PPm) as in the case of FIG. 11, the amplitude value is monitored while applying the offset of the focus servo to indicate the maximum value. The focus servo offset is fixed to the value. The phase difference between push-pull signals due to side spots is set to 180 ± 15 ° as a guide.

Further, whether or not each spot is at the center of the light receiving element is based on the following as in the case of FIG. That is, when the DC outputs of the divided outputs A to H of each light receiving element shown in FIG.
| Ad + Bd−Cd−Dd | / (Ad + Bd + Cd + Dd) ≦ 0.1
| Ad + Dd−Bd−Cd | / (Ad + Bd + Cd + Dd) ≦ 0.1
| Ed−Fd | / (Ed + Fd) ≦ 0.1
| Gd−Hd | / (Gd + Hd) ≦ 0.1
Meet.

  Thus, the two side spots can be adjusted so as to be positioned on the centers of the side spot light receiving elements 13 and 14, respectively, and an optical head suitable for the DPP method can be obtained.

It is a block diagram which shows one Embodiment of the optical head of this invention. It is a figure which shows the light-receiving surface of the photodetector of the optical head of FIG. It is a circuit diagram which shows the example of the arithmetic circuit for producing | generating a DPP signal in case a main spot traces on a land or a groove. It is a figure which shows arrangement | positioning of three beams on an optical disk in case a main spot traces on a land or a groove. It is a figure which shows arrangement | positioning of three beams on an optical disk in case a main spot traces on the boundary of a land and a groove. It is a circuit diagram which shows the example of the arithmetic circuit for producing | generating the DPP signal in case a main spot traces on the boundary of a land and a groove. It is a figure which shows beam arrangement | positioning when the side spot which is a problem of a prior art is not on the center of a side spot light receiving element. It is a figure which shows the beam arrangement | positioning at the time of adjusting the space | interval of a main spot and a side spot so that a side spot may be located on the center of a side spot light receiving element by position adjustment of the optical axis direction of the diffraction grating by this invention. It is a figure which shows the motion of the diffraction grating and the change of a diffracted light optical path for demonstrating the principle of this invention. It is a figure which shows the relationship between the apparent light emission point for demonstrating the principle of this invention, and the image formation point on a photodetector. 3 is a flowchart showing an optical head adjustment method of the present invention. It is a flowchart which shows the other adjustment method of the optical head of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser diode 2 Diffraction grating 3 Sensor for APC 4 Polarizing beam splitter 5 Collimator 6 1/4 wavelength plate 7 Objective lens 8 Optical disk 9 Sensor lens 10 Cylindrical lens 11 Photo detector 12 Main spot light receiving element 13, 14 Side spot light receiving element 15 Groove 16 Land 17 Main spot 18, 19 Side spot 21, 22 Grating surface of diffraction grating 24, 26 Decomposition optical system 25 Information surface of optical disk 27 Light receiving surface of photodetector

Claims (4)

A light source, an objective lens for condensing a light beam emitted from the light source on a recording medium, and disposed between the light source and the objective lens, and the light beam from the light source is divided into at least a main beam and two side beams on both sides thereof An optical head having a wavefront dividing element that performs the above and a photodetector that receives reflected light of each beam from the recording medium, wherein the wavefront dividing element has a side spot by the side beam that is a side spot light receiving element of the photodetector. An optical head characterized in that the position in the optical axis direction is adjusted so as to be located on the center of the optical axis. The optical head according to claim 1, wherein the wavefront splitting element is disposed in a non-parallel light beam. A light source, an objective lens for condensing a light beam emitted from the light source on a recording medium, and a light beam from the light source are arranged between at least a main beam and two side beams on both sides thereof. In the method of adjusting an optical head having a wavefront splitting element for splitting into beams and a photodetector for receiving the reflected light of each beam from the recording medium, the main spot by the main beam is the main spot light reception of the photodetector. A step of adjusting the position of the photodetector so that the tracking error signal obtained at the center of the element has a maximum amplitude; a push-pull signal by the main spot and a push-pull signal by the side spot of the side beam; Adjusting the rotation of the wavefront splitting element around the optical axis so that the phase difference from the Adjustment method for an optical head, which comprises a step of spot to adjust the position of the optical axis of the wavefront splitting element to be in the center of the side spot light receiving elements of the photodetector. A light source, an objective lens for condensing a light beam emitted from the light source on a recording medium, and a light beam from the light source are arranged between at least a main beam and two side beams on both sides thereof. In the method of adjusting an optical head having a wavefront splitting element for splitting into beams and a photodetector for receiving the reflected light of each beam from the recording medium, the main spot by the main beam is the main spot light reception of the photodetector. Adjusting the position of the photodetector so that the obtained tracking error signal has the maximum amplitude, and a phase difference between push-pull signals by two side spots of the two side beams; Adjusting the rotation of the wavefront splitting element about the optical axis so that the angle is approximately 180 degrees, and the side spot is the size of the photodetector. Adjustment method for an optical head, which comprises a step of adjusting the position of the optical axis of the wavefront splitting element to be in the center of the spot light receiving element.