JP3172360B2 - Reflection grating for beam splitting and optical pickup using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection type diffraction grating for dividing a beam into three beams which is used with an inclination with respect to the optical axis of a divergent laser light source, and an optical pickup using the same.

[0002]

2. Description of the Related Art A three-beam method is known as one of tracking servo methods for an optical pickup.
In the case of using the beam method, in order to avoid the disadvantage of the transmission type beam splitting diffraction grating that the signal light is incident on the diffraction grating again to cause a loss, a reflection type beam splitting diffraction grating is used. Have been. The reflection type diffraction grating forms irregularities on the substrate surface by etching to form Al or Cr and A.
It is manufactured by a manufacturing method such as evaporation of u.

FIG. 9 is a vertical sectional side view showing an optical pickup using a reflection type diffraction grating for dividing a beam into three. Reference numeral 1 in the figure denotes a good heat conductive substrate made of a conductive semiconductor material such as n ⁺ -type Si or a conductive metal such as copper. The first heat-conductive substrate 1 has a first flat portion 1a, a 45 ° inclined surface portion 1
b, and the second plane portion 1c. A semiconductor laser chip 4 is provided on the first plane portion 1a via a submount 2 having a PIN photodiode portion for adjusting light quantity. The 45-degree inclined surface portion 1b is provided with a reflective diffraction grating 6 for dividing the beam into three. Then, a six-division PIN photodiode 5 for signal detection is mounted on the second plane part 1c.

A hologram element 7 is arranged above these element arrangement surfaces by support means (not shown), and a condensing lens 8 attached to an actuator (not shown) is arranged above the hologram element 7. I have. When the light emitted from the semiconductor laser chip 4 is reflected by the three-beam reflective diffraction grating 6, it is split into three beams by a diffraction effect. And these three beams
After passing through the hologram element 7 (zero-order diffraction), the light is condensed on the signal recording surface of the optical disk 9 by the condenser lens 8.

The reflected light of the three beams on which the pit signal (HF signal), the tracking error signal, and the focusing error signal have passed passes through the condenser lens 8 again, and is then subjected to first-order diffraction (or-) by the hologram element 7. (First-order diffraction) and is incident on the PIN photodiode 5 for signal detection. Note that the focusing error signal can be detected by the astigmatism method using a 4-division PIN photodiode because the hologram element 7 has a function of giving astigmatism to the diffracted light.

In an optical pickup using such a three-beam reflective diffraction grating 6, the elements can be arranged on substantially the same plane except for the hologram element 7, and the semiconductor laser 4 and the photodiode 5 In such a case, it is easy to produce wire bonding and the like, and cost reduction can be achieved.

[0007]

By the way, the shape of the condensed spot of the semiconductor laser light becomes an elliptical shape reflecting the shape of the near-field image on the emission surface of the semiconductor laser, and the S / N ratio for pit signal detection is taken into consideration. Then, it is necessary to perform the three-beam division in the direction perpendicular to the semiconductor laser element bonding surface (diffraction in the plane of FIG. 9; see FIG. 10).

FIG. 10 shows how a light beam emitted from a semiconductor laser 4 is split and diffracted by a three-beam reflecting diffraction grating 6 arranged at an angle of 45 ° and reflected by a virtual light source. FIG. 4 is an explanatory diagram showing a state where is formed at a position symmetrical to the semiconductor laser 4 with respect to the diffraction grating surface. In the figure, 32a is a 0th-order virtual light source, 32b is a + 1st-order virtual light source, and 32c is a -1st-order virtual light source. In the drawing, X indicates the direction of inclination of the three-beam splitting reflection diffraction grating 6 and the direction in which the laser light incident angle becomes large. O indicates an origin (a point coinciding with the optical axis). x 'indicates the distance of the virtual light source in the direction corresponding to the direction X (0 is the origin), and d indicates the position of the virtual light source on x'.

As shown in FIG. 11, the conventional reflection grating 6 for dividing a beam into three beams is composed of a regular-period grating in which the grating pitch (Λ ₀ ) is all equal in the X direction in the figure. When the light intensity distribution of the virtual emission point of the ± 1st-order diffracted light is calculated, the distribution becomes considerably wide as shown in FIG.
That is, the light is not focused on one point and cannot be focused on a spot having a small diffraction limit. The position of 110 μm in FIG. 12 corresponds to the position of 20 μm on the optical disc (2
0 μm × lens magnification 5.5 = 110 μm).

