JP2010244588A - Diffraction element for two wavelength, and optical pickup using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively arrange the optical axis of a laser for two wavelengths with a simple configuration. <P>SOLUTION: The diffraction element 1 for two wavelengths on which light having a wavelength λ1 and light having a wavelength λ2 are made incident has a two-dimensional diffraction grating 3 provided on one surface of a transparent substrate 2. The two-dimensional diffraction grating 3 has a step periodic structure formed in a three step shape along a first direction parallel to the transparent substrate 2. The diffraction element 1 further has a one-dimensional diffraction grating 4 provided on the other surface of the transparent substrate 2. The two-dimensional diffraction grating 3 further has a projecting and recessed periodic structure formed in a projecting and recessed shape having a projecting and recessed depth equal to the height of each step of the step periodic structure 5 along a second direction parallel to the transparent substrate 2 and orthogonal to the first direction and the one-dimensional diffraction grating 4 has a one-dimensional projecting and recessed periodic structure formed in a projecting and recessed shape having a recessed depth different from the recessed depth of the projecting and recessed periodic structure along the second direction. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a two-wavelength diffractive element for an optical pickup used in an optical recording medium such as an optical disk using at least two wavelengths of light as a light source, a reproducing apparatus, and the like, and the optical pickup.

  In order to simplify the structure, an optical pickup mounted on an optical disc recording / reproducing apparatus that can use both a CD and a DVD has a light emitting unit in which two light emitting elements of laser beams having different wavelengths are integrally provided. Is installed. In the light emitting section, the light emitting elements are arranged at a minute interval, so that an optical path is formed with the optical axis of the laser light being shifted.

  FIG. 10 is a schematic diagram showing the configuration of a conventional optical pickup 90. As shown in FIG. The light emitting unit 91 provided in the optical pickup 90 includes light emitting elements 91a and 91b that emit laser beams having different wavelengths. The optical pickup 90 includes a collimating lens 93 that collimates laser light emitted from the light emitting unit 91, a polarization beam splitter 92 that transmits incident laser light and reflects light incident from the transmission direction, and 1 A quarter-wave plate 94, an objective lens 95, a cylindrical lens 97, and a light receiving unit 98 are provided.

In the optical pickup 90, one wavelength lambda ₁ of the light emitting element 91a is to design so as to pass through the center of the optical system, light of the wavelength lambda ₁ to form an optical path shown by the solid line in FIG. The light from the other wavelength lambda ₂ of the light-emitting element 1b to form an optical path shown by a broken line. As shown in FIG. 10, the optical axis of the wavelength λ ₂ is formed obliquely, and the return light reflected and returned to the optical disk 96 is also received by the light receiving unit 98 with the optical axis shifted. The detection accuracy of return light at 98 decreases.

Therefore, in order to eliminate this shift, the diffractive element 72 shown in FIG. 11 is disposed in front of the light receiving surface of the light receiving unit 98, whereby the positional shift of the optical axis can be corrected. The diffraction element 72 shown in FIG. 11 has a sawtooth diffraction grating formed on the laser beam exit surface side, and diffracts both laser beams so that their optical axes are received by the light receiving unit 98. It comes to agree on the surface. At this time, the diffraction angle of the wavelength lambda ₁ (780 nm) and theta _1, when the diffraction angle of the wavelength lambda ₂ (660 nm) and θ _2, θ _1> θ ₂ relationship is established. Similarly, there is a diffractive element disclosed in Patent Document 1 as an element for correcting the positional deviation of the optical axis.

  In ordinary optical pickups, various tracking methods have been developed in order to trace the track while following the rotation of the optical disc while the laser beam is focused on the track on the information recording surface of the optical disc. Yes.

  As these tracking methods, for example, a three-beam method is known. In this three-beam method, one beam emitted from a semiconductor laser as a light source is diffracted by a diffraction grating, and three diffracted lights of 0th order and ± 1st order are used as each beam from the diffracted light. . Of these, the 0th-order light is used as a main beam for signal reproduction of pits recorded on the track, but the remaining ± 1st-order light is used as two sub-beams for tracking. Then, it is arranged so as to irradiate with a shift of about 1/4 of the track pitch before and after the pit. As a result, the reflected light from the information recording surface of the three beams is incident on the light receiving elements disposed at appropriate positions, and is converted into electrical signals corresponding to the received light intensity.

In such a three-beam method, a tracking error signal for track tracking servo is obtained by taking the difference in the average value level of the electrical signals on the sub-beam side, and the main beam is moved from above the track using this tracking error signal. Servo control is performed so as not to deviate. That is, when the main beam is scanning the track center, the average level of the sub beams is the same level, but if the main beam deviates from the track center, a difference occurs in the average level of the sub beam, which is detected as a tracking error signal. Is done.

