JPH07302435A - Reflection type diffraction grating for trisecting beam and optical pickup formed by using the same - Google Patents

Abstract

PURPOSE:To form a reflection type diffraction grating for trisecting a beam by condensing the trisected and diffracted beams at symmetrical spot intervals to a small spot in the case of arranging the reflection type diffraction grating for trisecting the beam with a inclination with a light source. CONSTITUTION:This reflection type diffraction grating 11 for trisecting the beam is used by inclining the diffraction grating with the optical axis of the diverging laser beam source and eventually the incident angle of the laser beam varies in the inclination direction on the grating surface. The diffraction grating described above has the effect of diffracting the beam in this inclination direction. The period of the grating is so modulated that the period of the grating is longer on the grating surface 11a as the incident angle of the laser beam is larger within at least the effective aperture. The + or -1st order virtual light sources formed by the trisected diffraction effect are so formed as to be symmetrical with the zero order virtual light source as a center and to be narrow in the width of the light intensity distribution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflective diffraction grating for dividing a beam into three beams, which is used by being inclined 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.
When the beam method is used, in order to avoid the defect of the transmission type beam division grating for division of three beams that the signal light is incident on the diffraction grating again, a reflection type beam division grating for division of three beams is used. Has been. The reflection type diffraction grating is formed by etching the surface of the substrate to form irregularities, and
It is manufactured by a manufacturing method such as vapor deposition of u and the like.

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

A hologram element 7 is arranged by a supporting means (not shown) above the element arrangement surface, and a condenser lens 8 attached to an actuator (not shown) is arranged above the hologram element 7. There is. When the light emitted from the semiconductor laser chip 4 is reflected by the reflection type diffraction grating 6 for dividing the beam into three, it is divided into three beams by the diffractive action. And these three beams are
After passing through the hologram element 7 (zero-order diffraction), it is focused on the signal recording surface of the optical disc 9 by the focusing lens 8.

The reflected light of the three beams carrying the pit signal (HF signal), the tracking error signal, and the focusing error signal passes through the condenser lens 8 again, and is then diffracted by the hologram element 7 in the first order (or −). The light is incident on the PIN photodiode 5 for signal detection. 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 diffracted light astigmatism.

In the optical pickup using the reflection type diffraction grating 6 for dividing the beam into three beams, the respective elements except the hologram element 7 can be arranged on substantially the same plane, and the semiconductor laser 4 and the photodiode 5 can be arranged. It is easy to produce wire bonding and other wire bonding, and the cost can be reduced.

[0007]

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

FIG. 10 shows a state in which a light beam emitted from the semiconductor laser 4 is diffracted in three divisions by a reflection type diffraction grating 6 for dividing the beam into three portions and is reflected by a virtual light source. FIG. 6 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 addition, X in the figure indicates the direction in which the reflection diffraction grating 6 for dividing the beam into three beams is inclined and the laser light incident angle is increased. O indicates the origin (point that coincides with the optical axis). x'represents the distance (with 0 as the origin) in the direction corresponding to the above-mentioned direction X with respect to the virtual light source, and d represents the position of the virtual light source on x '.

As shown in FIG. 11, the conventional reflection type diffraction grating 6 for dividing the beam into three beams is composed of an equal period diffraction grating in which the grating pitches (Λ ₀ ) are all equal in the X direction in the figure. When the light intensity distribution of the virtual light emitting points of the ± first-order diffracted light is calculated, the distribution is considerably wide as shown in FIG.
That is, it is not a single point and cannot be focused on a spot with a small diffraction limit. The 110 μm position in FIG. 12 corresponds to the 20 μm position 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 are asymmetrical with the semiconductor laser optical axis 23 as the center, the + 1st-order virtual light source 32b and −
The spot spacing of the primary virtual light source 32c is also asymmetric. Therefore, when the tracking error signal is detected using such a beam, the signal amplitude becomes small,
When the optical disk is tilted, there is a drawback that an offset occurs in the signal.