Further, as shown in FIG. 10, since the incident angles on the upper side and the lower side of the diffraction grating with respect to the optical axis 23 of the semiconductor laser are asymmetrical, the + 1st-order virtual light source 32b and-
The spot interval of the primary virtual light source 32c is also asymmetric. Therefore, when a tracking error signal is detected using such a beam, the amplitude of the signal becomes small,
When the optical disk is tilted, there is a disadvantage that an offset occurs in the signal.

The present invention has been made in view of the above circumstances, and in the case where a reflection type diffraction grating for dividing a beam into three is arranged at an angle with respect to a light source, the spot intervals of the beam divided into three are symmetrically collected into small spots. It is an object of the present invention to provide a reflective diffraction grating for splitting a beam into three light beams and an optical pickup using the same.

[0012]

SUMMARY OF THE INVENTION Therefore, the present invention is not limited to this .
The configuration 1 is inclined with respect to the optical axis of the divergent laser light source.
Distance from the laser emission point on the lattice plane
Beam 3 in which the incident angle of the laser beam is different in the inclination direction
Diffraction in the tilt direction
The laser, at least within the effective aperture
The greater the distance from the light emitting point or the angle of incidence of the laser beam
The larger the is, the longer the lattice period on that lattice plane
And the divergence
It is Wakashi substantially constant grating period in the vicinity of the optical axis of the laser beam source crosses
In other words, the change is more gradual than other parts .

In each of the second and third structures, the reflection type diffraction grating for splitting the beam into three beams has a grating period Λ (x) modulated by a predetermined formula.

The optical pickup having the fourth configuration is the same as the first embodiment.
And a reflective diffraction grating for splitting the beam into three beams according to any one of the third to third configurations.

[0015]

According to the first to third configurations, the laser beam on the lattice plane is formed.
Even if the incident angle of the laser beam is different in the inclination direction, it is diffracted into three parts
Symmetrical and small spot spacing
Focusing is possible, and periodic changes near the origin
Is so loose that the diffraction grating
Of the optical pickup can be tolerated.
Easy assembly can be achieved.

According to the optical pickup of the fourth configuration, the amplitude of the tracking error signal can be increased,
Furthermore, even when the optical disk is inclined, it is possible to avoid occurrence of an offset in the signal, so that the reliability of the tracking servo can be improved.

[0017]

EXAMPLES (Example 1) First, the beam is divided into three reflection type diffraction according to the reference example of the present invention
The folded lattice will be described with reference to FIGS. It should be noted that the manufacturing method and the point that the semiconductor laser is used while being inclined with respect to the optical axis of the semiconductor laser are the same as those in the related art. That is, the reference example of the present invention and
The reflection diffraction grating for beam splitting according to the embodiment is arranged as shown in FIG. 10 shown in the conventional example. In the following description, FIG. 10 will be referred to as needed.

FIG. 1 is a longitudinal sectional side view showing a reflection type diffraction grating 11 for beam splitting according to the present embodiment . The direction X in the figure,
The origin (the point coincident with the optical axis) O coincides with the direction X and the origin O shown in FIG.

The above-described reflection diffraction grating for beam splitting 11
Are the ± first-order virtual light sources 3 formed by three-division diffraction
The periodic modulation of the grating is performed so that 1b and 31c are symmetric with respect to the 0th-order virtual light source 31a and the width of the light intensity distribution becomes narrow. Specifically, in the X direction passing through the origin O, as the incident angle of the laser beam increases (in other words, as the distance from the laser emission point increases), the lattice plane (the unevenness of the diffraction grating becomes larger) The grating is modulated so that the grating period Λ (x) on the formed surface 11a becomes longer. Here, x indicates position coordinates in the X direction on the lattice plane.

The lattice period Λ (x) is given by the following equation (1).
Is determined by

[0021]

１ (x) = (１ ／) [λ / h ₁ (x) + λ / h ₂ (x)] where h ₁ (x) = sin (tan ⁻¹ ((dx) / (l + x)) - π / 4 ) + sin (tan -1 (x / (l + x)) + π / 4) h 2 (x) = sin (tan -1 (-x / (l + x)) -π / 4) + sin (tan ^-1 ((d + x) / (l + x)) + π / 4) λ: wavelength of laser light l: distance between laser emission point and origin O d: ± 1 Position of the virtual diffracted light source in the x 'direction

FIG. 2 shows that in equation (1) above, λ = 0.7
It is a graph which showed the change of lattice period (xi) when 9 micrometers, l = 2000 micrometers, and d = 110 micrometers. In this figure, the side where the laser beam incident angle (the angle made with the lattice plane normal) is large, in other words, the side where the distance from the laser light source is far away is denoted as the plus side, and the opposite side Is indicated as a minus side. The dotted line in the drawing indicates the grating period of the conventional beam-reflection reflective diffraction grating. The same applies to the following Reference Examples and Examples.

There is a portion on the minus side of the position on the grating plane in the graph of FIG. 2 where the inclination of the graph is reversed, but this portion is outside the effective aperture of the diffraction grating. . The same applies to the following Reference Examples and Examples .

FIG. 3 is a graph showing the light intensity distribution of the virtual light emitting point of the ± 1st-order diffracted light in the case where the grating surface 11a is periodically modulated as described above. The ± 1st-order virtual light source is symmetrical with respect to the 0th-order virtual light source and the width of the light intensity distribution is narrow, which is considerably improved as compared with the conventional diffraction grating for splitting a beam shown in FIG. .

Embodiment 2 Hereinafter, another embodiment of the present invention will be described with reference to FIG.

The beam is divided into three reflection type diffraction grating of the present embodiment is to modulate the grating period by Equation 2 (Equation 2) shown below which approximates the equation 1 of Example 1 by a quadratic function.

[0027]

２ (x) = a (x−b) ² + c where a, b and c are constants

FIG. 4 shows that in the above equation 2, a = 1.3.
It is the graph which showed the change of the grating period (x) when 95 * 10 < ^-5 >, b = -720 and c = 12.75. A result almost similar to the result shown in FIG. 2 can be obtained.

[0029] (Reference Example 3) Hereinafter, another exemplary embodiment of the present invention will be described with reference to FIG.

The reflection type diffraction grating for beam splitting of the present embodiment modulates the grating period by the following equation (3) using a wave optics method.

[0031]

数 (x) = λh ₃ (x) / h ₄ (x) where h ₃ (x) = [｛(xx ₀ ) ² + y ₀ ² ｝ ｛(xx ₁ ) ² + y ₁ ^{2 _{}] 1/2 h 4 (x)}} = (xx 0) ^{_{[(xx 1) 2 + y 1}} 2 ] ^1/2 - (xx _{1) ^{[(xx 0) 2 + y 0}} 2 ] ^1/2 x ₀ = -Y ₀ = -l / 2 ^1/2 x ₁ = (-l + d) / 2 ^1/2 y ₁ = (-ld) / 2 ^1/2 or x ₁ = (-ld) / 2 ^{1 / 2} y ₁ = (− l + d) / 2 ^1/2 where λ, l, d, and x are defined in the same manner as in Reference Example 1 .

FIG. 5 shows that in equation (3) above, λ = 0.7
9 is a graph showing a change in lattice period Λ (x) when 9 μm, l = 2000 μm, and d = 110 μm. In this case as well, in the X direction passing through the origin O, the larger the incident angle of the laser light, The grating period on the grating surface 11a ａ
(X) becomes longer. In addition, the reflection type diffraction grating 11 for splitting the beam having the grating period obtained by the wave optical method could obtain the best result through these reference examples.

Embodiment 4 Hereinafter, another embodiment of the present invention will be described with reference to FIG.

In this embodiment , as shown in FIG. 6, the change of the grating period Λ (x) is linearly approximated. The figure shows a case where the inclination is set to 0.016. In the case of this linear approximation,
Although the spot width of the virtual light source is slightly widened, there is an advantage that the design of the lattice plane is easy.

[0035] (Example 1) Hereinafter, an embodiment of the present invention will be described with reference to FIG.

The reflective diffraction grating for beam splitting of the present embodiment can reduce the positional accuracy of the grating arrangement. That is, in Reference Examples 1 to 4, there is a disadvantage that if the mounting position of the diffraction grating is shifted in the x direction, the interval between the virtual light sources is shifted from the design value. Since it is gradual, it is possible to allow a slight shift of the mounting position of the diffraction grating, thereby facilitating the assembly of the optical pickup.

In this embodiment, the grating period Λ (x) is given by the following equation (4), which is a quartic function.

[0038]

４ (x) = a ₁ x ³ (b ₁ x−c ₁ ) + f ₁ where a ₁ , b ₁ , c ₁ , and f ₁ are constants

FIG. 6 shows that a ₁ = 2.
871 × 10 ⁻¹¹ , b ₁ = 3, c ₁ = −2880, f ₁
6 is a graph showing a change in lattice period Λ (x) when = 20.23. As is clear from the figure, the period change near the origin O is substantially constant.

Embodiment 2 Hereinafter, another embodiment of the present invention will be described with reference to FIG.

The beam-reflection reflective diffraction grating of this embodiment can relax the positional accuracy of the grating arrangement, as in the first embodiment . Then, the lattice period Λ (x) is given by the following equation 5.
It is given by (Equation 5) .

[0042]

## EQU5 ## In the region where x ≦ 0, Λ (x) = a ₂ cos (xπ / b ₂ ) + c ₂ x> 0, and in the region where Λ (x) = f ₂ x ^1.5 + g ₂ where a ₂ , b ₂ , c ₂ , f ₂ and g ₂ are constants

FIG. 8 shows that a ₂ = 3.
_{858, b 2 = 720, c} 2 = 16.37, f 2 = 1.
It is the graph which showed the change of lattice period Λ (x) when 230 × 10 ^-10 and g ₂ = 20.23. As is clear from the figure, the period change near the origin O is substantially constant.

Expressions 1 to 5 shown in the above description are examples, and the grating period may be expressed by another function. Of course, other numerical values may be used as variables or constants shown together.

Third Embodiment Next, an optical pickup will be described. The optical pickup of the present invention is provided with the above-described reflective diffraction grating 11 for dividing the beam into three. Needless to say, the reflection grating 11 for dividing the beam into three beams is used while being inclined with respect to the optical axis of the divergent laser light source. That is, the reflection grating for three-beam splitting of the present invention is arranged as shown in FIG. 9 shown in the conventional example, and other configurations can be configured similarly to FIG.

According to such an optical pickup, the amplitude of the tracking error signal can be increased, and furthermore,
It is possible to avoid occurrence of an offset in the signal even when the optical disk is tilted, so that the reliability of the tracking servo can be improved.

In the above embodiment, the period is modulated only with respect to the change in the distance between the light incident point and the laser emission point on the lattice plane passing through the origin O, and the period changes in the direction orthogonal to the X direction. Although it is not performed (that is, a parallel linear grid pattern), it is orthogonal to the X direction in consideration of the change in the distance between the laser emission point in the X direction that does not pass through the origin O and the light incident point on the grid surface. Alternatively, the grating period may be different at each position in the direction (ie, a curved grating pattern).

In addition, the present invention is not limited to the above embodiment. In a three-beam reflecting diffraction grating for reflecting and splitting an incident beam, the spots formed by the three split beams are arranged at equal intervals on a straight line and at respective intervals. It may be configured to have a diffraction grating shape in which the spot is reduced,
In this configuration, the incident beam may be a non-parallel beam.

[0049]

As described above, according to the reflection grating for beam splitting of the present invention, the spots formed by the beam split into three beams are equally spaced on a straight line and each spot is small. , It is possible to converge the spot interval of the beam divided into three parts symmetrically and to a small spot,
Further, by periodically changing near the origin is gentle,
Since it is possible to allow a slight shift in the mounting position of the diffraction grating, it is possible to easily assemble the optical pickup. Further, according to the optical pickup of the present invention, the amplitude of the tracking error signal is increased, and further, it is possible to avoid the occurrence of an offset in the signal even when the optical disk is tilted, thereby improving the reliability of the tracking servo. This has the effect that it can be performed.

[Brief description of the drawings]

1 is a vertical sectional side view of the beam is divided into three reflection type diffraction grating of Reference Example 1 in the onset bright.

2 is a graph showing the grating period of the beam is divided into three reflection type diffraction grating of Reference Example 1 in the onset bright.

FIG. 3 is a graph showing a light intensity distribution with respect to a virtual light source position by a three-beam reflecting diffraction grating having the grating period shown in FIG. 2;

FIG. 4 is a graph showing a grating period of a reflective diffraction grating for beam splitting according to Reference Example 2 of the present invention.

FIG. 5 is a graph showing a grating period of a reflective diffraction grating for beam splitting according to a third embodiment of the present invention.

FIG. 6 is a graph showing a grating period of a reflection grating for three-beam splitting according to a fourth embodiment of the present invention.

FIG. 7 is a graph showing a grating period of the reflection grating for beam splitting according to the first embodiment of the present invention.

FIG. 8 is a graph showing a grating period of a reflective diffraction grating for beam splitting according to a second embodiment of the present invention.

FIG. 9 is a schematic sectional view of an optical pickup.

FIG. 10 shows that the light beam emitted from the semiconductor laser is 4
The figure shows a state in which a beam is divided into three parts and reflected by a reflection grating for three divisions arranged at an angle of 5 °, and a virtual light source is used for a semiconductor laser 4 with reference to the diffraction grating surface.
FIG. 4 is an explanatory diagram showing a state where the light emitting device is formed at a position symmetrical with FIG.

FIG. 11 is a vertical sectional side view showing a conventional reflection diffraction grating for dividing a beam into three.

FIG. 12 is a graph showing a light intensity distribution with respect to a virtual light source position by a conventional three-beam reflective diffraction grating.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Good thermal conductive substrate 4 Semiconductor laser chip 5 PIN photodiode for signal detection 7 Hologram element 8 Condensing lens 9 Optical disk 9 11 Reflection type diffraction grating for beam splitting 11a grating surface

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takao Yamaguchi 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Tatsuhiko Niname 2-5-5 Keihanhondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd. (56) References JP-A-63-39146 (JP, A) JP-A-63-76119 (JP, A) JP-A-63-58629 (JP, A) JP-A-61 -265746 (JP, A) JP-A-3-212828 (JP, A) JP-A-61-151844 (JP, A) JP-A-60-173134 (JP, U) (58) Fields investigated (Int. .7, DB name) G11B ⁷ /12-7/22 G02B 5 / 18,5 / 30,5 / 32

Claims (4)

(57) [Claims] A diverging laser light source which is inclined with respect to an optical axis of the diverging laser light source;
The distance from the laser emission point on the lattice plane
Is a beam whose laser beam incident angle differs in the tilt direction
Diffraction in the inclination direction of the reflection grating for three divisions
At least within the effective aperture.
The greater the distance from the light emitting point or the more the laser beam
The larger the angle, the longer the lattice period on that lattice plane
The periodic modulation of the grating is
The grating period near the intersection of the optical axes of the scattered laser light source is almost constant.
Or a reflection diffraction grating for splitting a beam into three parts, the change of which is more gradual than other parts . The lattice period Λ (x) is expressed as follows: Λ (x) = a ₁ x ³ (b ₁ x−c ₁ ) + f ₁ where a ₁ , b ₁ , c ₁ , and f ₁ are constants and x is a constant. The optical axis of the laser beam
Distance in the direction of inclination on the grid plane from the point of intersection with the grid plane
The reflection grating for beam splitting according to claim 1 , wherein the reflection grating is modulated as follows . 3. In a region where the lattice period Λ (x) is x ≦ 0, in a region where Λ (x) = a ₂ cos (xπ / b ₂ ) + c ₂ x> 0, Λ (x) = f ₂ x ^1.5 + g ₂ where a ₂ , b ₂ , c ₂ , f ₂ and g ₂ are constants and x is the laser light
In the direction of inclination on the lattice plane from the point where the optical axis intersects the lattice plane
Characterized by being modulated as the distance of
2. The reflective diffraction grating for beam splitting according to item 1 . 4. An optical pickup comprising the reflection diffraction grating for splitting a beam into three beams according to claim 1 .