  Another signal detection method is a push in which a beam reflected on the information recording surface is received by a light receiving element divided into two parallel to the track, and a tracking error signal is detected from the output difference at this time. The pull method is also known. In this push-pull method, an offset occurs in the signal with only one beam and the tracking accuracy deteriorates, so the differential push-pull method is used to cancel the offset.

  That is, even in this differential push-pull method, three beams of zero-order light and ± first-order light generated by diffracting one beam emitted from a semiconductor laser by a diffraction grating in the same manner as the above-described three-beam method are obtained. Use. The 0th-order light of the main beam is arranged on the track, the ± 1st-order light is arranged as two sub-beams in an oblique direction with respect to the track line direction, and the track pitch is reduced before and after the pit where the main beam is arranged. The tracks are arranged so as to be irradiated while being shifted in the vertical direction by 1/2. Then, reflected light from the information recording surface is received by the two-divided light receiving elements arranged for the respective beams, and the received light amount is pushed and pulled in the two light receiving portions. In this differential push-pull method, the offset is canceled by subtracting the push-pull value of the main beam and the push-pull value of the sub beam.

  As described above, the three-beam method or the differential push-pull method requires a diffraction grating, and the diffraction grating 81 is provided behind the light emitting unit 1 in FIG. Further, since the wavelength and track pitch of light used for CD and DVD are different, it is necessary to set the grating pitch of the diffraction grating and the inclination direction of the grating to be different for each optical disc. As a laser diffraction grating, there is one disclosed in Patent Document 2, for example.

JP 2002-311221 A (published on October 23, 2002) JP 2001-290017 A (published on October 19, 2001)

  However, when a two-wavelength laser is used in the conventional optical pickup device, an element for aligning the optical axis of the two-wavelength laser and a diffraction grating whose diffraction direction is optimized for the laser light of each wavelength are provided. Since this is necessary, there is a problem that the number of parts increases. An optical element in which these functions are integrated is also disclosed in Patent Document 2. However, there is a problem that a birefringent material is necessary, and a multilayer structure makes it difficult to manufacture at a low cost. is there.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a two-wavelength diffractive element capable of aligning the optical axes of two-wavelength lasers at low cost with a simple configuration, and light using the same. To provide a pickup.

  In order to solve the above-described problems, a two-wavelength diffractive element according to the present invention is a two-wavelength diffractive element on which light having a wavelength λ1 and light having a wavelength λ2 are incident. The two-dimensional diffraction grating has a step periodic structure formed in a step shape of three or more steps along a first direction parallel to the transparent flat plate.

  Due to this feature, the stepped periodic structure of the two-dimensional diffraction grating formed in a stepped shape of three or more steps allows light of wavelength λ1 to pass through without being diffracted and diffracts only light of wavelength λ2. For this reason, it is possible to align the optical axes of the light of wavelength λ1 and the light of wavelength λ2 at a low cost with a simple configuration.

  The two-wavelength diffraction element according to the present invention further includes a one-dimensional diffraction grating provided on the other surface of the transparent flat plate, the two-dimensional diffraction grating being in a direction parallel to the transparent flat plate and the first The one-dimensional diffraction grating further includes a concavo-convex periodic structure formed in a concavo-convex shape having a concavo-convex depth equal to the height of each step of the staircase periodic structure along a second direction orthogonal to the direction. It is preferable to have a one-dimensional uneven periodic structure having an uneven depth different from the uneven depth of the uneven periodic structure and formed in an uneven shape along the second direction.

  According to the above configuration, the concavo-convex periodic structure of the two-dimensional diffraction grating can be configured to generate diffracted light with respect to light of wavelength λ2 and not generate diffracted light with respect to light of wavelength λ1. The one-dimensional concavo-convex periodic structure of the original diffraction grating can be configured not to generate diffracted light with respect to light with wavelength λ2, but to generate diffracted light with respect to light with wavelength λ1. For this reason, the diffraction element for two wavelengths which optimized the diffraction direction is realizable.

In the two-wavelength diffraction element according to the present invention, when the height of each step of the step periodic structure is h and the refractive index of the two-dimensional diffraction grating is n in the two-dimensional diffraction grating, (n−1) ) × h is an integer multiple of the wavelength λ1, and the staircase periodic structure is formed. The unevenness depth of the one-dimensional uneven periodic structure of the one-dimensional diffraction grating: d ₂ , and the refractive index of the one-dimensional diffraction grating: n _2, and the time, it is preferable to form the one-dimensional convex-concave periodic structure so as to be an integral multiple of (n _{2 -1)} × d 2 is the wavelength .lambda.2.

  According to the above configuration, it is possible to realize a two-wavelength diffraction element with a simple configuration and optimized diffraction direction.

  In the two-wavelength diffractive element according to the present invention, the stepped periodic structure of the two-dimensional diffraction grating diffracts the light having the wavelength λ2 among the light having the wavelength λ1 and the light having the wavelength λ2 having different emission points from each other. The optical axes of the light of λ1 and the light of wavelength λ2 are aligned, and the concave-convex periodic structure of the two-dimensional diffraction grating generates a sub-beam that diffracts the light of wavelength λ2 along the second direction. It is preferable that the one-dimensional uneven periodic structure of the diffraction grating generates a sub beam obtained by diffracting the light having the wavelength λ1 along the second direction.

  According to the above configuration, a two-wavelength diffractive element in which the optical axes of the light of wavelength λ1 and the light of wavelength λ2 are aligned and the diffraction direction is optimized can be realized with a simple configuration.

  In the two-wavelength diffraction element according to the present invention, it is preferable that the light of wavelength λ1 and the light of wavelength λ2 enter the two-dimensional diffraction grating and exit from the one-dimensional diffraction grating.

  Since the two-dimensional diffraction grating is optimally designed for a light beam incident at a specific wavelength and at a specific angle, a large amount of stray light is generated in a light beam incident at other angles. Therefore, in the configuration in which three beams are generated by the one-dimensional diffraction grating and then guided to the two-dimensional diffraction grating, a lot of stray light is generated. Therefore, stray light can be suppressed by configuring the light of wavelength λ1 and the light of wavelength λ2 to be incident on the two-dimensional diffraction grating and emitted from the one-dimensional diffraction grating.

The two-wavelength diffractive element according to the present invention is a two-wavelength diffractive element on which light of wavelength λ1 and light of wavelength λ2 are incident, and is a first two-dimensional diffraction grating provided on one surface of a transparent flat plate And a second two-dimensional diffraction grating provided on the other surface of the transparent flat plate, wherein the first two-dimensional diffraction grating has three or more steps along a first direction parallel to the transparent flat plate. The first two-dimensional diffraction grating has a stepped structure of three or more steps along a second direction opposite to the first direction. The stepwise inclination direction of the first stepwise periodic structure and the stepwise inclination direction of the second stepwise periodic structure are parallel to each other. Features.

  According to this feature, the light of wavelength λ2 is diffracted by the first step periodic structure and further diffracted by the second step periodic structure, so that the optical axis of the light of wavelength λ2 becomes the optical axis of the light of wavelength λ1. Can be aligned.

  In the two-wavelength diffraction element according to the present invention, the first two-dimensional diffraction grating is formed in a concavo-convex shape along a third direction that is parallel to the transparent flat plate and perpendicular to the first direction. The second two-dimensional diffraction grating further includes a second concave-convex periodic structure formed in a concave-convex shape along the third direction; The height of each step of the first staircase periodic structure is equal to the height of each step of the second staircase periodic structure, the unevenness depth of the first uneven periodic structure, and the height of the second uneven periodic structure. It is preferable that the depth of the unevenness is different.

  According to the above configuration, a two-wavelength diffraction element with an optimized diffraction direction can be realized.

In the two-wavelength diffraction element according to the present invention, in the first and second two-dimensional diffraction gratings, the height of each step of the first and second step periodic structures: h, the first and second When the refractive index of the two-dimensional diffraction grating is n, the first and second step periodic structures are formed so that (n−1) × h is an integral multiple of the wavelength λ1, and the second unevenness When the concave-convex depth of the periodic structure is d ₂ and the refractive index of the second two-dimensional diffraction grating is n ₂ , the (n ₂ −1) × d ₂ is an integral multiple of the wavelength λ ₂ . It is preferable to form 2 irregular periodic structures.

  In the two-wavelength diffractive element according to the present invention, the first and second step periodic structures diffract light having a wavelength λ2 out of light having a wavelength λ1 and light having a wavelength λ2 that have different emission points. The optical axes of the light of wavelength λ1 and the light of wavelength λ2 are aligned, and the first concave-convex periodic structure generates a sub-beam that diffracts the light of wavelength λ1 along the third direction. The concave-convex periodic structure preferably generates a sub beam in which the light with the wavelength λ2 is diffracted along the third direction.

  In the two-wavelength diffraction element according to the present invention, it is preferable that the light of wavelength λ1 and the light of wavelength λ2 are incident on the first two-dimensional diffraction grating and emitted from the second two-dimensional diffraction grating. .

  An optical pickup according to the present invention includes the two-wavelength diffraction element according to the present invention.

  The two-wavelength diffractive element according to the present invention has a step periodic structure formed in three or more steps along the first direction parallel to the transparent flat plate. There is an effect that the optical axes of the light and the light of wavelength λ2 can be aligned.

1 is a diagram illustrating a configuration of an optical pickup according to Embodiment 1. FIG. It is a perspective view which shows the structure of the diffraction element for two wavelengths provided in the said optical pick-up. (A) is sectional drawing which shows the structure of the staircase periodic structure formed in the said diffraction element for two wavelengths, (b) is sectional drawing for demonstrating operation | movement of the said diffraction element for two wavelengths. (A) is sectional drawing which shows the structure of the uneven | corrugated periodic structure formed in the said diffraction element for two wavelengths, (b) is sectional drawing for demonstrating other operation | movement of the said diffraction element for two wavelengths. (A) is a graph showing the relationship between the staircase depth H of the light of wavelength λ1 and the diffraction efficiency, and (b) shows the relationship between the staircase depth H of the light of wavelength λ2 and the diffraction efficiency. It is a graph. 6 is a diagram illustrating a configuration of an optical pickup according to Embodiment 2. FIG. It is a perspective view which shows the structure of the diffraction element for two wavelengths provided in the said optical pick-up. (A) is sectional drawing which shows the structure of the 1st step periodic structure formed in the said diffraction element for two wavelengths, (b) is sectional drawing for demonstrating operation | movement of the said diffraction element for two wavelengths. (A) is sectional drawing which shows the structure of the 1st and 2nd uneven | corrugated periodic structure formed in the said 2 wavelength diffraction element, (b) is for demonstrating other operation | movement of the said 2 wavelength diffraction element. It is sectional drawing. It is a figure which shows the structure of the conventional optical pick-up. It is a figure for demonstrating the function of the diffraction element mounted in the conventional optical pick-up.

  Hereinafter, embodiments of the present invention will be described in detail.

(Embodiment 1)
FIG. 1 is a diagram illustrating a configuration of the optical pickup 14 according to the first embodiment. An optical pickup 14 shown in FIG. 1 includes a light emitting unit 15 incorporating a semiconductor laser diode, a collimating lens 16 that collimates laser light emitted from the light emitting unit 15, a quarter-wave plate 17, and an optical disc. The objective lens 19 that condenses the light 18, the light receiving unit 20 that detects the return light from the optical disk 18, the cylindrical lens 21 that collects the return light to the light receiving unit 20, and the laser light emitted from the light emitting unit 15. A polarizing beam splitter 22 that reflects the return light from the optical disk 18 and guides it to the light receiving unit 20, and the two wavelengths of the present embodiment disposed between the light emitting unit 15 and the polarizing beam splitter 22. The diffraction element 1 is provided.

  In the light emitting section 15, a light emitting element 23a for forming a light emitting point for laser light with a wavelength of 660 nm (λ1) for DVD, and a light emitting element 23b for forming a light emitting point for laser light with a wavelength of 780 nm (λ2) for CD. However, they are arranged in a state of being spaced apart by a minute distance. For this reason, since the emission points are shifted, the optical axes of laser beams of different wavelengths emitted from the light emitting unit 15 are shifted from each other. The diffractive element 1 of the present embodiment is provided immediately after the light emitting unit 15, aligns the optical axes of the laser light, and generates three beams at an optimum angle with respect to light of each wavelength.

  The laser beams having the wavelengths λ1 and λ2 emitted from the light emitting unit 15 are transmitted through the two-wavelength diffraction element 1 so that their optical axes are aligned, and after further generating two sub beams, are transmitted through the polarization beam splitter 22. Then, after being collimated by the collimator lens 16, it is transmitted through the quarter-wave plate 17 and enters the objective lens 19. The laser light condensed by the objective lens 19 forms spot light on the optical disk 18. The return light reflected by the optical disk 18 is guided to the polarization beam splitter 22 through the objective lens 19, the quarter wavelength plate 17, and the collimator lens 16. The laser light emitted from the light emitting unit 15 is transmitted through the quarter-wave plate 17 in the forward path and the return path, whereby the direction of polarization is rotated by 90 degrees, and the return light is reflected by the polarization beam splitter 22 and passes through the cylindrical lens 21. Guided to the light receiving unit 20.

  FIG. 2 is a perspective view showing the configuration of the two-wavelength diffraction element 1. As shown in FIG. 2, the two-wavelength diffraction element 1 includes a transparent substrate (transparent flat plate) 2. A two-dimensional diffraction grating 3 is formed on the laser beam incident surface side of the transparent substrate 2, and a one-dimensional diffraction grating 4 is formed on the emission surface side. The two-dimensional diffraction grating 3 is in two directions, the X direction and the Y direction. The one-dimensional diffraction grating 4 has a periodic structure along the Y direction.