In view of the above-mentioned circumstances, the present invention provides that when the reflection diffraction grating for dividing the beam into three beams is tilted with respect to the light source, the spot intervals of the beams diffracted into three beams are symmetrically gathered into small spots. An object of the present invention is to provide a reflective diffraction grating for splitting a beam into three beams and an optical pickup using the same.

[0012]

Therefore, in the first configuration, in the reflection type diffraction grating for dividing the beam into three beams that reflects and splits the incident beam, the converging spot formed by the three-divided beam is on a straight line. It has a diffraction grating shape at equal intervals so that each spot becomes smaller. In the second configuration, the incident beam is a non-parallel beam.

The reflection diffraction grating for dividing the beam into three beams of the third structure is used while being inclined with respect to the optical axis of the diverging laser light source, and the distance from the laser emission point on the grating surface or the laser light incident angle is used. In the reflective diffraction grating for dividing the beam into three beams, which is different in the tilt direction, the beam has a function of diffracting in the tilt direction, and the position of the ± first-order virtual light source formed by the three-way diffraction function is the zero-order virtual light source. It is characterized in that the grating is periodically modulated so as to be symmetrical about the center and to narrow the width of the light intensity distribution of the ± first-order virtual light sources.

The reflection type diffraction grating for dividing the beam into three beams of the fourth structure is used by being inclined with respect to the optical axis of the divergent laser light source, and the distance from the laser emission point on the grating surface or the laser light incident angle is used. In the reflection type diffraction grating for dividing the beam into three beams, which has different tilt directions, it has a function of diffracting in the tilt direction, and as the distance from the laser emission point increases or the laser light enters, at least within the effective aperture. It is characterized in that the grating is periodically modulated so that the larger the angle, the longer the grating period on the grating surface.

The reflection type diffraction grating for dividing the beam into three beams according to the fifth configuration has the grating period near the intersection of the optical axes of the diverging laser light sources being substantially constant or changing from the other portions in the third or fourth configuration. May be loose.

In the reflection type diffraction grating for dividing the beam into three beams having the sixth to tenth configurations, the grating period Λ (x) is modulated by a predetermined formula.

An eleventh structure of an optical pickup is characterized by including the beam-dividing reflective diffraction grating of any one of the first to tenth structures.

[0018]

According to the first or second structure, the spots formed by the three-divided beam are arranged at equal intervals on a straight line and the respective spots are small.

According to the third to tenth configurations, even if the laser beam incident angle on the grating surface is different in the tilt direction, the spot intervals of the beams diffracted in three parts are symmetrically condensed into small spots. be able to.

According to the reflection type diffraction grating for dividing the beam into three beams of the fifth, ninth and tenth configurations, the periodical change near the origin is gradual. Therefore, in addition to the above operation, the mounting position of the diffraction grating It is possible to allow some deviation, which facilitates the assembly of the optical pickup.

According to the optical pickup having the eleventh structure,
Since the amplitude of the tracking error signal can be increased and the offset of the signal can be avoided even when the optical disc is tilted, the reliability of the tracking servo can be improved.

[0022]

【Example】

(Embodiment 1) A reflective diffraction grating for dividing a beam into three beams according to the present invention will be described below with reference to FIGS. It should be noted that the manufacturing method and the point of using the semiconductor laser inclined with respect to the optical axis of the semiconductor laser are the same as in the conventional case. That is, the reflection diffraction grating for dividing the beam into three beams according to the present invention is arranged as shown in FIG. In the following description, FIG. 10 will be referred to as necessary.

FIG. 1 is a vertical sectional side view showing a reflective diffraction grating 11 for dividing a beam into three parts according to this embodiment. Direction X in the figure,
And the origin (point that coincides with the optical axis) O coincides with the direction X and the origin O shown in FIG.

The above-mentioned reflective diffraction grating 11 for dividing the beam into three parts.
Is a ± first-order virtual light source 3 formed by three-division diffraction action
Periodic modulation of the gratings is performed so that 1b and 31c are symmetrical about the 0th virtual light source 31a and the width of the light intensity distribution is narrowed. Specifically, in the X direction passing through the origin O, the larger the incident angle of the laser light is (in other words, the farther the distance from the laser emission point is), the lattice surface (the unevenness of the diffraction grating is Periodic modulation of the grating is performed so that the grating period Λ (x) on the formed surface) 11a becomes long. However, x indicates the position coordinate in the X direction on the lattice plane.

