JP4308204B2 - Optical head device - Google Patents

Description

  The present invention relates to an optical head device mounted on an optical disk recording / reproducing apparatus, and more particularly to an optical head apparatus capable of suppressing the influence of wavefront aberration on recording / reproducing characteristics.

  The optical head device is mounted as an information reading / writing device in various optical disk recording / reproducing devices such as a CD player, a DVD player, and an MD player. As the optical disc recording / reproducing apparatus itself is reduced in size and price, the optical head apparatus is also required to have higher density, higher performance, smaller size, thinner thickness, and lower cost.

  The configuration of a conventional optical head device is shown in FIGS. 7 and 19 of Non-Patent Document 1, for example. In this configuration, a collimator lens converts a light beam emitted from a semiconductor laser as a light source into a parallel light beam. Then, the parallel light beam enters the objective lens. An optical system using such a parallel light beam is called an infinite optical system.

  While the optical head device of the infinite optical system as described above exists, there is also an optical head device that has a simple configuration by eliminating the collimator lens from the conventional optical head device. Such an optical head device that does not have a collimator lens employs a configuration in which a diffused light beam that is not a parallel light beam is incident on the objective lens. Such a configuration in which the diffused light beam is incident on the objective lens is called a finite optical system. An optical head device of a finite optical system is shown in FIG.

  In addition to Patent Document 1 and Non-Patent Document 1, the following documents are listed as prior art documents related to the present invention.

  A finite optical system optical head device does not require a collimator lens, and thus has a small number of components, which is advantageous for miniaturization, thickness reduction, and cost reduction. However, the optical head device of the finite optical system adopts a configuration in which the diffused light beam is incident on the objective lens. For this reason, when the light source is displaced from the lens optical axis of the objective lens and the incident angle of the light beam to the objective lens changes, wavefront aberration occurs. Then, the quality of the collected light beam deteriorates. This is because the incident surface of the objective lens has a spherical or aspherical shape, so that incident light deviating from the lens optical axis does not focus when passing through the objective lens.

  In the optical disc recording / reproducing apparatus, the wavefront aberration is generated as follows.

  First, on the recording surface of an optical disc information recording medium to be recorded / reproduced by an optical disc recording / reproducing apparatus, recording marks (pits) as data are arranged in a circular shape or a spiral shape, and this arrangement forms a track. is doing. The optical disk recording / reproducing apparatus scans the light beam condensed by the objective lens along the track, and reads data from the reflected light. The optical disk information recording medium is rotated by a motor or the like with the track center as the central axis.

  Here, if the shape of the optical disc information recording medium is decentered or if the motor mounting position has an error, the track is displaced in the radial direction (radial direction) of the optical disc information recording medium with rotation. Therefore, the optical disk recording / reproducing apparatus needs to control the focal position of the light beam so as to follow the displacement of the track.

  The optical head device itself can be displaced by a servo mechanism. However, since the track displacement as described above is a minute displacement, the position control of the light beam with respect to the track displacement by the servo mechanism for displacing the entire optical head device is not desirable from the viewpoint of accuracy and power consumption.

  Therefore, in order to accurately follow the position of the light beam with respect to the above-described track displacement, the objective lens can be independently displaced minutely in the radial direction of the optical disk information recording medium in the optical head device. Specifically, an objective lens displacement mechanism using an electromagnetic force generated by a coil or the like is provided in the optical head device so that the objective lens can be displaced. This objective lens displacement mechanism controls the position of the objective lens based on positional deviation information between the track and the light beam obtained from the reflected light from the optical disk information recording medium. Therefore, the objective lens displacement mechanism causes the objective lens to follow the track movement of the optical disc information recording medium.

  However, if the objective lens is caused to follow the track movement, the light source is displaced from the lens optical axis of the objective lens. For this reason, wavefront aberration occurs, and the recording / reproducing characteristics deteriorate.

  On the other hand, in the case of an optical head device of an infinite optical system, since parallel rays are incident on the objective lens, it can be considered that the position of the light source is substantially at infinity when viewed from the objective lens. Therefore, even if the objective lens is displaced, the optical axis of the light beam does not deviate from the optical axis of the objective lens (that is, the incident angle of the light beam to the objective lens does not change), and the wavefront aberration is reduced. Occurrence does not matter.

  In view of this point, it is difficult to employ a finite optical system in an optical head device that realizes high-density recording / reproduction using a short-wavelength light source, which is disadvantageous for downsizing, thinning, and cost reduction. I had to adopt an infinite optical system.

JP 2003-208731 A Heitaro Nakajima and Hiroshi Ogawa, “The Illustrated Compact Disc Reader (Revised 3rd Edition)” published by Ohmsha, May 20, 1996, p. 208 Arimoto Akira et al., “Examination of Allowable Aberration of Optical Systems in Optical Video Discs” Optics, Japan Optical Society (Applied Physics Society Subcommittee), Vol. 12, No. 6, p. 494 (1983) Warren J.H. Smith, “Modern Optical Engineering the design of optical systems” McGraw-Hill, 1966, pp. 196 83-84

  The present invention has been made in view of the above circumstances, and even when a reduction in size, thickness, and cost are achieved by adopting a configuration in which a diffused light beam that is not a parallel light beam is incident on an objective lens, An object of the present invention is to provide an optical head device capable of suppressing the influence of wavefront aberration on recording / reproducing characteristics.

A first aspect of the optical head device according to the present invention includes a light source that emits a light beam and a diffraction surface on which a groove-shaped diffraction grating is formed, and diffracts the light beam to generate a plurality of diffracted lights. A diffractive optical element, an objective lens for condensing the plurality of diffracted lights on an optical disc information recording medium, a light receiving region divided into a plurality of parts, and the optical disc information recording medium received through the objective lens A light detector that converts the light signal into an electrical signal corresponding to the amount of reflected light and an optical base that rotatably fixes the diffractive optical element, and a relative optical axis of the light source with respect to a lens optical axis of the objective lens as with astigmatism direction to cancel the astigmatism generated by deviation, the optical head apparatus der finite optical system where the back surface of the diffraction surface or the diffractive surface of the diffractive optical element and cylindrical shape Te, the optical base, and performs the fixing of the diffractive optical element is pressurized substantially parallel to a direction having a curvature of the cylindrical shape of the diffractive optical element.

  According to the first aspect of the optical head device of the present invention, the diffractive optical element not only generates a plurality of diffracted lights, but also the light source moves away from the lens optical axis of the objective lens in the radial direction of the optical disc information recording medium. Astigmatism in a direction to cancel out the astigmatism generated by this. Therefore, even when the light beam is incident on the objective lens as diffuse light that is not a parallel light beam, the influence of the wavefront aberration on the recording / reproducing characteristics can be suppressed by the diffractive optical element without using a collimator lens. It is.

  The objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description and the accompanying drawings.

FIG. 1 is a diagram showing a basic configuration of an optical head device according to Embodiment 1 of the present invention.
FIG. 2 is a diagram showing wavefront aberration caused by a relative optical axis shift from the lens optical axis of the objective lens to the light source in the optical head device according to Embodiment 1 of the present invention.
FIG. 3 is a schematic diagram showing a condensing position of a radial cross section and a tangential cross section in a state where the objective lens in the optical head device according to Embodiment 1 of the present invention is moved in the radial direction.
FIG. 4 is a perspective view showing a positional relationship between a radial direction, a tangential direction, and the like and an optical disc information recording medium.
FIG. 5 is a perspective view showing the shape of a diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 6 is another perspective view showing the shape of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 7 is a perspective view showing another shape of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 8 is another perspective view showing another shape of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 9 is a diagram showing a recording surface of a read-only optical disc information recording medium.
FIG. 10 is a diagram showing a recording surface of a writable optical disc information recording medium.
FIG. 11 is a diagram showing a change in a combined value of off-axis astigmatism generated by the radial movement of the objective lens according to astigmatism generated by the diffractive optical element.
FIG. 12 is a schematic diagram showing a condensing position of a radial cross section and a tangential cross section in a state in which the objective lens has moved in the radial direction in the optical head device according to Embodiment 1 of the present invention.
FIG. 13 is a perspective view showing another shape of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 14 is a perspective view showing another shape of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 15 is a perspective view showing another shape of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 16 is a perspective view showing another shape of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention.
FIG. 17 is a diagram showing the direction in which the grooves of the diffraction grating of the diffractive optical element employed in the optical head device according to Embodiment 1 of the present invention extend.
FIG. 18 is a diagram showing a basic configuration of an optical head device according to Embodiment 2 of the present invention.
FIG. 19 is a diagram showing the shape and arrangement of parallel plate diffractive optical elements in an optical head device according to Embodiment 2 of the present invention.
FIG. 20 is a diagram showing the shape and arrangement of parallel plate diffractive optical elements in an optical head device according to Embodiment 2 of the present invention.
FIG. 21 is a diagram showing another configuration example of the optical head apparatus according to Embodiment 2 of the present invention.
FIG. 22 is a diagram showing a basic configuration of an optical head device according to Embodiment 3 of the present invention.
FIG. 23 is a diagram showing a specific shape of a dichroic prism in the optical head device according to Embodiment 3 of the present invention.
FIG. 24 is a diagram showing a basic configuration of an optical head device according to Embodiment 4 of the present invention.
FIG. 25 is a diagram showing movement of the diffraction grating element according to the fourth embodiment of the present invention.
FIG. 26 is a diagram showing the position of the light beam on the photodetector according to the fourth embodiment of the present invention.
FIG. 27 is a diagram showing a basic configuration of an optical head device according to a fifth embodiment of the present invention.
FIG. 28 shows a hologram element according to Embodiment 5 of the present invention.
FIG. 29 is a diagram showing a condensing state of a semi-light beam according to Embodiment 5 of the present invention.
FIG. 30 is a diagram showing a light receiving area of a photodetector according to a fifth embodiment of the present invention.