3A is a cross-sectional view showing the configuration of the staircase periodic structure 5 formed in the two-wavelength diffraction element 1, and FIG. 3B is a cross-sectional view for explaining the operation of the two-wavelength diffraction element 1. FIG. . With respect to the XZ cross section, as shown in FIG. 3A, the step periodic structure 5 of the two-dimensional diffraction grating 3 has a plurality of convex portions 24a periodically formed in a saw-tooth shape. When viewed macroscopically, each convex portion 24a is formed in a right triangle shape with a vertical surface and an inclined surface, and when viewed microscopically, step portions S1, S2, and S3 are formed stepwise on each inclined surface. Yes.
The stepped portion has a shape in which the flat surface gradually changes in height and the number of steps is 3 or more, and the step depth, which is the distance from the lowermost flat surface to the uppermost flat surface, is H, 1 per step. The step depth is h, the period of the convex portion is P _x , and the width dimension W _{x in} the X direction of each step. W _x and h are formed at an equal pitch (that is, h = H / 2, Wx = Px / 3).
A phase difference (formed by a difference in refractive index from air) generated when incident light having a wavelength λ passes through the staircase periodic structure 5 is expressed as follows. If the refractive index of the two-dimensional diffraction grating 3 is n (λ), It is represented by Formula 1.

φ (λ) = 2π · (n (λ) −1) h / λ Formula 1
This phase difference φ (λ) is set so as to satisfy Equation 2 for _one wavelength λ ₁ of the laser beam and satisfy Equation 3 for the other wavelength λ ₂ . However, N ₁ and N ₂ are positive integers.

φ (λ ₁ ) = 2N ₁ π (N ₁ is an integer) Equation 2
φ (λ ₂ ) ≠ 2N ₂ π (N ₂ is an integer) Equation 3
As shown in Equation 2, by setting the depth dimension h so that (n (λ) −1) · h is an integral multiple of the wavelength λ, the laser light with the wavelength λ is reflected in the two-dimensional diffraction element 3. When the laser beam passes through the laser beam, the laser beam having the wavelength λ is not diffracted. So as to satisfy the equation 2 with respect to laser light having a wavelength of lambda _1, by setting the 1-stage depth h so as to satisfy the equation 3 with respect to laser light having a wavelength lambda _2, the wavelength lambda ₁ of the laser beam There can be transmitted without diffracting the laser light having entered into the two-dimensional diffractive element 3 0 in the state of the diffracted light, it can be diffracted only the laser beam having a wavelength lambda _2.

As shown in FIG. 3B, when a laser beam having a wavelength λ ₂ is incident on the two-dimensional diffractive element 3, the diffraction angle shown in Expression 4 (the zero-order optical axis and the generation direction axis of the first-order diffracted light form). Diffracted light is generated in the direction of (angle) θx.

2π / λ ₂ · sin (θ _x ) = 2π / P _x Equation 4
If each light emitting element is arranged so as to satisfy the relationship of Equation 5, where L is the distance from the light emitting point to the two-dimensional diffraction grating 3 and D is the distance between the two light emitting points, as shown in FIG. In addition, the optical axes of the light of wavelength λ _{1 and} the light of wavelength λ ₂ can be aligned.

L · tan (θ _x ) = D Equation 5
FIG. 5 shows the calculation results of the diffraction efficiency when the step depth H of the step periodic structure 5 is changed. The refractive index n of the diffraction grating layer (two-dimensional diffraction grating 3) was 1.55, the wavelength λ1 was 660 nm, and the wavelength λ2 was 780 nm. FIG. 5A shows the result of the wavelength λ1, and FIG. 5B shows the result of the wavelength λ2. It can be seen that near the staircase depth H = 4.7 μm, the 0th-order diffracted light with a wavelength of 660 nm can be maximized, and for the light with a wavelength of 780 nm, the −1st-order diffracted light can be maximized.

FIG. 4A is a cross-sectional view showing a configuration of an uneven periodic structure formed in the two-wavelength diffraction element 1, and FIG. 4B is a cross-sectional view for explaining another operation of the two-wavelength diffraction element 1. FIG. is there. As for the YZ cross section, as shown in FIG. 4A, the two-dimensional diffraction grating 3 has a rectangular uneven periodic structure 6 formed thereon. Similarly, the one-dimensional diffraction grating 4 has a rectangular primary shape. An original uneven periodic structure 7 is formed. As shown in FIG. 4B, the two-dimensional diffraction grating 3 generates diffracted light in the direction in the YZ plane with respect to light having the wavelength λ ₂ and diffracts light with wavelength λ _1. is configured so as not to generate light, one-dimensional diffraction grating 4, conversely to generate diffracted light in the direction of the Y-Z plane with respect to the wavelength lambda ₁ of light, the diffracted light with respect to the wavelength lambda ₂ of light It is configured not to generate.

  Regarding the shapes of the uneven periodic structure 6 and the one-dimensional uneven periodic structure 7 in the YZ section of the two-dimensional diffraction grating 3 and the one-dimensional diffraction grating 4, the uneven depth d of the uneven part, the convex refractive index n, and the period Py. The convex portion width Wy is individually set for each of the uneven periodic structure 6 and the one-dimensional uneven periodic structure 7. Using the unevenness depth d of the uneven portion of this diffraction grating and the refractive index n (λ) of the protruded portion, the phase difference formed by the difference in refractive index between the incident light of wavelength λ and air is expressed by Equation 6. .