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

[0026]

Λ (x) = (1/2) [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: Laser light wavelength l: Distance between laser emission point and origin O d: ± 1st order diffracted light On the x'direction of the virtual light source Position in

FIG. 2 shows that in the above equation 1, λ = 0.7.
9 is a graph showing changes in the grating period Λ (x) when 9 μm, l = 2000 μm, and d = 110 μm. In this figure, the side where the laser light incident angle (the angle formed by the normal to the lattice plane) is large, in other words, the side where the distance from the laser light source is far is described as the plus side, and the opposite side. Is shown as a minus side. The dotted line in the figure indicates the grating period of the conventional reflective diffraction grating for dividing the beam into three parts. The same applies to the following examples.

It should be noted that there is a portion where the inclination of the graph is reversed on the minus side of the position on the grating surface in the graph of FIG. 2, but this part is a part which is out of the effective aperture of the diffraction grating. . The same applies to the following examples.

FIG. 3 is a graph showing the light intensity distribution of the virtual light emitting points of the ± 1st order diffracted light in the case where the grating surface 11a which is periodically modulated as described above is provided. The ± first-order virtual light sources are symmetrical with respect to the zero-order virtual light source and the width of the light intensity distribution is narrowed, which is considerably improved as compared with the conventional reflective diffraction grating for dividing the beam into three beams shown in FIG. .

(Embodiment 2) Another embodiment of the present invention will be described below with reference to FIG.

In the reflective diffraction grating for dividing the beam into three beams according to the present embodiment, the grating period is modulated by the following expression 2 which is obtained by approximating the expression 1 of the embodiment 1 by a quadratic function.

[0032]

[Formula 2] Λ (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.
9 is a graph showing changes in the lattice period Λ (x) when 95 × 10 ⁻⁵ , b = −720, and c = 12.75. It is possible to obtain almost the same result as the result shown in FIG.

(Embodiment 3) Another embodiment of the present invention will be described below with reference to FIG.

The reflection type diffraction grating for dividing the beam into three beams according to the present embodiment modulates the grating period by the following equation 3 using a wave optical technique.

[0036]

## EQU3 ## Λ (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 Note that λ, l, d and x are defined as in the first embodiment.

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

(Embodiment 4) Another embodiment of the present invention will be described below with reference to FIG.

In this embodiment, as shown in FIG. 6, the change of the grating period Λ (x) is approximated by a straight line. The figure shows the case where the inclination is 0.016. For this linear approximation,
Although the spot width of the virtual light source is slightly widened, it has an advantage that the lattice plane can be easily designed.

(Embodiment 5) Another embodiment of the present invention will be described below with reference to FIG.

The reflection type diffraction grating for dividing the beam into three beams according to the present embodiment can relax the positional accuracy of the grating arrangement. That is, in Examples 1 to 4, there is a defect that the distance between the virtual light sources deviates from the design value when the mounting position of the diffraction grating deviates in the x direction. However, according to this Example, the periodic change near the origin O Since it is gradual, it is possible to allow a slight shift in the mounting position of the diffraction grating, which facilitates the assembly of the optical pickup.

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

[0043]

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.

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

(Embodiment 6) Another embodiment of the present invention will be described below with reference to FIG.

The reflection type diffraction grating for dividing the beam into three beams according to the present embodiment can relax the positional accuracy of the grating arrangement, as in the fifth embodiment. Then, the lattice period Λ (x) is expressed by the following equation 5
Is given by.

[0047]

In the region of x ≦ 0, Λ (x) = a ₂ cos (xπ / b ₂ ) + c _{2 In} the region of 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

FIG. 8 shows that a ₂ = 3.
858, b ₂ = 720, c ₂ = 16.37, f ₂ = 1.
6 is a graph showing changes in the grating period Λ (x) when 230 × 10 ⁻¹⁰ and g ₂ = 20.23. As is clear from the figure, the periodic change near the origin O is substantially constant.

