WO2014013719A1

WO2014013719A1 - Optical pickup device

Info

Publication number: WO2014013719A1
Application number: PCT/JP2013/004337
Authority: WO
Inventors: 有希古清水
Original assignee: 三洋電機株式会社; 三洋電子部品販売株式会社
Priority date: 2012-07-17
Filing date: 2013-07-16
Publication date: 2014-01-23
Also published as: JP2015172985A

Abstract

[Problem] To provide an optical pickup device capable of matching the optical axes of two out of three laser beams using a low-cost diffraction grating while achieving the desired diffraction efficiency. [Solution] An optical pickup device (1) is provided with a diffraction grating (200) on which a BD beam, a DVD beam and a CD beam travelling toward an optical detector (110) are incident. A rectangular-shaped diffraction groove (201) having a single bottom surface is formed in the diffraction grating (200). The diffraction grating (200) is arranged so that the diffraction direction of the diffraction grating (200) is parallel to the alignment direction of the optical axes of the BD beam, the DVD beam and the CD beam. The optical detector (110) is provided with a 4-divided sensor (S1) arranged on the position where the 0 order diffracted light of the BD beam and the +1st order diffracted light of the DVD beam are overlapped, and a 4-divided sensor (S4) arranged on the position onto which the +1st order diffracted light of the CD beam is irradiated.

Description

Optical pickup device

The present invention relates to an optical pickup device, and is particularly suitable for use in an optical pickup device that irradiates an optical disc with laser light having a plurality of wavelengths.

Conventionally, compatible optical pickup devices that support multiple types of optical disks have been developed. In such an optical pickup device, information is read and written using laser beams having different wavelengths. As a semiconductor laser used in this type of optical pickup device, a two-wavelength laser that emits two laser beams having different wavelengths is known (for example, Patent Document 1). Furthermore, in recent years, a three-wavelength laser that emits three laser beams having different wavelengths is being used.

By using a semiconductor laser that emits laser beams having different wavelengths in this way, the number of parts can be reduced. Furthermore, by aligning the optical axes of laser beams having different wavelengths on the photodetector, the sensor layout of the photodetector can be simplified. Here, for example, a diffractive optical element is used as an optical element for matching the optical axes of the two laser beams (for example, Patent Document 2).

JP 2001-230501 A JP 2006-99941 A

When a three-wavelength laser is used as the light source as described above, the diffractive optical element is usually designed to bend the optical axis of one of the three laser beams. In this case, a so-called blazed diffractive optical element is used to increase the diffraction efficiency of each laser beam.

However, the blazed diffractive optical element is expensive because the structure of the diffractive groove is complicated, and this increases the cost of the optical pickup device. In contrast, in a three-beam type optical pickup device, a so-called two-step type diffraction grating used to split a laser beam into a main beam and two sub beams is a diffraction groove formed by a rectangular groove having one bottom surface. Therefore, the structure of the diffraction groove is simple, and it is considerably cheaper than a blazed diffractive optical element. However, in this diffraction grating, two diffracted lights of the same order (for example, + 1st order diffracted light and −1st order diffracted light) are generated from one laser light, and the optical system is configured so that one of them is guided to the photodetector. Then, there is a problem that the diffraction efficiency of the diffracted light guided to the photodetector is lowered.

The present invention has been made in order to solve the above-described problems. It is possible to match the optical axes of two of the three laser beams with an inexpensive diffraction grating while achieving a desired diffraction efficiency. An object is to provide a possible optical pickup device.

An optical pickup device according to the present invention includes a first light emitting unit that emits a first laser beam having a first wavelength, and a second laser beam that has a second wavelength longer than the first wavelength. The second light emitting unit that emits light and the third light emitting unit that emits third laser light having a third wavelength longer than the second wavelength are arranged on one package so that they are arranged in a straight line. A laser light source, a photodetector for receiving the first, second and third laser lights, and the first, second and third laser lights emitted from the laser light source, respectively, The first, second, and third laser beams that are converged on the first, second, and third disks and reflected from the first, second, and third disks, respectively, are transmitted to the photodetector. An optical system for guiding. Here, the optical system includes a diffraction grating on which the first, second, and third laser beams incident on the photodetector are incident, and the diffraction grating has a rectangular diffraction pattern having one bottom surface. Grooves are formed. The diffraction grating is arranged so that the diffraction direction of the diffraction grating is parallel to the alignment direction of the optical axes of the first, second and third laser beams. The photodetector is arranged at a position where the first laser light not diffracted by the diffraction grating and one of the second and third laser lights diffracted by the diffraction grating overlap each other. A quadrant sensor, and a second quadrant sensor disposed at a position where the third laser beam diffracted by the diffraction grating is irradiated.

The diffraction grating has a zero-order diffraction efficiency of the first laser light higher than that of the other orders of the first laser light, and the first-order diffraction efficiency of the second and third laser lights. However, the depth of the diffraction groove is set so as to be higher than the diffraction efficiency of the other orders of the second and third laser beams.

ADVANTAGE OF THE INVENTION According to this invention, the optical pick-up apparatus which can make the optical axis of two laser beams match among three laser beams with an inexpensive diffraction grating can be provided, achieving desired diffraction efficiency. .

The characteristics of the present invention will be further clarified by the following embodiments. However, the following embodiment is merely one embodiment of the present invention, and the meaning of the term of the present invention or each constituent element is not limited at all by the following embodiment.

It is a figure which shows the optical system of the optical pick-up apparatus which concerns on embodiment. It is a figure explaining the irradiation position of the laser beam on the light-receiving surface which concerns on a comparative example, and a figure which shows the 4-part dividing sensor distribute | arranged on a light-receiving surface. It is a figure which shows the structure and diffraction efficiency of the diffractive optical element which concern on a comparative example. It is a figure explaining the irradiation position of the laser beam on the light-receiving surface which concerns on an Example, and the figure which shows the 4-part dividing sensor distribute | arranged on a light-receiving surface. It is a figure which shows the diffraction effect of the diffraction optical element which concerns on a comparative example, and the diffraction grating which concerns on an Example, and the structure of a photodetector. It is a figure which shows the structure and diffraction efficiency of the diffraction grating which concern on an Example. It is a figure explaining the adjustment method of the diffraction grating concerning an example. It is a figure which shows the state of the beam on the light-receiving surface which concerns on a comparative example and an Example. It is a figure which shows the structure and beam diameter of a 4-part dividing sensor used for verification of the focus error signal which concerns on a comparative example and an Example. It is a figure which shows the verification result of the focus error signal which concerns on a comparative example and an Example. It is a figure which shows the verification result of the focus error signal which concerns on a comparative example and an Example. It is a figure which shows the diffraction effect of the diffraction grating which concerns on the example 1 of a change, and the structure of a photodetector. It is a figure which shows the diffraction effect of the diffraction grating which concerns on the example 2 of a change, and the structure of a photodetector.

In this embodiment, the present invention is applied to an optical pickup device that irradiates a BD [Blu-ray (registered trademark) Disc], DVD (Digital Versatile Disc), and CD (Compact Disc) with laser light.

In the present embodiment, the

light emitting units

101a, 101b, and 101c each correspond to “first, second, and third light emitting units” recited in the claims, and the semiconductor laser 101 includes the claims. This corresponds to the “laser light source”. The diffraction grating 102 to the astigmatism plate 109 and the diffraction grating 200 correspond to the “optical system” recited in the claims. Each of the four-divided sensor S1 and the four-divided sensor S2 corresponds to “first and second four-divided sensors” recited in the claims. However, the configuration described in the embodiment exemplifies the configuration of the scope of claims, and does not limit the scope of claims.

1A and 1B are diagrams showing an optical system of an optical pickup device 1 according to an embodiment. 1A is a plan view of the optical system as viewed from the front side of the optical pickup device 1 (in the positive y-axis direction), and FIG. 1B is an internal perspective view of the peripheral portion of the objective lens actuator 122 as viewed from the side. FIG. 1C is a diagram showing an arrangement state of the light emitting units in the semiconductor laser 101. In FIGS. 1A to 1C, three-dimensional coordinate axes orthogonal to each other are added. Among these coordinate axes, the x-axis is parallel to the emission direction of the laser light source 101, the y-axis is parallel to the optical axis of the objective lens 108, and the z-axis is parallel to the optical axis of the collimator lens 105.

Referring to FIG. 1A, an optical pickup device 1 includes a semiconductor laser 101, a diffraction grating 102, a PBS mirror 103, a quarter wavelength plate 104, a collimator lens 105, and a lens actuator 106. A raising mirror 107, an objective lens 108, a diffraction grating 200, an astigmatism plate 109, and a photodetector 110 are provided.

The semiconductor laser 101 includes a laser beam with a wavelength of about 400 nm (hereinafter referred to as “BD light”), a laser beam with a wavelength of about 650 nm (hereinafter referred to as “DVD light”), and a laser beam with a wavelength of about 780 nm (hereinafter referred to as “CD light”). Light)) in the same direction.

As shown in FIG. 1C, the semiconductor laser 101 includes light emitting

units

101a, 101b, and 101c that emit BD light, DVD light, and CD light, respectively, in one package. The

light emitting units

101b and 101c are integrally formed on the substrate 101e so that the interval is w2. The light emitting unit 101a is formed on a substrate 101d different from the substrate 101e so that the interval between the light emitting

units

101a and 101b is w1 (w1 = w2). The

substrates

101d and 101e are installed on the submount 101f. The light emitting portions 101a to 101c are formed so as to be aligned on a straight line. In the present embodiment, the optical system after the semiconductor laser 101 is adjusted so that its optical axis is aligned with the optical axis of the DVD light. In the present embodiment, both w1 and w2 are designed to be 90 μm.

Here, the light emitting unit 101a and the

light emitting units

101b and 101c are formed on the

substrates

101d and 101e, respectively, by a semiconductor manufacturing process. For this reason, the positional accuracy of the light emitting unit 101a on the substrate 101d and the positional accuracy of the

light emitting units

101b and 101c on the substrate 101e are sufficiently enhanced. However, the

substrates

101d and 101e on which the light emitting portions are formed are bonded onto the submount 101f via electrodes, bonding layers, and the like. For this reason, the positional accuracy of the

substrates

101d and 101e in the submount 101f is lowered. For the above reasons, w2 includes only an error of about −1 to +1 μm, but w1 has an error of about −10 to +10 μm.

The diffraction grating 102 mainly divides BD light out of BD light, DVD light, and CD light emitted from the semiconductor laser 101 into a main beam and two sub beams. In the diffraction grating 102, diffraction grooves are formed so that the three beams of BD light follow the track of the disk. Note that DVD light and CD light are also diffracted by the diffraction grating 102, but the intensity of the sub-beams of these lights is extremely small. The diffraction grating 102 is a composite element of a half-wave plate and a diffraction grating.

The PBS mirror 103 reflects the laser beam incident from the diffraction grating 102 side. The PBS mirror 103 is a thin parallel plate having a square outline on the entrance surface and the exit surface, and a polarizing film is formed on the entrance surface. The diffraction grating 102 is arranged so that the polarization directions of the BD light, DVD light, and CD light are S-polarized with respect to the PBS mirror 103.

The quarter-wave plate 104 converts the laser light reflected by the PBS mirror 103 into circularly polarized light, and converts the reflected light from the disk into linearly polarized light that is orthogonal to the polarization direction toward the disk. As a result, the laser light reflected by the disk passes through the PBS mirror 103 and is guided to the photodetector 110.

The collimator lens 105 converts the laser light reflected by the PBS mirror 103 into parallel light. The lens actuator 106 drives the quarter wavelength plate 104 and the collimator lens 105 in the optical axis direction of the collimator lens 105. By moving the collimator lens 105, the aberration generated in the laser light is corrected.

The rising mirror 107 reflects the laser beam incident from the collimator lens 105 side in the direction toward the objective lens 108 (negative y-axis direction). The objective lens 108 is held by a holder 121, and the holder 121 is driven by an objective lens actuator 122 in a focus direction (y-axis direction) and a tracking direction (radial direction of the disc). By driving the holder 121 in this way, the objective lens 108 is driven in the focus direction and the tracking direction.

Note that the optical axis of the laser beam incident on the rising mirror 107 from the collimator lens 105 side is inclined by 67.5 degrees with respect to the tangential direction of the disk. Further, the alignment direction of the main beam and the two sub beams of the BD light traveling from the rising mirror 107 toward the disk coincides with the tangential direction of the disk.

The reflected light from the disk is converted into linearly polarized light that becomes P-polarized light with respect to the PBS mirror 103 by the quarter-wave plate 104. Thereby, the reflected light from the disk passes through the PBS mirror 103.

The diffraction grating 200 has a so-called two-step type diffraction groove. That is, the diffraction groove formed in the diffraction grating 200 is a rectangular groove having one bottom surface. The depth of the diffraction groove of the diffraction grating 200 is designed so that the zero-order diffraction efficiency is high for BD light and the first-order diffraction efficiency is high for CD light and DVD light. The diffraction grating 200 is arranged so that the diffraction direction is parallel to the x-axis direction. The configuration of the diffraction grating 200 will be described in detail later with reference to FIG.

When DVD light enters the diffraction grating 200, it is diffracted in the first-order diffracted light diffracted in the direction approaching the optical axis of the BD light (x-axis negative direction) and in the direction away from the optical axis of the BD light (x-axis positive direction). 1st order diffracted light is generated. Similarly, when the CD light is incident on the diffraction grating 200, the first-order diffracted light diffracted in the direction approaching the optical axis of the BD light (x-axis negative direction) and the direction away from the optical axis of the BD light (x-axis positive direction). First-order diffracted light is diffracted into Hereinafter, the first-order diffracted light diffracted in the direction approaching the optical axis of the BD light (x-axis negative direction) is referred to as “+ 1st-order diffracted light” and is diffracted in the direction away from the optical axis of the BD light (x-axis positive direction). The first-order diffracted light is expressed as “−1st-order diffracted light”.

The astigmatism plate 109 is a parallel plate and is arranged to be inclined by 45 degrees with respect to the optical axes of BD light, DVD light, and CD light. As a result, astigmatism is introduced to the BD light, DVD light, and CD light incident on the astigmatism plate 109 in a converged state. Since the PBS mirror 103 is a parallel plate and is inclined with respect to the optical axes of the BD light, DVD light, and CD light, the astigmatism is also caused by the PBS mirror 103 in each light. be introduced. The astigmatism plate 109 has a thickness, a refractive index, and an inclination direction so that an appropriate astigmatism is introduced into each light by the astigmatism introduced by itself and the astigmatism introduced by the PBS mirror 103. It has been adjusted.

The light receiving surface of the photodetector 110 is parallel to the xy plane, and a four-divided sensor is disposed on the light receiving surface at a position where BD light, DVD light, and CD light are irradiated. The sensor layout of the photodetector 110 will be described later with reference to FIG.

<Comparative example>
Prior to describing the configuration of the diffraction grating 200 and the photodetector 110 described above, first, the configuration of the diffractive optical element (DOE) 300 according to the comparative example and the sensor layout of the photodetector 110 when the DOE 300 is used. Will be described with reference to FIGS. 2 (a) and 2 (b). In this comparative example, the DOE 300 is used in place of the diffraction grating 200.

FIG. 2A is a diagram for explaining the diffraction action by the DOE 300. In FIG. 2A, the astigmatism plate 109 disposed between the DOE 300 and the light receiving surface of the photodetector 110 is not shown for convenience.

As shown in FIG. 2A, the BD light, the DVD light, and the CD light are incident on the DOE 300 in this order in the positive x-axis direction. The BD light incident on the DOE 300 is divided into a main beam and a sub beam by a diffraction grating 102 disposed immediately after the semiconductor laser 101. The alignment direction of these three beams is a direction corresponding to the tangential direction of the disk, and is slightly inclined with respect to the direction perpendicular to the diffraction direction (the positive x-axis direction) of the DOE 300.

As shown in FIG. 2A, the three beams of BD light incident on the DOE 300 are bent in the positive x-axis direction by the diffraction action of the DOE 300, and −1st order diffracted light is generated. In FIG. 2A, the diffracted light other than the −1st order of the BD light and the diffracted light other than the 0th order of the DVD light and the CD light are not shown for convenience.

The −1st order diffracted light of the main beam diffracted by the DOE 300 overlaps the 0th order diffracted light of the DVD light on the light receiving surface of the photodetector 110. Thus, the optical axis of the main beam of BD light and the optical axis of DVD light coincide on the light receiving surface.

FIG. 2 (b) is a diagram showing four-divided sensors S1 to S4 arranged on the light receiving surface of the photodetector 110. FIG. FIG. 2B shows the four-divided sensors S1 to S4 when the photodetector 110 is viewed from the back surface (in the negative z-axis direction).

The 4-split sensor S1 receives the -1st-order diffracted light of the main beam of BD light and the 0th-order diffracted light of the DVD light, and the 4-split sensors S2 and S3 receive the -1st-order diffracted light of the two sub-beams of BD light. The quadrant sensor S4 receives the 0th order diffracted light of the CD light. The four-divided sensors S1 to S3 are arranged so as to be inclined so that the arrangement direction of the three four-divided sensors matches the arrangement direction of the three beams of BD light shown in FIG. The quadrant sensor S4 is disposed adjacent to the quadrant sensor S1 in the same direction (x-axis direction) as the arrangement direction of the light emitting units 101a to 101c.

The quadrant sensors S1 to S4 are composed of sensors S11 to S14, sensors S21 to S24, sensors S31 to S34, and sensors S41 to S44, respectively. In FIG. 2B, assuming that the arrangement direction of the four-divided sensors S1 to S3 is the Y-axis direction, and the direction perpendicular to the Y-axis direction is the X-axis direction, the X-axis direction and the Y-axis direction are respectively the radial direction of the disc. The direction corresponds to the direction and the tangential direction. The dividing lines of the four-divided sensors S1 to S4 are configured to coincide with the X-axis direction and the Y-axis direction. A reproduction RF signal, a focus error signal, and a tracking error signal are generated by outputs from the respective sensors constituting the four-divided sensors S1 to S4.

Note that the distance between the optical axis of the 0th-order diffracted light of the DVD light and the optical axis of the 0th-order diffracted light of the CD light is equal to the distance w2 between the light emitting

units

101b and 101c shown in FIG. For this reason, the interval between the center of the four-divided sensor S1 and the center of the four-divided sensor S4 is also w2.

In the comparative example, the outlines of the quadrant sensors S1 and S4 are square, and the shapes of the sensors S11 to S14 and S41 to S44 constituting the quadrant sensors S1 and S4 are also square. The outlines of the quadrant sensors S2 and S3 have a shape in which square corners are dropped. Each of the four-divided sensors S1 and S4 is larger than the diameter of the spot (minimum circle of confusion) of the received laser beam.

FIG. 3A is a diagram schematically showing a cross-sectional view when the DOE 300 shown in FIG. 2A is cut along a plane parallel to the xz plane. As shown in FIG. 3A, the DOE 300 includes a blazed diffraction structure. That is, a plurality of diffraction grooves 301 extending linearly in the y-axis direction are formed on the incident surface of the DOE 300 at a constant pitch. As shown in FIG. 3A, the cross-sectional shape of each diffraction groove 301 is a triangle. Due to the diffraction groove 301, a sawtooth blaze is formed on the incident surface of the DOE 300. Strictly speaking, the slope of each diffraction groove 301 is formed in a stepped shape of 8 steps. In this comparative example, the DOE 300 is made of a glass material.

FIG. 3B is a diagram showing the diffraction efficiency of the DOE 300 when the depth of the diffraction groove 301 is changed. On the left side of FIG. 3B, when the pitch of the diffraction grooves 301 is 21.2 μm and the refractive index of the DOE 300 for each light (the refractive index of the glass) is set as shown in the table on the right side of FIG. The simulation results are shown. Here, the wavelengths of BD light, DVD light, and CD light are set to 405 nm, 660 nm, and 785 nm, respectively.

In the semiconductor laser 101, the emission power of BD light is weaker than that of DVD light and CD light. For this reason, the DOE 300 is set so that the −1st-order diffraction efficiency of BD light is higher than the 0th-order diffraction efficiency of DVD light and CD light. Usually, it is desirable that the diffraction efficiency for BD light is about 90%.

By setting the depth d of the diffraction groove 301 to the value of the arrow in FIG. 3B, as shown in the table on the right side of FIG. 3B, the −1st-order diffraction efficiency of the BD light is 88%. The zero-order diffraction efficiencies of DVD light and CD light can be set to 33% and 47%, respectively. Thereby, a desired diffraction efficiency can be achieved for each light.

<Example>
Next, the configuration of the diffraction grating 200 and the sensor layout of the photodetector 110 according to the embodiment will be described with reference to FIGS.

In addition, a present Example respond | corresponds to the structure of Claim 2, 4, and 5.

FIG. 4A is a diagram for explaining the diffraction action by the DOE 300. In FIG. 4A, the astigmatism plate 109 disposed between the diffraction grating 200 and the light receiving surface of the photodetector 110 is not shown for convenience.

As in the case of FIG. 2A, the alignment direction of the main beam and the sub beam of the BD light is a direction corresponding to the tangential direction of the disk, and the diffraction direction of the diffraction grating 20000 (x-axis positive direction). It is slightly inclined with respect to the direction perpendicular to

In this embodiment, the three beams of BD light incident on the diffraction grating 200 are not substantially diffracted by the diffraction grating 200 but pass through the diffraction grating 200 as they are. On the other hand, DVD light and CD light are bent in the negative x-axis direction by the diffraction action of the diffraction grating 200. In FIG. 4A, the diffracted light other than the 0th order of the BD light and the diffracted light other than the + 1st order of the DVD light and the CD light are not shown for convenience.

The + 1st order diffracted light of the DVD light diffracted by the DOE 300 overlaps with the 0th order diffracted light of the main beam of the BD light on the light receiving surface of the photodetector 110. Thus, the optical axis of the main beam of BD light and the optical axis of DVD light coincide on the light receiving surface.

FIG. 4B is a diagram showing the four-divided sensors S1 to S4 arranged on the light receiving surface of the photodetector 110. FIG. FIG. 4B shows the four-divided sensors S1 to S4 when the photodetector 110 is viewed from the back surface (in the negative z-axis direction).

The light received by the four-divided sensors S1 to S4 is the same as in the comparative example. The quadrant sensors S2 and S3 are configured in the same manner as in the comparative example. On the other hand, the four-divided sensors S1 and S4 have a reduced width in the X-axis direction compared to the comparative example.

The distance between the optical axis of the 0th-order diffracted light of the DVD light and the optical axis of the 0th-order diffracted light of the CD light is the distance between the light emitting

units

101b and 101c shown in FIG. Gradually smaller from w2. For this reason, the space | interval of the center of 4-part dividing sensor S1 and the center of 4-part dividing sensor S4 is smaller than w2.

FIGS. 5A and 5B are diagrams showing diffraction states of BD light, DVD light, and CD light in the comparative example and the example, respectively.

As shown in FIG. 5A, the DVD light and the CD light travel straight without being diffracted by the DOE 300. Therefore, as described above, the interval w3 between the optical axis of the DVD light and the optical axis of the CD light on the light receiving surface of the photodetector 110 is the same as the interval w2 between the light emitting

units

101b and 101c of the semiconductor laser 101. .

On the other hand, in this embodiment, the DVD light and the CD light are each diffracted by the diffraction grating 200, and the + 1st order diffracted light enters the photodetector 110. Here, the diffraction angle θ by the diffraction grating 200 is obtained by the following equation.

Sin θ = mλ / d (m: diffraction order)

Therefore, the diffraction angle θc of the + 1st order diffracted light of the CD light is larger than the diffraction angle θd of the + 1st order diffracted light of the DVD light. For this reason, the distance between the optical axis of the 0th-order diffracted light of the DVD light and the optical axis of the 0th-order diffracted light of the CD light gradually decreases as it advances from the diffraction grating 200 in the positive z-axis direction. As a result, in this embodiment, the interval w3 between the optical axis of the DVD light and the optical axis of the CD light on the light receiving surface of the photodetector 110 is smaller than the interval w2 between the light emitting

portions

101b and 101c of the semiconductor laser 101. .

As described above, in the present embodiment, the interval w3 is smaller than the interval w3 in the comparative example, so that the width in the X-axis direction of the four-divided sensors S1, S4 is reduced so that the four-divided sensors S1, S4 do not overlap. As shown in FIG. 4B, the outlines of the four-divided sensors S1 and S4 are rectangles that are long in the Y-axis direction.

Note that the four-divided sensors S1 and S4 are reduced only in the width in the X-axis direction and not reduced in the width in the Y-axis direction. This is for suppressing the influence of the lens shift of the objective lens 108. That is, during actual operation, when the objective lens 108 is displaced in the tracking direction (the radial direction of the disk), the beam spot of each light is displaced in the Y-axis direction in FIG. 4B on the photodetector 110. Thus, even when the beam spot is displaced, the width in the Y-axis direction of the four-divided sensors S1 and S4 is maintained at the same width as in the comparative example so that each beam spot can be properly covered.

FIG. 6A is a diagram schematically showing a cross-sectional view when the diffraction grating 200 shown in FIG. 4A is cut along a plane parallel to the xz plane. As shown in FIG. 3A, the diffraction grating 200 includes a rectangular diffraction groove 201 having one bottom surface. That is, the diffraction grating 200 is a two-step type diffraction grating similar to the diffraction grating 102 shown in FIG. A plurality of diffraction grooves 201 extending linearly in the y-axis direction are formed on the incident surface of the diffraction grating 200 at a constant pitch.

The diffraction groove 201 is formed by forming a protrusion 203 made of SiO _{2 on} the upper surface of the glass substrate 202. The protrusion 203 is formed by forming an SiO ₂ layer on the upper surface of the glass substrate 202 and then etching the SiO ₂ layer. This configuration is generally used for a two-step type diffraction grating at present.

FIG. 6B shows the diffraction efficiency of the diffraction grating 200 when the depth of the diffraction groove 201 is changed. On the left side of FIG. 6B, the pitch of the diffraction grooves 201 is set to 34 μm, and the refractive index of the projection 203 (the refractive index of SiO ₂ ) for each light is set as shown in the table on the right side of FIG. 6B. When the simulation results are shown. The refractive index of the glass substrate 202 is the same as the refractive index in the table on the right side of FIG. Here, the wavelengths of BD light, DVD light, and CD light are set to 405 nm, 660 nm, and 785 nm, respectively. Further, the width of the protrusion 203 in the x-axis direction and the width of the diffraction groove 201 in the x-axis direction are set to be equal.

As described above, in the semiconductor laser 101, the emission power of BD light is weaker than that of DVD light and CD light. For this reason, also in the present embodiment, the diffraction grating 200 is set so that the zero-order diffraction efficiency of the BD light is higher than the + 1st-order diffraction efficiency of the DVD light and the CD light. Usually, it is desirable that the diffraction efficiency for BD light is about 90%.

By setting the depth d of the diffraction groove 201 to the value of the arrow (862 nm) in FIG. 6B, the zero-order diffraction efficiency of the BD light is set to 97 as shown in the table on the right side of FIG. %, And the + 1st order diffraction efficiency of DVD light and CD light can be set to 35% and 40%, respectively. Thereby, although the diffraction efficiency of CD light falls 15% compared with a comparative example, the diffraction efficiency with respect to BD light and DVD light is raised 10% and 16%, respectively.

Thus, even with a two-step simple shape and inexpensive diffraction grating 200, it is possible to achieve a desired diffraction efficiency for each light.

In the present embodiment, the distance in the x-axis direction between the center of the four-divided sensor S1 and the center of the four-divided sensor S4 is a design value (90 μm in this case) shown in FIG. ) Is set equal to the interval w3. However, as described above, an error of about ± 10 μm may occur in the interval w1, and the position of the diffraction grating 200 in the z-axis direction needs to be adjusted according to this error.

7A to 7D are diagrams illustrating a method for adjusting the position of the diffraction grating 200. FIG. Before adjusting the position of the diffraction grating 200, the position of the photodetector 110 is adjusted in advance so that the main beam and the two sub beams of the BD light are respectively positioned at the centers of the four-divided sensors S1 to S3. .

For example, as shown in FIG. 7B, when the interval between the light emitting unit 101a and the light emitting unit 101b is larger than the design value w1 by Δw, as shown in FIG. The spots are shifted in the positive x-axis direction from the centers of the four-divided sensors S1 and S4, respectively. In FIG. 7B, the diffraction grating 200 is positioned at the design position (design position). In this case, as shown in FIG. 7D, the diffraction grating 200 is moved in the negative z-axis direction from the design position, and the spot of the DVD light and the spot of the CD light are moved in the negative x-axis direction. Thereby, as shown in FIG. 7C, the spot of the DVD light and the spot of the CD light are positioned at the centers of the four-divided sensors S1 and S4, respectively. Thus, the position adjustment of the diffraction grating 200 is completed.

Here, the case where the interval between the light emitting unit 101a and the light emitting unit 101b is larger than the design value w1 has been described. However, when the interval between the light emitting unit 101a and the light emitting unit 101b is smaller than the design value w1, diffraction is performed. The grating 200 may be moved from the design position in the positive z-axis direction. Thereby, the spot of DVD light and the spot of CD light are positioned at the centers of the four-divided sensors S1 and S4, respectively.

8A and 8B are diagrams showing the state of the beam spot of each light when the sensor layout according to the comparative example is used, and FIGS. 8C and 8D are the sensors according to the example. It is a figure which shows the state of the beam spot of each light at the time of using a layout.

Referring to FIGS. 8A and 8B, in the comparative example, even when the main beam of BD light, DVD light, and CD light are converged to a focal line state, the beam spot of these lights is divided into four. The size of the four-divided sensors S1 and S4 and the size of the beam spot of each light are adjusted so as to be within the sensors S1 and S4. The size of the beam spot is adjusted by changing the magnification of the return path of the optical system shown in FIG. 1A and the magnitude of the astigmatism action by the astigmatism plate 109.

Referring to FIG. 8C, when the size of the beam spot of each light is set in the same manner as in the comparative example, in the sensor layout of the example, the main beam of BD light, the DVD light, and the CD light are minimally confused. When in a circular state, the beam spots of these lights fall within the four-divided sensors S1 and S4. However, in the sensor layout of the embodiment, since the width in the X-axis direction of the four-divided sensors S1 and S4 is shortened as described above, the main beam of the BD light, the DVD light, and the CD light are converged to the focal line state. In this case, as shown in FIG. 8D, the beam spot of the main beam of BD light and the beam spot of DVD light and CD light protrude from the four-divided sensors S1 and S4. For this reason, in the sensor layout of the embodiment, there is a concern about deterioration of the focus error signal.