When the shape of the concavo-convex periodic structure 6 of the two-dimensional diffraction grating 3 is formed so as to satisfy the equation 7 for the incident light of the wavelength λ _{1 and} the equation 8 for the incident light of the wavelength λ ₂ , the two-dimensional The diffraction grating 3 diffracts only the light with the wavelength λ ₂ in the direction in the YZ plane.
φ (λ) = 2π · (n (λ) −1) d / λ Equation 6
φ (λ ₁ ) = 2N ₃ π (N ₃ is an integer) Equation 7
φ (λ ₂ ) ≠ 2N ₄ π (N ₄ is an integer)
Further, when the shape of the concavo-convex structure of the second diffraction grating is formed so as to satisfy Equation 9 for incident light of wavelength λ ₁ and Equation 10 for incident light of wavelength λ ₂ , the second diffraction grating is in the direction of the Y-Z plane diffracts only light of the wavelength lambda _1.

φ (λ ₁ ) ≠ 2N ₅ π (N ₅ is an integer) Equation 9
φ (λ ₂ ) = 2N ₆ π (N ₆ is an integer)
The efficiency of the light diffracted by the two-wavelength diffraction grating 1 formed in this way is obtained based on the scalar theory, and the zero-order light diffraction efficiency η ₀ (λ) with respect to the incident light having the wavelength λ and ± 1 next time The folding efficiency η _{± 1} (λ) is expressed by Expression 11 and Expression 12, respectively.

η ₀ (λ) = 4 (W / P−1 / 2) ² sin ² (φ (λ) / 2)
+ Cos ² (φ (λ) / 2) Equation 11
η _{± 1} (λ) = 4 / π ² · sin ² (πW / P) · sin ² (φ (λ) / 2) Equation 12
When Expressions 7 and 10 are satisfied, the 0th-order light diffraction efficiency is 100% and the first-order light diffraction efficiency is 0% regardless of the duty ratio (W / P).

Diffraction angle by the two-dimensional diffraction grating 3 (angle formed by the 0th-order optical axis and the generation direction axis of the 1st-order diffracted light) θ ₁ satisfies Expression 13.

2π / λ ₂ · sin (θ ₁ ) = 2π / P ₁ Formula 13
Similarly, in the one-dimensional diffraction grating 4,
2π / λ ₁ · sin (θ ₂ ) = 2π / P ₂ Formula 14
Diffracted light is generated in the angle direction θ ₂ that satisfies the above.

  Since the track pitch is different between the CD system and the DVD system optical disk, the direction of generation of the diffracted light needs to be optimized for each optical disk. In the two-wavelength diffraction element 1 according to the present embodiment, the shapes of the two-dimensional diffraction grating 3 and the one-dimensional diffraction grating 4 can be designed independently, so that the two-dimensional diffraction grating 3 satisfies the above conditions. And the characteristics (diffraction efficiency, diffraction angle) of the one-dimensional diffraction grating 4 can be optimized. Further, since the element for correcting the optical axis of the two-wavelength laser and the diffraction grating for generating three beams can be integrated, the number of components can be reduced.

  The configuration of the two-wavelength diffraction element 1 according to the present embodiment is such that a two-dimensional diffraction grating 3 and a one-dimensional diffraction grating 4 are provided on both surfaces of a transparent substrate 2. As a material for the diffraction grating 4, a thermosetting resin, a thermoplastic resin, a photocurable resin, or the like can be used, and the period of these diffraction gratings may be several μm to several tens of μm and the depth is several μm. Since it is easy to transfer with a mold, it can be manufactured at low cost. As a method for producing the two-wavelength diffraction element 1, the two-dimensional diffraction grating 3 and the one-dimensional diffraction grating 4 may be molded on both surfaces of the transparent substrate 2, or the transparent substrate 2, the two-dimensional diffraction grating 3, and the primary The original diffraction grating 4 may be the same member and may be integrally molded using a mold. In the two-wavelength diffraction element 1 described above, the number of steps of the step periodic structure 5 of the two-dimensional diffraction grating 3 is three, but the present invention is not limited to this. Further, the number of steps may be increased.

(Embodiment 2)
FIG. 6 is a diagram showing a configuration of the optical pickup 14a according to the second embodiment. The same reference numerals are given to the same components as those described in the first embodiment. Detailed description of these components will not be repeated. The optical pickup 14a includes a two-wavelength diffraction element 1a.

  FIG. 7 is a perspective view showing the configuration of the two-wavelength diffraction element 1a. As shown in FIG. 7, the two-wavelength diffraction element 1 a includes a transparent substrate 2. A two-dimensional diffraction grating 9 is formed on the incident surface side of the transparent substrate 2 and a two-dimensional diffraction grating 8 is formed on the exit surface side. The two-dimensional diffraction grating 9 and the two-dimensional diffraction grating 8 are both in the X direction and the Y direction. It has a periodic structure along two directions.

  FIG. 8A is a cross-sectional view showing the configuration of the staircase periodic structure 12 formed in the two-dimensional diffraction grating 9 of the two-wavelength diffraction element 1a, and FIG. 8B shows the operation of the two-wavelength diffraction element 1a. It is sectional drawing for demonstrating. Regarding the XZ cross section, as shown in FIG. 8A, the staircase periodic structure 12 has a plurality of convex portions 24b continuously formed in a saw-tooth shape, and each convex portion 24b as in the first embodiment. When viewed macroscopically, a vertical plane and an inclined surface are formed in a right triangle shape, and when viewed microscopically, stepped portions S1, S2, and S3 are formed on each inclined surface in a stepped manner. The second embodiment is different from the first embodiment in that the stepped periodic structure 10 similar to the two-dimensional diffraction grating 9 is used for the two-dimensional diffraction grating 8 on the exit surface side. The inclined surfaces of the staircase periodic structure 12 and the staircase periodic structure 10 are formed to be parallel to each other. Thus, the staircase-like inclination direction of the staircase periodic structure 12 and the staircase-like inclination direction of the staircase periodic structure 10 are parallel to each other.

As shown in FIG. 8B, both the two-dimensional diffraction gratings 8 and 9 do not generate diffracted light with respect to the laser light with the wavelength λ ₁ but generate diffracted light with respect to the laser light with the wavelength λ _2. Set the structure. With the shape shown in FIG. 8B, the first-order diffracted light of the λ ₂ laser light in the staircase periodic structure 12 of the two-dimensional diffraction grating 9 is diffracted to the optical axis side of the λ ₁ laser light, When the two-dimensional diffraction grating 8 provided on the opposite side of the two-dimensional diffraction grating 9 diffracts the laser beam having the wavelength λ _{2 to} the opposite side, the optical axis of the laser beam having the wavelength λ _{2 and} the laser beam having the wavelength λ ₁ are used. The optical axis can be made coincident with each other.

Assuming that the refractive index of the transparent substrate 2 is n _p , diffracted light is generated in the direction of the diffraction angle θx that satisfies the relationship of Expression 15.

2π / λ ₂ · n _p · sin (θ _x ) = 2π / P _x Equation 15
Assuming that the thickness of the transparent substrate 2 is T and the distance between both light emitting points is D, the optical axes of the wavelengths λ ₁ and λ ₂ can be aligned if the relationship of Expression 16 is satisfied.

T · tan (θ _x ) = D Equation 16
In a two-wavelength laser in which a CD laser element and a DVD laser element are monolithically integrated, variation in the emission point interval is managed with an accuracy of ± 1 μm or less. Therefore, the thickness T of the transparent substrate 2, by appropriately managing the diffraction angle theta _x, can be made unnecessary alignment of the two dimensional diffraction element 1a.

  For the YZ plane, as in the first embodiment, as shown in FIGS. 9A and 9B, the two-dimensional diffraction grating 9 is set so as to generate diffracted light only for light of one wavelength. Then, the two-dimensional diffraction grating 8 is set so that diffracted light is generated only for light of the other wavelength. The two-dimensional diffraction grating 9 has an uneven periodic structure 13. The two-dimensional diffraction grating 8 has an uneven periodic structure 11.

This two dimensional diffraction element 1a can be moved parallel to the optical axis of the wavelength lambda ₂ of the light to match the optical axis of the wavelength lambda ₁ of light. For this reason, adjustment of the distance between the light emitting point and the two-dimensional diffraction element 1a can be made unnecessary. Furthermore, since the element for correcting the optical axis of the two-wavelength laser and the diffraction grating for generating three beams can be integrated, the number of parts can be reduced.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  INDUSTRIAL APPLICABILITY The present invention can be used for a two-wavelength diffractive element for an optical pickup used for a recording device or a reproducing device for an optical recording medium such as an optical disk using light of at least two wavelengths as a light source and the optical pickup.

1 Two-wavelength diffraction element 2 Transparent substrate (transparent flat plate)
3 Two-dimensional diffraction grating 4 One-dimensional diffraction grating 5 Staircase periodic structure 6 Concavity and convexity structure 7 One-dimensional concavity and convexity structure 8 Two-dimensional diffraction grating 9 Two-dimensional diffraction grating 10 Staircase periodic structure 11 Concavity and convexity structure 12 Staircase periodic structure 13 Concavity and convexity structure 14 Optical pickup

Claims (11)

A two-wavelength diffractive element in which light of wavelength λ1 and light of wavelength λ2 are incident,
A two-dimensional diffraction grating provided on one surface of the transparent flat plate,
2. The two-wavelength diffraction element according to claim 2, wherein the two-dimensional diffraction grating has a stepped periodic structure formed in a stepped shape of three or more steps along a first direction parallel to the transparent flat plate. A one-dimensional diffraction grating provided on the other surface of the transparent flat plate;
The two-dimensional diffraction grating has a concavo-convex depth equal to a height of each step of the step periodic structure along a second direction that is parallel to the transparent flat plate and perpendicular to the first direction. It further has an uneven periodic structure formed in a shape,
The one-dimensional diffraction grating has a one-dimensional uneven periodic structure having an uneven depth different from the uneven depth of the uneven periodic structure and formed in an uneven shape along the second direction. The diffraction element for two wavelengths of description. In the two-dimensional diffraction grating,
Height of each step of the staircase periodic structure: h,
Refractive index of the two-dimensional diffraction grating: n,
Then, the step periodic structure is formed so that (n−1) × h is an integral multiple of the wavelength λ1,
Concave and convex depth of the one-dimensional concave and convex periodic structure of the one-dimensional diffraction grating: d ₂
Refractive index of the one-dimensional diffraction grating: n ₂
3. The two-wavelength diffraction element according to claim 2, wherein the one-dimensional concavo-convex periodic structure is formed such that (n ₂ −1) × d ₂ is an integral multiple of the wavelength λ2. The staircase periodic structure of the two-dimensional diffraction grating diffracts light of wavelength λ2 out of light of wavelength λ1 and light of wavelength λ2 having different light emission points to produce light of light of wavelength λ1 and light of wavelength λ2. Align axes,
The concave-convex periodic structure of the two-dimensional diffraction grating generates a sub-beam obtained by diffracting the light of the wavelength λ2 along the second direction,
3. The two-wavelength diffraction element according to claim 2, wherein the one-dimensional uneven periodic structure of the one-dimensional diffraction grating generates a sub beam obtained by diffracting the light of the wavelength λ <b> 1 along the second direction.   3. The two-wavelength diffraction element according to claim 2, wherein the light of wavelength λ <b> 1 and the light of wavelength λ <b> 2 enter the two-dimensional diffraction grating and exit from the one-dimensional diffraction grating. A two-wavelength diffractive element in which light of wavelength λ1 and light of wavelength λ2 are incident,
A first two-dimensional diffraction grating provided on one surface of the transparent flat plate, and a second two-dimensional diffraction grating provided on the other surface of the transparent flat plate,
The first two-dimensional diffraction grating has a first step periodic structure formed in a step shape of three or more steps along a first direction parallel to the transparent flat plate,
The second two-dimensional diffraction grating has a second step periodic structure formed in a step shape of three or more steps along a second direction opposite to the first direction,
The two-wavelength diffraction element, wherein the stepwise inclination direction of the first staircase periodic structure and the stepwise inclination direction of the second staircase periodic structure are parallel to each other. The first two-dimensional diffraction grating further includes a first uneven periodic structure formed in an uneven shape along a third direction that is parallel to the transparent flat plate and perpendicular to the first direction. And
The second two-dimensional diffraction grating further includes a second uneven periodic structure formed in an uneven shape along the third direction,
The height of each step of the first staircase periodic structure is equal to the height of each step of the second staircase periodic structure,
The two-wavelength diffraction element according to claim 6, wherein the uneven depth of the first uneven periodic structure is different from the uneven depth of the second uneven periodic structure. In the first and second two-dimensional diffraction gratings,
Height of each step of the first and second staircase periodic structures: h,
Refractive index of the first and second two-dimensional diffraction gratings: n,
The first and second step periodic structures are formed so that (n−1) × h is an integral multiple of the wavelength λ1,
Concave and convex depth of the second concave and convex periodic structure: d ₂
Refractive index of the second two-dimensional diffraction grating: n ₂
The two-wavelength diffraction element according to claim 7, wherein the second uneven periodic structure is formed so that (n ₂ −1) × d ₂ is an integral multiple of the wavelength λ2. The first and second staircase periodic structures diffract light of wavelength λ2 out of light of wavelength λ1 and light of wavelength λ2 having different emission points from each other to obtain light of wavelength λ1 and light of wavelength λ2. Align the optical axes,
The first concave-convex periodic structure generates a sub beam obtained by diffracting the light of the wavelength λ1 along the third direction,
The two-wavelength diffractive element according to claim 7, wherein the second uneven periodic structure generates a sub beam obtained by diffracting the light having the wavelength λ <b> 2 along the third direction.   The two-wavelength diffraction element according to claim 6, wherein the light of wavelength λ1 and the light of wavelength λ2 are incident on the first two-dimensional diffraction grating and are emitted from the second two-dimensional diffraction grating. An optical pickup comprising the two-wavelength diffraction element according to claim 1.

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