Embodiment 1 FIG.
In this embodiment, the diffractive optical element has not only a diffraction function but also a function of generating astigmatism, and the light source moves away from the lens optical axis of the objective lens in the radial direction of the optical disc information recording medium. This is an optical head device designed to cancel out the generated astigmatism.

  FIG. 1 is a diagram showing a basic configuration of an optical head device according to the present embodiment. This optical head device adopts a configuration of a finite optical system having no collimator lens.

  In FIG. 1, reference numeral 1 is a semiconductor laser as a light source, reference numeral 2 a is a light beam emitted from the semiconductor laser 1, reference numeral 3 is a flat glass serving as an emission window of the light beam 2 a, and reference numeral 4 is a semiconductor laser 1 and a flat glass 3. And a heat radiating metal package having a function of radiating heat generated when the semiconductor laser 1 emits light. The integrated structure of the semiconductor laser 1, the flat glass 3, and the heat dissipation metal package 4 is a light emitting component 5.

  Reference numeral 6 denotes a diffractive optical element for diffracting the light beam 2a to split it into a plurality of diffracted light beams 2b. Reference numeral 7 denotes a deflecting prism that deflects the light beam 2b to the light beam 2c by a reflecting surface provided inside. Reference numeral 8 denotes an optical disk information recording medium such as a CD, DVD, MD, etc., and reference numeral 9 denotes an objective lens that focuses the light beam 2c from the deflecting prism 7 on the optical disk information recording medium 8 as a light beam 2d. Since no collimator lens is provided in the path of the light beams 2a to 2c, the light beams 2a to 2c do not become parallel light beams but become diffuse light. Therefore, this optical head device is a finite optical system.

  The light beam 2d reflected by the optical disc information recording medium 8 is incident on the objective lens 9 again, converted into convergent light having the optical axes A1b and A1c as the central axes, and passes through the deflecting prism 7. The light beam 2e that has passed through the deflecting prism 7 passes through the detection optical element 11 and enters the light detector 10 as a light beam 2f.

  The photodetector 10 is a photodiode provided with a plurality of divided light receiving surfaces according to the optical system. The light detector 10 converts the light quantity of the light beam 2f incident on each light receiving surface from the objective lens 9 into an electrical signal and outputs it. The detection optical element 11 is an optical element for appropriately allowing the light beam 2e reflected by the optical disc information recording medium 8 to enter the divided light receiving surface of the light detector 10. The detection optical element 11 is composed of a concave lens, for example, and has a lens function and a beam splitting function.

  The optical disc information recording medium 8 is formed with tracks in which recording marks (pits) as data are arranged in a circle or spiral, and this arrangement forms a track. The optical disk recording / reproducing apparatus equipped with the optical head device according to the present embodiment scans the light beam 2d collected by the objective lens 9 along the track, and reads data from the reflected light. The optical disc information recording medium 8 is rotated by a motor (not shown) or the like with the track center as the central axis.

  Here, if the shape of the optical disc information recording medium 8 is decentered or there is an error in the mounting position of the motor, the track is displaced in the radial direction (radial direction) of the optical disc information recording medium 8 with rotation. Therefore, the optical disk recording / reproducing apparatus needs to control the radial position of the light beam 2d so as to follow the displacement of the track.

  The optical head device itself can be displaced by a servo mechanism (not shown). However, since the track displacement as described above is a minute displacement, the radial position control of the light beam by the servo mechanism for displacing the entire optical head device is not desirable from the viewpoint of accuracy and power consumption.

  Therefore, the objective lens 9 can be independently translated in the radial direction of the optical disk information recording medium 8 in the optical head device in order to accurately follow the radial position of the light beam 2d with respect to the track displacement. It is said that. Specifically, an objective lens displacement mechanism (not shown) using an electromagnetic force generated by a coil or the like is provided in the optical head device so that the objective lens 9 can be displaced. This objective lens displacement mechanism calculates the amount of positional deviation between the track and the light beam based on the signal output by the light detector 10 receiving the reflected light from the optical disk information recording medium 8 and controls the position of the objective lens 9. To do.

  The semiconductor laser 1 serving as the light source is substantially present on the lens optical axis of the objective lens 9 (“substantially” means that the light beam from the semiconductor laser 1 is taken into consideration without considering the deflection by the deflection prism 7. In many cases, the position of the objective lens 9 is assumed to be the initial design position of the objective lens 9 in the objective lens displacement mechanism. Here, the optical axes A1a and A1b determined by the principal point of the objective lens 9 and the position of the light source at the initial design position are called design optical axes.

  In the configuration of FIG. 1, the light beams 2a to 2c from the semiconductor laser 1 enter the objective lens 9 as diffused light. As described in the section of the problem to be solved by the invention, when the objective lens 9 is controlled to follow in the radial direction, the position of the semiconductor laser 1 as the light source is aligned with respect to the lens optical axis of the objective lens 9. It will be located outside. Therefore, wavefront aberration occurs when the light beam 2 c passes through the objective lens 9.

  FIG. 2 is a diagram illustrating a typical characteristic example of the wavefront aberration generated due to the relative optical axis shift of the light source with respect to the lens optical axis of the objective lens 9. Typical aberration components of wavefront aberration include astigmatism, coma, spherical aberration, and higher order aberration. The total aberration represents a combined value of all these aberrations. Astigmatism accounts for the largest proportion of the generated aberrations and is called off-axis astigmatism.

  FIG. 3 is a diagram schematically showing a cross section of the light beam 2 in the radial direction D1 and the tangential direction D2 orthogonal thereto. FIG. 4 is a perspective view showing the positional relationship between the optical disc information recording medium 8 and the radial direction D1, the tangential direction D2, and the like. In FIG. 3, for simplicity of explanation, it is assumed that the light beam 2 from the semiconductor laser 1 is directly incident on the objective lens 9 without considering the deflection by the deflection prism 7. In each figure, the same direction as the radial direction D1 is the y-axis, the same direction as the tangential direction D2 is the x-axis, and the direction orthogonal to both the x and y axes is the z-axis.

  Focusing on the off-axis astigmatism generated by the above relative optical axis deviation, the condensing position (focal line F1) in the radial cross section of the light beam 2d and the condensing position (focal line F2) in the tangential cross section. Is different. The front focal line F2 near the optical disc information recording medium 8 is oriented parallel to the radial direction, and the rear focal line F1 near the objective lens 9 is oriented parallel to the tangential direction.

  When the objective lens 9 is controlled to be displaced in the radial direction and a relative optical axis shift occurs between the semiconductor laser 1 serving as the light source and the objective lens 9, the condensing position of the light beam 2 d is changed from the optical disc information recording medium 8 to the objective. Since it moves in the direction close to the lens 9 side, the spot shape of the light beam 2d on the optical disc information recording medium 8 changes to an ellipse that is long in the radial direction.

  As a result, the spot of the light beam 2d spreads to a track adjacent to the track being read or written, and crosstalk from the adjacent track increases. Further, the resolution in the radial direction is deteriorated, and the detection ability of the light beam 2d across the track is also lowered.

  Also, if the position of the objective lens 9 is controlled in the z-axis direction so that the recording surface of the optical disc information recording medium 8 is positioned at a substantially intermediate position between the position of the front focal line F2 and the position of the rear focal line F1. The spot shape of the light beam 2d on the recording surface of the optical disc information recording medium 8 can be made substantially circular. It can be considered that this can prevent deterioration in resolution in the radial direction. However, the spot of the light beam 2d at that time is larger than the case where there is no relative optical axis deviation because of the presence of wavefront aberration. Therefore, the resolution in the tangential direction is lowered and the recording / reproducing characteristics are deteriorated.

  Therefore, in the present embodiment, the diffractive optical element 6 has the shape shown in FIGS. 5 and 6, or the shape shown in FIGS.

  First, the diffractive optical element 6a shown in FIGS. 5 and 6 has a diffractive surface 102a on which a diffraction grating DF is formed, and a back surface 101a thereof. The diffractive surface 102a is a flat surface. On the other hand, on the back surface 101a, the curvature in the γ-axis direction that is the minor axis direction of the diffractive optical element 6a is Cγ, and the curvature in the β-axis direction that is the major axis direction of the diffractive optical element 6a is Cβ. The back surface 101a is a convex toric surface having different curvatures Cγ and Cβ. The direction of the grooves of the diffraction grating DF formed on the diffraction surface 102a is substantially parallel to the β axis. The traveling direction of the light beam 2a is defined as the α axis. The α, β, and γ axes are orthogonal to each other.

  By designing the groove width and groove interval of the diffraction grating DF to appropriate values, the light beam 2a emitted from the light emitting component 5 is incident on the back surface 101a of the diffractive optical element 6a and then diffracted on the diffraction surface 102a. The light is split into at least two light beams 2b1 to 2b3 by the grating DF. The number of light beams to be dispersed varies depending on the application, but here, as an example, a case where the light beams are divided into three light beams (0th order diffracted light beam 2b1, + 1st order diffracted light beam 2b2, and −1st order diffracted light beam 2b3). Show.

  The 0th-order diffracted light beam 2b1 is transmitted light that is not diffracted, and is used for recording / reproducing on the optical disc information recording medium 8. The + 1st order diffracted light beam 2b2 and the −1st order diffracted light beam 2b3 are both diffracted light that is diffracted and has a curved optical path. All of the 0th-order diffracted light beam 2 b 1, the + 1st-order diffracted light beam 2 b 2, and the −1st-order diffracted light beam 2 b 3 are collected on the optical disc information recording medium 8 by the objective lens 9, reflected, and received by the photodetector 10. .

  The diffractive optical element 6b shown in FIGS. 7 and 8 has a diffractive surface 102b on which a diffraction grating DF is formed, and a back surface 101b. The diffraction surface 102b is a flat surface. On the other hand, on the back surface 101b, the curvature in the γ-axis direction that is the minor axis direction of the diffractive optical element 6b is Cγ, and the curvature in the β-axis direction that is the major axis direction of the diffractive optical element 6b is Cβ. The back surface 101b is a concave toric surface having different curvatures Cγ and Cβ. Note that the direction of the grooves of the diffraction grating DF formed on the diffraction surface 102b is substantially parallel to the β axis. The traveling direction of the light beam 2a is defined as the α axis. The α, β, and γ axes are orthogonal to each other.

  In the diffractive optical element 6b as well, as in the case of the diffractive optical element 6a, the light beam 2a emitted from the light emitting component 5 is incident from the back surface 101b of the diffractive optical element 6b, and then is reflected by the diffraction grating DF on the diffractive surface 102b. The light is split into at least two light beams 2b1 to 2b3.

  In any of the diffractive optical elements 6a and 6b, the diffraction grating DF is not provided on the back surfaces 101a and 101b of the diffraction surface, but the diffraction grating DF is provided not only on the diffraction surfaces 102a and 102b but also on the back surfaces 101a and 101b. May be. Alternatively, a diffraction grating DF may be provided only on the rear surfaces 101a and 101b, and this may be used as a diffraction surface.

  The + 1st order diffracted light beam 2b2 and the −1st order diffracted light beam 2b3 are used to detect how much the 0th order diffracted light beam 2b1 used for recording / reproduction is shifted from the track center on the optical disc information recording medium 8. As a typical example of such a detection method, a method called “3-beam method” and a method called “differential push-pull method” are widely known. These detection methods are described below. Three or more diffracted light beams may be generated and used for detection.

  The 0th-order diffracted light beam 2b1, + 1st-order diffracted light beam 2b2, and -1st-order diffracted light beam 2b3 dispersed by the diffraction grating DF on the diffractive surface 102a or 102b are respectively applied to the optical disc information recording medium 8 by the objective lens 9 on the optical disc information recording medium 8. , + 1st order diffracted light beam 2d2 and -1st order diffracted light beam 2d3.

  Then, as shown in FIG. 9 or FIG. 10, adjustment is made so that the 0th-order diffracted light beam 2d1 is arranged on the track TR on the optical disc information recording medium 8. FIG. 9 is a diagram showing a recording surface of the read-only optical disc information recording medium 8 on which information marks are recorded side by side in the tangential direction D2. FIG. 10 is a diagram showing a recording surface of a writable optical disc information recording medium 8 having a guide groove GR along which recording marks are recorded.

  The direction of the arrangement of the three diffracted light beams 2d1 to 2d3 can be adjusted by rotating the diffractive optical element 6a or 6b about the design optical axis. In this adjustment, the structure of various optical disc information recording media as shown in FIGS. 9 and 10 is different, and the difference in detection method between whether the track TR is used for detection of deviation or whether the guide groove GR is used for detection of deviation. Need to be considered.

  The three-beam method is often used for an optical disc information recording medium having a structure not having the guide groove GR as shown in FIG. In order to detect the deviation, the deviation angle φa in the arrangement direction of the three diffracted light beams 2d1 to 2d3 from the tangential direction D2 is set so that the radial arrangement distance of the diffracted light beams 2d2 and 2d3 is approximately the track period. The angle is adjusted to (N + 1/2) times (N is an arbitrary integer).

  On the other hand, when the differential push-pull method using the guide groove GR for detecting displacement is employed as shown in FIG. 10, the three diffracted light beams 2d1 to 2d3 from the tangential direction D2 are used. Is shifted to an angle at which the radial arrangement distance of the diffracted light beams 2d2 and 2d3 is approximately (2M + 1) times the track period (M is an arbitrary integer).

  As described above, the angle of deviation in the arrangement direction of the three diffracted light beams 2d1 to 2d3 varies depending on the difference in the structure of the optical disk information recording medium and the difference in deviation detection method, and may be set relatively large. If so, it may be set relatively small.

  As shown in FIGS. 5 to 8, the γ-axis is aligned with the direction rotated from the tangential direction by, for example, φa (in the case of FIG. 9, φb in the case of FIG. 10), and the β-axis is in the radial direction. For example, the alignment direction of the three diffracted light beams 2d1 to 2d3 can be adjusted by adjusting the rotation direction by φa (in the case of FIG. 9, φb in the case of FIG. 10).

  The 0th-order diffracted light beam 2d1, the + 1st-order diffracted light beam 2d2, and the -1st-order diffracted light beam 2d3 are reflected by the optical disc information recording medium 8, respectively. Then, each reflected diffracted light beam is received by the light detector 10, and each light amount is output as an electric signal. Among these, the electric signals corresponding to the + 1st order diffracted light beam 2d2 and the −1st order diffracted light beam 2d3 are subjected to a calculation according to the shift detection method, and the relative positional shift in the radial direction between the 0th order diffracted light beam 2d1 and the information mark row A tracking error signal representing is generated. For example, if the light amounts of the reflected light beams of the + 1st order diffracted light beam 2d2 and the −1st order diffracted light beam 2d3 are equal, the error is 0, and if there is a difference in the light amount, a tracking error signal corresponding to the difference is generated. Based on this tracking error signal, the position of the objective lens 9 is controlled in the radial direction. The above is the description of the deviation detection method using the three-beam method or the differential push-pull method.

  In the diffractive optical element 6a or 6b, when the light beam 2a passes through the toric surface of the back surface 101a or 101b of the diffractive surface, astigmatism occurs due to the difference in curvatures Cγ and Cβ between the γ-axis direction and the β-axis direction. Therefore, the diffractive optical element 6a or 6b has not only a diffractive function but also a function of generating astigmatism, so that the semiconductor laser 1 serving as the light source can be used as the radial direction of the optical disc information recording medium 8 from the lens optical axis of the objective lens 9. What is necessary is just to cancel the astigmatism generated by moving away in the direction. That is, the respective curvatures Cγ and Cβ are determined so that the diffractive optical element 6a or 6b has astigmatism in a direction that cancels off-axis astigmatism that occurs when the objective lens 9 moves in the radial direction.

  FIG. 11 is a diagram illustrating a change in the combined value Was of off-axis astigmatism generated by the radial movement of the objective lens 9 according to astigmatism generated by the diffractive optical element 6a or 6b. In FIG. 11, the curve (a) corresponds to the graph of astigmatism in FIG. Then, according to the difference in the amount of astigmatism generated in the diffractive optical element 6a or 6b, three states, ie, curves (b), (c), and (d) are illustrated. Note that the amount of astigmatism generated in the diffractive optical element 6a or 6b is ASg (corresponding to the value of the intercept on each curve in FIG. 11).

  When the value of the combined off-axis astigmatism value Was is positive, the relationship between the front focal line F2 and the rear focal line F1 is the relationship shown in FIG. As shown in FIG. 12, the position of the front focal line F2 and the position of the rear focal line F1 of the light beam are interchanged from the case of FIG.

  As described above, the recording / reproduction characteristics, the ability to detect the cross-track of the light beam, and the allowable range of astigmatism that can maintain all of the low crosstalk, and the design from the design optical axis of the objective lens 9 in the radial direction. When the required maximum movement amount is in the relationship shown in FIG. 11, a combined value of off-axis astigmatism within a rectangular region defined by the points P4 and P5 determined by the required maximum movement amount and the allowable range of astigmatism. It is only necessary that Was fit. That is, the characteristic of the combined value Was of off-axis astigmatism only needs to be between the curves (b) and (d), and the astigmatism amount ASg generated in the diffractive optical element 6a or 6b is set so as to be so. You only have to set it. The value of the astigmatism amount ASg is set by the values of the curvatures Cγ and Cβ of the diffractive optical elements 6a and 6b.

  Note that, as in the case of the curve (c), the average value of the maximum astigmatism Was (max) and the minimum astigmatism Was (min) within the allowable range of astigmatism 0.5 × {Was (max) + Was ( min)} as a reference, it is most desirable that the astigmatism change amount ΔAS2 up to the objective lens movement amount zero and the astigmatism change amount ΔAS1 up to the required maximum movement amount be approximately the same. This is because each margin up to the upper limit and the lower limit within the allowable range of the combined value Was of the off-axis astigmatism is maximized.

  In addition to the toric surface shape shown in FIGS. 5 to 8, the astigmatism similar to that in FIGS. 5 to 8 can be obtained even if the diffractive optical element 6 has a cylindrical surface shape as shown in FIGS. 13 to 16. The effect of generating is obtained.

  A diffractive optical element 6c shown in FIG. 13 has a diffraction cylindrical DF having a convex cylindrical surface having a curvature only in the γ-axis direction on the back surface 101c of the diffractive surface 102c, and a groove substantially parallel to the β-axis direction on the diffractive surface 102c. Is formed.

  In the diffractive optical element 6d shown in FIG. 14, a diffraction grating DF having a convex cylindrical surface having a curvature only in the γ-axis direction on the diffractive surface 102d and a groove substantially parallel to the β-axis direction is formed on the diffractive surface 102d. ing. The back surface 101d of the diffractive surface 102d is a flat surface.

  A diffractive optical element 6e shown in FIG. 15 has a diffraction grating DF having a concave cylindrical surface having a curvature only in the β-axis direction on the back surface 101e of the diffractive surface 102e, and a groove substantially parallel to the β-axis direction on the diffractive surface 102e. Is formed.

  In the diffractive optical element 6f shown in FIG. 16, a diffraction grating DF having a concave cylindrical surface having a curvature only in the β-axis direction on the diffraction surface 102f and a groove substantially parallel to the β-axis direction is formed on the diffraction surface 102f. Yes. The back surface 101f of the diffractive surface 102f is a flat surface.

  In any of the diffractive optical elements 6c to 6f shown in FIGS. 13 to 16, the back surface 101c to 101f of the diffractive surface is not provided with the diffraction grating DF, but not only the diffractive surfaces 102c to 102f but also the back surface 101c to A diffraction grating DF may also be provided in 101f. Alternatively, a diffraction grating DF may be provided only on the rear surfaces 101c to 101f, and this may be used as a diffraction surface.

  There is the following difference between the case where the toric surface shape shown in FIGS. 5 to 8 is adopted and the case where the cylindrical surface shape shown in FIGS. 13 to 16 is adopted.

  When the diverging light beam passes through an optical element having a thickness, in addition to astigmatism, spherical aberration and lens light of the objective lens 9 from the best image plane of the light beam condensed on the optical disc information recording medium 8 Aberration (referred to as defocus aberration) caused by defocus caused by a shift in the axial direction occurs.

  When the toric surface shape shown in FIGS. 5 to 8 is adopted, spherical and defocus aberrations in addition to astigmatism are obtained by optimizing the γ-direction and β-direction curved surfaces of the toric surface as aspherical surfaces. Can be reduced. In addition to this, spherical aberration generated when the distance between the objective lens 9 and the light source deviates from the design value of the objective lens 9, and the thickness error of the protective transparent substrate formed on the optical disc information recording medium 8 If the above-mentioned aspherical shape is optimized in consideration of the spherical aberration generated from the above, the quality of the light beam can be further improved, and the recording / reproducing characteristics can be improved. On the other hand, when the cylindrical surface shape shown in FIGS. 13 to 16 is employed, the amount of astigmatism can only be adjusted.

  That is, if the diffractive surface of the diffractive optical element 6 or the back surface of the diffractive surface is a toric surface or a cylindrical surface, the curvature of a meridian surface (for example, Cγ) in the toric surface and the curvature (for example, Cγ) of the other meridian plane perpendicular to it. The amount of astigmatism of the diffractive optical element 6 can be adjusted by adjusting Cβ) or the curvature of the cylindrical surface. In addition, in the case of a toric surface, a meridional surface in the toric surface and another meridian surface perpendicular to the meridian surface are made aspherical, so that not only the amount of astigmatism but also the light beam emitted from the light source It is also possible to adjust the amount of spherical aberration and defocus aberration that occur in the optical system until it is condensed on the information recording medium 8. Therefore, the quality of the light beam used for recording / reproduction can be further improved.

  As shown in FIGS. 9 and 10, there is an angle between the track direction, that is, the tangential direction D2, and the arrangement direction of the 0th-order diffracted light beam 2d1, the + 1st-order diffracted light beam 2d2, and the -1st-order diffracted light beam 2d3. When φa or φb is provided, the diffractive optical element 6 is rotated in the above.

  However, when the diffractive optical element 6 is rotated, the astigmatism of the diffractive optical element 6 is also rotated, and the off-axis astigmatism generated in the objective lens 9 may not be canceled efficiently.

  Therefore, the curvature of the toric surface or cylindrical surface of the diffractive optical element 6 is set so that the amount of astigmatism generated in the diffractive optical element 6 can most effectively cancel off-axis astigmatism generated in the objective lens 9. The holding direction is preferably parallel to at least one of the tangential direction D2 and the radial direction D1, and the extending direction of the grooves of the diffraction grating DF is desirably shifted from the β axis by φa or φb as shown in FIG.

  That is, if the direction of the groove of the diffraction grating DF is inclined from the radial direction (corresponding to the β-axis direction) in the tangential direction of the optical disc information recording medium 8 by an angle φa or φb, the optical disc information recording is performed. A plurality of diffracted lights can be arranged on the optical disc information recording medium 8 at an angle φa or φb with respect to the track direction of the medium 8, and the astigmatism of the diffractive optical element 6 is in the radial direction of the objective lens 9. The diffractive optical element 6 can be arranged so as to most efficiently cancel astigmatism generated by movement.

  According to the optical head device according to the present embodiment, the diffractive optical element 6 not only generates a plurality of diffracted lights, but also the semiconductor laser 1 serving as a light source can It also has astigmatism in a direction that cancels out astigmatism generated by moving away in the radial direction. Therefore, even when the light beam is incident on the objective lens 9 as diffused light that is not a parallel light flux, the diffractive optical element 6 suppresses the influence of the wavefront aberration on the recording / reproducing characteristics without using a collimator lens. Is possible. As a result, the optical head device can be reduced in size, thickness, and cost.

  That is, by providing the diffractive optical element 6 with an astigmatism generation function capable of canceling off-axis astigmatism that occurs during lens shift of the objective lens 9 and a spectral function by the diffraction grating DF, astigmatism is achieved in a desired direction. It becomes possible to provide the aberration ASg.

  In an optical disk device such as a DVD or a CD that is currently in practical use, the required maximum amount of lens shift is about 600 μm, and the off-axis astigmatism generated in the objective lens 9 at that time is about 300 mλpv alone. Therefore, the astigmatism ASg of the diffractive optical element 6 is desirably about 300 mλpv or less in the direction in which off-axis astigmatism is canceled.

  However, the amount of astigmatism takes into account off-axis astigmatism of the objective lens, and in the case of an optical configuration in which astigmatism is further added by a light source or other optical components, the astigmatism Sometimes it should be set larger than the amount.

  The optical configuration described above is an example of a typical finite optical system, but this embodiment can also be applied to a pseudo finite optical system. Here, the quasi-finite optical system means that an objective lens is provided in the optical path between the light source and the objective lens, even if the light beam emitted from the light source becomes a parallel light flux in the optical path. It refers to a configuration in which a light beam is converted to be diffused before entering the lens. For example, an optical configuration having a function of changing the incident condition of the light beam to the objective lens or the diffusion degree of the light beam by moving the lens in the optical path in the optical axis direction is applicable.

  Also in the case of the quasi-finite optical system, the light beam incident on the objective lens is the same as that in the case of the finite optical system, and thus this embodiment can be applied.

Embodiment 2. FIG.
The present embodiment is a modification of the optical head device according to the first embodiment, and adopts a parallel plate type diffractive optical element instead of the diffractive optical element 6 having a toric surface or a cylindrical surface, and the diffractive surface thereof. Is tilted from the radial direction toward the lens optical axis of the objective lens 9 to generate astigmatism.

  FIG. 18 is a diagram showing an optical head device according to the present embodiment. In FIG. 18, the apparatus configuration is the same as that in FIG. 1 except that the diffractive optical element 6 is changed to a parallel plate type diffractive optical element 61. Therefore, since the points other than the parallel plate type diffractive optical element 61 are the same as those of the optical head device according to the first embodiment, the description thereof is omitted.

  FIG. 19 and FIG. 20 show the shape and arrangement of the parallel plate type diffractive optical element 61 in FIG. 19 is a view of the parallel plate type diffractive optical element 61 viewed from the tangential direction, and FIG. 20 is a view of the parallel plate type diffractive optical element 61 viewed from the radial direction.

  The parallel plate type diffractive optical element 61 is made of a parallel plate type resin or glass crystal material, and a diffraction grating DF is formed on at least one of the surface on which the light beam 2a is incident and the surface on which it is emitted (FIGS. 19 and 20). Shows a case where the diffraction grating DF is provided only on the outgoing surface). The light beam 2a is split into at least two light beams 2b1 to 2b3 by the diffraction grating DF of the parallel plate type diffractive optical element 61.

  When the parallel plate type diffractive optical element 61 is given an inclination of an angle θ in the direction corresponding to the radial direction D1 with respect to the design optical axis, the emitted light beam 2a passes through the parallel plate type diffractive optical element 61. Astigmatism occurs. Therefore, as in the first embodiment, the off-axis described in the first embodiment occurs when the objective lens 9 moves in the radial direction D1 due to astigmatism generated in the parallel plate type diffractive optical element 61. Astigmatism can be canceled out.

  That is, in the parallel plate type diffractive optical element 61, the β axis that is the direction in which the grooves of the diffraction grating DF extend is aligned with the z axis, and the γ axis that is the direction in which the grooves of the diffraction grating DF are aligned are aligned with the x axis. The diffractive surface of the element 61 is disposed so as to be inclined by a predetermined angle θ from the radial direction D1 toward the lens optical axis A1a of the objective lens 9.

  Here, the amount of astigmatism W22pv generated by the parallel plate type diffractive optical element 61 is such that the numerical aperture of the light beam 2a transmitted through the parallel plate type diffractive optical element 61 is NA, the wavelength of the light beam 2a is λ, and astigmatism. Δz is the distance (astigmatic difference) between the front focal line F2 and the rear focal line F1.

It can be expressed as Equation 1 is based on Non-Patent Document 2.

  Here, with respect to the astigmatic difference Δz, the thickness d and the refractive index n of the parallel plate type diffractive optical element 61 and the angle θ from the radial direction D1 are used from Non-Patent Document 3.

It can be expressed as

  Therefore, the amount of astigmatism ASg generated by the parallel plate type diffractive optical element 61 can be determined by the following equation.

  As in the case of the first embodiment, the arrangement direction of the light beams 2b1 to 2b3 after the spectroscopy is set to make an angle φa or φb from the track direction of the optical disc information recording medium 8 shown in FIGS. The arrangement of the parallel plate type diffractive optical element 61 is adjusted with the design optical axis as the rotation axis.

  However, when the parallel plate type diffractive optical element 61 is rotated, the astigmatism of the parallel plate type diffractive optical element 61 is also rotated, and the off-axis astigmatism generated in the objective lens 9 cannot be canceled out efficiently. There is also.

  Therefore, the diffraction grating of the parallel plate type diffractive optical element 61 is designed so that the amount of astigmatism generated in the parallel plate type diffractive optical element 61 can most effectively cancel off-axis astigmatism generated in the objective lens 9. The direction of the DF groove is desirably shifted by an angle φa or φb from the radial direction D1 to the tangential direction D2 as in FIG.

  That is, if the direction of the groove of the diffraction grating DF is inclined from the radial direction (corresponding to the β-axis direction) in the tangential direction of the optical disc information recording medium 8 by an angle φa or φb, the optical disc information recording is performed. A plurality of diffracted lights can be arranged on the optical disc information recording medium 8 at an angle φa or φb with respect to the track direction of the medium 8, and the astigmatism of the parallel plate type diffractive optical element 61 is caused by the objective lens 9. The parallel plate type diffractive optical element 61 can be arranged so as to most efficiently cancel the astigmatism generated by the movement in the radial direction.

  According to the present embodiment, the diffractive surface of the parallel plate type diffractive optical element 61 is disposed so as to be inclined at a predetermined angle θ from the radial direction D1. Therefore, the amount of astigmatism of the diffractive optical element can be easily adjusted by adjusting the thickness d, the refractive index n, and the tilting degree θ of the parallel plate type diffractive optical element 61.

  In addition, as shown in FIG. 21, the flat glass 3 which is the exit window of the light beam 2a is a light emitting component which is a parallel plate exit window 3a inclined obliquely from the radial direction toward the lens optical axis of the objective lens 9. 5a exists. This parallel plate exit window 3a has the function of generating astigmatism as in the case of the parallel plate type diffractive optical element 61. However, since it is inclined obliquely from the radial direction, coma is likely to occur.

  However, the parallel plate type diffractive optical element 61 is applied to the light emitting component 5a of FIG. 21, and the diffractive surface of the parallel plate type diffractive optical element 61 is tilted in the direction opposite to the exit surface of the parallel plate exit window 3a. For example, part or all of the coma generated by the parallel plate exit window 3a can be canceled by the coma generated by the parallel plate type diffractive optical element 61. Therefore, the quality of the light beam used for recording / reproduction can be further improved.

  In an optical disk device such as a DVD or a CD that is currently in practical use, the required maximum amount of lens shift is about 600 μm, and the off-axis astigmatism generated in the objective lens 9 at that time is about 300 mλpv alone. Therefore, it is desirable that the amount of astigmatism ASg of the parallel plate type diffractive optical element 61 is about 300 mλpv or less in the direction to cancel off-axis astigmatism.

  However, the amount of astigmatism takes into account off-axis astigmatism of the objective lens, and in the case of an optical configuration in which astigmatism is further added by a light source or other optical components, the astigmatism Sometimes it should be set larger than the amount. In addition, this embodiment can be applied to a pseudo finite optical system.

Embodiment 3 FIG.
The present embodiment is also a modification of the optical head device according to the first embodiment, which has a two-light source configuration and does not cause the diffractive optical element to have an astigmatism generation function. A dichroic prism that emits the beam in the same direction has an astigmatism generation function.

  FIG. 22 is a diagram showing an optical head device according to the present embodiment. In FIG. 22, the semiconductor laser 1 as the first light source, the flat glass 3, the heat radiating metal package 4, the diffractive optical element 62, the deflection prism 7, the optical disk information recording medium 8, the objective lens 9, the photodetector 10, and the detection As for the optical element 11, a parallel plate type diffractive optical element 62 having a diffractive surface that does not tilt from the radial direction is used instead of the diffractive optical element 6, and a dichroic prism 12 is inserted in the optical path. Except for the point, it is the same as FIG. Therefore, description of the same components as those in FIG. 1 is omitted.

  Reference numeral 13 in FIG. 22 denotes a semiconductor laser as a second light source, which emits a light beam 14 (for example, blue light) having a wavelength different from that of the light beam 2a (for example, red light). Reference numeral 15 denotes a hologram element, and reference numeral 16 denotes a light receiving element formed on the same semiconductor substrate as the semiconductor laser 13.

  The semiconductor substrate on which the semiconductor laser 13 and the light receiving element 16 are formed is attached to a package 17 having a heat dissipation function. A power supply terminal 18 for the semiconductor laser 13 and the light receiving element 16 and an output terminal 19 for outputting an electrical signal from the light receiving element 16 are attached to the package 17.

  Further, the hologram element 15 is integrated by being bonded to the package 17. The light beam 14 emitted from the semiconductor laser 13 passes through the hologram element 15.

  Reference numeral 12 denotes a dichroic prism, which has a characteristic of deflecting the light beam 14 in the z direction by an internal reflection surface and transmitting the light beam 2c from the deflecting prism 7 as it is. The objective lens 9 is a wavelength compatible lens capable of condensing the light beam 2c and the light beam 14 having different wavelengths onto the optical disc information recording medium 8.

  The light beam 2 d output from the semiconductor laser 1 and reflected by the optical disk information recording medium 8 is converted into a light beam that is incident again on the objective lens 9 and converges, and passes through the dichroic prism 12 and the deflecting prism 7. Then, the light reaches the light detector 10 through the detection optical element 11. On the other hand, the light beam 14 output from the semiconductor laser 13 is reflected by the optical disk information recording medium 8 and incident again on the objective lens 9, then deflected by the dichroic prism 12, split by the hologram element 15 into the light receiving element 16, and Deflected.

  In the present embodiment, two light beams 2 a and 14 emitted from two light sources (semiconductor laser 1 and semiconductor laser 13) are incident on one objective lens 9, and in the radial direction of the optical disc information recording medium 8. It has the dichroic prism 12 which can give the astigmatism of this.

  A specific shape of the dichroic prism 12 is shown in FIG. The dichroic prism 12 has a structure in which a prism 12 a and a prism 12 b are attached by a transmission / reflection surface 105. The transmission / reflection surface 105 functions as a reflection surface for the light beam 14 and as a simple transmission surface for the light beam 2c. That is, the dichroic prism 12 receives the first surface 103 that receives the light beam 2 c, the second surface 106 that receives the light beam 14, and the second surface 106 while transmitting the light beam 2 c received by the first surface 103. The light beam 14 is reflected, and the light beam 2c transmitted through the transmission / reflection surface 105 and the light beam 14 reflected by the transmission / reflection surface 105 are emitted in the same direction in which the optical axis A1d extends. And three surfaces 104. Moreover, the refractive index of the prism 12a and the prism 12b is the same value (n2). Note that there is no change in the cross-sectional shape of the dichroic prism 12 in the yz plane with respect to the x-axis direction in FIG.

  In FIG. 23, with the Z axis parallel to the z axis in FIG. 22 and the Y axis parallel to the y axis in FIG. 22 as the origin, the intersection P7 between the optical axis A2 of the light beam 14 and the second surface 106 is the origin. Newly set.

  In the present embodiment, as described in the first and second embodiments, the astigmatism generated by moving the light source away from the lens optical axis of the objective lens 9 in the radial direction of the optical disc information recording medium 8 is canceled. Astigmatism ASg is generated by the dichroic prism 12. For this purpose, the perpendicular lines L4, L5, and L6 of the first surface 103, the third surface 104, and the second surface 106 of the dichroic prism 12 are set to the respective optical axes A1b, A1d, and A2 of the incident light beam. It is inclined with respect to the direction by angles of θ7, θ7, and θ1, respectively. The angles θ2 and θ6 are respectively formed by the optical axes A4 and A5 of the respective light beams inside the dichroic prism 12 and the perpendicular L6 of the second surface 106 and the perpendicular L4 or L5 of the first surface 103 or the third surface 104. Is an angle.

  In FIG. 23, the first surface 103, the third surface 104, and the straight line L1 passing through the reflection point P6 are parallel (indicated by the symbol //), and the second surface 106 and the reflection point P6 are The straight line L2 that passes through is parallel (the symbol /// indicates “parallel”). The angle formed by the optical axis A2 before incidence of the light beam 14 and the optical axis A1d after emission is 90 °.

  First, the transmission optical path of the dichroic prism 12 of the light beam 2c using the semiconductor laser 1 as a light source will be described.

  For the light beam 2c, the dichroic prism 12 is a parallel plate type optical element inclined obliquely from the radial direction to the lens optical axis of the objective lens 9 in the same manner as the parallel plate type diffractive optical element 61 in the second embodiment. Works. Therefore, by using the same equation as Equation 3 described in the second embodiment, the astigmatism ASg having characteristics between the curve (b) and the curve (d) in FIG. 11 can be generated. The angle d and the thickness d between the first surface 103 and the third surface 104 are determined. In Equation 3, θ = θ7 and n = n2 (refractive indexes of the prism 12a and the prism 12b) are respectively replaced. NA is the numerical aperture of the light beam 2c that passes through the dichroic prism 12, and λ is the wavelength of the light beam 2c.

  Next, the reflection system optical path of the dichroic prism 12 of the light beam 14 using the semiconductor laser 13 as a light source will be described.

  In the present embodiment, by setting the angle θ1 of the second surface 106 that is the incident surface and the angle θ7 of the third surface 104 that is the output surface to the same value, the second surface 106 and the third surface 104 are also In the same manner that the first surface 103 and the third surface 104 are parallel, the angles of the respective parts are designed so as to be equivalently parallel through the transmission / reflection surface 105. That is, when the light beam 14 passes through the dichroic prism 12, the light beam 14 receives an action equivalent to that passed through the parallel plate optical element having the inclination of θ7.

  Furthermore, if the angle θ1 of the second surface 106 that is the entrance surface and the angle θ7 of the third surface 104 that is the exit surface are set to different values, the amount of astigmatism generated in the light beam 14 also varies depending on this angle difference. It is possible to make it. In this case, a degree of freedom is given to the design of the amount of astigmatism generated in the light beam 14 and the shape of the dichroic prism 12. That is, when the light beam 14 passes through the dichroic prism 12, the incident surface and the exit surface are subjected to an action equivalent to that of passing through a non-parallel wedge-shaped flat plate optical element.

  Then, astigmatism can be given in the radial direction of the optical disc information recording medium 8, and astigmatism that occurs when the objective lens 9 moves in the radial direction can be canceled out. When the angle θ1 and the angle θ7 are set to the same value, the amount of astigmatism given to the light beam 14 can be obtained by Equation 3 as in the case of the light beam 2c. The distance dr1 between the second surface 106 and the straight line L2 and the distance dr2 between the third surface 104 and the straight line L1 in FIG. Since the distance dr2 is already determined in the transmission optical path, the desired astigmatism to be generated in the light beam 14 is independent of the amount of astigmatism generated in the light beam 2c by optimizing the distance dr1. The amount of aberration ASg can be obtained.

  The light beam 14 is refracted by the second surface 106 and further refracted by the third surface 104 when it exits the dichroic prism 12. If the angle θ1 and the angle θ7 are equal, the angle θ1 (or Regardless of θ7), by setting the transmission / reflection surface 105 to 45 ° (θ3 = 0) with respect to the Y axis, the deflection angle of the reflection optical path can be set to 90 ° with respect to the Y axis.

  On the other hand, when the angle θ1 and the angle θ7 are set to different values, since the refraction condition is different from that at the time of incidence on the third surface 104 when exiting the dichroic prism 12, the light beam 14 exiting from the dichroic prism 12 is emitted. Is required to be inclined by a certain angle θ3 from a 45 ° angle with respect to the Y axis. By setting the angle θ1 to a value different from the angle θ7, the amount of astigmatism generated in the light beam 14 can be made variable independently of the amount of astigmatism generated in the light beam 2c.

First, consider a straight line L3 having an inclination of 45 ° with respect to the Y axis in the ZY plane. Here, the angle θ _A formed by the straight line L3 and the Y axis is

It is. Now, the angle θ _B formed between the transmission / reflection surface 105 and the Y-axis is obtained by using the angle θ3 formed between the transmission / reflection surface 105 and the straight line L3.

It can be expressed.

Next, an angle θ _C formed between the transmission / reflection surface 105 and the optical axis A4 of the light beam 14 in the dichroic prism 12 is a straight line L7 parallel to the transmission / reflection surface 105 and the Y axis passing through the reflection point P6. Using the angle θ1-θ2 formed by

It can be expressed.

Here, since the light beam 14 is reflected at the point P6, the angle θ _C formed between the transmission / reflection surface 105 and the optical axis A4 and the angle formed between the transmission / reflection surface 105 and the reflected optical axis A5 are Are equal. Therefore, the angle θ _{D obtained} by subtracting the angle θ 3 from the angle formed by the transmission / reflection surface 105 and the optical axis A 5 after reflection is

It can be expressed.

  Now, the angle θ4 formed by the straight line L8 parallel to the Z axis passing through the reflection point P6 and the optical axis A5 after reflection is

It can be expressed.

  Further, the angle θ6 formed between the perpendicular L5 of the third surface 104 and the optical axis A5 after reflection is

It can be expressed. Here, n1 is the refractive index of the air outside the dichroic prism 12. Therefore, if Equation 9 is modified, the angle θ3 is

It can be expressed. Therefore, when the angle θ1 and the angle θ7 are set to different values, the light beam 14 and the light beam 2c can be obtained by setting the angle formed by the transmission / reflection surface 105 and the straight line L3 to the angle θ3 obtained by Expression 10. The angles emitted from the third surface 104 can be made equal.

  Note that the angle θ2 is

Therefore, when θ1 = θ7 described above, Equation 10 is

If θ3 = 0 and the transmission / reflection surface 105 is 45 degrees with respect to the Y-axis, the angles at which the light beam 14 and the light beam 2c are emitted from the third surface 104 can be made equal.

  In the optical head device according to the present embodiment, the first surface 103 and the third surface 104 of the dichroic prism 12 are parallel to each other, and the second surface 106 and the third surface 104 also pass through the transmission / reflection surface 105. The third surface 104 is disposed so as to be inclined from the radial direction by a predetermined angle θ7 in the direction of the lens optical axis of the objective lens 9. In particular, when the angle θ1 and the angle θ7 are equal, both the light beam 2c emitted from the semiconductor laser 1 serving as the first light source and the light beam 14 emitted from the semiconductor laser 13 serving as the second light source are both in the radial direction. Therefore, it will receive an action equivalent to passing through a parallel plate optical element inclined by an angle θ7.

  When the angle θ1 and the angle θ7 are different from each other, the second surface 106 and the third surface 104 of the dichroic prism 12 are reflected by the transmission / reflection surface 105 through the transmission / reflection surface. The light beam 14 is subjected to an action equivalent to that when the second surface 106 and the third surface 104 pass through a wedge-shaped flat optical element that functions as a non-parallel incident surface and output surface. The amount of astigmatism generated in the light beam 14 reflected by the transmission / reflection surface 105 can also be changed by the angle difference between the second surface 106 and the third surface 104, and the astigmatism generated in the light beam can be changed. It is possible to give a degree of freedom to the design of the amount of aberration and the shape of the dichroic prism.

  Therefore, the dichroic prism 12 has astigmatism in a direction that cancels astigmatism generated when the light source moves away from the lens optical axis of the objective lens 9 in the radial direction of the optical disc information recording medium 8. Therefore, even when the light beam 2c and the light beam 14 are incident on the objective lens 9 as diffused light that is not a parallel light flux, the influence of the wavefront aberration on the recording / reproducing characteristics by the dichroic prism 12 without using a collimator lens. Can be suppressed. As a result, the optical head device can be reduced in size, thickness, and cost.

  In an optical disk device such as a DVD or a CD that is currently in practical use, the required maximum amount of lens shift is about 600 μm, and the off-axis astigmatism generated in the objective lens 9 at that time is about 300 mλpv alone. Therefore, it is desirable that the amount of astigmatism ASg of the dichroic prism 12 is about 300 mλpv or less in the direction to cancel off-axis astigmatism.

  However, the amount of astigmatism takes into account off-axis astigmatism of the objective lens, and in the case of an optical configuration in which astigmatism is further added by a light source or other optical components, the astigmatism Sometimes it should be set larger than the amount. In addition, this embodiment can be applied to a pseudo finite optical system.

Embodiment 4 FIG.
The present embodiment is a modification of the optical head device shown in FIG. 1 in the first embodiment. In an optical system that performs astigmatism focus error detection, the curvature direction of the cylindrical surface of the diffraction grating element 6 is changed. By setting the tangential direction to a substantially tangential direction, the recording of the optical head device due to the deviation of the light beam on the light receiving surface of the photodetector 10 due to the position variation in the plane perpendicular to the design optical axis A1a of the diffraction grating element 6 is achieved. -It is intended to reduce the deterioration of reproduction characteristics.

  FIG. 24 is a perspective view showing the optical head device according to the fourth embodiment, and the basic optical configuration is the same as FIG. Hereinafter, the configuration and operation of the optical head device will be described with reference to FIG. 24. However, the configuration and operation in which there is no special point or change with respect to the contents described above with reference to FIG. A description of the operation is omitted.

  The diffraction grating element 6 has the shape shown in FIG. 13, and splits the light beam 2a emitted from the semiconductor laser 1 (not shown) inside the light emitting component 5 into diffracted light beams 2b1 to 2b3. In FIG. 24, only the path of the 0th-order diffracted light beam 2b1 among the diffracted light beams is shown by lines, and the ± 1st-order diffracted light beams 2b2 and 2b3 are not shown. Therefore, in FIG. 24, the path of each light beam is indicated by a line substantially the same as the design optical axis.

  In the fourth embodiment, instead of the deflecting prism 7 of the first embodiment, the light beams 2b1 to 2b3 are deflected in the + z direction by using a flat plate half mirror 120 that is disposed to be inclined with respect to the design optical axis A1b. The light is incident on the objective lens 9.

  The light beam 2d reflected by the optical disk information recording medium 8 is incident on the objective lens 9 again, converted into convergent light having the optical axes A1b and A1c as the central axes, and passes through the flat plate half mirror 120. The light beam 2e that has passed through the flat half mirror 120 passes through the detection optical element 121 and enters the light detector 122 as a light beam 2f.

  As shown in FIG. 24, the photodetector 122 has a light receiving surface pattern having at least a quadrant light receiving region pair composed of light receiving regions A to D and light receiving regions E and F arranged on both sides thereof. The light receiving areas A to D are divided into four by a dividing line substantially parallel to the x axis and a dividing line substantially parallel to the y axis. Each light receiving region has a function of receiving an electric signal having a magnitude corresponding to the amount of light when the light beam 2f converted by the detection optical element 121 is received.

  Astigmatism occurs when the reflected light beam 2c toward the optical detector 122 passes through the flat plate half mirror 120 arranged to be inclined with respect to the optical axis while converging. At this time, since the inclination of the flat half mirror 120 with respect to the design optical axis A1b is set to be oblique with respect to the x direction and the y direction, the direction of the light beam received on the photodetector 122 is 4 The direction is oblique with respect to the dividing line direction of the divided light receiving region pair. Assuming that the output of electrical signals from the respective light receiving areas A to D is A to D, the astigmatism method focus error detection can be realized by obtaining the focus error signal FES by the calculation of FES = (A + C) − (B + D). it can. At this time, if the direction in which the flat plate half mirror 120 is arranged is set to about 45 degrees with respect to the x direction and the y direction, astigmatism method focus error detection with high detection sensitivity can be performed.

  Focus control is performed by displacing the objective lens 9 in two axial directions so that the light beam 2d can be focused on the optical disc information recording medium 8 based on the focus error signal FES. Specifically, the objective lens 9 is displaced in the z direction by an objective lens displacement mechanism (not shown) using an electromagnetic force generated by a coil or the like.

  If a lens function is added to the detection optical element 121, the condensing position of the light beam 2f can be set to an arbitrary distance. Further, the detection optical element 121 has an astigmatism generation function such as a curved entrance surface or exit surface or a hologram, and is used in combination with the flat plate half mirror 120 to focus on the astigmatism method. It may be used to generate astigmatism having a size necessary for error detection. Further, the detection optical element 121 is generated in the flat plate half mirror 120 if the central axes of the incident surface and the output surface are shifted relative to each other or arranged to be inclined with respect to the design optical axis A1c. The coma aberration can be corrected, and it can be used to prevent the light intensity distribution on the photodetector 122 from becoming asymmetric. However, the detection optical element 121 is not an essential component for the optical head device. For example, when such a lens function, astigmatism generation function, and coma aberration correction are not performed, the optical head device does not include the detection optical element 121 in order to reduce the number of components. It doesn't matter.

  The light receiving areas E and F are arranged substantially parallel to the tangential direction D2, and receive the + 1st order diffracted light beam 2d2 and the −1st order diffracted light beam 2d3 reflected by the optical disc information recording medium 8, respectively. The optical head device shown in FIG. 24 has an optical configuration for detecting a three-beam method tracking error by using this, and by subtracting the output signals from the light receiving areas E and F, the three-beam method tracking error is obtained. A signal is generated, and radial position control of the light beam 2d is performed so as to follow the displacement of the track based on the signal.

  The optical head device can also use other methods as long as it is a tracking error detection method similarly performed using the diffracted light beams 2d1 to 2d3. For example, if the light receiving areas E and F each have a light receiving surface pattern divided into at least two in the y direction, differential push-pull tracking error detection can be performed.

  In order to optimally perform the focus error detection and tracking error detection described above, it is necessary to accurately adjust the position of the photodetector 122. Specifically, the position of the photodetector 122 in the z direction is in the vicinity where the light beam 2f becomes the minimum circle of confusion, and the positions in the x and y directions are (A + D) − (B + C) and (A + B) − ( It is necessary to perform the initial adjustment with high accuracy so that each calculation value of (C + D) becomes almost zero. However, in this case, in the case of an optical configuration in which coma aberration exists in the light beam 2f by the flat plate half mirror 120, the asymmetry of the intensity distribution of the light beam 2f on the photodetector 122 caused by the coma aberration is taken into consideration. Thus, the position adjustment may be performed so that the calculated value is not zero but a predetermined initial target value.

  When the photodetector 122 is arranged in the vicinity of the minimum circle of confusion as described above, the light beam 2f that is reflected by the optical disc information recording medium 8 and incident thereon is caused on the photodetector 122 by the effect of astigmatism. It is folded back in the tangential D2 direction which is approximately 90 degrees, that is, in the y direction. Therefore, in the fourth embodiment, when the objective lens 9 moves in the radial direction D1, the light beam is shifted in the y direction on the photodetector 122.

  On the other hand, in order to realize the above optical configuration, the light-emitting component 5, the diffraction grating element 6, the flat plate half mirror 120, the detection optical element 121, and the photodetector 122 are formed on an optical base made of resin, metal, or the like. Each is fixed and integrated.

  At the same time as the diffraction grating element 6 is fixed to the optical base, as described above in the first embodiment, the diffracted light beam 2d1 to the track TR on the optical disc information recording medium 8 or the guide groove GR is provided. It is necessary to rotate and adjust the diffraction grating element 6 so that 2d3 is arranged in a predetermined direction. Therefore, for example, a cylindrical diffraction grating element 6 or a cylindrical diffraction grating element holder that indirectly holds the diffraction grating element 6 is fitted into a cylindrical hole provided in the optical base. Thus, the diffraction grating element 6 can be rotated about the α-axis (however, in FIG. 24, the outer shape of the diffraction grating element 6 is not shown as a cylinder but is shown as a rectangle in order to make the cylindrical surface easy to see). However, the fixing method is not limited to the embodiment using the fitting structure as in the above example, and the structure in which the optical base and the diffraction grating element 6 or the diffraction grating element holder are fixed with an adhesive, Other modes, such as fixing via a fixing member such as a leaf spring or a screw, may be used.

  Since the structure is accompanied by rotation adjustment, in order to facilitate adjustment at the time of rotation adjustment of the diffraction grating element 6, there is some structural difference between the diffraction grating element 6 and the optical base or between the diffraction grating element holder and the optical base. Need a gap. Further, since the diffraction grating element 6, the optical base, and the diffraction grating element holder usually have an outer dimension tolerance of at least several tens of μm, there is a possibility that the gap is widened within this tolerance range.

  When fixing with adhesive, the gap will be filled with adhesive, or it will be fixed with the gap left, and when fixing with fixing members such as screws and leaf springs, the contact will be limited At this point, the diffraction grating element 6 is pressed against and held by the optical base.

  In such a fixed state of the diffraction grating element 6, the position of the diffraction grating element 6 with respect to the optical base is fixed because the adhesive and the fixing member expand and contract due to repeated external impacts and temperature shocks. There is a possibility of changing over time in the range of the previous gap.

  For example, consider a case where the diffraction grating element 6 moves along the curvature direction of the cylindrical surface. FIG. 25 is a cross-sectional view along the direction of curvature of the cylindrical surface of the diffraction grating element 6 at this time (that is, the γ-axis direction). When the diffraction grating element 6 moves in the γ direction, the light beam 2a Since the incident angle of the central optical axis on the incident surface 101c changes with respect to the initial incident angle, the central optical axis of the light beam 2b after passing through the diffraction grating element 6 is relative to the design optical axis A1a. B1a having an inclination. In FIG. 25, a state in which the central axis of the diffraction grating element 6 is located on the design optical axis A1a is represented as an outer shape 6a indicated by a dotted line.

  When the central optical axis of the light beam 2b changes due to such position movement of the diffraction grating element 6, the light beam 2f moves on the light detector 122 accordingly. The movement of the light beam 2f affects the operation such as the focus error detection described above, but the influence varies depending on the shift direction of the light beam 2f on the detector 122.

  FIG. 26A schematically shows a state in which the light beam 2f is shifted in the x direction on the photodetector 122. Even in this state, the calculated value of the focus error signal is FES = 0. Is maintained. Then, if the objective lens 9 is moved in the radial direction D1, the light beam is moved in the y direction as indicated by a dotted line in FIG. As a result, the light beam near the center of the four-divided light receiving region pair moves in an oblique direction with respect to the dividing line. From this state, if the focus control by the above-described focus error detection is performed so as to maintain the FES at 0, the minimum confusion circle of the light beam 2f adjusted at the initial stage cannot be maintained, and the control operating point changes, and the optical disc information recording is performed. The defocus of the light beam 2d occurs on the medium 8. The recording / reproducing characteristics of the optical head device deteriorate due to the influence of the defocus.

  On the other hand, when the light beam 2f deviates in the y direction, this coincides with the positional deviation direction of the light beam 2f caused by the movement of the objective lens 9 in the radial direction D1, and therefore, as shown in FIG. The light beam 2f does not move obliquely with respect to the dividing line of the divided light receiving region pair. Therefore, the defocus of the light beam 2d does not occur on the optical disc information recording medium 8.

  That is, in the case of an optical configuration that detects astigmatism focus error, an optical configuration that suppresses the deviation of the light beam in the x direction as much as possible is desirable.

  In the fourth embodiment, since the curvature direction of the cylindrical surface of the diffraction grating element 6 is arranged corresponding to the tangential direction D2, the diffraction grating element 6 is moved as described with reference to FIG. In addition, the shift direction of the light beam 2f on the photodetector 122 can be limited to the y direction, and the above-described deterioration of the recording / reproducing characteristics of the optical head device can be suppressed.

  The diffraction grating element 6 may have the shape shown in FIG. Even in this case, the direction of deviation of the light beam on the photodetector 122 caused by the position movement of the diffraction grating element 6 can be limited to the y direction, and the same effect as described above can be obtained.

  The diffraction grating element 6 may have the shape shown in FIG. 15 or FIG. In this case, since the cylindrical surface has a curvature in the β-axis direction, the light generated by the movement of the position of the diffraction grating element 6 is different from the case of using the shape of FIG. 13 or FIG. 14 having the cylindrical surface in the γ-axis direction. The direction of deviation of the light beam on the detector 122 is the x direction. As described above, the deviation of the light beam in the x direction causes deterioration of the recording / reproducing characteristics of the optical head device. Therefore, when the diffraction grating element 6 having the shape shown in FIG. 15 or FIG. 16 is used, the diffraction grating element is used when it is fixed using a fixing member or the like in order to suppress the occurrence of deviation of the light beam in the x direction. 6 is a state in which a pressure is applied in the β-axis direction, that is, a press-fitted state pressed against the optical base. This suppresses the positional movement of the diffraction grating element 6 in the direction of curvature of the cylindrical surface of the diffraction grating element 6 (that is, the β-axis direction, which is normally set as the radial direction D1) due to rotation adjustment of the diffraction grating element 6 and changes with time. it can. As a result, the shift of the light beam in the x direction on the photodetector 122 can be suppressed. It should be noted that such application of pressure when the diffraction grating element 6 is fixed is effective not only for the present embodiment but also for suppressing the deviation of the light beam in all optical head devices.

Embodiment 5 FIG.
The present embodiment is a modification of the optical head device according to the first embodiment. In the optical system that performs spot size method focus error detection, the direction of curvature of the cylindrical surface of the diffraction grating element 6 is set to a substantially radial direction. As a result, the deterioration of the recording / reproducing characteristics of the optical head device due to the deviation of the light beam on the light receiving surface of the photodetector 10 due to the position variation in the plane perpendicular to the design optical axis A1a of the diffraction grating element 6 is reduced. It is what I tried.

  FIG. 27 is a perspective view showing the optical head device according to the present embodiment, and the basic optical configuration is the same as FIG. In the following, the configuration and operation of the optical head device will be described with reference to FIG. 27. However, there is no point to be noted or changed with respect to the contents described above with reference to FIG. A description of the operation is omitted.

  The diffraction grating element 6 has the shape shown in FIG. 15, and splits the light beam 2a emitted from the semiconductor laser 1 (not shown) inside the light emitting component 5 into diffracted light beams 2b1 to 2b3. In FIG. 27, only the optical path of the 0th-order diffracted light beam 2b1 among the diffracted light beams is indicated by lines, and the ± 1st-order diffracted light beams 2b2 and 2b3 are not shown. Therefore, in FIG. 27, the path of each light beam is indicated by a line substantially the same as the design optical axis.

  The deflecting prism 140 corresponds to the deflecting prism 7 shown in FIG. 1 of the first embodiment, and deflects the light beams 2 b 1 to 2 b 3 to enter the objective lens 9.

  The light beam 2d reflected by the optical disc information recording medium 8 is incident on the objective lens 9 again, converted into convergent light having the optical axes A1b and A1c as central axes, and passes through the deflecting prism 140. The hologram element 141 has a function of dividing the light beam 2e transmitted through the deflecting prism 140 into four half-beam beams 2f1 to 2f4 in order to perform spot size method focus error detection.

  The semi-beam beams 2f1 to 2f4 that have passed through the condenser lens 142 enter the photodetector 143. The condensing function of the condensing lens 142 may be integrated in the hologram element 141.

  As shown in FIG. 28, the hologram element 141 generates concentric or curved hologram patterns having different centers in two regions with a line BL parallel to the tangential direction D2 of the optical disc information recording medium 8 as a boundary. 29, the half beam beams 2f1 and 2f2 diffracted in the hologram region 141a and transmitted through the condenser lens 142 converge to different focal lengths FC1 and FC2, respectively (here, FC1 <FC2). May be FC1> FC2.) Similarly, the half-beam beams 2f3 and 2f4 diffracted in the hologram region 141b and transmitted through the condenser lens 142 are similarly converged to different focal lengths FC1 and FC2. The focal lengths FC1 and FC2 are determined by the set value of the focus error detection range of the present optical head device. Similarly, the + 1st order diffracted light beam 2d2 and the −1st order diffracted light beam 2d3 are also divided into half beam beams, and then pass through the condenser lens 142 and enter the photodetector 143.

  As shown in FIG. 30, the photodetector 143 includes at least four pairs of strip-shaped light receiving areas P1a, P1b divided into three by a dividing line parallel to the radial direction D1 (that is, corresponding to the x-axis direction), P1c and P1d, and light receiving areas A2 to D2 and A3 to D3 for receiving the half beam beams generated by dividing the + 1st order diffracted light beam 2d2 and the −1st order diffracted light beam 2d3 by the hologram element 141, respectively.

  The photodetector 143 is arranged at an approximately intermediate distance between the focal lengths FC1 and FC2 of the half-beam beams 2f1 to 2f4 (that is, in the vicinity of (FC1 + FC2) / 2) and has a short focal length as shown in FIG. Since the beam is condensed once before the photodetector 143 and then spreads again, the arcs of the outer shape of the half-beam are in the directions shown in FIG.

  The spot size method focus error detection signal FES is generated by FES = (A1 + C1 + E1) − (B1 + D1 + F1) + (G1 + I1 + K1) − (H1 + J1 + L1) using a detection signal from each light receiving surface, and the photodetector 143 is the FES. The position is initially adjusted so that becomes almost zero.

  On the other hand, for the tracking error detection signal, the operation push-pull method tracking error detection signal TES can be obtained by calculating TES = (A1 + B1 + C1 + D1 + E1 + F1) − (G1 + H1 + I1 + J1 + K1 + L1) + MG × (A2 + B2 + A3 + B3-C2-D2-C3-D3). Here, MG is a gain, which is optimally set so that no offset of the TES signal occurs even if the objective lens 9 is moved in the radial direction.

  Note that the light detector 143 shown in FIG. 30 is provided with a light receiving region or a light receiving region pair for each of the half-beam beams, but the light detector 143 has the light receiving region or the light receiving region pair. It is also possible to adopt a mode having a light receiving surface pattern in which is shared.

  In addition, FIG. 27 shows an optical configuration that performs differential push-pull tracking error detection, but a mode in which a 3-beam tracking error detection method can also be used may be used. In this case, as described above in the first embodiment, by selecting the “3-beam method” or the “differential push-pull method”, the arrangement direction of the diffracted light beams 2d1 to 2d3 with respect to the track direction on the optical disc information recording medium 9 Is set to φa or φb.

  In the spot size method focus error detection performed by using the pair of strip light receiving areas P1a to P1d as described above, the deviation of the half beam beams 2f1 to 2f4 in the direction parallel to the dividing line of each strip light receiving area, or the light intensity The distribution deviation hardly affects the calculation of the focus error detection because the light amount ratio between the inner light receiving region and the outer light receiving region of each received half-beam is not changed.

  However, the shift in the direction orthogonal to the dividing line (that is, the y-axis direction) greatly affects the calculation of the focus error detection.

  If the half-beam is shifted in the tangential direction D2, the sum of the amount of light received in the inner light receiving region of the pair of strip-shaped light receiving regions and the amount of light received in the outer light receiving regions on both sides thereof are not equal. The calculation result of the error signal is different from the ideal calculation result. As a result, if the control is performed so that the calculation of the focus error detection is maintained at 0, there is a problem that the operating point changes and defocusing occurs, and the recording / reproducing characteristics deteriorate.

  For the above reasons, in general, in an optical configuration that performs spot size method focus error detection, the deviation of the light beam that occurs when the objective lens 9 moves in the radial direction does not affect the calculation of the focus error detection as much as possible. The dividing line direction of the strip-shaped light receiving region pair is set to the radial direction.

  Therefore, in the case of an optical configuration that detects the beam size method focus error as in the present embodiment, an optical configuration that suppresses the deviation of the light beam in the y direction, that is, the direction corresponding to the tangential direction D2 as much as possible is desirable.

  As shown in FIG. 27, when the direction of curvature of the cylindrical surface of the diffraction grating element 6 is made to correspond to the radial direction D1, the shift direction of each light beam on the photodetector 143 caused by the position movement of the diffraction grating element 6 can be expressed as x. It becomes possible to limit to the direction, and the deterioration of the recording / reproducing characteristics of the optical head device as described above can be suppressed.

  Note that the diffraction grating element 6 may have the shape shown in FIG. Even in this case, the direction of deviation of the light beam on the photodetector 143 caused by the movement of the position of the diffraction grating element 6 can be limited to the x direction, and the same effect as described above can be obtained.

  In addition, a knife edge method may be used as a focus error detection method. In this case, the hologram element 141 has a function of splitting the half beam beam parallel to at least the x-axis direction corresponding to the radial direction D1, and each of the half beam beams generated by the previous hologram element 141 is used. The light detector 143 having a light receiving pattern divided into two by a dividing line parallel to the x-axis direction corresponding to the radial direction may be used.

  When performing the knife edge method focus error detection, as in the spot size method focus error detection, each half-beam is detected by a light receiving pattern having a dividing line parallel to the x-axis direction corresponding to the radial direction. Therefore, an optical configuration having as much likelihood as possible with respect to a shift in the y-direction of the semi-beam beam, that is, a direction corresponding to the tangential direction D2, is desirable. This has been described in the fourth embodiment. It can be realized by the method.

  Further, the diffraction grating element 6 is held on the optical base in the same manner as described in the fourth embodiment, but the diffraction grating element 6 has the shape of FIG. 13 or FIG. 14 having a cylindrical surface curvature in the γ-axis direction. When using one, unlike the case of using the shape of FIG. 15 or FIG. 16 having a cylindrical surface curvature in the β-axis direction, the light beam shift on the photodetector 143 caused by the position movement of the diffraction grating element 6 is used. The direction is the y direction. In this case, in the same manner as described in the fourth embodiment, when fixing is performed using a fixing member or the like, a state in which a pressure is applied in the γ-axis direction of the diffraction grating element 6, that is, pressed against the optical base. Keep in a pressed-in state. As a result, the position of the diffraction grating element 6 in the direction of curvature of the cylindrical surface of the diffraction grating element 6 (that is, the β-axis direction, which is normally set as the tangential direction D2) due to the rotation adjustment or a change with time is changed. Can be suppressed. As a result, the shift of the light beam in the y direction on the photodetector 143 can be suppressed. It should be noted that such application of pressure when the diffraction grating element 6 is fixed is effective not only for the present embodiment but also for suppressing the deviation of the light beam in all optical head devices.

Although the present invention has been described in detail, the above description is illustrative in all aspects, and the present invention is not limited thereto. It is understood that countless variations that are not illustrated can be envisaged without departing from the scope of the present invention.
Industrial applicability

  The present invention can be used in an apparatus for recording / reproducing information on an information recording medium using a light beam. For example, optical disc information such as a CD (Compact Disc), a DVD (Digital Versatile Disc), and an MD (Mini Disc). The present invention can be applied to an optical disc recording / reproducing apparatus that performs recording / reproduction via a recording medium.

Claims (3)

A light source that emits a light beam;
A diffractive optical element having a diffractive surface on which a groove-like diffraction grating is formed, and diffracting the light beam to generate a plurality of diffracted lights;
An objective lens for condensing the plurality of diffracted lights on an optical disk information recording medium;
A light detector having a light receiving region divided into a plurality of light detectors, and converting the light into an electric signal corresponding to the amount of reflected light from the optical disc information recording medium received through the objective lens;
An optical base for rotatably fixing the diffractive optical element;
As with astigmatism direction to cancel the astigmatism generated by the relative displacement of the optical axis of the light source with respect to the lens optical axis of the objective lens, the back surface of the diffraction surface or the diffractive surface of the diffractive optical element An optical head device of a finite optical system having a cylindrical shape,
The optical base is configured to fix the diffractive optical element by applying pressure substantially parallel to a direction having a cylindrical curvature of the diffractive optical element,
Optical head device. The optical head device according to claim 1,
Characterized by further comprising astigmatism adding means for constituting an astigmatism method for focus error detection;
Optical head device. The optical head device according to claim 1,
A focusing lens for configuring either the spot size method or the knife edge method for focus error detection;
Optical head device.

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