The expressions 1 to 5 shown in the above description are examples, and the lattice period may be expressed by another function. Of course, other numerical values may be used as the variables or constants shown with these.

(Embodiment 7) Next, an optical pickup will be described. The optical pickup of the present invention comprises the reflection type diffraction grating 11 for dividing the beam into three beams as described above. Of course, the beam-divided reflection type diffraction grating 11 is used while being inclined with respect to the optical axis of the diverging laser light source. That is,
The reflection type diffraction grating for dividing the beam into three beams according to the present invention is arranged as shown in FIG. 9 showing the conventional example, and other configurations are also as follows.
It can be configured similarly to FIG.

According to such an optical pickup, the amplitude of the tracking error signal can be increased, and further,
Even when the optical disc is tilted, it is possible to avoid the occurrence of an offset in the signal, so that it is possible to improve the reliability of the tracking servo.

In the above embodiment, the cycle 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 cycle changes in the direction orthogonal to the X direction. Although not (that is, parallel linear grid pattern), 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 is taken into consideration, and is orthogonal to the X direction. The lattice period may be different at each position in the direction (that is, a curved lattice pattern).

Further, the present invention is not limited to the above-mentioned embodiments, but in the reflection type diffraction grating for dividing the beam into three beams which reflects and splits the incident beam, the spots formed by the three-divided beam are arranged on a straight line at equal intervals. It may be configured to have a diffraction grating shape with a small spot,
Further, in this configuration, the incident beam may be a non-parallel beam.

[0054]

As described above, according to the reflection type diffraction grating for splitting a beam into three beams according to the present invention, the spots formed by the beam split into three beams are arranged at equal intervals on a straight line, and the respective spots become smaller. The spot spacing of the beam diffracted in three divisions can be focused symmetrically and into a small spot,
Further, in the configuration in which the periodical change near the origin is gradual, it is possible to allow a slight shift in the mounting position of the diffraction grating, so that the assembly of the optical pickup can be facilitated.
Further, according to the optical pickup of the present invention, the amplitude of the tracking error signal becomes large, and further, it is possible to avoid the occurrence of offset in the signal even when the optical disc is tilted, so that the reliability of the tracking servo is improved. There is an effect that can be.

[Brief description of drawings]

FIG. 1 is a vertical sectional side view of a reflective diffraction grating for dividing a beam into three beams according to the present invention.

FIG. 2 is a graph showing a grating period of a reflection type diffraction grating for dividing a beam into three beams according to the present invention.

FIG. 3 is a graph showing a light intensity distribution with respect to a virtual light source position by a beam-dividing reflective diffraction grating having the grating period of FIG.

FIG. 4 is a graph showing a grating period of a reflective diffraction grating for dividing a beam into three beams according to another embodiment of the present invention.

FIG. 5 is a graph showing a grating period of a reflective diffraction grating for dividing a beam into three beams according to another embodiment of the present invention.

FIG. 6 is a graph showing a grating period of a reflective diffraction grating for dividing a beam into three beams according to another embodiment of the present invention.

FIG. 7 is a graph showing a grating period of a reflective diffraction grating for dividing a beam into three beams according to another embodiment of the present invention.

FIG. 8 is a graph showing a grating period of a reflective diffraction grating for dividing a beam into three beams according to another 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 semiconductor laser 4 shows a state in which a virtual light source is diffracted and reflected by the reflection type diffraction grating for dividing the beam into three beams, which is arranged at an angle of 5 °, and the virtual light source is based on the diffraction grating surface.
FIG. 6 is an explanatory view showing a state of being formed at a position symmetrical with.

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

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

[Explanation of symbols]

 1 Good Thermal Conductivity Substrate 4 Semiconductor Laser Chip 5 Signal Detection PIN Photodiode 7 Hologram Element 8 Condensing Lens 9 Optical Disc 9 11 Reflective Diffraction Grating for Beam 3 Division 11a Lattice Surface

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takao Yamaguchi 2-5-5 Keihan Hondori, Moriguchi City, Osaka Prefecture Sanyo Electric Co., Ltd. (72) Inventor Tatsuhiko Shinmei 2-chome, Keihan Hondori, Moriguchi City, Osaka Prefecture No. 5-5 Sanyo Electric Co., Ltd.