Therefore, the inventor of the present application obtained a focus error signal generated in the case of the comparative example and the example by simulation, and evaluated the deterioration of the focus error signal in the example. FIGS. 9A to 9D are diagrams for explaining simulation conditions. FIGS. 9A and 9B are diagrams showing the dimensions of the sensor layouts according to the comparative example and the example, respectively. In FIG. 9A, D1 and D2 are the widths in the Y-axis direction and the X-axis direction of the quadrant sensor S1, D3 and D4 are the widths in the Y-axis direction and the X-axis direction of the quadrant sensor S4, and D5 is This is the distance between the center of the quadrant sensor S1 and the quadrant sensor S4 in the X-axis direction. In FIG. 9B, D11 and D12 are the widths in the Y-axis direction and the X-axis direction of the quadrant sensor S1, D13 and D14 are the widths in the Y-axis direction and the X-axis direction of the quadrant sensor S4, and D15. Is the distance between the center of the quadrant sensor S1 and the quadrant sensor S4 in the X-axis direction.

FIG. 9C is a diagram showing a state where the main beam and the sub beam of the BD light are the minimum circle of confusion. In FIG. 9C, Db is the diameter of the minimum circle of confusion of BD light. FIG. 9D is a diagram illustrating a state when the DVD light and the CD light are each in a minimum circle of confusion. In FIG. 9D, Dd and Dc are the diameters of the minimum circle of confusion of DVD light and CD light, respectively.

In this simulation, D1 to D5 are set as follows.

D1 = 90 μm,
D2 = 90 μm
D3 = 60 μm
D4 = 60 μm
D5 = 88 μm

Also, D11 to D15 are set as follows.

D11 = 90 μm
D12 = 76 μm
D13 = 60 μm
D14 = 49 μm
D15 = 72 μm

Furthermore, Db, Dd, and Dc are set as follows.

Db = 63 μm
Dd = 53 μm
Dc = 40 μm

Note that Db, Dd, and Dc are set to the same value in both the comparative example and the example. Further, the length of the beam when the main beam of the BD light is in the focal line state is equal to the length of the diagonal line of the quadrant sensor S1 in the comparative example, and when the CD light is in the focal line state. The length of the beam is equal to the length of the diagonal line of the quadrant sensor S4 in the comparative example.

10 (a) to 10 (f) are diagrams showing simulation results of the focus error signal when BD light is used. 10A to 10C are diagrams showing simulation results according to the comparative example, and FIGS. 10D to 10F are diagrams showing simulation results according to the example. In this simulation, it is assumed that a focus search is performed on a BD having two recording layers.

FIG. 10A shows the waveform of the focus error signal (main FE) generated based on the output from the quadrant sensor S1 that receives the main beam of BD light. FIG. 10B shows the waveform of the BD light. FIG. 10C shows the waveform of the focus error signal (sub FE) generated based on the outputs from the four-divided sensors S2 and S3 that receive the sub beam. FIG. 10C shows a focus error signal (main FE and sub FE added) ( The waveform of the FE signal) is shown. Similarly, FIGS. 10D, 10E, and 10F show main FE, sub FE, and FE signals according to the embodiment.

Referring to FIGS. 10C and 10F, SL1 and SL2 are Peak-to-Peak values of S-shaped curves for the first recording layer and the second recording layer, respectively. Comparing FIGS. 10C and 10F, the S-shaped level SL1 in the example (the peak-to-peak value of the S-shaped curve for the first recording layer) is 97 of the S-shaped level SL1 in the comparative example. %Met. Therefore, it can be seen that the same focus error signal as in the comparative example can be obtained even if the width in the X-axis direction of the four-divided sensors S1 and S4 is reduced as in the embodiment.

In FIGS. 10C and 10F, S-shaped signals (Fake signals) also appear in the areas A1 and A2. The Fake signal is generated when stray light from the recording layer leaks into the 4-minute sensors S1 to S4. If the amplitude of the Fake signal is large, the Fake signal may be erroneously detected as an S-shaped signal for the second recording layer during focus search. Therefore, the amplitude of the Fake signal needs to be sufficiently smaller than the S-shaped level SL2 for the second recording layer.

In the comparative example, the ratio of the Fake signal to the S-shaped level SL2 was 36%, and in the example, the ratio of the Fake signal to the S-shaped level SL2 was 34%. Therefore, it can be seen that the Fake signal is suppressed as compared with the comparative example by reducing the width in the X-axis direction of the four-divided sensors S1 and S4 as in the embodiment.

11 (a) and 11 (b) are diagrams showing simulation results of a focus error signal when DVD light is used. In this simulation, it is assumed that a focus search is performed on a DVD having two recording layers. FIGS. 11C and 11D are diagrams showing simulation results of the focus error signal when CD light is used. In this simulation, it is assumed that a focus search is performed on a CD having one recording layer. 11A and 11C are diagrams illustrating simulation results according to the comparative example, and FIGS. 11B and 11D are diagrams illustrating simulation results according to the example.

Referring to FIGS. 11A to 11D, although the S-shaped level of the embodiment is slightly lower than the S-shaped level of the comparative example, a focus error signal substantially equivalent to that of the comparative example is obtained. . Therefore, it can be seen that the same focus error signal as in the comparative example can be obtained even if the width in the X-axis direction of the four-divided sensors S1 and S4 is reduced as in the embodiment.

From the above verification, it was confirmed that a focus error signal substantially similar to that of the comparative example can be obtained also by the sensor layout in the example.

<Effect of Example>
According to the present embodiment, the following effects can be achieved.

Since the diffraction grating 200 has a simple structure in which a rectangular diffraction groove 201 having a single bottom surface is formed, it can be generated at a lower cost than the DOE 300 in the comparative example. Further, by diffracting DVD light and CD light without diffracting BD light, as shown in FIG. 6B, the diffraction efficiency for BD light, DVD light and CD light can be increased to the same level as the comparative example. A solution for the groove depth can be obtained. Furthermore, even if DVD light and CD light are diffracted in this way, as verified with reference to FIGS. 10 and 11, the quality of the focus error signal substantially equivalent to that of the comparative example can be obtained. Therefore, according to the present embodiment, the optical pickup device 1 capable of matching the optical axes of two of the three laser beams with an inexpensive diffraction grating 200 while achieving a desired diffraction efficiency. Can be realized.

Furthermore, according to the present embodiment, as shown in FIG. 4B, the width of the quadrant sensor S1, S4 in the X-axis direction is reduced, so that the sensor layout can be made compact. As verified with reference to FIGS. 10 and 11, even with such a compact sensor layout, the quality of the focus error signal substantially equivalent to that of the comparative example can be obtained. Further, as shown in FIG. 4B, since the width in the X-axis direction of the four-divided sensors S1 and S4 is not reduced, even if a lens shift occurs in the objective lens 108 as described above, the sensor output is deteriorated. It does not occur. Thus, according to the present embodiment, the objective lens is configured by configuring the four-divided sensors S1, S4 so that the width in the arrangement direction of the four-divided sensors S1, S4 is smaller than the width in the direction perpendicular to the arrangement direction. The sensor layout can be made compact while suppressing the influence of the lens shift 108.

<Modification 1>
FIGS. 12A and 12B show the configuration of the photodetector 110a according to the first modification. In the first modification, the same diffraction grating 200 as that in the first embodiment is used.

The first modification example exemplifies the configuration described in claim 6. However, the first modification does not limit the configuration of claim 6 in any way.

As shown in FIG. 12A, in the first modification, the optical axis of the 0th-order diffracted light of the main beam of the BD light and the optical axis of the + 1st-order diffracted light of the CD light coincide on the light receiving surface of the photodetector 101a. Two diffracted lights are received by the quadrant sensor S1. As for DVD light, −1st order diffracted light is received by the four-divided sensor S4. The photodetector 110a is arranged at a position away from the diffraction grating 200 in the z-axis positive direction as compared with the above embodiment so that each diffracted light can be received by the corresponding four-divided sensor in this way.

As shown in FIG. 12B, the four-divided sensor S4 receives DVD light having a spot diameter larger than that of CD light, and thus is larger in size than the comparative example. Further, since the interval w3 between the optical axis of the BD light and the optical axis of the DVD light on the light receiving surface is larger than that in the above-described example and the comparative example, the interval between the four-divided sensors S1 and S4 is also the above example and the comparative example. Is bigger than. For this reason, in the first modification, the sensor layout is larger than that in the above-described embodiment and the comparative example.

On the other hand, since a space is generated between the four-divided sensors S1 and S4, the size of the four-divided sensors S1 and S4 and the BD light received by these four-divided sensors S1 and S4, compared to the above-described embodiment and the comparative example. The size of the beam spot of CD light and DVD light can be set large. As described above, by increasing the size of the four-divided sensors S1 and S4 and the size of the beam spot of the BD light, the CD light, and the DVD light, the RF signal for the positional deviation in the xy plane of the photodetector 110a, Degradation of the tracking error signal and the focus error signal can be suppressed. As described above, the size of the beam spot can be adjusted by changing the magnification of the return path of the optical system shown in FIG. 1A and the magnitude of the astigmatism action by the astigmatism plate 109.

Therefore, the configuration of Modification 1 is preferable when the photodetector 110a can be increased in size. In this modified example as well, the optical axes of two of the three laser beams can be made to coincide with each other by the inexpensive diffraction grating 200 while achieving the desired diffraction efficiency, as in the above embodiment. The optical pickup device 1 can be realized.

<Modification 2>
FIGS. 13A and 13B show the configuration of the photodetector 110b according to the second modification. In the second modification, the same diffraction grating 200 as that in the first embodiment is used.

Note that the second modification exemplifies the configuration described in claim 7. However, this modified example 2 does not limit the configuration of claim 7 in any way.

As shown in FIG. 13A, in the modified example 2, the optical axis of the 0th-order diffracted light of the main beam of the BD light and the optical axis of the + 1st-order diffracted light of the DVD light are the light receiving surface of the photodetector 101b, as in the above embodiment. These two diffracted lights coincide with each other and are received by the four-divided sensor S1. In the second modification, the −1st order diffracted light of CD light is received by the four-divided sensor S4. The photodetector 110a is separated from the diffraction grating 200 in the positive z-axis direction by the same distance as in the above embodiment.

In FIG. 13B, the size of the four-divided sensors S1 and S4 is the same as that of the comparative example. In addition, since the interval w3 between the optical axis of the BD light and the optical axis of the CD light on the light receiving surface is larger than that of the above-described embodiment and the comparative example, the interval between the four-divided sensors S1, S4 is also the above-described embodiment and comparative example Is bigger than. For this reason, in the modified example 2, as in the modified example 2, the sensor layout is larger than that in the above-described example and comparative example.

On the other hand, since a space is generated between the four-divided sensors S1 and S4, the size of the four-divided sensors S1 and S4 and the size of the beam spots of BD light, CD light, and DVD light are set large as in the first modification. can do. As a result, as in the first modification, it is possible to suppress degradation of the RF signal, tracking error signal, and focus error signal due to the positional deviation of the photodetector 110a in the xy plane. As described above, the size of the beam spot can be adjusted by changing the magnification of the return path of the optical system shown in FIG. 1A and the magnitude of the astigmatism action by the astigmatism plate 109.

Therefore, the configuration of the second modification is also preferable when the enlargement of the photodetector 110a is allowed, as in the first modification. In this modified example as well, the optical axes of two of the three laser beams can be made to coincide with each other by the inexpensive diffraction grating 200 while achieving the desired diffraction efficiency, as in the above embodiment. The optical pickup device 1 can be realized.

The embodiments and examples of the present invention and the modified examples 1 and 2 have been described above. However, the present invention is not limited to the above-described embodiments and examples, and modified examples 1 and 2, and The embodiment of the present invention can be variously modified in addition to the above.

For example, in the above embodiment, the semiconductor laser 101 is configured such that the BD light emitting unit 101a and the CD light emitting unit 101c sandwich the DVD light emitting unit 101b, but the arrangement order of the light emitting units 101a to 101c is not limited. May be changed to other orders. For example, the semiconductor laser 101 may be configured such that the light emitting unit 101a for BD light and the light emitting unit 101b for DVD light sandwich the light emitting unit 101c for CD light.

In the case of this arrangement order, the light-emitting portion 101c of the CD light whose restriction on the angle of view with respect to the objective lens 108 is the gentlest is positioned at the center of the three light-emitting portions 101a to 101c. Considering the above, this arrangement order is not necessarily a desirable arrangement order. That is, in this arrangement order, when the optical axis of the CD light is made coincident with the optical axis of the objective lens 108, both the BD light and the DVD light are incident on the objective lens 108 with a predetermined angle of view. If one of the optical axes of the light and the DVD light is made coincident with the optical axis of the objective lens 108, the other laser light is incident on the objective lens 108 with a large angle of view. In this case, there is a possibility that a large aberration occurs in the BD light or DVD light.

Therefore, in consideration of the angle of view characteristics with respect to the objective lens 108, the semiconductor laser 101 is preferably configured such that the light emitting unit 101a for BD light and the light emitting unit 101c for CD light sandwich the light emitting unit 101b for DVD light.

In the above embodiment, the so-called three-beam method is applied to the BD light. However, the BD light may be a one-beam method, or the three-beam method is appropriately applied to laser light of other wavelengths. May be.

Further, in the above embodiment, the angle between the optical axis of the laser beam incident on the rising mirror 107 from the collimator lens 105 side and the tangential direction of the disk is 67.5 degrees. It is not limited to. The present invention can also be appropriately applied to an optical pickup device including an optical system having another configuration.

In addition, the embodiment of the present invention can be variously modified as appropriate within the scope of the technical idea shown in the claims.

DESCRIPTION OF SYMBOLS 1 ... Optical pick-up apparatus 101 ... Semiconductor laser (laser light source)
101a ... Light emitting part (first light emitting part)
101b ... Light emitting part (second light emitting part)
101c ... Light emitting part (third light emitting part)
110, 110a, 110b ... Photodetector 200 ... Diffraction grating S1 ... Quadrant sensor (first quadrant sensor)
S4: Quadrant sensor (second quadrant sensor)

Claims

A first light emitting unit that emits a first laser beam having a first wavelength; a second light emitting unit that emits a second laser beam having a second wavelength longer than the first wavelength; A laser light source installed on one package so that a third light emitting unit emitting a third laser beam having a third wavelength longer than the second wavelength is arranged in a straight line;
A photodetector for receiving the first, second and third laser beams;
The first, second, and third laser beams emitted from the laser light source are converged on the first, second, and third discs, respectively, and the first, second, and third discs are converged. An optical system that guides the first, second, and third laser beams reflected from each to the photodetector,
The optical system includes a diffraction grating on which the first, second, and third laser beams entering the photodetector are incident, and a rectangular diffraction groove having one bottom surface is formed on the diffraction grating. And
The diffraction grating is arranged so that the diffraction direction of the diffraction grating is parallel to the alignment direction of the optical axes of the first, second and third laser beams,
The photodetector is arranged at a position where the first laser light not diffracted by the diffraction grating and one of the second and third laser lights diffracted by the diffraction grating overlap each other. A four-divided sensor and a second four-divided sensor disposed at a position where the third laser light diffracted by the diffraction grating is irradiated.
An optical pickup device characterized by that.
The optical pickup device according to claim 1,
The diffraction grating has a zero-order diffraction efficiency of the first laser light higher than that of the other orders of the first laser light, and the first-order diffraction efficiency of the second and third laser lights. However, the depth of the diffraction groove is set to be higher than the diffraction efficiency of the other orders of the second and third laser beams, respectively.
An optical pickup device characterized by that.
The optical pickup device according to claim 2,
The first, second, and third light emitting units are installed in the package such that the first and third light emitting units sandwich the second light emitting unit.
An optical pickup device characterized by that.
The optical pickup device according to claim 3,
Of the two first-order diffracted lights generated from the second laser light by the diffraction grating, the one approaching the 0th-order diffracted light of the first laser light is represented as + 1st-order diffracted light, and the one away from the 0th-order diffracted light is −1. Of the two first-order diffracted lights generated from the third laser light by the diffraction grating, the one approaching the 0th-order diffracted light is represented as the + 1st-order diffracted light, and the one away from the 0th-order diffracted light is − When expressed as first-order diffracted light,
The first four-divided sensor is disposed at a position where the 0th-order diffracted light of the first laser light and the + 1st-order diffracted light of the second laser light overlap, and the second 4-divided sensor is It is arranged at a position where the + 1st order diffracted light of the third laser light is irradiated.
An optical pickup device characterized by that.
The optical pickup device according to claim 4,
In each of the first and second four-divided sensors, one of the two dividing lines is parallel to the diffraction direction by the diffraction grating,
In the second quadrant sensor, the width in the arrangement direction of the first and second quadrant sensors is smaller than the width in the direction perpendicular to the arrangement direction.
An optical pickup device characterized by that.
The optical pickup device according to claim 3,
Of the two first-order diffracted lights generated from the second laser light by the diffraction grating, the one approaching the 0th-order diffracted light of the first laser light is represented as + 1st-order diffracted light, and the one away from the 0th-order diffracted light is −1. Of the two first-order diffracted lights generated from the third laser light by the diffraction grating, the one approaching the 0th-order diffracted light is represented as the + 1st-order diffracted light, and the one away from the 0th-order diffracted light is − When expressed as first-order diffracted light,
The first four-divided sensor is disposed at a position where the 0th-order diffracted light of the first laser light and the + 1st-order diffracted light of the third laser light overlap, and the second 4-divided sensor is Arranged at a position where the −1st order diffracted light of the second laser light is irradiated;
An optical pickup device characterized by that.
The optical pickup device according to claim 3,
Of the two first-order diffracted lights generated from the second laser light by the diffraction grating, the one approaching the 0th-order diffracted light of the first laser light is represented as + 1st-order diffracted light, and the one away from the 0th-order diffracted light is −1. Of the two first-order diffracted lights generated from the third laser light by the diffraction grating, the one approaching the 0th-order diffracted light is represented as the + 1st-order diffracted light, and the one away from the 0th-order diffracted light is − When expressed as first-order diffracted light,
The first four-divided sensor is disposed at a position where the 0th-order diffracted light of the first laser light and the + 1st-order diffracted light of the second laser light overlap, and the second 4-divided sensor is Arranged at a position where the −1st order diffracted light of the third laser light is irradiated;
An optical pickup device characterized by that.
The optical pickup device according to any one of claims 1 to 7,
The first, second and third discs are BD, DVD and CD, respectively.
An optical pickup device characterized by that.