Claims (11)

[Claims] 1. A reflective diffraction grating for splitting a beam into three beams, which reflects and splits an incident beam, so that condensed spots formed by the three-split beam are arranged at equal intervals on a straight line and each spot becomes small. A reflective diffraction grating for dividing the beam into three beams, which has the above-mentioned diffraction grating shape. 2. The reflection diffraction grating for splitting a beam into three beams according to claim 1, wherein the incident beam is a non-parallel beam. 3. A reflection type for dividing into three beams, which is used by being inclined with respect to the optical axis of a divergent laser light source, and the distance from the laser emission point on the grating surface or the laser light incident angle is different in the inclination direction. In the diffraction grating, it has a function of diffracting in its tilt direction, and is formed by a three-division diffraction function ± 1
The beam is divided into three parts, wherein the position of the secondary virtual light source is symmetrical about the zeroth virtual light source, and the periodic modulation of the grating is performed so that the width of the light intensity distribution of the ± primary virtual light sources is narrowed. Reflective diffraction grating. 4. A reflection type for dividing into three beams, which is used while being inclined with respect to the optical axis of a divergent laser light source, and the distance from the laser emission point on the grating surface or the laser light incident angle is different in the inclination direction. In the diffraction grating, it has a function of diffracting in its tilt direction, and at least within the effective aperture, the larger the distance from the laser emission point or the larger the laser light incident angle, the grating period on the grating surface. A reflection type diffraction grating for dividing a beam into three beams, characterized in that the period of the grating is modulated so as to be longer. 5. The beam 3 according to claim 3, wherein the grating period in the vicinity of the intersection of the optical axes of the diverging laser light source is substantially constant or the change is gentler than in other portions. Reflective diffraction grating for division. 6. The lattice period Λ (x) is Λ (x) = (1/2) [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) x: Distance in the tilt direction on the lattice plane from the point O where the laser emission point and its optical axis intersect the lattice plane λ: Wavelength of laser light l: Distance between laser emission point and point O where its optical axis intersects the lattice plane d: ± 1st-order diffracted light Virtual light source is modulated like 0th-order diffracted light virtual light source The reflective diffraction grating for dividing the beam into three beams according to claim 3 or 4. 7. The grating period Λ (x) is Λ (x) = a (x−b) ² + c, where a, b, and c are constants, and x is a point from which the optical axis of the laser light intersects the lattice plane. 5. The reflection type diffraction grating for dividing the beam into three parts according to claim 3 or 4, wherein the reflection type diffraction grating is modulated like a distance in a tilt direction on the grating surface. 8. The lattice period Λ (x) is Λ (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 x: On the lattice plane from the point O where the laser emission point and its optical axis intersect the lattice plane In the tilt direction at λ: wavelength of laser light l: distance between laser emission point and point O where its optical axis intersects the lattice plane d: ± first-order diffracted light virtual light source distance from zero-order diffracted light virtual light source 5. The beam-divided reflection type diffraction grating according to claim 3, wherein the reflection diffraction grating is modulated as follows. 9. The lattice period Λ (x) is Λ (x) = a ₁ x ³ (b ₁ x−c ₁ ) + f ₁ where a ₁ , b ₁ , c ₁ , f ₁ are constants and x is 6. The reflection diffraction grating for beam splitting according to claim 5, wherein the optical axis of the laser light is modulated like a distance in a tilt direction on the grating surface from a point where the optical axis intersects the grating surface. 10. A region where the lattice period Λ (x) is x ≦ 0, and 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 distance in the tilt direction on the lattice plane from the point where the optical axis of the laser beam intersects the lattice plane. The reflective diffraction grating for dividing the beam into three beams according to claim 5, wherein the reflective diffraction grating is modulated. 11. An optical pickup comprising the reflective diffraction grating for splitting a beam into three beams according to claim 1. Description: